Patent Publication Number: US-2007118920-A1

Title: Herbicide-resistant sunflower plants, polynucleotides encoding herbicide-resistant acetohydroxyacid synthase large subunit proteins, and methods of use

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
      This application claims the benefit of U.S. Provisional Application Ser. No. 60/735,045, filed Nov. 9, 2005, the contents of which are herein incorporated by reference in its entirety. 
    
    
     FIELD OF THE INVENTION  
      This invention relates to the field of agricultural biotechnology, particularly to herbicide-resistant sunflower plants and novel polynucleotide sequences that encode wild-type and herbicide-resistant sunflower acetohydroxyacid synthase large subunit proteins.  
     BACKGROUND OF THE INVENTION  
      Acetohydroxyacid synthase (AHAS; EC 4.1.3.18, also known as acetolactate synthase or ALS), is the first enzyme that catalyzes the biochemical synthesis of the branched chain amino acids valine, leucine and isoleucine (Singh (1999) “Biosynthesis of valine, leucine and isoleucine,” in  Plant Amino Acid,  Singh, B. K., ed., Marcel Dekker Inc. New York, N.Y., pp. 227-247). AHAS is the site of action of five structurally diverse herbicide families including the sulfonylureas (LaRossa and Falco (1984)  Trends Biotechnol.  2:158-161), the imidazolinones (Shaner et al. (1984)  Plant Physiol.  76:545-546), the triazolopyrimidines (Subramanian and Gerwick (1989) “Inhibition of acetolactate synthase by triazolopyrimidines,” in  Biocatalysis in Agricultural Biotechnology,  Whitaker, J. R. and Sonnet, P. E. eds., ACS Symposium Series, American Chemical Society, Washington, D.C., pp. 277-288), and the pyrimidyloxybenzoates (Subramanian et al. (1990)  Plant Physiol.  94: 239-244). Imidazolinone and sulfonylurea herbicides are widely used in modem agriculture due to their effectiveness at very low application rates and relative non-toxicity in animals. By inhibiting AHAS activity, these families of herbicides prevent further growth and development of susceptible plants including many weed species. Several examples of commercially available imidazolinone herbicides are PURSUIT® (imazethapyr), SCEPTER® (imazaquin) and ARSENAL® (imazapyr). Examples of sulfonylurea herbicides are chlorsulfuron, metsulfuron methyl, sulfometuron methyl, chlorimuron ethyl, thifensulfuron methyl, tribenuron methyl, bensulfuron methyl, nicosulfuron, ethametsulfuron methyl, rimsulfuron, triflusulfuron methyl, triasulfuron, primisulfuron methyl, cinosulfuron, amidosulfiuon, fluzasulfuron, imazosulfuron, pyrazosulfuron ethyl and halosulfuron.  
      Due to their high effectiveness and low-toxicity, imidazolinone herbicides are favored for application by spraying over the top of a wide area of vegetation. The ability to spray a herbicide over the top of a wide range of vegetation decreases the costs associated with plant establishment and maintenance, and decreases the need for site preparation prior to use of such chemicals. Spraying over the top of a desired tolerant species also results in the ability to achieve maximum yield potential of the desired species due to the absence of competitive species. However, the ability to use such spray-over techniques is dependent upon the presence of imidazolinone-resistant species of the desired vegetation in the spray over area.  
      Among the major agricultural crops, some leguminous species such as soybean are naturally resistant to imidazolinone herbicides due to their ability to rapidly metabolize the herbicide compounds (Shaner and Robinson (1985)  Weed Sci.  33:469-471). Other crops such as corn (Newhouse et al. (1992)  Plant Physiol.  100:882-886) and rice (Barrett et al. (1989)  Crop Safeners for Herbicides,  Academic Press, New York, pp. 195-220) are somewhat susceptible to imidazolinone herbicides. The differential sensitivity to the imidazolinone herbicides is dependent on the chemical nature of the particular herbicide and differential metabolism of the compound from a toxic to a non-toxic form in each plant (Shaner et al. (1984)  Plant Physiol.  76:545-546; Brown et al., (1987)  Pestic. Biochem. Physiol.  27:24-29). Other plant physiological differences such as absorption and translocation also play an important role in sensitivity (Shaner and Robinson (1985)  Weed Sci.  33:469-471).  
      Plants resistant to imidazolinones, sulfonylureas, triazolopyrimidines, and pyrimidyloxybenzoates have been successfully produced using seed, microspore, pollen, and callus mutagenesis in  Zea mays, Arabidopsis thaliana, Brassica napus  (i.e., canola)  Glycine max, Nicotiana tabacum,  sugarbeet ( Beta vulgaris ) and  Oryza sativa  (Sebastian et al. (1989)  Crop Sci.  29:1403-1408; Swanson et al., 1989  Theor. Appl. Genet.  78:525-530; Newhouse et al. (1991)  Theor. Appl. Genet.  83:65-70; Sathasivan et al. (1991)  Plant Physiol.  97:1044-1050; Mourand et al. (1993)  J. Heredity  84:91-96; Wright and Penner (1998)  Theor. Appl. Genet.  96:612-620; U.S. Pat. No. 5,545,822). In all cases, a single, partially dominant nuclear gene conferred resistance. Four imidazolinone resistant wheat plants were also previously isolated following seed mutagenesis of  Triticum aestivum  L. cv. Fidel (Newhouse et al. (1992)  Plant Physiol.  100:882-886). Inheritance studies confirmed that a single, partially dominant gene conferred resistance. Based on allelic studies, the authors concluded that the mutations in the four identified lines were located at the same locus. One of the Fidel cultivar resistance genes was designated FS-4 (Newhouse et al. (1992)  Plant Physiol.  100:882-886).  
      Naturally occurring plant populations that were discovered to be resistant to imidazolinone and/or sulfonylurea herbicides have also been used to develop herbicide-resistant sunflower breeding lines. Recently, two sunflower lines that are resistant to a sulfonylurea herbicide were developed using germplasm originating from a wild population of common sunflower ( Helianthus annuus ) as the source of the herbicide-resistance trait (Miller and Al-Khatib (2004)  Crop Sci.  44:1037-1038). Previously, White et al. ((2002)  Weed Sci.  50:432-437) had reported that individuals from a wild population of common sunflower from South Dakota, U.S.A. were cross-resistant to an imidazolinone and a sulfonylurea herbicide. Analysis of a portion of the coding region of the acetohydroxyacid synthase large subunit (AHASL) genes of individuals from this population revealed a point mutation that results in an Ala-to-Val amino acid substitution in the sunflower AHASL protein that corresponds to Ala 205  in the wild-type  Arabidopsis thaliana  AHASL protein (White et al. (2003)  Weed Sci.  51:845-853).  
      Computer-based modeling of the three dimensional conformation of the AHAS-inhibitor complex predicts several amino acids in the proposed inhibitor binding pocket as sites where induced mutations would likely confer selective resistance to imidazolinones (Ott et al. (1996)  J. Mol. Biol.  263:359-368). Wheat plants produced with some of these rationally designed mutations in the proposed binding sites of the AHAS enzyme have in fact exhibited specific resistance to a single class of herbicides (Ott et al. (1996)  J. Mol. Biol.  263:359-368).  
      Plant resistance to imidazolinone herbicides has also been reported in a number of patents. U.S. Pat. Nos. 4,761,373, 5,331,107, 5,304,732, 6,211,438, 6,211,439 and 6,222,100 generally describe the use of an altered AHAS gene to elicit herbicide resistance in plants, and specifically discloses certain imidazolinone resistant corn lines. U.S. Pat. No. 5,013,659 discloses plants exhibiting herbicide resistance due to mutations in at least one amino acid in one or more conserved regions. The mutations described therein encode either cross-resistance for imidazolinones and sulfonylureas or sulfonylurea-specific resistance, but imidazolinone-specific resistance is not described. U.S. Pat. No. 5,731,180 and U.S. Pat. No. 5,767,361 discuss an isolated gene having a single amino acid substitution in a wild-type monocot AHAS amino acid sequence that results in imidazolinone-specific resistance. In addition, rice plants that are resistant to herbicides that interfere with AHAS have been developed by mutation breeding and also by the selection of herbicide-resistant plants from a pool of rice plants produced by anther culture. See, U.S. Pat. Nos. 5,545,822, 5,736,629, 5,773,703, 5,773,704, 5,952,553 and 6,274,796.  
      In plants, as in all other organisms examined, the AHAS enzyme is comprised of two subunits: a large subunit (catalytic role) and a small subunit (regulatory role) (Duggleby and Pang (2000)  J. Biochem. Mol. Biol.  33:1-36). The AHAS large subunit (also referred to herein as AHASL) may be encoded by a single gene as in the case of  Arabidopsis,  rice, and sugar beet or by multiple gene family members as in maize, canola, and cotton. Specific, single-nucleotide substitutions in the large subunit confer upon the enzyme a degree of insensitivity to one or more classes of herbicides (Chang and Duggleby (1998)  Biochem J.  333:765-777).  
      For example, bread wheat,  Triticum aestivum  L., contains three homoeologous acetohydroxyacid synthase large subunit genes. Each of the genes exhibit significant expression based on herbicide response and biochemical data from mutants in each of the three genes (Ascenzi et al. (2003) International Society of Plant Molecular Biologists Congress, Barcelona, Spain, Ref. No. S10-17). The coding sequences of all three genes share extensive homology at the nucleotide level (WO 03/014357). Through sequencing the AHASL genes from several varieties of  Triticum aestivum,  the molecular basis of herbicide tolerance in most IMI-tolerant (imidazolinone-tolerant) lines was found to be the mutation S653(At)N, indicating a serine to asparagine substitution at a position equivalent to the serine at amino acid 653 in  Arabidopsis thaliana  (WO 03/01436; WO 03/014357). This mutation is due to a single nucleotide polymorphism (SNP) in the DNA sequence encoding the AHASL protein.  
      Multiple AHASL genes are also know to occur in dicotyledonous plants species. Recently, Kolkman et al. ((2004)  Theor. Appl. Genet.  109: 1147-1159) reported the identification, cloning, and sequencing for three AHASL genes (AHASL1, AHASL2, and AHASL3) from herbicide-resistant and wild type genotypes of sunflower ( Helianthus annuus  L.). Kolkman et al. reported that the herbicide-resistance was due either to the Pro197Leu (using the  Arabidopsis  AHASL amino acid position nomenclature) substitution or the Ala205Val substitution in the AHASL1 protein and that each of these substitutions provided resistance to both imidazolinone and sulfonylurea herbicides.  
      Given their high effectiveness and low-toxicity, imidazolinone herbicides are favored for agricultural use. However, the ability to use imidazolinone herbicides in a particular crop production system depends upon the availability of imidazolinone-resistant varieties of the crop plant of interest. To produce such imidazolinone-resistant varieties, plant breeders need to develop breeding lines with the imidazolinone-resistance trait. Thus, additional imidazolinone-resistant breeding lines and varieties of crop plants, as well as methods and compositions for the production and use of imidazolinone-resistant breeding lines and varieties, are needed.  
     SUMMARY OF THE INVENTION  
      The present invention provides sunflower plants having increased resistance to herbicides when compared to a wild-type sunflower plant. In particular, the sunflower plants of the invention have increased resistance to imidazolinone herbicides, when compared to a wild-type sunflower plant. The herbicide-resistant sunflower plants of the invention comprise at least one copy of a gene or polynucleotide that encodes a herbicide-resistant acetohydroxyacid synthase large subunit (AHASL). Such a herbicide-resistant AHASL protein comprises a isoleucine at amino acid position 188 or equivalent position. The herbicide-resistant sunflower plant of the invention can contain one, two, three, four, five, six, or more copies of a gene or polynucleotide encoding a herbicide-resistant AHASL protein of the invention. The sunflower plants of the invention also include seeds and progeny plants that comprise at least one copy of a gene or polynucleotide encoding a herbicide-resistant AHASL protein of the invention.  
      In one embodiment, the present invention provides herbicide-resistant sunflower plants that are from the sunflower line that has been designated as MUT9. A sample of seeds of the MUT9 line has been deposited with the American Type Culture Collection (ATCC) as ATCC Patent Deposit No. PTA-6325. Such MUT9 sunflower plants comprise in their genomes an AHASL1 gene that comprises the nucleotide sequence set forth in SEQ ID NO: 1 and that encodes the AHASL1 protein comprising, the amino acid sequence set forth in SEQ ID NO: 2. When compared to the amino acid sequence of the AHASL1 protein (SEQ ID NO: 4) that is encoded by an AHASL 1 gene (SEQ ID NO: 3) from a wild-type sunflower plant, the amino acid sequence set forth in SEQ ID NO: 2 has a single amino acid difference from the wild-type amino acid sequence. In the amino acid sequence set forth in SEQ ID NO: 2, there is an isoleucine at amino acid position 102. This position corresponds to position 188 in the full-length sunflower AHASL1 protein encoded by the nucleotide sequence set froth in SEQ ID NO: 24 (Acession No. AY541451). In the wild-type AHASL1 amino acid sequence of the invention (SEQ ID NO: 4), this equivalent amino acid position has a threonine. Unless otherwise indicated, the amino acid positions referred to herein for sunflower AHASL proteins correspond to the amino acid positions of the full-length amino acid sequence set forth in SEQ ID NO: 24.  
      The present invention further provides isolated polynucleotides and isolated polypeptides for sunflower ( Helianthus annuus ) AHASL proteins. The polynucleotides of the invention encompass nucleotide sequences that encode herbicide-resistant and wild-type AHASL proteins, including, but not limited to, the proteins encoded by the sunflower AHASL1, AHASL2, and AHASL3 genes. The herbicide-resistant sunflower AHASL proteins of the invention are imidazolinone-resistant AHASL proteins that comprise an amino acid other than threonine at position 188 of a full-length sunflower AHASL1 protein or equivalent position. Preferrably, the amino acid at position 188 or equivalent position is an isoleucine. The polynucleotides of the invention encompass the nucleotide sequences set forth in SEQ ID NOS: 1 and 3, nucleotide sequences encoding the amino acid sequences set forth in SEQ ID NOS: 2 and 4, and fragments and variants of said nucleotide sequences that encode proteins comprising AHAS activity.  
      The present invention provides expression cassettes for expressing the polynucleotides of the invention in plants, plant cells, and other, non-human host cells. The expression cassettes comprise a promoter expressible in the plant, plant cell, or other host cells of interest operably linked to a polynucleotide of the invention that encodes either a wild-type or herbicide-resistant AHASL protein. If necessary for targeting expression to the chloroplast, the expression cassette can also comprise an operably linked chloroplast-targeting sequence that encodes of a chloroplast transit peptide to direct an expressed AHASL protein to the chloroplast. The expression cassettes of the invention find use in a method for enhancing the herbicide tolerance of a plant and a host cell. The method involves transforming the plant or host cell with an expression cassette of the invention, wherein the expression cassette comprises a promoter that is expressible in the plant or host cell of interest and the promoter is operably linked to a polynucleotide of the invention that encodes an herbicide-resistant AHASL protein of the invention. The method further comprises regenerating a transformed plant from the transformed plant cell.  
      The present invention provides a method for increasing AHAS activity in a plant comprising transforming a plant cell with a polynucleotide construct comprising a nucleotide sequence operably linked to a promoter that drives expression in a plant cell and regenerating a transformed plant from the transformed plant cell. The nucleotide sequence is selected from those nucleotide sequences that encode the herbicide-resistant or wild-type AHASL proteins of the invention, particularly the nucleotide sequences set forth in SEQ ID NOS: 1 and 3, and nucleotide sequences encoding the amino acid sequences set forth in SEQ ID NOS: 2, and 4, and fragments and variants thereof. A plant produced by this method comprises increased AHAS activity or increased herbicide-resistant AHAS activity, when compared to an untransformed plant.  
      The present invention provides a method for producing a herbicide-resistant plant comprising transforming a plant cell with a polynucleotide construct comprising a nucleotide sequence operably linked to a promoter that drives expression in a plant cell and regenerating a transformed plant from said transformed plant cell. The nucleotide sequence is selected from those nucleotide sequences that encode the herbicide-resistant AHASL proteins of the invention, particularly the nucleotide sequence set forth in SEQ ID NO: 1, the nucleotide sequence encoding the amino acid sequence set forth in SEQ ID NO: 2, and fragments and variants thereof. A herbicide-resistant plant produced by this method comprises enhanced resistance to at least one herbicide, particularly an imidazolinone herbicide, when compared to an untransformed plant.  
      The present invention provides a method for enhancing herbicide-tolerance in a herbicide-tolerant plant. The method finds use in enhancing the resistance of a plant that already is resistant to a level of a herbicide that would kill or significantly injure a wild-type plant. Such a herbicide-tolerant plant can be a herbicide-tolerant plant that has been genetically engineered for herbicide-tolerance or a herbicide-tolerant plant that was developed by means that do not involve recombinant DNA such as, for example, the MUT9 sunflower plants of the present invention. The method comprises transforming a herbicide-tolerant plant with a polynucleotide construct comprising a nucleotide sequence operably linked to a promoter that drives expression in a plant cell and regenerating a transformed plant from the transformed plant cell. The nucleotide sequence is selected from those nucleotide sequences that encode the herbicide-resistant AHASL proteins of the invention, particularly the nucleotide sequence set forth in SEQ ID NO: 1, nucleotide sequences encoding the amino acid sequence set forth in SEQ ID NO: 2, and fragments and variants thereof.  
      The present invention provides transformation vectors comprising a selectable marker gene of the invention. The selectable marker gene comprises a promoter that drives expression in a host cell operably linked to a polynucleotide comprising a nucleotide sequence that encodes an herbicide-resistant AHASL protein of the invention. The transformation vector can additionally comprise a gene of interest to be expressed in the host cell and can also, if desired, include a chloroplast-targeting sequence that is operably linked to the polynucleotide of the invention.  
      The present invention further provides methods for using the transformation vectors of the invention to select for cells transformed with the gene of interest. Such methods involve the transformation of a host cell with the transformation vector, exposing the cell to a level of an imidazolinone herbicide that would kill or inhibit the growth of a non-transformed host cell, and identifying the transformed host cell by its ability to grow in the presence of the herbicide. In one embodiment of the invention, the host cell is a plant cell and the selectable marker gene comprises a promoter that drives expression in a plant cell.  
      The present invention provides a method for controlling weeds in the vicinity of the herbicide-resistant plants of the invention, including the herbicide-resistant sunflower plants described above and plants transformed with the herbicide-resistant AHASL polynucleotides of the invention. Such transformed plants comprise in their genomes at least one expression cassette comprising a promoter that drives gene expression in a plant cell, wherein the promoter is operably linked to an AHASL polynucleotide of the invention. The method comprises applying an effective amount of an herbicide to the weeds and to the herbicide-resistant plant, wherein the herbicide-resistant plant has increased resistance to at least one herbicide, particularly an imidazolinone herbicide, when compared to a wild-type or untransformed plant.  
      The plants of the present invention can be transgenic or non-transgenic. An example of a non-transgenic sunflower plant having increased resistance to imidazolinone and/or sulfonylurea herbicides includes the sunflower plant (MUT9) having ATCC Patent Deposit No. PTA-6325; or mutant, recombinant, or a genetically engineered derivative of the plant having ATCC Patent Deposit No. PTA-6325; or of any progeny of the plant having ATCC Patent Deposit No. PTA-6325; or a plant that is a progeny of any of these plants; or a plant that comprises the herbicide resistance characteristics of the plant having ATCC Patent Deposit No. PTA-6325.  
