WO2004076625A2

WO2004076625A2 - Method of identifying plant cells transformed with a gene

Info

Publication number: WO2004076625A2
Application number: PCT/US2004/004332
Authority: WO
Inventors: Mark Conkling; Xia Li; Kheng Cheah; Jiahua Xie
Original assignee: Vector Tobacco Ltd.
Priority date: 2003-02-21
Filing date: 2004-02-13
Publication date: 2004-09-10
Also published as: WO2004076625A3

Abstract

Several approaches to differentiate transformed plant cells from non-transformed plant cells, in the absence of a selectable marker, are aspects of the invention described herein. It was discovered that transformed plant cells experience detectable morphological changes (e.g., a reduced rate of growth or biomass) that can be readily identified and used to differentiate the transformed cell from the non-transformed cell in the absence of a selectable marker. Embodiments include approaches for differentiating, identifying, selecting, and regenerating plant cells that have been transformed with a gene of interest, with or without a selectable marker by comparing the growth rate or morphological feature of the transformed plant cells with the growth rate or a morphological feature of non-transformed plant cells that are cultured under similar conditions.

Description

METHOD OF IDENTIFYING PLANT CELLS TRANSFORMED WITH A GENE

FIELD OF THE INVENTION

The present invention generally relates to the field of plant genetics and molecular biology. More specifically, embodiments described herein concern approaches for identifying, selecting, and regenerating plant cells that have been transformed with a gene of interest.

BACKGROUND OF THE INVENTION

There are many techniques in plant molecular biology that allow for the introduction of foreign DNA into the nuclear genome of plants resulting in the modification (gain or loss) of gene function. Typically, these techniques involve gene transfer to a recipient plant tissue or cells, identification or selection of transformed tissue or cells and regeneration of a transgenic plant. Subsequent to gene transfer, however, there generally exists a mixture of both transformed and non-transformed plant tissue or cells and identification or selection of positive transformants is desired so that only transgenic plants are regenerated. Over the years, several approaches have been developed to facilitate the identification or selection of transformed plants.

Many selection protocols involve the co-introduction of a gene that confers resistance to a chemical that is deleterious to the host plant cell (e.g., an antibiotic or herbicide resistance gene placed in cis with respect to the gene of interest). Regenerating cells transformed with the resistance gene in a growth media that contains the antibiotic, for example, permits identification or selection of only those cells that have incorporated the chemical resistance gene and the gene of interest. The literature is replete with examples of selectable marker genes, including the bar gene and the pat gene, which confer resistance to glufosinate herbicides; (see, e.g., DeBlock et al., EMBO J., 6:2513-2518, 1987), neomycin phosphotransferase genes, which confer resistance to the kanamycin, G418, and neomycin (see, e.g., Fraley et al, CRC Critical Reviews in Plant Science 4: 1-25, 1986), and the Hml gene, which confers resistance to the HC toxin of Cochliobolus carbonum (see e.g., U.S. Patent No. 5,589,611). See also Bowen, B.A., "Markers for plant gene transfer", in Transgenic Plants - Engineering and Utilization, Vol. 1, Academic Press, N.Y., pp. 89-123.

Additionally, WOO 1/64023 describes a selection protocol that involves the use of a gene, which encodes a phytotoxin detoxifying enzyme. Growth of plant tissue or cells in a medium containing the phytotoxin (e.g., an herbicide) permits selection against those plants that have not taken up the gene for the detoxifying enzyme. Similarly, U.S. Patent No. 5,633,153 describes a method of using an aldehyde dehydrogenase as a selectable marker, U.S. Patent No. 5,589,611 describes transformant selection based on genetically conferred disease resistance, and several patents describe selection protocols based on genetically conferred osmoprotection. (See e.g., U.S. Patent No. 5,780,709). International Publication WO 98/51806 also describes selection methods that involve the use of assayable transgenes and selection approaches based on phenotype restoration have been described in U.S. Patent Application No. 2002/0004939). Methods of identifying or selecting transformed plants, which are based on the control of cell division, have also been described. (See e.g., International Publication WO 00/52168 and International Publication WO01/59086).

Many of these approaches are not desirable for the production of agricultural crops, however, because the selectable marker genes may continue to produce their gene products in the regenerated plant. One of the primary concerns regarding genetically modified crops, for example, is the presence of clinically important antibiotic resistance gene products in transgenic plants that could inactivate oral doses of the antibiotic (reviewed by Puchta, H. (2000) TIPS 5:273-274; Daniell, H. (1999) TIPS 4:467-69). Members of the public are also concerned that the antibiotic resistant genes could be transferred to pathogenic microbes in the gastrointestinal tract or soil rendering them resistant to treatment with such antibiotics. Widespread transfer of antibiotic resistance genes may further the propagation of drug resistant bacteria. Many more countries may follow Germany's lead in prohibiting the release of genetically modified crops containing antibiotic resistant genes. (Peerenboorn, E. (2000) Nature Biotechnol. 18:374). Despite recent advances in the field, the need for more approaches to isolate, identify, select, and regenerate transformed plant cells is manifest.

SUMMARY OF THE INVENTION

Several approaches for differentiating a transformed plant cell from a non-transformed plant cell, in the absence of a selectable marker, are provided herein. It was discovered that transformed plant cells experience detectable morphological changes (e.g., a reduced rate of growth or biomass) that can be readily identified and used to differentiate the transformed cell from the non-transformed cell.

In some embodiments, for example, a candidate plant cell is provided; a nucleic acid is introduced into said candidate plant cell thereby producing a transformed plant cell; said transformed plant cell is cultured; and said transformed plant cell is then identified by comparing a morphological feature of said transformed plant cell with a morphological feature of a nontransformed plant cell that is cultured under similar conditions, wherein said morphological feature of said transfoπned plant cell is different when compared to said moiphological feature of said nontransformed plant cell.

In a preferred embodiment, the morphological feature is the growth rate of the plant cells. The growth rate can be determined, for example, by measuring a change in biomass over a predetermined time interval. Alternatively, the growth rate can be determined by evaluating a moiphological feature that is characteristic of a developmental stage after a predetermined time interval, h some embodiments, the growth rate of the transfoπned cell lags the growth rate of the nontransfoπned cell by at least about one hour, two hours, three hours, four hours, five hours, ten hours, fifteen hours, eighteen hours, one day, three days, five days, one week. In other embodiments, the morphological feature is tissue differentiation.

In still more embodiments, the candidate plant cell is a protoplast. The transfonned plant cell can also be, for example, a monocot cell, a dicot cell, or a gymnosperm cell. Additionally, the transfonned plant cell can be obtained from a plant meristem. hi preferred embodiments, the transformed plant cell is a tobacco cell, preferably a Nicotiana tobacum cell, hi one embodiment, the tobacco cell produces a reduced level of nicotine.

The nucleic acid can be introduced into the candidate plant cell by, for example, electroporation, microprojectile bombardment, co-cultivation with Agrobacterium, or microinjection. The nucleic acid can be DNA or RNA (e.g., RNAi). The DNA can encode a protein. Further, the DNA can be operably linked to a promoter in a sense or antisense orientation, hi some embodiments, the DNA integrates into the genome of the transformed candidate plant cell.

More embodiments of the invention concern the transformed plant cell and the nontransformed plant cell present together in a mixture or as part of a plant tissue. Desirably, the transformed plant cell and the nontransformed plant cell are cultured on the same growth medium. The medium can be solid or a liquid. Further, in some embodiments, the transformed plant cell and the nontransformed plant cell are cultured under the same environmental conditions. The transformed plant cell and the nontransfoπned plant cell can be cultured for a sufficient amount of time to permit growth of a transfoπned cell mass and a nontransformed cell mass. Preferably, the transfoπned plant cell and the nontransfoπned plant cell are cultured for a sufficient amount of time to permit regeneration of a transformed plant and a nontransformed plant.

Other embodiments concern methods of producing transgenic plants. Some methods comprise, for example, the steps of: introducing a nucleic acid into a candidate plant cell; determining whether said candidate plant cell is transformed by comparing a morphological feature of said candidate plant cell with a morphological feature of a nontransfoπned plant cell, wherein said candidate plant cell is transformed if the moiphological feature of said candidate plant cell is different when compared to the morphological feature of said nontransformed cell; and regenerating a transgenic plant from said transfoπned candidate plant cell, hi still more embodiments, the transgenic plant is a tobacco plant, preferably Nicotiana tobacum and some of the transgenic plants produce a reduced level of nicotine and nitrosamine.

Aspects of the invention described herein also concern transgenic plant cells produced by introducing a nucleic acid into a candidate plant cell thereby producing a transformed plant cell; culturing said transfonned plant cell; and identifying said transformed plant cell by comparing a moiphological feature of the transfoπned plant cell with a morphological feature of a nontransformed plant cell that is cultured under similar conditions, wherein said moiphological feature of the transformed plant cell is different when compared to said morphological feature of the nontransfoπned plant cell. In some of these embodiments, the transfoπned plant cell is a tobacco cell, preferably a Nicotiana tobacum cell and, preferably, the transgenic plant cell is a tobacco cell that produces a reduced level of nicotine.

DETAILED DESCRIPTION OF THE INVENTION

It was discovered that transformed plant cells exhibit a reduced rate of growth for a short time after transformation, which can be exploited to identify and/or select positive transfonnants from non-transfonned cells, hi some embodiments, the identification or selection process can be perfoπned by monitoring or assessing the difference in the rate of cell growth between a control (a non-transformed cell or culture of cells) and the transfoπned cell or culture of cells (e.g., cells or tissue that have been infected with an Agrobacterium containing a vector comprising a gene of interest or biolistically transfonned cells or tissue). The difference in cell growth can be monitored, for example, by assessing the difference in the accumulation in biomass (e.g., weight), relative size of the cells, or by measuring one or more metabolic processes associated with particular stages of growth (e.g., differentiation of a specific cell type or tissue type). The selection protocols described herein are preferably practiced in the absence of a selectable marker gene allowing for the regeneration of transgenic plants that are free of the selectable marker gene, however, one or more selectable marker genes can also be employed. The selection methods described herein are applicable to all plants that can be transfoπned with a nucleic acid, however, preferred plants include members of Nicotiana, most preferably, Nicotiana tobacum. The section below describes several approaches that can be used to transfer genetic material to a plant cell.

