WO2009060418A2 - Transgenic plants and modulators for improved stress tolerance - Google Patents

Abstract

Isolated nucleic acids and proteins and plants expressing the same for increased stress tolerance.

Description

TRANSGENIC PLANTS AND MODULATORS FOR IMPROVED STRESS TOLERANCE

RELATED APPLICATIONS

Priority is claimed to Chinese Application No. 200710176995.7, filed November 8, 2007, which is incorporated herein by reference in its entirety.

FIELD OF THE INVENTION

The present invention generally relates to compositions and methods for modulating plant characteristics such as plant stress tolerance.

BACKGROUND OF THE INVENTION

One of the main objects of plant cultivation is the cultivation of a plant with increased stress tolerance. Genes related to abiotic stress tolerance of plants have been reported extensively, including both effector and regulator genes. Effector genes involved in the response to abiotic stress include the betaine-aldehyde dehydrogenase gene (BADH), which is the second key enzyme involved in biosynthesis of the osmoregulator betaine, the pyrroline-5-carboxylate synthetase (P5CS) (a proline synthase gene), the gene of late embryogenesis abundant protein (LEA), the H⁺-ATPase gene (a membrane transporter gene), the Na⁺ZH⁺ reverse transporter gene, the aquaporin gene, and cell cycle related genes. Regulator genes involved in the response to abiotic stress include transcription factors, for example, OsbHLH and OsDREBL. Transformation of such genes into the plants Oryza Sativa or Arabidopsis thaliana leads to the improvement of abiotic stress tolerance of the transgenic plants.

Rice is an important grain plant and its yield can be severely affected by various environmental conditions. Accordingly, there is considerable practical significance in identifying mediators of stress tolerance in rice.

To meet this need, the present invention provides OsSIKl (Stress Inducible Kinase 1) nucleic acids and proteins from rice. The present invention additionally provides plants that overexpress OsSIKl nucleic acids and proteins to thereby increase stress tolerance, including increased tolerance to salinity and drought. Also provided are methods for making such plants and methods for mimicking a stress tolerance phenotype using an OsSIKl modulator. SUMMARY OF THE INVENTION

The present invention provides isolated OsSIKl nucleic acids and proteins, vectors, and cells expressing the disclosed nucleic acids, and antibodies that specifically bind to the disclosed proteins. Representative OsSIKl nucleic acids of the invention include nucleic acids comprising (a) a nucleotide sequence set forth as SEQ ID NO: 1; (b) a nucleic acid that specifically hybridizes to the complement of SEQ ID NO: 1 under stringent hybridization conditions; (c) an open reading frame (ORF) of a gene encoding an OsSIKl protein, where the open reading frame comprises nucleotides 57-3056 of SEQ ID NO: 1; (d) a nucleotide sequence encoding an OsSIKl protein comprising an amino acid sequence set forth as SEQ ID NO: 2; (e) a nucleotide sequence complementary to that of the nucleic acid of (a)-(d); and a functional fragment of (a)-(e). Representative nucleic acids of the invention also include nucleic acids encoding an OsSIKl protein as described herein below. Representative OsSIKl proteins of the invention include proteins comprising (a) an amino acid sequence of SEQ ID NO: 2; (b) an amino acid sequence at least 80% identical to SEQ ID NO: 2; (c) an amino acid sequence of SEQ ID NO: 2 and further comprising one or more substitutions, deletions, or additions; and (d) a functional fragment of (a)-(c). Representative OsSIKl proteins of the invention also include proteins which are related to stress tolerance of plants and derived from SEQ ID NO: 2, with one or more substitutions, deletions, and/or additions of the amino acid residues in SEQ ID NO: 2. In some instances, substitutions, deletions, and/or addition of the amino acid residues of SEQ ID NO: 2 do not occur in 13 LRR domains, a transmembrane domain, and a kinase domain, which domains consist of sequences having amino acid residues at positions 98-122 (LRR-I), 146-169 (LRR-2), 170-194 (LRR-3), 218-242 (LRR-4), 244-266 (LRR-5), 267-288 (LRR-6), 291-313 (LRR-7), 315-337 (LRR-8), 361-385 (LRR-9), 409-433 (LRR-IO), 457-480 (LRR-I l), 481-504 (LRR- 12), 505-529 (LRR- 13); 606-628 (transmembrane domain), and 672-942 (kinase domain) of SEQ ID NO: 2, respectively.

Also provided are plants overexpressing OsSIKl, including monocot and dicot plants and methods of producing such plants using the OsSIKl nucleotide sequences disclosed herein. OsSIKl transgenic plants are characterized by increased OsSIKl expression and increased stress tolerance.

Further provided are methods of identifying OsSIKl binding agents and activators. In one aspect of the invention, the method can comprise (a) providing an OsSIKl protein; (b) contacting the OsSIKl protein with one or more test agents or a control agent under conditions sufficient for binding; (c) assaying binding of a test agent to the isolated OsSIKl protein; and (d) selecting a test agent that demonstrates specific binding to the OsSIKl protein. In another aspect of the invention, the method can comprise (a) providing a cell expressing an OsSIKl protein; (b) contacting the cell with one or more test agents or a control agent; (c) assaying expression of an OsSIKl nucleic acid or protein; and (d) selecting a test agent that induces elevated expression of the OsSIKl nucleic acid or protein when the cell is contacted with the test agent as compared to the control agent. In another aspect of the invention, the method can comprise (a) providing a plant expressing an OsSIKl protein; (b) contacting the plant with one or more test agents or a control agent; (c) assaying survival of the plants under abiotic stress; and (d) selecting a test agent that promotes survival of the plants under abiotic stress when in the presence of the test agent as compared to the control agent. In another aspect of the invention, the method cam comprise (a) providing a cell expressing an OsSIKl protein; (b) contacting the cell with one or more test agents or a control agent; (c) assaying expression of an OsSIKl target gene; and (d) selecting a test agent that induces a change in expression of the target gene, which gene is normally subject to OsSIKl control, when the cell is contacted with the test agent as compared to the control agent. Methods are also providing for conferring abiotic stress tolerance to a plant by contacting the plant with an OsSIKl activator identified as described herein.

Also provided are methods for preparing a transgenic plant having stress tolerance, including salt tolerance and drought tolerance, comprising introducing an OsSIKl nucleic acid as disclosed herein into a plant cell to obtain a plant having stress tolerance.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 shows the expression levels of OsSIKl at different time points (h, hour) under conditions of salt stress (NaCl), drought stress, low temperature stress, H₂O₂ stress, ABA stress, and ACC stress as determined by RT-PCR. See Example 2.

Figure 2 shows a schematic diagram of the structure of the pBin438-0&S7A7 vector. NPT II, neomycin phosphotransferase II gene; 35S, CaMV 35S promoter; Ω, tobacco mosaic virus Ω sequence; GmWRKY54, GmWRKY54 open reading frame; NOS, terminator sequence.

Figure 3 shows the expression level of OsSIKl normalized to actin expression in transgenic plants overexpressing the OsSIKl gene, as determined by real-time PCR.. Figure 4 shows the expression level of OsSIKl normalized to actin expression in transgenic plants with inhibited expression of the OsSIKl gene, as determined by real-time PCR.

Figure 5 A shows growth of a control plant (TP 309 transformed with blank vector), OsSIKl-OX (overexpressing) plant strains 7-2, 7-1, and 3-4, and OsSIKl-RNAi plant strains 83- 1, 12-3, and 20-3, grown in a solution of 0.55% (0.55g/100 ml) NaCl for 10 days under normal conditions. OsSIKl-RNAi plant strain 20-3 also showed loss of green color in some leaves.

Figure 5B shows growth of a control plant (TP 309 transformed with blank vector), OsSIKl-OX (overexpressing) plant strains 7-2, 7-1, and 3-4, OsSIKl-RNAi plant strains 83-1, 12-3, and 20-3, grown in a solution of 0.55% (0.55g/100 ml) NaCl for 10 days under normal conditions and a further 10 days recovery. The control plant and OsSIKl-RNAi plant strains 83- 1, 12-3, and 20-3 also showed substantial loss of green color in most or all leaves. OsSIKl-OX (overexpressing) plant strains 7-2, 7-1, and 3-4 showed slight loss of green color in some leaves.

Figure 5 C shows the survival rate of control plants (TP 309 transformed with blank vector), OsSIKl-OX (overexpressing) plant strains 7-2, 7-1, and 3-4, and OsSIKl-RNAi plant strains 83-1, 12-3, and 20-3, grown in a solution of 0.55% (0.55g/100 ml) NaCl for 10 days under normal conditions and a further 10 days recovery.

Figure 6 A shows growth of a control plant (TP 309 transformed with blank vector), OsSIKl-OX (overexpressing) plants 7-2, 7-1, and 3-4, and OsSIKl-KNAi plant strains 83-1, 12- 3, and 20-3, grown without water for 8 days. The control plant and OsSIKl-RNAi plant strains 83-1, 12-3, and 20-3 also showed loss of green color in some leaves.

Figure 6B shows growth of a control plant (TP 309 transformed with blank vector), OsSIKl-OX (overexpressing) plant strains 7-2, 7-1, and 3-4, and OsSIKl-RNAi plant strains 83- 1, 12-3, and 20-3, grown without water for 8 days, followed by 4 days of recovery.

Figure 6C shows growth of control plants (TP 309 transformed with blank vector), OsSIKl-OX (overexpressing) plant strains 7-2, 7-1, and 3-4, and OsSIKl-KNAi plant strains 83- 1, 12-3, and 20-3, grown without water for 8 days, followed by 15 days of recovery. The control plant showed loss of green color in some leaves, and OsSIKl-RNAi plant strains 83-1, 12-3, and 20-3 showed substantial loss of green color in most or all leaves.

Figure 6D shows the survival rate of control plants (TP 309 transformed with blank vector), OsSIKl-OX (overexpressing) plant strains 7-2, 7-1, and 3-4, and OsSIKl-KNAi plant strains 83-1, 12-3, and 20-3, grown without water for 8 days, followed by 15 days of recovery. DETAILED DESCRIPTION I. OsSIKl Nucleic Acids And Proteins

The present invention provides OsSIKl nucleic acids and proteins, variants thereof, and activators thereof. Previously described nucleic acids and proteins have not taught how to use such molecules for promoting stress tolerance in plants, as presently disclosed.

A representative OsSIKl nucleic acid is set forth as SEQ ID NO: 1, which contains an open reading frame (ORF) at nucleotides 57-3056 that encodes the representative OsSIKl protein of SEQ ID NO: 2. OsSIKl nucleic acid variants encompassed by the present invention include nucleic acids encoding a functional OsSIKl protein.

LA. OsSIKl Nucleic Acids

Nucleic acids are deoxyribonucleotides or ribonucleotides and polymers thereof in single- stranded, double-stranded, or triplexed form. Unless specifically limited, the term encompasses nucleic acids containing known analogues of natural nucleotides that have similar properties as the reference natural nucleic acid. The terms nucleic acid molecule or nucleic acid may also be used in place of gene, cDNA, mRNA, or cRNA. Nucleic acids may be synthesized, or may be derived from any biological source, including any organism.

Representative nucleic acids of the invention comprise the nucleotide sequence of SEQ ID NO: 1 and substantially identical sequences encoding functional OsSIKl proteins with substantially identical activity, for example, sequences at least 50% identical to SEQ ID NO: 1, such as at least 55% identical; or at least 60% identical; or at least 65% identical; such as at least 70% identical; or at least 75% identical; or at least 80% identical; or at least 85% identical; or at least 90% identical, or as at least 91% identical; or at least 92% identical; or at least 93% identical; or at least 94% identical; or at least 95% identical; or at least 96% identical; or at least 97% identical; or at least 98% identical; or at least 99% identical. Sequences are compared for maximum correspondence using a sequence comparison algorithm using the full-length sequence of SEQ ID NO: 1 as the query sequence, as described herein below, or by visual inspection.

Substantially identical sequences may be polymorphic sequences, i.e., alternative sequences or alleles in a population. An allelic difference may be as small as one base pair.

Substantially identical sequences may also comprise mutagenized sequences, including sequences comprising silent mutations. A mutation may comprise one or more residue changes, a deletion of one or more residues, or an insertion of one or more additional residues. Substantially identical nucleic acids are also identified as nucleic acids that hybridize specifically to or hybridize substantially to the full length of SEQ ID NO: 1 under stringent conditions. In the context of nucleic acid hybridization, two nucleic acid sequences being compared may be designated a probe and a target. A probe is a reference nucleic acid molecule, and a target is a test nucleic acid molecule, often found within a heterogeneous population of nucleic acid molecules. A target sequence is synonymous with a test sequence.

A particular nucleotide sequence employed for hybridization studies or assays includes probe sequences that are complementary to at least an about 14 to 40 nucleotide sequence of a nucleic acid molecule of the present invention. For example, probes may comprise 14 to 20 nucleotides, or even longer where desired, such as 30, 40, 50, 60, 100, 200, 300, or 500 nucleotides or up to the full length of SEQ ID NO: 1. Such fragments may be readily prepared, for example by chemical synthesis of the fragment, by application of nucleic acid amplification technology, or by introducing selected sequences into vectors for recombinant production.

