WO2005112608A2

WO2005112608A2 - Modified plant transcription factors

Info

Publication number: WO2005112608A2
Application number: PCT/US2005/017583
Authority: WO
Inventors: Michael Thomashow; Sarah J. Gilmour; Daniel D. Cook; Donatella Canella
Original assignee: Michigan State University
Priority date: 2004-05-21
Filing date: 2005-05-19
Publication date: 2005-12-01
Also published as: WO2005112608A3; EP1766019A4; EP1766019A2

Abstract

The invention relates to modified plant transcription factor polypeptides, polynucleotides that encode them, homologs from a variety of plant species, and methods of producing transgenic plants having advantageous properties, including increased abiotic stress tolerance, as compared to wild-type or control plants. Without the disclosed modifications, the transcription factor sequences, when overexpressed in plants, often produce adverse morphological and developmental effects. The disclosed modifications mitigate these adverse morphological and developmental effects.

Description

REGULATION OF PLANT STRESS TOLERANCE BY MODIFIED TRANSCRIPTION FACTORS

FIELD OF THE INVENTION The present invention relates to compositions and methods for producing plants with improved abiotic stress tolerance. BACKGROUND OF THE INVENTION Abiotic stresses, including freezing temperatures, drought and high salinity, cause significant productivity losses and greatly limit the geographical locations where crops can be grown. A common goal of plant breeding programs is to improve the abiotic stress tolerance of crop and horticultural plants. Identifying the genetic and molecular mechanisms for abiotic stress tolerance has led to the discovery of the CBF regulatory pathways that have vital roles in freezing and drought tolerance. CBF regulatory pathways were discovered during the study of the cold acclimation response, in which some plants increase in freezing tolerance in response to low, non-freezing temperatures (Guy (1990) Annu. Rev. Plant Physiol. Plant Mol. Biol. 41: 187-223; Thomashow (1999) Annu. Rev. Plant Physiol. Plant Mol. Biol. 50: 571-599). This cold acclimation involves the CBF cold-response pathway (Thomashow (2001) Plant Physiol. 125: 89-93). Within 15 minutes of exposing plants to low temperature, transcripts for a family of genes designated either CBF1, CBF2 and CBF3 accumulate (Gilmour et al. (1998) Plant J. 16: 433-442; Jaglo-Ottosen et al. (1998) Science 280: 104-106; Medina et al. (1999) Plant Physiol. 119: 463^170), oxDREBlb, DREBlc and DREB1 a (Liu et al. (1998) Plant Cell 10: 1391-1406). These genes encode transcriptional activators that are members of the AP2/EREBP family of DNA binding proteins (Riechmann and Meyerowitz (1998) Biol. Chem. 319: 633-646). The CBF/DREB1 proteins bind to the cold- and dehydration-responsive DNA regulatory element designated the CRT (C-repeat)/DRE (dehydration response element) present in the promoters of COR (cold- regulated) and other cold responsive genes and stimulate their transcription (Baker et al. (1994) Plant. Mol. Biol. 24: 701-713; Yamaguchi-Shinozaki and Shinozaki (1994) Plant Cell 6: 251-264; Stockinger et al. (1997) Proc. Natl. Acad. Sci. 94: 1035-1040). Expression of this group of target genes, designated the CBF regulon, results in an increase in freezing tolerance (Jaglo-Ottosen et al. (1998) supra; Liu et al,

(1998) supra; Kasuga et al. (1999) Nature Biotechnol. 17: 287-291; Gilmour et al. (2000) Plant Physiol. 124: 1854-1865). Multiple mechanisms contribute to this increase in stress tolerance, including the synthesis of cryoprotective polypeptides such as COR15a (Artus et al. (1996) Proc. Natl. Acad. Sci. 93: 13404-13409; Steponkus et al. (1998) Proc. Natl. Acad. Sci. 95: 14570-14575), and the accumulation of compatible solutes with cryoprotective properties, including sucrose, raffinose and proline (Nanjo et al.

(1999) FEBSLett. 461: 205-210; Gilmour et al. (2000) supra; Taji et al. (2002) Plant J. 29: 417-426). When CBF1, CBF2 or CBF3 are constitutively overexpressed in Arabidopsis or canola, they induce expression of downstream target genes, the CBF regulon, and increase freezing tolerance of non- acclimated and cold-acclimated plants (Kasuga et al. (1999) supra, Jaglo-Ottosen et al. (1998) supra, Gilmour et al. (2000) supra). Constitutive overexpression of CBF1 also increases the tolerance of plants to drought and high salinity (Liu et al. (1998) supra; Kasuga et al. (1999) Nature Biotechnol. 17: 287- 291). This cross-protection occurs because the injury caused by freezing, drought and high salinity largely result from cellular dehydration and low water activities associated with these stresses. Activation of the CBF regulatory pathway in response to low temperature is brought about by the CBF1, CBF2 and CBF 3 genes that are induced in response to low temperature. Activation of the CBF regulatory pathway by drought and high salinity is brought about by action of CBF4 and DREB2 genes, which are also AP2 domain proteins that are induced in response to drought and high salinity and bind to the CRT/DRE element. In addition to increased abiotic stress tolerance, high level expression of CBF1, CBF2, CBF3 and CBF4 may also cause adverse developmental and/or morphological effects, including stunted growth, delayed flowering, and a decrease in yield as evidenced by decreased biomass, fruit or seed size or number. These undesirable effects may be alleviated by expressing CBF1, CBF2, CBF3 or CBF4 and related genes in a conditional manner. Expression of CBF3 under control of a stress-inducible promoter can improve stress tolerance, but r nimize other potentially negative growth traits, since the gene is only expressed under stress conditions when action of the CBF regulon of genes is required. While modifying the expression of CBF genes has great utility in improving the stress tolerance of plants, additional approaches are also desirable. Here we describe an invention that leads to modified versions of CBF and other genes that provide improve the abiotic stress tolerance of plants with adverse morphological and/or developmental effects either reduced or eliminated.

SUMMARY OF THE INVENTION The present invention is directed to compositions and methods for increased yield and abiotic stress tolerance. Polynucleotides of the invention may be used to transform plants to achieve increased yield. The polynucleotides will typically comprise a nucleotide sequence that encodes a CBF transcription factor polypeptide. CBF transcription factor polypeptides are now well known in the art. CBF polypeptides possess an /VP2 domain that binds to a cold or dehydration transcription-regulating region of DNA comprising the sequence CCG. However, when CBF transcription factor polypeptides (that is, polypeptides within the CBF clade) are overexpressed in plants, the increased abiotic stress tolerance that often results comes at a cost; the plant may be quite small, have low fertility, and other developmental abnormalities or undesirable morphological characteristics. However, the polynucleotides of the present invention have at least one mutation and encode a variant CBF polypeptide. This variation may be in one or more amino acid residues, and may lie within or outside of the conserved domain (the AP2 domain) of the polypeptide. The mutation may be introduced into the CBF-encoding polynucleotide in vitro, prior to transformation of a plant with the polynucleotide, or in vivo, or after the plant has been transformed with a polynucleotide encoding a native CBF clade member, the plant is subjected to a mutagenesis process sufficient to mutagenize a region of the polynucleotide. The transformed plant may then be identified by the use of a selectable marker or by increased abiotic stress tolerance. Whether mutagenesis is carried out in vivo or in vitro, the transformed plant has greater tolerance to at least one abiotic stress than a control plant, such as a wild-type plant or a plant transformed with an empty vector. 5 The transformed plant also has fewer adverse morphological and/or developmental effects than a plant that overexpresses a non-mutant form of a member of the CBF clade of transcription factors. Transgenic plants produced by these methods, and seed produced from these transgenic plants, are also encompassed by the invention.

10 BRIEF DESCRIPTION OF THE SEQUENCE LISTING AND FIGURES The Sequence Listing provides exemplary polynucleotide and polypeptide sequences of the invention. The traits associated with the use of the sequences are included in the Examples. CD-ROMs Copy 1 and Copy 2 are read-only memory computer-readable compact discs that each contain a copy of the Sequence Listing in ASCII text format. The Sequence Listing 15 "MBI0075PCT.ST25.txt" is 268 kilobytes in size and is identical in content to the paper copy of the Sequence Listing. Figure 1 shows a conservative estimate of phylogenetic relationships among the orders of flowering plants (modified from Angiosper Phylogeny Group (1998) Ann. Missouri Bot. Gard. 84: 1- 49). Those plants with a single cotyledon (monocots) are a monophyletic clade nested within at least two 20 major lineages of dicots; the eudicots are further divided into rosids and asterids. Arabidopsis is a rosid eudicot classified within the order Brassicales; rice is a member of the monocot order Poales. Figure 1 was adapted from Daly et al. (2001) Plant Physiol. 127: 1328-1333. Figure 2 shows a phylogenic dendogram depicting phylogenetic relationships of higher plant _{ taxa, including clades containing tomato and Arabidopsis; adapted from Ku et al. (2000) Proc. Natl.

25 Acad. Sci. 97: 9121-9126; and Chase et al. (1993) Ann. Missouri Bot. Gard. 80: 528-580. Figures 3A-3G represent a multiple sequence alignment of some related polypeptides within the CBF clade of transcription factor polypeptides. The amino acid residues in the boxes correspond to seven residues of the non-mutant form of CBF2, and in corresponding residues in paralogous and orthologous sequences (examples in Figures 4 or 5, or in the Sequence Listing), that have been substituted to produce 30 a mutant form of the polypeptide. When overexpressed, the mutant forms may confer abiotic stress tolerance with fewer adverse morphological and/or developmental effects than the native polypeptides. Figure 4 is a phylogenetic tree and multiple sequence alignment of CBFs and related full-length proteins, constructed using ClustalW (CLUSTAL W Multiple Sequence Alignment Program version 1.83, 2003) andMEGA2 (http://www.megasoftware.net) software. ClustalW multiple alignment 35 parameters were as follows: Gap Opening Penalty : 10.00 Gap Extension Penalty :0.20 Delay divergent sequences :30 % DNA Transitions Weight :0.50 Protein weight matrix :Gonnet series DNA weight matrix :IUB Use negative matrix :OFF A FastA formatted alignment was then used to generate a phylogenetic tree in MEGA2 using the neighbor joining algorithm and a p-distance model. A test of phylogeny was done via bootstrap with 100 replications and Random Speed set to default. Cut off values of the bootstrap tree were set to 50%. Figure 5 shows an alignment of subsequences of several members of the CBF clade. The major domains of the CBF polypeptides are shown, including the CBF signature sequences (brackets) conserved among CBF proteins from diverse plant species (At refers to Arabidopsis thaliana, AD refers to the activation domain of CBF polypeptides). Figure 6 shows amino acid substitutions found in several CBF2 transgene mutants after random mutagenesis. Particularly noteworthy are the mutations that converted the glutamic acid residue at position 84 to a lysine residue, the aspartic acid residue at position 108 to an asparagine residue, and the serine residue at position 118 to a leucine residue. The first substitution is within the conserved AP2 domain, and the second and third substitutions are outside of (e.g., trailing, or closer to the C-terminus) the AP2 domain. Each of these three mutations has been shown to confer increased abiotic stress tolerance in overexpressing plants that retain morphological and developmental similarity to control or wild-type plants. The first CBF signature sequence, the AP2 domain, and the second CBF signature sequence are indicated as CBF_a, AP2, and CBF_b, respectively, in this figure. Figure 7 A compares two wild-type Arabidopsis (Ws-2) plants on the left of the figure with four CBF2-overexpressing plants overexpressing CBF2 (line E2) on the right. The four overexpressors were stunted and flowering was delayed with respect to the wild-type plants. Figure 7B is a Northern analysis showing the CBF gene target COR15a is expressed in warm-(W; about 22° C) and cold-grown (C; about 4° C) E2 plants, whereas it is only expressed in cold- grown (e.g., stressed) wild-type Ws-2 plants. Figure 8A shows a number of Arabidopsis mutants in the M2 population that more closely resembled wild-type Ws-2 plants in growth and development than plants overexpressing non-mutated CBF polypeptides. The mutant lines designated 7-2 and 5-4 were much bigger and flowered earlier that the E2 plants. There was very little difference, if any, between the 7-2 plants and the wild-type Ws-2 plants. Figure 8B displays the results of a Northern analysis of the levels of COR15a transcripts in plants overexpressing wild-type CBF2 (E2 plants) and mutant CBF polypeptides (e.g., the 7-2 and 5-4 plants). Figure 9 is a photograph showing three Arabidopsis lines, wild-type (W), CBF-overexpressing line 5-4 (5-4) and E2 plants (E) grown at a warm temperature on plates, then exposed to freezing conditions. The 5-4 plants harboring the mutant CBF2 sequence having a serine to leucine substitution at position 118 were more freezing tolerant than wild-type Ws plants, and appeared to have a similar level of freezing tolerance as the E2 line. Figure 10 shows amino acid substitutions found in numerous CBF1 mutants after site-specific saturation mutagenesis. In these mutants, each of the four non-alanine residues in the CBF signature sequence DSAWR were substituted with an alanine residue to yield a mutant form of the CBF polypeptide that, when overexpressed, conferred increased abiotic stress tolerance in plants that retained morphological and developmental similarity to control or wild-type plants. The first CBF signature sequence, the AP2 domain, and the second CBF signature sequence are indicated as CBF_a, AP2, and CBF_b, respectively, in this figure. Figure 11 compares the expression of COR15 in transgenic Arabidopsis lines overexpressing four wild-type (WT) and four mutant (dsawr) versions of CBF 1 under the regulatory control of the cauliflower 35 S promoter. The dsawr mutants were created by mutagenizing a wild-type CBF sequence with a site-specific saturation mutagenesis procedure. Total RNA was isolated from transgenic lines and northern blots prepared and hybridized with probes for CBF1 and COR15. As shown in this figure, transgenic lines were obtained that expressed CBF 1 -dsawr at much higher levels than obtained with overexpression of the wild-type CBF1. In these CBFl-dsawr plants, COR15 was expressed at levels approximating those found in the lines overexpressing wild-type CBF1. Transgenic plants overexpressing WT8, a wild-type version of CBF1, displayed a dwarf phenotype, as seen in the example on the left in Figure 12. Plants expressing the CBFl-dsawr mutation, however, did not display a dwarf phenotype, as exemplified by the plant on the right in Figure 12. Figures 13A-13F show various mutant and wild-type plants in plate-based freezing assays. The freezing tolerance of non-acclimated non-transgenic Arabidopsis (Wassilewskija) plants (WS-2) and transgenic lines overexpressing either wild-type CBF1 (G-26) or mutant CBFl-dsawr under the regulatory control of the CaMV 35S promoter are shown (dsawr 6.2, dsawr 2.1, and dsawr 3.1 plants are shown, methods provided in Example III, below). The photographs in the left column of plates (Figures 13 A, 13C and 13E) were taken prior to a freezing step, and the photographs in the right column of plates (Figures 13B, 13D and 13F) were taken after the freezing and recovery steps. All of the plants in the left column appeared relatively green and healthy, although the G-26 plants overexpressing CBF1 were noticeably smaller than the WS-2 wild-type plants or the dsawr plants overexpressing the mutant CBFs. hi all of the experimental examples in the right column, the wild-type WS-2 plants were severely affected by the freezing process, as the plants were almost entirely wilted and chlorotic. The G-26 plants overexpressing a wild-type CBF were generally greener and healthier in appearance than the wild-type plants. The dsawr mutants in these experiments were at least as healthy in appearance as the G-26 wild- type CBF overexpressors after freezing; almost all of these plants were turgid with a healthy green color.

DESCRIPTION OF THE INVENTION The data presented herein represent the results obtained in experiments with modified transcription factor polynucleotides and polypeptides that may be expressed in plants for the purpose of reducing yield losses that arise from abiotic stress. In an important aspect, the present invention relates to polynucleotides and polypeptides, for example, for modifying phenotypes of plants, particularly those associated with increased abiotic stress tolerance. Throughout this disclosure, various information sources are referred to and/or are specifically incorporated. The information sources include scientific journal articles, patent documents, textbooks, and World Wide Web browser-active and inactive page addresses, for example. While the reference to these information sources clearly indicates that they can be used by one of skill in the art, each and every one of the information sources cited herein are specifically incorporated in their entirety, whether or not a specific mention of "incorporation by reference" is noted. The contents and teachings of each and every one of the information sources can be relied on and used to make and use embodiments of the invention. As used herein and in the appended claims, the singular forms "a," "an," and "the" include plural reference unless the context clearly dictates otherwise. Thus, for example, a reference to "a plant" includes a plurality of such plants, and a reference to "a stress" is a reference to one or more stresses and equivalents thereof known to those skilled in the art, and so forth.