Transformation

A variety of methods have been developed for the introduction of nucleic acids into plants. These techniques are commonly termed plant cell transformation methods. Examples of plant cell transfoπnation methods include, but are not limited to, microprojectile bombardment, electroporation, direct uptake, induced uptake, and introduction of nucleic acids mediated by Agrobacterium tumefaciens. (See generally, Mil et ah, "Procedures for Introducing Foreign DNA into Plants", in Methods in Plant Molecular Biology and Biotechnology, Glick, B.R. and Thompson, J. E., Eds. (CRC Press, Inc., Boca Raton, 1993), pages 67-88).

A nucleic acid of interest can be obtained from a donor species or it can be synthetic. There is no requirement that the transformed nucleic acid originate from a donor that is a different species than the recipient. That is, a nucleic acid can be isolated from a donor species then reintroduced into that same species. The nucleic acid that is transfoπned into the plant cells is not limited to any certain form of nucleic acid. As used herein, "nucleic acid" means DNA, RNA, or modified nucleic acids. Thus, the terminology "the nucleic acid of SEQ ID NO: X" or "the nucleic acid comprising the nucleotide sequence" includes both the DNA sequence of SEQ ID NO: X, the complement thereof, and an RNA sequence in which the thymidines in the DNA sequence have been substituted with uridines in the RNA sequence and in which the deoxyribose backbone of the DNA sequence has been substituted with a ribose backbone in the RNA sequence, i addition, in some contexts, the tenn "polynucleotide" has the same meaning as nucleic acid, hi those cases where the tenn "exogenous" is used in connection with a nucleic acid or polynucleotide, it can be meant that the nucleic acid or polynucleotide is introduced into an environment where it is not nonnally present. Accordingly, in one instance, a nucleic acid that is isolated from the genome of an organism then reintroduced into the same organism in a different context, such as at a new location in the genome, constitutes an exogenous nucleic acid.

Although a nucleic acid of interest can be directly transfonned into a plant cell, many approaches utilize a transformation vector, which for the purposes of the embodiments described herein, can comprise several prokaryotic and eukaryotic genetic elements including, but not limited to, promoters, regulatory elements, terminators, selectable markers, and antibiotic resistance genes in addition to the nucleic acid of interest. Some transformation vectors, however, only contain the nucleic acid of interest and suitable promoters and tenninators.

One of the most often used transfoπnation methods for plants involves co-cultivation of plant cells or tissue with Agrobacterium tumefaciens containing a plant transfonnation vector (see U.S. Patent No. 5,530,196). The transfer of nucleic acids from Agrobacterium to the plant host is mediated the Ti plasmid or a derivative thereof, which has been modified to increase its utility as a cloning vector. The components of the Ti plasmid required for such transfer are the right and left transfer borders that flank the T-DNA region and the so called vir region, which provides for mobilization of the T-DNA.

Agrobacterium cloning vectors have been developed by replacing the tumor-forming genes present within the T-DNA region with nucleic acid sequences that facilitate gene cloning and expression. For example, the T-DNA region often includes a multiple cloning site into which a nucleic acid of interest, such as an exogenous nucleic acid, can be introduced. More often, the T- DNA region is a construct that includes an expression cassette, which comprises a multiple cloning site that is flanked by a promoter, which is active in a plant cell and a transcriptional teπninator. The nucleic acid of interest is operably linked to the promoter so that it can be expressed after introduction into the appropriate host cell or tissue. A variety of promoters may be used depending on the level of control that is desired with respect to gene expression. For example, types of promoters that can be used include, but are not limited to, strong constitutive promoters, regulatable promoters, tissue specific promoters and developmentally regulated promoters.

Several different Agrobacterium cloning systems have been developed. One of the most common systems is the binary vector system. By this approach, the T-DNA region containing a nucleic acid of interest is contained in one vector and the vir region is located in a separate disarmed (without tumor-genes) Ti plasmid. The plasmids co-reside in Agrobacterium but remain independent. Another approach utilizes a co-integrated vector. These vectors can be constructed by recombining an Agrobacterium Ti plasmid lacking tumor-causing genes ("disarmed" Ti plasmid) and a small vector plasmid, which is engineered to cany a gene of interest between a right and a left T-DNA border of the T-DNA region (engineered or modified T-DNA region). Recombination takes place through a single crossover event in a homologous region present in both plasmids. Refinement of each of the above systems have been developed. For example, the vir region has been integrated into the chromosomal DNA of some Agrobacterium strains thus alleviating the need for two independent vectors in the binary system.

Most Agrobacterium cloning vectors also incorporate one or more selectable marker genes, which allows for the identification of host plant cells that are successfully transfonned with the nucleic acid of interest. These vectors have a T-DNA region that contains, in addition to the expression cassette, selectable markers such as antibiotic resistance genes, e.g., Nptll gene for kanamycin resistance, a gene that mediates resistance to herbicides, a positive selectable marker, such as a gene which enables the plant to utilize a new energy source such as mannose or xylose (see, U.S. Patent No. 5,767,378) or a reporter gene such as GUS or GFP.

It will be appreciated, however, that the methods of selection described herein do not require the presence of a selectable marker gene and, preferably, they are not present on the construct, other words, the methods of the present invention can be practiced without introducing into a plant cell or plant tissue an antibiotic resistance gene, a gene that mediates herbicide resistance, a gene for positive selection, a reporter gene, or any other gene that is used introduced into a plant cell or plant tissue, which is used to identify positive transformants. It should be understood, however, that the vectors that are useful with the approaches described herein can also include features that are common to most shuttle vectors including, but not limited to, an origin of replication for both Agrobacterium and E. coli, prokaryotic selectable marker genes, such as a β-lactamase, and appropriate regulatory regions.

In some plants, such as tobacco, transformation by Agrobacterium co-cultivation is perfonned by removing a sample of leaf tissue in the form of a leaf disc from the plant. The leaf disc is placed on a nutrient medium and allowed to form callus. A culture of Agrobacterium that possesses a Ti-based plasmid having the nucleic acid of interest is also prepared. Callus cells contained on the nutrient medium are then contacted with a portion of the Agrobacterium culture. The co-cultivation of the tobacco cells with the Agrobacterium results in the mobilization of the T- DNA and its transfer into the plant cell.

Monocotyledonous species, such as rice, can also be transfoπned via Agrobacterium co- cultivation. A common approach for rice is to excise immature rice flowers and place them on a nutrient agar medium containing an auxin (e.g., 2,4-D). After 7 to 10 days in the dark, the rice seed scutellum develops callus tissue that can be subcultured to fresh medium then contacted with Agrobacterium containing a Ti plasmid that is engineered to contain the desired genes of interest. As described above, this co-cultivation results in the mobilization of the T-DNA and its transfer into the plant cell.

Not all species of plants are amenable to the introduction of nucleic acids using Agrobacterium-medi&teά transfonnation. For such species, alternative methods for the introduction of nucleic acids exist. However, it will be appreciated that these alternative methods of nucleic acid introduction, can also be used with species that can be transfoπned by Agrobacterium.

In cases where Agrobacterium is not used to introduce the nucleic acid of interest into the plant host, microprojectile bombardment (biolistics) may be used as an alternative transfonnation approach (for example, see U.S. Patent No. 4,945,050 and U.S. Patent No. 5,036,006). hi this method, small metal particles, such as tungsten or gold, are coated with a layer of the nucleic acid of interest. These coated particles are then propelled into the host plant tissue (usually callus) using a gene transfonnation gun (for example the device described in U.S. Patent No. 5,036,006 and U.S. Patent No. 5,302,523) that utilizes helium or other suitable propellant. Many devices for biolistic transfonnation exist and one of ordinary skill in the art would recognize that any biolistic device that peπnits the delivery of the nucleic acid into the host plant cell is acceptable.

The nucleic acid that coats the microprojectile may comprise only the gene of interest or it may comprise an entire expression cassette, including regulatory elements and a selectable marker. Integration of the nucleic acid of interest into the chromosomal DNA of the host plant can be facilitated using methods such as those described in U.S. Patent No. 6,410,329. hi general, such methods utilize a nucleic acid construct comprised of the nucleic acid of interest flanked by Agrobacterium T-DNA border regions. This construct is co-transformed into the cell along with a gene that encodes the sequence-specific Agrobacterium recombinase. The recombinase recognizes the T-DNA border regions thereby promoting the integration of the DNA construct into the plant genome. It will be appreciated that nucleic acid that is inserted between the T-DNA border region is not limited to only a single gene of interest but rather could comprise multiple genes, expression cassettes or combinations thereof.

Other methods of introducing a nucleic acid of interest can also be used with the method of the present invention. Many of these alternative methods are for use with plant cells that have been converted into protoplasts (i.e. plant cells having degraded cell walls), h general protoplasts can be generated by enzymatic treatment the degrades the cell wall. Usually, the process begins by transfeπing undifferentiated plant cells to an osmotically balanced medium so that the cells will be isotonic with the medium when the cell wall is degraded. The resulting cell suspension is then heated with a mixture of cellulose degrading enzymes. Once the cell wall has been sufficiently degraded, the plant cell protoplasts are isolated then contacted with the nucleic acid of interest. Methods of introducing nucleic acid into protoplasts or other plant cells are known as direct uptake. Direct uptake can either be induced or uninduced. hi uninduced direct uptake, the nucleic acid of interest is introduced into the plant protoplast or cell without the aid of additional chemical or physical stimulus. Uninduced direct uptake generally results in low transfonnation efficiency.