Specific hybridization refers to the binding, duplexing, or hybridizing of a molecule only to a particular nucleotide sequence under stringent conditions when that sequence is present in a complex nucleic acid mixture (e.g., total cellular DNA or RNA). Specific hybridization may accommodate mismatches between the probe and the target sequence depending on the stringency of the hybridization conditions.

Stringent hybridization conditions and stringent hybridization wash conditions in the context of nucleic acid hybridization experiments such as Southern and Northern blot analysis are both sequence-dependent and environment-dependent. Longer sequences hybridize specifically at higher temperatures. An extensive guide to the hybridization of nucleic acids is found in Tijssen, Laboratory Techniques in Biochemistry and Molecular Biology-Hybridization with Nucleic Acid Probes, 1993, part I chapter 2, Elsevier, New York, New York. Generally, highly stringent hybridization and wash conditions are selected to be about 5⁰C lower than the thermal melting point (Tm) for the specific sequence at a defined ionic strength and pH. Typically, under stringent conditions a probe will hybridize specifically to its target subsequence, but to no other sequences.

The Tm is the temperature (under defined ionic strength and pH) at which 50% of the target sequence hybridizes to a perfectly matched probe. Very stringent conditions are selected to be equal to the Tm for a particular probe. An example of stringent hybridization conditions for Southern or Northern Blot analysis of complementary nucleic acids having more than about 100 complementary residues is overnight hybridization in 50% formamide with 1 mg of heparin at 42⁰C. An example of highly stringent wash conditions is 15 minutes in 0.1X SSC at 65⁰C. An example of stringent wash conditions is 15 minutes in 0.2X SSC buffer at 65⁰C. See Sambrook et al, eds., Molecular Cloning: A Laboratory Manual, 1989, Cold Spring Harbor Laboratory Press, Cold Spring Harbor, New York, for a description of SSC buffer. Often, a high stringency wash is preceded by a low stringency wash to remove background probe signal. An example of medium stringency wash conditions for a duplex of more than about 100 nucleotides, is 15 minutes in IX SSC at 45⁰C. An example of low stringency wash for a duplex of more than about 100 nucleotides, is 15 minutes in 4X to 6X SSC at 4O⁰C. For short probes (e.g., about 10 to 50 nucleotides), stringent conditions typically involve salt concentrations of less than about IM Na+ ion, typically about 0.01 to IM Na+ ion concentration (or other salts) at pH 7.0-8.3, and the temperature is typically at least about 3O⁰C. Stringent conditions may also be achieved with the addition of destabilizing agents such as formamide. In general, a signal to noise ratio of 2-fold (or higher) than that observed for an unrelated probe in the particular hybridization assay indicates detection of specific hybridization.

The following are examples of hybridization and wash conditions that may be used to identify nucleotide sequences that are substantially identical to reference nucleotide sequences of the present invention: a probe nucleotide sequence hybridizes to a target nucleotide sequence in 7% sodium dodecyl sulphate (SDS), 0.5M NaPO4, ImM EDTA at 50⁰C followed by washing in 2X SSC, 0.1% SDS at 50⁰C; such as, a probe and target sequence hybridize in 7% sodium dodecyl sulphate (SDS), 0.5M NaPO4, ImM EDTA at 50⁰C followed by washing in IX SSC, 0.1% SDS at 50⁰C; such as, a probe and target sequence hybridize in 7% sodium dodecyl sulphate (SDS), 0.5M NaPO4, ImM EDTA at 50⁰C followed by washing in 0.5X SSC, 0.1% SDS at 50⁰C; such as, a probe and target sequence hybridize in 7% sodium dodecyl sulphate (SDS), 0.5M NaPO4, ImM EDTA at 50⁰C followed by washing in 0.1X SSC, 0.1% SDS at 50⁰C; such as, a probe and target sequence hybridize in 7% sodium dodecyl sulphate (SDS), 0.5M NaPO4, ImM EDTA at 50⁰C followed by washing in 0.1X SSC, 0.1% SDS at 65°C; or such as, a probe and target sequence hybridize in a solution of 6X SSC (0.5% SDS) at 65⁰C followed by washing in 2X SSC (0.1%SDS) and IX SSC (0.1%SDS). A further indication that two nucleic acid sequences are substantially identical is that proteins encoded by the nucleic acids are substantially identical, share an overall three- dimensional structure, or are biologically functional equivalents, as described further herein below. Nucleic acid molecules that do not hybridize to each other under stringent conditions are still substantially identical if the corresponding proteins are substantially identical. This may occur, for example, when two nucleotide sequences comprise conservatively substituted variants as permitted by the genetic code.

The term conservatively substituted variants refers to nucleic acid sequences having degenerate codon substitutions wherein the third position of one or more selected (or all) codons is substituted with mixed-base and/or deoxyinosine residues. See Batzer et al., Nucleic Acids Res., 1991, 19:5081; Ohtsuka et al., J. Biol. Chem., 1985, 260:2605-2608; and Rossolini et al. MoI. Cell Probes, 1994, 8:91-98.

Nucleic acids of the invention also comprise nucleic acids complementary to SEQ ID NO: 1. Complementary sequences are two nucleotide sequences that comprise antiparallel nucleotide sequences capable of pairing with one another upon formation of hydrogen bonds between base pairs. As used herein, the term complementary sequences means nucleotide sequences which are substantially complementary, as may be assessed by the same nucleotide comparison methods set forth below, or is defined as being capable of hybridizing to the nucleic acid segment in question under relatively stringent conditions such as those described herein. A particular example of a complementary nucleic acid segment is an antisense oligonucleotide.

Nucleic acids of the invention also comprise nucleic acids of SEQ ID NO: 1, which have been altered for expression in organisms other than plants to account for differences in codon usage between plants and the other organism. For example, the specific codon usage in plants differs from the specific codon usage in certain microorganisms. Comparison of the usage of codons within a cloned microbial ORF to usage in plant genes (and in particular genes from the target plant) will enable an identification of the codons within the ORF that should specifically be changed. Typically plant evolution has tended towards a strong preference of the nucleotides C and G in the third base position of monocotyledons, whereas dicotyledons often use the nucleotides A or T at this position. By modifying a gene to incorporate specific codon usage for a particular target transgenic species, problems associated with GC/AT content and illegitimate splicing will be overcome. Plant genes typically have a GC content of more than 35%. ORF sequences which are rich in A and T nucleotides can cause several problems in plants. Firstly, motifs of ATTTA are believed to cause destabilization of messages and are found at the 3 ' end of many short-lived mRNAs. Secondly, the occurrence of polyadenylation signals such as AATAAA at inappropriate positions within the message is believed to cause premature truncation of transcription. In addition, monocotyledons may recognize AT -rich sequences as splice sites (see below).

Nucleic acids of the invention also includes subsequences of SEQ ID NO: 1, i.e., nucleic acids that comprise a part of a longer nucleic acid. An exemplary subsequence is a probe, described herein above, or a primer. The term primer as used herein refers to a contiguous sequence comprising about 8 or more deoxyribonucleotides or ribonucleotides, such as 10-20 nucleotides, or 20-30 nucleotides of a selected nucleic acid molecule. The primers of the invention encompass oligonucleotides of sufficient length and appropriate sequence so as to provide initiation of polymerization on a nucleic acid molecule of the present invention.

Nucleic acids of the invention also comprise nucleic acids encoding an OsSIKl protein set forth as SEQ ID NO: 2 or encoding an OsSIKl protein derived from SEQ ID NO: 2 containing one or more substitutions, deletions, and/or additions of amino acid residues. Such nucleic acids encoding an OsSIKl protein derived from SEQ ID NO: 2 may be obtained by deleting one or more codons from, making one or more missense mutations in, and/or linking one or more codons to the nucleic acid sequence of SEQ ID NO: 1.

Representative nucleic acids of the invention also include nucleic acids consisting essentially of SEQ ID NO: 1, or the ORF contained in SEQ ID NO: 1, in as much as the nucleic acids are isolated from and do not contain additional nucleotide sequences with which the nucleic acids may be normally associated.

The invention also provides vectors comprising the disclosed nucleic acids, including vectors for heterologous expression, wherein a nucleic acid of the invention is operably linked to a functional promoter. When operably linked to a nucleic acid, a promoter is in functional combination with the nucleic acid such that the transcription of the nucleic acid is controlled and regulated by the promoter region. Vectors refer to nucleic acids capable of replication in a host cell, such as plasmids, cosmids, and viral vectors. Nucleic acids of the present invention may be cloned, synthesized, altered, mutagenized, or combinations thereof. Standard recombinant DNA and molecular cloning techniques used to isolate nucleic acids are known in the art. Site-specific mutagenesis to create base pair changes, deletions, or small insertions is also known in the art. See e.g., Sambrook et al. (eds.) Molecular Cloning: A Laboratory Manual, 1989, Cold Spring Harbor Laboratory Press, Cold Spring Harbor, New York; Silhavy et al., Experiments with Gene Fusions, 1984, Cold Spring Harbor Laboratory Press, Cold Spring Harbor, New York; Glover & Hames, DNA Cloning: A Practical Approach, 2nd ed., 1995, IRL Press at Oxford University Press, Oxford/New York; Ausubel (ed.) Short Protocols in Molecular Biology, 3rd ed., 1995, Wiley, New York.

In another aspect of the invention, a method is provided for detecting a nucleic acid molecule that encodes an OsSIKl protein. Such methods may be used to detect OsSIKl gene variants or altered gene expression. Sequences detected by methods of the invention may be detected, subcloned, sequenced, and further evaluated by any measure well known in the art using any method usually applied to the detection of a specific DNA sequence. Thus, the nucleic acids of the present invention may be used to clone genes and genomic DNA comprising the disclosed sequences. Alternatively, the nucleic acids of the present invention may be used to clone genes and genomic DNA of related sequences. Levels of an OsSIKl nucleic acid molecule may be measured, for example, using an RT-PCR assay. See Chiang, J. Chromatogr. A., 1998, 806:209-218, and references cited therein.

In another aspect of the invention, genetic assays using OsSIKl nucleic acids may be performed for quantitative trait loci (QTL) analysis and to screen for genetic variants, for example by allele-specific oligonucleotide (ASO) probe analysis (Conner et al., Proc. Natl. Acad. ScL USA, 1983, 80(l):278-282), oligonucleotide ligation assays (OLAs) (Nickerson et al., Proc. Natl. Acad. ScL USA, 1990, 87(22):8923-8927), single-strand conformation polymorphism (SSCP) analysis (Orita et al., Proc. Natl. Acad. ScL USA, 1989, 86(8):2766-2770), SSCP/heteroduplex analysis, enzyme mismatch cleavage, direct sequence analysis of amplified exons (Kestila et al., MoI. Cell, 1998, l(4):575-582; Yuan et al., Hum. Mutat., 1999, 14(5):440- 446), allele-specific hybridization (Stoneking et al., Am. J. Hum. Genet., 1991, 48(2):370-382), and restriction analysis of amplified genomic DNA containing the specific mutation. Automated methods may also be applied to large-scale characterization of single nucleotide polymorphisms (Wang et al., Am. J. Physiol, 1998, 274(4 Pt 2):H1132-1140; Brookes, Gene, 1999, 234(2):177- 186). Useful detection methods may be non-electrophoretic, including, for example, the TAQMAN™ allelic discrimination assay, PCR-OLA, molecular beacons, padlock probes, and well fluorescence. See Landegren et al., Genome Res., 1998, 8:769-776 and references cited therein.

LB. OsSIKl Proteins

The present invention also provides isolated OsSIKl polypeptides. Polypeptides and proteins each refer to a compound made up of a single chain of amino acids joined by peptide bonds. A representative OsSIKl polypeptide is set forth as SEQ ID NO: 2. Additional polypeptides of the invention include OsSIKl proteins with substantially identical activity, for example, sequences at least 50% identical to SEQ ID NO: 2, such as at least 55% identical; or at least 60% identical; or at least 65% identical; such as at least 70% identical; or at least 75% identical; or at least 80% identical; or at least 85% identical; or at least 90% identical, or as at least 91% identical; or at least 92% identical; or at least 93% identical; or at least 94% identical; or at least 95% identical; or at least 96% identical; or at least 97% identical; or at least 98% identical; or at least 99% identical. Sequences are compared for maximum correspondence using a sequence comparison algorithm using the full-length sequence of SEQ ID NO: 2 as the query sequence, as described herein below, or by visual inspection. Representative proteins of the invention also include polypeptides consisting essentially of SEQ ID NO: 2, in as much as the polypeptides are isolated from and do not contain additional amino acid sequences with which the polypeptides may be normally associated. The invention further encompasses polypeptides encoded by any one of the nucleic acids disclosed herein.