DEFINITIONS "Nucleic acid molecule" refers to an oligonucleotide, polynucleotide or any fragment thereof. It may be DNA or RNA of genomic or synthetic origin, double-stranded or single-stranded, and combined with carbohydrate, lipids, protein, or other materials to perform a particular activity such as transformation or form a useful composition such as a peptide nucleic acid (PNA). A "polynucleotide" is a nucleic acid comprising a plurality of polymerized nucleotides, e.g., at least about 15 consecutive polymerized nucleotides, and optionally at least about 30 consecutive nucleotides. A polynucleotide may be a nucleic acid, oligonucleotide, nucleotide, or any fragment thereof. A polynucleotide may comprise a nucleotide sequence encoding a polypeptide, or a domain or fragment thereof. Additionally, the polynucleotide may comprise a promoter, an intron, an enhancer region, a polyadenylation site, a translation initiation site, 5' or 3' untranslated regions, a reporter gene, a selectable marker, or the like. The polynucleotide can be single stranded or double stranded DNA or RNA. The polynucleotide optionally comprises modified bases or a modified backbone. The polynucleotide can be, e.g., genomic DNA or RNA, a transcript (such as an mRNA), a cDNA, a PCR product, a cloned DNA, a synthetic DNA or RNA, or the like. The polynucleotide can comprise a sequence in either sense or antisense orientations. "Oligonucleotide" is substantially equivalent to the terms amplimer, primer, oligomer, element, target, and probe and is preferably single stranded. "Gene" or "gene sequence" refers to the partial or complete coding sequence of a gene, its complement, and its 5' or 3' untranslated regions. A gene is also a functional unit of inheritance, and in physical terms is a particular segment or sequence of nucleotides along a molecule of DNA (or RNA, in the case of RNA viruses) involved in producing a polypeptide chain. The latter may be subjected to subsequent processing such as splicing and folding to obtain a functional protein or polypeptide. A gene may be isolated, partially isolated, or be found with an organism's genome. By way of example, a transcription factor gene encodes a transcription factor polypeptide, which may be functional or require processing to function as an initiator of transcription. Operationally, genes may be defined by the cis-trans test, a genetic test that determines whether two mutations occur in the same gene and which may be used to determine the limits of the genetically active unit (Rieger et al. (1976) Glossary of Genetics and Cvtogenetics: Classical and Molecular, 4th ed., Springer Verlag, Berlin). A gene generally includes regions preceding ("leaders"; upstream) and following ("trailers"; downstream) the coding region. A gene may also mclude intervening, non-coding sequences, referred to as "introns", located between individual coding segments, referred to as "exons". Most genes have an associated promoter region, a regulatory sequence 5' of the transcription initiation codon (there are some genes that do not have an identifiable promoter). The function of a gene may also be regulated by enhancers, operators, and other regulatory elements. A "recombinant polynucleotide" is a polynucleotide that is not in its native state, e.g., the polynucleotide comprises a nucleotide sequence not found in nature, or the polynucleotide is in a context other than that in which it is naturally found, e.g., separated from nucleotide sequences with which it typically is in proximity in nature, or adjacent (or contiguous with) nucleotide sequences with which it typically is not in proximity. For example, the sequence at issue can be cloned into a vector, or otherwise recombined with one or more additional nucleic acid. Transgenic plants of the present invention comprise recombinant, overexpressed mutant polynucleotides that encodes a member of the CBF clade of transcription factor polypeptides. These transgenic plants exhibit fewer or reduced adverse morphological or developmental effects than plants that overexpress a wild-type form of the recombinant mutant polynucleotide (the "wild-type form" of the recombinant mutant polynucleotide is the polynucleotide sequence prior to its being mutated in vivo or in vitro to the mutant polynucleotide): An "isolated polynucleotide" is a polynucleotide whether naturally occurring or recombinant, that is present outside the cell in which it is typically found in nature, whether purified or not. Optionally, an isolated polynucleotide is subject to one or more enrichment or purification procedures, e.g., cell lysis, extraction, centrifugation, precipitation, or the like. A "polypeptide" is an amino acid sequence comprising a plurality of consecutive polymerized amino acid residues e.g., at least about 15 consecutive polymerized amino acid residues, optionally at least about 30 consecutive polymerized amino acid residues, at least about 50 consecutive polymerized amino acid residues, hi many instances, a polypeptide comprises a polymerized amino acid residue sequence that is a transcription factor or a domain or portion or fragment thereof. Additionally, the polypeptide may comprise 1) a localization domain, 2) an activation domain, 3) a repression domain, 4) an oligomerization domain, or 5) a DNA-binding domain, or the like. The polypeptide optionally comprises modified amino acid residues, naturally occurring amino acid residues not encoded by a codon, non-naturally occurring amino acid residues. "Protein" refers to an amino acid sequence, oligopeptide, peptide, polypeptide or portions thereof whether naturally occurring or synthetic. "Portion", as used herein, refers to any part of a protein used for any purpose, but especially for the screening of a library of molecules which specifically bind to that portion or for the production of antibodies. A "recombinant polypeptide" is a polypeptide produced by translation of a recombinant polynucleotide. A "synthetic polypeptide" is a polypeptide created by consecutive polymerization of isolated amino acid residues using methods well known in the art. An "isolated polypeptide," whether a naturally occurring or a recombinant polypeptide, is more enriched in (or out of) a cell than the polypeptide in its natural state in a wild-type cell, e.g., more than about 5% enriched, more than about 10% enriched, or more than about 20%, or more than about 50%, or more, enriched, i.e., alternatively denoted: 105%, 110%, 120%, 150% or more, enriched relative to wild type standardized at 100%). Such an enrichment is not the result of a natural response of a wild-type plant. Alternatively, or additionally, the isolated polypeptide is separated from other cellular components with which it is typically associated, e.g., by any of the various protein purification methods herein. "Homology" refers to sequence similarity between a reference sequence and at least a fragment of a newly sequenced clone insert or its encoded amino acid sequence. Additionally, the terms "homology" and "homologous sequence(s)" may refer to one or more polypeptide sequences that are modified by chemical or enzymatic means. The homologous sequence may be a sequence modified by lipids, sugars, peptides, organic or inorganic compounds, by the use of modified amino acids or the like. Protein modification techniques are illustrated in Ausubel et al. (eds.) Current Protocols in Molecular Biology, John Wiley & Sons (1998). The terms "essentially homologous" or "sufficiently homologous" refer to polynucleotide or polypeptide sequences that are sufficiently duplicative of one another that the sequences produce the same or similar results when similarly expressed in plants. An example of a similar result is a comparable degree of a particular abiotic stress tolerance conferred when two sufficiently homologous sequences are expressed in two different plants. These sequences may include a sequence of the Sequence Listing of this application, or other comparatively similar sequences that confer similar functions in plants. Such sequences can also be used as a probe to isolate DNA's in other plants. "Identity" or "similarity" refers to sequence similarity between two polynucleotide sequences or between two polypeptide sequences, with identity being a more strict comparison. The phrases "percent identity" and "% identity" refer to the percentage of sequence similarity found in a comparison of two or more polynucleotide sequences or two or more polypeptide sequences. "Sequence similarity" refers to the percent similarity in base pair sequence (as detennined by any suitable method) between two or more polynucleotide sequences. Two or more sequences can be anywhere from 0-100% similar, or any integer value therebetween. Identity or similarity can be determined by comparing a position in each sequence that may be aligned for purposes of comparison. When a position in the compared sequence is occupied by the same nucleotide base or amino acid, then the molecules are identical at that position. A degree of similarity or identity between polynucleotide sequences is a function of the number of identical or matching nucleotides at positions shared by the polynucleotide sequences. A degree of identity of polypeptide sequences is a function of the number of identical amino acids at positions shared by the polypeptide sequences. A degree of homology or similarity of polypeptide sequences is a function of the number of amino acids at positions shared by the polypeptide sequences. With regard to polypeptides, the terms "substantial identity" or "substantially identical" may refer to sequences of sufficient similarity and structure to the transcription factors in the Sequence Listing to produce similar function when expressed, overexpressed, or knocked-out in a plant; in the present invention, this function is increased tolerance to conditions of limited light. Polypeptide sequences that are at least about 55% identical to the instant polypeptide sequences are considered to have "substantial identity" with the latter. Sequences having lesser degrees of identity but comparable biological activity are considered to be equivalents. The structure required to maintain proper functionality is related to the tertiary structure of the polypeptide. There are discreet domains and motifs within a transcription factor that must be present within the polypeptide to confer function and specificity. These specific structures are required so that interactive sequences will be properly oriented to retain the desired activity. "Substantial identity" may thus also be used with regard to subsequences, for example, motifs, that are of sufficient structure and similarity, being at least about 55% identical to similar motifs in other related sequences. Thus, CBF polypeptides have the physical characteristics of substantial identity along their full length and within their AP2 domains (by way of example, the AP2 domain for CBF1 is provided as SEQ ID NO: 10, and the AP2 domain for CBF2 is shown in Figures 6 and 10). These polypeptides also share functional characteristics, as the polypeptides within this clade bind to a transcription-regulating region of DNA and increase abiotic tolerance in a plant when the polypeptides are overexpressed. "Alignment" refers to a number of nucleotide or amino acid residue sequences aligned by lengthwise comparison so that components in common (i.e., nucleotide bases or amino acid residues) may be visually and readily identified. The fraction or percentage of components in common is related to the homology or identity between the sequences. Alignments may be used to identify conserved domains and relatedness within these domains. An alignment may suitably be determined by means of computer programs known in the art, such as MacVector (1999) (Accelrys, Inc., San Diego, CA). A "conserved domain" or "conserved region" as used herein refers to a region in heterologous polynucleotide or polypeptide sequences where there is a relatively high degree of sequence identity between the distinct sequences. AP2 domains are examples of conserved domains. With respect to polynucleotides encoding presently disclosed transcription factors, a conserved domain is preferably at least 10 base pairs (bp) in length. A "conserved domain", with respect to presently disclosed polypeptides refers to a domain within a transcription factor family that exhibits a higher degree of sequence homology, such as at least 55% sequence similarity, including conservative substitutions, and more preferably at least 60% sequence identity, and even more preferably at least 62%, or at least about 64%, or at least about 68%, or at least about 71%, or at least about 75%, or at least about 80%, or at least about 82%, or at least about 85%, or at least about 87%, or at least about 90%, or at least about 95%, or at least about 98% amino acid residue sequence identity of a polypeptide of consecutive amino acid residues. A fragment or domain can be referred to as outside a conserved domain, outside a consensus sequence, or outside a consensus DNA-binding site that is known to exist or that exists for a particular transcription factor class, family, or sub-family. In this case, the fragment or domain will not include the exact amino acids of a consensus sequence or consensus DNA-binding site of a transcription factor class, family or sub-family, or the exact amino acids of a particular transcription factor consensus sequence or consensus DNA-binding site. Furthermore, a particular fragment, region, or domain of a polypeptide, or a polynucleotide encoding a polypeptide, can be "outside a conserved domain" if all the amino acids of the fragment, region, or domain fall outside of a defined conserved domain(s) for a polypeptide or protein. Sequences having lesser degrees of identity but comparable biological activity are considered to be equivalents. As one of ordinary skill in the art recognizes, conserved domains may be identified as regions or domains of identity to a specific consensus sequence. Thus, by using alignment methods well known in the art, the conserved domains of plant transcription factors may be determined. "Complementary" refers to the natural hydrogen bonding by base pairing between purines and pyrimidines. For example, the sequence A-C-G-T (5' -> 3') forms hydrogen bonds with its complements A-C-G-T (5' -> 3') or A-C-G-U (5' -> 3'). Two single-stranded molecules may be considered partially complementary, if only some of the nucleotides bond, or "completely complementary" if all of the nucleotides bond. The degree of complementarity between nucleic acid strands affects the efficiency and strength of the hybridization and amplification reactions. "Fully complementary" refers to the case where bonding occurs between every base pair and its complement in a pair of sequences, and the two sequences have the same number of nucleotides. The terms "highly stringent" or "highly stringent condition" refer to conditions that permit hybridization of DNA strands whose sequences are highly complementary, wherein these same conditions exclude hybridization of significantly mismatched DNAs. Polynucleotide sequences capable of hybridizing under stringent conditions with the polynucleotides of the present invention may be, for example, variants of the disclosed polynucleotide sequences, including allelic or splice variants, or sequences that encode orthologs or paralogs of presently disclosed polypeptides. Nucleic acid hybridization methods are disclosed in detail by Kashima et al. (1985) Nature 313:402-404, and Sambrook et al. (1989) Molecular Cloning: A Laboratory Manual. 2nd Ed., Cold Spring Harbor Laboratory, Cold Spring Harbor, N.Y. ("Sambrook"); and by Haymes et al., "Nucleic Acid

Hybridization: A Practical Approach", IRL Press, Washington, D.C. (1985), which references are incorporated herein by reference. In general, stringency is determined by the temperature, ionic strength, and concentration of denaturing agents (e.g., formamide) used in a hybridization and washing procedure (for a more detailed description of establishing and determining stringency, see below). The degree to which two nucleic acids hybridize under various conditions of stringency is correlated with the extent of their similarity. Similar nucleic acid sequences from a variety of sources, such as within a plant's genome (e.g., paralogs) or from another plant (e.g., orthologs) that may perform similar functions can be isolated on the basis of their ability to hybridize with known transcription factor sequences. Numerous variations are possible in the conditions and means by which nucleic acid hybridization can be performed to isolate transcription factor sequences having similarity to transcription factor sequences known in the art and are not limited to those explicitly disclosed herein. Such an approach may be used to isolate polynucleotide sequences having various degrees of similarity with disclosed transcription factor sequences, such as, for example, transcription factors having 60% identity, or more preferably greater than about 70% identity, or greater than about 75% identity, or greater than about 80% identity, or greater than about 82% identity, or greater than about 85% identity, or greater than about 87% identity, or greater than about 90% identity, or greater than about 95% identity, or greater identity with disclosed transcription factors. Regarding the terms "paralog" and "ortholog", homologous polynucleotide sequences and homologous polypeptide sequences may be paralogs or orthologs of the claimed polynucleotide or polypeptide sequence. Orthologs and paralogs are evolutionarily related genes that have similar sequence and similar functions. Orthologs are structurally related genes in different species that are derived by a speciation event. Paralogs are structurally related genes within a single species that are derived by a duplication event. Sequences that are sufficiently similar to one another will be appreciated by those of skill in the art and may be based upon percentage identity of the complete sequences, percentage identity of a conserved domain or sequence within the complete sequence, percentage similarity to the complete sequence, percentage similarity to a conserved domain or sequence within the complete sequence, and/or an arrangement of contiguous nucleotides or peptides particular to a conserved domain or complete sequence. Sequences that are sufficiently similar to one another will also bind in a similar manner to the same DNA binding sites of transcriptional regulatory elements using methods well known in the art. The term "equivalog" describes members of a set of homologous proteins that are conserved with respect to function since their last common ancestor. Related proteins are grouped into equivalog families, and otherwise into protein families with other hierarchically defined homology types. This definition is provided at the Institute for Genomic Research (TIGR) World Wide Web (www) website, "www.tigr.org" or "http://www.tigr.org/TIGRFAMs/Explanations.shtml" under the heading "Terms associated with TIGRFAMs". In general, the term "variant" refers to molecules with some differences, generated synthetically or naturally, in their base or amino acid sequences as compared to a reference (native) polynucleotide or polypeptide, respectively. These differences include substitutions, insertions, deletions or any desired combinations of such changes in a native polynucleotide of amino acid sequence. With regard to polynucleotide variants, differences between presently disclosed polynucleotides and polynucleotide variants are limited so that the nucleotide sequences of the former and the latter are closely similar overall and, in many regions, identical. Due to the degeneracy of the genetic code, differences between the former and latter nucleotide sequences may be silent (i.e., the amino acids encoded by the polynucleotide are the same, and the variant polynucleotide sequence encodes the same amino acid sequence as the presently disclosed polynucleotide. Variant nucleotide sequences may encode different amino acid sequences, in which case such nucleotide differences will result in amino acid substitutions, additions, deletions, insertions, truncations or fusions with respect to the similar disclosed polynucleotide sequences. These variations result in polynucleotide variants encoding polypeptides that share at least one functional characteristic. The degeneracy of the genetic code also dictates that many different variant polynucleotides can encode identical and/or substantially similar polypeptides in addition to those sequences illustrated in the Sequence Listing. Also within the scope of the invention is a variant of a transcription factor nucleic acid listed in the Sequence Listing, that is, one having a sequence that differs from the one of the polynucleotide sequences in the Sequence Listing, or a complementary sequence, that encodes a functionally equivalent polypeptide (i.e., a polypeptide having some degree of equivalent or similar biological activity) but differs in sequence from the sequence in the Sequence Listing, due to degeneracy in the genetic code. Included within this definition are polymorphisms that may or may not be readily detectable using a particular oligonucleotide probe of the polynucleotide encoding polypeptide, and improper or unexpected hybridization to allelic variants, with a locus other than the normal chromosomal locus for the polynucleotide sequence encoding polypeptide. "Allelic variant" or "polynucleotide allelic variant" refers to any of two or more alternative forms of a gene occupying the same chromosomal locus. Allelic variation arises naturally through mutation, and may result in phenotypic polymorphism within populations. Gene mutations may be "silent" or may encode polypeptides having altered amino acid sequence. "Allelic variant" and "polypeptide allelic variant" may also be used with respect to polypeptides, and in this case the terms refer to a polypeptide encoded by an allelic variant of a gene. "Splice variant" or "polynucleotide splice variant" as used herein refers to alternative forms of RNA transcribed from a gene. Splice variation naturally occurs as a result of alternative sites being spliced within a single transcribed RNA molecule or between separately transcribed RNA molecules, and may result in several different forms of mRNA transcribed from the same gene. Thus, splice variants may encode polypeptides having different amino acid sequences, which may or may not have similar functions in the organism. "Splice variant" or "polypeptide splice variant" may also refer to a polypeptide encoded by a splice variant of a transcribed mRNA. "Polynucleotide variants" may also refer to polynucleotide sequences that encode paralogs and orthologs of the presently disclosed polypeptide sequences. "Polypeptide variants" may refer to polypeptide sequences that are paralogs or orthologs of presently disclosed polypeptide sequences. Differences between presently disclosed polypeptides and polypeptide variants are limited so that the sequences of the former and the latter are closely similar overall and, in many regions, identical. Presently disclosed polypeptide sequences and similar polypeptide variants may differ in amino acid sequence by one or more substitutions, additions, deletions, fusions and truncations, which may be present in any combination. These differences may produce silent changes and result in a functionally equivalent transcription factor. Thus, it will be readily appreciated by those of skill in the art, that any of a variety of polynucleotide sequences is capable of encoding the transcription factors and transcription factor homolog polypeptides of the invention. A polypeptide sequence variant may have "conservative" changes, wherein a substituted amino acid has similar structural or chemical properties. Deliberate amino acid substitutions may thus be made on the basis of similarity in polarity, charge, solubility, hydrophobicity, hydrophilicity, and/or the amphipathic nature of the residues, as long as the functional or biological activity of the transcription factor is retained. For example, negatively charged amino acids may include aspartic acid and glutamic acid, positively charged amino acids may include lysine and arginine, and amino acids with uncharged polar head groups having similar hydrophilicity values may include leucine, isoleucine, and valine; glycine and alanine; asparagine and glutamine; serine and threonine; and phenylalanine and tyrosine (for more detail on conservative substitutions, see Table 2). More rarely, a variant may have "non-conservative" changes, for example, replacement of a glycine with a tryptophan. Similar minor variations may also include amino acid deletions or insertions, or both. Related polypeptides may comprise, for example, additions and/or deletions of one or more N-linked or O-linked glycosylation sites, or an addition and/or a deletion of one or more cysteine residues. Guidance in determining which and how many amino acid residues may be substituted, inserted or deleted without abolishing functional or biological activity may be found using computer programs well known in the art, for example, DNASTAR software (see USPN 5,840,544). "Modulates" refers to a change in activity (biological, chemical, or immunological) or lifespan resulting from specific binding between a molecule and either a nucleic acid molecule or a protein. The term "plant" includes whole plants, shoot vegetative organs/structures (for example, leaves, stems and tubers), roots, flowers and floral organs/structures (for example, bracts, sepals, petals, stamens, carpels, anthers and ovules), seed (including embryo, endosperm, and seed coat), progeny plants derived from seed, fruit (the mature ovary), plant tissue (for example, vascular tissue, ground tissue, and the like) and cells (for example, guard cells, egg cells, and the like), and progeny derived from tissue or cells. The class of plants that can be used in the method of the invention is generally as broad as the class of higher and lower plants amenable to transformation techniques, including angiosperms (monocotyledonous and dicotyledonous plants), gymnosperms, ferns, horsetails, psilophytes, lycophytes, bryophytes, and multicellular algae. (See for example, Figure 1, adapted from Daly et al. (2001) Plant Physiol. 127: 1328- 1333; Figure 2, adapted from Ku et al. (2000) Proc. Natl. Acad. Sci. 97: 9121-9126; and see also Tudge in The Variety of Life. Oxford University Press, New York, NY (2000) pp. 547-606). A "transgenic plant" or "transformed plant" refers to a plant that contains genetic material not found in a wild-type plant of the same species, variety or cultivar. The genetic material may include a transgene, an insertional mutagenesis event (such as by transposon or T-DNA insertional mutagenesis), an activation tagging sequence, a mutated sequence, a homologous recombination event or a sequence modified by chimeraplasty. Typically, the foreign genetic material has been introduced into the plant by human manipulation, but any method can be used as one of skill in the art recognizes. A transgenic plant may contain an expression vector or cassette. The expression cassette typically comprises a polypeptide-encoding sequence operably linked (i.e., under regulatory control of) to appropriate inducible or constitutive regulatory sequences that allow for the expression of the polypeptide. The expression cassette can be introduced into a plant by transformation or by breeding after transformation of a parent plant. A plant refers to a whole plant as well as to a plant part, such as seed, fruit, leaf, or root, plant tissue, plant cells or any other plant material, for example, a plant explant, as well as to progeny thereof, and to in vitro systems that mimic biochemical or cellular components or processes in a cell. A "member of the CBF clade of transcription factor polypeptides" generally refers to a polypeptide comprising the subsequence K-K/R-R/P-A-G-R-X-X-F-X-E-T-R-H-P, where X is any amino acid residue, and an AP2 domain. A "mutant member of the CBF clade of transcription factor polypeptides" refers to a CBF clade member polypeptide encoded by a polynucleotide that has been mutagenized, naturally or artificially, in vivo or in vitro, to yield a molecule that encodes at least one substituted, inserted, deleted, or concatenated amino acid residue. The mutant CBF clade member polypeptide will generally comprise the subsequence K-K/R-R/P-A-G-R-X-X-F-X-E-T-R-H-P, although it is envisioned that conservative or similar substitutions or neutral insertions or deletions may take place in this subsequence to yield a polypeptide that retains much of the function of the wild-type polypeptide for conferring abiotic stress tolerance. "Wild type" or "wild-type", as used herein, refers to a plant cell, seed, plant component, plant tissue, plant organ or whole plant that has not been genetically modified or treated in an experimental sense. Wild-type cells, seed, components, tissue, organs or whole plants may be used as controls to compare levels of expression and the extent and nature of trait modification with cells, tissue or plants of the same species in which a transcription factor expression is altered, e.g., in that it has been knocked out, overexpressed, or ectopically expressed. "Wild-type" may also refer to a native form of a molecule. A "control plant" as used in the present invention refers to a plant cell, seed, plant component, plant tissue, plant organ or whole plant used to compare against transgenic or genetically modified plant for the purpose of identifying an enhanced phenotype in the transgenic or genetically modified plant. A control plant may be a wild-type plant or in some cases be a transgenic plant line that comprises an empty vector or marker gene, but does not contain the recombinant polynucleotide of the present invention that is expressed in the transgenic or genetically modified plant being evaluated. In general, a control plant is a plant of the same line or variety as the transgenic or genetically modified plant being tested. A suitable control plant would include a genetically unaltered or non-transgenic plant of the parental line used to generate a transgenic plant herein. "Fragment", with respect to a polynucleotide, refers to a clone or any part of a polynucleotide molecule that retains a usable, functional characteristic. Useful fragments include oligonucleotides and polynucleotides that may be used in hybridization or amplification technologies or in the regulation of replication, transcription or translation. A "polynucleotide fragment" refers to any subsequence of a polynucleotide, typically, of at least about 9 consecutive nucleotides, preferably at least about 30 nucleotides, more preferably at least about 50 nucleotides, of any of the sequences provided herein. Fragments may also include subsequences of polypeptides and protein molecules, or a subsequence of the polypeptide. Fragments may have uses in that they may have antigenic potential. In some cases, the fragment or domain is a subsequence of the polypeptide which performs at least one biological function of the intact polypeptide in substantially the same manner, or to a similar extent, as does the intact polypeptide. For example, a polypeptide fragment can comprise a recognizable structural motif or functional domain such as a DNA-binding site or domain that binds to a DNA promoter region, an activation domain, or a domain for protein-protein interactions, and may initiate transcription. Fragments can vary in size from as few as 3 amino acid residues to the full length of the intact polypeptide, but are preferably at least about 30 amino acid residues in length and more preferably at least about 60 amino acid residues in length. Exemplary polypeptide fragments are the first twenty consecutive amino acids of a mammalian protein encoded by are the first twenty consecutive amino acids of the transcription factor polypeptides listed in the Sequence Listing. Exemplary polynucleotide fragments are the first sixty consecutive nucleotides of the transcription factor polynucleotides listed in the Sequence Listing. Exemplary fragments also include fragments that comprise a region that encodes a conserved domain of a transcription factor, for example, an AP2 domain such as found at amino acid residues 48-115 of SEQ ID NO: 13. The invention also encompasses production of DNA sequences that encode transcription factors and transcription factor derivatives, or fragments thereof, entirely by synthetic chemistry. After production, the synthetic sequence may be inserted into any of the many available expression vectors and cell systems using reagents well known in the art. Moreover, synthetic chemistry may be used to introduce mutations into a sequence encoding transcription factors or any fragment thereof. "Derivative" refers to the chemical modification of a nucleic acid molecule or amino acid sequence. Chemical modifications can include replacement of hydrogen by an alkyl, acyl, or amino group or glycosylation, pegylation, or any similar process that retains or enhances biological activity or lifespan of the molecule or sequence. A "trait" refers to a physiological, morphological, biochemical, or physical characteristic of a plant or particular plant material or cell, hi some instances, this characteristic is visible to the human eye, such as seed or plant size, or can be measured by biochemical techniques, such as detecting the protein, starch, or oil content of seed or leaves, or by observation of a metabolic or physiological process, e.g. by measuring tolerance to water deprivation or particular salt or sugar concentrations, or by the observation of the expression level of a gene or genes, for example, by employing Northern analysis, RT-PCR, microarray gene expression assays, or reporter gene expression systems, or by agricultural observations such as limited light conditions or other abiotic stress tolerance or yield. Any technique can be used to measure the amount of, comparative level of, or difference in any selected chemical compound or macromolecule in the transgenic plants, however. "Trait modification" refers to a detectable difference in a characteristic in a plant ectopically expressing a polynucleotide or polypeptide of the present invention relative to a plant not doing so, such as a wild-type plant. In some cases, the trait modification can be evaluated quantitatively. For example, the trait modification can entail at least about a 2% increase or decrease in an observed trait (difference), at least a 5% difference, at least about a 10% difference, at least about a 20% difference, at least about a 30%, at least about a 50%, at least about a 70%, or at least about a 100%, or an even greater difference compared with a wild-type plant. It is known that there can be a natural variation in the modified trait. Therefore, the trait modification observed entails a change of the normal distribution of the trait in the plants compared with the distribution observed in wild-type plants. When two or more plants are "morphologically similar" they have comparable forms or appearances, including analogous features such as dimension, height, width, mass, root mass, shape, glossiness, color, stem diameter, leaf size, leaf dimension, leaf density, inte node distance, branching, root branching, number and form of inflorescences, and other macroscopic characteristics at a particular stage of growth. If the plants are morphologically similar at all stages of growth, they are also "developmentally similar". It may be difficult to distinguish two plants that are genotypically distinct but morphologically similar based on morphological characteristics alone. The term "transcript profile" refers to the expression levels of a set of genes in a cell in a particular state, particularly by comparison with the expression levels of that same set of genes in a cell of the same type in a reference state. For example, the transcript profile of a particular transcription factor in a suspension cell is the expression levels of a set of genes in a cell repressing or overexpressing that transcription factor compared with the expression levels of that same set of genes in a suspension cell that has normal levels of that transcription factor. The transcript profile can be presented as a list of those genes whose expression level is significantly different between the two treatments, and the difference ratios. Differences and similarities between expression levels may also be evaluated and calculated using statistical and clustering methods. "Ectopic expression" or "altered expression" with respect to a polynucleotide indicates that the pattern of expression in a transgenic plant or plant tissue is different from the expression pattern in a wild-type or reference plant of the same species. The pattern of expression may also be compared with a reference expression pattern in a wild-type plant of the same species. For example, the polynucleotide or polypeptide is expressed in a cell or tissue type other than a cell or tissue type in which the sequence is expressed in the wild-type plant, or by expression at a time other than at the time the sequence is expressed in the wild-type plant, or by a response to different inducible agents, such as hormones or environmental signals, or at different expression levels (either higher or lower) compared with those found in a wild-type plant. The term also refers to altered expression patterns that are produced by lowering the levels of expression to below the detection level or completely abolishing expression. The resulting expression pattern can be transient or stable, constitutive or inducible. In reference to a polypeptide, the term "ectopic expression or altered expression" further may relate to altered activity levels resulting from the interactions of the polypeptides with exogenous or endogenous modulators or from interactions with factors or as a result of the chemical modification of the polypeptides. The term "overexpression" as used herein refers to a greater expression level of a gene in a plant, plant cell or plant tissue, compared to expression in a wild-type plant, cell or tissue, at any developmental or temporal stage for the gene. Overexpression can occur when, for example, the genes encoding one or more transcription factors are under the control of a strong expression signal, such as one of the promoters described herein (for example, the cauliflower mosaic virus 35 S transcription initiation region). Overexpression may occur throughout a plant or in specific tissues of the plant, depending on the promoter used, as described below. Overexpression may take place in plant cells normally lacking expression of polypeptides functionally equivalent or identical to the present transcription factors.