      The present invention also provides plants, plant organs, plant tissues, plant cells, seeds, and non-human host cells that are transformed with the at least one polynucleotide, expression cassette, or transformation vector of the invention. Such transformed plants, plant organs, plant tissues, plant cells, seeds, and non-human host cells have enhanced tolerance or resistance to at least one herbicide, at levels of the herbicide that kill or inhibit the growth of an untransformed plant, plant tissue, plant cell, or non-human host cell, respectively. Preferably, the transformed plants, plant tissues, plant cells, and seeds of the invention are  Arabidopsis thaliana  and crop plants.  
      The present invention further provides isolated polypeptides comprising imidazolinone-resistant and wild-type sunflower AHASL proteins. The isolated polypeptides comprise the amino acid sequences set forth in SEQ ID NOS: 2 and 4, the amino acid sequences encoded by nucleotide sequences set forth in SEQ ID NOS: 1 and 3, and fragments and variants of said amino acid sequences that encode proteins comprising AHAS activity. 
    
    
     BRIEF DESCRIPTION THE DRAWINGS  
       FIG. 1  is a nucleotide sequence alignment of the coding sequences of the herbicide-resistant sunflower AHASL1 gene (MUT9 SUNALS1; SEQ ID NO: 1), the wild-type sunflower AHASL1 gene (RHA266; SEQ ID NO: 3), an  Arabidopsis thaliana  AHASL gene (GenBank Accession No. NM — 114714; SEQ ID NO: 9), a  Zea mays  AHASL gene (GenBank Accession No. X63553; SEQ ID NO: 25) the herbicide-resistant AHASL gene from  Xanthium  sp. (GenBank Accession No. U16279; SEQ ID NO: 5), and the wild-type AHASL gene from  Xanthium  sp. (GenBank Accession No. U 16280; SEQ ID NO: 7). The site of the mutation in MUT SUNALS 1 (SEQ ID NO: 1) is indicated by an asterisk. The mutation is a C-to-T transition at nucleotide position 305 of SEQ ID NO: 1.  
       FIG. 2  is an amino acid sequence alignment of the herbicide-resistant sunflower AHASL I protein (MUT9 SUNALS 1; SEQ ID NO: 2), the wild-type sunflower AHASL1 protein (RHA266; SEQ ID NO: 4), an  Arabidopsis thaliana  AHASL protein (GenBank Accession No. NM — 114714; SEQ ID NO: 10), a  Zea mays  AHASL protein (GenBank Accession No. X63553; SEQ ID NO: 26) the herbicide-resistant AHASL protein from  Xanthium  sp. (GenBank Accession No. U16279; SEQ ID NO: 6), and the wild-type AHASL protein from  Xanthium  sp. (GenBank Accession No. U16280; SEQ ID NO: 8). The asterisk indicates the site of the single amino acid substitution (Thr-to-Ile) found in the herbicide-resistant sunflower AHASL1 protein. This site of the substitution corresponds to amino acid position 102 in the partial-length AHASL1 amino acid sequence set forth in SEQ ID NO: 2. The equivalent position of this substitution in the full-length sunflower AHASL1 amino acid sequence set forth in SEQ ID NO: 24 is 188. 
    
    
     SEQUENCE LISTING  
      The nucleotide and amino acid sequences listed in the accompanying sequence listing are shown using standard letter abbreviations for nucleotide bases, and three-letter code for amino acids. The nucleotide sequences follow the standard convention of beginning at the 5′ end of the sequence and proceeding forward (i.e., from left to right in each line) to the 3′ end. Only one strand of each nucleic acid sequence is shown, but the complementary strand is understood to be included by any reference to the displayed strand. The amino acid sequences follow the standard convention of beginning at the amino terminus of the sequence and proceeding forward (i.e., from left to right in each line) to the carboxy terminus.  
      SEQ ID NO: 1 sets forth the partial length nucleotide sequence encoding a herbicide-resistant AHASL1 protein from the sunflower line MUT9.  
      SEQ ID NO: 2 sets forth the partial length amino acid sequence of the herbicide-resistant AHASL1 protein encoded by the nucleotide sequence set forth in SEQ ID NO: 1.  
      SEQ ID NO: 3 sets forth the partial length nucleotide sequence encoding the wild-type AHASL1 protein from sunflower line RHA266.  
      SEQ ID NO: 4 sets forth the partial length amino acid sequence of the wild-type AHASL1 protein encoded by the nucleotide sequence set forth SEQ ID NO: 3.  
      SEQ ID NO: 5 is the nucleotide sequence of GenBank Accession No. U16279.  
      SEQ ID NO: 6 is the amino acid sequence encoded by the nucleotide sequence of GenBank Accession No. U16279.  
      SEQ ID NO: 7 is the nucleotide sequence of GenBank Accession No. U16280.  
      SEQ ID NO: 8 is the amino acid sequence encoded by the nucleotide sequence of GenBank Accession No. U16280.  
      SEQ ID NO: 9 is the nucleotide sequence of GenBank Accession No. NM — 114714.  
      SEQ ID NO: 10 is the amino acid sequence encoded by the nucleotide sequence of GenBank Accession No. NM — 114714.  
      SEQ ID NO: 11 sets forth the nucleotide sequence of the C2359-FI primer that is described in Example 2.  
      SEQ ID NO: 12 sets forth the nucleotide sequence of the ALSRI primer that is described in Example 2.  
      SEQ ID NO: 13 sets forth the nucleotide sequence of the SUNALS1-F1 primer that is described in Example 2.  
      SEQ ID NO: 14 sets forth the nucleotide sequence of the ALS1-8R primer that is described in Example 2.  
      SEQ ID NO: 15 sets forth the nucleotide sequence of the ALS-2R primer that is described in Example 2.  
      SEQ ID NO: 16 sets forth the nucleotide sequence of the ALS-2F primer that is described in Example 2.  
      SEQ ID NO: 17 sets forth the nucleotide sequence of the ALS-3R primer that is described in Example 2.  
      SEQ ID NO: 18 sets forth the nucleotide sequence of the ALS-3F primer that is described in Example 2.  
      SEQ ID NO: 19 sets forth the nucleotide sequence of the ALS-4F primer that is described in Example 2.  
      SEQ ID NO: 20 sets forth the nucleotide sequence of the ALS-6R primer that is described in Example 2.  
      SEQ ID NO: 21 sets forth the nucleotide sequence of the ALS-7F primer that is described in Example 2.  
      SEQ ID NO: 22 sets forth the nucleotide sequence of the ALS-8F primer that is described in Example 2.  
      SEQ ID NO: 23 is the nucleotide sequence of GenBank Accession No. AY541451.  
      SEQ ID NO: 24 is the amino acid sequence encoded by the nucleotide sequence of GenBank Accession No. AY541451.  
      SEQ ID NO: 25 is the nucleotide sequence of GenBank Accession No. X63553.  
      SEQ ID NO: 26 is the amino acid sequence encoded by the nucleotide sequence of GenBank Accession No. X63553.  
     DETAILED DESCRIPTION OF THE INVENTION  
      The present invention relates to sunflower plants having increased resistance to herbicides when compared to a wild-type sunflower plant. Herbicide-resistant sunflower plants were produced as described hereinbelow by exposing wild-type (with respect to herbicide resistance) sunflower plants to a mutagen, allowing the plants to mature and reproduce, and selecting progeny plants that displayed enhanced resistance to an imidazolinone herbicide, relative to the resistance of a wild-type sunflower plant. The invention provides a herbicide-resistant sunflower line that is referred to herein as MUT9.  
      From the MUT9 herbicide-resistant sunflower plants and wild-type sunflower plants, the coding region of an acetohydroxyacid synthase large subunit gene (designated as AHASL1) was isolated by polymerase chain reaction (PCR) amplification and sequenced. By comparing the polynucleotide sequences of the herbicide-resistant and wild-type sunflower plants, it was discovered that the coding region of the AHASL1 polynucleotide sequence from the herbicide-resistant sunflower plant differed from the AHASL1 polynucleotide sequence of the wild type plant by a single nucleotide, a C-to-T transition at nucleotide 305 (SEQ ID NO: 1). This C-to-T transition in the AHASL1 polynucleotide sequence results in a threonine-to-isoleucine substitution at amino acid 102 in a conserved region of the predicted amino acid sequence of the herbicide-resistant sunflower AHASL1 protein (SEQ ID NO: 2), relative to the equivalent amino acid position of the wild-type AHASL1 protein from sunflower line RHA266 (amino acid 108 of SEQ ID NO: 4).  
      Because the nucleotide sequence set forth in SEQ ID NO: 1 does not correspond to a full-length coding region of an AHASL protein, the amino acid sequence encoded thereby that is set forth in SEQ ID NO: 2 is also less than full-length. To facilitate comparison with other sunflower ASHAL amino acid sequences, the amino acid positions of sunflower AHASL proteins referred to herein, unless otherwise indicated or apparent for the context in which such positions appear, correspond to the amino acid positions of the full-length amino acid sequence of the sunflower AHASL1 protein encoded by the nucleotide sequence having GenBank Accession No. AY541451 (SEQ ID NO: 24). Accordingly, the threonine-to-isoleucine substitution at amino acid position 102 of SEQ ID NO: 2 corresponds to amino acid position 188 in the amino acid sequence of SEQ ID NO: 24.  
      While a variety of amino acid substitutions in wild-type AHASL proteins are known to confer herbicide resistance on a plant expressing such a substituted AHASL protein, an AHASL protein comprising an amino acid substitution at position 188 or equivalent position (e.g., position 203 in the  Arabidopsis  AHASL protein encoded by the nucleotide sequence set forth in GenBank Accession No. X51514) and that confers herbicide resistance on a plant comprising such an AHASL protein has not been previously disclosed. See, Example 5 below. See also, Hartnett et al. (1990) “Herbicide-resistant plants carrying mutated acetolactate synthase genes,” In:  Managing Resistance to Agrochemicals: Fundamental Research to Practical Strategies,  Green et al. (eds.), American Chemical Soc. Symp., Series No. 421, Washington, D.C., USA; Simpson (1998) Down to Earth 53(1):26-35; Bruniard (2001) Inheritance of imidazolinone resistance, characterization of cross-resistance pattern, and identification of molecular markers in sunflower ( Helianthus annuus  L.), Ph.D. Thesis, North Dakota State University, Fargo, N. Dak., USA, pp 1-78.; White et al. (2002)  Weed Sci.  50:432-437; and Kolkman et al. (2004)  Theor. Appl. Genet.  109: 1147-1159; all of which are herein incorporated by reference.  
      Thus, the present invention discloses a novel amino substitution that can be used to produce herbicide-resistant sunflower AHASL proteins, the polynucleotides encoding such proteins, and herbicide-resistant plants, plant tissues, plant cells, and seeds. Because the threonine that is found at amino acid position 188 in wild-type sunflower AHASL1 proteins, or the equivalent position in other AHASL proteins, is within a region of amino acids that is conserved across plant species, it is expected that the substitution of another amino acid, preferably isoleucine, for this same conserved threonine in other AHASL proteins from sunflower (e.g., sunflower AHASL2 and AHASL3) or from other plant species will result in herbicide-resistant AHASL proteins. Accordingly, such herbicide-resistant AHASL proteins and the polynucleotides encoding them find use in the production of herbicide-resistant plants, plant cells, plant tissues, and seeds by the methods disclosed herein.  
      The present invention additionally encompasses isolated sunflower AHASL2 and AHASL3 polynucleotides that encode herbicide-resistant AHASL2 and AHASL3 proteins, respectively. Such herbicide-resistant AHASL2 and AHASL3 proteins each comprise an amino acid other than threonine at position 188 or equivalent position. Preferably in such herbicide-resistant AHASL2 and AHASL3 proteins, the amino acid at position 188 or equivalent position is isoleucine.  
      The invention further relates to isolated polynucleotide molecules comprising nucleotide sequences that encode acetohydroxyacid synthase large subunit (AHASL) proteins and to such AHASL proteins. The invention discloses the isolation and nucleotide sequence of a polynucleotide encoding a herbicide-resistant sunflower AHASL1 protein from an herbicide-resistant sunflower plant that was produced by chemical mutagenesis of wild-type sunflower plants. The herbicide-resistant sunflower AHASL1 proteins of the invention possess a threonine-to-isoleucine substitution at position 188 in their respective amino acid sequences, when compared to the corresponding wild-type amino acid sequence. The invention further discloses the isolation and nucleotide sequence of a polynucleotide molecule encoding a wild-type sunflower AHASL1 protein.  
      The present invention provides isolated polynucleotide molecules that encode AHASL proteins from sunflower ( Helianthus annuus  L.). Specifically, the invention provides isolated polynucleotide molecules comprising: the nucleotide sequences set forth in SEQ ID NOS: 1 and 3, nucleotide sequences encoding AHASL proteins comprising the amino acid sequences set forth in SEQ ID NOS: 2 and 4, and fragments and variants of such nucleotide sequences that encode functional AHASL proteins.  
      The isolated herbicide-resistant AHASL polynucleotide molecules of the invention comprise nucleotide sequences that encode herbicide-resistant AHASL proteins. Such polynucleotide molecules can be used in polynucleotide constructs for the transformation of plants, particularly crop plants, to enhance the resistance of the plants to herbicides, particularly herbicides that are known to inhibit AHAS activity, more particularly imidazolinone herbicides. Such polynucleotide constructs can be used in expression cassettes, expression vectors, transformation vectors, plasmids and the like. The transgenic plants obtained following transformation with such polynucleotide constructs show increased resistance to AHAS-inhibiting herbicides such as, for example, imidazolinone and sulfonylurea herbicides.  
      Compositions of the invention include nucleotide sequences that encode AHASL proteins. In particular, the present invention provides for isolated polynucleotide molecules comprising nucleotide sequences encoding the amino acid sequences shown in SEQ ID NOS: 2 and 4, and fragments and variants thereof that encode polypeptides comprising AHAS activity. Further provided are polypeptides having an amino acid sequence encoded by a polynucleotide molecule described herein, for example those set forth in SEQ ID NOS: 1 and 3, and fragments and variants thereof that encode polypeptides comprising AHAS activity.  
      The invention encompasses isolated or substantially purified nucleic acid or protein compositions. An “isolated” or “purified” polynucleotide molecule or protein, or biologically active portion thereof, is substantially or essentially free from components that normally accompany or interact with the polynucleotide molecule or protein as found in its naturally occurring environment. Thus, an isolated or purified polynucleotide molecule or protein is substantially free of other cellular material, or culture medium when produced by recombinant techniques, or substantially free of chemical precursors or other chemicals when chemically synthesized. Preferably, an “isolated” nucleic acid is free of sequences (preferably protein encoding sequences) that naturally flank the nucleic acid (i.e., sequences located at the 5′ and 3′ ends of the nucleic acid) in the genomic DNA of the organism from which the nucleic acid is derived. For example, in various embodiments, the isolated polynucleotide molecule can contain less than about 5 kb, 4 kb, 3 kb, 2 kb, 1 kb, 0.5 kb, or 0.1 kb of nucleotide sequences that naturally flank the polynucleotide molecule in genomic DNA of the cell from which the nucleic acid is derived. A protein that is substantially free of cellular material includes preparations of protein having less than about 30%, 20%, 10%, 5%, or 1% (by dry weight) of contaminating protein. When the protein of the invention or biologically active portion thereof is recombinantly produced, preferably culture medium represents less than about 30%, 20%, 10%, 5%, or 1% (by dry weight) of chemical precursors or non-protein-of-interest chemicals.  
      The present invention provides isolated polypeptides comprising AHASL proteins. The isolated polypeptides comprise an amino acid sequence selected from the group consisting of the amino acid sequences set forth in SEQ ID NOS: 2 and 4, the amino acid sequences encoded by nucleotide sequences set forth in SEQ ID NOS: 1 and 3, and finctional fragments and variants of said amino acid sequences that encode an AHASL polypeptide comprising AHAS activity. By “functional fragments and variants” is intended fragments and variants of the exemplified polypeptides that comprise AHAS activity.  
      In certain embodiments of the invention, the methods involve the use of herbicide-tolerant or herbicide-resistant plants. By an “herbicide-tolerant” or “herbicide-resistant” plant, it is intended that a plant that is tolerant or resistant to at least one herbicide at a level that would normally kill, or inhibit the growth of, a normal or wild-type plant. In one embodiment of the invention, the herbicide-tolerant plants of the invention comprise a herbicide-tolerant or herbicide-resistant AHASL protein. By “herbicide-tolerant AHASL protein” or “herbicide-resistant AHASL protein”, it is intended that such an AHASL protein displays higher AHAS activity, relative to the AHAS activity of a wild-type AHASL protein, when in the presence of at least one herbicide that is known to interfere with AHAS activity and at a concentration or level of the herbicide that is to known to inhibit the AHAS activity of the wild-type AHASL protein. Furthermore, the AHAS activity of such a herbicide-tolerant or herbicide-resistant AHASL protein may be referred to herein as “herbicide-tolerant” or “herbicide-resistant” AHAS activity.  
      For the present invention, the terms “herbicide-tolerant” and “herbicide-resistant” are used interchangeable and are intended to have an equivalent meaning and an equivalent scope. Similarly, the terms “herbicide-tolerance” and “herbicide-resistance” are used interchangeable and are intended to have an equivalent meaning and an equivalent scope. Likewise, the terms “imidazolinone-resistant” and “imidazolinone-resistance” are used interchangeable and are intended to be of an equivalent meaning and an equivalent scope as the terms “imidazolinone-tolerant” and “imidazolinone-tolerance”, respectively.  
      The invention encompasses herbicide-resistant AHASL polynucleotides and herbicide-resistant AHASL proteins. By “herbicide-resistant AHASL polynucleotide” is intended a polynucleotide that encodes a protein comprising herbicide-resistant AHAS activity. By “herbicide-resistant AHASL protein” is intended a protein or polypeptide that comprises herbicide-resistant AHAS activity.  
      Further, it is recognized that a herbicide-tolerant or herbicide-resistant AHASL protein can be introduced into a plant by transforming a plant or ancestor thereof with a nucleotide sequence encoding a herbicide-tolerant or herbicide-resistant AHASL protein. Such herbicide-tolerant or herbicide-resistant AHASL proteins are encoded by the herbicide-tolerant or herbicide-resistant AHASL polynucleotides. Alternatively, a herbicide-tolerant or herbicide-resistant AHASL protein may occur in a plant as a result of a naturally occurring or induced mutation in an endogenous AHASL gene in the genome of a plant or progenitor thereof.  
      The present invention provides plants, plant tissues, plant cells, and host cells with increased resistance or tolerance to at least one herbicide, particularly an imidazolinone or sulfonylurea herbicide. The preferred amount or concentration of the herbicide is an “effective amount” or “effective concentration.” By “effective amount” and “effective concentration” is intended an amount and concentration, respectively, that is sufficient to kill or inhibit the growth of a similar, wild-type, plant, plant tissue, plant cell, or host cell, but that said amount does not kill or inhibit as severely the growth of the herbicide-resistant plants, plant tissues, plant cells, and host cells of the present invention. Typically, the effective amount of a herbicide is an amount that is routinely used in agricultural production systems to kill weeds of interest. Such an amount is known to those of ordinary skill in the art, or can be easily determined using methods known in the art.  