The OsSIKl polypeptide set forth in SEQ ID NO: 2 contains 999 amino acids and shows similarity to Leucine -rich Repeat Receptor- like Kinase (LRR-RLK) proteins. It comprises a signal peptide, several LRR domains, a transmembrane domain, and a kinase domain. The signal peptide consists of the amino acid residues at positions 1-24 of SEQ ID NO: 2. The LRR domains include 13 such domains, which individually consist of the amino acid residues at positions 98-122, 146-169, 170-194, 218-242, 244-266, 267-288, 291-313, 315-337, 361-385, 409-433, 457-480, 481-504, and 505-529 of SEQ ID NO: 2. The transmembrane domain consists of amino acid residues at positions 606-628 of SEQ ID NO: 2. The kinase domain consists of amino acid residues at positions 672-942 of SEQ ID NO: 2. Polypeptides of the invention may comprise substitutions, deletions, and/or additions of amino acid residues, which substitutions, deletions, and/or additions of the amino acid residues do not occur in the above domains of SEQ ID NO: 2.

Polypeptides of the invention may comprise naturally occurring amino acids, synthetic amino acids, genetically encoded amino acids, non-genetically encoded amino acids, and combinations thereof. Polypeptides may include both L-form and D-form amino acids.

Representative non-genetically encoded amino acids include but are not limited to 2- aminoadipic acid; 3-aminoadipic acid; β-aminopropionic acid; 2-aminobutyric acid; 4- aminobutyric acid (piperidinic acid); 6-aminocaproic acid; 2-aminoheptanoic acid; 2- aminoisobutyric acid; 3-aminoisobutyric acid; 2-aminopimelic acid; 2,4-diaminobutyric acid; desmosine; 2,2'-diaminopimelic acid; 2,3-diaminopropionic acid; N-ethylglycine; N- ethylasparagine; hydroxy Iy sine; allo-hydroxylysine; 3-hydroxyproline; 4-hydroxyproline; isodesmosine; allo-isoleucine; N-methylglycine (sarcosine); N-methylisoleucine; N- methylvaline; norvaline; norleucine; and ornithine.

Representative derivatized amino acids include, for example, those molecules in which free amino groups have been derivatized to form amine hydrochlorides, p-toluene sulfonyl groups, carbobenzoxy groups, t-butyloxycarbonyl groups, chloroacetyl groups or formyl groups. Free carboxyl groups may be derivatized to form salts, methyl and ethyl esters or other types of esters or hydrazides. Free hydroxyl groups may be derivatized to form O-acyl or O-alkyl derivatives. The imidazole nitrogen of histidine may be derivatized to form N-im- benzylhistidine.

The present invention also provides functional fragments of an OsSIKl polypeptide, for example, fragments that have activity similar to that of a full-length OsSIKl protein. Functional polypeptide sequences that are longer than the disclosed sequences are also provided. For example, one or more amino acids may be added to the N-terminus or C-terminus of a polypeptide. Such additional amino acids may be employed in a variety of applications, including but not limited to purification applications. Methods of preparing elongated proteins are known in the art.

OsSIKl proteins of the invention include proteins comprising amino acids that are conservatively substituted variants of SEQ ID NO: 2. A conservatively substituted variant refers to a polypeptide comprising an amino acid in which one or more residues have been conservatively substituted with a functionally similar residue. Examples of conservative substitutions include the substitution of one non-polar (hydrophobic) residue such as isoleucine, valine, leucine or methionine for another; the substitution of one polar (hydrophilic) residue for another such as between arginine and lysine, between glutamine and asparagine, between glycine and serine; the substitution of one basic residue such as lysine, arginine or histidine for another; or the substitution of one acidic residue, such as aspartic acid or glutamic acid for another.

Isolated polypeptides of the invention may be purified and characterized using a variety of standard techniques that are known to the skilled artisan. See e.g., Schroder et al., The Peptides, 1965, Academic Press, New York; Bodanszky, Principles of Peptide Synthesis, 2nd rev. ed. 1993, Springer-Verlag, Berlin/ New York; Ausubel (ed.), Short Protocols in Molecular Biology, 3rd ed., 1995, Wiley, New York.

The present invention further provides methods for detecting an OsSIKl polypeptide. The disclosed methods can be used, for example, to determine altered levels of OsSIKl protein, for example, induced levels of OsSIKl protein.

For example, the method may involve performing an immunochemical reaction with an antibody that specifically recognizes an OsSIKl protein. Techniques for detecting such antibody-antigen conjugates or complexes are known in the art and include but are not limited to centrifugation, affinity chromatography and other immunochemical methods. See e.g., Ishikawa Ultrasensitive and Rapid Enzyme Immunoassay, 1999, Elsevier, Amsterdam/New York, United States of America; Law, Immunoassay: A Practical Guide, 1996, Taylor & Francis, London/Bristol, Pennsylvania, United States of America; Liddell et al., Antibody Technology, 1995, Bios Scientific Publishers, Oxford, United Kingdom; and references cited therein.

LC. Nucleotide and Amino Acid Sequence Comparisons

The terms identical or percent identity in the context of two or more nucleotide or protein sequences, refer to two or more sequences or subsequences that are the same or have a specified percentage of amino acid residues or nucleotides that are the same, when compared and aligned for maximum correspondence, as measured using one of the sequence comparison algorithms disclosed herein or by visual inspection.

The term substantially identical in regards to a nucleotide or protein sequence means that a particular sequence varies from the sequence of a naturally occurring sequence by one or more deletions, substitutions, or additions, the net effect of which is to retain biological function of an OsSIKl nucleic acid or protein.

For comparison of two or more sequences, typically one sequence acts as a reference sequence to which one or more test sequences are compared. When using a sequence comparison algorithm, test and reference sequences are entered into a computer program, subsequence coordinates are designated if necessary, and sequence algorithm program parameters are selected. The sequence comparison algorithm then calculates the percent sequence identity for the designated test sequence(s) relative to the reference sequence, based on the selected program parameters.

Optimal alignment of sequences for comparison may be conducted, for example, by the local homology algorithm of Smith & Waterman, Adv. Appl. Math, 1981, 2:482-489, by the homology alignment algorithm of Needleman & Wunsch, J. MoI. Biol., 1970, 48:443-453, by the search for similarity method of Pearson & Lipman, Proc. Natl. Acad. Sci. USA, 1988, 85:2444- 2448, by computerized implementations of these algorithms (GAP, BESTFIT, FASTA, and TFASTA in the Wisconsin Genetics Software Package, Genetics Computer Group, Madison, Wisconsin), or by visual inspection. See generally, Ausubel (ed.), Short Protocols in Molecular Biology, 3rd ed., 1995, Wiley, New York.

A useful algorithm for determining percent sequence identity and sequence similarity is the BLAST algorithm, which is described in Altschul et al, J. MoI. Biol, 1990, 215:403-410. Software for performing BLAST analyses is publicly available through the National Center for Biotechnology Information (http://www.ncbi.nlm.nih.gov/). The BLAST algorithm parameters determine the sensitivity and speed of the alignment. For comparison of two nucleotide sequences, the BLASTn default parameters are set at W=I l (wordlength) and E=IO (expectation), and also include use of a low-complexity filter to mask residues of the query sequence having low compositional complexity. For comparison of two amino acid sequences, the BLASTp program default parameters are set at W=3 (wordlength), E=IO (expectation), use of the BLOSUM62 scoring matrix, gap costs of existence=l 1 and extension=l, and use of a low- complexity filter to mask residues of the query sequence having low compositional complexity. II. System for Recombinant Expression of an OsSIKl Protein

The present invention further provides a system for recombinant expression of an OsSIKl protein. Such a system may be used for subsequent purification and/or characterization of an OsSIKl protein. A system for recombinant expression of an OsSIKl protein may also be used for identification of activators, or targets of an OsSIKl protein, as described further herein below. An expression system refers to a host cell comprising a recombinant nucleic acid and the protein encoded by the recombinant nucleic acid. For example, a recombinant expression system may comprise a host cell transfected with a construct comprising an OsSIKl nucleic acid encoding an OsSIKl protein operably linked to a promoter, or a cell line produced by introduction of OsSIKl nucleic acids into a host cell genome. The expression system may further comprise one or more additional recombinant nucleic acids relevant to OsSIKl function, such as targets of OsSIKl activity. These additional nucleic acids may be expressed as a single construct or multiple constructs. A recombinant expression system may be used for heterologous expression (i.e., expression of a nucleic acid in a cell other than the origin of the nucleic acid), including overexpression in a cell line that endogenously expresses a same nucleic acid.

Isolated proteins and recombinantly produced proteins may be purified and characterized using a variety of standard techniques that are known to the skilled artisan. See e.g., Schroder et al, The Peptides, 1965, Academic Press, New York; Bodanszky, Principles of Peptide Synthesis, 2nd rev. ed. 1993, Springer- Verlag, Berlin/ New York; Ausubel (ed.), Short Protocols in Molecular Biology, 3rd ed., 1995, Wiley, New York. Additionally, recombinantly produced proteins may be purified by the addition of tags to the protein. Such tags may include Poly-Arg (RRRRR); Poly-His (HHHHHH); FLAG (DYKDDDDK); Strep-tag II (WSHPQFEK); and, c- myc (EQKLISEEDL).

ILA. Expression Constructs

A construct for expression of an OsSIKl protein may include a vector sequence and an OsSIKl nucleotide sequence, wherein the OsSIKl nucleotide sequence is operably linked to a promoter sequence. A construct for recombinant OsSIKl expression may also comprise transcription termination signals and sequences required for proper translation of the nucleotide sequence. Preparation of an expression construct, including addition of translation and termination signal sequences, is known to one skilled in the art. The promoter may be any polynucleotide sequence which shows transcriptional activity in the chosen plant cells, plant parts, or plants. The promoter may be native or analogous, or foreign or heterologous, to the plant host and/or to the DNA sequence of the invention. Where the promoter is native or endogenous to the plant host, it is intended that the promoter is found in the native plant into which the promoter is introduced. Where the promoter is foreign or heterologous to the DNA sequence of the invention, the promoter is not the native or naturally occurring promoter for the operably linked DNA sequence of the invention. The promoter may be inducible or constitutive. It may be naturally-occurring, may be composed of portions of various naturally-occurring promoters, or may be partially or totally synthetic. Guidance for the design of promoters is provided by studies of promoter structure, such as that of Harley et al, Nucleic Acids Res., 1987, 15:2343-61. Also, the location of the promoter relative to the transcription start may be optimized. See e.g., Roberts et al., Proc. Natl. Acad. Sci. USA, 1979, 76:760-4. Many suitable promoters for use in plants are well known in the art.

For example, suitable constitutive promoters for use in plants include the promoters from plant viruses, such as the peanut chlorotic streak caulimovirus (PClSV) promoter (U.S. Patent No. 5,850,019); the 35S promoter from cauliflower mosaic virus (CaMV) (Odell et al., Nature, 1985, 313:810-812); promoters of Chlorella virus methyltransferase genes (U.S. Patent No. 5,563,328) and the full-length transcript promoter from figwort mosaic virus (FMV) (U.S. Patent No. 5,378,619); the promoters from such genes as rice actin (McElroy et al. (1990) Plant Cell 2:163-171); ubiquitin (Christensen et al., Plant MoI. Biol., 1989, 12:619-632 and Christensen et al., Plant MoI. Biol., 1992, 18:675-689); pEMU (Last et al., Theor. Appl. Genet., 1991, 81 :581- 588); MAS (Velten et al., EMBO J, 1984, 3:2723-2730); maize H3 histone (Lepetit et al., MoI. Gen. Genet., 1992, 231 :276-285 and Atanassova et al., Plant J., 1992, 2(3):291-300); Brassica napus ALS3 (PCT International Publication No. WO 97/41228); and promoters of various Agrobacterium genes (see U.S. Patent Nos. 4,771,002; 5,102,796; 5,182,200; and 5,428,147).

Suitable inducible promoters for use in plants include the promoter from the ACEl system which responds to copper (Mett et al., Proc. Natl. Acad. Sci. USA, 1993, 90:4567-4571); the promoter of the maize In2 gene which responds to benzenesulfonamide herbicide safeners (Hershey et al., MoI. Gen. Genetics, 1991, 227:229-237 and Gatz et al., MoI. Gen. Genetics, 1994, 243:32-38); and the promoter of the Tet repressor from TnIO (Gatz et al., MoI. Gen. Genet., 1991, 227:229-237). Another inducible promoter for use in plants is one that responds to an inducing agent to which plants do not normally respond. An exemplary inducible promoter of this type is the inducible promoter from a steroid hormone gene, the transcriptional activity of which is induced by a glucocorticosteroid hormone (Schena et al., Proc. Natl. Acad. Sci. USA, 1991, 88:10421) or the recent application of a chimeric transcription activator, XVE, for use in an estrogen receptor-based inducible plant expression system activated by estradiol (Zuo et al., Plant J, 2000, 24:265-273). Other inducible promoters for use in plants are described in EP 332104, PCT International Publication Nos. WO 93/21334 and WO 97/06269. Promoters composed of portions of other promoters and partially or totally synthetic promoters can also be used. See e.g., Ni et al., Plant J., 1995, 7:661-676 and PCT International Publication No. WO 95/14098 describing such promoters for use in plants.

The promoter may include, or be modified to include, one or more enhancer elements to thereby provide for higher levels of transcription. Suitable enhancer elements for use in plants include the PClSV enhancer element (U.S. Patent No. 5,850,019), the CaMV 35S enhancer element (U.S. Patent Nos. 5,106,739 and 5,164,316) and the FMV enhancer element (Maiti et al., Transgenic Res., 1997, 6:143-156). See also PCT International Publication No. WO 96/23898.