Alternative methods have been developed to enhance the uptake of nucleic acids by plant cell protoplasts and increase the general applicability of protoplast based methods (for example, see U.S. Patent No. 5,231,019). One method of induced uptake, for example, utilizes chemical agents, such as polyethylene glycol. Incubation of the protoplasts with such agents creates a surface charge on the exposed protoplast membrane thereby facilitating transfer of the nucleic acid of interest across the cell membrane, hi general, polyethylene glycols having a molecular weight between 1000 and 10,000 can be used in these procedures, however, it will be appreciated that PEG of other sizes may also be used.

Physical methods, such as electroporation, have also been applied to plant cells (for example, see Neumann, E., et al. (1982). EMBO 7:841-845 and U.S. Patent No. 5,231,019). In electroporation methods protoplasts are transferred to an osmoticum, for example a mannitol/magnesiuni solution and the protoplast suspension is introduced into the electroporator chamber between two electrodes. By discharging a condenser over the suspension, the protoplasts are subjected to an electrical impulse of high voltage and brief duration, thereby effecting polarization of the protoplast membrane and causing the opening of pores in the membrane.

Heat shock can also be used to increase the uptake of nucleic acids by protoplasts (see U.S. Patent No. 5,231,019). i such methods the protoplast and nucleic acid of interest are mixed together then heat shocked by rapidly increasing the temperature of the mixture to approximately 45°C for about five minutes. It will be appreciated that the induced uptake methods can be used alone or together, as described in U.S. Patent No. 5,231,019. For example, PEG induced protoplasts may be additionally subjected to heat shock and/or electroporation.

An alternative method for facilitating transfonnation of a nucleic acid of interest into protoplasts utilizes lipofection (see U.S. Patent No. 4,394,448). In lipofection procedures, the nucleic acid of interest is encapsulated into a liposome or other lipid vesicle using methods well lmown in the art. The lipid-encapsulated nucleic acid is then contacted with plant cell protoplasts, thereby mediating fusion between the protoplast membrane and the lipid vesicle. The nucleic acid carried within the vesicle is ultimately released into the cell.

Nucleic acids can also be introduced into plant cells by microinjection methods (see, Graessmann, M. et a (1983). "Microinjection of Tissue Culture Cells", Meth. Enzymol.101: 482- 492). It will be appreciated that nucleic acids of interest that are introduced into plant cells via any of the above methods can be flanked by Agrobacterium T-DNA border regions to facilitate integration of the transfoπned nucleic acid into the chromosomal DNA of the host cell as described in U.S. Patent No. 6,410, 329).

It will be appreciated that the expression of nucleic acids that are transfoπned using any of the above-described methods can be achieved by creating a transcriptionally operative construct, which includes the nucleic acid of interest inserted downstream of a plant-functional promoter and upstream of transcriptional tenninator sequence. Transcripts corresponding to both the sense and antisense strands of the nucleic acid of interest can be produced depending on the orientation in which the nucleic acid of interest is inserted between the promoter and operator elements. Transcriptionally operative constructs can include for example, genes, gene fragments, antisense nucleic acids complementary to genes or portions of genes, hi some embodiments, the transcriptionally operative construct includes an antisense nucleic acid that has a length that is at least 15 nucleotides, at least 20 nucleotides, at least 25 nucleotides, at least 30 nucleotides, at least 35 nucleotides, at least 40 nucleotides, at least 45 nucleotides, at least 50 nucleotides, at least 55 nucleotides, at least 60 nucleotides, at least 65 nucleotides, at least 70 nucleotides, at least 75 nucleotides, at least 80 nucleotides, at least 85 nucleotides, at least 90 nucleotides, at least 95 nucleotides, at least 100 nucleotides, at least 200 nucleotides, at least 300 nucleotides, at least 400 nucleotides, or at least 500 nucleotides. hi a prefeπed embodiment, an antisense nucleic acid of a length described above is complementary to at least a portion of a nucleic acid encoding the enzyme quinolate phosphosribosyltransferase (QPTase) or its 5' upstream sequence or 3' downstream sequence (e.g., SEQ. ID.No.l or portions thereof). Preferably, the QPTase antisense sequence contains at least 15, 20, 25, 30, 35, 40, 45, 50, 55, 60, 65, 70, 75, 80, 85, 90, 95, 100, 200, 3300, 400, or 500 consecutive nucleotides of the gene described in International Publication Number W098/56923 corresponding to PCT Application Number PCT/US98/11893.

The transcriptionally active construct can be part of a plasmid, transfer vector or other nucleic acid vector and is capable of both transient and stable expression. In a prefeπed embodiment, the transcriptionally active construct is inserted into the genome of the target plant.

Expression level of the nucleic acid of interest can be modulated by selecting an appropriate promoter. For example, a strong constitutively active promoter, such as the CaMV 35 S promoter, may be used to achieve high-level expression of the nucleic acid of interest Alternatively, the promoter may be a root-cortex specific promoter, such as TobRD2 (as described in International Publication Number W098/56923 corresponding to PCT Application Number PCT/US98/11893) or other root specific promoters such as those described in U.S. Patent No. 5,459,252. It will be appreciated that in addition to the above-described promoters, any plant- functional promoter capable of achieving the desired expression effect can be used in the methods of the present invention. Notably, all of the preceding transformation approaches can be used in conjunction with the embodiments described herein. Although the nucleic acid of interest or the transformation vector harboring the nucleic acid of interest may or may not contain a selectable marker or nucleic acid encoding a selectable marker, the methods described herein do not require using a selectable marker as the basis for determining whether or not transformation was successful. Accordingly, the section below describes how selection can be accomplished in the absence of analysis of a selectable marker.

Identification of Transformants

It has been discovered that transformed cells require a period of time to recover from a gene integration event when plated on regeneration medium, while non-transfoπned cells do not experience such a growth lag. The differences in growth rate can be measured by analyzing aspects of plant cell development (e.g., growth stage or differentiation) and/or the proliferation of biomass. The lag period can be as little as an hour or portion thereof, or as short as a day or as long as several weeks. The growth of transfoπned tobacco cells, for example, can lag that of nontransfoπned cells by anywhere from less than 1 hour (with sufficiently sensitive instruments and or depending on growth conditions, observations of differences on a scale of minutes are contemplated) to about 1 hour, 2 hours, 3 hours, 4 hours, 5 hours, 6 hours, 7 hours, 8 hours, 9 hours, 10 hours, 11 hours, 12 hours, 13 hours, 14 hours, 15 hours, 16 hours, 17 hours, 18 hours, 19 hours, 20 hours, 21 hours, 22 hours, 23 hours or up to 24 hours. Additionally, the growth of transfoπned tobacco cells can lag that of non-transfoπned cells by one day, by two day, by three days, by four days, by five days, by six days, by seven days, by eight days, by nine days, by ten days, by eleven days, by twelve days, by thirteen days, by fourteen days, by fifteen days, by sixteen days, by seventeen days, by eighteen days, by nineteen days, by twenty days, by twenty-one days, or by more than twenty one days.

By monitoring the differences in the rate of growth in plant cells or tissue that has undergone transformation, one can rapidly identify and select cells that have been successfully transformed with a nucleic acid of interest. That is, transformed cells can be rapidly distinguished from non-transformed cells in a mixture of transformed and non-transformed cells by carefully analyzing differences in growth behavior, including but not limited to a lag in proliferation, accumulation of mass, and cell differentiation.

One embodiment concerns a process for enriching the number of transformed plants regenerated from a mixture of transformed and non-transformed plant cells cultured under nonselective conditions. By one approach, for example, tissue explants that have been exposed to Agrobacterium containing the nucleic acid of interest are cultured on non-selection regeneration medium for a time sufficient for the cells to recover and proliferate. The time required for regeneration depends on the plant variety that is transformed. For example, two to four weeks is usually sufficient to regenerate tobacco. After the regeneration period, the edge portions of leaf explants having buds are cut and discarded. Such samples are the fast growing (non-transfonned) tissues present in the mixture of transfonned and non-transfonned tissues.

The slower growing transformed tissue is cultured until it develops buds then it is transferred onto Murashige and Skoog medium (MS) containing 6-benzylaminopurine (shoot elongation medium) for a time period sufficient for the transformed plant cells to grow and generate shoots, the case of tobacco, a time period of 7-10 days is usually sufficient for shoot elongation. Individual nonnal shoots are then harvested and placed into rooting medium. The initial culture is then examined to identify any additional slow growing buds that have formed since the initial harvest of transformants. These buds are then transfeπed to shoot elongation media for a time period sufficient for the transfoπned explant to grow and proliferate. The average number of shoots per explant is about 20 shoots.

Rather than monitoring plant development, positive transformants can also be distinguished from non-transfoπnants by monitoring the accumulation of biomass. This can be accomplished by monitoring either differences in size or weight between the cell masses present on the plate (positive transformants will be smaller and lighter than the non-transformed cells). hi tobacco, there is approximately a two-week lag in the stage of growth between transfoπned and non-transfonned cells. In certain cases, it may be desirable to increase the difference between the growth rate of the plant cell transfonned with the nucleic acid of interest and the nontransfoπned plant cell. The difference in the growth rate between a plant cell that has been transfonned with the nucleic acid of interest and a plant cell that has not been transfonned can be adjusted by altering environmental conditions. Conditions that can be altered include, but are not limited to, composition of the growth medium, temperature, humidity, carbon dioxide levels and oxygen levels.