      The herbicides of the present invention are those that interfere with the activity of the AHAS enzyme such that AHAS activity is reduced in the presence of the herbicide. Such herbicides may also referred to herein as “AHAS-inhibiting herbicides” or simply “AHAS inhibitors.” As used herein, an “AHAS-inhibiting herbicide” or an “AHAS inhibitor” is not meant to be limited to single herbicide that interferes with the activity of the AHAS enzyme. Thus, unless otherwise stated or evident from the context, an “AHAS-inhibiting herbicide” or an “AHAS inhibitor” can be a one herbicide or a mixture of two, three, four, or more herbicides, each of which interferes with the activity of the AHAS enzyme.  
      By “similar, wild-type, plant, plant tissue, plant cell or host cell” is intended a plant, plant tissue, plant cell, or host cell, respectively, that lacks the herbicide-resistance characteristics and/or particular polynucleotide of the invention that are disclosed herein. The use of the term “wild-type” is not, therefore, intended to imply that a plant, plant tissue, plant cell, or other host cell lacks recombinant DNA in its genome, and/or does not possess herbicide-resistant characteristics that are different from those disclosed herein.  
      As used herein unless clearly indicated otherwise, the term “plant” intended to mean a plant at any developmental stage, as well as any part or parts of a plant that may be attached to or separate from a whole intact plant. Such parts of a plant include, but are not limited to, organs, tissues, and cells of a plant. Examples of particular plant parts include a stem, a leaf, a root, an inflorescence, a flower, a floret, a fruit, a pedicle, a peduncle, a stamen, an anther, a stigma, a style, an ovary, a petal, a sepal, a carpel, a root tip, a root cap, a root hair, a leaf hair, a seed hair, a pollen grain, a microspore, a cotyledon, a hypocotyl, an epicotyl, xylem, phloem, parenchyma, endosperm, a companion cell, a guard cell, and any other known organs, tissues, and cells of a plant. Furthermore, it is recognized that a seed is a plant.  
      The plants of the present invention include both non-transgenic plants and transgenic plants. By “non-transgenic plant” is intended to mean a plant lacking recombinant DNA in its genome. By “transgenic plant” is intended to mean a plant comprising recombinant DNA in its genome. Such a transgenic plant can be produced by introducing recombinant DNA into the genome of the plant. When such recombinant DNA is incorporated into the genome of the transgenic plant, progeny of the plant can also comprise the recombinant DNA. A progeny plant that comprises at least a portion of the recombinant DNA of at least one progenitor transgenic plant is also a transgenic plant.  
      For the purposes of the present invention unless otherwise expressly indicated or apparent from the context, a “progeny plant” is any plant that is descended from at least one plant of the invention and includes, but is not limited to, first, second, third, fourth, fifth, sixth, seventh, eight, ninth, and tenth generation descendents of the plant of the invention. Preferably, such progeny or descendents comprise increased resistance to at least one herbicide when compared to a wild-type plant. More preferably, such progeny or descendents comprise increased resistance to at least one imidazolinone herbicide when compared to a wild-type plant. Even more preferably, such progeny or descendents comprise the herbicide-resistance characteristics of the plant with ATCC Patent Deposit Number PTA-6325, and/or a herbicide-resistant AHASL protein comprising an isoleucine at amino acid position 188 or equivalent position.  
      The present invention provides the herbicide-resistant sunflower line that is referred to herein as MUT9. A deposit of at least 2375 seeds from sunflower line RHA266 MUT-9-1-1-2 (referred to herein as “MUT9”) with the Patent Depository of the American Type Culture Collection (ATCC), Manassas, Va. 20110 USA was made on Nov. 22, 2004 and assigned ATCC Patent Deposit Number PTA-6325. Due to a shortage of seeds of the MUT9 line, less than 2500 seeds of the MUT9 line were initially submitted to the ATCC. On Feb. 4, 2005, Applicants made a supplemental deposit of 125 seeds to the ATCC to reach a total of at least 2500 seeds. The deposit of sunflower line MUT9 was made for a term of at least 30 years and at least 5 years after the most recent request for the furnishing of a sample of the deposit is received by the ATCC. The deposit will be maintained under the terms of the Budapest Treaty on the International Recognition of the Deposit of Microorganisms for the Purposes of Patent Procedure. The deposit of sunflower line MUT9 was made for a term of at least 30 years and at least 5 years after the most recent request for the furnishing of a sample of the deposit is received by the ATCC. Additionally, Applicants have satisfied all the requirements of 37 C.F.R. §§ 1.801-1.809, including providing an indication of the viability of the sample.  
      The present invention provides herbicide-resistant sunflower plants of the MUT9 line that were produced by a mutation breeding. Wild-type sunflower plants were mutagenized by exposing the plants to a mutagen, particularly a chemical mutagen, more particularly ethyl methanesulfonate (EMS). However, the present invention is not limited to herbicide-resistant sunflower plants that are produced by a mutagenesis method involving the chemical mutagen EMS. Any mutagenesis method known in the art may be used to produce the herbicide-resistant sunflower plants of the present invention. Such mutagenesis methods can involve, for example, the use of any one or more of the following mutagens: radiation, such as X-rays, Gamma rays (e.g., cobalt 60 or cesium 137), neutrons, (e.g., product of nuclear fission by uranium 235 in an atomic reactor), Beta radiation (e.g., emitted from radioisotopes such as phosphorus 32 or carbon 14), and ultraviolet radiation (preferably from 2500 to 2900 nm), and chemical mutagens such as base analogues (e.g., 5-bromo-uracil), related compounds (e.g., 8-ethoxy caffeine), antibiotics (e.g., streptonigrin), alkylating agents (e.g., sulfur mustards, nitrogen mustards, epoxides, ethylenamines, sulfates, sulfonates, sulfones, lactones), azide, hydroxylamine, nitrous acid, or acridines. Herbicide-resistant plants can also be produced by using tissue culture methods to select for plant cells comprising herbicide-resistance mutations and then regenerating herbicide-resistant plants therefrom. See, for example, U.S. Pat. Nos. 5,773,702 and 5,859,348, both of which are herein incorporated in their entirety by reference. Further details of mutation breeding can be found in “Principals of Cultivar Development” Fehr, 1993 Macmillan Publishing Company the disclosure of which is incorporated herein by reference.  
      Analysis of the AHASL1 gene of the sunflower plant of the MUT9 line revealed that the mutation that results in the substitution of a isoleucine for the threonine that is found at amino acid position 188 in the wild-type AHASL1 amino acid sequence for SEQ ID NO: 4. Thus, the present invention discloses that substituting another amino acid for the threonine at position 188 can cause a sunflower plant to have enhanced resistance to a herbicide, particularly an imidazolinone and/or sulfonylurea herbicide. As disclosed in Example 5 below, the threonine at amino acid position 188 occurs within a conserved region of AHASL proteins. Similarly, amino acid substitutions in other conserved regions of AHASL proteins have been disclosed that are known to confer herbicide resistance on a plant that comprises such an AHASL protein. Accordingly, the herbicide-resistant sunflower plants of the invention include, but are not limited to those sunflower plants which comprise in their genomes at least one copy of an AHASL polynucleotide that encodes a herbicide-resistant AHASL protein that comprises a isoleucine at amino acid position 188 or equivalent position.  
      The sunflower plants of the invention further include plants that comprise, relative to the wild-type AHASL protein, an isoleucine or another amino acid other than threonine at amino acid position 188 or equivalent position and one or more additional amino acid substitutions in the AHASL protein relative to the wild-type AHASL protein, wherein such a sunflower plant has increased resistance to at least one herbicide when compared to a wild-type sunflower plant. Such sunflower plants comprise AHASL proteins that comprise at least one member selected from the group consisting of: a threonine at amino acid position 107 or equivalent position; an alanine, threonine, histidine, leucine, arginine, isoleucine, glutamine, or serine at amino acid position 182 or equivalent position; an aspartate or valine at amino acid position 190 or equivalent position; a leucine at amino acid position 559 or equivalent position; and an asparagine, threonine, or phenylalanine at amino acid position 638 or equivalent position.  
      The present invention provides AHASL proteins with amino acid substitutions at particular amino acid positions within conserved regions of the sunflower AHASL proteins disclosed herein. Furthermore, those of ordinary skill will recognize that such amino acid positions can vary depending on whether amino acids are added or removed from, for example, the N-terminal end of an amino acid sequence. Thus, the invention encompasses the amino substitutions at the recited position or equivalent position (e.g., “amino acid position 188 or equivalent position”). By “equivalent position” is intended to mean a position that is within the same conserved region as the exemplified amino acid position. Such conserved regions are know in the art (see Table 6 below) or can be determined by multiple sequence alignments as disclosed herein or by methods known in the art.  
      In addition, the present invention provides AHASL polypeptides comprising amino acid substitutions that are known to confer resistance on a plant to at least one herbicide, particularly an imidazolinone herbicide and/or a sulfonylurea herbicide. Such AHASL polypeptides include, for example, those that comprise at least one member selected from the group consisting of: an isoleucine or another amino acid other than threonine at amino acid position 188 or equivalent position; a threonine at amino acid position 107 or equivalent position; an alanine, threonine, histidine, leucine, arginine, isoleucine, glutamine, or serine at amino acid position 182 or equivalent position; an aspartate or valine at amino acid position 190 or equivalent position; an aspartate or valine at amino acid position 190 or equivalent position; a leucine at amino acid position 559 or equivalent position; and an asparagine, threonine, or phenylalanine at amino acid position 638 or equivalent position. The invention further provides isolated polynucleotides encoding such AHASL polypeptides, as well as expression cassettes, transformation vectors, transformed host cells, transformed plants, and methods comprising such polynucleotides.  
      The present invention provides methods for enhancing the tolerance or resistance of a plant, plant tissue, plant cell, or other host cell to at least one herbicide that interferes with the activity of the AHAS enzyme. Preferably, such an AHAS-inhibiting herbicide is an imidazolinone herbicide, a sulfonylurea herbicide, a triazolopyrimidine herbicide, a pyrimidinyloxybenzoate herbicide, a sulfonylamino-carbonyltriazolinone herbicide, or mixture thereof. More preferably, such a herbicide is an imidazolinone herbicide, a sulfonylurea herbicide, or mixture thereof. For the present invention, the imidazolinone herbicides include, but are not limited to, PURSUIT® (imazethapyr), CADRE® (imazapic), RAPTOR® (imazamox), SCEPTER® (imazaquin), ASSERT® (imazethabenz), ARSENAL® (imazapyr), a derivative of any of the aforementioned herbicides, and a mixture of two or more of the aforementioned herbicides, for example, imazapyr/imazamox (ODYSSEY®). More specifically, the imidazolinone herbicide can be selected from, but is not limited to, 2-(4-isopropyl-4-methyl-5-oxo-2-imidiazolin-2-yl)-nicotinic acid, [2-(4-isopropyl)-4-][methyl-5-oxo-2-imidazolin-2-yl)-3-quinolinecarboxylic] acid, [5-ethyl-2-(4-isopropyl-]4-methyl-5-oxo-2-imidazolin-2-yl)-nicotinic acid, 2-(4-isopropyl-4-methyl-5-oxo-2-imidazolin-2-yl)-5-(methoxymethyl)-nicotinic acid, [2-(4-isopropyl-4-methyl-5-oxo-2-]imidazolin-2-yl)-5-methylnicotinic acid, and a mixture of methyl [6-(4-isopropyl-4-]methyl-5-oxo-2-imidazolin-2-yl)-m-toluate and methyl [2-(4-isopropyl-4-methyl-5-]oxo-2-imidazolin-2-yl)-p-toluate. The use of 5-ethyl-2-(4-isopropyl-4-methyl-5-oxo-2-imidazolin-2-yl)-nicotinic acid and [2-(4-isopropyl-4-methyl-5-oxo-2-imidazolin-2-]yl)-5-(methoxymethyl)-nicotinic acid is preferred. The use of [2-(4-isopropyl-4-]methyl-5-oxo-2-imidazolin-2-yl)-5-(methoxymethyl)-nicotinic acid is particularly preferred.  
      For the present invention, the sulfonylurea herbicides include, but are not limited to, chlorsulfuron, metsulfuron methyl, sulfometuron methyl, chlorimuron ethyl, thifensulfuron methyl, tribenuron methyl, bensulfuron methyl, nicosulfuron, ethametsulfuron methyl, rimsulfuron, triflusulfuron methyl, triasulfuron, primisulfuron methyl, cinosulfuron, amidosulfiuon, fluzasulfuron, imazosulfuron, pyrazosulfuron ethyl, halosulfuron, azimsulfuron, cyclosulfuron, ethoxysulfuron, flazasulfuron, flupyrsulfuron methyl, foramsulfuron, iodosulfuron, oxasulfuron, mesosulfuron, prosulfuron, sulfosulfuron, trifloxysulfuron, tritosulfuron, a derivative of any of the aforementioned herbicides, and a mixture of two or more of the aforementioned herbicides. The triazolopyrimidine herbicides of the invention include, but are not limited to, cloransulam, diclosulam, florasulam, flumetsulam, metosulam, and penoxsulam. The pyrimidinyloxybenzoate herbicides of the invention include, but are not limited to, bispyribac, pyrithiobac, pyriminobac, pyribenzoxim and pyriftalid. The sulfonylamino-carbonyltriazolinone herbicides include, but are not limited to, flucarbazone and propoxycarbazone.  
      It is recognized that pyrimidinyloxybenzoate herbicides are closely related to the pyrimidinylthiobenzoate herbicides and are generalized under the heading of the latter name by the Weed Science Society of America. Thereofore, unless indicated otherwise herein or apparent from the context, pyrimidinylthiobenzoate herbicides of the present invention include, but are not limited to, pyrimidinyloxybenzoate herbicides, pyrimidinylthiobenzoate herbicides, and mixtures thereof. Likewise, unless indicated otherwise herein or apparent from the context, pyrimidinyloxybenzoate herbicides of the present invention include, but are not limited to, pyrimidinyloxybenzoate herbicides, pyrimidinylthiobenzoate herbicides, and mixtures thereof.  
      The present invention provides methods for enhancing AHAS activity in a plant comprising transforming a plant with a polynucleotide construct comprising a promoter operably linked to an AHASL1 nucleotide sequence of the invention. The methods involve introducing a polynucleotide construct of the invention into at least one plant cell and regenerating a transformed plant therefrom. The methods involve the use of a promoter that is capable of driving gene expression in a plant cell. Preferably, such a promoter is a constitutive promoter or a tissue-preferred promoter. The methods find use in enhancing or increasing the resistance of a plant to at least one herbicide that interferes with the catalytic activity of the AHAS enzyme, particularly an imidazolinone herbicide.  
      The present invention provides expression cassettes for expressing the polynucleotides of the invention in plants, plant tissues, plant cells, and other host cells. The expression cassettes comprise a promoter expressible in the plant, plant tissue, plant cell, or other host cells of interest operably linked to a polynucleotide of the invention that comprises a nucleotide sequence encoding either a full-length (i.e. including the chloroplast transit peptide) or mature AHASL1 protein (i.e. without the chloroplast transit peptide). If expression is desired in the plastids or chloroplasts of plants or plant cells, the expression cassette may also comprise an operably linked chloroplast-targeting sequence that encodes a chloroplast transit peptide.  
      The expression cassettes of the invention find use in a method for enhancing the herbicide tolerance of a plant or a host cell. The method involves transforming the plant or host cell with an expression cassette of the invention, wherein the expression cassette comprises a promoter that is expressible in the plant or host cell of interest and the promoter is operably linked to a polynucleotide of the invention that comprises a nucleotide sequence encoding an imidazolinone-resistant AHASL1 protein of the invention.  
      The use of the term “polynucleotide constructs” herein is not intended to limit the present invention to polynucleotide constructs comprising DNA. Those of ordinary skill in the art will recognize that polynucleotide constructs, particularly polynucleotides and oligonucleotides, comprised of ribonucleotides and combinations of ribonucleotides and deoxyribonucleotides may also be employed in the methods disclosed herein. Thus, the polynucleotide constructs of the present invention encompass all polynucleotide constructs that can be employed in the methods of the present invention for transforming plants including, but not limited to, those comprised of deoxyribonucleotides, ribonucleotides, and combinations thereof. Such deoxyribonucleotides and ribonucleotides include both naturally occurring molecules and synthetic analogues. The polynucleotide constructs of the invention also encompass all forms of polynucleotide constructs including, but not limited to, single-stranded forms, double-stranded forms, hairpins, stem-and-loop structures, and the like. Furthermore, it is understood by those of ordinary skill in the art that each nucleotide sequences disclosed herein also encompasses the complement of that exemplified nucleotide sequence.  
      Furthermore, it is recognized that the methods of the invention may employ a polynucleotide construct that is capable of directing, in a transformed plant, the expression of at least one protein, or at least one RNA, such as, for example, an antisense RNA that is complementary to at least a portion of an mRNA. Typically such a polynucleotide construct is comprised of a coding sequence for a protein or an RNA operably linked to 5′ and 3′ transcriptional regulatory regions. Alternatively, it is also recognized that the methods of the invention may employ a polynucleotide construct that is not capable of directing, in a transformed plant, the expression of a protein or an RNA.  
      Further, it is recognized that, for expression of a polynucleotides of the invention in a host cell of interest, the polynucleotide is typically operably linked to a promoter that is capable of driving gene expression in the host cell of interest. The methods of the invention for expressing the polynucleotides in host cells do not depend on particular promoter. The methods encompass the use of any promoter that is known in the art and that is capable of driving gene expression in the host cell of interest.  
      The present invention encompasses AHASL polynucleotide molecules and fragments and variants thereof. Polynucleotide molecules that are fragments of these nucleotide sequences are also encompassed by the present invention. By “fragment” is intended a portion of the nucleotide sequence encoding an AHASL protein of the invention. A fragment of an AHASL nucleotide sequence of the invention may encode a biologically active portion of an AHASL protein, or it may be a fragment that can be used as a hybridization probe or PCR primer using methods disclosed below. A biologically active portion of an AHASL protein can be prepared by isolating a portion of one of the AHASL nucleotide sequences of the invention, expressing the encoded portion of the AHASL protein (e.g., by recombinant expression in vitro), and assessing the activity of the encoded portion of the AHASL1 protein. Polynucleotide molecules that are fragments of an AHASL nucleotide sequence comprise at least about 15, 20, 50, 75, 100, 200, 300, 350, 400, 450, 500, 550, 600, 650, 700, 750, 800, 850, 900, 950, 1000, 1050, 1100, 1150, 1200, 1250, 1300, 1350, 1400, 1450, 1500, 1550, 1600, 1650, or 1700 nucleotides, or up to the number of nucleotides present in a full-length nucleotide sequence disclosed herein (for example, 1684 and 1706 nucleotides for SEQ ID NOS: 1 and 3, respectively) depending upon the intended use.  
      A fragment of an AHASL nucleotide sequence that encodes a biologically active portion of an AHASL protein of the invention will encode at least about 15, 25, 30, 50, 75, 100, 125, 150, 175, 200, 250, 300, 350, 400, 450, 500, or 550 contiguous amino acids, or up to the total number of amino acids present in a full-length AHASL1 protein of the invention (for example, 561 and 568 amino acids for SEQ ID NOS: 2 and 4, respectively). Fragments of an AHASL1 nucleotide sequence that are useful as hybridization probes for PCR primers generally need not encode a biologically active portion of an AHASL1 protein.  