Such constructs can contain a 'signal sequence' or 'leader sequence' to facilitate co- translational or post-translational transport of the peptide of interest to certain intracellular structures such as the chloroplast (or other plastid), endoplasmic reticulum, or Golgi apparatus, or to be secreted. For example, the construct can be engineered to contain a signal peptide to facilitate transfer of the peptide to the endoplasmic reticulum. A signal sequence is known or suspected to result in cotranslational or post-translational peptide transport across the cell membrane. In eukaryotes, this typically involves secretion into the Golgi apparatus, with some resulting glycosylation. A leader sequence refers to any sequence that, when translated, results in an amino acid sequence sufficient to trigger co-translational transport of the peptide chain to a sub-cellular organelle. Thus, this includes leader sequences targeting transport and/or glycosylation by passage into the endoplasmic reticulum, passage to vacuoles, plastids including chloroplasts, mitochondria, and the like. Plant expression cassettes may also contain an intron, such that mRNA processing of the intron is required for expression.

Such constructs can also contain 5' and 3' untranslated regions. A 3' untranslated region is a polynucleotide located downstream of a coding sequence. Polyadenylation signal sequences and other sequences encoding regulatory signals capable of affecting the addition of polyadenylic acid tracts to the 3' end of the mRNA precursor are 3' untranslated regions. A 5' untranslated region is a polynucleotide located upstream of a coding sequence.

The termination region may be native with the transcriptional initiation region, may be native with the sequence of the present invention, or may be derived from another source. Convenient termination regions are available from the Ti-plasmid of A. tumefaciens, such as the octopine synthase and nopaline synthase termination regions, or the termination region of a plant gene, such as soybean storage protein. See also Guerineau et al, MoI Gen. Genet., 1991, 262:141-144; Proudfoot, Cell, 1991, 64:671-674; Sanfacon et al., Genes Dev. 1991, 5:141-149; Mogen et al., Plant Cell, 1990, 2:1261-1272; Munroe et al., Gene, 1990, 91 :151-158; Ballas et al., Nucleic Acids Res., 1989, 17:7891-7903; and Joshi et al., Nucleic Acid Res., 1987, 15:9627- 9639.

Where appropriate, the vector and OsSIKl sequences may be optimized for increased expression in the transformed host cell. That is, the sequences can be synthesized using host cell-preferred codons for improved expression, or may be synthesized using codons at a host- preferred codon usage frequency. Generally, the GC content of the polynucleotide will be increased. See e.g., Campbell et al., Plant Physiol, 1990, 92:1-11 for a discussion of host- preferred codon usage. Methods are known in the art for synthesizing host-preferred polynucleotides. See e.g., U.S. Patent Nos. 6,320,100; 6,075,185; 5,380,831; and 5,436,391, U.S. Published Application Nos. 20040005600 and 20010003849, and Murray et al., Nucleic Acids Res., 1989, 17:477-498, herein incorporated by reference.

For example, polynucleotides of interest can be targeted to the chloroplast for expression. In this manner, where the polynucleotide of interest is not directly inserted into the chloroplast, the expression cassette may additionally contain a polynucleotide encoding a transit peptide to direct the nucleotide of interest to the chlorop lasts. Such transit peptides are known in the art. See e.g., Von Heijne et al., Plant MoI. Biol. Rep., 1991, 9:104-126; Clark et al. J. Biol. Chem., 1989, 264:17544-17550; Della-Cioppa et al., Plant Physiol, 1987, 84:965-968; Romer et al., Biochem. Biophys. Res. Commun., 1993, 196:1414-1421; and Shah et al., Science, 1986, 233 :478-481. The polynucleotides of interest to be targeted to the chloroplast may be optimized for expression in the chloroplast to account for differences in codon usage between the plant nucleus and this organelle. In this manner, the polynucleotides of interest may be synthesized using chloroplast-preferred codons. See e.g., U.S. Patent No. 5,380,831, herein incorporated by reference.

A plant expression cassette {i.e., an OsSIKl open reading frame operably linked to a promoter) can be inserted into a plant transformation vector, which allows for the transformation of DNA into a cell. Such a molecule may consist of one or more expression cassettes, and may be organized into more than one vector DNA molecule. For example, binary vectors are plant transformation vectors that utilize two non-contiguous DNA vectors to encode all requisite cis- and trans-acting functions for transformation of plant cells (Hellens et al., Trends in Plant Science, 2000, 5:446-451).

A plant transformation vector comprises one or more DNA vectors for achieving plant transformation. For example, it is a common practice in the art to utilize plant transformation vectors that comprise more than one contiguous DNA segment. These vectors are often referred to in the art as binary vectors. Binary vectors as well as vectors with helper plasmids are most often used for Agrobacterium-mediated transformation, where the size and complexity of DNA segments needed to achieve efficient transformation is quite large, and it is advantageous to separate functions onto separate DNA molecules. Binary vectors typically contain a plasmid vector that contains the cis-acting sequences required for T-DNA transfer (such as left border and right border), a selectable marker that is engineered to be capable of expression in a plant cell, and a polynucleotide of interest (i.e., a polynucleotide engineered to be capable of expression in a plant cell for which generation of transgenic plants is desired). Also present on this plasmid vector are sequences required for bacterial replication. The cis-acting sequences are arranged in a fashion to allow efficient transfer into plant cells and expression therein. For example, the selectable marker sequence and the sequence of interest are located between the left and right borders. Often a second plasmid vector contains the trans-acting factors that mediate T- DNA transfer from Agrobαcterium to plant cells. This plasmid often contains the virulence functions (Vir genes) that allow infection of plant cells by Agrobαcterium, and transfer of DNA by cleavage at border sequences and vir-mediated DNA transfer, as in understood in the art (Hellens et al., 2000). Several types of Agrobαcterium strains (e.g., LBA4404, GV3101, EHAlOl, EHA 105, etc.) can be used for plant transformation. The second plasmid vector is not necessary for introduction of polynucleotides into plants by other methods such as microprojection, microinjection, electroporation, polyethylene glycol, etc. ILB. Host Cells

Host cells are cells into which a recombinant nucleic acid molecule of the invention may be introduced. Representative eukaryotic host cells include yeast and plant cells, as well as prokaryotic hosts such as E. coli and Bacillus subtilis. Preferred host cells for functional assays substantially or completely lack endogenous expression of an OsSIKl protein.

A host cell strain may be chosen which affects the expression of the recombinant sequence, or modifies and processes the gene product in a specific manner. For example, different host cells have characteristic and specific mechanisms for the translational and post- translational processing and modification {e.g., glycosylation, phosphorylation of proteins). Appropriate cell lines or host cells may be chosen to ensure the desired modification and processing of the foreign protein expressed. For example, expression in a bacterial system may be used to produce a non-glycosylated core protein product, and expression in yeast will produce a glycosylated product.

The present invention further encompasses recombinant expression of an OsSIKl protein in a stable cell line. Methods for generating a stable cell line following transformation of a heterologous construct into a host cell are known in the art. See e.g., Joyner, Gene Targeting: A Practical Approach, 1993, Oxford University Press, Oxford/New York. Thus, transformed cells, tissues, and plants are understood to encompass not only the end product of a transformation process, but also transgenic progeny or propagated forms thereof.

III. OsSIKl Transgenic Plants

The present invention also provides transgenic plants comprising an overexpressed OsSIKl nucleic acid and protein, including conditional or inducible expression of OsSIKl . In particular, OsSIKl transgenic plants show increased stress tolerance as compared to wild type plants, including increased tolerance to drought and high salt concentrations (see Examples 6 and 7). OsSIKl transgenic plants may also be used to increase stress tolerance to other abiotic stress conditions, such as high light intensity, low- and high-temperature, continuous light, continuous dark, and mechanical wounding.

OsSIKl transgenic plants may be prepared in monocot or dicot plants, for example, in corn (maize), sorghum, wheat, sunflower, tomato, crucifers, peppers, potato, cotton, rice, soybean, sugarbeet, sugarcane, tobacco, barley, and oilseed rape, Brassica sp., alfalfa, rye, millet, safflower, peanuts, sweet potato, cassava, coffee, coconut, pineapple, citrus trees, cocoa, tea, banana, avocado, fig, guava, mango, olive, papaya, cashew, macadamia, almond, oats, vegetables, ornamentals, and conifers. Representative vegetables include tomatoes, lettuce, green beans, lima beans, peas, yams, onion, and members of the genus Curcumis such as cucumber, cantaloupe, and musk melon. Ornamentals include, but are not limited to, azalea, hydrangea, hibiscus, roses, tulips, daffodils, petunias, carnation, poinsettia, and chrysanthemum. As used herein, a plant refers to a whole plant, a plant organ (e.g., leaves, stems, roots, etc.), a seed, a plant cell, a propagule, an embryo, and progeny of the same. Plant cells can be differentiated or undifferentiated (e.g., callus, suspension culture cells, protoplasts, leaf cells, root cells, phloem cells, pollen).

The OsSIKl transgenic plants may be further modified at a locus other than OsSIKl to confer increased stress tolerance or other trait of interest. Representative desired traits include improved crop yield; increased seed yield; increased amino acid content; increased nitrate content; increased tolerance to stress; insect resistance; tolerance to broad-spectrum herbicides; resistance to diseases caused by viruses, bacteria, fungi, and worms; and enhancement of mechanisms for protection from environmental stresses such as heat, cold, drought, and high salt concentration. Additional desired traits include output traits that benefit consumers, for example, nutritionally enhanced foods that contain more starch or protein, more vitamins, more antioxidants, and/or fewer trans-fatty acids; foods with improved taste, increased shelf-life, and better ripening characteristics; trees that make it possible to produce paper with less environmental damage; nicotine-free tobacco; ornamental flowers with new colors, fragrances, and increased longevity; etc. Still further, desirable traits that may be used in accordance with the invention include gene products produced in plants as a means for manufacturing, for example, therapeutic proteins for disease treatment and vaccination; textile fibers; biodegradable plastics; oils for use in paints, detergents, and lubricants; etc. For genetic modifications that confer traits associated with altered gene expression, or increased stress tolerance, the combination of an OsSIKl overexpressing plant and a second genetic modification can produce a synergistic effect, i.e., a change in gene expression, or increased stress tolerance that is greater than the change elicited by either genetic modification alone.

For preparation of an OsSIKl transgenic plant, introduction of a polynucleotide into plant cells is accomplished by one of several techniques known in the art, including but not limited to electroporation or chemical transformation (See e.g., Ausubel, ed. (1994) Current Protocols in Molecular Biology, John Wiley and Sons, Inc., Indianapolis, Indiana. Markers conferring resistance to toxic substances are useful in identifying transformed cells (having taken up and expressed the test polynucleotide sequence) from non-transformed cells (those not containing or not expressing the test polynucleotide sequence). In one aspect of the invention, genes are useful as a marker to assess introduction of DNA into plant cells. Transgenic plants, transformed plants, or stably transformed plants, or cells, tissues or seed of any of the foregoing, refer to plants that have incorporated or integrated exogenous polynucleotides into the plant cell. Stable transformation refers to introduction of a polynucleotide construct into a plant such that it integrates into the genome of the plant and is capable of being inherited by progeny thereof.

In general, plant transformation methods involve transferring heterologous DNA into target plant cells (e.g., immature or mature embryos, suspension cultures, undifferentiated callus, protoplasts, etc.), followed by applying a maximum threshold level of appropriate selection (depending on the selectable marker gene) to recover the transformed plant cells from a group of untransformed cell mass. Explants are typically transferred to a fresh supply of the same medium and cultured routinely. Subsequently, the transformed cells are differentiated into shoots after placing on regeneration medium supplemented with a maximum threshold level of selecting agent (i.e., temperature and/or herbicide). The shoots are then transferred to a selective rooting medium for recovering rooted shoot or plantlet. The transgenic plantlet then grow into mature plant and produce fertile seeds (e.g., Hiei et al., Plant J., 1994, 6:271-282; Ishida et al., Nat. BiotechnoL, 1996, 14:745-750). A general description of the techniques and methods for generating transgenic plants are found in Ayres et al., CRC Crit. Rev. Plant Sci., 1994. 13:219- 239, and Bommineni et al., Maydica, 1997, 42:107-120. Since the transformed material contains many cells, both transformed and non-transformed cells are present in any piece of subjected target callus or tissue or group of cells. The ability to kill non-transformed cells and allow transformed cells to proliferate results in transformed plant cultures. Often, the ability to remove non-transformed cells is a limitation to rapid recovery of transformed plant cells and successful generation of transgenic plants. Then molecular and biochemical methods can be used for confirming the presence of the integrated nucleotide(s) of interest in the genome of transgenic plant. For example, selectable markers, such as, enzymes leading to changes of colors or luminescent molecules (e.g. , GUS and luciferase), antibiotic-resistant genes (e.g. , gentamicin and kanamycin-resistance genes) and chemical-resistant genes (e.g. , herbicide-resistance genes) may be used to confirm the integration of the nucleotide(s) of interest in the genome of transgenic plant. Alternatively, considering safety of the transgenic plants, the transformed plants can be selected under environmental stresses avoiding incorporation of any selectable marker genes.