Plant cells can be cultured on a variety of media, which differ in their individual components or groups of components. However, the composition of most media is in accordance with the following principle: they contain a group of inorganic ions in the concentration range from about 10 mg/1 to several hundred mg/1 (so-called macroelements such as nitrate, phosphate, sulfate, potassium, magnesium, iron), a further group of inorganic ions in maximum concentrations of several mg/1 (so-called microelements such as cobalt, zinc, copper, manganese), then a number of vitamins (for example inositol, folic acid, thiamine), a source of energy and carbon, for example saccharose or glucose, and also growth regulators in the form of natural or synthetic phytohormones of the auxin and cytokinin classes in a concentration range from 0.01 to 10 mg/1. The culture media are additionally stabilized osmotically with sugar alcohols (for example mannitol) or sugar (for example glucose) or salt ions (for example CaCl₂), and are adjusted to a pH in the range from 5.6 to 6.5. Alteration of the composition of the growth medium can differentially influence the growth rate of both transformed and nontransfoπned cells. For example, in cases where the growth rate of transfonned cells is only slightly less than that of nontransfoπned cells, growth of each culture on a "lean" medium may result in a more pronounced difference between the growth rates of the transfoπned and nontransformed cells thereby facilitating detection of transformants.

Other potential ways to influence the growth of transformed and non-transfonned plant cells, include, for example, but not limited to, varying the temperature, humidity, carbon dioxide levels, and oxygen levels. The section below describes several approaches that can be used to regenerate transformed plant cells.

Regeneration of Transformed Plants

After the transformants have been allowed to propagate on shoot elongation medium they are ready for root induction. Rooting is perfonned by transferring the transfoπned shoot culture to an appropriate rooting medium such as T medium (see e.g., Science 163, 85-87 (1969)). hi the case of tobacco, root initials are generally visible within one week, and viable roots form within 2- 3 weeks. Plantlets of about 5 cm are suitable for transplantation to soil. hi general, soil mixtures that are useful for the rooting of plant initials are those that provide for substantial aeration. For example a soil mixture comprising 2:1:1 soil:ρerlite:vermiculite is adequate. Such a soil mixture can be conveniently sterilized by directly autoclaving in Magenta boxes. Alternatively, the soil mixture can be autoclaved separately and transferred to sterile PlantCondos. Initial fertilization of the soil mixture can be attained by adding to each container approximately 5-10 mL 1/4 strength Gamborg's B5 without hormones. Single transformed plantlets are then transferred to the soil mixture and cultured in a growth chamber. The growing plants are acclimated to atmospheric humidity by providing small openings in the box id.

In a prefeπed embodiment, tobacco plants are regenerated into mature plants after induction of rooting. The tobacco plants described herein are suitable for conventional growing and harvesting techniques (e.g., topping or no topping, bagging the flowers or not bagging the flowers, cultivation in manure rich soil or without manure) and the harvested leaves and stems are suitable for use in any traditional tobacco product including, but not limited to, pipe, cigar and cigarette tobacco and chewing tobacco in any form including leaf tobacco, shredded tobacco or cut tobacco, h a more prefeπed embodiment, genetically modified low nicotine and/or TSNA tobacco varieties can be produced and regenerated using the methods described herein. For example, tobacco cells can be transformed with an antisense nucleic acid corresponding to at least a portion of to the quinolate phosphoribosyl transferase (QTPase) gene. Providing such antisense nucleic acids to tobacco has been shown to reduce QPTase levels as well as nicotine levels in tobacco (see United States Patent Number 6,586,661, entitled REGULATION OF QUINOLATE PHOSPHORIBOSYL TRANSFERASE EXPRESSION BY TRANSFORMATION WITH A TOBACCO QUINOLATE PHOSPHORIBOSYL TRANSFERASE NUCLEIC ACID, filed on February 10, 1998). Tobacco cells transformed with the QPTase antisense nucleic acid are then selected using the methods described herein. Tobacco plants producing low levels of nicotine are then regenerated and subsequently grown under conditions conducive to low nicotine and/or TSNA production. Such plants can be harvested using techniques well known in the art and processed into a variety of tobacco products.

Tobacco having low nicotine and/or TSNA content can be processed as described above then blended with other tobacco to create a wide-range of tobacco products containing varying amounts of nicotine and/or TSNAs. These blended tobacco products can be used in tobacco product cessation programs so as to slowly move a consumer from a high nicotine and TSNA product to a low nicotine and TSNA product. For example, the blended tobacco products as described herein provide, but are not limited to, smoking materials (e.g., cigarettes, cigars, pipe tobacco), snuff, chewing tobacco, gum, and lozenges. The section below describes several approaches that can be used to verify the transmission of the gene and/or construct.

Verification of Transformation in Regenerated Plants

Plants that have been successfully transfonned with a nucleic acid of interest are referred to as transgenic plants or "genetically modified plants." As used herein, "transgenic" refers to the possession a foreign nucleic acid either transiently or permanently. A "transgenic plant" or "genetically modified plant" encompasses all descendants, hybrids, and crosses thereof, whether reproduced sexually or asexually. These plants further encompass seeds and/or other propagative tissues generated from one or more transgenic plants. Although a transgenic plant or genetically modified plant often has the nucleic acid of interest integrated into its genome, in some cases, the nucleic acid of interest may not be incorporated into the genome of the host plant.

Plants generated from cells that have been identified as transformants on the basis of having reduced growth rate when compared to a non-transfonned control cell can be verified by a variety of methods. Such methods include, but are not limited to, the use of visually screenable reporter genes such as, GUS or GFP; direct nucleic acid identification techniques such as, Southern blotting, Northern blotting or amplification of specific portion of the fransformed nucleic acid of interest using PCR and specific primers; or direct display of the transgenic phenotype. Genetically modified plants obtained by using the selection methods described herein, in a prefeπed embodiment, tobacco, are aspects of the invention.

Once the growing plantlets are mature, they can be used to produce transgenic progeny. For example, to breed progeny from plants transfoπned and identified according to the methods of the present invention, a procedure such as that which follows may be used. Tobacco plants produced using the methods described herein are grown in pots in a greenhouse or in soil, as is known in the art, and pennitted to flower. Pollen, which is obtained from the mature anthers of the transgenic plant, is used to pollinate the pistil of the same plant, sibling plants, or any desirable tobacco plant. Transgenic progeny obtained by this method may be distinguished from nontransfoπned progeny by the presence of the introduced gene(s) and/or accompanying DNA (genotype), or the phenotype confeπed. The transformed progeny may similarly be selfed or crossed to other plants, as is nonnally done with any plant carrying a desirable trait. Similarly, other transgenic plants produced by the methods described herein may be selfed or crossed as is lαiown in the art in order to produce progeny with desired characteristics. Transgenic progeny that result from transgenic plants obtained by the selection methods described herein, in a preferred embodiment tobacco, are also aspects of the invention. The selection method described herein can be used with various monocots and dicots, however, plants of the species Nicotiana, in particular Nicotiana tabacum, preferably Burley and Virginia flue cultivars are prefeπed. The section below provides more detail on the types of plants that can be used with the selection approaches described herein.

Applicability

Any tissue type or source of plant cells, which can serve as a target for transfonnation by any one or more of the various biological or non-biological delivery mechanisms available in the art, can be subject to the methods described herein. Such tissue types or cell sources include, but are not limited to, immature and mature embryos, pollen, protoplasts, suspension culture cells, callus cells, cotyledons, seeds or seedling parts, and leaves or leaf pieces and meristematic tissues.

Potential target plants compatible with the methods described herein include both monocotyledonous and dicotyledonous plants including, but not limited to, Acacia spp., Acer spp., Actinidia spp., Aesculus spp., Agathis australis, Albizia amara, Alsophila tricolor, Andropogon spp., Arachis spp., Areca catechu, Astelia fragrans, Astragalus cicer, Baikiaea plurijuga, Betula spp., Brassica spp., Brugniera gymnorrhiza, Burkea africana, Butea frondosa, Cadaba farinosa, Calandra spp., Camellia sinensis, Canna indica, Capsicum spp., Cassia spp., Centroema pubescens, Chaenomeles spp., Cinnamomum cassia, Cojfea arabica, Colophospermum mopane, Coronillia varia, Cotoneaster serotina, Crataegus spp., Cucumis spp., Cupressus spp., Cyathea dealbata, Cydonia oblonga, Cryptomeria japonica, Cymbopogon spp., Cynthea dealbata, Cydonia oblonga, Dalbergia monetaria, Davallia divaricata, Desmodium spp., Diclsonia squarosa, Diheteropogon amplectens, Dioclea spp., Dolichos spp., Dorycnium rectum, Echinochloa pyramidalis, Ehrartia spp., Eleusine coracana, Eragrestis spp., Erythήna spp., Eucalyptus spp., Euclea sc imperi, Eulalia villosa, Fagopyrum spp., Feijoa sellowiana, Fragaria spp., Flemingia spp., Freycinetia banksii, Geranium thunbergii, Ginkgo biloba, Glycine javanica, Glycine max, Gliricidia spp., Gossypium hirsutum, Grevillea spp., Guiboiirtia coleosperma, Hedysarum spp., Hemarthia altissima, Heteropogon contortus, Hordeum vulgare, Hyparrhenia rufa, ypericum erectum, Hyperthelia dissoluta, Indigo incamata, Iris spp., Leptarrhena pyrolifolia, Lespediza spp., Lettuca spp., Leucaena leucocephala, Loudetia simplex, Lotonus bainesii, Lotus spp., Macrotyloma axillare, Mains spp., Manihot esculenta, Medicago sativa, Metasequoia glyptostroboides, Musa sapientum, Nicotianum tobacum, Nicotianwn spp., Onobrychis spp., Omithopus spp., Oryza spp., Peltophorum africanum, Pennisetum spp., Persea gratissima, Petunia spp., Phaseolus spp., Phoenix canariensis, Phormium cookianum, Photinia spp., Picea glauca, Pinus spp., Pisum sativum, Podocarpus totara, Pogonarthria fleckii, Pogonarthria squarrosa, Populus spp., Prosopis cineraria, Pseudotsuga menziesii, Pterolobium stellatum, Pyrus communis, Quercus spp., Rhaphiolepsis umbeϊlata, Rhopalostylis sapida, Rhus natalensis, Ribes grossidaria, Ribes spp., Robinia pseudoacacia, Rosa spp., Rubus spp., Salix spp., Schyzachyrium sanguineum, Sciadopitys verticillata, Sequoia sempervirens, Sequoiadendron giganteum, Sorghum bicolor, Spinacia spp., Sporobolus fmbriatus, Stiburus alopecuroides, Stylosanthos humilis, Tadehagi spp., Taxodium distichum, Themeda triandra, Trifolium spp., Tritium spp., Tsuga heterophylla, Vaccinium spp., Vicia spp. Vitis vinifera, Watsonia pyramidata, Zantedeschia aethiopica, Zea mays, amaranth, artichoke, asparagus, broccoli, brassel sprout, cabbage, canola, caπot, cauliflower, celery, collard greens, flax, kale, lentil, oilseed rape, okra, onion, potato, rice, soybean, straw, sugarbeet, sugar cane, sunflower, tomato, squash, and tea, amongst others, or the seeds of any plant specifically named above or a tissue, cell or organ culture of any of the above species. Agronomically important crop plants such as tobacco, maize and cereal crops such as wheat, oats, rye, sorghum, rice, barley, millet, turf and forage grasses, and the like, as well as cotton, sugar cane, sugar beet, oilseed rape, bananas, poplar, walnut and soybeans are particularly prefeπed. Other potential target plants include gymnospeπns such as fir, pine, cedar, hemlock, spruce, and yew. The above lists of target plants are meant to be non-exclusive and can include other varieties of plants that can be transfonned with one or more nucleic acids and subsequently regenerated or otherwise propagated as a proliferative cell mass.