      Polynucleotide molecules that are variants of the nucleotide sequences disclosed herein are also encompassed by the present invention. “Variants” of the AHASL nucleotide sequences of the invention include those sequences that encode the AHASL proteins disclosed herein but that differ conservatively because of the degeneracy of the genetic code. These naturally occurring allelic variants can be identified with the use of well-known molecular biology techniques, such as polymerase chain reaction (PCR) and hybridization techniques as outlined below. Variant nucleotide sequences also include synthetically derived nucleotide sequences that have been generated, for example, by using site-directed mutagenesis but which still encode the AHASL1 protein disclosed in the present invention as discussed below. Generally, nucleotide sequence variants of the invention will have at least about 70%, 75%, 80%, 85%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, or 99% identity to a particular nucleotide sequence disclosed herein. A variant AHASL nucleotide sequence will encode an AHASL protein, respectively, that has an amino acid sequence having at least about 70%, 75%, 80%, 85%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, or 99% identity to the amino acid sequence of an AHASL protein disclosed herein.  
      In addition, the skilled artisan will further appreciate that changes can be introduced by mutation into the nucleotide sequences of the invention thereby leading to changes in the amino acid sequence of the encoded AHASL proteins without altering the biological activity of the AHASL proteins. Thus, an isolated polynucleotide molecule encoding an AHASL protein having a sequence that differs from that of SEQ ID NOS: 1 or 3, respectively, can be created by introducing one or more nucleotide substitutions, additions, or deletions into the corresponding nucleotide sequence disclosed herein, such that one or more amino acid substitutions, additions or deletions are introduced into the encoded protein. Mutations can be introduced by standard techniques, such as site-directed mutagenesis and PCR-mediated mutagenesis. Such variant nucleotide sequences are also encompassed by the present invention.  
      For example, preferably, conservative amino acid substitutions may be made at one or more predicted, preferably nonessential amino acid residues. A “nonessential” amino acid residue is a residue that can be altered from the wild-type sequence of an AHASL protein (e.g., the sequence of SEQ ID NOS: 2 and 4, respectively) without altering the biological activity, whereas an “essential” amino acid residue is required for biological activity. A “conservative amino acid substitution” is one in which the amino acid residue is replaced with an amino acid residue having a similar side chain. Families of amino acid residues having similar side chains have been defined in the art. These families include amino acids with basic side chains (e.g., lysine, arginine, histidine), acidic side chains (e.g., aspartic acid, glutamic acid), uncharged polar side chains (e.g., glycine, asparagine, glutamine, serine, threonine, tyrosine, cysteine), nonpolar side chains (e.g., alanine, valine, leucine, isoleucine, proline, phenylalanine, methionine, tryptophan), beta-branched side chains (e.g., threonine, valine, isoleucine) and aromatic side chains (e.g., tyrosine, phenylalanine, tryptophan, histidine). Such substitutions would not be made for conserved amino acid residues, or for amino acid residues residing within a conserved motif.  
      The proteins of the invention may be altered in various ways including amino acid substitutions, deletions, truncations, and insertions. Methods for such manipulations are generally known in the art. For example, amino acid sequence variants of the AHASL proteins can be prepared by mutations in the DNA. Methods for mutagenesis and nucleotide sequence alterations are well known in the art. See, for example, Kunkel (1985)  Proc. Natl. Acad. Sci. USA  82:488-492; Kunkel et al. (1987)  Methods in Enzymol.  154:367-382; U.S. Pat. No. 4,873,192; Walker and Gaastra, eds. (1983)  Techniques in Molecular Biology  (MacMillan Publishing Company, New York) and the references cited therein. Guidance as to appropriate amino acid substitutions that do not affect biological activity of the protein of interest may be found in the model of Dayhoff et al. (1978)  Atlas of Protein Sequence and Structure  (Natl. Biomed. Res. Found., Washington, D.C.), herein incorporated by reference. Conservative substitutions, such as exchanging one amino acid with another having similar properties, may be preferable.  
      Alternatively, variant AHASL nucleotide sequences can be made by introducing mutations randomly along all or part of an AHASL coding sequence, such as by saturation mutagenesis, and the resultant mutants can be screened for AHAS activity to identify mutants that retain AHAS activity, including herbicide-resistant AHAS activity. Following mutagenesis, the encoded protein can be expressed recombinantly, and the activity of the protein can be determined using standard assay techniques.  
      Thus, the nucleotide sequences of the invention include the sequences disclosed herein as well as fragments and variants thereof. The AHASL nucleotide sequences of the invention, and fragments and variants thereof, can be used as probes and/or primers to identify and/or clone AHASL homologues in other plants. Such probes can be used to detect transcripts or genomic sequences encoding the same or identical proteins.  
      In this manner, methods such as PCR, hybridization, and the like can be used to identify such sequences having substantial identity to the sequences of the invention. See, for example, Sambrook et al. (1989)  Molecular Cloning: Laboratory Manual  (2d ed., Cold Spring Harbor Laboratory Press, Plainview, N.Y.) and Innis, et al. (1990)  PCR Protocols: A Guide to Methods and Applications  (Academic Press, N.Y.). AHASL nucleotide sequences isolated based on their sequence identity to the AHASL1 nucleotide sequences set forth herein or to fragments and variants thereof are encompassed by the present invention.  
      In a hybridization method, all or part of a known AHASL nucleotide sequence can be used to screen cDNA or genomic libraries. Methods for construction of such cDNA and genomic libraries are generally known in the art and are disclosed in Sambrook et al. (1989)  Molecular Cloning: A Laboratory Manual  (2d ed., Cold Spring Harbor Laboratory Press, Plainview, N.Y.). The so-called hybridization probes may be genomic DNA fragments, cDNA fragments, RNA fragments, or other oligonucleotides, and may be labeled with a detectable group such as  32 P, or any other detectable marker, such as other radioisotopes, a fluorescent compound, an enzyme, or an enzyme co-factor. Probes for hybridization can be made by labeling synthetic oligonucleotides based on the known AHASL nucleotide sequence disclosed herein. Degenerate primers designed on the basis of conserved nucleotides or amino acid residues in a known AHASL nucleotide sequence or encoded amino acid sequence can additionally be used. The probe typically comprises a region of nucleotide sequence that hybridizes under stringent conditions to at least about 12, preferably about 25, more preferably about 50, 75, 100, 125, 150, 175, 200, 250, 300, 350, 400, 500, 600, 700, 800, 900, 1000, 1100, 1200, 1300, 1400, 1500, 1600, or 1700 consecutive nucleotides of an AHASL nucleotide sequence of the invention or a fragment or variant thereof. Preparation of probes for hybridization is generally known in the art and is disclosed in Sambrook et al. (1989)  Molecular Cloning: A Laboratory Manual  (2d ed., Cold Spring Harbor Laboratory Press, Plainview, N.Y.), herein incorporated by reference.  
      For example, the entire AHASL sequence disclosed herein, or one or more portions thereof, may be used as a probe capable of specifically hybridizing to corresponding AHASL sequences and messenger RNAs. Hybridization techniques include hybridization screening of plated DNA libraries (either plaques or colonies; see, for example, Sambrook et al. (1989)  Molecular Cloning: A Laboratory Manual  (2d ed., Cold Spring Harbor Laboratory Press, Plainview, N.Y.).  
      Hybridization of such sequences may be carried out under stringent conditions. By “stringent conditions” or “stringent hybridization conditions” is intended conditions under which a probe will hybridize to its target sequence to a detectably greater degree than to other sequences (e.g., at least 2-fold over background). Stringent conditions are sequence-dependent and will be different in different circumstances.  
      Typically, stringent conditions will be those in which the salt concentration is less than about 1.5 M Na ion, typically about 0.01 to 1.0 M Na ion concentration (or other salts) at pH 7.0 to 8.3 and the temperature is at least about 30° C. for short probes (e.g., 10 to 50 nucleotides) and at least about 60° C. for long probes (e.g., greater than 50 nucleotides). Stringent conditions may also be achieved with the addition of destabilizing agents such as formamide. Exemplary low stringency conditions include hybridization with a buffer solution of 30 to 35% formamide, 1 M NaCl, 1% SDS (sodium dodecyl sulphate) at 37° C., and a wash in 1× to 2×SSC (20×SSC=3.0 M NaCl/0.3 M trisodium citrate) at 50 to 55° C. Exemplary moderate stringency conditions include hybridization in 40 to 45% formamide, 1.0 M NaCl, 1% SDS at 37° C., and a wash in 0.5× to 1×SSC at 55 to 60° C. Exemplary high stringency conditions include hybridization in 50% formamide, 1 M NaCl, 1% SDS at 37° C., and a wash in 0.1×SSC at 60 to 65° C. Optionally, wash buffers may comprise about 0.1% to about 1% SDS. The duration of hybridization is generally less than about 24 hours, usually about 4 to about 12 hours.  
      Specificity is typically the function of post-hybridization washes, the critical factors being the ionic strength and temperature of the final wash solution. For DNA-DNA hybrids, the Tm can be approximated from the equation of Meinkoth and Wahl (1984)  Anal. Biochem.  138:267-284: T m =81.5° C.+16.6 (log M)+0.41 (% GC)−0.61 (% form)−500/L; where M is the molarity of monovalent cations, % GC is the percentage of guanosine and cytosine nucleotides in the DNA, % form is the percentage of formamide in the hybridization solution, and L is the length of the hybrid in base pairs. The T m  is the temperature (under defined ionic strength and pH) at which 50% of a complementary target sequence hybridizes to a perfectly matched probe. T m  is reduced by about 1° C. for each 1% of mismatching; thus, T m , hybridization, and/or wash conditions can be adjusted to hybridize to sequences of the desired identity. For example, if sequences with ≧90% identity are sought, the T m  can be decreased 10° C. Generally, stringent conditions are selected to be about 5° C. lower than the thermal melting point (T m ) for the specific sequence and its complement at a defined ionic strength and pH. However, severely stringent conditions can utilize a hybridization and/or wash at 1, 2, 3, or 4° C. lower than the thermal melting point (T m ); moderately stringent conditions can utilize a hybridization and/or wash at 6, 7, 8, 9, or 10° C. lower than the thermal melting point (T m ); low stringency conditions can utilize a hybridization and/or wash at 11, 12, 13, 14, 15, or 20° C. lower than the thermal melting point (T m ). Using the equation, hybridization and wash compositions, and desired T m , those of ordinary skill will understand that variations in the stringency of hybridization and/or wash solutions are inherently described. If the desired degree of mismatching results in a T m  of less than 45° C. (aqueous solution) or 32° C. (formamide solution), it is preferred to increase the SSC concentration so that a higher temperature can be used. An extensive guide to the hybridization of nucleic acids is found in Tijssen (1993)  Laboratory Techniques in Biochemistry and Molecular Biology—Hybridization with Nucleic Acid Probes,  Part I, Chapter 2 (Elsevier, N.Y.); and Ausubel et al., eds. (1995)  Current Protocols in Molecular Biology,  Chapter 2 (Greene Publishing and Wiley-Interscience, New York). See Sambrook et al. (1989)  Molecular Cloning: A Laboratory Manual  (2d ed., Cold Spring Harbor Laboratory Press, Plainview, N.Y.).  
      It is recognized that the polynucleotide molecules and proteins of the invention encompass polynucleotide molecules and proteins comprising a nucleotide or an amino acid sequence that is sufficiently identical to the nucleotide sequence of SEQ ID NOS: 1 and/or 3, or to the amino acid sequence of SEQ ID NOS: 2 and/or 4. The term “sufficiently identical” is used herein to refer to a first amino acid or nucleotide sequence that contains a sufficient or minimum number of identical or equivalent (e.g., with a similar side chain) amino acid residues or nucleotides to a second amino acid or nucleotide sequence such that the first and second amino acid or nucleotide sequences have a common structural domain and/or common functional activity. For example, amino acid or nucleotide sequences that contain a common structural domain having at least about 45%, 55%, or 65% identity, preferably 75% identity, more preferably 85%, 95%, or 98% identity are defined herein as sufficiently identical.  
      To determine the percent identity of two amino acid sequences or of two nucleic acids, the sequences are aligned for optimal comparison purposes. The percent identity between the two sequences is a function of the number of identical positions shared by the sequences (i.e., percent identity=number of identical positions/total number of positions (e.g, overlapping positions)×100). In one embodiment, the two sequences are the same length. The percent identity between two sequences can be determined using techniques similar to those described below, with or without allowing gaps. In calculating percent identity, typically exact matches are counted.  
      The determination of percent identity between two sequences can be accomplished using a mathematical algorithm. A preferred, nonlimiting example of a mathematical algorithm utilized for the comparison of two sequences is the algorithm of Karlin and Altschul (1990)  Proc. Natl. Acad. Sci. USA  87:2264, modified as in Karlin and Altschul (1993)  Proc. Natl. Acad. Sci. USA  90:5873-5877. Such an algorithm is incorporated into the NBLAST and XBLAST programs of Altschul et al. (1990)  J. Mol. Biol.  215:403. BLAST nucleotide searches can be performed with the NBLAST program, score=100, wordlength=12, to obtain nucleotide sequences homologous to the polynucleotide molecules of the invention. BLAST protein searches can be performed with the XBLAST program, score=50, wordlength=3, to obtain amino acid sequences homologous to protein molecules of the invention. To obtain gapped alignments for comparison purposes, Gapped BLAST can be utilized as described in Altschul et al. (1997)  Nucleic Acids Res.  25:3389. Alternatively, PSI-Blast can be used to perform an iterated search that detects distant relationships between molecules. See Altschul et al. (1997) supra. When utilizing BLAST, Gapped BLAST, and PSI-Blast programs, the default parameters of the respective programs (e.g., XBLAST and NBLAST) can be used. See http://www.ncbi.nlm.nih.gov. Another preferred, non-limiting example of a mathematical algorithm utilized for the comparison of sequences is the algorithm of Myers and Miller (1988)  CABIOS  4:11-17. Such an algorithm is incorporated into the ALIGN program (version 2.0), which is part of the GCG sequence alignment software package. When utilizing the ALIGN program for comparing amino acid sequences, a PAM120 weight residue table, a gap length penalty of 12, and a gap penalty of 4 can be used. Alignment may also be performed manually by inspection.  
      Unless otherwise stated, sequence identity/similarity values provided herein refer to the value obtained using the full-length sequences of the invention and using multiple alignment by mean of the algorithm Clustal W (Nucleic Acid Research, 22(22):4673-4680, 1994) using the program AlignX included in the software package Vector NTI Suite Version 7 (InforMax, Inc., Bethesda, Md., USA) using the default parameters; or any equivalent program thereof. By “equivalent program” is intended any sequence comparison program that, for any two sequences in question, generates an alignment having identical nucleotide or amino acid residue matches and an identical percent sequence identity when compared to the corresponding alignment generated by AlignX in the software package Vector NTI Suite Version 7.  
      The AHASL nucleotide sequences of the invention include both the naturally occurring sequences as well as mutant forms, particularly mutant forms that encode AHASL proteins comprising herbicide-resistant AHAS activity. Likewise, the proteins of the invention encompass both naturally occurring proteins as well as variations and modified forms thereof. Such variants will continue to possess the desired AHAS activity. Obviously, the mutations that will be made in the DNA encoding the variant must not place the sequence out of reading frame and preferably will not create complementary regions that could produce secondary mRNA structure. See, EP Patent Application Publication No. 75,444.  
      The deletions, insertions, and substitutions of the protein sequences encompassed herein are not expected to produce radical changes in the characteristics of the protein. However, when it is difficult to predict the exact effect of the substitution, deletion, or insertion in advance of doing so, one skilled in the art will appreciate that the effect will be evaluated by routine screening assays. That is, the activity can be evaluated by AHAS activity assays. See, for example, Singh et al. (1988)  Anal. Biochem.  171:173-179, herein incorporated by reference.  
      Variant nucleotide sequences and proteins also encompass sequences and proteins derived from a mutagenic and recombinogenic procedure such as DNA shuffling. With such a procedure, one or more different AHASL coding sequences can be manipulated to create a new AHASL protein possessing the desired properties. In this manner, libraries of recombinant polynucleotides are generated from a population of related sequence polynucleotides comprising sequence regions that have substantial sequence identity and can be homologously recombined in vitro or in vivo. For example, using this approach, sequence motifs encoding a domain of interest may be shuffled between the AHASL gene of the invention and other known AHASL genes to obtain a new gene coding for a protein with an improved property of interest, such as an increased K m  in the case of an enzyme. Strategies for such DNA shuffling are known in the art. See, for example, Stemmer (1994)  Proc. Natl. Acad. Sci. USA  91:10747-10751; Stemmer (1994)  Nature  370:389-391; Crameri et al. (1997)  Nature Biotech.  15:436-438; Moore et al. (1997)  J. Mol. Biol.  272:336-347; Zhang et al. (1997)  Proc. Natl. Acad. Sci. USA  94:4504-4509; Crameri et al. (1998)  Nature  391:288-291; and U.S. Pat. Nos. 5,605,793 and 5,837,458.  
      The nucleotide sequences of the invention can be used to isolate corresponding sequences from other organisms, particularly other plants, more particularly other dicots. In this manner, methods such as PCR, hybridization, and the like can be used to identify such sequences based on their sequence homology to the sequences set forth herein. Sequences isolated based on their sequence identity to the entire AHASL polynucleotide sequences set forth herein or to fragments thereof are encompassed by the present invention. Thus, isolated polynucleotide sequences that encode for an AHASL protein and which hybridize under stringent conditions to the sequence disclosed herein, or to fragments thereof, are encompassed by the present invention.  
      In a PCR approach, oligonucleotide primers can be designed for use in PCR reactions to amplify corresponding DNA sequences from cDNA or genomic DNA extracted from any plant of interest. Methods for designing PCR primers and PCR cloning are generally known in the art and are disclosed in Sambrook et al. (1989)  Molecular Cloning: A Laboratory Manual  (2d ed., Cold Spring Harbor Laboratory Press, Plainview, N.Y.). See also Innis et al., eds. (1990)  PCR Protocols: A Guide to Methods and Applications  (Academic Press, New York); Innis and Gelfand, eds. (1995)  PCR Strategies  (Academic Press, New York); and Innis and Gelfand, eds. (1999)  PCR Methods Manual  (Academic Press, New York). Known methods of PCR include, but are not limited to, methods using paired primers, nested primers, single specific primers, degenerate primers, gene-specific primers, vector-specific primers, partially-mismatched primers, and the like.  
      The AHASL polynucleotide sequences of the invention are provided in expression cassettes for expression in the plant of interest. The cassette will include 5′ and 3′ regulatory sequences operably linked to an AHASL polynucleotide sequence of the invention. By “operably linked” is intended a functional linkage between a promoter and a second sequence, wherein the promoter sequence initiates and mediates transcription of the DNA sequence corresponding to the second sequence. Generally, operably linked means that the nucleic acid sequences being linked are contiguous and, where necessary to join two protein coding regions, contiguous and in the same reading frame. The cassette may additionally contain at least one additional gene to be cotransformed into the organism. Alternatively, the additional gene(s) can be provided on multiple expression cassettes.  
      Such an expression cassette is provided with a plurality of restriction sites for insertion of the AHASL polynucleotide sequence to be under the transcriptional regulation of the regulatory regions. The expression cassette may additionally contain selectable marker genes.  