Overexpression may also occur in plant cells where endogenous expression of the present transcription factors or functionally equivalent molecules normally occurs, but such normal expression is at a lower level. Overexpression thus results in a greater than normal production, or "overproduction" of the transcription factor in the plant, cell or tissue. The term "transcription-regulating region" refers to a DNA regulatory sequence that regulates expression of one or more genes in a plant when a transcription factor having one or more specific binding domains binds to the DNA regulatory sequence. Transcription factors of the present invention possess for example, an AP2 domain that comprises a transcription-regulating region. The transcription factors of the invention also comprise an amino acid subsequence that forms a transcription activation domain that regulates expression of abiotic stress tolerance genes in a plant when the transcription factor binds to the regulating region. The term "C-repeat cold and drought regulation element" or "C-repeat/DRE" refers to a sequence which includes CCG and functions as a binding domain in a plant to regulate expression of one or more environmental stress tolerance genes, such as cold or dehydration stress tolerance genes. The term "cold stress" refers to a decrease in ambient temperature, including a decrease to freezing temperatures, which causes a plant to attempt to acclimate itself to the decreased ambient temperature. The term "dehydration stress" refers to drought, high salinity and other conditions that cause a decrease in cellular water potential in a plant. "Adverse morphological or developmental effects" refers to physical or growth characteristics of a plant that occur in an undesirable manner or to an undesirable extent. These effects may include one or more of the following characteristics that are associated with native CBF overexpression: reduced internode length, reduced fertility, smaller rosettes, narrow leaves, curled leaves, wrinkled leaves, reduced leaf size, reduced seed number, altered flowering time, stunting, decreased fruit yield, and reduced biomass. A characteristic that is undesirable in one instance may be neutral or desirable in another (for example, stunting may be a desirable attribute in certain crops or ornamentals, whereas increased biomass and yield may be desirable for others). Thus, a first plant that has "fewer or reduced adverse morphological or developmental effects" possesses superior physical or growth characteristics relative to a second plant depending on the attributes required for a particular plant or environment.

DETAILED DESCRIPTION A transcription factor may mclude, but is not limited to, any polypeptide that can activate or repress transcription of a single gene or a number of genes. As one of ordinary skill in the art recognizes, transcription factors can be identified by the presence of a region or domain of structural similarity or identity to a specific consensus sequence or the presence of a specific consensus DNA-binding site or DNA-binding site motif (see, for example, Riechmann et al. (2000) Science 290: 2105-2110). The plant transcription factors may belong to, for example, the AP2 or other transcription factor families. Generally, the transcription factors encoded by the present sequences are involved in cell differentiation and proliferation and the regulation of growth. Accordingly, one skilled in the art would recognize that by expressing the present sequences in a plant, one may change the expression of autologous genes or induce the expression of introduced genes. By affecting the expression of similar autologous sequences in a plant that have the biological activity of the present sequences, or by introducing the present sequences into a plant, one may alter a plant's phenotype to one with improved traits related to abiotic stress tolerance. The sequences of the invention may also be used to transform a plant and introduce desirable traits not found in the wild-type cultivar or strain. Plants may then be selected for those that produce the most desirable degree of over- or under-expression of target genes of interest and coincident trait improvement. The sequences of the present invention may be from any species, particularly plant species, in a naturally occurring form or from any source whether natural, synthetic, semi-synthetic or recombinant. The sequences of the invention may also include fragments of the present amino acid sequences that function in a manner similar to the present amino acid sequences. Where "amino acid sequence" is recited to refer to an amino acid sequence of a naturally occurring protein molecule, "amino acid sequence" and like terms are not meant to limit the amino acid sequence to the complete native amino acid sequence associated with the recited protein molecule. In addition to methods for modifying a plant phenotype by employing one or more polynucleotides and polypeptides of the invention described herein, the polynucleotides and polypeptides of the invention have a variety of additional uses. These uses include their use in the recombinant production (i.e., expression) of proteins; as regulators of plant gene expression, as diagnostic probes for the presence of complementary or partially complementary nucleic acids (including for detection of natural coding nucleic acids); as substrates for further reactions, for example, mutation reactions, PCR reactions, or the like; as substrates for cloning for example, including digestion or ligation reactions; and for identifying exogenous or endogenous modulators of the transcription factors. In many instances, a polynucleotide comprises a nucleotide sequence encoding a polypeptide (or protein) or a domain or fragment thereof. Additionally, the polynucleotide may comprise a promoter, an intron, an enhancer region, a polyadenylation site, a translation initiation site, 5 ' or 3' untranslated regions, a reporter gene, a selectable marker, or the like. The polynucleotide can be single stranded or double stranded DNA or RNA. The polynucleotide optionally comprises modified bases or a modified backbone. The polynucleotide can be, for example, genomic DNA or RNA, a transcript (such as an mRNA), a cDNA, a PCR product, a cloned DNA, a synthetic DNA or RNA, or the like. The polynucleotide can comprise a sequence in either sense or antisense orientations. Expression of genes that encode transcription factors that modify expression of endogenous genes, polynucleotides, and proteins are well known in the art. In addition, transgenic plants comprising isolated polynucleotides encoding transcription factors may also modify expression of endogenous genes, polynucleotides, and proteins. Examples include Peng et al. (1997) Genes Development 11: 3194-3205, and Peng et al. (1999) Nature, 400: 256-261). In addition, many others have demonstrated that an Arabidopsis transcription factor expressed in an exogenous plant species elicits the same or very similar phenotypic response (see, for example, Fu et al. (2001) Plant Cell 13: 1791-1802; Nandi et al. (2000) Curr. Biol. 10: 215-218; Coupland (1995) Nature 311: 482-483; and Weigel and Nilsson (1995) Nature 377: 482-500). In another example, Mandel et al. (Mandel et al. (1992) Cell 71-133-143)) and Suzuki et al. (Suzuki et al. (2001) Plant J. 28: 409-418) teach that a transcription factor expressed in another plant species elicits the same or very similar phenotypic response of the endogenous sequence, as often predicted in earlier studies of Arabidopsis transcription factors in Arabidopsis (see Mandel et al. (1992) supra; Suzuki et al. (2001) supra). Other examples include Mϋller et al. ((2001) Plant J. 28: 169-179)), Kim et al. ((2001) Plant J. 25: 247-259), Kyozuka and Shimamoto ((2002) Plant Cell Physiol. 43: 130- 135), Boss and Thomas ((2002) Nature, 416: 847-850)), He et al. ((2000) Transgenic Res. 9: 223-227)), and Robson et al. ((2001) Plant J. 28: 619-631). h yet another example, Gilmour et al. (1998) supra, teach an Arabidopsis AP2 transcription factor, CBFl (SEQ ID NO: 2), which, when overexpressed in transgenic plants, increases plant freezing tolerance. Jaglo et al. (2001) Plant Physiol. 127: 910-917, further identified sequences in Brassica napus that encode CBF-like genes and that transcripts for these genes accumulated rapidly in response to low temperature. Transcripts encoding CBF-like proteins were also found to accumulate rapidly in response to low temperature in wheat, as well as in tomato. An alignment of the CBF proteins from Arabidopsis, B. napus, wheat, rye, and tomato revealed the presence of conserved consecutive amino acid residues, P- K-K/R-R/P-A-G-R-x-K-F x E-T-R-H-P and D-S-A-W-R, that bracket the AP2/EREBP DNA binding domains of the proteins and distinguish them from other members of the AP2/EREBP protein family (Jaglo et al. (2001) supra). Transcription factors mediate cellular responses and control traits through altered expression of genes containing cis-acting nucleotide sequences that are targets of the introduced transcription factor. It is well appreciated in the art that the effect of a transcription factor on cellular responses or a cellular trait is determined by the particular genes whose expression is either directly or indirectly (for example, by a cascade of transcription factor binding events and transcriptional changes) altered by transcription factor binding. In a global transcription analysis comparing a standard condition with one in which a transcription factor is overexpressed, the resulting transcript profile associated with transcription factor overexpression is related to the trait or cellular process controlled by that transcription factor. For example, the PAP2 gene and other genes in the MYB family have been shown to control anthocyanin biosynthesis through regulation of the expression of genes known to be involved in the anthocyanin biosynthetic pathway (Bruce et al. (2000) Plant Cell, 12: 65-79; Borevitz et al. (2000) Plant Cell 12: 2383-93). Further, global transcript profiles have been used successfully as diagnostic tools for specific cellular states (for example, cancerous vs. non-cancerous; Bhattacharjee et al. (2001) Proc Natl. Acad. Sci., USA, 98: 13790-13795; Xu et al. (2001) Proc. Natl Acad. Sci., USA, 98: 15089-15094). Consequently, it is evident to one skilled in the art that similarity of transcript profile upon overexpression of different transcription factors would indicate similarity of transcription factor function.

Polypeptides and Polynucleotides of the Invention The present invention relates to polynucleotides and polypeptides that may be used to increase a tolerance to environmental stress in a plant that is morphologically and developmentally similar to a control plant. The present invention provides, among other things, CBF clade transcription factors (TFs), and transcription factor homolog polypeptides, and isolated or recombinant polynucleotides encoding the polypeptides, or novel sequence variant polypeptides or polynucleotides encoding novel variants of transcription factors derived from the specific sequences provided here. The CBF clade of transcription factor polypeptides comprises sequences derived from a common ancestral node (denoted by the arrow in Figure 4). Many of these sequences, when overexpressed in plants, confer abiotic stress tolerance, including tolerance to hyperosmotic stresses such as high salinity, drought or freezing, or other abiotic stresses such as heat or cold. Major domains of the CBF polypeptides include two signature sequences (Figure 5) that are highly conserved in CBF polypeptides from diverse species. Examples of CBF polypeptides are also found in the Sequence Listing. Sequences within this clade may be distinguished structurally by these signature sequences, and particularly the first signature sequence; clade member polypeptides generally comprise the subsequence K-K/R-R/P-A-G-R- X-X-F-X-E-T-R-H-P, where X is any amino acid residue (Figure 5). Wild-type and mutant members of the CBF clade of transcription factor polypeptides will generally comprise this subsequence. However, it is envisioned that some conservative or similar substitutions in this signature sequence may be introduced by a mutagenesis process that yields a functional mutant polypeptide that, when overexpressed in a plant, also confers abiotic stress tolerance without adverse developmental or morphological effects. Environmental stresses for which stress tolerance genes are known to exist include, but are not limited to, cold tolerance, dehydration tolerance, and salinity tolerance. As used herein, environmental stress tolerance genes refer to genes that function to acclimate a plant to an environmental stress. For example, cold tolerance genes, also referred to as COR genes (cold regulated), refer to genes which function to acclimate a plant to a cold temperature environment. These genes typically are activated when a plant is exposed to cold temperatures. Dehydration tolerance genes refer to genes that function to acclimate a plant to dehydration stress. These genes typically are activated in response to dehydration conditions that can be associated with drought or cold temperatures. Sufficiently cold conditions may cause water in the plant to freeze and thereby dehydrate the plant tissue. Some cold tolerance genes may function by providing a plant with a degree of dehydration tolerance, and visa versa. For example, COR genes are known to be activated by dehydration stress. This application is intended to encompass genes that regulate one or more environmental stress tolerance genes such as cold tolerance genes, dehydration tolerance genes, and genes that perform dual functions of cold and dehydration tolerance. The polynucleotides of the invention were ectopically expressed in plant cells, and the changes in the expression levels of a number of genes, polynucleotides, and/or proteins of the plant cells observed. Therefore, the polynucleotides and polypeptides can be employed to change expression levels of a genes, polynucleotides, and/or proteins of plants. These polypeptides and polynucleotides may be employed to modify a plant's characteristics, particularly abiotic stress tolerance. The present invention thus relates to DNA, including isolated DNA, that encodes mutant or variant CBF polypeptides capable of binding to a DNA regulatory sequence that regulates expression of one or more environmental stress tolerance genes in a plant. The mutant or variant CBF polypeptides confer abiotic stress tolerance to a plant when overexpressed, but the plant retains morphological and developmental similarity to a control or wild-type plant of the same species that does not overexpress a CBF polypeptide. This is often not the case with plants that overexpress native CBF polypeptides; the latter overexpressors often have a number of defects including low fertility, reduced seed production, small size, or reduced yield relative to wild-type plants. The isolated DNA sequence may exist in a variety of forms, including in a plasmid or vector. The plasmid or vector can include a promoter that regulates expression of the regulatory gene, hi one variation of this embodiment, the DNA regulatory sequence is a C-repeat cold and drought regulation element (C-repeat/DRE). C-repeat/DRE regulatory sequences appear to be conserved in plants with some degree of variability from plant to plant. C-repeat/DRE regulatory sequences native to different plants can be identified as well as the native stress tolerance regulatory genes that encode for proteins that bind to the C-repeat/DRE DNA regulatory sequences. Hence, although the examples provided herein are described with regard to the Arabidopsis C-repeat/DRE DNA regulatory sequence, the present invention is not intended to be limited to the Arabidopsis C-repeat/DRE DNA regulatory sequence. Rather, the Arabidopsis C-repeat/DRE DNA regulatory sequence is a member of a class of environmental stress response regulatory elements that includes the subsequence CCGAC, which in turn is a member of a class of environmental stress response regulatory elements that includes the subsequence CCG. Other different classes of environmental stress response regulatory elements may also exist. The teachings of the present invention may be used to identify sequences that bind to these and other classes of environmental stress response regulatory elements. In one variation of this embodiment, the gene sequence of the invention encodes a mutant or variant CBF polypeptide that selectively binds to a member of a class of DNA regulatory sequences that includes the subsequence CCG. In another variation, the gene sequence encodes a CBF polypeptide that selectively binds to a member of a class of DNA regulatory sequences that includes the subsequence CCGAC. The CCGAC subsequence has been found to be present in the C-repeat/DRE DNA regulatory sequences of Arabidopsis and Brassica and to function in tobacco based on the ability of the C- repeat/DRE to direct cold and tolerance regulated gene expression. Promoters can be used to overexpress the mutant or variant CBF polypeptide, change the environmental conditions under which the mutant or variant CBF polypeptide is expressed, or enable the expression of the mutant or variant CBF polypeptide to be induced, for example by the addition of an exogenous inducing agent. Promoters can also be used to cause the mutant or variant CBF polypeptide to be expressed at selected times during a plant's life. Tissue-specific promoters can be used to cause the mutant or variant CBF polypeptide to be expressed in selected tissues. For example, flower-, fruit- and seed-specific promoters can be used to cause the mutant or variant CBF polypeptide to be selectively expressed in flowers, fruits or seeds of the plant. The present invention also relates to methods for using the DNA and mutant (variant) CBF polypeptides to regulate expression of one or more native or non-native environmental stress tolerance genes in a plant. These methods may include introducing DNA encoding a variant CBF polypeptide capable of binding to a DNA regulatory sequence into a plant, introducing a promoter into a plant that regulates expression of the CBF polypeptide, introducing a DNA regulatory sequence into a plant to which a variant CBF polypeptide can bind, and/or introducing one or more environmental stress tolerance genes into a plant whose expression is regulated by a DNA regulatory sequence. The present invention relates to recombinant cells, plants and plant materials (e.g., plant tissue, seeds) into which one or more gene sequences encoding a variant CBF polypeptide have been introduced, as well as cells, plants and plant materials within which recombinant CBF polypeptides encoded by these gene sequences are expressed. By introducing a gene sequence encoding a variant CBF polypeptide into a plant, the variant CBF polypeptide can be overexpressed or ectopically expressed within the plant. The variant CBF polypeptide is capable of regulating expression of one or more stress tolerance genes in the plant, which is morphologically and developmentally similar to a control or wild-type plant. Regulation of expression can include causing one or more stress tolerance genes to be expressed under different conditions than would alter the expression of those genes in the plant's native state, increasing a level of expression of one or more stress tolerance genes, and/or causing the expression of one or more stress tolerance genes to be inducible by an exogenous agent or environmental condition. The present invention also relates to variant CBF polypeptides. The DNA and variant CBF polypeptides may be naturally occurring (a naturally occurring mutation has taken place within a plant), or artificially mutagenized or varied (for the latter, a number of possible methods may be used such as by creating truncations or fusions). One embodiment of the invention relates to a variant CBF polypeptide capable of selectively binding to a DNA regulatory sequence that regulates expression of one or more environmental stress tolerance genes in a plant, preferably by selectively binding to a DNA regulatory sequence that regulates the environmental stress tolerance genes. Because of the nature of the mutation or variation, this plant retains morphological and developmental similarity to a control or wild-type plant of the same species. In one variation, the variant CBF polypeptide is a non-naturally occurring polypeptide formed by combining an amino acid sequence capable of binding to a CCG regulatory sequence, preferably a CCGAC regulatory sequence, with an amino acid sequence that forms a transcription activation region that regulates expression of one or more environmental stress tolerance genes. hi yet another variation, the stress tolerance regulatory gene sequence encodes a mutant or variant CBF polypeptide that includes an AP2 domain. It is believed that a significant class of environmental stress tolerance regulatory genes encodes for CBF polypeptides with an AP2 domain capable of binding to the DNA regulatory sequence. The AP2 domain of the mutant or variant CBF polypeptide is preferably a homolog of the AP2 domain of one of the mutant or variant CBF polypeptides described herein. The subsequence encoding the AP2 domain is preferably a homolog of a subsequence of one of the mutant or variant CBF genes described herein that encodes an AP2 domain. In another variation, the DNA sequence encoding the mutant or variant CBF polypeptide comprises an AP2 domain that comprises a sequence sufficiently homologous to SEQ ID NO: 13 that the mutant or variant CBF polypeptide is capable of binding to a CCG regulatory sequence, preferably a CCGAC regulatory sequence; and the mutant or variant CBF polypeptide comprises a mutant sequence or variation such that a plant overexpressing the CBF polypeptide is morphologically similar to a wild- type plant that does not overexpress a CBF polypeptide. The variant CBF polypeptide may be derived from any plant having a genome encoding a CBF polypeptide that confers increased abiotic stress tolerance when the polypeptide is overexpressed. The Examples provided below demonstrate that different variations that may occur within the AP2 domain, or outside of the AP2 domain, confer abiotic stress tolerance in plants of wild-type or nearly wild-type morphology and fertility. Mutations in other CBF polypeptides at corresponding positions to those shown in Figures 6 and 10 are expected to behave similarly in most instances and are also encompassed by the ^■ invention. In this context, "corresponding position" refers to a similar or the same position in an alignment of two similar or identical subsequences of distinct CBF polypeptides. Figure 3C and 3D are part of an alignment of CBF polypeptides that may also be used to determine corresponding residues. For example, Figure 3C shows the glutamic acid residue (position 84) in the subsequence of the AP2 domain ofCBF2 (SEQ ID NO: 13): Gin Thr Ala Glu Met Ala Ala

occurs in corresponding positions in SEQ ID NO: 13 and orthologous and paralogous CBF polypeptides. The same is true for corresponding aspartic acid residue (position 108) found outside of the AP2 domain but in a CBF signature sequence (Figure 10) in the subsequence shown in Figure 3C: Asn Phe Ala Asp Ser Ala Trp Arg

and in the serine residue (position 118) outside of the AP2 domain of CBF2 (SEQ ID NO: 13) in the Figure 3D subsequence: He Pro Glu Ser Thr Cys Ala.