Transgenic plants obtained as described herein may take a variety of forms. The plants may be chimeras of transfonned cells and non-transfonned cells; the plants may be clonal transformants (e.g., all cells transformed to contain the transformed gene); the plants may comprise grafts of transformed and untransfonned tissues (e.g., a transfoπned root stock grafted to an untransfoπned scion in citrus species). The transgenic plants may be propagated by a variety of means, such as by clonal propagation or classical breeding techniques. For example, first generation (or T,) transformed plants may be selfed to give homozygous second generation (or T₂) • fransformed plants and the T₂ plants further propagated through classical breeding techniques.

As used herein, a crop comprises a plurality of plants of the present invention, and of the same genus, planted together in an agricultural field. By "agricultural field" is meant a common plot of soil or a greenhouse. Thus, the present invention provides a method of producing a crop of transgenic plants, h a prefeπed embodiment, the methods described herein are use to produce a crop of transgenic tobacco plants which have altered phenotypic traits such as having lowered QPTase activity and thus having decreased nicotine and/or TSNA levels, as compared to a similar crop of non-transfoπned plants of the same species and variety. The examples that follow are set forth to illusfrate several embodiments described above and are not intended to limit the scope of the invention.

EXAMPLE 1 Transfonnation of Burley 21 Tobacco Cells with a Reporter Gene Construction of Binary Vector Containing Reporter Gene

An Agrobacterium binary vector is prepared by removing the selectable marker gene present between the Ti-border region. The entire coding region of the GUS gene or the GFP gene is then inserted between the Ti-border regions under control of CaMV 35S promoter and the 35S tenninator (see e.g., ReicheL et al. (1996) PNAS 93:5888-5893). Each of the binary vector reporter constructs are transfonned into Agrobacterium as described below.

Transformation of Agrobacterium

Agrobacterium tumefaciens strain LBA4404, as described by Hoekema et al. (1983) Nature, 303: 179-180, is used to transfonn Burley 21 tobacco. Prior to the transfonnation of Burely 21, Agrobacterium is transfoπned with the binary vector caπying either a GUS or an GFP reporter gene construct. Transfonnation is by electroporation in a 1mm gap cuvette at a charging voltage of approximately 1.4kV with a pulse length of 5 ms. After the transfonnation, the Agrobacterium are cultured on a solid YEB medium containing Beef extract, yeast extract, peptone, salt and sucrose. Sucrose is the energy source and agar is used as the gelling agent. Proper antibiotics, Kanamycin (50 mg/L) and rifampicin (25 mg/L) are included in the medium in order to select the transformed Agrobacterium caπying the binary vector containing the reporter construct. Agrobacterium colonies appear after about about 2 to 3 days when incubated in the dark at about 28°C and a single nonnal growing colony of Agrobacterium is isolated from the plate and inoculated into 10 ml liquid YEB medium, as described above, without gelling agent. The Agrobacterium culture is grown on a shaker overnight in the dark at 28°C then the overnight cultures are transfeπed to fresh YEB medium and grown without antibiotics for another 10 to 12 l rs. Agrobacterium cell density is deteπnined by the OD₆₀o and a cell density of about 2 x 10⁹ cells/ml at 1.0 of OD_fi00 is typically obtained. Acetosyringone, a virulence inducing agent, is included in the medium at concentration from about 10 to 200 μM to enhance infection of Agrobacterium. Transformation of tobacco leaves

Young leaves (2.5-3.5 cm in length) from in vitro maintained shoots of Nicotiana tabacum (Burley 21) are used as the leaf explant sources after about 3 weeks on rooting medium containing MS salts (Murashige 1962), sucrose and B5 vitamins. Leaves are harvested and then cut into about 0.8 cm squares and immersed in Agrobacterium culture for about 20 minutes (see, for example, Horsch et ah, (1986) Science 227: 1229-1231 for a description of leaf disk fransfonnation). During the inoculation period, the plates are gently rotated to ensure that all the leaf disks were exposed to the Agrobacterium culture.

Leaf disks are then removed from the inoculum and each of the explants are blotted on several layers of sterile paper, hifected leaf explants are then cultured for 3 days on MS basal medium without antibiotics. Following this incubation, leaf disks are then transferred onto nonselective regeneration medium comprising MS basal medium (Sigma), BA 1 mg/L and 0.1 mg/L NAA and agar at PH 5.7 for shoot induction. By following the protocol described above, one will obtain Burley 21 tobacco cells that have been transfoπned with a marker gene (e.g., GFP or GUS). The following example describes an antisense fransfonnation experiments conducted in Nicotiana tabacum.

EXAMPLE 2 Transfonnation of Burley 21 Tobacco Cells with an Antisense Nucleic Acid

Cells of Burley 21 tobacco plants are transfonned with a nucleic acid complementary to the gene encoding the enzyme quinolate phosphosribosyltransferase (QPTase) (SEQ ID NO: 1), as described in International Publication Number W098/56923 coπesponding to PCT Application Number PCT/US98/11893. Specifically, the nucleic acid of SEQ ID NO: 1, in antisense orientation, is operably linked to a plant promoter (CaMV 35S or TobRD2 root-cortex specific promoter) to produce two different transcriptionally operative constructs: CaMV 35S promoter/antisense SEQ ID NO: 1 and TobRD2 promoter/antisense SEQ ID NO: 1.

A wild-type tobacco line and a low-nicotine tobacco line are selected for fransfonnation, e.g., wild-type Burley 21 tobacco (Nicl^l+/Nz^'c2⁺) and homozygous Nicl^~INic2^~ Burley 21. A plurality of tobacco plant cells from each line are transfoπned using each of the transcriptionally operative constructs inserted between the Ti-border sequences of an Agrobacterium-bmary vector, as described in the previous example. By following the protocol described above, Burley 21 tobacco cells containing a QPTase antisense construct can be obtained. The following example describes more fransfonnation experiments conducted in Nicotiana tabacum. EXAMPLE 3

Transformation of Pooled QPTase Antisense Nucleic

Acids in Burley and Virginia Flue Varieties of Tobacco

Sets of nucleic acid fragments of the QPTase gene (SEQ ID NO: 1), as described in International Publication Number W098/56923 corresponding to PCT Application Number PCT/US98/11893, consisting of, consisting essentially of, or comprising at least 20, 25, 30, 40, 50, 75 or 100 consecutive nucleotides of SEQ. ID NO: 1, are generated using PCR or oligonucleotide synthesis. Primers can be constructed so that convenient restriction sites are present to facilitate cloning. Longer nucleic acids, such as 100-mers, and 75-mers are more conveniently synthesized using by amplifying specific regions of SEQ ID NO: 1 using PCR. Shorter fragments such are 20- mers are more conveniently generated by synthesizing a single stranded portion of SEQ ID NO: 1 and its nucleic acid complement. The two strands are then annealed to form the double stranded oligonucleotide. Such nucleic acid sets are generated so that each nucleic acid in the set contains at least a five base pair region of overlap with at least one other nucleic acid in the set. Each set of nucleic acids is constructed so as to contain a sufficient number of overlapping fragments to represent the entire length of SEQ ID NO: 1.

Each of the above-described sets of nucleic acids are inserted into a binary Agrobacterium vector by operably linking the nucleic acids in each set either to the CaMV 35S promoter or to the TobRD2 promoter in an antisense orientation, as described in the previous example. The binary vectors are then transformed in Agrobacterium by electroporation, as described above.

Burley and Virginia Flue tobacco leaf disks are prepared and transfoπned using the Agrobacterium fransfonnation procedure described in the previous examples. By following the approach detailed in this example, Burley and/or Virginia Flue tobacco cells containing pools of antisense fragments of the QPTase gene can be obtained. The following example describes an approach that can be used to identify positively transfonned Nicotiana tabacum and to differentiate and select such positively transfonned Nicotiana tabacum tissue from non-transfonned plant tissue.