      The expression cassette will include in the 5′-3′ direction of transcription, a transcriptional and translational initiation region (i.e., a promoter), an AHASL polynucleotide sequence of the invention, and a transcriptional and translational termination region (i.e., termination region) functional in plants. The promoter may be native or analogous, or foreign or heterologous, to the plant host and/or to the AHASL polynucleotide sequence of the invention. Additionally, the promoter may be the natural sequence or alternatively a synthetic sequence. Where the promoter is “foreign” or “heterologous” to the plant host, it is intended that the promoter is not found in the native plant into which the promoter is introduced. Where the promoter is “foreign” or “heterologous” to the AHASL polynucleotide sequence of the invention, it is intended that the promoter is not the native or naturally occurring promoter for the operably linked AHASL polynucleotide sequence of the invention. As used herein, a chimeric gene comprises a coding sequence operably linked to a transcription initiation region that is heterologous to the coding sequence.  
      While it may be preferable to express the AHASL polynucleotides of the invention using heterologous promoters, the native promoter sequences may be used. Such constructs would change expression levels of the AHASL protein in the plant or plant cell. Thus, the phenotype of the plant or plant cell is altered.  
      The termination region may be native with the transcriptional initiation region, may be native with the operably linked AHASL sequence of interest, may be native with the plant host, or may be derived from another source (i.e., foreign or heterologous to the promoter, the AHASL polynucleotide sequence of interest, the plant host, or any combination thereof). Convenient termination regions are available from the Ti-plasmid of  A. tumefaciens,  such as the octopine synthase and nopaline synthase termination regions. See also Guerineau et al. (1991)  Mol. Gen. Genet.  262:141-144; Proudfoot (1991)  Cell  64:671-674; Sanfacon et al. (1991)  Genes Dev.  5:141-149; Mogen et al. (1990)  Plant Cell  2:1261-1272; Munroe et al. (1990)  Gene  91:151-158; Ballas et al. (1989)  Nucleic Acids Res.  17:7891-7903; and Joshi et al. (1987)  Nucleic Acid Res.  15:9627-9639.  
      Where appropriate, the gene(s) may be optimized for increased expression in the transformed plant. That is, the genes can be synthesized using plant-preferred codons for improved expression. See, for example, Campbell and Gowri (1990)  Plant Physiol.  92:1-11 for a discussion of host-preferred codon usage. Methods are available in the art for synthesizing plant-preferred genes. See, for example, U.S. Pat. Nos. 5,380,831, and 5,436,391, and Murray et al. (1989)  Nucleic Acids Res.  17:477-498, herein incorporated by reference.  
      Additional sequence modifications are known to enhance gene expression in a cellular host. These include elimination of sequences encoding spurious polyadenylation signals, exon-intron splice site signals, transposon-like repeats, and other such well-characterized sequences that may be deleterious to gene expression. The G-C content of the sequence may be adjusted to levels average for a given cellular host, as calculated by reference to known genes expressed in the host cell. When possible, the sequence is modified to avoid predicted hairpin secondary mRNA structures.  
      Nucleotide sequences for enhancing gene expression can also be used in the plant expression vectors. These include the introns of the maize AdhI, intronl gene (Callis et al.  Genes and Development  1:1183-1200, 1987), and leader sequences, (W-sequence) from the Tobacco Mosaic virus (TMV), Maize Chlorotic Mottle Virus and Alfalfa Mosaic Virus (Gallie et al.  Nucleic Acid Res.  15:8693-8711, 1987 and Skuzeski et al.  Plant Mol. Biol.  15:65-79, 1990). The first intron from the shrunken-1 locus of maize, has been shown to increase expression of genes in chimeric gene constructs. U.S. Pat. Nos. 5,424,412 and 5,593,874 disclose the use of specific introns in gene expression constructs, and Gallie et al. ( Plant Physiol.  106:929-939, 1994) also have shown that introns are useful for regulating gene expression on a tissue specific basis. To further enhance or to optimize AHAS large subunit gene expression, the plant expression vectors of the invention may also contain DNA sequences containing matrix attachment regions (MARs). Plant cells transformed with such modified expression systems, then, may exhibit overexpression or constitutive expression of a nucleotide sequence of the invention.  
      The expression cassettes may additionally contain 5′ leader sequences in the expression cassette construct. Such leader sequences can act to enhance translation. Translation leaders are known in the art and include: picomavirus leaders, for example, EMCV leader (Encephalomyocarditis 5′ noncoding region) (Elroy-Stein et al. (1989)  Proc. Natl. Acad. Sci. USA  86:6126-6130); potyvirus leaders, for example, TEV leader (Tobacco Etch Virus) (Gallie et al. (1995)  Gene  165(2):233-238), MDMV leader (Maize Dwarf Mosaic Virus) ( Virology  154:9-20), and human immunoglobulin heavy-chain binding protein (BiP) (Macejak et al. (1991)  Nature  353:90-94); untranslated leader from the coat protein mRNA of alfalfa mosaic virus (AMV RNA 4) (Jobling et al. (1987)  Nature  325:622-625); tobacco mosaic virus leader (TMV) (Gallie et al. (1989) in  Molecular Biology of RNA,  ed. Cech (Liss, N.Y.), pp. 237-256); and maize chlorotic mottle virus leader (MCMV) (Lommel et al. (1991)  Virology  81:382-385). See also, Della-Cioppa et al. (1987)  Plant Physiol.  84:965-968. Other methods known to enhance translation can also be utilized, for example, introns, and the like.  
      In preparing the expression cassette, the various DNA fragments may be manipulated, so as to provide for the DNA sequences in the proper orientation and, as appropriate, in the proper reading frame. Toward this end, adapters or linkers may be employed to join the DNA fragments or other manipulations may be involved to provide for convenient restriction sites, removal of superfluous DNA, removal of restriction sites, or the like. For this purpose, in vitro mutagenesis, primer repair, restriction, annealing, resubstitutions, e.g., transitions and transversions, may be involved.  
      A number of promoters can be used in the practice of the invention. The promoters can be selected based on the desired outcome. The nucleic acids can be combined with constitutive, tissue-preferred, or other promoters for expression in plants.  
      Such constitutive promoters include, for example, the core promoter of the Rsyn7 promoter and other constitutive promoters disclosed in WO 99/43838 and U.S. Pat. No. 6,072,050; the core CaMV 35S promoter (Odell et al. (1985)  Nature  313:810-812); rice actin (McElroy et al. (1990)  Plant Cell  2:163-171); ubiquitin (Christensen et al. (1989)  Plant Mol. Biol.  12:619-632 and Christensen et al. (1992)  Plant Mol. Biol.  18:675-689); pEMU (Last et al. (1991)  Theor. Appl. Genet.  81:581-588); MAS (Velten et al. (1984)  EMBO J.  3:2723-2730); ALS promoter (U.S. Pat. No. 5,659,026), and the like. Other constitutive promoters include, for example, U.S. Pat. Nos. 5,608,149; 5,608,144; 5,604,121; 5,569,597; 5,466,785; 5,399,680; 5,268,463; 5,608,142; and 6,177,611.  
      Tissue-preferred promoters can be utilized to target enhanced AHASL1 expression within a particular plant tissue. Such tissue-preferred promoters include, but are not limited to, leaf-preferred promoters, root-preferred promoters, seed-preferred promoters, and stem-preferred promoters. Tissue-preferred promoters include Yamamoto et al. (1997)  Plant J.  12(2):255-265; Kawamata et al. (1997)  Plant Cell Physiol.  38(7):792-803; Hansen et al. (1997)  Mol. Gen Genet.  254(3):337-343; Russell et al. (1997)  Transgenic Res.  6(2):157-168; Rinehart et al. (1996)  Plant Physiol.  112(3):1331-1341; Van Camp et al. (1996)  Plant Physiol.  112(2):525-535; Canevascini et al. (1996)  Plant Physiol.  112(2):513-524; Yamamoto et al. (1994)  Plant Cell Physiol.  35(5):773-778; Lam (1994)  Results Probl. Cell Differ.  20:181-196; Orozco et al. (1993)  Plant Mol Biol.  23(6):1129-1138; Matsuoka et al. (1993)  Proc Natl. Acad. Sci. USA  90(20):9586-9590; and Guevara-Garcia et al. (1993)  Plant J.  4(3):495-505. Such promoters can be modified, if necessary, for weak expression.  
      In one embodiment, the nucleic acids of interest are targeted to the chloroplast for expression. In this manner, where the nucleic acid of interest is not directly inserted into the chloroplast, the expression cassette will additionally contain a chloroplast-targeting sequence comprising a nucleotide sequence that encodes a chloroplast transit peptide to direct the gene product of interest to the chloroplasts. Such transit peptides are known in the art. With respect to chloroplast-targeting sequences, “operably linked” means that the nucleic acid sequence encoding a transit peptide (i.e., the chloroplast-targeting sequence) is linked to the AHASL polynucleotide of the invention such that the two sequences are contiguous and in the same reading frame. See, for example, Von Heijne et al. (1991)  Plant Mol. Biol. Rep.  9:104-126; Clark et al. (1989)  J. Biol. Chem.  264:17544-17550; Della-Cioppa et al. (1987)  Plant Physiol.  84:965-968; Romer et al. (1993)  Biochem. Biophys. Res. Commun.  196:1414-1421; and Shah et al. (1986)  Science  233:478-481. While the AHASL proteins of the invention include a native chloroplast transit peptide, any chloroplast transit peptide known in the art can be fused to the amino acid sequence of a mature AHASL protein of the invention by operably linking a choloroplast-targeting sequence to the 5′-end of a nucleotide sequence encoding a mature AHASL protein of the invention.  
      Chloroplast targeting sequences are known in the art and include the chloroplast small subunit of ribulose-1,5-bisphosphate carboxylase (Rubisco) (de Castro Silva Filho et al. (1996)  Plant Mol. Biol.  30:769-780; Schnell et al. (1991)  J. Biol. Chem.  266(5):3335-3342); 5-(enolpyruvyl)shikimate-3-phosphate synthase (EPSPS) (Archer et al. (1990)  J. Bioenerg. Biomemb.  22(6):789-810); tryptophan synthase (Zhao et al. (1995)  J. Biol. Chem.  270(11):6081-6087); plastocyanin (Lawrence et al. (1997)  J. Biol. Chem.  272(33):20357-20363); chorismate synthase (Schmidt et al. (1993)  J. Biol. Chem.  268(36):27447-27457); and the light harvesting chlorophyll a/b binding protein (LHBP) (Lamppa et al. (1988)  J. Biol. Chem.  263:14996-14999). See also Von Heijne et al. (1991)  Plant Mol. Biol. Rep.  9:104-126; Clark et al. (1989)  J. Biol. Chem.  264:17544-17550; Della-Cioppa et al. (1987)  Plant Physiol.  84:965-968; Romer et al. (1993)  Biochem. Biophys. Res. Commun.  196:1414-1421; and Shah et al. (1986)  Science  233:478-481.  
      Methods for transformation of chloroplasts are known in the art. See, for example, Svab et al. (1990)  Proc. Natl. Acad. Sci. USA  87:8526-8530; Svab and Maliga (1993)  Proc. Natl. Acad. Sci. USA  90:913-917; Svab and Maliga (1993)  EMBO J.  12:601-606. The method relies on particle gun delivery of DNA containing a selectable marker and targeting of the DNA to the plastid genome through homologous recombination. Additionally, plastid transformation can be accomplished by transactivation of a silent plastid-borne transgene by tissue-preferred expression of a nuclear-encoded and plastid-directed RNA polymerase. Such a system has been reported in McBride et al. (1994)  Proc. Natl. Acad. Sci. USA  91:7301-7305.  
      The nucleic acids 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 nucleic acids of interest may be synthesized using chloroplast-preferred codons. See, for example, U.S. Pat. No. 5,380,831, herein incorporated by reference.  
      As disclosed herein, the AHASL nucleotide sequences of the invention find use in enhancing the herbicide tolerance of plants that comprise in their genomes a gene encoding a herbicide-tolerant AHASL protein. Such a gene may be an endogenous gene or a transgene. Additionally, in certain embodiments, the nucleic acid sequences of the present invention can be stacked with any combination of polynucleotide sequences of interest in order to create plants with a desired phenotype. For example, the polynucleotides of the present invention may be stacked with any other polynucleotides encoding polypeptides having pesticidal and/or insecticidal activity, such as, for example, the  Bacillus thuringiensis  toxin proteins (described in U.S. Pat. Nos. 5,366,892; 5,747,450; 5,737,514; 5,723,756; 5,593,881; and Geiseretal. (1986)  Gene  48:109). The combinations generated can also include multiple copies of any one of the polynucleotides of interest.  
      It is recognized that with these nucleotide sequences, antisense constructions, complementary to at least a portion of the messenger RNA (mRNA) for the AHASL polynucleotide sequences can be constructed. Antisense nucleotides are constructed to hybridize with the corresponding mRNA. Modifications of the antisense sequences may be made as long as the sequences hybridize to and interfere with expression of the corresponding mRNA. In this manner, antisense constructions having 70%, preferably 80%, more preferably 85% sequence identity to the corresponding antisensed sequences may be used. Furthermore, portions of the antisense nucleotides may be used to disrupt the expression of the target gene. Generally, sequences of at least 50 nucleotides, 100 nucleotides, 200 nucleotides, or greater may be used.  
      The nucleotide sequences of the present invention may also be used in the sense orientation to suppress the expression of endogenous genes in plants. Methods for suppressing gene expression in plants using nucleotide sequences in the sense orientation are known in the art. The methods generally involve transforming plants with a DNA construct comprising a promoter that drives expression in a plant operably linked to at least a portion of a nucleotide sequence that corresponds to the transcript of the endogenous gene. Typically, such a nucleotide sequence has substantial sequence identity to the sequence of the transcript of the endogenous gene, preferably greater than about 65% sequence identity, more preferably greater than about 85% sequence identity, most preferably greater than about 95% sequence identity. See, U.S. Pat. Nos. 5,283,184 and 5,034,323; herein incorporated by reference.  
      While the herbicide-resistant AHASL1 polynucleotides of the invention find use as selectable marker genes for plant transformation, the expression cassettes of the invention can include another selectable marker gene for the selection of transformed cells. Selectable marker genes, including those of the present invention, are utilized for the selection of transformed cells or tissues. Marker genes include, but are not limited to, genes encoding antibiotic resistance, such as those encoding neomycin phosphotransferase II (NEO) and hygromycin phosphotransferase (HPT), as well as genes conferring resistance to herbicidal compounds, such as glufosinate ammonium, bromoxynil, imidazolinones, and 2,4-dichlorophenoxyacetate (2,4-D). See generally, Yarranton (1992)  Curr. Opin. Biotech.  3:506-511; Christopherson etal. (1992)  Proc. Natl. Acad. Sci. USA  89:6314-6318; Yao et al. (1992)  Cell  71:63-72; Reznikoff (1992)  Mol. Microbiol.  6:2419-2422; Barkley et al. (1980) in  The Operon,  pp. 177-220; Hu et al. (1987)  Cell  48:555-566; Brown et al. (1987)  Cell  49:603-612; Figge et al. (1988)  Cell  52:713-722; Deuschle et al. (1989)  Proc. Natl. Acad. Aci. USA  86:5400-5404; Fuerst et al. (1989)  Proc. Natl. Acad. Sci. USA  86:2549-2553; Deuschle et al. (1990)  Science  248:480-483; Gossen (1993) Ph.D. Thesis, University of Heidelberg; Reines et al. (1993)  Proc. Natl. Acad. Sci. USA  90:1917-1921; Labow et al. (1990)  Mol. Cell. Biol.  10:3343-3356; Zambretti et al. (1992)  Proc. Natl. Acad. Sci. USA  89:3952-3956; Baim et al. (1991)  Proc. Natl. Acad Sci. USA  88:5072-5076; Wyborski et al. (1991)  Nucleic Acids Res.  19:4647-4653; Hillenand-Wissman (1989)  Topics Mol. Struc. Biol.  10:143-162; Degenkolb et al. (1991)  Antimicrob. Agents Chemother.  35:1591-1595; Kleinschnidt et al. (1988)  Biochemistry  27:1094-1104; Bonin (1993) Ph.D. Thesis, University of Heidelberg; Gossen et al. (1992)  Proc. Natl. Acad Sci. USA  89:5547-5551; Oliva et al. (1992)  Antimicrob. Agents Chemother.  36:913-919; Hlavka et al. (1985)  Handbook of Experimental Pharmacology,  Vol. 78 (Springer-Verlag, Berlin); Gill et al. (1988)  Nature  334:721-724. Such disclosures are herein incorporated by reference.  
      The above list of selectable marker genes is not meant to be limiting. Any selectable marker gene can be used in the present invention.  
      The isolated polynucleotide molecules comprising nucleotide sequence that encode the AHASL proteins of the invention can be used in vectors to transform plants so that the plants created have enhanced resistant to herbicides, particularly imidazolinone herbicides. The isolated AHASL polynucleotide molecules of the invention can be used in vectors alone or in combination with a nucleotide sequence encoding the small subunit of the AHAS (AHASS) enzyme in conferring herbicide resistance in plants. See, U.S. Pat. No. 6,348,643; which is herein incorporated by reference.  
      The invention also relates to a plant expression vector comprising a promoter that drives expression in a plant operably linked to an isolated polynucleotide molecule of the invention. The isolated polynucleotide molecule comprises a nucleotide sequence encoding an AHASL protein, particularly an AHASL protein comprising an amino sequence that is set forth in SEQ ID NO: 2, 4, 6, or 8, or a functional fragment and variant thereof. The plant expression vector of the invention does not depend on a particular promoter, only that such a promoter is capable of driving gene expression in a plant cell. Preferred promoters include constitutive promoters and tissue-preferred promoters.  
      The transformation vectors of the invention can be used to produce plants transformed with a gene of interest. The transformation vector will comprise a selectable marker gene of the invention and a gene of interest to be introduced and typically expressed in the transformed plant. Such a selectable marker gene comprises a herbicide-resistant AHASL polynucleotide of the invention operably linked to a promoter that drives expression in a host cell. For use in plants and plant cells, the transformation vector comprises a selectable marker gene comprising a herbicide-resistant AHASL polynucleotide of the invention operably linked to a promoter that drives expression in a plant cell.  
      The genes of interest of the invention vary depending on the desired outcome. For example, various changes in phenotype can be of interest including modifying the fatty acid composition in a plant, altering the amino acid content of a plant, altering a plant&#39;s insect and/or pathogen defense mechanisms, and the like. These results can be achieved by providing expression of heterologous products or increased expression of endogenous products in plants. Alternatively, the results can be achieved by providing for a reduction of expression of one or more endogenous products, particularly enzymes or cofactors in the plant. These changes result in a change in phenotype of the transformed plant.  
      In one embodiment of the invention, the genes of interest include insect resistance genes such as, for example,  Bacillus thuringiensis  toxin protein genes (U.S. Pat. Nos. 5,366,892; 5,747,450; 5,736,514; 5,723,756; 5,593,881; and Geiser et al. (1986)  Gene  48:109).  
      The AHASL proteins or polypeptides of the invention can be purified from, for example, sunflower plants and can be used in compositions. Also, an isolated polynucleotide molecule encoding an AHASL protein of the invention can be used to express an AHASL protein of the invention in a microbe such as  E. coli  or a yeast The expressed AHASL protein can be purified from extracts of  E. coli  or yeast by any method known to those of ordinary skill in the art.  