Specific substitutions of these residues, as noted below, confer abiotic stress tolerance when the mutant polypeptide is overexpressed, with fewer adverse morphological and/or developmental effects that occur when the non-mutant form of the CBF2 polypeptide is overexpressed. Other variations in the amino acid sequence of the CBF polypeptide may be considered that also confer abiotic stress tolerance in plants of wild-type or nearly wild-type morphology and fertility, and are also encompassed by the present invention. Additionally, the substitution of the aspartic acid, serine, tryptophan or arginine residues (at positions 108-109 and 111-112 in SEQ ID NO: 2) with alanine residues in the CBF signature sequence: D-S-A-W-R (replaced with A-A-A-A-A as seen in SEQ ID NO: 220)

has been shown to yield a CBF mutant polypeptide that, when overexpressed in plants, conferred increased freezing stress tolerance without the adverse morphological or developmental effects often associated with wild-type CBF polypeptide overexpression. It is envisioned that other similar substitutions may also be used to produce useful polypeptides of the invention. In addition to alanine, different combinations of similar amino acids that may be substituted for the aspartic acid, serine, tryptophan, or arginine residues in this signature sequence may mclude, for example, serine, threonine, glycine, valine, leucine, or isoleucine residues.

Producing Polypeptides The polynucleotides of the invention mclude sequences that encode transcription factors and transcription factor homolog polypeptides and sequences complementary thereto, as well as unique fragments of coding sequence, or sequence complementary thereto. Such polynucleotides can be, for example, DNA or RNA, the latter including mRNA, cRNA, synthetic RNA, genomic DNA, cDNA synthetic DNA, oligonucleotides, etc. The polynucleotides are either double-stranded or single-stranded, and include either, or both sense (i.e., coding) sequences and antisense (i.e., non-coding, complementary) sequences. The polynucleotides include the coding sequence of a transcription factor, or transcription factor homolog polypeptide, in isolation, in combination with additional coding sequences (e.g., a purification tag, a localization signal, as a fiision-protein, as a pre-protein, or the like), in combination with non-coding sequences (for example, introns or inteins, regulatory elements such as promoters, enhancers, terminators, and the like), and/or in a vector or host environment in which the polynucleotide encoding a transcription factor or transcription factor homolog polypeptide is an endogenous or exogenous gene. A variety of methods exist for producing the polynucleotides of the invention. Procedures for identifying and isolating DNA clones are well known to those of skill in the art, and are described in, for example, Berger and Kimmel, Guide to Molecular Cloning Techniques, Methods in Enzymology, vol. 152 Academic Press, Inc., San Diego, CA ("Berger and Kimmel''); Sambrook et al. Molecular Cloning - A Laboratory Manual (2nd Ed.), Vol. 1-3, Cold Spring Harbor Laboratory, Cold Spring Harbor, New York, 1989 ("Sambrook") and Current Protocols in Molecular Biology, Ausubel et al. eds., Current Protocols, a joint venture between Greene Publishing Associates, hie. and John Wiley & Sons, hie, (supplemented through 2000) ("Ausubel"). Alternatively, polynucleotides of the invention can be produced by a variety of in vitro amplification methods adapted to the present invention by appropriate selection of specific or degenerate primers. Examples of protocols sufficient to direct persons of skill through in vitro amplification methods, including the polymerase chain reaction (PCR), the ligase chain reaction (LCR), Qβ-replicase amplification and other RNA polymerase mediated teclmiques (for example, NASBA). Protocols for the production of the homologous nucleic acids of the invention are found in Berger and Kimmel (supra), Sambrook (supra), and Ausubel (supra), as well as Mullis et al. (1987) PCR Protocols A Guide to Methods and Applications (Innis et al. eds) Academic Press Inc. San Diego, CA (1990) (hmis). Improved methods for cloning in vitro amplified nucleic acids are described in Wallace et al. US Pat. No. 5,426,039. Improved methods for amplifying large nucleic acids by PCR are summarized in Cheng et al. (1994) Nature 369: 684-685 and the references cited therein, in which PCR amplicons of up to 40 kb are generated. One of skill will appreciate that essentially any RNA can be converted into a double stranded DNA suitable for restriction digestion, PCR expansion and sequencing using reverse transcriptase and a polymerase. See, e.g., Ausubel, Sambrook and Berger, all supra. Alternatively, polynucleotides and oligonucleotides of the invention can be assembled from fragments produced by solid-phase synthesis methods. Typically, fragments of up to approximately 100 bases are individually synthesized and then enzymatically or chemically ligated to produce a desired sequence, e.g., a polynucleotide encoding all or part of a transcription factor. For example, chemical synthesis using the phosphoramidite method is described, e.g., by Beaucage et al. (1981) Tetrahedron Letters 22: 1859-1869; and Matthes et al. (1984) EMBOJ. 3: 801-805. According to such methods, oligonucleotides are synthesized, purified, annealed to their complementary strand, ligated and then optionally cloned into suitable vectors. And if so desired, the polynucleotides and polypeptides of the invention can be custom ordered from any of a number of commercial suppliers.

Homologous Sequences Sequences homologous, i.e., that share significant sequence identity or similarity, to those provided in the Sequence Listing, derived from Arabidopsis thaliana or from other plants of choice, are also an aspect of the invention. Homologous sequences can be derived from any plant including monocots and dicots and in particular agriculturally important plant species, including but not limited to: crops such as soybean, wheat, corn (maize), potato, cotton, rice, rape, oilseed rape (including canola), sunflower, alfalfa, clover, sugarcane, and turf; or fruits and vegetables, such as banana, blackberry, blueberry, strawberry, and raspberry, cantaloupe, carrot, cauliflower, coffee, cucumber, eggplant, grapes, honeydew, lettuce, mango, melon, onion, papaya, peas, peppers, pineapple, pumpkin, spinach, squash, sweet corn, tobacco, tomato, tomatillo, watermelon, rosaceous fruits and fruit trees (such as apple, peach, pear, cherry and plum) and brassicas (such as broccoli, cabbage, cauliflower, Brussels sprouts, and kohlrabi). Other crops, including fruits and vegetables, whose phenotype can be changed and which comprise homologous sequences include barley; rye; millet; sorghum; currant; avocado; citrus fruits such as oranges, lemons, grapefruit and tangerines, artichoke, cherries; nuts such as the walnut and peanut; endive; leek; roots such as arrowroot, beet, cassava, turnip, radish, yam, and sweet potato; and beans. The homologous sequences may also be derived from woody species, such pine, poplar and eucalyptus, or mint or other labiates. In addition, homologous sequences may be derived from plants that are evolutionarily related to crop plants, but which may not have yet been used as crop plants. Examples include deadly nightshade (Atropa belladona), related to tomato; jimson weed (Datura strommium), related to peyote; and teosinte (Zea species), related to corn (maize).

Orthologs and Paralogs Homologous sequences as described above can comprise orthologous or paralogous sequences.

Several different methods are known by those of skill in the art for identifying and defining these functionally homologous sequences. Three general methods for defining orthologs and paralogs are described; an ortholog, paralog or homolog may be identified by one or more of the methods described below. Orthologs and paralogs are evolutionarily related genes that have similar sequence and similar functions. Orthologs are structurally related genes in different species that are derived by a speciation event. Paralogs are structurally related genes within a single species that are derived by a duplication event. Within a single plant species, gene duplication may cause two copies of a particular gene, giving rise to two or more genes with similar sequence and often similar function known as paralogs. A paralog is therefore a similar gene formed by duplication within the same species. Paralogs typically cluster together or in the same clade (a group of similar genes) when a gene family phylogeny is analyzed using programs such as CLUSTAL (Thompson et al. (1994) Nucleic Acids Res. 22: 4673-4680; Higgins et al. (1996) Methods Enzymol. 266: 383-402). Groups of similar genes can also be identified with pair-wise BLAST analysis (Feng and Doolittle (1987) J. Mol. Evol. 25: 351-360). For example, a clade of very similar MADS domain transcription factors from Arabidopsis all share a common function in flowering time (Ratcliffe et al. (2001) Plant Physiol. 126: 122-132), and a group of very similar AP2 domain transcription factors from Arabidopsis are involved in tolerance of plants to freezing (Gilmour et al. (1998) supra). Analysis of groups of similar genes with similar function that fall within one clade can yield sub-sequences that are particular to the clade. These sub-sequences, known as consensus sequences, can not only be used to define the sequences within each clade, but define the functions of these genes; genes within a clade may contain paralogous sequences, or orthologous sequences that share the same function (see also, for example, Mount (2001), in Bioinformatics: Sequence and Genome Analysis Cold Spring Harbor Laboratory Press, Cold Spring Harbor, New York, page 543.) Speciation, the production of new species from a parental species, can also give rise to two or more genes with similar sequence and similar function. These genes, termed orthologs, often have an identical function within their host plants and are often interchangeable between species without losing function. Because plants have common ancestors, many genes in any plant species will have a corresponding orthologous gene in another plant species. Once a phylogenic tree for a gene family of one species has been constructed using a program such as CLUSTAL (Thompson et al. (1994) Nucleic Acids Res. 22: 4673-4680; Higgins et al. (1996) supra) potential orthologous sequences can be placed into the phylogenetic tree and their relationship to genes from the species of interest can be determined.

Orthologous sequences can also be identified by a reciprocal BLAST strategy. Once an orthologous sequence has been identified, the function of the ortholog can be deduced from the identified function of the reference sequence. Transcription factor gene sequences are conserved across diverse eukaryotic species lines (Goodrich et al. (1993) Cell 75: 519-530; Lin et al. (1991) Nature 353: 569-571; Sadowski et al. (1988) Nature 335: 563-564). Plants are no exception to this observation; diverse plant species possess transcription factors that have similar sequences and functions. Orthologous genes from different organisms have highly conserved functions, and very often essentially identical functions (Lee et al. (2002) Genome Res. 12: 493-502; Remm et al. (2001) J. Mol Biol. 314: 1041-1052). Paralogous genes, which have diverged through gene duplication, may retain similar functions of the encoded proteins. In such cases, paralogs can be used interchangeably with respect to certain embodiments of the instant invention (for example, transgenic expression of a coding sequence). An example of such highly related paralogs is the CBF family, with four well-defined members in Arabidopsis, CBFs 1, 2 and 3 (SEQ ID NOs: 2, 13, 15), CBF4 (G912, SEQ ID NO: 97; GenBank accession number BABl 1047) and at least one ortholog in Brassica napus, (SEQ ID NO: 17), all of which control pathways involved in both freezing and drought stress (Gilmour et al. (1998) supra; Jaglo et al. (1998) Plant Physiol. 127: 910-917). The following references represent a small sampling of the many studies that demonstrate that conserved transcription factor genes from diverse species are likely to function similarly (i.e., regulate similar target sequences and control the same traits), and that transcription factors may be transformed into diverse species to confer or improve traits. (1) The Arabidopsis NPRl gene regulates systemic acquired resistance (SAR) (Cao et al. (1997) Cell 88: 57-63); over-expression of NPRl leads to enhanced resistance in Arabidopsis. When either Arabidopsis NPRl or the rice NPRl ortholog was overexpressed in rice (which, as a monocot, is diverse from Arabidopsis), challenge with the rice bacterial blight pathogen Xanthomonas oryzae pv. Oryzae, the transgenic plants displayed enhanced resistance (Chern et al. (2001) Plant J. 27: 101-113). NPRl acts through activation of expression of transcription factor genes, such as TGA2 (Fan and Dong (2002) Plant Cell 14: 1377-1389). (2) E2F genes are involved in transcription of plant genes for proliferating cell nuclear antigen (PCNA). Plant E2Fs share a high degree of similarity in amino acid sequence between monocots and dicots, and are even similar to the conserved domains of the animal E2Fs. Such conservation indicates a functional similarity between plant and animal E2Fs. E2F transcription factors that regulate meristem development act through common cis-elements, and regulate related (PCNA) genes (Kosugi and Ohashi, (20Q2) Plant J. 29: 45-59). (3) The ABI5 gene (ABA insensitive 5) encodes a basic leucine zipper factor required for ABA response in the seed and vegetative tissues. Co-transformation experiments with ABI5 cDNA constructs in rice protoplasts resulted in specific transactivation of the ABA-inducible wheat, Arabidopsis, bean, and barley promoters. These results demonstrate that sequentially similar ABI5 transcription factors are key targets of a conserved ABA signaling pathway in diverse plants. (Gampala et al. (2001) J. Biol. Chem. 277: 1689-1694). (4) Sequences of three Arabidopsis GAMYB-like genes were obtained on the basis of sequence similarity to GAMYB genes from barley, rice, and L. temulentum. These three Arabidopsis genes were determined to encode transcription factors (AtMYB33, AtMYB65, and A MYBlOl) and could substitute for a barley GAMYB and control α-amylase expression (Gocal et al. (2001) Plant Physiol. 127: 1682- 1693). (5) The floral control gene LEAFY from Arabidopsis can dramatically accelerate flowering in numerous dictoyledonous plants. Constitutive expression of Arabidopsis LEAFY also caused early flowering in transgenic rice (a monocot), with a heading date that was 26-34 days earlier than that of wild-type plants. These observations indicate that floral regulatory genes from Arabidopsis are useful tools for heading date improvement in cereal crops (He et al. (2000) Transgenic Res. 9: 223-227). (6) Bioactive gibberellins (GAs) are essential endogenous regulators of plant growth. GA signaling tends to be conserved across the plant kingdom. GA signaling is mediated via GAI, a nuclear member of the GRAS family of plant transcription factors. Arabidopsis GAI has been shown to function in rice to inhibit gibberellin response pathways (Fu et al. (2001) Plant Cell 13: 1791-1802). (7) The Arabidopsis gene SUPERMAN (SUP), encodes a putative transcription factor that maintains the boundary between stamens and carpels. By over-expressing Arabidopsis SUP in rice, the effect of the gene's presence on whorl boundaries was shown to be conserved. This demonstrated that SUP is a conserved regulator of floral whorl boundaries and affects cell proliferation (Nandi et al. (2000) Curr. Biol. 10: 215-218). (8) Maize, petunia and Arabidopsis myb transcription factors that regulate flavonoid biosynthesis are very genetically similar and affect the same trait in their native species, therefore sequence and function of these myb transcription factors correlate with each other in these diverse species (Borevitz et al. (2000) Plant Cell 12: 2383-2394). (9) Wheat reduced height- 1 (Rht-Bl/Rht-Dl) and maize dwarf-8 (d8) genes are orthologs of the Arabidopsis gibberellin insensitive (GAI) gene. Both of these genes have been used to produce dwarf grain varieties that have improved grain yield. These genes encode proteins that resemble nuclear transcription factors and contain an SH2-like domain, indicating that phosphotyrosine may participate in gibberellin signaling. Transgenic rice plants containing a mutant GAI allele from Arabidopsis have been shown to produce reduced responses to gibberellin and are dwarfed, indicating that mutant GAI orthologs could be used to increase yield in a wide range of crop species (Peng et al. (1999) Nature 400: 256-261). Transcription factors that are homologous to the listed sequences will typically share at least about 70% amino acid sequence identity in the conserved domain. More closely related transcription factors can share at least about 79% or about 90% or about 95% or about 98% or more sequence identity with the listed sequences, or with the listed sequences but excluding or outside a known consensus sequence or consensus DNA-binding site, or with the listed sequences excluding one or all conserved domains. Factors that are most closely related to the listed sequences share, e.g., at least about 85%, about 90% or about 95% or more % sequenceidentity to the listed sequences, or to the listed sequences but excluding or outside a known consensus sequence or consensus DNA-binding site or outside the conserved domain. At the nucleotide level, the sequences will typically share at least about 40% nucleotide sequence identity, preferably at least about 50%, about 60%, about 70% or about 80% sequence identity, and more preferably about 85%, about 90%, about 95% or about 97% or more sequence identity to one or more of the listed sequences, or to a listed sequence but excluding or outside a known consensus sequence or consensus DNA-binding site, or outside one or all conserved domain. The degeneracy of the genetic code enables major variations in the nucleotide sequence of a polynucleotide while maintaining the amino acid sequence of the encoded protein. AP2 domains within the AP2 transcription factor family may exhibit a higher degree of sequence homology, such as at least 70%) amino acid sequence identity including conservative substitutions, and preferably at least 80% sequence identity, and more preferably at least 85%, or at least about 86%, or at least about 87%, or at least about 88%, or at least about 90%, or at least about 95%, or at least about 98% sequence identity. Transcription factors that are homologous to the listed sequences should share at least 30%, or at least about 60%, or at least about 75%, or at least about 80%, or at least about 90%, or at least about 95% amino acid sequence identity over the entire length of the polypeptide or the homolog. Percent identity can be determined electronically, e.g., by using the MEGALIGN program (DNASTAR, Inc. Madison, Wis.). The MEGALIGN program can create alignments between two or more sequences according to different methods, for example, the clustal method. (See, for example, Higgins and Sharp (1988) Gene 73: 237-244.) The clustal algorithm groups sequences into clusters by examining the distances between all pairs. The clusters are aligned pairwise and then in groups. Other alignment algorithms or programs may be used, including FASTA, BLAST, or ENTREZ, FASTA and BLAST, and which may be used to calculate percent similarity. These are available as a part of the GCG sequence analysis package (University of Wisconsin, Madison, Wis.), and can be used with or without default settings. ENTREZ is available through the National Center for Biotechnology Information. In one embodiment, the percent identity of two sequences can be determined by the GCG program with a gap weight of 1, e.g., each amino acid gap is weighted as if it were a single amino acid or nucleotide mismatch between the two sequences (see USPN 6,262,333). Other techniques for alignment are described in Methods Enzymol, vol. 266, "Computer Methods for Macromolecular Sequence Analysis" (1996), ed. Doolittle, Academic Press, Inc., San Diego, Calif, USA. Preferably, an alignment program that permits gaps in the sequence is utilized to align the sequences. The Smith- Waterman is one type of algorithm that permits gaps in sequence alignments (see Shpaer (1997) Methods Mol. Biol. 70: 173-187). Also, the GAP program using the Needleman and Wunsch alignment method can be utilized to align sequences. An alternative search strategy uses MPSRCH software, which runs on a MASPAR computer. MPSRCH uses a Smith-Waterman algorithm to score sequences on a massively parallel computer. This approach improves ability to pick up distantly related matches, and is especially tolerant of small gaps and nucleotide sequence errors. Nucleic acid- encoded amino acid sequences can be used to search both protein and DNA databases. The percentage similarity between two polypeptide sequences, e.g., sequence A and sequence B, is calculated by dividing the length of sequence A, minus the number of gap residues in sequence A, minus the number of gap residues in sequence B, into the sum of the residue matches between sequence A and sequence B, times one hundred. Gaps of low or of no similarity between the two amino acid sequences are not included in determining percentage similarity. Percent identity between polynucleotide sequences can also be counted or calculated by other methods known in the art, e.g., the Jotun Hein method (see, e.g., Hein (1990) Methods Enzymol. 183: 626-645) . Identity between sequences can also be determined by other methods known in the art, e.g., by varying hybridization conditions (see US Patent Application No. 20010010913). Thus, the invention provides methods for identifying a sequence similar or paralogous or orthologous or homologous to one or more polynucleotides as noted herein, or one or more target polypeptides encoded by the polynucleotides, or otherwise noted herein and may include linking or associating a given plant phenotype or gene fiinction with a sequence. In the methods, a sequence database is provided (locally or across an internet or intranet) and a query is made against the sequence database using the relevant sequences herein and associated plant phenotypes or gene functions. In addition, one or more polynucleotide sequences or one or more polypeptides encoded by the polynucleotide sequences may be used to search against a BLOCKS (Bairoch et al. (1997) Nucleic Acids Res. 25: 217-221), PFAM, and other databases which contain previously identified and annotated motifs, sequences and gene functions. Methods that search for primary sequence patterns with secondary structure gap penalties (Smith et al. (1992) Protein Engineering 5 : 35-51) as well as algorithms such as Basic Local Alignment Search Tool (BLAST; Altschul (1993) J. Mol. Evol 36: 290-300; Altschul et al. (1990) J. Mol. Biol. 215: 403-410), BLOCKS (Henikoff and Henikoff (1991) Nucleic Acids Res. 19: 6565-6572), Hidden Markov Models (HMM; Eddy (1996) Curr. Opin. Str. Biol 6: 361-365; Sonnhammer et al. (1997) Proteins 28: 405-420), and the like, can be used to manipulate and analyze polynucleotide and polypeptide sequences encoded by polynucleotides. These databases, algorithms and other methods are well known in the art and are described in Ausubel et al. (1997; Short Protocols in Molecular Biology, John Wiley & Sons, New York, NY, unit 7.7) and in Meyers (1995; Molecular Biology and Biotechnology, Wiley VCH, New York, NY, p 856-853). Another method for identifying or confirming that specific homologous sequences control the same function is by comparison of the transcript profile(s) obtained upon overexpression or knockout of two or more related transcription factors. Since transcript profiles are diagnostic for specific cellular states, one skilled in the art will appreciate that genes that have a highly similar transcript profile (e.g., with greater than 50% regulated transcripts in common, more preferably with greater than 70% regulated transcripts in common, most preferably with greater than 90% regulated transcripts in common) will have highly similar functions. Fowler et al. (2002, Plant Cell, 14: 1675-79) have shown that three paralogous AP2 family genes (CBFl, CBF2 and CBF3), each of which is induced upon cold treatment, and each of which can condition improved freezing tolerance, have highly similar transcript profiles. Once a transcription factor has been shown to provide a specific function, its transcript profile becomes a diagnostic tool to determine whether putative paralogs or orthologs have the same function. Furthermore, methods using manual alignment of sequences similar or homologous to one or more polynucleotide sequences or one or more polypeptides encoded by the polynucleotide sequences may be used to identify regions of similarity and AP2 domains. Such manual methods are well-known of those of skill in the art and can include, for example, comparisons of tertiary structure between a polypeptide sequence encoded by a polynucleotide which comprises a known function with a polypeptide sequence encoded by a polynucleotide sequence which has a function not yet determined. Such examples of tertiary structure may comprise predicted α-helices, β-sheets, amphipathic helices, leucine zipper motifs, zinc finger motifs, proline-rich regions, cysteine repeat motifs, and the like. Orthologs and paralogs of presently disclosed transcription factors may be cloned using compositions provided by the present invention according to methods well known in the art. cDNAs can be cloned using mRNA from a plant cell or tissue that expresses one of the present transcription factors. Appropriate mRNA sources may be identified by interrogating Northern blots with probes designed from the present transcription factor sequences, after which a library is prepared from the mRNA obtained from a positive cell or tissue. Transcription factor-encoding cDNA is then isolated using, for example, PCR, using primers designed from a presently disclosed transcription factor gene sequence, or by probing with a partial or complete cDNA or with one or more sets of degenerate probes based on the disclosed sequences. The cDNA library may be used to transform plant cells. Expression of the cDNAs of interest is detected using, for example, methods disclosed herein such as microarrays, Northern blots, quantitative PCR, or any other technique for monitoring changes in expression. Genomic clones may be isolated using similar techniques to those.