EXAMPLE 4 Identification and Differentiation of Tobacco Transformants

Burley 21 and Virginia Flue tobacco leaf disks are transformed using the Agrobacterium co-cultivation procedure, as described in previous examples. Subsequent to cocultivation, the leaf disk explant is cultured on non-selective regeneration medium for 2 to 4 weeks. During this period, the culture is observed to determine regions of the explant that are growing at a relatively high rate, as well as, areas that are growing at a relatively slow rate. Growth rate is conveniently measured by examining the change in morphology of the edges of the explant over time. In particular, rapidly proliferating nontransformed edges of the explant will produce buds sooner than the slower growing transfonned edges. After a time sufficient to permit clear differentiation between budding and nonbudding regions of the explant, budding edges are cut and discarded. Nonbudding portions of the explant that remain are enriched in transformed cells. Such slower growing transfonned explant tissue is pennitted to grow until it begins budding.

The slow growing buds are then transferred to Murashige and Skoog medium (MS) containing 6-benzylaminopurine (shoot elongation medium) and allowed to grow for 7-10 days, which is typically the amount of time necessary for tobacco to generate shoots. After this incubation period, additional slow growing buds that have continued to develop from the initial explant culture are also transferred to shoot elongation medium. Typically about 20 transformed shoots are obtained for tobacco. By following this selection approach, transfoπned tobacco cells can be readily identified, selected, and differentiated from nontransformed tobacco cells. The following example describes an approach that can be used to regenerate transfonned Nicotiana tabacum tissue into a transgenic plant.

EXAMPLE 5 Regeneration of Transgenic Tobacco Plants

The transformants selected by using the approaches described in Example 4 can be regenerated into transgenic plants using the following approach. Shoots are transferred to a rooting medium containing indolebutyric acid (IBA) or other auxin such as 2,4-D. For both Burley and Virginia Flue varieties, root initials are generally visible within one week, and viable roots form within 2-3 weeks. When plantlets reach about 5 to 6 centimeters in length they are transferred to soil supplemented with 25% (w/w) perlite and 25% (w/w) vermiculite. The transgenic tobacco plants (T₀ generation) are grown in Magenta boxes in a growth chamber until they reach maturity. The following example describes ways to verify that the selected transfonnants contain the gene of interest.

EXAMPLE 6

Verification of Transfonnation, Gene Expression and

Gene Propagation in Transgenic Burley 21 and Virginia Flue Tobacco Varieties

The T₀ plants are tested to detennine whether they have been successfully transfoπned with either the GUS or the GFP reporter gene constructs (Example 1). GUS activity is measured in plant leaves using 5-bromo-4-chloro-indolyl glucuronide or in tissues extracts by fluorimefry using 4-methylumbelliferyl b-D-glucuronide. GFP activity is measured by visual detection of fluorescence using a long-wave UV lamp.

The T₀ plants transfoπned with a binary vector containing SEQ ID NO: 1 or a portion thereof operably linked to either the CaMV 35S promoter or the TobRD2 promoter in an antisense orientation (see Examples 2 and 3) are grown to maturity and tested for levels of nicotine. T₀ plants are then selfed and the segregation of the transgene is analyzed in next generation, the Tj progeny. Ti progeny are grown to maturity and selfed; segregation of the transgene among T₂ progeny indicates which Ti plants are homozygous for the transgene.

Nicotine levels of T, progeny segregating 3:1 are measured qualitatively using a micro- assay technique. Approximately 200 mg fresh tobacco leaves are collected and ground in 1 ml extraction solution (extraction solution: 1 ml Acetic acid in 100 ml H₂0). Homogenate is centrifuged for 5 min at 14,000 x g and supernatant removed to a clean tube, to which the following reagents are added: 100 μL NH₄OAC (5 g/100 ml H₂0 + 50 μL Brij 35); 500 μL Cyanogen Bromide (Sigma C-6388, 0.5 g/100 ml H₂0 + 50 μL Brij 35); 400 μL Aniline (0.3 ml buffered Aniline in 100 ml NH₄OAC + 50 μL Brij 35). A nicotine standard stock solution of 10 mg/ml in extraction solution is prepared and diluted to create a standard series for calibration. Absorbance at 460 ran is read and nicotine content of test samples are deteπnined using the standard calibration curve.

Ti progeny that have less than 10%> of the nicotine levels of the Burley 21 parent are allowed to self to produce T₂ progeny. Homozygous T₂ progeny are identified by using real time PCR (homozygous progeny have approximately twice the copy number of the transgene). Nicotine levels in homozygous and heterozygous T progeny are qualitatively deteπnined using the micro- assay are expected to show levels less than 10% of the Burley 21 parent. Leaf samples of homozygous T₂ progeny are then processed for quantitative analysis of nicotine levels using Gas Chromatography/Flame Ionization Detection (GC/FID).

The transgenic plants producing the lowest nicotine levels are allowed to self-cross, producing T₃ progeny. T₃ progeny are grown and nicotine levels assayed qualitatively and quantitatively. T₃ progeny are allowed to self-cross, producing T₄ progeny. Samples of the bulked seeds of the T₄ progeny are grown and nicotine levels tested. hi general, the transgenic Burley 21 is expected to be similar to nontransformed Burley 21 in all assessed characteristics, with the exception of alkaloid content and total reducing sugars (e.g., nicotine and nor-nicotine). The transgenic Burley 21 can be distinguished from wildtype Burley 21 by its substantially reduced content of nicotine, nor-nicotine and total alkaloids. Similar results are expected using Virginia Flue. By following these approaches, the transgenic plants obtained according to the methods described in the previous examples, can be verified as positive transformants. The following example describes the manufacture of tobacco products from the transgenic plants described in Examples 5 and 6.

EXAMPLE 7 Blended Tobacco Products Containing Low Nicotine/TSNA Producing Transgenic Tobacco

The following example describes several ways to create tobacco products having specific amounts of nicotine and/or TSNAs through blending. Some blending approaches begin with tobacco prepared from varieties that have extremely low amounts of nicotine and/or TSNAs. By blending prepared tobacco from a low nicotine/TSNA variety (e.g., low nicotine varieties described above) with a conventional tobacco (e.g., Burley, which has 30,000 parts per million (ppm) nicotine and 8,000 parts per billion (ppb) TSNA; Flue-Cured, which has 20,000 ppm nicotine and 300 ppb TSNA; and Oriental, which has 10,000 ppm nicotine and 100 ppb TSNA), tobacco products having virtually any desired amount of nicotine and/or TSNAs can be manufactured. Tobacco products having various amounts of nicotine and/or TSNAs can be incorporated into tobacco use cessation kits and programs to help tobacco users reduce or eliminate their dependence on nicotine and reduce the carcinogenic potential.

For example, a step 1 tobacco product can comprise a blend of low nicotine Burley and conventional Virginia flue so as to obtain a final amount of nicotine that is approximately l.Omg/g. A step 2 tobacco product can comprise a slightly different blend of low nicotine Burley and conventional Virginia flue so as to obtain a final amount of nicotine that is approximately 0.6mg/g and a step 3 tobacco product can comprise either 100%) low nicotine Burley or a blend of low nicotine Burley and low nicotine Virginia flue so as to obtain a final amount of nicotine that is O.lmg/g or less.

It will be appreciated that tobacco products are often a blend of many different types of tobaccos, which were grown in many different parts of the world under various growing conditions. As a result, the amount of nicotine and TSNAs will differ from crop to crop. Nevertheless, by using conventional techniques one can easily detennine an average amount of nicotine and TSNA per crop so as to create any desired blend. By adjusting the amount of each type of tobacco that makes up the blend one can balance the amount of nicotine and/or TSNA with other considerations such as appearance and flavor., and smokeability. In this manner, a variety of types of tobacco products having varying level of nicotine and/or nitrosamine, as well as varying appearance and flavor and smokeability can be created.

A tobacco use cessation kit can comprise an amount of tobacco product from each of the aforementioned blends to satisfy a consumer for a single month program. That is, if the consumer is a one pack a day smoker, for example, a single month kit would provide 7 packs from each step, a total of 21 packs of cigarettes. Each tobacco use cessation kit would include a set of instructions that specifically guide the consumer through the step-by-step process. Of course, tobacco products having specific amounts of nicotine and/or TSNAs would be made available in conveniently sized amounts (e.g., boxes of cigars, packs of cigarettes, tins of snuff, and pouches or twists of chew) so that consumers could select the amount of nicotine and/or TSNA they individually desire. There are many ways to obtain various low nicotine/low TSNA tobacco blends using the teachings described herein and this example is intended to merely guide one of skill in the art through the process. The following example describes an approach that can be used to transform cells from a variety of plant species. EXAMPLE 8 Transformation of Monocots, Dicots, and Conifers

Plant species that are amenable to transformation of nucleic acids can be used in the selection methods described herein. Monocots

Methods for the transformation and regeneration of wheat are described in U.S. Patent No. 5,955,362. Methods for the transformation and regeneration of corn, rice and barley are provided in U.S. Patent No. 6,372,963. Methods for the transformation and regeneration of gramineous plants such as corn, wheat, rice, and ιye are provided in U.S. Patent No. 6,002,070.