Generation of transgenic plants may be performed by one of several methods, including but not limited to introduction of heterologous DNA by Agrobacterium into plant cells (Agrobacterium -mediated transformation), bombardment of plant cells with heterologous foreign DNA adhered to particles, and various other non-particle direct-mediated methods, including microinjection, electroporation, application of Ti plasmid, Ri plasmid, or plant virus vector and direct DNA transformation (e.g., Hiei et al, Plant J, 1994, 6:271-282; Ishida et al, Nat. Biotechnol, 1996, 14:745-750; Ayres et al., CRC Crit. Rev. Plant ScL, 1994, 13:219-239; Bommineni et al., Maydica, 1997, 42:107-120) to transfer DNA.

There are three common methods to transform plant cells with Agrobacterium. The first method is co-cultivation of Agrobacterium with cultured isolated protoplasts. This method requires an established culture system that allows culturing protoplasts and plant regeneration from cultured protoplasts. The second method is transformation of cells or tissues with Agrobacterium. This method requires (a) that the plant cells or tissues can be transformed by Agrobacterium and (b) that the transformed cells or tissues can be induced to regenerate into whole plants. The third method is transformation of seeds, apices or meristems with Agrobacterium. This method requires micropropagation.

The efficiency of transformation by Agrobacterium may be enhanced by using a number of methods known in the art. For example, the inclusion of a natural wound response molecule such as acetosyringone (AS) to the Agrobacterium culture has been shown to enhance transformation efficiency with Agrobacterium tumefaciens (Shahla et al., Plant Molec. Biol, 1987, 8:291-298). Alternatively, transformation efficiency may be enhanced by wounding the target tissue to be transformed. Wounding of plant tissue may be achieved, for example, by punching, maceration, bombardment with microprojectiles, etc. See e.g., Bidney et al., Plant Molec. Biol., 1992, 18:301-313.

In another aspect of the invention, the plant cells are transfected with vectors via particle bombardment (i.e., with a gene gun). Particle mediated gene transfer methods are known in the art, are commercially available, and include, but are not limited to, the gas driven gene delivery instrument described in U.S. Patent No. 5,584,807, the entire contents of which are herein incorporated by reference. This method involves coating the polynucleotide sequence of interest onto heavy metal particles, and accelerating the coated particles under the pressure of compressed gas for delivery to the target tissue.

Other particle bombardment methods are also available for the introduction of heterologous polynucleotide sequences into plant cells. Generally, these methods involve depositing the polynucleotide sequence of interest upon the surface of small, dense particles of a material such as gold, platinum, or tungsten. The coated particles are themselves then coated onto either a rigid surface, such as a metal plate, or onto a carrier sheet made of a fragile material such as mylar. The coated sheet is then accelerated toward the target biological tissue. The use of the flat sheet generates a uniform spread of accelerated particles that maximizes the number of cells receiving particles under uniform conditions, resulting in the introduction of the polynucleotide sample into the target tissue.

Specific initiation signals may also be used to achieve more efficient translation of sequences encoding the polypeptide of interest. Such signals include the ATG initiation codon and adjacent sequences. In cases where sequences encoding the polypeptide of interest, its initiation codon, and upstream sequences are inserted into the appropriate expression vector, no additional transcriptional or translational control signals may be needed. However, in cases where only coding sequence, or a portion thereof, is inserted, exogenous translational control signals including the ATG initiation codon should be provided. Furthermore, the initiation codon should be in the correct reading frame to ensure translation of the entire insert. Exogenous translational elements and initiation codons may be of various origins, both natural and synthetic. The efficiency of expression may be enhanced by the inclusion of enhancers that are appropriate for the particular cell system that is used, such as those described in the literature (Scharf et al., Results Probl. Cell Differ., 1994, 20:125).

In another aspect of the present invention, at least one genomic copy corresponding to a nucleotide sequence of the present invention is modified in the genome of the plant by homologous recombination as further illustrated in Paszkowski et al., EMBO Journal, 1988, 7:4021-26. This technique uses the property of homologous sequences to recognize each other and to exchange nucleotide sequences between each by a process known in the art as homologous recombination. Homologous recombination can occur between the chromosomal copy of a nucleotide sequence in a cell and an incoming copy of the nucleotide sequence introduced in the cell by transformation. Specific modifications are thus accurately introduced in the chromosomal copy of the nucleotide sequence. In one aspect, the regulatory elements of the nucleotide sequences of the present invention are modified. Such regulatory elements are easily obtainable by screening a genomic library using the nucleotide sequences of the present invention, or a portion thereof, as a probe. The existing regulatory elements are replaced by different regulatory elements, thus altering expression of the nucleotide sequence.

The cells that have been transformed may be grown into plants in accordance with conventional ways. See e.g., McCormick et al., Plant Cell Rep., 1986, 5:81-84. These plants may then be grown, and either pollinated with the same transformed strain or different strains, and the resulting hybrid having constitutive expression of the desired phenotypic characteristic identified. Two or more generations may be grown to ensure that expression of the desired phenotypic characteristic is stably maintained and inherited and then seeds harvested to ensure expression of the desired phenotypic characteristic has been achieved. In this manner, the present invention provides transformed seed (also referred to as transgenic seed) having a polynucleotide of the invention, for example, an expression cassette of the invention, stably incorporated into their genome.

Transgenic plants of the invention can be homozygous for the added polynucleotides; i.e., a transgenic plant that contains two added sequences, one sequence at the same locus on each chromosome of a chromosome pair. A homozygous transgenic plant can be obtained by sexually mating (selfing) an independent segregant transgenic plant that contains the added sequences according to the invention, germinating some of the seed produced and analyzing the resulting plants produced for enhanced enzyme activity (i.e., herbicide resistance) and/or increased plant yield relative to a control (native, non-transgenic) or an independent segregant transgenic plant.

It is to be understood that two different transgenic plants can also be mated to produce offspring that contain two independently segregating added, exogenous polynucleotides. Selfing of appropriate progeny can produce plants that are homozygous for all added, exogenous polynucleotides that encode a polypeptide of the present invention. Back-crossing to a parental plant and outcrossing with a non-transgenic plant are also contemplated.

Following introduction of DNA into plant cells, the transformation or integration of the polynucleotide into the plant genome is confirmed by various methods such as analysis of polynucleotides, polypeptides and metabolites associated with the integrated sequence.

IV. OsSIKl Binding Partners and Activators

The present invention further discloses assays to identify OsSIKl binding partners and OsSIKl activators. OsSIKl activators are agents that alter chemical and biological activities or properties of an OsSIKl protein. Such chemical and biological activities and properties may include, but are not limited to, OsSIKl nucleic acid expression levels and expression levels of nucleic acids subject to OsSIKl regulation. Methods of identifying activators involve assaying an enhanced level or quality of OsSIKl function in the presence of one or more test agents. Representative OsSIKl activators include small molecules as well as biological entities, as described herein below.

A control level or quality of OsSIKl activity refers to a level or quality of wild type OsSIKl activity, for example, when using a recombinant expression system comprising expression of SEQ ID NO: 2. When evaluating the activating capacity of a test agent, a control level or quality of OsSIKl activity comprises a level or quality of activity in the absence of the test agent.

Significantly changed activity of an OsSIKl protein refers to a quantifiable change in a measurable quality that is larger than the margin of error inherent in the measurement technique. For example, significant enhancement refers to OsSIKl activity that is increased by about 2-fold or greater relative to a control measurement, or an about 5 -fold or greater increase, or an about 10-fold or greater increase. An assay of OsSIKl function may comprise determining a level of OsSIKl expression; determining DNA binding activity of a recombinantly expressed OsSIKl protein; determining transcriptional regulatory activity of an OsSIKl protein; determining an active conformation of an OsSIKl protein; or determining activation of signaling events in response to binding of an OsSIKl activator (e.g., increased stress tolerance).

In accordance with the present invention there is also provided a rapid and high throughput screening method that relies on the methods described herein. This screening method comprises separately contacting an OsSIKl protein with a plurality of test agents. In such a screening method the plurality of test agents may comprise more than about 10⁴ samples, or more than about 10⁵ samples, or more than about 10⁶ samples.

The in vitro and cellular assays of the invention may comprise soluble assays, or may further comprise a solid phase substrate for immobilizing one or more components of the assay. For example, an OsSIKl protein, or a cell expressing an OsSIKl protein, may be bound directly to a solid state component via a covalent or non-covalent linkage. Optionally, the binding may include a linker molecule or tag that mediates indirect binding of an OsSIKl protein to a substrate.

Cellular assays, including expression assays and functional assays as described herein below, may employ a cell that endogenously expresses OsSIKl, a cell that expresses a heterologous OsSIKl nucleic acid but otherwise lacks endogenous OsSIKl expression, or a cell that expresses endogenous OsSIKl as well as a heterologous OsSIKl nucleic acid.

IV.A. Test Agents

A test agent refers to any agent that potentially interacts with an OsSIKl nucleic acid or protein, including any synthetic, recombinant, or natural product. A test agent suspected to interact with a protein may be evaluated for such an interaction using the methods disclosed herein.

Representative test agents include but are not limited to peptides, proteins, nucleic acids, small molecules {e.g., organic and inorganic chemical compounds), antibodies or fragments thereof, nucleic acid-protein fusions, any other affinity agent, and combinations thereof. A test agent to be tested may be a purified molecule, a homogenous sample, or a mixture of molecules or compounds.

A small molecule refers to a compound, for example an organic compound, with a molecular weight of less than about 1,000 daltons, such as less than about 750 daltons, or less than about 600 daltons, or less than about 500 daltons. A small molecule may have a computed log octanol-water partition coefficient in the range of about -4 to about +14, such as in the range of about -2 to about +7.5.

Test agents may be obtained or prepared as a library or collection of molecules. A library may contain a few or a large number of different molecules, varying from about ten molecules to several billion molecules or more. A molecule may comprise a naturally occurring molecule, a recombinant molecule, or a synthetic molecule. A plurality of test agents in a library may be assayed simultaneously. Optionally, test agents derived from different libraries may be pooled for simultaneous evaluation.

Representative libraries include but are not limited to a peptide library (U.S. Patent Nos. 6,156,511, 6,107,059, 5,922,545, and 5,223,409), an oligomer library (U.S. Patent Nos. 5,650,489 and 5,858,670), an aptamer library (U.S. Patent Nos. 7,338,762; 7,329,742; 6,949,379; 6,180,348; and 5,756,291), a small molecule library (U.S. Patent Nos. 6,168,912 and 5,738,996), a library of antibodies or antibody fragments (U.S. Patent Nos. 6,174,708, 6,057,098, 5,922,254, 5,840,479, 5,780,225, 5,702,892, and 5,667988), a library of nucleic acid-protein fusions (U.S. Patent No. 6,214,553), and a library of any other affinity agent that may potentially bind to an OsSIKl protein.

A library may comprise a random collection of molecules. Alternatively, a library may comprise a collection of molecules having a bias for a particular sequence, structure, or conformation, for example, as for inhibitory nucleic acids. See e.g., U.S. Patent Nos. 5,264,563 and 5,824,483. Methods for preparing libraries containing diverse populations of various types of molecules are known in the art, for example as described in U.S. patents cited herein above. Numerous libraries are also commercially available.

IV.B. Expression Assays

In one aspect of the invention, an activator of an OsSIKl protein may be identified by assaying expression of an OsSIKl nucleic acid. For example, a method of identifying an OsSIKl activator useful for promoting stress tolerance may include the steps of (a) providing a cell recombinantly expressing an OsSIKl protein; (b) contacting the cell with one or more test agents or a control agent; (c) assaying expression of an OsSIKl nucleic acid or protein; and (d) selecting a test agent that induces elevated expression of the OsSIKl nucleic acid or nucleic acid when the cell is contacted with the test agent as compared to the control agent.

IV. C. Binding Assays

In another aspect of the invention, a method of identifying of an OsSIKl activator comprises determining specific binding of a test agent to an OsSIKl protein. For example, a method of identifying an OsSIKl binding partner may comprise: (a) providing an OsSIKl protein of SEQ ID NO: 2; (b) contacting the OsSIKl protein with one or more test agents under conditions sufficient for binding; (c) assaying binding of a test agent to the isolated OsSIKl protein; and (d) selecting a test agent that demonstrates specific binding to the OsSIKl protein. Specific binding may also encompass a quality or state of mutual action such that binding of a test agent to an OsSIKl protein is activating or inducing. The activating nature of specific binding can be assessed by the additional assays of OsSIKl function described herein. Specific binding refers to a binding reaction which is determinative of the presence of the protein in a heterogeneous population of proteins and other biological materials. The binding of a test agent to an OsSIKl protein may be considered specific if the binding affinity is about IxIO⁴M^"1 to about IxIO⁶M^"1 or greater. Specific binding also refers to saturable binding. To demonstrate saturable binding of a test agent to an OsSIKl protein, Scatchard analysis may be carried out as described, for example, by Mak et al., J. Biol. Chem., 1989, 264:21613-21618.