Identifying Polynucleotides or Nucleic Acids by Hybridization Polynucleotides homologous to the sequences illustrated in the Sequence Listing and tables can be identified, e.g., by hybridization to each other under stringent or under highly stringent conditions. Single stranded polynucleotides hybridize when they associate based on a variety of well characterized physical-chemical forces, such as hydrogen bonding, solvent exclusion, base stacking and the like. The stringency of a hybridization reflects the degree of sequence identity of the nucleic acids involved, such that the higher the stringency, the more similar are the two polynucleotide strands. Stringency is influenced by a variety of factors, including temperature, salt concentration and composition, organic and non-organic additives, solvents, etc. present in both the hybridization and wash solutions and incubations (and number thereof), as described in more detail in the references cited above. Encompassed by the invention are polynucleotide sequences that are capable of hybridizing to the claimed polynucleotide sequences, including any of the transcription factor polynucleotides within the Sequence Listing, and fragments thereof under various conditions of stringency (See, for example, Wahl and Berger (1987) Methods Enzymol. 152: 399-407; and Kimmel (1987) Methods Enzymol. 152: 507-511). In addition to the nucleotide sequences in the Sequence Listing, full length cDNA, orthologs, and paralogs of the present nucleotide sequences may be identified and isolated using well-known methods. The cDNA libraries, orthologs, and paralogs of the present nucleotide sequences may be screened using hybridization methods to determine their utility as hybridization target or amplification probes. With regard to hybridization, conditions that are highly stringent, and means for achieving them, are well known in the art. See, for example, Sambrook et al. (1989) "Molecular Cloning: A Laboratory Manual" (2nd ed., Cold Spring Harbor Laboratory); Berger and Kimmel, eds., (1987) "Guide to

Molecular Cloning Techniques", mMethods Enzymol. 152: 467-469; and Anderson and Young (1985) "Quantitative Filter Hybridisation." In: Hames and Higgins, ed., Nucleic Acid Hybridisation. A Practical Approach. Oxford, IRL Press, 73-111. Stability of DNA duplexes is affected by such factors as base composition, length, and degree of base pair mismatch. Hybridization conditions may be adjusted to allow DNAs of different sequence relatedness to hybridize. The melting temperature (T_m) is defined as the temperature when 50% of the duplex molecules have dissociated into their constituent single strands. The melting temperature of a perfectly matched duplex, where the hybridization buffer contains formamide as a denaturing agent, may be estimated by the following equations:

(I) DNA-DNA: T_m(° C)=81.5+16.6(log [Na+])+0.41(% G+C)- 0.62(% formamide)-500/Z

(II) DNA-RNA:

T_m(° C)=79.8+18.5(log [Na+])+0.58(% G+C)+ 0.12(%G+C)²- 0.5(% formamide) - 820/L

(TTI) RNA-RNA:

T_m(° C)=79.8+18.5(log [Na+])+0.58(% G+C)+ 0.12(%G+C)²- 0.35(% formamide) - 820/L where L is the length of the duplex formed, [Na+] is the molar concentration of the sodium ion in the hybridization or washing solution, and % G+C is the percentage of (guanine+cytosme) bases in the hybrid. For imperfectly matched hybrids, approximately 1° C is required to reduce the melting temperature for each 1% mismatch. Hybridization experiments are generally conducted in a buffer of pH between 6.8 to 7.4, although the rate of hybridization is nearly independent of pH at ionic strengths likely to be used in the hybridization buffer (Anderson and Young (1985) supra), hi addition, one or more of the following may be used to reduce non-specific hybridization: sonicated salmon sperm DNA or another non- complementary DNA, bovine serum albumin, sodium pyrophosphate, sodium dodecylsulfate (SDS), polyvinyl-pyrrolidone, ficoll and Denhardt's solution. Dextran sulfate and polyethylene glycol 6000 act to exclude DNA from solution, thus raising the effective probe DNA concentration and the hybridization signal within a given unit of time. In some instances, conditions of even greater stringency may be desirable or required to reduce non-specific and/or background hybridization. These conditions may be created with the use of higher temperature, lower ionic strength and liigher concentration of a denaturing agent such as formamide. Stringency conditions can be adjusted to screen for moderately similar fragments such as _\ homologous sequences from distantly related organisms, or to highly similar fragments such as genes that duplicate functional enzymes from closely related organisms. The stringency can be adjusted either during the hybridization step or in the post-hybridization washes. Salt concentration, formamide concentration, hybridization temperature and probe lengths are variables that can be used to alter stringency (as described by the formula above). As a general guidelines high stringency is typically performed at T_m-5° C to T_m-20° C, moderate stringency at T_m-20° C to T_m-35° C and low stringency at T_m-35° C to T_m-50° C for duplex >150 base pairs. Hybridization may be performed at low to moderate stringency (25-50° C below T_m), followed by post-hybridization washes at increasing stringencies. Maximum rates of hybridization in solution are determined empirically to occur at T_m-25° C for DNA- DNA duplex and T_m-15° C for RNA-DNA duplex. Optionally, the degree of dissociation may be assessed after each wash step to determine the need for subsequent, higher stringency wash steps. High stringency conditions may be used to select for nucleic acid sequences with high degrees of identity to the disclosed sequences. An example of stringent hybridization conditions obtained in a filter- based method such as a Southern or northern blot for hybridization of complementary nucleic acids that have more than 100 complementary residues is about 5°C to 20°C lower than the thermal melting point (T_m) for the specific sequence at a defined ionic strength and pH. Conditions used for hybridization may include about 0.02 M to about 0.15 M sodium chloride, about 0.5% to about 5% casein, about 0.02% SDS or about 0.1% N-laurylsarcosine, about 0.001 M to about 0.03 M sodium citrate, at hybridization temperatures between about 50° C and about 70° C. More preferably, high stringency conditions are about 0.02 M sodium chloride, about 0.5% casein, about 0.02% SDS, about 0.001 M sodium citrate, at a temperature of about 50° C. Nucleic acid molecules that hybridize under stringent conditions will typically hybridize to a probe based on either the entire DNA molecule or selected portions, e.g., to a unique subsequence, of the DNA. Stringent salt concentration will ordinarily be less than about 750 mM NaCl and 75 mM trisodium citrate. Increasingly stringent conditions may be obtained with less than about 500 mM NaCl and 50 mM trisodium citrate, to even greater stringency with less than about 250 mM NaCl and 25 mM trisodium citrate. Low stringency hybridization can be obtained in the absence of organic solvent, e.g., formamide, whereas high stringency hybridization may be obtained in the presence of at least about 35% formamide, and more preferably at least about 50% formamide. Stringent temperature conditions will ordinarily mclude temperatures of at least about 30° C, more preferably of at least about 37° C, and most preferably of at least about 42° C with formamide present. Varying additional parameters, such as hybridization time, the concentration of detergent, e.g., sodium dodecyl sulfate (SDS) and ionic strength, are well known to those skilled in the art. Various levels of stringency are accomplished by combining these various conditions as needed. The washing steps that follow hybridization may also vary in stringency; the post-hybridization wash steps primarily determine hybridization specificity, with the most critical factors being temperature and the ionic strength of the final wash solution. Wash stringency can be increased by decreasing salt concentration or by increasing temperature. Stringent salt concentration for the wash steps will preferably be less than about 30 mM NaCl and 3 mM trisodium citrate, and most preferably less than about 15 mM NaCl and 1.5 mM trisodium citrate. Thus, hybridization and wash conditions that may be used to bind and remove polynucleotides with less than the desired homology to the nucleic acid sequences or their complements that encode the present transcription factors include, for example: 6X SSC at 65° C; 50% formamide, 4X SSC at 42° C; or 0.5X SSC, 0.1% SDS at 65° C; with, for example, two wash steps of 10 - 30 minutes each. Useful variations on these conditions will be readily apparent to those skilled in the art. A person of skill in the art would not expect substantial variation among polynucleotide species encompassed within the scope of the present invention because the highly stringent conditions set forth in the above formulae yield structurally similar polynucleotides. If desired, one may employ wash steps of even greater stringency, including about 0.2X SSC, 0.1% SDS at 65° C and washing twice, each wash step being about 30 min, or about 0.1 X SSC, 0.1 % SDS at 65° C and washing twice for 30 min. The temperature for the wash solutions will ordinarily be at least about 25° C, and for greater stringency at least about 42° C. Hybridization stringency may be increased further by using the same conditions as in the hybridization steps, with the wash temperature raised about 3° C to about 5° C, and stringency may be increased even further by using the same conditions except the wash temperature is raised about 6° C to about 9° C. For identification of less closely related homologs, wash steps may be performed at a lower temperature, e.g., 50° C. An example of a low stringency wash step employs a solution and conditions of at least 25° C in 30 mM NaCl, 3 mM trisodium citrate, and 0.1% SDS over 30 min. Greater stringency may be obtained at 42° C in 15 mM NaCl, with 1.5 mM trisodium citrate, and 0.1% SDS over 30 min. Even liigher stringency wash conditions are obtained at 65° C -68° C in a solution of 15 mM NaCl, 1.5 mM trisodium citrate, and 0.1% SDS. Wash procedures will generally employ at least two final wash steps. Additional variations on these conditions will be readily apparent to those skilled in the art (see, for example, US Patent Application No.20010010913). Stringency conditions can be selected such that an oligonucleotide that is perfectly complementary to the coding oligonucleotide hybridizes to the coding oligonucleotide with at least about a 5-1 Ox higher signal to noise ratio than the ratio for hybridization of the perfectly complementary oligonucleotide to a nucleic acid encoding a transcription factor known as of the filing date of the application. It may be desirable to select conditions for a particular assay such that a higher signal to noise ratio, that is, about 15x or more, is obtained. Accordingly, a subject nucleic acid will hybridize to a unique coding oligonucleotide with at least a 2x or greater signal to noise ratio as compared to hybridization of the coding oligonucleotide to a nucleic acid encoding known polypeptide. The particular signal will depend on the label used in the relevant assay, e.g., a fluorescent label, a colorimetric label, a radioactive label, or the like. Labeled hybridization or PCR probes for detecting related polynucleotide sequences may be produced by oligolabeling, nick translation, end-labeling, or PCR amplification using a labeled nucleotide. Estimates of homology are provided by either DNA-DNA or DNA-RNA hybridization under conditions of stringency as is well understood by those skilled in the art (Hames and Higgins, Eds. (1985) Nucleic Acid Hybridisation. JJ L Press, Oxford, U.K.). Stringency conditions can be adjusted to screen for moderately similar fragments, such as homologous sequences from distantly related organisms, to highly similar fragments, such as genes that duplicate functional enzymes from closely related organisms. Post-hybridization washes determine stringency conditions.

Identifying Polynucleotides or Nucleic Acids with Expression Libraries In addition to hybridization methods, transcription factor homolog polypeptides can be obtained by screening an expression library using antibodies specific for one or more transcription factors. With the provision herein of the disclosed transcription factor, and transcription factor homolog nucleic acid sequences, the encoded polypeptide(s) can be expressed and purified in a heterologous expression system (e.g., E. coli) and used to raise antibodies (monoclonal or polyclonal) specific for the polypeptide(s) in question. Antibodies can also be raised against synthetic peptides derived from the amino acid sequences or subsequences of a transcription factor or transcription factor homolog. Methods of raising antibodies are well known in the art and are described in Harlow and Lane (1988), Antibodies: A Laboratory Manual, Cold Spring Harbor Laboratory, New York. Such antibodies can then be used to screen an expression library produced from the plant from which it is desired to clone additional transcription factor homologs, using the methods described above. The selected cDNAs can be confirmed by sequencing and enzymatic activity.

Sequence Variations It will readily be appreciated by those of skill in the art, that a significant variety of polynucleotide sequences are capable of encoding the transcription factors and transcription factor homolog polypeptides of the invention. Due to the degeneracy of the genetic code, many different polynucleotides can encode identical and/or substantially similar polypeptides in addition to those sequences illustrated in the Sequence Listing. Nucleic acids having a sequence that differs from the sequences shown in the Sequence Listing, or complementary sequences, that encode functionally equivalent peptides (i.e., peptides having some degree of equivalent or similar biological activity) but differ in sequence from the sequence shown in the Sequence Listing due to degeneracy in the genetic code, are also within the scope of the invention. Altered polynucleotide sequences encoding polypeptides include those sequences with deletions, insertions, or substitutions of different nucleotides, resulting in a polynucleotide encoding a polypeptide that confers abiotic stress tolerance in a plant that is morphologically and developmentally similar to wild-type. Included within this definition are polymorphisms that may or may not be readily detectable using a particular oligonucleotide probe of the polynucleotide encoding the instant polypeptides, and improper or unexpected hybridization to allelic variants, with a locus other than the normal chromosomal locus for the polynucleotide sequence encoding the instant polypeptides. It is expected that these distinctions from wild-type will be in residues other than the specific mutated residues encompassed by the present invention, although it is anticipated that conservative or similar substitutions of the mutated residues (as identified in Tables 2 and 3) may allow the polypeptide to retain similar structural and functional roles in plants by conferring abiotic stress tolerance morphological and developmental similarity to wild-type plants. Allelic variant refers to any of two or more alternative forms of a gene occupying the same chromosomal locus. Allelic variation arises naturally through mutation, and may result in phenotypic polymorphism within populations. Gene mutations can be silent (i.e., no change in the encoded polypeptide) or may encode polypeptides having altered amino acid sequence. The term allelic variant is also used herein to denote a protein encoded by an allelic variant of a gene. Splice variant refers to alternative forms of RNA transcribed from a gene. Splice variation arises naturally through use of alternative splicing sites within a transcribed RNA molecule, or less commonly between separately transcribed RNA molecules, and may result in several mRNAs transcribed from the same gene. Splice variants may encode polypeptides having altered amino acid sequence. The term splice variant is also used herein to denote a protein encoded by a splice variant of an mRNA transcribed from a gene. Those skilled in the art would recognize that, for example, CBF2, SEQ ID NO: 13, represents a single transcription factor; allelic variation and alternative splicing may be expected to occur. Allelic variants of SEQ ID NO: 12 can be cloned by probing cDNA or genomic libraries from different individual organisms according to standard procedures. Allelic variants of the DNA sequence shown in SEQ ID NO: 12, including those containing silent mutations and those in which mutations result in amino acid sequence changes, are within the scope of the present invention, as are proteins which are allelic variants of SEQ E) NO: 13. cDNAs generated from alternatively spliced mRNAs, which retain the properties of the transcription factor, are included within the scope of the present invention, as are polypeptides encoded by such cDNAs and mRNAs. Allelic variants and splice variants of these sequences can be cloned by probing cDNA or genomic libraries from different individual organisms or tissues according to standard procedures known in the art (see USPN 6,388,064). Thus, in addition to the sequences set forth in the Sequence Listing, the invention also encompasses related nucleic acid molecules that are allelic or splice variants of the sequences of the invention, polynucleotides that encode orthologs, paralogs, variants, and fragments thereof that function in conferring abiotic stress tolerance in plants that are morphologically and developmentally similar to wild type, and include sequences that are complementary to any of the above nucleotide sequences. The invention also includes sequences that encode allelic or splice variants of the polypeptide sequences of the invention, orthologs, paralogs, variants, and fragments thereof that confer abiotic stress tolerance in plants that are morphologically and developmentally similar to wild type. Related nucleic acid molecules also include nucleotide sequences encoding a polypeptide comprising or consisting essentially of a substitution, modification, addition and/or deletion of one or more amino acid residues compared to the polypeptide sequences of the invention. Such related polypeptides may comprise, for example, additions and/or deletions of one or more N-linked or O-linked glycosylation sites, or an addition and/or a deletion of one or more cysteine residues. For example, Table 1 illustrates, e.g., that the codons AGC, AGT, TCA, TCC, TCG, and TCT all encode the same amino acid: serine. Accordingly, at each position in the sequence where there is a codon encoding serine, any of the above trinucleotide sequences can be used without altering the encoded polypeptide. Table 1 Amino acid Possible Codons Alanine Ala A GCA GCC GCG GCT Cysteine Cys C TGC TGT Aspartic acid Asp D GAC GAT Glutamic acid Glu E GAA GAG Phenylalanine Phe F TTC TTT Glycine Gly G GGA GGC GGG GGT Histidine His H CAC CAT Isoleucine lie I ATA ATC ATT Lysine Lys K AAA AAG Leucine Leu L TTA TTG CTA CTC CTG CTT Methionine Met M ATG Asparagine Asn N AAC AAT Proline Pro P CCA CCC CCG CCT Glutamine Gin Q CAA CAG Arginine ^'Arg R AGA AGG CGA CGC CGG CGT Serine Ser S AGC AGT TCA TCC TCG TCT Threonine Thr T ACA ACC ACG ACT Valine Val V GTA GTC GTG GTT Tryptophan Trp w TGG Tyrosine Tyr Y TAC TAT

Sequence alterations that do not change the amino acid sequence encoded by the polynucleotide are termed "silent" variations. With the exception of the codons ATG and TGG, encoding methionine and tryptophan, respectively, any of the possible codons for the same amino acid can be substituted by a variety of techniques, e.g., site-directed mutagenesis (see methods listed below), available in the art. Accordingly, any and all such variations of a sequence selected from the above table are a feature of the invention. hi addition to silent variations, other conservative variations that alter one, or a few amino acid residues in the encoded polypeptide, can be made without altering the function of the polypeptide, these conservative variants are, likewise, a feature of the invention. For example, substitutions, deletions and insertions introduced into the sequences provided in the Sequence Listing, are also envisioned by the invention. Such sequence modifications can be engineered into a sequence by site-directed mutagenesis (Wu (ed.) Methods Enzymol. (1993) vol. 217, Academic Press) or the other methods noted below. Amino acid substitutions are typically of single residues; insertions usually will be on the order of about from 1 to 10 amino acid residues; and deletions will range about from 1 to 30 residues, hi preferred embodiments, deletions or insertions are made in adjacent pairs, e.g., a deletion of two residues or insertion of two residues. Substitutions, deletions, insertions or any combination thereof can be combined to arrive at a sequence. The mutations that are made in the polynucleotide encoding the transcription factor should not place the sequence out of reading frame and should not create complementary regions that could produce secondary mRNA structure. Preferably, the polypeptide encoded by the DNA performs the desired function. Conservative substitutions are those in which at least one residue in the amino acid sequence has been removed and a different residue inserted in its place. Such substitutions generally are made in accordance with the Table 2 when it is desired to maintain the activity of the protein. Table 2 shows amino acids which can be substituted for an amino acid in a protein and which are typically regarded as conservative substitutions.

Table 2

Similar substitutions are those in which at least one residue in the amino acid sequence has been removed and a different residue inserted in its place. Such substitutions generally are made in accordance with the Table 3 when it is desired to maintain the activity of the protein. Table 3 shows amino acids which can be substituted for an amino acid in a protein and which are typically regarded as structural and functional substitutions. For example, a residue in column 1 of Table 3 may be substituted with a residue in column 2; in addition, a residue in column 2 of Table 3 may be substituted with the residue of column 1.

Table 3

Substitutions that are less conservative than those in Table 3 can be selected by picking residues that differ more significantly in their effect on maintaining (a) the structure of the polypeptide backbone in the area of the substitution, for example, as a sheet or helical conformation, (b) the charge or hydrophobicity of the molecule at the target site, or (c) the bulk of the side chain. The substitutions which in general are expected to produce the greatest changes in protein properties will be those in which (a) a hydrophilic residue, e.g., seryl or threonyl, is substituted for (or by) a hydrophobic residue, e.g., leucyl, isoleucyl, phenylalanyl, valyl or alanyl; (b) a cysteine or proline is substituted for (or by) any other residue; (c) a residue having an electropositive side chain, e.g., lysyl, arginyl, or histidyl, is substituted for (or by) an electronegative residue, e.g., glutamyl or aspartyl; or (d) a residue having a bulky side chain, e.g., phenylalanine, is substituted for (or by) one not having a side chain, e.g., glycine.