From monocotyledonous plants generally, the compact embryogenic callus can be obtained by in vitro culture of explant sources such as immature zygotic embryos, mature seeds, leaf bases, anthers, microspores, young inflorescences, etc. hi com, the type I callus can be most efficiently generated from immature zygotic embryos. The compact embryogenic callus can be induced from the appropriate explants and maintained in culture according to well-established methods (see e.g., Hodges et al (1986) Bio/Technology 4:219). During maintenance of the callus culture, care should be taken to select and subculture only the embryogenic sectors of the calli in which are the embryogenic cells. Such cells can generally be characterized as small, tightly packed, thin-walled, richly cytoplasmic, highly basophilic cells containing many small vacuoles, lipid droplets and starch grains (Vasil (1988) supra). The most convenient way to remove, from a plant, tissues that are lαiown to be capable of foπning the compact embryogenic callus is by means of dissection.

Competent cells can be obtained directly from a monocotyledonous plant by cutting from the plant, in a conventional manner, intact tissue that is capable of foπning compact embryogenic callus. The cells of such wounded intact tissue can then be stably transfonned. Alternatively, wounded intact tissue can be cut into smaller fragments to wound further such tissue and provide more competent cells for transfonnation. The average maximum dimension of the tissue fragments is preferably 0.1 to 5 mm long, particularly 1 to 2.5 mm long, more particularly 1.25 to 1.75 mm long. The wounded intact tissue can be any piece of tissue that is cut from the plant or any fragments thereof (e.g., cut pieces). Thus, the tenn "intact tissue" should be understood as refeπing to aggregates of monocot plant cells that are obtained from a naturally occurring plant part, without a tissue-culturing stage in between.

"Mechanical disruption" and "wounding" of the cell wall encompasses the significant damaging of the cell wall of one or more cells of the intact tissue in order to expose the cell(s) and render the cell(s) open to insertion of a DNA fragment. This can be accomplished, for example, by cutting the cell wall, physically removing one or more portions of the cell wall, or rendering the cell wall discontinuous in one or more places, such as by abrading, squeezing or striking the cell wall. Mechanical disruption or wounding of the intact tissue can be supplemented or even replaced by a treatment of the intact tissue with an enzyme or enzyme mixture to degrade the plant cell walls, especially when the intact tissue is relatively large. The enzyme treatment can be canied out in a conventional manner. The enzyme can be applied to the intact tissue primarily to generate pores in its cell walls. The enzyme treatment can be relatively short (e.g., from 1 to 10 minutes depending upon the nature and the consistency of the intact tissue) so as not to cause a complete disruption of the tissue. Depending upon the type of plant, various enzymes or enzyme solutions can be used such as those listed by Powell and Chapman (1985) "Plant Cell Culture, A Practical Approach", R. A. Dixon ed., Chapter 3.

When the intact tissue, obtainable from the plant, is too small to be wounded (e.g., cut) or wounded intact tissue is too small to be further wounded (e.g., cut into smaller pieces), the enzyme treatment can be used to generate additional competent cells. Such an enzyme treatment can also be particularly useful, by itself, for foπning competent cells in embryos, particularly in immature zygotic embryos isolated from developing seeds and in mature zygotic embryos isolated from mature (e.g., dry) seeds of, for example, corn. Embryos are generally not cut to remove them from seeds and generally cannot be cut into significantly smaller fragments without destroying their ability to generate compact embryogenic callus. Immature embryos are particularly important in com as they are the only convenient and reliable source of compact embryogenic callus, rice and other monocots, mature embryos can also be used. For plants such as corn, it is preferced that the intact tissue (e.g., immature corn embryos) have a maximum length of about 0.5 to 2 mm, preferably 0.5 to 1.5 mm, even though smaller lengths of 0.5 to 1 mm can be used.

The intact tissue can also be subjected to a period of, for example, about 15 minutes or more, preferably about 30 minutes to about 5 hours, particularly 2 to 3 hours, of preplasmolysis which involves placing the tissue in a conventional hypertonic solution, such as the electroporation buffer discussed below. The purpose of this preplasmolysis treatment is to separate at least partly, in the cells of the intact tissue, their protoplasts, preferably all or at least part of their cell membranes, from their cell walls. The preplasmolysis should be caπied out after any wounding of the intact tissue but before any enzyme treatment of the intact tissue. When the intact tissue has already been degraded by an enzyme treatment, any subsequent preplasmolysis should be only for a brief period, and after the enzyme treatment of immature embryos of corn, as discussed above, it is preferred that such preplasmolysis not be caπied out at all.

Competent cells can also be obtained by culturing in vitro the intact tissue of this invention to produce compact embryogenic callus; and then cutting the callus into smaller fragments. The resulting callus fragments should comprise, wholly or at least in part, the embryogenic sectors or parts of the callus. The callus fragments also preferably have an average maximum length of 0.5 to 2.5 mm, particularly 1 to 2 mm, more particularly 1.25 to 1.75 mm, and preferably have a minimum length of about 0.1 mm. To obtain sufficient amounts of compact embryogenic callus, it is preferred to propagate the primary callus, as obtained from tissue explants, for at least one month and to subculture the embryogenic sectors of such primary callus at least once during this period. Mechanical disruption of the callus may be supplemented or replaced by an enzyme treatment to degrade the callus cell walls, especially when the compact embryogenic callus fragments remain relatively large. This enzyme treatment can be carried out in a conventional manner. The enzyme treatment preferably serves primarily to generate pores in the cell walls of the cells of the callus fragments, and it is therefore recommended that the enzyme treatment be relatively short, preferably from 1 to 10 minutes depending upon the consistency of the callus fragments, so as not to cause a complete disruption of the tissues. Depending upon the monocot plant, various enzymes or enzyme solutions can be used such as those listed by Powell and Chapman (1985) supra. Preferably, the compact embryogenic callus fragments are also subjected to a period (e.g., 2 to 3 hours) of preplasmolysis, as discussed above.

The wounded and/or degraded, intact tissue or compact embryogenic callus fragments, particularly their embryogenic sectors, obtained as described above, are then brought into contact with one or more nucleic acid fragments (e.g., DNA or RNA or RNAi) containing gene(s) of interest in order to transfonn their competent monocot plant cells of this invention. Direct gene transfer can be accomplished for example, by electroporation, direct gene transfer using polyethyleneglycol, bombardment with DNA-coated microprojectiles (i.e., biolistic transfonnation using, for example, a particle gun), and Agrobacteriwn-mediateά transfonnation.

The compact embryogenic callus, used in carrying out the plant transfonnation can have certain characteristics of a friable embryogenic callus. In this regard, a compact embryogenic callus or a friable embryogenic callus can change or be caused to change into a type of callus that has some of the characteristics of compact embryogenic callus as well as some characteristics of friable embryogenic callus. As a result, such an intennediate type of callus and embryogenic portions thereof can sometimes be transformed. Indeed, somatic embryos that develop on such an intermediate type of callus, as well as on friable embryogenic callus, can be isolated and can be wounded and/or degraded and then transfoπned as described above. Thus, such somatic embryos obtained from an intennediate type callus or a friable embryogenic callus can be regarded as equivalent to immature or mature zygotic embryos obtained from developing or mature seeds, particularly when electroporation is used as the means for transforming cells of the somatic embryos.

Electroporation can be caπied out in a conventional manner. In this regard, the wounded and/or degraded intact tissue or callus fragments, particularly meristematic or embryogenic sections thereof, quite particularly embryogenic sections thereof, can be transferred to a cuvette suitable for use in an electroporation apparatus (e.g., as described by Dekeyser et al (1990) The Plant Cell 2:591). Preferably, about 10 to 500 mg, particularly about 50 to 200 mg, most particularly about 100 to 150 mg, of intact tissue or callus fragments per 200 μl of electroporation buffer are transfeπed to the cuvette. For cereals, such as com, (where it is prefeπed to use intact enzyme-treated immature embryos), it is prefeπed that about 10 to 500 embryos, particularly about 50 to 150 embryos, more particularly about 75 to 125 embryos, in 200 μl of electroporation buffer are transferred to the cuvette. The nucleic acid is then added to the cuvette, and the electroporation is carried out. Preferably, the nucleic acid is coincubated (e.g., for about 1 hour) with the intact tissue or callus fragments prior to electroporation. It is believed that best results can be obtained with linear, rather than circular, DNA of relatively small size, preferably smaller than about 20 kb, especially smaller than 15 kb, particularly smaller than 10 kb, quite particularly smaller than 6 kb (e.g., down to about 2-3 kb). hi this regard, multiple linear DNA fragments of different composition can be used to transform the competent cells of this invention with multiple genes of interests. Preferably, about 5 to 30 μg, particularly about 10-25 μg, quite particularly about 20 μg, of DNA is added to the cuvette containing the intact tissue or callus fragments. Particular electroporation conditions are not believed to be critical, and good results can be obtained, for example, with a discharge of one pulse with a field strength of 375 V/cm from a 900 μF capacitor using an electroporation buffer containing 150 mM NaCl or 80 niM KC1 (Dekeyser et al (1990) supra). Dicots

Methods for the transformation and regeneration of brassica species are provided in U.S. Patent No. 6,201,169. Methods for the transfonnation and regeneration of soybean are provided in U.S. Patent No. 6,384,301. Methods for the transfonnation and regeneration of sunflower are provided in U.S. Patent No. 6,265,638. Methods for the transfonnation and regeneration of tomato are provided in U.S. Patent No. 6,225,528. Methods for the fransfonnation and regeneration of cacao are provided in U.S. Patent No. 6,150,587. Methods for the fransfonnation and regeneration of cotton and kenaf are provided in U.S. Patent No. 6,150,587. Conifers

Methods for the transformation and regeneration of pine are provided in U.S. Patent No. 4,886,937. Higher Plants Generally

Methods for the transfonnation and regeneration of higher plants, such as tobacco, rice, tomato, potato, petunia, maize, rape, and the like, are provided in U.S. Patent No. 5,731,179. For example, plants can be transfoπned with a single Agrobacterium or a binary vector system can be used. By one approach, a hybrid vector is used.