      The invention also relates to a method for creating a transgenic plant that is resistant to herbicides, comprising transforming a plant with a plant expression vector comprising a promoter that drives expression in a plant operably linked to an isolated polynucleotide molecule of the invention. The isolated polynucleotide molecule comprises a nucleotide sequence encoding an AHASL protein of the invention, particularly an AHASL protein comprising: an amino sequence that is set forth in SEQ ID NO: 2, an amino acid sequence encoded by SEQ ID NO: 1, or a functional fragment and variant of said amino acid sequences.  
      The invention also relates to the non-transgenic sunflower plants, transgenic plants produced by the methods of the invention, and progeny and other descendants of such non-transgenic and transgenic plants, which plants exhibit enhanced or increased resistance to herbicides that interfere with the AHAS enzyme, particularly imidazolinone and sulfonylurea herbicides.  
      The AHASL polynucleotides of the invention, particularly those encoding herbicide-resistant AHASL proteins, find use in methods for enhancing the resistance of herbicide-tolerant plants. In one embodiment of the invention, the herbicide-tolerant plants comprise a herbicide-tolerant or herbicide-resistant AHASL protein. The herbicide-tolerant plants include both plants transformed with a herbicide-tolerant AHASL nucleotide sequences and plants that comprise in their genomes an endogenous gene that encodes a herbicide-tolerant AHASL protein. Nucleotide sequences encoding herbicide-tolerant AHASL proteins and herbicide-tolerant plants comprising an endogenous gene that encodes a herbicide-tolerant AHASL protein include the polynucleotides and plants of the present invention and those that are known in the art. See, for example, U.S. Pat. Nos. 5,013,659, 5,731,180, 5,767,361, 5,545,822, 5,736,629, 5,773,703, 5,773,704, 5,952,553 and 6,274,796; all of which are herein incorporated by reference. Such methods for enhancing the resistance of herbicide-tolerant plants comprise transforming a herbicide-tolerant plant with at least one polynucleotide construction comprising a promoter that drives expression in a plant cell that is operably linked to a herbicide-resistant AHASL polynucleotide of the invention, particularly the polynucleotide encoding a herbicide-resistant AHASL protein set forth in SEQ ID NO: 1, polynucleotides encoding the amino acid sequence set forth in SEQ ID NO: 2, and fragments and variants said polynucleotides that encode polypeptides comprising herbicide-resistant AHAS activity.  
      Numerous plant transformation vectors and methods for transforming plants are available. See, for example, An, G. et al. (1986)  Plant Pysiol.,  81:301-305; Fry, J., et al. (1987)  Plant Cell Rep.  6:321-325; Block, M. (1988)  Theor. Appl Genet.  76:767-774; Hinchee, et al. (1990)  Stadler. Genet. Symp.  203212.203-212; Cousins, et al. (1991)  Aust. J. Plant Physiol.  18:481-494; Chee, P. P. and Slightom, J. L. (1992)  Gene.  118:255-260; Christou, et al. (1992)  Trends. Biotechnol.  10:239-246; D&#39;Halluin, et al. (1992)  Bio/Technol.  10:309-314; Dhir, et al. (1992)  Plant Physiol.  99:81-88; Casas et al. (1993)  Proc. Nat. Acad Sci. USA  90:11212-11216; Christou, P. (1993)  In Vitro Cell. Dev. Biol.—Plant;  29P:119-124; Davies, et al. (1993)  Plant Cell Rep.  12:180-183; Dong, J. A. and Mchughen, A. (1993)  Plant Sci.  91:139-148; Franklin, C. I. and Trieu, T. N. (1993)  Plant. Physiol.  102:167; Golovkin, et al. (1993)  Plant Sci.  90:41-52; Guo Chin Sci. Bull. 38:2072-2078; Asano, et al. (1994)  Plant Cell Rep.  13; Ayeres N. M. and Park, W. D. (1994)  Crit. Rev. Plant. Sci.  13:219-239; Barcelo, et al. (1994)  Plant. J.  5:583-592; Becker, et al. (1994)  Plant. J.  5:299-307; Borkowska et al. (1994)  Acta. Physiol Plant.  16:225-230; Christou, P. (1994)  Agro. Food. Ind. Hi Tech.  5: 17-27; Eapen et al. (1994)  Plant Cell Rep.  13:582-586; Hartman, et al. (1994)  Bio - Technology  12: 919923; Ritala, et al. (1994)  Plant. Mol. Biol.  24:317-325; and Wan, Y. C. and Lemaux, P. G. (1994)  Plant Physiol.  104:3748.  
      The methods of the invention involve introducing a polynucleotide construct into a plant. By “introducing” is intended presenting to the plant the polynucleotide construct in such a manner that the construct gains access to the interior of a cell of the plant. The methods of the invention do not depend on a particular method for introducing a polynucleotide construct to a plant, only that the polynucleotide construct gains access to the interior of at least one cell of the plant. Methods for introducing polynucleotide constructs into plants are known in the art including, but not limited to, stable transformation methods, transient transformation methods, and virus-mediated methods.  
      By “stable transformation” is intended that the polynucleotide construct introduced into a plant integrates into the genome of the plant and is capable of being inherited by progeny thereof. By “transient transformation” is intended that a polynucleotide construct introduced into a plant does not integrate into the genome of the plant.  
      For the transformation of plants and plant cells, the nucleotide sequences of the invention are inserted using standard techniques into any vector known in the art that is suitable for expression of the nucleotide sequences in a plant or plant cell. The selection of the vector depends on the preferred transformation technique and the target plant species to be transformed. In an embodiment of the invention, an AHASL1 nucleotide sequence is operably linked to a plant promoter that is known for high-level expression in a plant cell, and this construct is then introduced into a plant that is susceptible to an imidazolinone herbicide and a transformed plant is regenerated. The transformed plant is tolerant to exposure to a level of an imidazolinone herbicide that would kill or significantly injure an untransformed plant. This method can be applied to any plant species; however, it is most beneficial when applied to crop plants.  
      Methodologies for constructing plant expression cassettes and introducing foreign nucleic acids into plants are generally known in the art and have been previously described. For example, foreign DNA can be introduced into plants, using tumor-inducing (Ti) plasmid vectors. Other methods utilized for foreign DNA delivery involve the use of PEG mediated protoplast transformation, electroporation, microinjection whiskers, and biolistics or microprojectile bombardment for direct DNA uptake. Such methods are known in the art. (U.S. Pat. No. 5,405,765 to Vasil et al.; Bilang et al. (1991)  Gene  100: 247-250; Scheid et al., (1991)  Mol. Gen. Genet.,  228: 104-112; Guerche et al., (1987)  Plant Science  52: 111-116; Neuhause et al., (1987)  Theor. Appl Genet.  75: 30-36; Klein et al., (1987)  Nature  327: 70-73; Howell et al., (1980)  Science  208:1265; Horsch et al., (1985)  Science  227: 1229-1231; DeBlock et al., (1989)  Plant Physiology  91: 694-701; Methods for Plant Molecular Biology (Weissbach and Weissbach, eds.) Academic Press, Inc. (1988) and  Methods in Plant Molecular Biology  (Schuler and Zielinski, eds.) Academic Press, Inc. (1989). The method of transformation depends upon the plant cell to be transformed, stability of vectors used, expression level of gene products and other parameters.  
      Other suitable methods of introducing nucleotide sequences into plant cells and subsequent insertion into the plant genome include microinjection as Crossway et al. (1986)  Biotechniques  4:320-334, electroporation as described by Riggs et al. (1986)  Proc. Natl. Acad. Sci. USA  83:5602-5606,  Agrobacterium -mediated transformation as described by Townsend et al., U.S. Pat. No. 5,563,055, Zhao et al., U.S. Pat. No. 5,981,840, direct gene transfer as described by Paszkowski et al. (1984)  EMBO J.  3:2717-2722, and ballistic particle acceleration as described in, for example, Sanford et al., U.S. Pat. No. 4,945,050; Tomes et al., U.S. Pat. No. 5,879,918; Tomes et al., U.S. Pat. No. 5,886,244; Bidney et al., U.S. Pat. No. 5,932,782; Tomes et al. (1995) “Direct DNA Transfer into Intact Plant Cells via Microprojectile Bombardment,” in  Plant Cell, Tissue, and Organ Culture: Fundamental Methods,  ed. Gamborg and Phillips (Springer-Verlag, Berlin); McCabe et al. (1988)  Biotechnology  6:923-926); and Lec1 transformation (WO 00/28058). Also see, Weissinger et al. (1988)  Ann. Rev. Genet.  22:421-477; Sanford et al. (1987)  Particulate Science and Technology  5:27-37 (onion); Christou et al. (1988)  Plant Physiol.  87:671-674 (soybean); McCabe et al. (1988)  Bio/Technology  6:923-926 (soybean); Finer and McMullen (1991)  In Vitro Cell Dev. Biol.  27P:175-182 (soybean); Singh et al. (1998)  Theor. Appl. Genet.  96:319-324 (soybean); Datta et al. (1990)  Biotechnology  8:736-740 (rice); Klein et al. (1988)  Proc. Natl. Acad. Sci. USA  85:4305-4309 (maize); Klein et al. (1988)  Biotechnology  6:559-563 (maize); Tomes, U.S. Pat. No. 5,240,855; Buising et al., U.S. Pat. Nos. 5,322,783 and 5,324,646; Tomes et al. (1995) “Direct DNA Transfer into Intact Plant Cells via Microprojectile Bombardment,” in  Plant Cell, Tissue, and Organ Culture: Fundamental Methods,  ed. Gamborg (Springer-Verlag, Berlin) (maize); Klein et al. (1988)  Plant Physiol.  91:440-444 (maize); Fromm et al. (1990)  Biotechnology  8:833-839 (maize); Hooykaas-Van Slogteren et al. (1984)  Nature  (London) 311:763-764; Bowen et al., U.S. Pat. No. 5,736,369 (cereals); Bytebier et al. (1987)  Proc. Natl. Acad. Sci. USA  84:5345-5349 (Liliaceae); De Wet et al. (1985) in  The Experimental Manipulation of Ovule Tissues,  ed. Chapman et al. (Longman, N.Y.), pp. 197-209 (pollen); Kaeppler et al. (1990)  Plant Cell Reports  9:415-418 and Kaeppler et al. (1992)  Theor. Appl. Genet.  84:560-566 (whisker-mediated transformation); D&#39;Halluin et al. (1992)  Plant Cell  4:1495-1505 (electroporation); Li et al. (1993)  Plant Cell Reports  12:250-255 and Christou and Ford (1995)  Annals of Botany  75:407-413 (rice); Osjoda et al. (1996)  Nature Biotechnology  14:745-750 (maize via  Agrobacterium tumefaciens ); all of which are herein incorporated by reference.  
      The polynucleotides of the invention may be introduced into plants by contacting plants with a virus or viral nucleic acids. Generally, such methods involve incorporating a polynucleotide construct of the invention within a viral DNA or RNA molecule. It is recognized that the an AHASL protein of the invention may be initially synthesized as part of a viral polyprotein, which later may be processed by proteolysis in vivo or in vitro to produce the desired recombinant protein. Further, it is recognized that promoters of the invention also encompass promoters utilized for transcription by viral RNA polymerases. Methods for introducing polynucleotide constructs into plants and expressing a protein encoded therein, involving viral DNA or RNA molecules, are known in the art. See, for example, U.S. Pat. Nos. 5,889,191, 5,889,190, 5,866,785, 5,589,367 and 5,316,931; herein incorporated by reference.  
      The cells that have been transformed may be grown into plants in accordance with conventional ways. See, for example, McCormick et al. (1986)  Plant Cell Reports  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 construct of the invention, for example, an expression cassette of the invention, stably incorporated into their genome.  
      The present invention may be used for transformation of any plant species, including, but not limited to, monocots and dicots. Examples of plant species of interest include, but are not limited to, corn or maize ( Zea mays ),  Brassica  sp. (e.g.,  B. napus, B. rapa, B. juncea ), particularly those  Brassica  species useful as sources of seed oil, alfalfa ( Medicago sativa ), rice ( Oryza sativa ), rye ( Secale cereale ), sorghum ( Sorghum bicolor, Sorghum vulgare ), millet (e.g., pearl millet ( Pennisetum glaucum ), proso millet ( Panicum miliaceum ), foxtail millet ( Setaria italica ), finger millet ( Eleusine coracana )), sunflower ( Helianthus annuus ), safflower ( Carthamus tinctorius ), wheat ( Triticum aestivum, T. Turgidum  ssp.  durum ), soybean ( Glycine max ), tobacco ( Nicotiana tabacum ), potato ( Solanum tuberosum ), peanuts ( Arachis hypogaea ), cotton ( Gossypium barbadense, Gossypium hirsutum ), sweet potato ( Ipomoea batatus ), cassava ( Manihot esculenta ), coffee ( Coffea  spp.), coconut ( Cocos nucifera ), pineapple ( Ananas comosus ), citrus trees ( Citrus  spp.), cocoa ( Theobroma cacao ), tea ( Camellia sinensis ), banana ( Musa  spp.), avocado ( Persea americana ), fig ( Ficus casica ), guava ( Psidium guajava ), mango ( Mangifera indica ), olive ( Olea europaea ), papaya ( Carica papaya ), cashew ( Anacardium occidentale ), macadamia ( Macadamia integrifolia ), almond ( Prunus amygdalus ), sugar beets ( Beta vulgaris ), sugarcane ( Saccharum  spp.), oats, barley, vegetables, ornamentals, and conifers. Preferably, plants of the present invention are crop plants (for example, sunflower,  Brassica  sp., cotton, sugar, beet, soybean, peanut, alfalfa, safflower, tobacco, corn, rice, wheat, rye, barley triticale, sorghum, millet, etc.).  
      The herbicide resistant plants of the invention find use in methods for controlling weeds. Thus, the present invention further provides a method for controlling weeds in the vicinity of a herbicide-resistant plant of the invention. The method comprises applying an effective amount of a herbicide to the weeds and to the herbicide-resistant plant, wherein the plant has increased resistance to at least one herbicide, particularly an imidazolinone or sulfonylurea herbicide, when compared to a wild-type plant. In such a method for controlling weeds, the herbicide-resistant plants of the invention are preferably crop plants, including, but not limited to, sunflower, alfalfa,  Brassica  sp., soybean, cotton, safflower, peanut, tobacco, tomato, potato, wheat, rice, maize, sorghum, barley, rye, millet, and sorghum.  
      By providing plants having increased resistance to herbicides, particularly imidazolinone and sulfonylurea herbicides, a wide variety of formulations can be employed for protecting plants from weeds, so as to enhance plant growth and reduce competition for nutrients. A herbicide can be used by itself for pre-emergence, post-emergence, pre-planting and at planting control of weeds in areas surrounding the plants described herein or an imidazolinone herbicide formulation can be used that contains other additives. The herbicide can also be used as a seed treatment. That is an effective concentration or an effective amount of the herbicide, or a composition comprising an effective concentration or an effective amount of the herbicide can be applied directly to the seeds prior to or during the sowing of the seeds. Additives found in an imidazolinone or sulfonylurea herbicide formulation or composition include other herbicides, detergents, adjuvants, spreading agents, sticking agents, stabilizing agents, or the like. The herbicide formulation can be a wet or dry preparation and can include, but is not limited to, flowable powders, emulsifiable concentrates and liquid concentrates. The herbicide and herbicide formulations can be applied in accordance with conventional methods, for example, by spraying, irrigation, dusting, coating, and the like.  
      The present invention provides non-transgenic and transgenic seeds with increased resistance to at least one herbicide, particularly an AHAS-inhibiting herbicide. Such seeds include, for example, non-transgenic sunflower seeds comprising the herbicide-resistance characteristics of the plant with ATCC Patent Deposit Number PTA-6325, and transgenic seeds comprising a polynucleotide molecule of the invention that encodes a herbicide-resistant AHASL1 protein.  
      The present invention provides methods for producing a herbicide-resistant plant, particularly a herbicide-resistant sunflower plant, through conventional plant breeding involving sexual reproduction. The methods comprise crossing a first plant that is resistant to a herbicide to a second plant that is not resistant to the herbicide. The first plant can be any of the herbicide resistant plants of the present invention including, for example, transgenic plants comprising at least one of the polynucleotides of the present invention that encode a herbicide resistant AHASL and non-transgenic sunflower plants that comprise the herbicide-resistance characteristics of the sunflower plant with ATCC Patent Deposit Number PTA-6325. The second plant can be any plant that is capable of producing viable progeny plants (i.e., seeds) when crossed with the first plant. Typically, but not necessarily, the first and second plants are of the same species. The methods of the invention can further involve one or more generations of backcrossing the progeny plants of the first cross to a plant of the same line or genotype as either the first or second plant. Alternatively, the progeny of the first cross or any subsequent cross can be crossed to a third plant that is of a different line or genotype than either the first or second plant. The methods of the invention can additionally involve selecting plants that comprise the herbicide resistance characteristics of the first plant.  
      The present invention further provides methods for increasing the herbicide-resistance of a plant, particularly a herbicide-resistant sunflower plant, through conventional plant breeding involving sexual reproduction. The methods comprise crossing a first plant that is resistant to a herbicide to a second plant that may or may not be resistant to the herbicide or may be resistant to different herbicide or herbicides than the first plant. The first plant can be any of the herbicide resistant plants of the present invention including, for example, transgenic plants comprising at least one of the polynucleotides of the present invention that encode a herbicide resistant AHASL and non-transgenic sunflower plants that comprise the herbicide-resistance characteristics of the sunflower plant with ATCC Patent Deposit Number PTA-6325. The second plant can be any plant that is capable of producing viable progeny plants (i.e., seeds) when crossed with the first plant. Typically, but not necessarily, the first and second plants are of the same species. The progeny plants produced by this method of the present invention have increased resistance to a herbicide when compared to either the first or second plant or both. When the first and second plants are resistant to different herbicides, the progeny plants will have the combined herbicide resistance characteristics of the first and second plants. The methods of the invention can further involve one or more generations of backcrossing the progeny plants of the first cross to a plant of the same line or genotype as either the first or second plant. Alternatively, the progeny of the first cross or any subsequent cross can be crossed to a third plant that is of a different line or genotype than either the first or second plant. The methods of the invention can additionally involve selecting plants that comprise the herbicide resistance characteristics of the first plant, the second plant, or both the first and the second plant.  
      The plants of the present invention can be transgenic or non-transgenic. An example of a non-transgenic sunflower plant having increased resistance to imidazolinone and/or sulfonylurea herbicides includes the sunflower plant (MUT9) having ATCC Patent Deposit No. PTA-6325; or mutant, recombinant, or a genetically engineered derivative of the plant having ATCC Patent Deposit No. PTA-6325; or of any progeny of the plant having ATCC Patent Deposit No. PTA-6325; or a plant that is a progeny of any of these plants; or a plant that comprises the herbicide resistance characteristics of the plant having ATCC Patent Deposit No. PTA-6325.  
      The present invention also provides plants, plant organs, plant tissues, plant cells, seeds, and non-human host cells that are transformed with the at least one polynucleotide molecule, expression cassette, or transformation vector of the invention. Such transformed plants, plant organs, plant tissues, plant cells, seeds, and non-human host cells have enhanced tolerance or resistance to at least one herbicide, at levels of the herbicide that kill or inhibit the growth of an untransformed plant, plant tissue, plant cell, or non-human host cell, respectively. Preferably, the transformed plants, plant tissues, plant cells, and seeds of the invention are  Arabidopsis thaliana  and crop plants.  