Several techniques may be used to detect interactions between an OsSIKl protein and a test agent without employing a known competitive inhibitor. Representative methods include, but are not limited to, Fluorescence Correlation Spectroscopy, Surface-Enhanced Laser Desorption/Ionization Time-Of-flight Spectroscopy, and BIACORE® technology, each technique described herein below. These methods are amenable to automated, high-throughput screening.

Fluorescence Correlation Spectroscopy (FCS) measures the average diffusion rate of a fluorescent molecule within a small sample volume. The sample size may be as low as 10³ fluorescent molecules and the sample volume as low as the cytoplasm of a single bacterium. The diffusion rate is a function of the mass of the molecule and decreases as the mass increases. FCS may therefore be applied to protein-ligand interaction analysis by measuring the change in mass and therefore in diffusion rate of a molecule upon binding. In a typical experiment, the target to be analyzed {e.g., an OsSIKl protein) is expressed as a recombinant protein with a sequence tag, such as a poly-histidine sequence, inserted at the N-terminus or C-terminus. The expression is mediated in a host cell, such as E. coli, yeast, Xenopus oocytes, or mammalian cells. The protein is purified using chromatographic methods. For example, the poly-histidine tag may be used to bind the expressed protein to a metal chelate column such as Ni2+ chelated on iminodiacetic acid agarose. The protein is then labeled with a fluorescent tag such as carboxytetramethylrhodamine or BODIP Y™ reagent (available from Molecular Probes of Eugene, Oregon). The protein is then exposed in solution to the potential ligand, and its diffusion rate is determined by FCS using instrumentation available from Carl Zeiss, Inc. (Thornwood of New York, New York). Ligand binding is determined by changes in the diffusion rate of the protein.

Surface-Enhanced Laser Desorption/Ionization (SELDI) was developed by Hutchens & Yip, Rapid Commun. Mass Spectrom., 1993, 7:576-580. When coupled to a time-of-flight mass spectrometer (TOF), SELDI provides a technique to rapidly analyze molecules retained on a chip. It may be applied to ligand-protein interaction analysis by covalently binding the target protein, or portion thereof, on the chip and analyzing by mass spectrometry the small molecules that bind to this protein (Worrall et al, Anal Chem., 1998, 70(4):750-756). In a typical experiment, a target protein (e.g., an OsSIKl protein) is recombinantly expressed and purified. The target protein is bound to a SELDI chip either by utilizing a poly-histidine tag or by other interaction such as ion exchange or hydrophobic interaction. A chip thus prepared is then exposed to the potential ligand via, for example, a delivery system able to pipet the ligands in a sequential manner (autosampler). The chip is then washed in solutions of increasing stringency, for example a series of washes with buffer solutions containing an increasing ionic strength. After each wash, the bound material is analyzed by submitting the chip to SELDI-TOF. Ligands that specifically bind a target protein are identified by the stringency of the wash needed to elute them.

BIACORE® relies on changes in the refractive index at the surface layer upon binding of a ligand to a target protein (e.g., an OsSIKl protein) immobilized on the layer. In this system, a collection of small ligands is injected sequentially in a 2-5 microliter cell, wherein the target protein is immobilized within the cell. Binding is detected by surface plasmon resonance (SPR) by recording laser light refracting from the surface. In general, the refractive index change for a given change of mass concentration at the surface layer is practically the same for all proteins and peptides, allowing a single method to be applicable for any protein. In a typical experiment, a target protein is recombinantly expressed, purified, and bound to a BIACORE® chip. Binding may be facilitated by utilizing a poly-histidine tag or by other interaction such as ion exchange or hydrophobic interaction. A chip thus prepared is then exposed to one or more potential ligands via the delivery system incorporated in the instruments sold by Biacore (Uppsala, Sweden) to pipet the ligands in a sequential manner (autosampler). The SPR signal on the chip is recorded and changes in the refractive index indicate an interaction between the immobilized target and the ligand. Analysis of the signal kinetics of on rate and off rate allows the discrimination between non-specific and specific interaction. See also Homola et al., Sensors and Actuators, 1999, 54:3-15 and references therein.

IV.D. Conformational Assay

The present invention also provides methods of identifying OsSIKl binding partners and activators that rely on a conformational change of an OsSIKl protein when bound by or otherwise interacting with a test agent. For example, application of circular dichroism to solutions of macromolecules reveals the conformational states of these macromolecules. The technique may distinguish random coil, alpha helix, and beta chain conformational states.

To identify binding partners and activators of an OsSIKl protein, circular dichroism analysis may be performed using a recombinantly expressed OsSIKl protein. An OsSIKl protein is purified, for example by ion exchange and size exclusion chromatography, and mixed with a test agent. The mixture is subjected to circular dichroism. The conformation of an OsSIKl protein in the presence of a test agent is compared to a conformation of an OsSIKl protein in the absence of the test agent. A change in conformational state of an OsSIKl protein in the presence of a test agent identifies an OsSIKl binding partner or activator. Representative methods are described in U.S. Patent Nos. 5,776,859 and 5,780,242. Activity of the binding partner or activator may be assessed using functional assays, such assays include nitrate content, nitrate uptake, lateral root growth, seed yield, amino acid content or plant biomass, as described herein.

IV.E. Functional Assays

In another aspect of the invention, a method of identifying an OsSIKl activator employs a functional OsSIKl protein, for example, an OsSIKl protein having an amino acid sequence as set forth in SEQ ID NO: 2. Representative methods for determining OsSIKl function include assaying a physiological change elicited by OsSIKl activity, such as transcriptional regulatory activity and increased stress tolerance (see e.g., Examples 6 and 7).

With respect to determining expression of an OsSIKl target gene, an OsSIKl activator is an agent that increases the normal transcriptional regulatory activity of an OsSIKl protein, including both transcriptional activating and transcriptional repressive activities. Thus, a method of identifying an OsSIKl activator useful for promoting stress tolerance may also include the steps of (a) providing a cell recombinantly expressing an OsSIKl protein; (b) contacting the cell with one or more test agents or a control agent; (c) assaying expression of an OsSIKl target gene, which normally functions as a positive regulator of a stress response and is subject to transcriptional activation by an OsSIKl protein; and (d) selecting a test agent that induces elevated expression of the target gene when the cell is contacted with the test agent as compared to the control agent. As another example, a method of identifying an OsSIKl activator useful for promoting stress tolerance may also include the steps of (a) providing a cell recombinantly expressing an OsSIKl protein; (b) contacting the cell with one or more test agents or a control agent; (c) assaying expression of an OsSIKl target gene, which normally functions as a negative regulator of a stress response and is subject to transcriptional repression by an OsSIKl protein; and (d) selecting a test agent that induces reduced expression of the target gene when the cell is contacted with the test agent as compared to the control agent.

Assays of OsSIKl activity that employ transiently transfected cells may include a marker that distinguishes transfected cells from non-transfected cells. A marker may be encoded by or otherwise associated with a construct for OsSIKl expression, such that cells are simultaneously transfected with a nucleic acid molecule encoding OsSIKl and the marker. Representative detectable molecules that are useful as markers include but are not limited to a heterologous nucleic acid, a protein encoded by a transfected construct (e.g., an enzyme or a fluorescent protein), a binding protein, and an antigen. In accordance with the disclosed methods, cells expressing OsSIKl may be provided in the form of a kit useful for performing an assay of OsSIKl function. For example, a test kit for detecting an OsSIKl activator may include cells transfected with DNA encoding a full-length OsSIKl protein and a medium for growing the cells.

A method of identifying an OsSIKl activator may also comprise (a) providing a plant expressing an OsSIKl protein; (b) contacting the plant with one or more test agents or a control agent; (c) assaying survival of the plants under abiotic stress; and (d) selecting a test agent that promotes survival of the plants under abiotic stress when in the presence of the test agent as compared to the control agent.

Assays employing cells expressing OsSIKl or plants expressing OsSIKl may additionally employ control cells or plants that are substantially devoid of native OsSIKl and, optionally, proteins substantially similar to an OsSIKl protein. When using transiently transfected cells, a control cell may comprise, for example, an untransfected host cell. When using a stable cell line expressing an OsSIKl protein, a control cell may comprise, for example, a parent cell line used to derive the OsSIKl -expressing cell line. When using plants, a negative control may comprise a plant transformed with vector lacking an OsSIKl transgene. Also when using plants, a positive control plant may include an OsSIKl overexpressing plant, such that an OsSIKl activator elicits a phenotype similar to an OsSIKl overexpressing plant. IV.F. Rational Design

The knowledge of the structure of a native OsSIKl protein provides an approach for rational design of OsSIKl activators. In brief, the structure of an OsSIKl protein may be determined by X-ray crystallography and/or by computational algorithms that generate three- dimensional representations. See Saqi et al., Bioinformatics, 1999, 15:521-522; Huang et al., Pac. Symp. Biocomput, 2000, 230-241; and PCT International Publication No. WO 99/26966. Alternatively, a working model of an ENOD93 protein structure may be derived by homology modeling (Maalouf et al., J. Biomol Struct. Dyn., 1998, 15(5):841-851). Computer models may further predict binding of a protein structure to various substrate molecules that may be synthesized and tested using the assays described herein above. Additional compound design techniques are described in U.S. Patent Nos. 5,834,228 and 5,872,011.

An OsSIKl protein is a soluble protein, which may be purified and concentrated for crystallization. The purified OsSIKl protein may be crystallized under varying conditions of at least one of the following: pH, buffer type, buffer concentration, salt type, polymer type, polymer concentration, other precipitating ligands, and concentration of purified OsSIKl. Methods for generating a crystalline protein are known in the art and may be reasonably adapted for determination of an OsSIKl protein as disclosed herein. See e.g., Deisenhofer et al., J. MoI. Biol, 1984, 180:385-398; Weiss et al., FEBS Lett., 1990, 267:268-272; or the methods provided in a commercial kit, such as the CRYSTAL SCREEN™ kit (available from Hampton Research of Riverside, California, USA).

A crystallized OsSIKl protein may be tested for functional activity and differently sized and shaped crystals are further tested for suitability in X-ray diffraction. Generally, larger crystals provide better crystallography than smaller crystals, and thicker crystals provide better crystallography than thinner crystals. For example, OsSIKl crystals may range in size from 0.1- 1.5 mm. These crystals diffract X-rays to at least 10 A resolution, such as 1.5-10.0 A or any range of value therein, such as 1.5, 1.6, 1.7, 1.8, 1.9, 2.0, 2.1, 2.2, 2.3, 2.4, 2.5, 2.6, 2.7, 2.8, 2.9, 3.0, 3.1, 3.2, 3.3, 3.4, 3.5 or 3, with 3.5 A or less being preferred for the highest resolution.

IV.G. OsSIKl Antibodies

In another aspect of the invention, a method is provided for producing an antibody that specifically binds an OsSIKl protein. According to the method, a full-length recombinant OsSIKl protein is formulated so that it may be used as an effective immunogen, and used to immunize an animal so as to generate an immune response in the animal. The immune response is characterized by the production of antibodies that may be collected from the blood serum of the animal.

An antibody is an immunoglobulin protein, or antibody fragments that comprise an antigen binding site (e.g., Fab, modified Fab, Fab', F(ab')₂ or Fv fragments, or a protein having at least one immunoglobulin light chain variable region or at least one immunoglobulin heavy chain region). Antibodies of the invention include diabodies, tetrameric antibodies, single chain antibodies, tretravalent antibodies, multispecifϊc antibodies (e.g., bispecifϊc antibodies), and domain-specific antibodies that recognize a particular epitope. Cell lines that produce anti- OsSIKl antibodies are also encompassed by the invention.

Specific binding of an antibody to an OsSIKl protein refers to preferential binding to an OsSIKl protein in a heterogeneous sample comprising multiple different antigens. Substantially lacking binding describes binding of an antibody to a control protein or sample, i.e., a level of binding characterized as non-specific or background binding. The binding of an antibody to an antigen is specific if the binding affinity is at least about 10 ⁷ M or higher, such as at least about 10^~8 M or higher, including at least about 10^~9 M or higher, at least about 10^~π M or higher, or at least about 10^~12 M or higher.

OsSIKl antibodies prepared as disclosed herein may be used in methods known in the art relating to the expression and activity of OsSIKl proteins, e.g., for cloning of nucleic acids encoding an OsSIKl protein, immunopurification of an OsSIKl protein, and detecting an OsSIKl protein in a plant sample, and measuring levels of an OsSIKl protein in plant samples. To perform such methods, an antibody of the present invention may further comprise a detectable label, including but not limited to a radioactive label, a fluorescent label, an epitope label, and a label that may be detected in vivo. Methods for selection of a label suitable for a particular detection technique, and methods for conjugating to or otherwise associating a detectable label with an antibody are known to one skilled in the art. OsSIKl antibodies as disclosed herein may also be useful as OsSIKl activators.

V. Modulation OfAn OsSIKl Protein in Plants

The disclosed OsSIKl binding partners and OsSIKl activators are useful both in vitro and in vivo for applications generally related to assessing responses to abiotic stress and for increasing stress tolerance. In particular, OsSIKl activators may be used to induce transcription of OsSIKl and to increase stress tolerance, including increased tolerance to drought and high salt concentrations (see Examples 6 and 7). OsSIKl activators may also be used to increase stress tolerance to other abiotic stress conditions, such as high light intensity, low- and high- temperature, continuous light, continuous dark, and mechanical wounding.