Mutagenesis Numerous well-known methods exist to mutagenize nucleic acids. These methods may be conducted in vitro, that is, prior to transformation of a plant with the now mutated form of a polynucleotide, or after transformation, that is, after transforming a plant with an unaltered form of a polynucleotide of interest, in order to produce a mutated form of the polynucleotide within the plant, hi one iteration of the invention, a polynucleotide encoding a wild-type member of the CBF clade is mutagenized in vitro, after which the mutagenized polynucleotide is used to transform a target plant. The wild-type CBF polynucleotide may be amplified prior to in vitro mutagenesis. Examples of in vitro mutagenesis techniques, in this case, methods for altering DNA outside of plants or plant cells; can be random or specific for a particular site and base change, depending on the technique used. In vitro mutagenesis methods include, by way of a few examples, transposon mutagenesis, random nucleic acid mutagenesis using exo-DNA polymerases (Hogrefe and Cline USPN 6,803,216, October 12, 2004), site directed mutagenesis (e.g., Kunkel et al. (1985) Proc. Natl Acad. Sci. USA 82: 488-492; Taylor et al. (1985) Nucleic Acids Res. 13: 8764-8785; Vandeyar et al. (1988) Gene 65: 129-133; Sugimoto et al. (1989) Anal Biochem. 179: 309-311. Taylor and Eckstein (1985) Nucleic Acids Res. 13: 8764-8765), alanine scan mutagenesis (Cunningham and Wells (1989) Science 244: 1081-1085), Taq-based PCR mutagenesis (Leung et al. (1989) Technique 1 : 11-15), Pfu-based mutagenesis (Papworth et al. (1996) Strategies 9: 3-4; Bergseid et al. (1991) Strategies 4: 34-35), saturation mutagenesis (Lim and Sauer (1989) Nature 339: 31-36), gene-shuffling (Stemmer (1994) Proc. Natl Acad. Sci. USA 91: 10747- 10751; Stemmer (1994) Nature 370: 389-391), error-prone PCR (Cadwell et al. (1994) PCRMethods Appl. 3 : S 136-S 140; Vartanian et al. (1996) Nucleic Acids Res.24: 2627-2631 ), assembly PCR (Meyers, R.M., editor, (1996) The Encyclopedia of Molecular Biology and Molecular Medicine, vol. 5, pp. 447 457. VCH, New York), "oligonucleotide primer-directed mutagenesis (O'Donohue and Kneale (1996) Mol. Biotechnol. 6:179-189), recursive ensemble mutagenesis (Arkin and Youvan (1992) Proc. Natl. Acad. Sci. USA 89:7811-7815), codon cassette mutagenesis (Kegler-Ebo et al. (1994) Nucleic Acids Res. 22: 1593-1599), and crossover PCR (Link et al. (1997) J. Bacteriol. 179: 6228-6237). A mutagenized polynucleotide may be incorporated into an expression cassette or vector in advance of the transformation step that would follow in vitro mutagenesis. The polynucleotide may contain a selectable marker to identify transformants. Transformed plants that overexpress a mutant form of the CBF clade member and that have desirable developmental and morphological characteristics may be selected. In the present instance, these characteristics may include less stunting, a desirable flowering time, and greater yield in the form of, for example, greater biomass, fruit of seed yield. Alternatively, a plant may be transformed with a polynucleotide encoding a wild-type member of the CBF clade, using methods described herein or others well-known in the art. Generally, a wild-type polynucleotide will be incorporated into an expression cassette or vector in advance of a transformation step that would be followed by in vivo mutagenesis. The polynucleotide may contain a selectable marker that may be used to identify transformants. Following transformation, a CBF clade polynucleotide may be mutagenized in vivo to produce a mutated CBF clade polynucleotide. Plants comprising a mutated CBF clade polynucleotide and overexpressing a mutant CBF clade member polypeptide may be selected for desirable morphological and developmental properties such as those noted above. In vivo mutagenesis methods for plants are also well known in the art, and have been described. These include methanesulfonic acid ethyl ester (EMS)- based methods (McCallum et al. (2000) Nat. Biotechnol. 18: 455-457; Somerville and Ogren (1982) in Edelman, Hallick, and Chua, eds, Methods in Chloroplast Molecular Biology. Elsevier Biomedical Press, Amsterdam, The Netherlands, pp 129-138) or "chimeroplast transformation" (where a hybrid RNA- DNA-oligonucleotide chimeroplast is transformed into a cell (e.g., Kipp et al., (1997) "5th International Congress of Plant Molecular Biology, 21-27, Singapore; Dixon and Arntzen, "Metabolic Engineering in Transgenic Plants", Keystone Symposia, Copper Mountain, Colo., USA, TIBTECH 15 (1997), 441-447; Patent Application WO9515972; Kren et al. (1997) Hepatology 25: 1462-1468; Cole-Strauss et al.,

Science (1996) 273: 1386-1389). For in vivo mutagenesis methods, see also Neuffer (1982) hi: Maize for Biological Research. Sheridan, ed. Univ. Press, Grand Forks, N.Dak., pp. 61-64. Mutagenized seed (which may be mutagenized with EMS, gamma rays or fast neutrons) may be plated and grown (for Arabidopsis, seed may be planted in flats of soil, or plated at densities of up to 10,000 seeds per 10 cm diameter plate) on minimal salts medium containing an appropriate concentration of inhibitor to select for resistance (Ward et al. USPN 6,307,129). The plants that are selected may be grown and used to fertilize plants and the resulting mutant progeny seeds collected. Progeny of these seeds may also be tested for resistance to the selection marker, or selected for desirable morphology or tolerance to one or more abiotic stresses.

Further Modifying Sequences of the Invention - Mutation/Forced Evolution In addition to generating silent or conservative substitutions as noted, above, the present invention optionally includes methods of modifying the sequences of the Sequence Listing, hi the methods, nucleic acid or protein modification methods are used to alter the given sequences to produce new sequences and/or to chemically or enzymatically modify given sequences to change the properties of the nucleic acids or proteins. Thus, in one embodiment, given nucleic acid sequences are modified, e.g., according to standard mutagenesis or artificial evolution methods to produce modified sequences. The modified sequences may be created using purified natural polynucleotides isolated from any organism or may be synthesized from purified compositions and chemicals using chemical means well know to those of skill in the art. For example, Ausubel, supra, provides additional details on mutagenesis methods. Artificial forced evolution methods are described, for example, by Stemmer (1994) Nature 370: 389-391, Stemmer (1994) Proc. Natl Acad. Sci. 91: 10747-10751, and US Patents 5,811,238, 5,837,500, and 6,242,568. Methods for engineering synthetic transcription factors and other polypeptides are described, for example, by Zhang et al. (2000) J. Biol Chem. 275: 33850-33860, Liu et al. (2001) J. Biol. Chem. 276: 11323-11334, and Isalan et al. (2001) Nature Biotechnol. 19: 656-660. Many other mutation and evolution methods are also available and expected to be within the skill of the practitioner. Similarly, chemical or enzymatic alteration of expressed nucleic acids and polypeptides can be performed by standard methods. For example, a sequence can be modified by addition of lipids, sugars, peptides, organic or inorganic compounds, by the inclusion of modified nucleotides or amino acids, or the like. For example, protein modification teclmiques are illustrated in Ausubel, supra. Further details on chemical and enzymatic modifications can be found herein. These modification methods can be used to modify any given sequence, or to modify any sequence produced by the various mutation and artificial evolution modification methods noted herein. Accordingly, the invention provides for modification of any given nucleic acid by mutation, evolution, chemical or enzymatic modification, or other available methods, as well as for the products produced by practicing such methods, e.g., using the sequences herein as a starting substrate for the various modification approaches. For example, optimized coding sequence containing codons preferred by a particular prokaryotic or eukaryotic host can be used e.g., to increase the rate of translation or to produce recombinant RNA transcripts having desirable properties, such as a longer half-life, as compared with transcripts produced using a non-optimized sequence. Translation stop codons can also be modified to reflect host preference. For example, preferred stop codons for Saccharomyces cerevisiae and mammals are TAA and TGA, respectively. The preferred stop codon for monocotyledonous plants is TGA, whereas insects and E. coli prefer to use TAA as the stop codon. The polynucleotide sequences of the present invention can also be engineered in order to alter a coding sequence for a variety of reasons, including but not limited to, alterations which modify the sequence to facilitate cloning, processing and/or expression of the gene product. For example, alterations are optionally introduced using techniques which are well known in the art, e.g., site-directed mutagenesis, to insert new restriction sites, to alter glycosylation patterns, to change codon preference, to introduce splice sites, etc. Furthermore, a fragment or domain derived from any of the polypeptides of the invention can be combined with domains derived from other transcription factors or synthetic domains to modify the biological activity of a transcription factor. For instance, a DNA-binding domain derived from a transcription factor of the invention can be combined with the activation domain of another transcription factor or with a synthetic activation domain. A transcription activation domain assists in initiating transcription from a DNA-binding site. Examples include the transcription activation region of VP 16 or GAL4 (Moore et al. (1998) Proc. Natl. Acad. Sci. 95: 376-381; Aoyama et al. (1995) Plant Cell 1: 1773- 1785), peptides derived from bacterial sequences (Ma and Ptashne (1987) Cell 51: 113-119) and synthetic peptides (Giniger and Ptashne (1987) Nature 330: 670-672).

Expression and Modification of Polypeptides Typically, polynucleotides of the invention are incorporated into recombinant DNA (or RNA) molecules that direct expression of polypeptides of the mvention in appropriate host cells, transgenic plants, in vitro translation systems, or the like. Due to the inherent degeneracy of the genetic code, nucleic acids that encode substantially the same or a functionally equivalent amino acid sequence can be substituted for any listed sequence to provide for cloning and expressing the relevant homolog. The transgenic plants of the present invention comprising recombinant polynucleotide sequences are generally derived from parental plants, which may themselves be non-transformed (or non- transgenic) plants. These transgenic plants may either have a transcription factor gene "knocked out" (for example, with a genomic insertion by homologous recombination, an antisense or ribozyme construct) or expressed to a normal or wild-type extent. However, overexpressing transgenic "progeny" plants will exhibit greater mRNA levels, wherein the mRNA encodes a transcription factor, that is, a DNA-binding protein that is capable of binding to a DNA regulatory sequence and inducing transcription, and preferably, expression of a plant trait gene, such as a gene that increases abiotic stress tolerance. Preferably, the mRNA expression level will be at least three-fold greater than that of the parental plant, or more preferably at least ten-fold greater mRNA levels compared to said parental plant, and most preferably at least fifty-fold greater compared to said parental plant.

Vectors, Promoters, and Expression Systems The present invention includes recombinant constructs comprising one or more of the nucleic acid sequences herein. The constructs typically comprise a vector, such as a plasmid, a cosmid, a phage, a virus (e.g., a plant virus), a bacterial artificial chromosome (BAC), a yeast artificial chromosome (YAC), or the like, into which a nucleic acid sequence of the invention has been inserted, in a forward or reverse orientation, hi a preferred aspect of this embodiment, the construct further comprises regulatory sequences, including, for example, a promoter, operably linked to the sequence. Large numbers of suitable vectors and promoters are known to those of skill in the art, and are commercially available. General texts that describe molecular biological techniques useful herein, including the use and production of vectors, promoters and many other relevant topics, include Berger, supra, Sambrook, supra and Ausubel, supra. Any of the identified sequences can be incoφorated into a cassette or vector, e.g., for expression in plants (in this case, the cassette or vector is thus an "expression cassette" or "expression vector"). A number of expression vectors suitable for stable transformation of plant cells or for the establishment of transgenic plants have been described including those described in Weissbach and

Weissbach (1989) Methods for Plant Molecular Biology, Academic Press, and Gelvin et al. (1990) Plant Molecular Biology Manual Kluwer Academic Publishers. Specific examples include those derived from a Ti plasmid of Agrobacterium tumefaciens, as well as those disclosed by Herrera-Estrella et al. (1983) Nature 303: 209, Bevan (1984) Nucleic Acids Res. 12: 8711-8721, Klee (1985) Bio/Technology 3: 637- 642, for dicotyledonous plants. Alternatively, non-Ti vectors can be used to transfer the DΝA into monocotyledonous plants and cells by using free DΝA delivery techniques. Such methods can involve, for example, the use of liposomes, electroporation, microprojectile bombardment, silicon carbide whiskers, and viruses. By using these methods transgenic plants such as wheat, rice (Christou (1991) Bio/Technology 9: 957-962) and corn (Gordon-Kamm (1990) Plant Cell 2: 603-618) can be produced. An immature embryo can also be a good target tissue for monocots for direct DΝA delivery techniques by using the particle gun (Weeks et al. (1993) Plant Physiol 102: 1077-1084; Vasil (1993) Bio/Technology 10: 667-674; Wan and Lemeaux (1994) Plant Physiol. 104: 37-48, and for Agrobacterium-mediated DΝA transfer (Ishida et al. (1996) Nature Biotechnol 14: 745-750). Typically, plant transformation vectors include one or more cloned plant coding sequence (genomic or cDΝA) under the transcriptional control of 5' and 3' regulatory sequences and a dominant selectable marker. Such plant transformation vectors typically also contain a promoter (e.g., a regulatory region controlling inducible or constitutive, environmentally-or developmentally-regulated, or cell- or tissue-specific expression), a transcription initiation start site, an RΝA processing signal (such as intron splice sites), a transcription termination site, and/or a polyadenylation signal. A potential utility for the transcription factor polynucleotides disclosed herein is the isolation of promoter elements from these genes that can be used to program expression in plants of any genes. Each transcription factor gene disclosed herein is expressed in a unique fashion, as determined by promoter elements located upstream of the start of translation, and additionally within an intron of the transcription factor gene or downstream of the termination codon of the gene. As is well known in the art, for a significant portion of genes, the promoter sequences are located entirely in the region directly upstream of the start of translation. In such cases, typically the promoter sequences are located within 2.0 kb of the start of translation, or within 1.5 kb of the start of translation, frequently within 1.0 kb of the start of translation, and sometimes within 0.5 kb of the start of translation. Promoter sequences can be isolated according to well-known methods. Examples of constitutive plant promoters which can be useful for expressing the TF sequence include: the cauliflower mosaic virus (CaMV) 35S promoter, which confers constitutive, high-level expression in most plant tissues (see, e.g., Odell et al. (1985) Nature 313: 810-812); the nopaline synthase promoter (An et al. (1988) Plant Physiol. 88: 547-552); and the octopine synthase promoter (Fromm et al. (1989) Plant Cell 1: 977-984). The transcription factors of the invention may be operably linked with a specific promoter that causes the transcription factor to be expressed in response to environmental, tissue-specific or temporal signals. A variety of plant gene promoters that regulate gene expression in response to environmental, hormonal, chemical, developmental signals, and in a tissue-active manner can be used for expression of a TF sequence in plants. Choice of a promoter is based largely on the phenotype of interest and is determined by such factors as tissue (e.g., seed, fruit, root, pollen, vascular tissue, flower, carpel, etc.), inducibility (e.g., in response to wounding, heat, cold, drought, light, pathogens, etc.), timing, developmental stage, and the like. Numerous known promoters have been characterized and can favorably be employed to promote expression of a polynucleotide of the invention in a transgenic plant or cell of interest. For example, tissue specific promoters include: seed-specific promoters (such as the napin, phaseolin or DC3 promoter described in US Pat. No. 5,773,697), fruit-specific promoters that are active during fruit ripening, such as the dru 1 promoter (US Pat. No. 5,783,393), or the 2A11 promoter (US Pat. No. 4,943,674) and the tomato polygalacturonase promoter (Bird et al. (1988) Plant Mol Biol. 11: 651-662), root-specific promoters, such as those disclosed in US Patent Nos. 5,618,988, 5,837,848 and 5,905,186, pollen-active promoters such as PTA29, PTA26 and PTA13 (US Pat. No. 5,792,929), promoters active in vascular tissue (Ringli and Keller (1998) Plant Mol. Biol. 37: 977-988), flower- specific (Kaiser et al. (1995) Plant Mol Biol. 28: 231-243), pollen (Baerson et al. (1994) Plant Mol. Biol 26: 1947-1959), carpels (Ohl et al. (1990) Plant Cell 2: 837-848), pollen and ovules (Baerson et al.

(1993) Plant Mol. Biol. 22: 255-267), auxin-inducible promoters (such as that described in van der Kop et al. (1999) Plant Mol Biol 39: 979-9901 or Baumann et al., (1999) Plant Cell 11 : 323-334), cytokinin- inducible promoter (Guevara-Garcia (1998) Plant Mol. Biol. 38: 743-753), promoters responsive to gibberellin (Shi et al. (1998) Plant Mol. Biol 38: 1053-1060, Willmott et al. (1998) Plant Molec. Biol. 38: 817-825) and the like. Additional promoters are those that elicit expression in response to heat (Ainley et al. (1993) Plant Mol. Biol. 22: 13-23), light (e.g., the pea rbcS-3 A promoter, Kuhlemeier et al. (1989) Plant Cell 1: 471-478, and the maize rbcS promoter, Schaffiier and Sheen (1991) Plant Cell 3: 997-1012); wounding (e.g., wunl, Siebertz et al. (1989) Plant Cell 1: 961-968); pathogens (such as the PR-1 promoter described in Buchel et al. (1999) Plant Mol. Biol. 40: 387-396, and the PDF1.2 promoter described in Manners et al. (1998) Plant Mol. Biol. 38: 1071-1080), and chemicals such as methyl jasmonate or salicylic acid (Gatz (1997) Annu. Rev. Plant Physiol. Plant Mol. Biol. 48: 89-108). In addition, the timing of the expression can be controlled by using promoters such as those acting at senescence (Gan and Amasino (1995) Science 270: 1986-1988); or late seed development (Odell et al.

(1994) Plant Physiol. 106: 447-458). Plant expression vectors can also include RNA processing signals that can be positioned within, upstream or downstream of the coding sequence. In addition, the expression vectors can include additional regulatory sequences from the 3 '-untranslated region of plant genes, e.g., a 3' terminator region to increase mRNA stability of the mRNA, such as the PI-II teπninator region of potato or the octopine or nopaline synthase 3' terminator regions.

Additional Expression Elements Specific initiation signals can aid in efficient translation of coding sequences. These signals can include, e.g., the ATG initiation codon and adjacent sequences. In cases where a coding sequence, its initiation codon and upstream sequences are inserted into the appropriate expression vector, no additional translational control signals may be needed. However, in cases where only coding sequence (e.g., a mature protein coding sequence), or a portion thereof, is inserted, exogenous transcriptional control signals including the ATG initiation codon can be separately provided. The initiation codon is provided in the correct reading frame to facilitate transcription. Exogenous transcriptional elements and initiation codons can be of various origins, both natural and synthetic. The efficiency of expression can be enhanced by the inclusion of enhancers appropriate to the cell system in use.

Expression Hosts The present invention also relates to host cells which are transduced with vectors of the invention, and the production of polypeptides of the mvention (including fragments thereof) by recombinant techniques. Host cells are genetically engineered (i.e., nucleic acids are introduced, e.g., transduced, transformed or transfected) with the vectors of this invention, which may be, for example, a cloning vector or an expression vector comprising the relevant nucleic acids herein. The vector is optionally a plasmid, a viral particle, a phage, a naked nucleic acid, etc. The engineered host cells can be cultured in conventional nutrient media modified as appropriate for activating promoters, selecting transformants, or amplifying the relevant gene. The culture conditions, such as temperature, pH and the like, are those previously used with the host cell selected for expression, and will be apparent to those skilled in the art and in the references cited herein, including, Sambrook, supra and Ausubel, supra. The host cell can be a eukaryotic cell, such as a yeast cell, or a plant cell, or the host cell can be a prokaryotic cell, such as a bacterial cell. Plant protoplasts are also suitable for some applications. For example, the DNA fragments are introduced into plant tissues, cultured plant cells or plant protoplasts by standard methods including electroporation (Fromm et al. (1985) Proc. Natl. Acad. Sci. 82: 5824-5828, infection by viral vectors such as cauliflower mosaic virus (CaMV) (Hohn et al. (1982) Molecular Biology of Plant Tumors Academic Press, New York, NY, pp. 549-560; US 4,407,956), high velocity ballistic penetration by small particles with the nucleic acid either within the matrix of small beads or particles, or on the surface (Klein et al. (1987) Nature 327: 70-73), use of pollen as vector (WO 85/01856), or use of Agrobacterium tumefaciens ox A. rhizogenes carrying a T-DNA plasmid in which DNA fragments are cloned. The T-DNA plasmid is transmitted to plant cells upon infection by Agrobacterium tumefaciens, and a portion is stably integrated into the plant genome (Horsch et al. (1984) Science 233: 496-498; Fraley et al. (1983) Proc. Natl. Acad. Sci. 80: 4803-4807). The cell can include a nucleic acid of the invention that encodes a polypeptide, wherein the cell expresses a polypeptide of the invention. The cell can also include vector sequences, or the like. Cells and transgenic plants that include any polypeptide or nucleic acid above or throughout this specification, e.g., produced by transduction of a vector of the invention, are an additional feature of the invention. For long-term, high-yield production of recombinant proteins, stable expression can be used.

Host cells transformed with a nucleotide sequence encoding a polypeptide of the invention are optionally cultured under conditions suitable for the expression and recovery of the encoded protein from cell culture. The protein or fragment thereof produced by a recombinant cell may be secreted, membrane- bound, or contained intracellularly, depending on the sequence and/or the vector used. As will be understood by those of skill in the art, expression vectors containing polynucleotides encoding mature proteins of the invention can be designed with signal sequences which direct secretion of the mature polypeptides through a prokaryotic or eukaryotic cell membrane.

Modified Amino Acid Residues Polypeptides of the invention may contain one or more modified amino acid residues. The presence of modified amino acids maybe advantageous in, for example, increasing polypeptide half-life, reducing polypeptide antigenicity or toxicity, increasing polypeptide storage stability, or the like. Amino acid residue(s) are modified, for example, co-translationally or post-translationally during recombinant production or modified by synthetic or chemical means. Non-limiting examples of a modified amino acid residue include incorporation or other use of acetylated amino acids, glycosylated amino acids, sulfated amino acids, prenylated (e.g., farnesylated, geranylgeranylated) amino acids, PEG modified (e.g., "PEGylated") amino acids, biotinylated amino acids, carboxylated amino acids, phosphorylated amino acids, etc. References adequate to guide one of skill in the modification of amino acid residues are replete throughout the literature. The modified amino acid residues may prevent or increase affinity of the polypeptide for another molecule, including, but not limited to, polynucleotide, proteins, carbohydrates, lipids and lipid derivatives, and other organic or synthetic compounds.