The hybrid vector is prepared by homologous recombination of an acceptor vector and an intennediate vector, which homologous recombination occurs in a bacterium belonging to genus Agrobacterium. The acceptor vector is a plasmid which can replicate in both Agrobacterium bacterium and Escherichia coli. The acceptor vector contains a DNA fragment which is homologous to a DNA fragment in the intermediate vector. Utilizing this DNA fragment, the acceptor vector can incorporate the intennediate vector therein by homologous recombination in an Agrobacterium bacterium. The intermediate vector is a plasmid which can replicate in Escherichia coli but can not replicate by itself in an Agrobacterium bacterium. The intermediate vector contains a DNA fragment which is homologous to the DNA fragment contained in the acceptor vector. The intennediate vector can be incorporated into the acceptor vector by homologous recombination through the DNA fragment. Once incoiporated into the acceptor vector, the intermediate vector can be maintained in the bacterium belonging to the genus Agrobacterium. The following example describes an approach that can be used to identify and differentiate transfoπned cells and/or tissues transfoπned from a variety of plant species.

EXAMPLE 9 Identification of Plant Transformants

The methods described in Example 4 can be applied to identify and differentiate fransformed plant cells and/or tissues from nonfransfonned cells and/or tissues from the plants species that are transformed according to the approach in Example 8. hi cases where comparing morphological differences is not convenient, the accumulation of cell mass over time can be compared to differentiate slow growing transfonned cells from rapidly growing nontransfoπned cells. The following example describes an approach that can be used to regenerate transgenic plants from a variety of plant species.

EXAMPLE 10 Regeneration of Transgenic Plants

Methods for regenerating plants that have been transfoπned in Example 8 and identified as transformants in Example 9 can be regenerated as follows. Methods for the regeneration of wheat are described in U.S. Patent No. 5,955,362. Methods for the regeneration of brassica species are provided in U.S. Patent No. 6,201,169. Methods for the regeneration of pine are provided in U.S. Patent No. 4,886,937. Methods for the regeneration of soybean are provided in U.S. Patent No. 6,384,301. Methods for the regeneration of corn, rice and barley are provided in U.S. Patent No. 6,372,963. Methods for the regeneration of sunflower are provided in U.S. Patent No. 6,265,638. Methods for the regeneration of tomato are provided in U.S. Patent No. 6,225,528. Methods for the regeneration of cacao are provided in U.S. Patent No. 6,150,587. Methods for the regeneration of cotton and lcenaf are provided in U.S. Patent No. 6,150,587.

Although the invention has been described with reference to embodiments and examples, it should be understood that various modifications can be made without departing from the spirit of the invention. Accordingly, the invention is limited only by the following claims.

Claims

WHAT IS CLAIMED IS:

1. A method of identifying a transformed plant cell or differentiating said transfoπned plant cell from a non-transformed plant cell, said method comprising: providing a candidate plant cell; introducing a nucleic acid into said candidate plant cell thereby producing a fransfomied plant cell; culturing said transfonned plant cell; and identifying said transfoπned plant cell by comparing a growth rate or a moiphological feature of said transfonned plant cell with a growth rate or a morphological feature of a non-transformed plant cell that is cultured under similar conditions, wherein said growth rate or the moiphological feature of said transfoπned plant cell is different when compared to said growth rate or the morphological feature of said non-transfonned plant cell.

2. The method of Claim 1, wherein said candidate plant cell is a protoplast.

3. The method of Claim 1, wherein said nucleic acid is introduced into said candidate plant cell by electroporation.

4. The method of Claim 1, wherein said nucleic acid is introduced into said candidate plant cell by microprojectile bombardment.

5. The method of Claim 1, wherein said nucleic acid is introduced into said candidate plant cell by co-cultivation with Agrobacterium.

6. The method of Claim 1, wherein said nucleic acid is introduced into said candidate plant cell by microinjection.

7. The method of Claim 1, wherein said fransfomied plant cell and said nonfransfomied plant cell are present together in a mixture.

8. The method of Claim 1, wherein said transfonned plant cell and said nontransformed plant cell are present as part of a plant tissue.

9. The method of Claim 1 , wherein said transformed plant cell is a monocot cell.

10. The method of Claim 1, wherein said transfonned plant cell is a dicot cell.

11. The method of Claim 1 , wherein said transformed plant cell is gymnosperm cell.

12. The method of Claim 1 , wherein said transfonned plant cell is from a plant nieristem.

13. The method of Claim 1 , wherein said transfoπned plant cell is a tobacco cell.

14. The method of Claim 13, wherein said tobacco cell is a Nicotiana tobacum cell.

15. The method of Claim 13, wherein said tobacco cell produces a reduced level of nicotine.

16. The method of Claim 1 , wherein said nucleic acid is a DNA.

17. The method of Claim 16, wherein said DNA encodes a protein.

18. The method of Claim 16, wherein said DNA is operably linked to a promoter in a sense orientation.

19. The method of Claim 16, wherein said DNA is operably linked to a promoter in an antisense orientation.

20. The method of Claim 16, wherein said DNA integrates into the genome of said fransfomied candidate plant cell.

21. The method of Claim 1, wherein said transformed plant cell and said nonfransfonned plant cell are cultured on the same growth medium.

22. The method of Claim 21 , wherein said growth medium is a solid.

23. The method of Claim 21 , wherein said growth medium is a liquid.

24. The method of Claim 1, wherein said transformed plant cell and said nontransformed plant cell are cultured under the same environmental conditions.

25. The method of Claim 1 , wherein said transfoπned plant cell and said nonfransfonned plant cell are cultured for a sufficient amount of time to pennit growth of a transfoπned cell mass and a nontransfoπned cell mass.

26. The method of Claim 1, wherein said transformed plant cell and said nontransformed plant cell are cultured for a sufficient amount of time to permit regeneration of a transformed plant and a nonfransfonned plant.

27. The method of Claim 1, wherein said growth rate is detennined by measuring a change in biomass over a predeteπnined time interval.

28. The method of Claim 1, wherein said growth rate is determined by evaluating a moiphological feature that is characteristic of developmental stage after a predetermined time interval.

29. The method of Claim 28, wherein said morphological feature is tissue differentiation.

30. The method of Claim 1, wherein said growth rate of said fransfomied cell lags the growth rate of said nontransformed cell by at least about one hour.

31. The method of Claim 1, wherein said growth rate of said transfonned cell lags the growth rate of said nontransformed cell by at least three hours.

32. The method of Claim 1, wherein said growth rate of said fransfomied cell lags the growth rate of said nontransformed cell by at least 6 hours.

33. A method of producing a transgenic plant said method comprising: introducing a nucleic acid into a candidate plant cell; deteni ining whether said candidate plant cell is transformed by comparing a growth rate or a morphological feature of said candidate plant cell with a growth rate or moiphological feature of a nonfransfonned plant cell, wherein said candidate plant cell is fransfomied if the growth rate or morphological feature of said candidate plant cell is different when compared to the growth rate of said nonfransfonned cell; and regenerating a transgenic plant from said fransfomied candidate plant cell.

34. The method of Claim 33, wherein said transgenic plant is a tobacco plant.

35. The method of Claim 34, wherein said transgenic plant is Nicotiana tobacum.

36. The method of Claim 33, wherein said transgenic plant is produces a reduced level of nicotine.

37. The method of Claim 33, wherein said nucleic acid is a DNA.

38. The method of Claim 37, wherein said DNA encodes a protein.

39. The method of Claim 37, wherein said DNA is operably linked to a promoter in a sense orientation.

40. The method of Claim 37, wherein said DNA is operably linked to a promoter in an antisense orientation.

41. The method of Claim 37, wherein said DNA integrates into the genome of said transfonned candidate plant cell.

42. A transgenic plant cell produced by a method comprising: introducing a nucleic acid into a candidate plant cell thereby producing a transformed plant cell; culturing said transfonned plant cell; and identifying said transfonned plant cell by comparing a growth rate or morphological feature of said transfoπned plant cell with a growth rate or moiphological feature of a nonfransfonned plant cell that is cultured under similar conditions, wherein said growth rate or morphological feature of said transformed plant cell is different when compared to said growth rate or morphological feature of said nonfransfonned plant cell.

43. The transgenic plant cell of Claim 42, wherein said transfoπned plant cell is a tobacco cell.

44. The transgenic plant cell of Claim 43, wherein said tobacco cell is a Nicotiana tobacum cell.

45. The transgenic plant cell of Claim 43, wherein said tobacco cell produces a reduced level of nicotine.

46. The transgenic plant cell of Claim 42, wherein said nucleic acid is a DNA.

47. The transgenic plant cell of Claim 46, wherein said DNA encodes a proteinΛ

48. The transgenic plant cell of Claim 46, wherein said DNA is operably linked to a promoter in a sense orientation.

49. The transgenic plant cell of Claim 46, wherein said DNA is operably linked to a promoter in an antisense orientation.

50. The transgenic plant cell of Claim 46, wherein said DNA integrates into the genome of said fransfomied plant cell.

51. A transgenic plant containing a transgenic plant cell of Claim 42.

52. The transgenic plant of Claim 51 , wherein said transformed plant cell is a tobacco cell.

53. The transgenic plant of Claim 52, wherein said tobacco cell is a Nicotiana tobacum cell.

54. The transgenic plant of Claim 52, wherein said tobacco cell produces a reduced level of nicotine.

55. The transgenic plant of Claim 51 , wherein said nucleic acid is a DNA.

56. The transgenic plant of Claim 51, wherein said DNA encodes a protein.

57. The transgenic plant of Claim 56, wherein said DNA is operably linked to a promoter in a sense orientation.

58. The transgenic plant of Claim 56, wherein said DNA is operably linked to a promoter in an antisense orientation.

59. The transgenic plant of Claim 56, wherein said DNA integrates into the genome of said transfoπned plant cell.