      The present invention provides methods that involve the use of an AHAS-inhibiting herbicide. In these methods, the AHAS-inhibiting herbicide can be applied by any method known in the art including, but not limited to, seed treatment, soil treatment, and foliar treatment.  
      Prior to application, the AHAS-inhibiting herbicide can be converted into the customary formulations, for example solutions, emulsions, suspensions, dusts, powders, pastes and granules. The use form depends on the particular intended purpose; in each case, it should ensure a fine and even distribution of the compound according to the invention.  
      The formulations are prepared in a known manner (see e.g. for review U.S. Pat. No. 3,060,084, EP-A 707 445 (for liquid concentrates), Browning, “Agglomeration”, Chemical Engineering, Dec. 4, 1967, 147-48, Perry&#39;s Chemical Engineer&#39;s Handbook, 4th Ed., McGraw-Hill, N.Y., 1963, pages 8-57 and et seq. WO 91/13546, U.S. Pat. No. 4,172,714, U.S. Pat. No. 4,144,050, U.S. Pat. No. 3,920,442, U.S. Pat. No. 5,180,587, U.S. Pat. No. 5,232,701, U.S. Pat. No. 5,208,030, GB 2,095,558, U.S. Pat. No. 3,299,566, Klingman, Weed Control as a Science, John Wiley and Sons, Inc., New York, 1961, Hance et al., Weed Control Handbook, 8th Ed., Blackwell Scientific Publications, Oxford, 1989 and Mollet, H., Grubemann, A., Formulation technology, Wiley VCH Verlag GmbH, Weinheim (Germany), 2001, 2. D. A. Knowles, Chemistry and Technology of Agrochemical Formulations, Kluwer Academic Publishers, Dordrecht, 1998 (ISBN 0-7514-0443-8), for example by extending the active compound with auxiliaries suitable for the formulation of agrochemicals, such as solvents and/or carriers, if desired emulsifiers, surfactants and dispersants, preservatives, antifoaming agents, anti-freezing agents, for seed treatment formulation also optionally colorants and/or binders and/or gelling agents.  
      Examples of suitable solvents are water, aromatic solvents (for example Solvesso products, xylene), paraffins (for example mineral oil fractions), alcohols (for example methanol, butanol, pentanol, benzyl alcohol), ketones (for example cyclohexanone, gamma-butyrolactone), pyrrolidones (NMP, NOP), acetates (glycol diacetate), glycols, fatty acid dimethylamides, fatty acids and fatty acid esters. In principle, solvent mixtures may also be used.  
      Examples of suitable carriers are ground natural minerals (for example kaolins, clays, talc, chalk) and ground synthetic minerals (for example highly disperse silica, silicates).  
      Suitable emulsifiers are nonionic and anionic emulsifiers (for example polyoxyethylene fatty alcohol ethers, alkylsulfonates and arylsulfonates).  
      Examples of dispersants are lignin-sulfite waste liquors and methylcellulose.  
      Suitable surfactants used are alkali metal, alkaline earth metal and ammonium salts of lignosulfonic acid, naphthalenesulfonic acid, phenolsulfonic acid, dibutylnaphthalenesulfonic acid, alkylarylsulfonates, alkyl sulfates, alkylsulfonates, fatty alcohol sulfates, fatty acids and sulfated fatty alcohol glycol ethers, furthermore condensates of sulfonated naphthalene and naphthalene derivatives with formaldehyde, condensates of naphthalene or of naphthalenesulfonic acid with phenol and formaldehyde, polyoxyethylene octylphenol ether, ethoxylated isooctylphenol, octylphenol, nonylphenol, alkylphenol polyglycol ethers, tributylphenyl polyglycol ether, tristearylphenyl polyglycol ether, alkylaryl polyether alcohols, alcohol and fatty alcohol ethylene oxide condensates, ethoxylated castor oil, polyoxyethylene alkyl ethers, ethoxylated polyoxypropylene, lauryl alcohol polyglycol ether acetal, sorbitol esters, lignosulfite waste liquors and methylcellulose.  
      Substances which are suitable for the preparation of directly sprayable solutions, emulsions, pastes or oil dispersions are mineral oil fractions of medium to high boiling point, such as kerosene or diesel oil, furthermore coal tar oils and oils of vegetable or animal origin, aliphatic, cyclic and aromatic hydrocarbons, for example toluene, xylene, paraffin, tetrahydronaphthalene, alkylated naphthalenes or their derivatives, methanol, ethanol, propanol, butanol, cyclohexanol, cyclohexanone, isophorone, highly polar solvents, for example dimethyl sulfoxide, N-methylpyrrolidone or water.  
      Also anti-freezing agents such as glycerin, ethylene glycol, propylene glycol and bactericides such as can be added to the formulation.  
      Suitable antifoaming agents are for example antifoaming agents based on silicon or magnesium stearate.  
      Suitable preservatives are for example Dichlorophen und enzylalkoholhemiformal.  
      Seed Treatment formulations may additionally comprise binders and optionally colorants.  
      Binders can be added to improve the adhesion of the active materials on the seeds after treatment. Suitable binders are block copolymers EO/PO surfactants but also polyvinylalcoholsl, polyvinylpyrrolidones, polyacrylates, polymethacrylates, polybutenes, polyisobutylenes, polystyrene, polyethyleneamines, polyethyleneamides, polyethyleneimines (Lupasol®, Polymin®), polyethers, polyurethans, polyvinylacetate, tylose and copolymers derived from these polymers.  
      Optionally, also colorants can be included in the formulation. Suitable colorants or dyes for seed treatment formulations are Rhodamin B, C.I. Pigment Red 112, C.I. Solvent Red 1, pigment blue 15:4, pigment blue 15:3, pigment blue 15:2, pigment blue 15: 1, pigment blue 80, pigment yellow 1, pigment yellow 13, pigment red 112, pigment red 48:2, pigment red 48:1, pigment red 57:1, pigment red 53:1, pigment orange 43, pigment orange 34, pigment orange 5, pigment green 36, pigment green 7, pigment white 6, pigment brown 25, basic violet 10, basic violet 49, acid red 51, acid red 52, acid red 14, acid blue 9, acid yellow 23, basic red 10, basic red 108.  
      An examples of a suitable gelling agent is carrageen (Satiagel®)  
      Powders, materials for spreading, and dustable products can be prepared by mixing or concomitantly grinding the active substances with a solid carrier.  
      Granules, for example coated granules, impregnated granules and homogeneous granules, can be prepared by binding the active compounds to solid carriers. Examples of solid carriers are mineral earths such as silica gels, silicates, talc, kaolin, attaclay, limestone, lime, chalk, bole, loess, clay, dolomite, diatomaceous earth, calcium sulfate, magnesium sulfate, magnesium oxide, ground synthetic materials, fertilizers, such as, for example, ammonium sulfate, ammonium phosphate, ammonium nitrate, ureas, and products of vegetable origin, such as cereal meal, tree bark meal, wood meal and nutshell meal, cellulose powders and other solid carriers.  
      In general, the formulations comprise from 0.01 to 95% by weight, preferably from 0.1 to 90% by weight, of the AHAS-inhibiting herbicide. In this case, the AHAS-inhibiting herbicides are employed in a purity of from 90% to 100% by weight, preferably 95% to 100% by weight (according to NMR spectrum). For seed treatment purposes, respective formulations can be diluted 2-10 fold leading to concentrations in the ready to use preparations of 0.01 to 60% by weight active compound by weight, preferably 0.1 to 40% by weight.  
      The AHAS-inhibiting herbicide can be used as such, in the form of their formulations or the use forms prepared therefrom, for example in the form of directly sprayable solutions, powders, suspensions or dispersions, emulsions, oil dispersions, pastes, dustable products, materials for spreading, or granules, by means of spraying, atomizing, dusting, spreading or pouring. The use forms depend entirely on the intended purposes; they are intended to ensure in each case the finest possible distribution of the AHAS-inhibiting herbicide according to the invention.  
      Aqueous use forms can be prepared from emulsion concentrates, pastes or wettable powders (sprayable powders, oil dispersions) by adding water. To prepare emulsions, pastes or oil dispersions, the substances, as such or dissolved in an oil or solvent, can be homogenized in water by means of a wetter, tackifier, dispersant or emulsifier. However, it is also possible to prepare concentrates composed of active substance, wetter, tackifier, dispersant or emulsifier and, if appropriate, solvent or oil, and such concentrates are suitable for dilution with water.  
      The active compound concentrations in the ready-to-use preparations can be varied within relatively wide ranges. In general, they are from 0.0001 to 10%, preferably from 0.01 to 1% per weight.  
      The AHAS-inhibiting herbicide may also be used successfully in the ultra-low-volume process (ULV), it being possible to apply formulations comprising over 95% by weight of active compound, or even to apply the active compound without additives.  
      The following are examples of formulations: 
          1. Products for dilution with water for foliar applications. For seed treatment purposes, such products may be applied to the seed diluted or undiluted. 
            A) Water-soluble concentrates (SL, LS)     Ten parts by weight of the AHAS-inhibiting herbicide are dissolved in 90 parts by weight of water or a water-soluble solvent. As an alternative, wetters or other auxiliaries are added. The AHAS-inhibiting herbicide dissolves upon dilution with water, whereby a formulation with 10% (w/w) of AHAS-inhibiting herbicide is obtained.     B) Dispersible concentrates (DC)     Twenty parts by weight of the AHAS-inhibiting herbicide are dissolved in 70 parts by weight of cyclohexanone with addition of 10 parts by weight of a dispersant, for example polyvinylpyrrolidone. Dilution with water gives a dispersion, whereby a formulation with 20% (w/w) of AHAS-inhibiting herbicide is obtained.     C) Emulsifiable concentrates (EC)     Fifteen parts by weight of the AHAS-inhibiting herbicide are dissolved in 7 parts by weight of xylene with addition of calcium dodecylbenzenesulfonate and castor oil ethoxylate (in each case 5 parts by weight). Dilution with water gives an emulsion, whereby a formulation with 15% (w/w) of AHAS-inhibiting herbicide is obtained.     D) Emulsions (EW, EO, ES)     Twenty-five parts by weight of the AHAS-inhibiting herbicide are dissolved in 35 parts by weight of xylene with addition of calcium dodecylbenzenesulfonate and castor oil ethoxylate (in each case 5 parts by weight). This mixture is introduced into 30 parts by weight of water by means of an emulsifier machine (e.g. Ultraturrax) and made into a homogeneous emulsion. Dilution with water gives an emulsion, whereby a formulation with 25% (w/w) of AHAS-inhibiting herbicide is obtained.     E) Suspensions (SC, OD, FS)     In an agitated ball mill, 20 parts by weight of the AHAS-inhibiting herbicide are comminuted with addition of 10 parts by weight of dispersants, wetters and 70 parts by weight of water or of an organic solvent to give a fine AHAS-inhibiting herbicide suspension. Dilution with water gives a stable suspension of the AHAS-inhibiting herbicide, whereby a formulation with 20% (w/w) of AHAS-inhibiting herbicide is obtained.     F) Water-dispersible granules and water-soluble granules (WG, SG)     Fifty parts by weight of the AHAS-inhibiting herbicide are ground finely with addition of 50 parts by weight of dispersants and wetters and made as water-dispersible or water-soluble granules by means of technical appliances (for example extrusion, spray tower, fluidized bed). Dilution with water gives a stable dispersion or solution of the AHAS-inhibiting herbicide, whereby a formulation with 50% (w/w) of AHAS-inhibiting herbicide is obtained.     G) Water-dispersible powders and water-soluble powders (WP, SP, SS, WS)     Seventy-five parts by weight of the AHAS-inhibiting herbicide are ground in a rotor-stator mill with addition of 25 parts by weight of dispersants, wetters and silica gel. Dilution with water gives a stable dispersion or solution of the AHAS-inhibiting herbicide, whereby a formulation with 75% (w/w) of AHAS-inhibiting herbicide is obtained.     I) Gel-Formulation (GF)     In an agitated ball mill, 20 parts by weight of the AHAS-inhibiting herbicide are comminuted with addition of 10 parts by weight of dispersants, 1 part by weight of a gelling agent wetters and 70 parts by weight of water or of an organic solvent to give a fine AHAS-inhibiting herbicide suspension. Dilution with water gives a stable suspension of the AHAS-inhibiting herbicide, whereby a formulation with 20% (w/w) of AHAS-inhibiting herbicide is obtained. This gel formulation is suitable for us as a seed treatment.    
            2. Products to be applied undiluted for foliar applications. For seed treatment purposes, such products may be applied to the seed diluted. 
            A) Dustable powders (DP, DS)     Five parts by weight of the AHAS-inhibiting herbicide are ground finely and mixed intimately with 95 parts by weight of finely divided kaolin. This gives a dustable product having 5% (w/w) of AHAS-inhibiting herbicide.     B) Granules (GR, FG, GG, MG)     One-half part by weight of the AHAS-inhibiting herbicide is ground finely and associated with 95.5 parts by weight of carriers, whereby a formulation with 0.5% (w/w) of AHAS-inhibiting herbicide is obtained. Current methods are extrusion, spray-drying or the fluidized bed. This gives granules to be applied undiluted for foliar use.    
               

      Conventional seed treatment formulations include for example flowable concentrates FS, solutions LS, powders for dry treatment DS, water dispersible powders for slurry treatment WS, water-soluble powders SS and emulsion ES and EC and gel formulation GF. These formulations can be applied to the seed diluted or undiluted. Application to the seeds is carried out before sowing, either directly on the seeds.  
      In a preferred embodiment a FS formulation is used for seed treatment. Typcially, a FS formulation may comprise 1-800 g/l of active ingredient, 1-200 g/l Surfactant, 0 to 200 g/l antifreezing agent, 0 to 400 g/l of binder, 0 to 200 g/l of a pigment and up to 1 liter of a solvent, preferably water.  
      The present invention non-transgenic and transgenic seeds of the herbicide-resistant plants of the present invention. Such seeds include, for example, non-transgenic sunflower seeds comprising the herbicide-resistance characteristics of the plant with ATCC Patent Deposit Number PTA-6325, and transgenic seeds comprising a polynucleotide molecule of the invention that encodes a herbicide-resistant AHASL1 protein.  
      For seed treatment, seeds of the herbicide resistant plants according of the present invention are treated with herbicides, preferably herbicides selected from the group consisting of AHAS-inhibiting herbicides such as amidosulfuron, azimsulfuron, bensulfuron, chlorimuron, chlorsulfuron, cinosulfuron, cyclosulfamuron, ethametsulfuron, ethoxysulfuron, flazasulfuron, flupyrsulfuron, foramsulfuron, halosulfuron, imazosulfuron, iodosulfuron, mesosulfuron, metsulfuron, nicosulfuron, oxasulfuron, primisulfuron, prosulfuron, pyrazosulfuron, rimsulfuron, sulfometuron, sulfosulfuron, thifensulfuron, triasulfuron, tribenuron, trifloxysulfuron, triflusulfuron, tritosulfuron, imazamethabenz, imazamox, imazapic, imazapyr, imazaquin, imazethapyr, cloransulam, diclosulam, florasulam, flumetsulam, metosulam, penoxsulam, bispyribac, pyriminobac, propoxycarbazone, flucarbazone, pyribenzoxim, pyriftalid, pyrithiobac, and mixtures thereof, or with a formulation comprising a AHAS-inhibiting herbicide.  
      The term seed treatment comprises all suitable seed treatment techniques known in the art, such as seed dressing, seed coating, seed dusting, seed soaking, and seed pelleting.  
      In accordance with one variant of the present invention, a further subject of the invention is a method of treating soil by the application, in particular into the seed drill: either of a granular formulation containing the AHAS-inhibiting herbicide as a composition/formulation (e.g. a granular formulation, with optionally one or more solid or liquid, agriculturally acceptable carriers and/or optionally with one or more agriculturally acceptable surfactants. This method is advantageously employed, for example, in seedbeds of cereals, maize, cotton, and sunflower.  
      The present invention also comprises seeds coated with or containing with a seed treatment formulation comprising at least one ALS inhibitor selected from the group consisting of amidosulfuron, azimsulfuron, bensulfuron, chlorimuron, chlorsulfuron, cinosulfuron, cyclosulfamuron, ethametsulfuron, ethoxysulfuron, flazasulfuron, flupyrsulfuron, foramsulfuron, halosulfuron, imazosulfuron, iodosulfuron, mesosulfuron, metsulfuron, nicosulfuron, oxasulfuron, primisulfuron, prosulfuron, pyrazosulfuron, rimsulfuron, sulfometuron, sulfosulfuron, thifensulfuron, triasulfuron, tribenuron, trifloxysulfuron, triflusulfuron, tritosulfuron, imazamethabenz, imazamox, imazapic, imazapyr, imazaquin, imazethapyr, cloransulam, diclosulam, florasulam, flumetsulam, metosulam, penoxsulam, bispyribac, pyriminobac, propoxycarbazone, flucarbazone, pyribenzoxim, pyriftalid and pyrithiobac.  
      The term seed embraces seeds and plant propagules of all kinds including but not limited to true seeds, seed pieces, suckers, corms, bulbs, fruit, tubers, grains, cuttings, cut shoots and the like and means in a preferred embodiment true seeds.  
      The term “coated with and/or containing” generally signifies that the active ingredient is for the most part on the surface of the propagation product at the time of application, although a greater or lesser part of the ingredient may penetrate into the propagation product, depending on the method of application. When the said propagation product is (re)planted, it may absorb the active ingredient.  
      The seed treatment application with the AHAS-inhibiting herbicide or with a formulation comprising the AHAS-inhibiting herbicide is carried out by spraying or dusting the seeds before sowing of the plants and before emergence of the plants.  
      In the treatment of seeds, the corresponding formulations are applied by treating the seeds with an effective amount of the AHAS-inhibiting herbicide or a formulation comprising the AHAS-inhibiting herbicide. Herein, the application rates are generally from 0.1 g to 10 kg of the a.i. (or of the mixture of a.i. or of the formulation) per 100 kg of seed, preferably from 1 g to 5 kg per 100 kg of seed, in particular from 1 g to 2.5 kg per 100 kg of seed. For specific crops such as lettuce the rate can be higher.  
      The present invention provides a method for combating undesired vegetation or controlling weeds comprising contacting the seeds of the resistant plants according to the present invention before sowing and/or after pregermination with an AHAS-inhibiting herbicide. The method can further comprise sowing the seeds, for example, in soil in a field or in a potting medium in greenhouse. The method finds particular use in combating undesired vegetation or controlling weeds in the immediate vicinity of the seed.  
      The control of undesired vegetation is understood as meaning the killing of weeds and/or otherwise retarding or inhibiting the normal growth of the weeds. Weeds, in the broadest sense, are understood as meaning all those plants which grow in locations where they are undesired.  