Identification of Additional Protein Factors A transcription factor provided by the present invention can also be used to identify additional endogenous or exogenous molecules that can affect a phenotype or trait of interest. Such molecules include endogenous molecules that are acted upon either at a transcriptional level by a transcription factor of the invention to modify a phenotype as desired. For example, the transcription factors can be employed to identify one or more downstream genes that are subject to a regulatory effect of the transcription factor. In one approach, a transcription factor or transcription factor homolog of the invention is expressed in a host cell, e.g., a transgenic plant cell, tissue or explant, and expression products, either RNA or protein, of likely or random targets are monitored, e.g., by hybridization to a microarray of nucleic acid probes corresponding to genes expressed in a tissue or cell type of interest, by two-dimensional gel electrophoresis of protein products, or by any other method known in the art for assessing expression of gene products at the level of RNA or protein. Alternatively, a transcription factor of the invention can be used to identify promoter sequences (such as binding sites on DNA sequences) involved in the regulation of a downstream target. After identifying a promoter sequence, interactions between the transcription factor and the promoter sequence can be modified by changing specific nucleotides in the promoter sequence or specific amino acids in the transcription factor that interact with the promoter sequence to alter a plant trait. Typically, transcription factor DNA-binding sites are identified by gel shift assays. After identifying the promoter regions, the promoter region sequences can be employed in double-stranded DNA arrays to identify molecules that affect the interactions of the transcription factors with their promoters (Bulyk et al. (1999) Nature Biotechnol 17: 573-577). The identified transcription factors are also useful to identify proteins that modify the activity of the transcription factor. Such modification can occur by covalent modification, such as by phosphorylation, or by protein-protein (homo or-heteropolymer) interactions. Any method suitable for detecting protein-protein interactions can be employed. Among the methods that can be employed are co- immunoprecipitation, cross-linking and co-purification through gradients or chromatographic columns, and the two-hybrid yeast system. The two-hybrid system detects protein interactions in vivo and is described in Chien et al. ((1991) Proc. Natl. Acad. Sci. 88: 9578-9582) and is commercially available from Clontech (Palo Alto, Calif.). In such a system, plasmids are constructed that encode two hybrid proteins: one consists of the DNA-binding domain of a transcription activator protein fused to the TF polypeptide and the other consists of the transcription activator protein's activation domain fused to an unknown protein that is encoded by a cDNA that has been recombined into the plasmid as part of a cDNA library. The DNA- binding domain fusion plasmid and the cDNA library are transformed into a strain of the yeast Saccharomyces cerevisiae that contains a reporter gene (e.g., lacZ) whose regulatory region contains the transcription activator's binding site. Either hybrid protein alone cannot activate transcription of the reporter gene. Interaction of the two hybrid proteins reconstitutes the functional activator protein and results in expression of the reporter gene, which is detected by an assay for the reporter gene product. Then, the library plasmids responsible for reporter gene expression are isolated and sequenced to identify the proteins encoded by the library plasmids. After identifying proteins that interact with the transcription factors, assays for compounds that interfere with the TF protein-protein interactions can be performed.

Subsequences Also contemplated are uses of polynucleotides, also referred to herein as oligonucleotides, typically having at least 12 bases, preferably at least 15, more preferably at least 20, 30, or 50 bases, which hybridize under at least highly stringent (or ultra-high stringent or ultra-ultra-high stringent conditions) conditions to a polynucleotide sequence described above. The polynucleotides may be used as probes, primers, sense and antisense agents, and the like, according to methods as noted supra. Subsequences of the polynucleotides of the invention, including polynucleotide fragments and oligonucleotides are useful as nucleic acid probes and primers. An oligonucleotide suitable for use as a probe or primer is at least about 15 nucleotides in length, more often at least about 18 nucleotides, often at least about 21 nucleotides, frequently at least about 30 nucleotides, or about 40 nucleotides, or more in length. A nucleic acid probe is useful in hybridization protocols, e.g., to identify additional polypeptide homologs of the invention, including protocols for microarray experiments. Primers can be annealed to a complementary target DNA strand by nucleic acid hybridization to form a hybrid between the primer and the target DNA strand, and then extended along the target DNA strand by a DNA polymerase enzyme. Primer pairs can be used for amplification of a nucleic acid sequence, e.g., by the polymerase chain reaction (PCR) or other nucleic-acid amplification methods. See Sambrook, supra, and Ausubel, supra. In addition, the invention includes an isolated or recombinant polypeptide including a subsequence of at least about 15 contiguous amino acids encoded by the recombinant or isolated polynucleotides of the invention. For example, such polypeptides, or domains or fragments thereof, can be used as immunogens, e.g., to produce antibodies specific for the polypeptide sequence, or as probes for detecting a sequence of interest. A subsequence can range in size from about 15 amino acids in length up to and including the full length of the polypeptide. To be encompassed by the present invention, an expressed polypeptide which comprises such a polypeptide subsequence performs at least one biological function of the intact polypeptide in substantially the same manner, or to a similar extent, as does the intact polypeptide. For example, a polypeptide fragment can comprise a recognizable structural motif or functional domain such as a DNA binding domain that activates transcription, e.g., by binding to a specific DNA promoter region an activation domain, or a domain for protein-protein interactions.

Production of Transgenic Plants Modification of Traits. The polynucleotides of the invention are used to produce transgenic plants with various traits, or characteristics, that have been modified in a desirable manner, e.g., to improve the abiotic stress tolerance characteristics of a plant. For example, alteration of expression levels or patterns (e.g., spatial or temporal expression patterns) of one or more of the transcription factors (or transcription factor homologs) of the invention, as compared with the levels of the same protein found in a wild-type plant, can be used to modify a plant's traits. An illustrative example of trait modification, improved characteristics, by altering expression levels of a particular transcription factor is described further in the Examples and the Sequence Listing. Arabidopsis as a model system. Arabidopsis thaliana is a favorite model for genetics and metabolism in plants. Arabidopsis has a small genome, and well-documented studies are available. It is easy to grow in large numbers and mutants defining important genetically controlled mechanisms are either available, or can readily be obtained. Various methods to introduce and express isolated homologous genes are available (see Koncz et al., eds., Methods in Arabidopsis Research (1992) World Scientific, New Jersey, NJ, in "Preface"). Because of its small size, short life cycle, obligate autogamy and high fertility, Arabidopsis is also a choice organism for the isolation of mutants and studies in morphogenetic and development pathways, and control of these pathways by transcription factors (Koncz (1992) supra, p. 72). A number of studies introducing transcription factors into A. thaliana have demonstrated the utility of this plant for understanding the mechanisms of gene regulation and trait alteration in plants. (See, for example, Koncz (1992) supra, and U.S. Patent No. 6,417,428). Homologous genes introduced into transgenic plants. Homologous genes that may be derived from any plant, or from any source whether natural, synthetic, semi-synthetic or recombinant, and that share significant sequence identity or similarity to those provided by the present invention, may be introduced into plants, for example, crop plants, to confer desirable or improved traits. Consequently, transgenic plants may be produced that comprise a recombinant expression vector or cassette with a promoter operably linked to one or more sequences homologous to presently disclosed sequences. The promoter may be, for example, a plant or viral promoter. The invention thus provides for methods for preparing transgenic plants, and for modifying plant traits. These methods include introducing into a plant a recombinant expression vector or cassette comprising a functional promoter operably linked to one or more sequences homologous to presently disclosed sequences. Plants and kits for producing these plants that result from the application of these methods are also encompassed by the present invention.

Genes, traits and utilities that affect plant characteristics. Plant transcription factors can modulate gene expression, and, in turn, be modulated by the environmental experience of a plant. Significant alterations in a plant's enviromnent invariably result in a change in the plant's transcription factor gene expression pattern. Altered transcription factor expression patterns generally result in phenotypic changes in the plant. Transcription factor gene product(s) in transgenic plants then differ(s) in amounts or proportions from that found in wild-type or non-, transformed plants, and those transcription factors likely represent polypeptides that are used to alter the response to the environmental change. By way of example, it is well accepted in the art that analytical methods based on altered expression patterns may be used to screen for phenotypic changes in a plant far more effectively than can be achieved using traditional methods.

Potential Applications of the Presently Disclosed Sequences that Regulate Abiotic Stress Tolerance. Genes identified by the presently disclosed experiments represent potential regulators of responses to abiotic stress. These genes or their orthologs and paralogs could be applied to commercial species in order to improve yield and allow certain crops to be grown under conditions of hyperosmotic (e.g., drought, freezing, high salinity) or other abiotic stresses (e.g., heat, cold). Arabidopsis plants that overexpress the CBF transcriptional activators can be stunted in their growth and delayed in flowering, e.g., when the activators are expressed at high levels. An example of this is the CBF2-overexpressing line, E2. The northern analysis in Figure 7B shows that the CBF- targeted gene, COR15a, is expressed in warm-grown E2 plants, whereas it is only expressed in cold- treated wild-type Ws-2 plants. As noted in the Examples below, it is possible that mutant versions of some genes suppress the potentially "negative" traits associated with CBF overexpression (e.g., the stunted and delayed flowering phenotypes found in plants that constitutively overexpress a CBF), but retain the positive effects that CBF overexpression on stress tolerance. Such mutants have now been invented, as shown in the Examples listed below.

EXAMPLES The invention, now being generally described, will be readily understood by reference to the following examples, which are included for purposes of illustration of certain aspects and embodiments of the present invention and are not intended to limit the invention. It will be recognized by one of skill in the art that a transcription factor that is associated with a particular first trait may also be associated with at least one other, unrelated and inherent second trait that was not predicted by the first trait.

EXAMPLE I. Production of plants overexpressing CBF In the example that follows, the transformation methods used followed the procedure of Fowler and Thomashow (2002) Plant Cell 14: 1675-1690). It is envisioned that other transformation methods may also be used to generate transformed plants; examples of such methods are noted above. Arabidopsis (L.) Heynh. ecotype Wassilewskija (Ws)-2, transgenic plants in the Ws-2 background expressing CBF2 (line E2; Fowler and Thomashow (2002) supra), and EMS mutants in these CBF2-overexpressing plants were grown in controlled environment chambers at 20° C under constant illumination from cool-white fluorescent lights (100-150 μmol m^"2 s^"1) in pots containing Baccto planting mix (Michigan Peat Co., Houston, TX) as described (Gilmour et al. (2000) supra). Plants were also grown on Gamborg's B-5 medium (Caisson Laboratories, Rexburg, ID) at pH 5.7 solidified with

0.8% phytagar (Life Technologies, Inc., Gaithersburg, MD) under sterile conditions in Petri plates at 22° C under continuous illumination of 100 μmol m^~2 s^"1. Plants were cold acclimated by placing pots or plates at 5° C under continuous light (20-60 μmol m^"2 s^"1) as described previously (Gilmour et al. (2000) supra).

EXAMPLE II. Mutagenesis In the example that follows, the mutagenesis step is conducted in vivo by mutagenizing CBF2- overexpressing Arabidopsis seeds. It is envisioned that other mutagenesis methods may also be used to generate functional CBF clade variants; examples of such methods are noted above. CBF2-overexpressing Arabidopsis seeds (line E2) were mutagenized with EMS as described

(Somerville and Ogren (1982) supra) and planted out into 40 separate flats of soil (~1,500 seeds per flat) and grown. The M2 seed was collected individually from each flat and M2 plants were screened for those that were phenotypically larger than E2 plants. M3 seed was collected from individual putative mutants and retained for further study. Where populations of M3 plants from an individual M2 plant were not uniform in size, M4 or M5 seeds were collected from individual M3 or M4 plants.

EXAMPLE IH. Freeze Tests Whole plant freezing tests and electrolyte leakage freeze tests were performed as described (Haake et al. (2002) Plant Physiol. 130: 639-48; Gilmour et al. (2000) supra)). For Petri plate freeze tests, seeds were germinated and grown at 24° C under continuous light at about 100 μmol m^"2 s^"1 for ten days on Gamborg's B-5 medium without sucrose, solidified with 0.8% agar. After 10 days the plates were placed in a dark chamber at -2° C for 2 hr, then ice nucleated with a single ice chip placed in each plate. The plates were incubated in the dark for 22 hr at -2° C, then for 48 hr at -5° C and 24 hr at 4° C. The plates were then placed in continuous light at 24° C and scored for survival 48 hrs later.

EXAMPLE IV. Analyses of RNA Blots Total RNA was extracted from Arabidopsis plants using RNeasy Plant Mini Kit (Qiagen,

Valencia, CA). Northern blots were prepared and hybridized as described using high stringency wash conditions (Gilmour et al. (2000) supra). Gene-specific probes for CBF2 were prepared as described previously (Gilmour et al. (1998) supra). Probes for COR78, COR15a and full-length CBF2 were inserts isolated from cDNA clones encoding these genes obtained by restriction enzyme digestion. The probes were radiolabeled with ³²P by random priming (Feinberg and Vogelstein 1983). Membranes were exposed to a phosphorimager plate that was then visualized by scanning the plate in a Fluor-S Multilmager (BioRad, Hercules, CA). Quantitation was performed using Quantity One software, version 4.2.2 (BioRad, Hercules, CA).

EXAMPLE V. Isolation and Sequencing of CBF2 Transgene Genomic DNA was extracted from wild type (E2) plants and mutant Arabidopsis plants using a DNeasy Plant Mini Kit (Qiagen, Valencia, CA). The transgene was amplified by PCR using primers: 5'- AAGTTCATTTCATTTGGAGAGGAC-3' (forward; SEQ ID NO: 199) and 5'- ATTGCCGTAGATGAAAGACTGAG-3' (reverse; SEQ ID NO: 200) which were present in the pGA643 vector. The resulting DNA fragment was gel purified using a QIAquick Gel Extraction Kit (Qiagen, Valencia, CA), then cloned into the pGEM-T Vector using a pGEM-T Easy Vector System I (Promega, Madison, WI). DNA from the resulting cDNA clones was extracted and sequenced. Sequence analysis was performed using DNAStar software.

EXAMPLE VI. Suppressor mutation screen phenotypic analyses We hypothesized that it might be possible that mutant versions of some genes would suppress potentially "negative" traits often associated with a high level of CBF overexpression, such as stunting, low seed production, or delayed flowering, but retain the positive effects that CBF overexpression has on abiotic stress tolerance. To test this hypothesis, seeds from E2 plants were chemically mutagenized and then grown. The resulting M2 populations were screened for plants that more closely resembled wild- type Ws-2 plants in growth and development. Examples of these mutants that were identified are shown on the right side of Figure 8 A. Mutant lines designated 7-2 and 5-4 were much bigger and flowered earlier that the E2 plants. There were few, if any, morphological or developmental differences between the 7-2 plants and the wild-type Ws-2 plants. Although the 5-4 plants were clearly bigger and flowering earlier than the E2 plants, but were not completely wild-type in their growth characteristics. The difference in phenotypes observed in the 7-2 and 5-4 plants may be due to mutations that resulted in the CBF2 transgene not being expressed. To test this, CBF2 transcript levels were detennined in both lines and were found to be unaffected (Figure 8B). Expression of the CBF-targeted gene COR15a was analyzed in the mutants, hi the 7-2 plants, there was little COR15a transcript when the plants were grown at warm temperature. However, there were high levels of COR15a transcripts in the 7-2 plants that had been cold-treated. The simplest interpretation of these results was that the CBF2 transgene had suffered a mutation that had largely inactivated the protein. To test this, the CBF2 transgene was amplified out of the 7-2 line using PCR and the nucleic acid sequence of the CBF2 transgene was determined. The results indicated that the CBF2 transgene in the 7-2 plants had a mutation that converted the alanine residue at position 127 to a threonine residue (Figure 6). Additional mutants were subsequently analyzed and found to behave similarly to 7-2 and to have mutations within the CBF2 protein: these include mutants 3-3, 2-4, 3-2 and 1-2 (Figure 6). In each of these cases, it appears that the mutations resulted in CBF2 proteins that were largely inactive. In contrast to the 7-2 mutant, COR15a transcript levels were not significantly altered in the 5-4 mutant line indicating that the CBF2 transgene was still active. The mutation that suppressed the stunted phenotype could have either been in another gene independent from the CBF2 transgene, which is what we anticipated, or in a position of the CBF2 transgene that had little effect on the function of the CBF2 protein. Surprising, the latter was the case. The CBF2 transgene was amplified out of the 5-4 line using by PCR (3 independent times) and the nucleic acid sequence was determined. The results indicated that the CBF2 transgene in the 5-4 mutant line had a mutation converting the serine at position 118 to a leucine (Figures 3D and 6). Additional testing indicated that 5-4 plants grown at warm temperature were more freezing tolerant than wild-type Ws plants grown at warm temperature (Figure 9). Electrolyte leakage experiments indicated that the E2 and 5-4 plants had a similar level of freezing tolerance. These results indicated that specific amino acid substitutions within the CBF2 protein result in proteins that continue to have significant activity in regard to activating expression of CBF-targeted genes that impart increased stress tolerance, but at the same time, have much reduced effects on plant growth including stature and time to flowering. Thus, one may reduce or eliminate adverse effects such as small stature, altered flowering time, or reduced biomass, fruit yield or fertility in plants that are abiotic stress tolerant by introducing a mutation in a CBF polynucleotide that provides for one of these specific amino acid substitutions. The nature of the mutation must be such that the CBF polypeptide retains the ability to bind to a cold or dehydration transcription regulating region comprising the sequence CCG (in a manner similar to CBF2), and the polypeptide, when overexpressed, confers greater abiotic stress tolerance than that of a wild-type plant. Additional screening resulted in two more mutants like 5-4: these were 17-2 and 6-4. Like the 5- 4 and 7-2 mutants, these mutants were much bigger and flowered earlier than E2 plants (plants of line 6-4 are shown in Figure 8A), yet the levels of CBF2 and COR15a transcripts in these mutants were about the same as in E2 plants (this is shown for 6-4 plants in Figure 8B). Like the 5-4 mutant plants, non- acclimated 17-2 and 6-4 plants were more freezing tolerant than non-acclimated wild-type plants. The EL₅₀ (the temperature at which 50% of the tissue electrolytes are release by freezing) values for 17-2 and 6-4 were -8.5° C and -9° C, respectively, whereas that for wild-type plants was -5° C. hi the case of the 6-4 mutant, the mutation converted the glutamic acid residue at position 84 of the CBF2 protein to a lysine residue (Figures 3C and 6). In the 17-2 mutant, the aspartic acid residue at position 108 was converted to an asparagine residue (Figures 3C and 6). Additional mutant lines, designated 16-1, 12-6, 14-2, 2-8 and 8-3, were identified that produced plants that were much bigger and flowered earlier than the E2 plants, yet still expressed the CBF and CBF-target gene COR15a under non-stressed conditions. An analysis of the CBF2 transgene in these lines indicated that there were no mutations within the transgene. Thus, second site mutations (i.e., in a non-CSE gene) can occur that can suppress the negative effects of CBF2 overexpression. hi light of the above discoveries, it has been determined that modified versions of the CBF2, paralogous and orthologous proteins may be isolated that: 1) activate expression of the CBF regulon; 2) increase abiotic stress tolerance; and 3) have much reduced adverse morphological characteristics including effects on plant growth and development. These results also indicate that non-CSE genes can be mutated such that they do not significantly affect the ability of the CBF proteins to activate expression of the CBF regulon and increase abiotic stress tolerance, but can suppress the potentially negative effects that CBF expression has on plant growth and development. These altered CBF and non-CBF genes can be used to improve stress tolerance of plants with fewer or reduced secondary effects on plant growth and development. Regulation of these modified genes with tissue-specific or inducible promoters, for example, stress-inducible promoters, could provide increased tolerance to environmental stresses without significantly impacting a plant's phenotype in a negative manner, such as by decreasing seed production, reducing plant size, and/or delaying flowering).

EXAMPLE VH. Site-specific saturation mutagenesis of the DSAWR subsequence Results from the suppressor mutation screen indicated that mutations within the highly conserved

DSAWR sequence of the CBF transcription factors (Figures 3C and 6) reduced adverse effects of CBF overexpression on plant growth and development. To explore this issue further, residues within the CBFl DSAWR sequence were substituted with alanine residues. This mutagenesis method was performed with the in vitro "QuikChange mutagenesis" developed by Sfratagene (La Jolla, CA). The methods were conducted according to the manufacturer's instructions. Primers containing mutations were synthesized to only one strand of the double-stranded CBF nucleotide template; one strand bearing multiple mutations and nicks was created and the nicks were sealed with "multi-enzyme blend" (QuikChange^® Multi Enzyme Blend). This reaction mixture was treated with Dpn I to digest the parental DNA template, enriching for the multiply mutated single-stranded DNA. This mixture was transformed into a competent E. coli strain (XLIO-Gold^® ultracompetent cells), where the mutant closed circle single-stranded DNA was converted into duplex form in vivo. ds-DNA was then prepared from the transformants. The primers used were designed to introduce alanine residues in place of the original DSAWR amino acids as well as a Notl restriction site to facilitate identification of mutant sequences: MT 817 (forward primer; SEQ ID NO: 215) 5' GTC TCA ACT TCG CTG CCG CGG CCG CGG CGC TAC GAA TCC CGG AG 3'; and MT 818 (reverse primer; SEQ HO NO: 216) 5' CTC CGG GAT TCG TAG CGC CGC GGC CGC GGC AGC GAA GTT GAG AC 3'. The various mutagenized CBFl sequences, each then possessing five consecutive alanine residues in place of the DSAWR subsequence (that is, "DSAWR" was changed to "AAAAA"), were overexpressed in transgenic Arabidopsis plants. The effects of these transformations on COR gene expression, plant growth and freezing tolerance (Table 4) were then determined. The mutated CBFl genes (each referred to as CBFl-dsawr), as well as wild-type CBFl, were placed under control of the cauliflower 35S promoter and transformed into Arabidopsis as previously described for CBF2. Transgenic lines were obtained that expressed CBFl-dsawr at much higher levels than obtained with overexpression of the wild-type CBFl (Figure 11). hi these CBFl-dsawr lines, COR15 was expressed at levels approximating those found in the lines overexpressing wild-type CBFl . Unlike the plants overexpressing wild-type CBFl, such as line WT8, which displayed a marked dwarf phenotype (Figure 12), those expressing the mutant CBFl-dsawr protein displayed little, if any, effect on growth. The freezing tolerance of the CBFl-dsawr plants, however, was much greater than non- transformed plants and at least as much freezing tolerance as transgenic plants overexpressing wild-type CBFl (Figures 13A-13F and Table 4).