The present invention provides that an effective amount of an OsSIKl activator is applied to a plant, i.e., an amount sufficient to elicit a desired biological response. For example, an effective amount of an OsSIKl activator may comprise an amount sufficient to elicit elevated expression of OsSIKl, altered levels of OsSIKl target genes (with the change in expression in the direction of normal OsSIKl transcriptional regulation, e.g., activation or repression), and increased tolerance to drought and high salt concentrations.

Plants that may benefit from OsSIKl activation include, but are not limited to, corn (maize), sorghum, wheat, sunflower, tomato, crucifers, peppers, potato, cotton, rice, soybean, sugarbeet, sugarcane, tobacco, barley, and oilseed rape, Brassica sp., alfalfa, rye, millet, safflower, peanuts, sweet potato, cassava, coffee, coconut, pineapple, citrus trees, cocoa, tea, banana, avocado, fig, guava, mango, olive, papaya, cashew, macadamia, almond, oats, Arabidopsis thaliana, poplar, turfgrass, vegetables, ornamentals, and conifers. Representative vegetables include tomatoes, lettuce, green beans, lima beans, peas, yams, onions, and members of the genus Curcumis such as cucumber, cantaloupe, and musk melon. Representative ornamentals include, but are not limited to, azalea, hydrangea, hibiscus, roses, tulips, daffodils, petunias, carnation, poinsettia, and chrysanthemum. Any of the afore-mentioned plants may be wild type, inbred, or transgenic, e.g., plants strains and genetically modified plants as used in agricultural settings.

Plants treated with an OsSIKl activator may be transgenic, i.e., genetically modified at OsSIKl, or at a locus other than OsSIKl to confer increased stress tolerance or other trait of interest. Representative desired traits include improved crop yield; increased seed yield; increased amino acid content; increased nitrate content; increased tolerance to stress; insect resistance; tolerance to broad-spectrum herbicides; resistance to diseases caused by viruses, bacteria, fungi, and worms; and enhancement of mechanisms for protection from environmental stresses such as heat, cold, drought, and high salt concentration. Additional desired traits include output traits that benefit consumers, for example, nutritionally enhanced foods that contain more starch or protein, more vitamins, more anti-oxidants, and/or fewer trans-fatty acids; foods with improved taste, increased shelf-life, and better ripening characteristics; trees that make it possible to produce paper with less environmental damage; nicotine-free tobacco; ornamental flowers with new colors, fragrances, and increased longevity; etc. Still further, desirable traits that may be used in accordance with the invention include gene products produced in plants as a means for manufacturing, for example, therapeutic proteins for disease treatment and vaccination; textile fibers; biodegradable plastics; oils for use in paints, detergents, and lubricants; etc. For genetic modifications that confer traits associated with increased stress tolerance, use of an OsSIKl activator or OsSIKl overexpression in combination with the genetic modification can produce a synergistic effect, i.e., a change in gene expression, or increased stress tolerance that is greater than the change elicited by either an OsSIKl activator/0SiS7A7 overexpression or the genetic modification alone.

EXAMPLES

The following examples have been included to illustrate modes of the invention. Certain aspects of the following examples are described in terms of techniques and procedures found or contemplated by the present co-inventors to work well in the practice of the invention. In light of the present disclosure and the general level of skill in the art, those of skill will appreciate that the following examples are intended to be exemplary only and that numerous changes, modifications, and alterations may be employed without departing from the scope of the invention.

Example 1. Cloning Of Rice OsSIKl cDNA

Oryza sativa var. Taipei 309 was used to isolate receptor-like kinase genes and to examine expression patterns of such genes under various growth conditions. A BLAST search of the rice whole genome database was performed to identify gene fragments related to receptor- like kinase genes. The fragments were then assembled to generate 267 receptor-like kinase genes. RT-PCR was used to assess gene expression in response to abiotic stress conditions, and some of the genes were identified as induced by abiotic stress. One gene was selected for further study. Λn OiSlKi cDMA was isolated as follows. Seeds were grown in pctri dishes for 2-3 weeks. Fresh leaves (Ig) were ground down in liquid nitrogen and suspended in 4 mol/L guanidinium thiocyanate. The resulting mixture was extracted by acidic phenol and chloroform, and absolute alcohol was added into the supernatant to precipitate the total RNA. The total RNA was used to obtain cDNA by reverse transcriptase. The following primers, specific for the gene of interest were used to amplify the gene from the cDNA using PCR:

5'-CGC GGA TCC AGG AGC GGC TTG AGA GGG-3' (SEQ ID NO: 3)

5'-CGG GGT ACC TGG AGG TTT GGA GAA GAT-3' (SEQ ID NO: 4).

The total volume of the PCR. mixture was 20 μL, and contained 1 μL of first-strand cDNA, lμl of primers (20 μM), 2 μl of 1OX PCE buffer, 0.4 μl of dNTP (10 mM), and 1 unit Taq DNA polymerase. The reaction mixture was ovcrlayed with liquid paraffin. The reaction was denatured at 94°C for 5 minutes, followed by 30-32 cycles of 1 minute at 94°C 1 minute at 56°C, and 1 minute at 72^CC, with a final extension for 10 minutes at 72°C. The product of the PCR amplification was resolved by electrophoresis on a 0.8% agarose gel. A band of approximately 3 kb was observed, which was consistent with the expected transcript size. This band was recovered using an agarose gel recovering kit (TIANGEN BIOTECH, Beijing, CHINA) and cloned into plasmid PGEM®-T Easy (Promega, Madison, Wisconsin, USA). The recombinant vector was transformed into E. coli DH5α competent cells, essentially as described in Cohen et al, Proc. Natl. Acad. ScL USA, 1972, 69:2110-2114). Cells containing the fragment recovered from the agarose gel were selected by using the ampicillin resistance gene and disruption of the β-galactosidase enzyme encoding gene on the PGEM®-T Easy plasmid. A recombinant plasmid containing the fragment was subsequently isolated and the fragment was sequenced with primers complementary to the T7 and SP6 promoter sequences, which flank the multiple cloning site on the PGEM®-T Easy plasmid. The isolated fragment, designated OsSIKl (Stress Inducible Kinase 1), contained 3069 deoxyribonucleotides. The open reading frame of the cDNA has a nucleotide sequence set forth as nucleotides 57-3069 of SEQ ID NO: 1, and encodes a polypeptide with an amino acid sequence set forth as SEQ ID NO: 2. The recombinant vector containing the OsSIKl (Stress Inducible Kinase 1) cDNA, which comprises nucleotides 8-3076 of SEQ ID NO: 1, was designated as pTE-OsSIKl. Example 2. Expression Of OsSIKl Under Abiotic Stresses And Administration Of Hormones

Seeds of rice Oryza sativa var. Taipei 309 were grown in petri dishes for two weeks and subjected to the following conditions: (1) salt stress condition (the seedlings were placed into a 0.6% NaCl solution); (2) drought condition (the seedlings were removed from the culture medium, dehydrated, and kept in the air); (3) low temperature condition (the seedlings were grown at 4⁰C); (4) ACC (1-amiocyclopropane-l-carboxylic acid) condition (the seedlings were transferred into a solution containing 100 μm ACC); (5) ABA condition (the seedlings were transferred into a solution containing 100 μm ABA); or (6) H₂O₂ condition (the seedlings were transferred into a solution containing 100 μm H₂O₂). The seedlings were light-cultivated, and fresh leaves were taken at 0, 1, 3, 6, 12, and 24 hours during these treatments. Total RNA was extracted from 1 g of collected fresh leaves. RT-PCR was carried out for determining the expression level of OsSIKl, and actin expression was used as a control. The following primers, specific for OsSIKl, were used for the RT-PCR:

5'-AGG AGC GGC TTG AGA GGG-3' (SEQ ID NO: 5)

5'-TGG AGG TTT GGA GAA GAT-3' (SEQ ID NO: 6)

As shown in Fig. 1, the expression level of OsSIKl was increased in plants exposed to the stress of salt, drought, low temperature (4⁰C), ABA, and H₂O₂. A slight increase in OsSIKl expression was observed in response to ACC.

Example 3. Construction Of The OsSIKl Expression Vector pBin438- OsSIKl

The OsSIKl cDNA obtained as described in Example 1 was amplified by PCT using the following primers specific for the OsSIKl cDNA and also containing BamHI and Kpnl linker sequences:

5'-CGC GGA TCC AGG AGC GGC TTG AGA GGG-3' (SEQ ID NO: 7) 5'-CGG GGT ACC TGG AGG TTT GGA GAA GAT-3' (SEQ ID NO: 8). The amplified product was digested with BamHI and Kpnl, recovered, and inserted between the BamHI and Kpnl restriction sites in the sense orientation downstream of the CaMV

35S promoter in the plant binary expression vector pBin438 (Li et al., J. Science in China

(Column B), 1994, 24(3): 276-282). The resulting vector was designated as pBin438-

OsSIKl (Stress Inducible Kinase 1). See Figure 2. Example 4. Preparation Of Transgenic Plants Overexpressing OsSIKl

Agrobacterium tumefaciens AGLl competent cells were prepared essentially as described in Sambrook et al., A Laboratory Manual, 3^rd edition, 2001, Cold Spring Harbor Laboratory Press, New York. Briefly, a single colony of Agrobacterium tumefaciems AGLl was inoculated in 10ml LB (10g/L NaCl, 5g/L yeast extracts, 10g/L tryptone) and cultured with shaking at 28⁰C until late log phase. Then 0.5ml of this bacterial suspension was added to 50ml of fresh LB liquid medium and cultured with shaking at 28⁰C until ODβoo reached approximately 0.5. The mixture was transferred to a 50ml centrifuge tube and incubated in an ice bath for 20 minutes, followed by centrifugation at 4000 rpm for 10 minutes at 4⁰C. The pellet was collected and resuspended in 20ml of 10% cold glycerol, followed by centrifugation at 4000 rpm for 10 minutes at 4⁰C. The supernatant was removed and the pellet was resuspended and divided into 50μl aliquots in 1.5ml centrifuge tubes. The aliquots were stored at -7O⁰C for further use.

The Agrobacterium tumefaciens AGLl competent cells were transformed with the pBin438-0SiS7A7 vector by electroporation. Briefly, 0.5μg of the plasmid was added to 50μl of the competent cells and mixed gently. The mixture was kept on ice and subjected to a voltage of 2500V for 5 seconds. Then 800μl of LB medium was added immediately. The mixture was cultured for 45 minutes at 150 rpm, 28⁰C. Cells were plated on a dish containing selective medium and ventilated in a sterile hood to remove the liquid on the surface, followed by a 2-day culture. After transformation, a single colony was used to inoculate 2ml LB YEB liquid medium (containing kanamycin at a final concentration of 50μg/ml and rifampicin at a final concentration of 25μg/ml), and cultured with shaking overnight at 28⁰C. Extraction of the plasmid was carried out by the alkaline lysis method, followed by PCR and enzyme digestion to confirm the presence ofpBin438-OftSΪK7.

Seeds of Oryza sativa var. Taipei 309 were sterilized in 70% alcohol for 1 minute, followed by washing in sterile water 2-3 times, a hypochloric acid solution (2% Cl) with shaking, and further washing in sterile water 4-5 times. The callus embryo was isolated under sterile conditions and used to inoculate N6D2 medium (N6 components and vitamins, 500g/L casein hydrolysate, 30g/L sucrose, 2mg/L 2,4-D, and 2.5g/L coagulant (gelrite), pH 5.8). The culture was incubated in the dark for 4 days.

Transformed Agrobacterium tumefaciens AGLl was plated and a single colony was transferred to 20ml of LB liquid medium, containing kanamycin at a final concentration of 50μg/ml and rifampicin at a final concentration of 25μg/ml. The bacteria were incubated with shaking at 28⁰C until the late log phase. A volume of 0.5ml of the suspension was transferred to 50ml of fresh LB medium supplemented with kanamycin and rifampicin as noted above. The mixture was further cultured with shaking at 28⁰C until OD600 reached approximately 0.5.

The transformed Agrobacterium was centrifuged at 4000Xg for 10 minutes. The pellet was resuspended in an equal volume of AAM-AS medium (amino acids and amino acid components, MS vitamin, and 10OmM Acetosyringone (AS), pH 5.2). The callus embryo of Oryza sativa var. Taipei 309, which had been cultured for 4 days, was immersed in the AAM-AS suspension for 20 minutes, followed by drying with sterile filter paper and transferring to N6D2C medium (N6D2, 10g/L glucose, 10OmM Acetosyringone, pH 5.2). The culture was incubated at 25⁰C in the dark for 3 days. The callus embryo was then washed in sterile water containing 300mg/L cephamycin (cef)) 4-5 times, dried with sterile filter paper and transferred to N6D2S1 medium (N6D2, 25mg/L hygromycine, and 600mg/L cefotaxime, pH 5.8). It was subcultured for one generation. After 2 weeks, the seedling was transferred to N6D2S2 medium (N6D2, 50mg/L hygromycine, and 300mg/L cefotaxime, pH 5.8) to subculture for another generation (2 weeks/subculture).