      The weeds of the present invention include, for example, dicotyledonous and monocotyledonous weeds. Dicotyledonous weeds include, but are not limited to, weeds of the genera:  Sinapis, Lepidium, Galium, Stellaria, Matricaria, Anthemis, Galinsoga, Chenopodium, Urtica, Senecio, Amaranthus, Portulaca, Xanthium, Convolvulus, Ipomoea, Polygonum, Sesbania, Ambrosia, Cirsium, Carduus, Sonchus, Solanum, Rorippa, Rotala, Lindemia, Lamium, Veronica, Abutilon, Emex, Datura, Viola, Galeopsis, Papaver, Centaurea, Trifolium, Ranunculus, and Taraxacum.  Monocotyledonous weeds include, but are not limited to, weeds of of the genera:  Echinochloa, Setaria, Panicum, Digitaria, Phleum, Poa, Festuca, Eleusine, Brachiaria, Lolium, Bromus, Avena, Cyperus, Sorghum, Agropyron, Cynodon, Monochoria, Fimbristyslis, Sagittaria, Eleocharis, Scirpus, Paspalum, Ischaemum, Sphenoclea, Dactyloctenium, Agrostis, Alopecurus,  and  Apera.    
      In addition, the weeds of the present invention can include, for example, crop plants that are growing in an undesired location. For example, a volunteer maize plant that is in a field that predominantly comprises soybean plants can be considered a weed, if the maize plant is undesired in the field of soybean plants.  
      The articles “a” and “an” are used herein to refer to one or more than one (i.e., to at least one) of the grammatical object of the article. By way of example, “an element” means one or more elements.  
      As used herein, the word “comprising,” or variations such as “comprises” or “comprising,” will be understood to imply the inclusion of a stated element, integer or step, or group of elements, integers or steps, but not the exclusion of any other element, integer or step, or group of elements, integers or steps.  
      The following examples are offered by way of illustration and not by way of limitation.  
     EXAMPLE 1  
     Mutagenesis of  Helianthus annuus  Line RHA266 and Selection of Imidazolinone-Resistant Plants  
      In the spring of growing season 1, sunflower plants ( Helianthus annuus ) of the line RHA266 were treated with ethyl methanesulfonate (EMS, also referred to as methanesulfonic acid ethyl ester). EMS is a known mutagen that typically induces G•C-to-A•T transitions in DNA (Jander et al. (2003)  Plant Physiol.  131:139-146). Two separate experiments were conducted, which differed by planting date and EMS concentrations used. In the first experiment, two concentrations of EMS were used. Plants were treated with a solution comprising 0.5%, or 5%, (w/v) EMS. For each EMS treatment, 21 rows of seeds were sown outdoors at the Advanta Semillas Biotech Research Station in Balcarce, BsAs, Argentina.  
      In the second experiment, 42 rows of line RHA266 sunflower seeds were sown outdoors at the Advanta Semillas Biotech Research Station, BsAs, Argentina one week after the seeds were sown in the first experiment. Of these 42 rows, 21 rows were treated with 10% EMS and 21 rows were treated with 15% EMS, as described above.  
      For each of the experiments, all M 0  plants were bagged prior to flowering in order to ensure that the resulting M1 seeds were the product of self-pollination. The seed heads from each EMS treatment were harvested and threshed in bulk. The following growing season, the mutated M 1  seeds from plants that were treated with 0.5%, 5.0%, 10.0% or 15.0% EMS were sown outdoors with each treatment in a separate plot. Twenty nine days later, when the plants were at the 2-4 leaf pair developmental stage, all of the EMS-treated plants were sprayed with 2× of Sweeper 70DG (100 g a.i./ha). The active ingredient in Sweeper is imazamox. After the herbicide spraying, a total of 14 plants survived and were selected as putative resistant. The distribution of resistant plants per EMS treatment is indicated in Table 1.  
               TABLE 1                          Number of M 1  Imidazolinone-Resistant Sunflower Plants       Recovered from Each EMS Treatment                             EMS Concentration (%)   No. of Resistant Plants Recovered                                         0.5   4           5.0   6           10   4           15   0                      
 
      Tissue samples were taken from each individual surviving M 1  plant and DNA from each sample was extracted for PCR amplification and sequencing studies described below in Example 2.  
      The 14 putative resistant plants (Table 1) were transplanted to pots, placed in the greenhouse, and allowed to grow to maturity. These plants produced seed (M 2 ), and the seed was harvested. Shortly thereafter, seed of two of these M 1:2  families were sown in separate pots in the greenhouse.  
     EXAMPLE 2  
     PCR Amplification and Sequencing of Sunflower Polynucleotides Encoding Imidazolinone-Resistant and Wild-Type AHASL1 Proteins  
      To determine the origin of the imidazolinone tolerance in the sunflower plants of Example, polymerase chain reaction (PCR) amplification of genomic DNA and DNA sequencing were employed to determine the origin of the imidazolinone tolerance described above in Example 1.  
      From an M 1  plant designated as MUT9, genomic DNA was isolated. Genomic DNA was also isolated from tissue of a wild-type RHA266 plant. The isolated DNA samples from MUT9 and RHA266 were each diluted to a stock concentration of 100 ng/μL for use as template DNA for PCR amplifications. The entire coding region of the sunflower AHASL1 gene was amplified from the MUT9 and RHA266 DNA samples. The specific primers used to obtain each amplicon are set forth in Table 2.  
                   TABLE 2                          PCR Primers for Amplifying the Coding           Region of the Sunflower AHASL1 Gene                             Region                   of AHAS1   Primer Name   Primer Sequence               1 st  amplicon   C2359-F1   AGATCAACCGAGAAAAGGCG           (1036 bp)       (SEQ ID NO:11)                   ALSR1   CGAAAAATCAAGATTAGTCACC               (SEQ ID NO:12)               2 nd  amplicon   SUNALS1-F1   GAGGAAAAGAATTCGGTGACT               (SEQ ID NO:13)                   ALS-8R   CGGTGATCACATCCGAGAAA               (SEQ ID NO:14)                  
 
      From comparisons of the nucleotide sequences of known AHASL1, AHASL2, and AHASL3 genes, PCR primers were designed to specifically amplify the AHASL1 gene from sunflower. The following PCR conditions were used in a total reaction volume of 50 μl: 1× buffer (Gibco-BRL), 0.2 mM dNTPs (Gibco-BRL), 2.5 mM MgCl 2  (Gibco-BRL), 0.2 μM of each primer, 0.5 μL of Platinium Taq (5 U/μL) (Gibco-BRL) and 100 ng of genomic DNA. PCR reactions were carried out in a GeneAmp PCR System 9700 (PerkinElmer, Inc., Boston, Mass., USA). Cycling conditions were: an initial denaturation step at 94° C. for 1 minute followed by a program of 40 cycles consisting of 10 cycles of touch-down PCR (94° C. for 30 seconds, 67° C. to 58° C. for 30 seconds and 72° C. for 30 seconds), followed by 30 cycles of the same cycling regime with a fixed annealing temperature of 58° C. and a final elongation step of 72° C. for 10 minutes. Ten microliters of each resulting PCR product were then analyzed by agarose gel electrophoresis and the remaining 40 μL of the PCR product was purified using the Concert™ Rapid PCR Purification System (Gibco-BRL) prior to sequencing. The purified PCR products were then cycle-sequenced using a BigDye® Terminator v3.1 Cycle Sequencing Kit (Perkin Elmer) following the manufacturer&#39;s instructions. In addition to the primers used for the PCR amplifications (Table 2), additional primers set forth in Table 3 were used to complete the sequencing of the entire coding region of the sunflower AHASL1 gene.  
                   TABLE 3                          Additional Primers for Sequencing the Coding           Region of the Sunflower AHASL1 gene                             Region                   of AHAS1   Primer Name   Primer Sequence               1 st  amplicon   ALS-2R   CATCGGTTCCGATCATTCCTC           (1036 bp)       (SEQ ID NO:15)                   ALS-2F   GTGTCACCGACGTCTTCGC               (SEQ ID NO:16)                   ALS-3R   CTAGACAAATAACCCGGTAACC               (SEQ ID NO:17)                   ALS-3F   GCGCTGTTAGACAGTGTCC               (SEQ ID NO:18)                   ALSRI   CGAAAAATCAAGATTAGTCACC               (SEQ ID NO:12)                   ALS4F   ACAGCAACAGTTAGTGGTG               (SEQ ID NO:19)               2 nd  amplicon   ALS-6R   CGGCAGATTTTCAACACGGA       (709 bp)       (SEQ ID NO:20)                   ALS-7F   AGACAATGGCTGACGTCGG               (SEQ ID NO:21)                   ALS-8F   TATGGTGGTTCAGTGGGAGG               (SEQ ID NO:22)                  
 
      Fluorescent-labeled products from sequencing reactions were resolved on an ABI 377 automatic sequencer (Perkin Elmer). The sunflower AHASL1 sequencing files obtained from each amplicon were assembled using the Vector NTI Suite-Contig Express software, version 7.0 (InforMax, Frederick, Md., USA). The resulting DNA sequences were aligned with AHASL1 polynucleotide sequences of the RHA266 sunflower line and  Xanthium  sp. ( FIG. 1 ). The predicted amino acid sequence from the new mutant sunflower AHASL1 gene was aligned with the AHASL1 amino acid sequences of RHA266 and  Xanthium  sp. ( FIG. 2 ) using Vector NTI Suite-AlignX software, version 7.0 (InforMax) was used with default parameters. Single nucleotide polymorphisms and amino acid changes were then identified in the nucleotide and amino acids sequence from the MUT9 plant. Relative to the equivalent positions in the wild-type nucleotide (SEQ ID NO: 2) and amino acid (SEQ ID NO: 4) sequences of RHA266, the MUT nucleotide sequence (SEQ ID NO: 1) has a C-to-T transition at nucleotide position 305 and its amino acid sequence (SEQ ID NO: 2) has a threonine-to-isoleucine substitution at amino acid position 102. The site of the C-to-T transition in the MUT9 nucleotide sequence corresponds to nucleotide 563 in the full-length sunflower AHASL coding sequence having GenBank Accession No. AY541451 (SEQ ID NO: 23). The site of the threonine-to-isoleucine substitution in the MUT9 amino acid sequence corresponds to amino acid 188 in the full-length amino acid sequence of the sunflower AHASL1 protein encoded by the nucleotide sequence having GenBank Accession No. AY541451 (SEQ ID NO: 24).  
     EXAMPLE 3  
     Fertile Herbicide-Resistant Sunflower Plants  
      Twenty M 2  seeds were obtained from the M 1  plant known as MUT9. These M 2  seeds were sown in the greenhouse. Of the six plantlets that resulted from these seeds, two grew into fertile plants. One of the fertile plants yielded 13 seeds and was determined by DNA sequencing to be heterozygous for the mutant AHASL1 allele described in Example 2. The other fertile plant yield 12 seeds but was homozygous for the wild-type allele at the AHASL1 gene.  
     EXAMPLE 4  
     The Herbicide-Resistance of MUT9 Sunflower Plants  
      To evaluate the resistance of MUT9 sunflower plants to an imidazolinone herbicides, RHA266 (wild-type) and homozygous M 5  MUT9 sunflower plants were planted outdoors in Balcarce, Argentina during the growing season in a randomized complete block design (RCBD) field trial with two replications to evaluate the tolerance of MUT9 plants to three rates of Sweeper 70DG: 1×, 2×, and 3× at the six-leaf stage (i.e., 3 pairs of leaves). The active ingredient in Sweeper is imazamox and the 1× dose is 50 g a.i./ha. The results are presented in Table 4.  
               TABLE 4                          Imidazolinone Tolerance of MUT9 Sunflower Plants       (% Injury 1 )                             RATE                                             LINE   0X   1X   2X   3X                       RHA266   0   75   75   75           MUT9   0   15   31   39                           1 Injury Scale: 0%, no injury; 5-10%, chlorosis, yellow flash; 10-20%, growth rate reduction, shortening of internodes; 20-30%, leaf deformations; 30-45%, necrosis; and &gt;50%, dead plants.             
 
      Compared to the wild-type RHA266 sunflower plants, the MUT9 sunflower plants had significantly less injury at the 1×, 2×, and 3× does of Sweeper. The results of this trial demonstrate that the MUT9 plants have increased resistance or tolerance to imidazolinone herbicides.  
      In another similar trial, RHA266 plants and MUT9 sunflower plants that were either heterozygous and homozygous for the herbicide-resistant AHASL1 allele were treated 0×, 1×, and 2× rates of Sweeper. The homozygous MUT9 plants displayed a higher level of tolerance than the heterozygous plants. As expected, the RHA266 plants (wild-type) were susceptible to the 1× and 2× rates of Sweeper.  
      To evaluate the resistance of MUT9 sunflower plants to sulfonylurea herbicides, RHA266 (wild-type) and MUT9 sunflower plants were planted outdoors in Balcarce, Argentina during the growing season in an RCBD field trial with two replications to evaluate the tolerance of homozygous M 5  MUT9 plants to the sulfonylurea herbicides tribenuron methyl (TBM) and thifensulfuron (TFS) at 1× and 2× rates applied at the six-leaf stage. Due to the lack of MUT9 seed, one herbicide was sprayed in each replication. The 1× rates for TBM and TFS are 13.15 g a.i./ha and 4.4 g a.i./ha, respectively. The results are presented in Table 5.  
               TABLE 5                          Tolerance of MUT9 Sunflower Plants to the Sulfonylurea Herbicides       Tribenuron Methyl (TBM) and Thifensulfuron (TFS)       (% Injury 1 )                         RATE                                 0X   1X   2X                                                     LINE   TBM   TFS   Avg. 2     TBM   TFS   Avg.   TBM   TFS   Avg.               RHA266   0   0   0       75   75   45   75   60       MUT9   0   0   0   75   75   75   75   75   75                   1 Injury scale same as Table 4.              2 Average of TBM and TFS injury values.             
 
     EXAMPLE 5  
     Herbicide-Resistant Sunflower AHASL Proteins  
      The present invention discloses both the nucleotide and amino acid sequences for wild-type and herbicide-resistant sunflower AHASL polypeptides. Plants comprising herbicide-resistant AHASL polypeptides have been previously identified, and a number of conserved regions of AHASL polypeptides that are the sites of amino acids substitutions that confer herbicide resistance have been described. See, Devine and Eberlein (1997) “Physiological, biochemical and molecular aspects of herbicide resistance based on altered target sites”. In:  Herbicide Activity: Toxicology, Biochemistry and Molecular Biology,  Roe et al. (eds.), pp. 159-185, IOS Press, Amsterdam; and Devine and Shukla, (2000)  Crop Protection  19:881-889.  
      Using the AHASL sequences of the invention and methods known to those of ordinary skill in art, one can produce additional polynucleotides encoding herbicide-resistant AHASL polypeptides having one, two, three, or more amino acid substitutions at the identified sites in these conserved regions. Table 6 provides the conserved regions of AHASL proteins, the amino acid substitutions known to confer herbicide resistance within these conserved regions, and the corresponding amino acids in the sunflower AHASL protein set forth in SEQ ID NO: 4.  
               TABLE 6                          Mutations in conserved regions of AHASL polypeptides known to confer       herbicide-resistance and their equivalent position in sunflower AHASL1                                                 Conserved region 1     Mutation 2     Reference   Position in sunflower               VFAYPGG   A   SMEIHQALTRS 3     Ala 122  to Thr   Bernasconi et al. 6     Ala 107                 Wright &amp; Penner 15         AITGQV   P   RRMIG   T     4     Pro 197  to Ala   Boutsalis et al. 7     Pro 182             Pro 197  to Thr   Guttieri et al. 8             Pro 197  to His   Guttieri et al. 9             Pro 197  to Leu   Guttieri et al. 8                 Kolkman et al. 16             Pro 197  to Arg   Guttieri et al. 8             Pro 197  to Ile   Boutsalis et al. 7             Pro 197  to Gln   Guttieri et al. 8             Pro 197  to Ser   Guttieri et al. 8             Thr 203  to Ile   Disclosed   Thr 188                 herein 14             A   FQETP 3     Ala 205  to Asp   Hartnett et al. 10     Ala 190             Ala 205  to Val   Simpson 11                 Kolkman et al. 16                 White et al. 17         Q   W   ED 3     Trp 574  to Leu   Bruniard 12     Trp 559                 Boutsalis et al. 7         IP   S   GG 5     Ser 653  to Asn   Devine &amp;   Ala 638                 Eberlein 13                 Lee et al. 18             Ser 653  to Thr   Chang &amp;           Ser 653  to Phe   Duggleby 19                     1 Conserved regions from Devine and Eberlein (1997) “Physiological, biochemical and molecular aspects of herbicide resistance based on altered target sites”. In: Herbicide Activity: Toxicology, Biochemistry and Molecular Biology, Roe et al. (eds.), pp. 159-185, IOS Press, Amsterdam and Devine and Shukla, (2000) Crop Protection 19: 881-889.              2 Amino acid numbering corresponds to the sequence of the  Arabidopsis thaliana  AHASL1 polypeptide.              3 Sunflower AHASL1 (SEQ ID NO: 4) has the same conserved region.              4 The region of the sunflower AHASL amino acid sequence set forth in SEQ ID NO: 2 that corresponds to this conserved region includes the site of the T188-to-I substitution and thus has the sequence AITGQVPRRMIGI. The same region of the wild-type sunflower AHASL amino acid sequence that is set forth in SEQ ID NO: 4 is identical to the conserved region indicated in Table 6.              5 The region of the sunflower AHASL1 (SEQ ID NO: 4) corresponding to this conserved region has the sequence IPAGG.              6 Bernasconi et al. (1995) J. Biol. Chem. 270(29): 17381-17385.              7 Boutsalis et al. (1999) Pestic. Sci. 55: 507-516.              8 Guttieri et al. (1995) Weed Sci. 43: 143-178.              9 Guttieri et al. (1992) Weed Sci. 40: 670-678.              10 Hartnett et al. (1990) “Herbicide-resistant plants carrying mutated acetolactate synthase genes,” In: Managing Resistance to Agrochemicals: Fundamental Research to Practical Strategies, Green et al. (eds.), American Chemical Soc. Symp., Series No. 421, Washington, DC, USA              11 Simpson (1998) Down to Earth 53(1): 26-35.              12 Bruniard (2001) Inheritance of imidazolinone resistance, characterization of cross-resistance pattern, and identification of molecular markers in sunflower (Helianthus annuus L.). Ph.D. Thesis, North Dakota State University, Fargo, ND, USA, pp 1-78.              13 Devine and Eberlein (1997) “Physiological, biochemical and molecular aspects of herbicide resistance based on altered target sites”. In: Herbicide Activity: Toxicology, Biochemistry and Molecular Biology, Roe et al. (eds.), pp. 159-185, IOS Press, Amsterdam              14 The present invention discloses the amino acid sequence of a herbicide-resistant AHASL1 with the Thr 188  to Ile substitution (SEQ ID NO: 2) and a polynucleotide sequence encoding this herbicide-resistant AHASL1 (SEQ ID NO: 1).              15 Wright and Penner (1998) Theor. Appl. Genet. 96: 612-620.              16 Kolkman et al. (2004) Theor. Appl. Genet. 109: 1147-1159.              17 White et al. (2003) Weed Sci. 51: 845-853.              18 Lee et al. (1999) FEBS Lett. 452: 341-345.              19 Chang and Duggleby (1998) Biochem J. 333: 765-777.             
 
      All publications and patent applications mentioned in the specification are indicative of the level of those skilled in the art to which this invention pertains. All publications and patent applications are herein incorporated by reference to the same extent as if each individual publication or patent application was specifically and individually indicated to be incorporated by reference.  
      Although the foregoing invention has been described in some detail by way of illustration and example for purposes of clarity of understanding, it will be obvious that certain changes and modifications may be practiced within the scope of the appended claims.