Table 4. Survival of trans enic lants overex ressin CBFl G-26 and mutant (dsawr) versions of CBF 1

EXAMPLE VHI. Mutagenesis of non-Arabidopsis CBFs Similar to the methods used in Example II, seeds overexpressing a CBF derived from any of a number of diverse plant species are mutagenized with EMS as described above and in Somerville and Ogren (1982) supra. The CBF sequences or the seeds can be derived from any plant including monocots and dicots and in particular agriculturally important plant species, including but not limited to, crops such as soybean, wheat, corn (maize), potato, cotton, rice, rape, oilseed rape (including canola), sunflower, alfalfa, clover, sugarcane, and turf; or fruits or fruit trees, vegetables such as banana, blackberry, blueberry, strawberry, and raspberry, cantaloupe, carrot, cauliflower, coffee, cucumber, eggplant, grapes, honeydew, lettuce, mango, melon, onion, papaya, peas, peppers, pineapple, pumpkin, spinach, squash, sweet corn, tobacco, tomato, tomatillo, watermelon, rosaceous fruits (such as apple, peach, pear, cherry and plum) and brassicas (such as broccoli, cabbage, cauliflower, Brussels sprouts, and kohlrabi). Seeds of other crops, including fruits and vegetables, whose phenotype can be changed and which comprise homologous CBF sequences include barley; rye; millet; sorghum; currant; avocado; citrus fruits such as oranges, lemons, grapefruit and tangerines, artichoke, cherries; nuts such as the walnut and peanut; endive; leek; roots such as arrowroot, beet, cassava, turnip, radish, yam, and sweet potato; and beans. The CBF sequences or seeds may also be derived from woody species, such pine, poplar and eucalyptus, or mint or other labiates. In addition, seeds or CBF sequences may be derived from plants that are evolutionarily related to crop plants, but which may not have yet been used as crop plants. Examples include deadly nightshade (Atropa belladona), related to tomato; jimson weed (Datura strommium), related to peyote; and teosinte (Zea species), related to corn (maize). After mutagenesis, the seeds are planted out into 40 separate flats of soil (~1,500 seeds per flat) and grown as described above. The M2 seed is collected individually from each flat and M2 plants are screened for those that are phenotypically larger than plants constitutively overexpressing a homologous CBF. M3 seed is collected from individual putative mutants and retained for further study. Where populations of M3 plants from an individual M2 plant are not uniform in size, M4 or M5 seeds are collected from individual M3 or M4 plants. Alternatively, site specific mutagenesis may be used to produce mutations in a CBF gene that encode changes in the amino acid residues corresponding the substitutions that have been shown to be of interest in CBF2 (e.g., a mutation that converts a glutamic acid residue corresponding to position 84 of to a lysine residue; a mutation that converts an aspartic acid residue conesponding to position 108 to an asparagine residue; other mutations in the second CBF signature sequence (CBF_b), or a mutation that converts a serine residue conesponding to position 118 of CBF2 to a leucine residue. Site-specific mutagenesis uses oligonucleotide sequences that encode the DNA sequence with the desired mutation, in this case, a transcription factor sequence, as well as a sufficient number of adjacent nucleotides, to provide a primer sequence of sufficient size to form a stable duplex on both sides of the deletion junction being traversed. Typically, a primer of about 17 to about 75 nucleotides or more in length is prefened, with about 10 to about 25 or more residues on both sides of the junction of the sequence being altered. Site-specific mutagenesis procedures are well known, and may be performed using a phage vector such as Ml 3. This phage exists in both a single stranded and double stranded form. Alternatively, one may use a double-stranded plasmid rather than a phage, which would eliminate the recombinant method steps involved in transferring the gene of interest from a plasmid to a phage. The next step is to prepare a vector using recombinant methods, where the vector contains the

DNA sequence of interest encoding the subject transcription factor. A single-stranded vector may be used, which may be obtained by melting a double stranded vector. An oligonucleotide primer that harbors the desired mutated sequence is then prepared, for example, by synthesis or with recombinant methods. This primer is then annealed with the single-stranded vector, followed by a treatment with a DNA polymerizing enzyme (e.g., polymerase 1 from E. coli, Klenow fragment). The DNA-polymerizing enzyme treatment causes a heteroduplex to be completed where one strand encodes the non-mutated sequence, and the second strand harbors the sequence containing the mutation. Cells of a plant species of interest are transformed with the vector. This heteroduplex vector is then used to transform or transfect cells, and cells are selected that include recombinant vectors bearing the mutated sequence arrangement. Alternatively, a gene amplification method (e.g., PCR) using, e.g., Taq polymerase, may be used to incoφorate a oligonucleotide primer harboring the mutation if interest into an amplified DNA fragment that can then be cloned into an appropriate expression vector. A gene amplification method that makes use of a thermostable ligase and a thermostable polymerase may also be used to incoφorate a phosphorylated mutagenic oligonucleotide into an amplified DNA fragment, that may then be cloned into an appropriate cloning or expression vector used to transform plant cells. For a further description of these methods and references, see, e.g., USPN 6,635,806 or USPN 6,620,988, from which the above site- specific mutagenesis methods were derived, Wu (ed.) Methods Enzymol. (1993) vol. 217, Academic Press, or Das, et al (1995) Plant Cell. 7:287-294.

EXAMPLE X. Application of altered Arabidopsis and non-Arabidopsis CBFs in plants The CBF polynucleotide sequences of the invention, including CBF-encoding polynucleotides from diverse species, harbor mutations for amino acid residue substitutions in the encoded CBF. These CBF sequences include, but are not limited to mutant forms of the polypeptide sequences of the Sequence Listing, and include SEQ ID NO: 2, 13, 15, 17, 39, 41, 43, 45, 47, 49, 51, 53, 55, 57, 59, 61, 63, 65, 67, 69, 71, 73, 75, 77, 79, 81, 83, 85, 87, 89, 91, 93, 95, 97, 99, 101, 103, 116, 118, 120, 122, 124, 126, 128, 138, 140, 142, 144, 146, 148, 150, 152, 154, 156, 158, 160, 162, 164, 166, 168, 170, 172, 174, 176, 178, 180, 182, 184, 186, 202, 204, 206, 208, 210, 212 and 214, orthologs, paralogs, variants, and fragments thereof that function in the manner of SEQ ID NO: 13 in conferring tolerance or resistance to an abiotic stress These sequences and their encoded polypeptides confer abiotic stress tolerance in plants that are moφhologically and developmentally similar to wild type. Examples of such diverse species are identified in the above section of this disclosure entitled "Homologous Sequences", and examples of specific orthologous CBF-encoding sequences include, but are not limited to, SEQ ID NO: 1, 12, 14, 18, 38, 40, 42, 44, 46, 48, 50, 52, 54, 56, 58, 60, 62, 64, 66, 68, 70, 72, 74, 76, 78, 80, 82, 84, 86, 88, 90, 92, 94, 96, 98, 100, 102, 115, 117, 119, 121, 123, 125, 127, 137, 139, 141, 143, 145, 147, 149, 151, 153, 155, 157, 159, 161, 163, 165, 167, 169, 171, 173, 175, 177, 179, 181, 183, 185, 201, 203, 205, 207, 209, 211 and 213, polynucleotides that encode orthologs, paralogs, variants, and fragments thereof. The specific residues that may be substituted in CBFs include a lysine residue for the glutamic acid residue (e.g., SEQ ID NO: 217) that corresponds to position 84, the glutamic acid residue in the AP2 domain of CBF2 (SEQ ID NO: 2): ...Gin Thr Ala Glu Met Ala Ala ... is replaced by ...Gin Thr Ala Lys Met Ala Ala...

the asparagine residue for the aspartic acid residue (e.g., SEQ ID NO: 218) that conesponds to position

108, the aspartic acid residue in the CBF signature sequence of CBF2 (SEQ ID NO: 2) after the AP2 domain: ...Asn Phe Ala Asp Ser Ala... is replaced by ...Asn Phe Ala Asn Ser Ala...

and the leucine residue for the serine residue (e.g., SEQ ID NO: 219) that conesponds to position 118, the serine residue after the AP2 domain of CBF2 (SEQ ID NO: 2) in the subsequence: ...He Pro Glu Ser Thr... is replaced by ...He Pro Glu Leu Thr... . These particular examples, identified in Figures 3C and 3D by the boxes around the residues, and in Figure 6, have been shown to confer abiotic stress tolerance when overexpressed with fewer adverse moφhological and/or developmental effects than when the non-mutant form of the CBF2 polypeptide is overexpressed. Other variations in the amino acid sequence of the CBF polypeptide may be considered that also confer abiotic stress tolerance in plants of wild-type or nearly wild-type moφhology and fertility, and are also encompassed by the present invention. These mutant orthologous sequences function similarly to SEQ ID NO: 13 in conferring abiotic stress tolerance, but due to the nature of the mutations, the transformed plants are moφhologically and developmentally similar to wild type plants of the same species. These sequences, including truncated sequences, or these sequences combined with an artificial activation domain, may be recombined into an expression vector (for example, the pGEM-T Vector, pMEN20 or pMEN65) and transformed into a plant of a species of interest. Transgenic plants that are abiotic stress tolerant and yet do not possess the developmental or moφhological defects found in plants that overexpress wild-type CBF proteins may be produced by the substitution of the aspartic acid, serine, tryptophan and/or arginine residues of the CBF signature sequence. For example: ... Asp-Ser-Ala-Tφ-Arg... is replaced by ... Ala-Ala-Ala-Ala-Ala...

or substitutions to a similar CBF signature sequence found in related polypeptides are encompassed by the invention. These substitutions (e.g., SEQ ID NO: 220) have been shown to yield a CBF mutant polypeptide that, when overexpressed in plants, confers increased abiotic stress tolerance without the adverse moφhological or developmental effects often associated with wild-type CBF polypeptide overexpression. The specific mutations may be alanine residues substituted for the aspartic acid, serine, tryptophan or arginine residues, although it is envisioned that other similar substitutions may also be used to produce useful polypeptides of the invention that confer abiotic stress tolerance without adverse moφhological or developmental effects. Amino acids that are similar to alanine and may be substituted for the original residues in this signature sequence to produce the useful polypeptides of the invention include serine, threonine, glycine, valine, leucine and isoleucine. The vector may be introduced into a variety of monocot plants by well known means, including direct DNA transfer or Agrobacterium tumefaciens-mediated transformation. It is routine to produce transgenic plants using dicot plants (see Weissbach and Weissbach,

(1989) supra; Gelvin et al. (1990) supra; Henera-Estrella et al. (1983) supra; Bevan (1984) supra; and Klee (1985) supra). For example, numerous protocols for the transformation of tomato and soy plants have been previously described, and are well known in the art. Gruber et al. ((1993) in Methods in Plant Molecular Biology and Biotechnology, p. 89-119, Glick and Thompson, eds., CRC Press, Inc., Boca Raton) describe several expression vectors and culture methods that may be used for cell or tissue transformation and subsequent regeneration. There are a substantial number of alternatives to Agrobacterium-mediated transformation protocols for transferring exogenous genes into soybeans or tomatoes. For soybean transformation, methods are described by Miki et al. (1993) in Methods in Plant Molecular Biology and Biotechnology, p. 67-88, Glick and Thompson, eds., CRC Press, h e, Boca Raton; and U.S. Pat. No. 5,563,055, (Townsend and Thomas), issued Oct. 8, 1996. One such method is microprojectile-mediated transformation, in which DNA on the surface of microprojectile particles is driven into plant tissues with a biolistic device (see, for example, Sanford et al., (1987) Part. Sci. Technol. 5:27-37; Christou et al. (1992) Plant. J. 2: 275-281; Sanford (1993) Methods Enzymol. 217: 483-509; Klein et al. (1987) Nature 327: 70-73; U.S. Pat. No. 5,015,580 (Christou et al), issued May 14, 1991; and U.S. Pat. No. 5,322,783 (Tomes et al), issued Jun. 21, 1994). Alternatively, sonication methods (see, for example, Zhang et al. (199\)Bio/Technology 9: 996- 997); direct uptake of DNA into protoplasts using CaCl₂ precipitation, polyvinyl alcohol or poly-L- ornithine (see, for example, Hain et al. (1985) Mol. Gen. Genet. 199: 161-168; Draper et al., Plant Cell Physiol. 23: 451-458 (1982)); liposome or spheroplast fusion (see, for example, Deshayes et al. (1985) EMBO J., 4: 2731-2737; Christou et al. (1987) Proc. Natl. Acad. Sci. U.S.A. 84: 3962-3966); and electroporation of protoplasts and whole cells and tissues (see, for example, Donn et al.(1990) in Abstracts of Vflth International Congress on Plant Cell and Tissue Culture IAPTC. A2-38: 53; D'Halluin et al. (1992) Plant Cell 4: 1495-1505; and Spencer et al. (1994) Plant Mol Biol. 24: 51-61) have been used to introduce foreign DNA and expression vectors into plants. After plants or plant cells are transformed (and the latter regenerated into plants) the transgenic plant thus generated may be crossed with itself ("selfing") or a plant from the same line, a non- transformed or wild-type plant, or another transformed plant from a different transgenic line of plants. Crossing provides the advantages of being able to produce new and perhaps stable transgenic varieties. Genes and the traits they confer that have been introduced into a tomato or soybean line may be moved into distinct line of plants using traditional backcrossing techniques well known in the art. Transformation of tomato plants may be conducted using the protocols of Koornneef et al (1986), in Tomato Biotechnology: Alan R. Liss, hie, 169-178, and in U.S. Patent 6,613,962, the latter method described in brief here. Eight day old cotyledon explants are precultured for 24 hours in Petri dishes containing a feeder layer of Petunia hybrida suspension cells plated on MS medium with 2% (w/v) sucrose and 0.8% agar supplemented with 10 μM α-naphthalene acetic acid and 4.4 μM 6- benzylaminopurine. The explants are then infected with a diluted overnight culture of Agrobacterium tumefaciens containing an expression vector comprising a polynucleotide of the invention for 5-10 minutes, blotted dry on sterile filter paper and cocultured for 48 hours on the original feeder layer plates. Culture conditions are as described above. Overnight cultures of Agrobacterium tumefaciens are diluted in liquid MS medium with 2% (w/v/) sucrose, pH 5.7) to an OD₆₀o of 0.8. Following the cocultivation, the cotyledon explants are transfened to Petri dishes with selective medium comprising MS medium with 4.56 μM zeatin, 67.3 μM vancomycin, 418.9 μM cefotaxime and 171.6 μM kanamycin sulfate, and cultured as described above. The explants are subcultured every three weeks onto fresh medium. Emerging shoots are dissected from the underlying,callus and transfened to glass jars with selective medium without zeatin to form roots. The formation of roots in a medium containing kanamycin sulfate is regarded as a positive indication of a successful transformation. Transformation of soybean plants may be conducted using methods found in, for example, U.S. Patent 5,563,055 (to Townsend et al.), described in brief here. In this method soybean seed is surface- sterilized by exposure to chlorine gas evolved in a glass bell jar. Seeds are germinated by plating on 1/10 strength agar solidified medium without plant growth regulators and culturing at 28° C, 16 hour day length. After 3-4 days, seed may be prepared for cocultivation. The seedcoat is removed and the elongating radicle removed 3-4 mm below the cotyledons. Overnight cultures of Agrobacterium tumefaciens harboring the expression vector comprising a polynucleotide of the invention are grown to log phase, pooled, and concentrated by centrifugation. Inoculations are conducted in batches such that each plate of seed was treated with a newly resuspended pellet of Agrobacterium. The pellets are resuspended in 20 ml inoculation medium. The inoculum is poured into a Petri dish containing prepared seed and the cotyledonary nodes are macerated with a surgical blade. After 30 minutes the explants are transfened to plates of the same medium which has been solidified. Explants are embedded with the adaxial side up and level with the surface of the medium and cultured at 22° C for three days under white fluorescent light. These plants may then be regenerated according to methods well established in the art, such as by moving the explants after three days to a liquid counter-selection medium (see U.S. Patent 5,563,055). Explants may be picked, embedded and cultured in solidified selection medium. After one month on selective media, transformed tissue is visible as green sectors of regenerating tissue against a background of bleached, less healthy tissue. Explants with green sectors are transfened to an elongation medium. Explants are transfened to fresh elongation medium plates every two weeks. When shoots are 0.5 cm in length, they may be excised at the base and placed in a rooting medium. The polynucleotide and polypeptide sequences derived from monocots may be used to transform both monocot and dicot plants, and those derived from dicots may be used to transform either group, although some of these sequences will function best if the gene is transformed into a plant from the same group as that from which the sequence is derived. Abiotic stress-tolerant, transformed plants may be identified by, for example, subjecting seeds of these transformed plants to abiotic stress assays, including germination assays. One example may be a high sucrose germination assay to measure sucrose sensing. Sterile monocot seeds, including, but not limited to, corn, rice, wheat, rye and sorghum, as well as dicots including, but not limited to soybean and alfalfa, are sown on 80% MS medium plus vitamins with 9.4% sucrose; control media lack sucrose. All assay plates are then incubated at 22° C under 24-hour light, 120-130 μEin/m²/s, in a growth chamber. Evaluation of germination and seedling vigor is then conducted three days after planting. Overexpressors of these genes may be found to be more tolerant to high sucrose by having better germination, longer radicles, and more cotyledon expansion. These results would indicate that overexpressors of mutant CBFs are involved in sucrose-specific sugar sensing. Plants overexpressing these orthologs may also be subjected to soil-based drought assays to identify those lines that are more tolerant to water deprivation than wild-type control plants. Generally, plants that overexpress a CBF mutant polypeptide of the invention, including CBFs from diverse species, will appear significantly larger and greener, with less tissue damage, wilting, desiccation, or necrosis, than wild-type controls plants, particularly after a period of freezing or water deprivation. Abiotic stress- tolerant plants that are moφhologically and developmentally similar to wild-type plants may then be used to generate lines for commercial development.

All publications and patent applications mentioned in this specification are herein incoφorated by reference to the same extent as if each individual publication or patent application was specifically and individually indicated to be incoφorated by reference. The present invention is not limited by the specific embodiments described herein. The invention now being fully described, it will be apparent to one of ordinary skill in the art that many changes and modifications can be made thereto without departing from the spirit or scope of the appended claims. Modifications that become apparent from the foregoing description and accompanying figures fall within the scope of the claims, or claims that may derive from the claims.

Claims

What is claimed is:

1. A method for producing a transformed plant; said method comprising: (a) providing a first polynucleotide that encodes a first polypeptide, wherein the first polypeptide is a member of the CBF clade of transcription factors; (b) transforming a target plant with the first polynucleotide, wherein the first polynucleotide is mutagenized: in an in vitro mutagenesis step prior to the transforming step, to produce a second polynucleotide that encodes a second polypeptide, wherein the second polypeptide has an amino acid sequence different than the first polypeptide; or in an in vivo mutagenesis step after the transforming step, to produce a third polynucleotide that encodes a third polypeptide, wherein the third polypeptide has an amino acid sequence different than the first polypeptide; and (c) identifying a transformed plant comprising said second polynucleotide or said third polynucleotide; wherein said in vitro or in vivo mutagenesis step is sufficient to mutagenize a region of the first polynucleotide such that when the second or third polypeptide is overexpressed: the transformed plant has greater tolerance to an abiotic stress than a wild-type control plant; and the transformed plant has fewer or reduced adverse moφhological or developmental effects than a plant overexpressing the first polypeptide.

2. The method of claim 1 , wherein the second or third polypeptide is capable of binding to a cold or dehydration transcription regulating region comprising a sequence CCG.

3. The method of claim 1, wherein the abiotic stress is drought, salt, cold, heat or freezing.

4. The method of claim 1 , wherein the target plant is a monocot or a dicot.

5. The method of Claim 1 , wherein the adverse moφhological or developmental effects are selected from the group consisting of reduced internode length, reduced fertility, smaller rosettes, nanow leaves, curled leaves, wrinkled leaves, reduced leaf size, reduced seed number, altered flowering time, stunting, decreased fruit yield, and reduced biomass.

6. The method of Claim 1, wherein the second polypeptide or third polypeptide comprises at least one mutation within its AP2 domain.

7. The method of Claim 1 , wherein the second polypeptide or third polypeptide comprises at least one mutation outside of its AP2 domain.

8. The method of Claim 1, wherein the in vitro or in vivo mutagenesis step produces a mutation selected from the group consisting of: (a) a substitution of a glutamic acid residue to a lysine residue at a position conesponding to position 84 of SEQ ID NO: 13; (b) a substitution of an aspartic acid residue to an asparagine residue at a position corresponding to position 108 of SEQ ID NO: 13; and (c) a substitution of a serine residue to a leucine residue at a position conesponding to position 118 of SEQ HO NO: 13.

9. The method of Claim 1, wherein the in vitro or in vivo mutagenesis step produces at least one substitution in a subsequence D-S-A-W-R of the first or second mutant member of the CBF clade of transcription factor.

10. A transgenic plant produced by a method according to any of the preceding claims.

11. Seed produced by the transgenic plant of claim 10,ι wherein the seed comprises the second or third polynucleotide.

12. A transgenic plant comprising a recombinant, overexpressed mutated polynucleotide that encodes a mutant member of the CBF clade of transcription factor polypeptides; wherein the transgenic plant exhibits fewer or reduced adverse moφhological or developmental effects than a plant that overexpresses a wild-type form of the mutated polynucleotide.

13. The transgenic plant of claim 12, wherein the mutated polynucleotide is introduced to the transgenic plant as a mutated CBF nucleic acid sequence.

14. The transgenic plant of claim 12, wherein the mutated polynucleotide is introduced to the transgenic plant as a wild-type CBF nucleic acid sequence and the mutated polynucleotide is produced by mutagenesis of the transgenic plant.

15. The transgenic plant of claim 12, wherein the mutated polynucleotide encodes an amino acid substitution in a polypeptide sequence of the wild-type CBF polypeptide selected from the group consisting of: (a) a substitution of a glutamic acid residue to a lysine residue at a position conesponding to position 84 of SEQ ED NO: 13; (b) a substitution of an aspartic acid residue to an asparagine residue at a position corresponding to position 108 of SEQ ID NO: 13; and (c) a substitution of a serine residue to a leucine residue at a position conesponding to position 118 of SEQ ID NO: 13.

16. The transgenic plant of claim 12, wherein the mutated polynucleotide encodes one or more amino acid substitutions in a subsequence D-S-A-W-R.