The well-grown subcultured selected callus (the second generation) were transferred to a differentiating medium (MS components and vitamins, 300g/L casein hydrolysate, 50mg/L hygromycin, 3g/L 6-BA, 2.5 mg/L KT, 0.2mg/L ZT, and 2.5g/L coagulant (gelrite), pH 5.8), and cultured in a differentiating incubator (12 hours light, 28⁰C / 12 hours dark, 25⁰C) for 7 days. The plants were then transferred to the differentiating medium and cultured to obtain regenerated seedlings. The regenerated plant was cultured in a rooting and propagation medium (1/4 MS components, MS vitamins, lmg/L Paclobutrazol, 0.5mg/L NAA, 6.5g/L agar powder, pH 5.8). When the seedlings grew to about 10cm, the vessel was unsealed for culturing for 2-3 days. The positive seedlings were transplanted in an Artificial Atmospheric Phenomena Simulator. After selection on kanamycin (at a final concentration of 50 μg/ml) and rifampicin (at a final concentration of 25 μg/ml), 56 positive OsSIKl -transgenic plants of T₀ generation were produced. The Ti generation represented seeds which were generated by selfing of To generation, and plants there from. Control plants of the To generation transformed with the control vector pBin438, were produced as described above. A total of 8 control plants of To generation transformed with the control vector pBin438 were obtained. Real-time PCR using total RNA template was performed to identify OsSIKl transgenic plants of T₀ generation, wild type plants and control plants of T₀ generation transformed by blank vector using the following primers:

5'-TAT GAC CTC GCC GAA CCT-3' (SEQ ID NO: 9)

5'-CGC CTC AAG CAC TGT CTA-3' (SEQ ID NO: 10).

OsSIKl expression was detected in 56 OsSIKl transgenic plants of T₀ generation, and 27 plants showed OsSIKl expression at 10-100 times the expression level seen in wild type plants. The control plants did not show any increased expression of OsSIKl compared to wild type plants. Three strains of transgenic plants, OsSIKl-OX (overexpressing) plants 7-2, 7-1, and 3-4 were selected for further study based on the highest expression of the OsSIKl gene normalized to actin as an internal control. See Figure 3.

Example 5. Preparation Of Transgenic Plants Expressing OsSIKl-RNAi

PCR was performed on cDNA of Oryza sativa var. Taipei 309 obtained from reverse- transcribed total RNA as the template using the following primers:

5'-CAA GTC GAC GGT ACC GTG GCT GTG GTT GAT TGG TT-3' (SEQ ID NO: 11)

5'-TCT TCT AGA GAG CTC GGT AGT CTG CCC GAG GAA-3' (SEQ ID NO: 12).

A 500 base pair fragment was amplified, digested with the enzymes Kpn\ and Sad, and cloned into pZHOl (Xiao et al., Plant Molecular Biology, 2003, 52: 957-966) in the sense orientation, between the restriction sites of Kpn\ and Sad to obtain the recombinant vector

The PCR products were also digested by the enzymes Sail and Xbal, and inserted into pZHOl-OsSIKl in the anti-sense orientation, between the restriction sites of Sail and Xbal to obtain the plant expression vector, pZHOl-OsSIKl-RNAi, containing OsSIKl-RNAi. This vector was then used to transform and prepare an OsSIKl-RNAi transgenic plant using the methods disclosed in Example 4 for producing OsSIKl overexpressing transgenic plants. Realtime PCR was performed to identify OsSIKl-RNAi transgenic plants of To generation, wild type plants and control plants of To generation transformed by blank vector using the following primers:

5'-TAT GAC CTC GCC GAA CCT-3' (SEQ ID NO: 9)

5'-CGC CTC AAG CAC TGT CTA-3' (SEQ ID NO: 10).

OsSIKl transcripts were detected in 25 of 48 OsSIKl-RNAi transgenic plants of To generation at 0.038-0.2 times the expression level seen in wild type plants. The control plants did not show any change in expression levels of OsSIKl compared to wild type plants. Three strains of transgenic plants, OsSIKl-RNAi plants 83-1, 12-3, and 20-3, were selected for further study based on the lowest expression of the OsSIKl gene normalized to actin as an internal control. See Figure 4.

Example 6. Salt Stress Tolerance Of OsSIKl Overexpressing And OsSIKl-RNAi Transgenic Plants

Three-week seedlings of OsSIKl-OX (overexpressing) plants 7-2, 7-1, and 3-4, OsSIKl- RNAi plants 83-1, 12-3, and 20-3, and control plants transformed by blank vector were grown in a solution of 0.55% (0.55g/100ml) NaCl for 10 days under normal conditions. The experiment was performed in triplicate, and 30 plants of each strain were used for each experiment. After 10 days, photographs were taken to record leaf morphology (e.g., wilting). See Figure 5A. Plants were then removed from the solution of 0.55% NaCl and allowed to recover for 10 days and photographed again to record leaf morphology. See Figure 5B.

After the 10-day recovery, the control plants had a survival rate of 38%±5%, the 3 strains of OsSIKl-RNAi plants had a survival rate of 12%±2%, 10%±2%, and 15%±2% respectively, and the 3 strains of OsSIKl-OX plants have a survival rate of 65%±5%, 62%±4%, and 69%±6%. See Figure 5C.

Example 7. Drought Stress Tolerance Of OsSIKl Overexpressing And OsSIKl-RNAi Transgenic Plants

Three-week seedlings of OsSIKl-OX (overexpressing) plants 7-2, 7-1, and 3-4, OsSIKl- RNAi plants 83-1, 12-3, and 20-3, and control plants transformed by blank vector were grown without water for 8 days and then grown under normal conditions for 4 or 15 days. The experiment was performed in triplicate, and 30 plants of each strain were used for each experiment. Following growth under normal conditions for 4 or 15 days, photographs were taken to record leaf morphology (e.g., wilting). See Figures 6A-6C. With either recovery period, OsSIKl-OX rice exhibited superior drought tolerance as compared to control plants, and OsSIKl-KNAi rice showed inferior drought tolerance as compared to control plants. See Figures 6A-6C. After the 8-day drought treatment and 15-day recovery, the control plants had a survival rate of 43%±6%, the 3 strains of OsSIKl -RNAi plants had a survival rate of 8%±2%, 11%±2%, and 7%±2% respectively, and the 3 strains of OsSIKl-OX plants had a survival rate of 90%±7%, 93%±2%, and 87%±5%, respectively. See Figure 6D.

Claims

1. An isolated OsSIKl nucleic acid comprising

(a) a nucleotide sequence set forth as SEQ ID NO: 1;

(b) a nucleic acid that specifically hybridizes to the complement of SEQ ID NO: 1 under stringent hybridization conditions;

(c) an open reading frame (ORF) of a gene encoding an OsSIKl protein, wherein the open reading frame comprises nucleotides 57-3056 of SEQ ID NO: 1;

(d) a nucleotide sequence encoding an OsSIKl protein comprising an amino acid sequence set forth as SEQ ID NO: 2;

(e) a nucleotide sequence complementary to that of a nucleic acid of (a)-(d); or

(f) a functional fragment of a nucleic acid of (a)-(e).

2. A vector comprising the nucleic acid of claim 1.

3. A host cell which expresses the vector of claim 2.

4. The host cell of claim 3, which is a plant cell.

5. An isolated OsSIKl protein comprising:

(a) an amino acid sequence set forth as SEQ ID NO: 2;

(b) an amino acid sequence set forth as SEQ ID NO: 2 and further comprising one or more amino acid substitutions, deletions, or additions; or

(c) a functional fragment of (a) or (b).

6. An antibody or antibody fragment which specifically binds to the isolated OsSIKl protein of claim 5.

7. A transgenic plant expressing a nucleic acid of claim 1.

8. The plant of claim 7, which is a monocot.

9. The plant of claim 7, which is a dicot.

10. The plant of claim 7, which shows an increase in abiotic stress tolerance.

11. The plant of claim 7, wherein the abiotic stress is increased salt concentration.

12. The plant of claim 7, wherein the abiotic stress is decreased water concentration.

13. A method of producing a transgenic plant comprising introducing into a plant cell an OsSIKl nucleic acid, wherein the OsSIKl nucleic acid comprises:

(a) a nucleotide sequence set forth as SEQ ID NO: 1; (b) a nucleic acid that specifically hybridizes to the complement of SEQ ID NO: 1 under stringent hybridization conditions;

(c) an open reading frame (ORF) of a gene encoding an OsSIKl protein, where the open reading frame comprises nucleotides 57-3056 of SEQ ID NO: 1;

(d) a nucleotide sequence encoding an OsSIKl protein comprising an amino acid sequence set forth as SEQ ID NO: 2; or

(e) a functional fragment of (a)-(d).

14. The method of claim 13, wherein the plant cell is a monocot plant cell.

15. The method of claim 13, wherein the plant cell is a dicot plant cell.

16. The method of claim 13, wherein the OsSIKl nucleic acid is introduced into the plant cell using a method selected from the group consisting of microparticle bombardment, Agrobacterium-mediatGd transformation, and whiskers-mediated transformation.

17. The method of claim 13, wherein the introduction of the OsSIKl nucleic acid results in constitutive overexpression of the OsSIKl nucleic acid.

18. The method of claim 13, wherein the plant shows an increase in abiotic stress tolerance.

19. The method of claim 18, wherein the abiotic stress is increased salt concentration.

20. The method of claim 18, wherein the abiotic stress is decreased water concentration.

21. A method of identifying an agent that enhances the expression level of a nucleic acid encoding an OsSIKl protein comprising:

(a) providing a cell expressing an OsSIKl protein;

(b) contacting the cell with one or more test agents or a control agent;

(c) assaying expression of an OsSIKl nucleic acid or protein;

(d) selecting a test agent that induces elevated expression of the OsSIKl nucleic acid or protein when contacted the cell is contacted with the test agent as compared to the control agent.

22. The method of claim 21, wherein the one or more test agents is a peptide, a protein, an oligomer, a nucleic acid, a small molecule, inorganic chemical, organic chemical, or an antibody.

23. A method of increasing the expression level of a nucleic acid encoding an OsSIKl protein comprising contacting a plant or cell expressing an OsSIKl protein with an agent identified according to claim 21.

24. The method of claim 21 , wherein the nucleic acid shows tissue-specific expression.

25. A method of identifying a binding partner of an OsSIKl protein, the method comprising the steps of:

(a) providing an OsSIKl protein of claim 5;

(b) contacting the OsSIKl protein with one or more test agents or a control agent under conditions sufficient for binding;

(c) assaying binding of a test agent to the isolated OsSIKl protein; and

(d) selecting a test agent that demonstrates specific binding to the OsSIKl protein.

26. The method of claim 25, wherein the one or more test agents is a peptide, a protein, an oligomer, a nucleic acid, a small molecule, inorganic chemical, organic chemical, or an antibody.

27. A method of identifying an OsSIKl activator comprising the steps of:

(a) providing a cell expressing an OsSIKl protein;

(b) contacting the cell with one or more test agents or a control agent;

(c) assaying expression of an OsSIKl target gene; and

(d) selecting a test agent that induces a change in expression of the target gene, which gene is normally subject to OsSIKl control, when the cell is contacted with the test agent as compared to the control agent.

28. The method of claim 27, wherein the change in expression is an increase in target gene expression.

29. The method of claim 27, wherein the change in expression is a decrease in target gene expression.

30. The method of claim 27, wherein the one or more test agents is a peptide, a protein, an oligomer, a nucleic acid, a small molecule, inorganic chemical, organic chemical, or an antibody.

31. A method of identifying an OsSIKl activator comprising the steps of:

(a) providing a plant expressing a OsSIKl protein;

(b) contacting the plant with one or more test agents or a control agent;

(c) assaying survival of the plants under abiotic stress; and

(d) selecting a test agent that promotes survival of the plants under abiotic stress when in the presence of the test agent as compared to the control agent.

32. The method of claim 31, wherein the one or more test agents is a peptide, a protein, an oligomer, a nucleic acid, a small molecule, inorganic chemical, organic chemical, or an antibody.

33. The method of claim 31 , wherein the abiotic stress is increased salt concentration.

34. The method of claim 31 , wherein the abiotic stress is decreased water concentration.

35. A method of improving abiotic stress tolerance in a plant expressing an OsSIKl protein comprising contacting the plant with an agent identified according to claim 19.

36. The method of claim 35, wherein the abiotic stress is increased salt concentration.

37. The method of claim 35, wherein the abiotic stress is decreased water concentration.

38. A method of improving abiotic stress tolerance in a plant expressing an OsSIKl protein comprising contacting the plant with an agent identified according to claim 25.

39. The method of claim 38, wherein the abiotic stress is increased salt concentration.

40. The method of claim 38, wherein the abiotic stress is decreased water concentration.

41. A method of improving abiotic stress tolerance in a plant expressing an OsSIKl protein comprising contacting the plant with an agent identified according to claim 29.

42. The method of claim 41, wherein the abiotic stress is increased salt concentration.

43. The method of claim 41 , wherein the abiotic stress is decreased water concentration.