Abstract:
The invention relates to a method for improving the kernel productivity of a maize plant, wherein said method comprises overexpressing in said plant at least one glutamine synthetase isoenzyme, in order to increase the number and/or size of kernels.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a division of U.S. application Ser. No. 12/444,030, filed Jun. 22, 2009, which is a national stage application filed under 35 U.S.C. 371 of International Application No. PCT/IB2007/004195, filed Oct. 9, 2007, which claims priority to European Application No. 06291570.7, filed Oct. 9, 2006. 
     
    
     FIELD AND BACKGROUND OF THE INVENTION 
       [0002]    The present invention relates to methods for controlling yield of plants, in particular cereal crops, preferably maize crops, through the modulation of glutamine synthetase (GS) activity. 
         [0003]    The cereals including maize, wheat and rice account for 70% of worldwide food production. When such crops are grown for protein content, they require large quantities of nitrogenous fertilizers to attain maximal yields. In the past few years, there has been considerable interest in nitrogen use efficiency (NUE), which can be defined as the kernel yield per unit of nitrogen (N) in the soil and the N utilization efficiency (NutE), which is the yield per N taken up (Hirel and Lemaire, A. Basra, and S. Goyal, eds. (Haworth&#39;s Food Product Press. Binghamton, N.Y.), 15: 213-257, 2005). A number of physiological and agronomic studies have been undertaken to identify which are the limiting steps in the control of N uptake, assimilation and recycling during plant growth and development (Jeuffroy et al., J. Exp. Bot., 53: 809-823, 2002) including cereals such as maize (Hirel et al., Physiol. Plant, 124: 178-188, 2005b), rice (Yamaya et al., J. Exp. Bot., 53: 917-925, 2002) and more recently wheat (Kichey et al., New Phytol., 169: 265-278, 2006). Using maize as a model crop, Hirel et al. (aforementioned, 2005b) have investigated the changes in metabolite concentration and enzyme activities involved in N metabolism within a single leaf, at different stages of leaf growth and at different periods of plant development during the kernel-filling period. It was concluded that total N, chlorophyll, soluble protein content and GS activity are strongly interrelated and are indicators that mainly reflect the metabolic activity of individual leaves with regards to N assimilation and recycling, whatever the level of N fertilization. 
         [0004]    Glutamine synthetase (GS; E.C.6.3.1.2) catalyzes the conversion of inorganic nitrogen (ammonium) into glutamine, according to the following reaction: 
         [0000]      ATP+L-glutamate+NH 3 =&gt;ADP+phosphate+L-glutamine. 
         [0005]    All of the N in a plant, whether derived initially from nitrate, ammonium ions, N fixation, or generated by other reactions within the plant that release ammonium (decarboxylation of glycine during photorespiration, metabolism of N transport compounds and the action of phenylalanine ammonia lyase) is channeled through the reactions catalyzed by GS. Thus, this enzyme is likely to be a major check point controlling plant growth and productivity (Miflin and Habash, J. Exp. Bot., 53(370): 979-87, 2002). 
         [0006]    In maize, the putative role of GS for kernel productivity has also been highlighted using quantitative genetic approaches. QTLs for the leaf enzyme activity have been shown to be coincident with QTLs for yield (Hirel et al., Plant Physiol., 125: 1258-1270, 2001), and a positive correlation has been observed between kernel yield and GS activity (Gallais and Hirel, J. Exp. Bot., 55: 295-306, 2004). Further QTLs for GS gene loci have been identified in relation to remobilization of N from the leaf, stem and whole plant, post-anthesis N uptake (Gallais and Hirel, aforementioned, 2004) and germination efficiency (Limami et al., Plant Physiol., 130: 1860-1870, 2002). The importance of GS in controlling cereal productivity has recently been strengthened by a study performed in rice, in which a strong reduction in both growth rate and kernel yield was observed in a mutant deficient in cytosolic GS (Tabuchi et al., Plant J., 42: 641-655, 2005). 
         [0007]    GS in higher plants including maize can be readily separated by standard chromatographic, localization and western blotting techniques into cytoplasmic (GS 1) and plastidic (GS2) forms (Hirel and Lea, In: Plant Nitrogen, P. J. Lea and J. F. Morot-Gaudry, eds (INRA, Springer), pp. 79-99, 2001). However, this distinction is not as simple as was first thought, as although only one gene has been shown to encode the plastidic form, a small family of up to five genes is now known to encode the cytoplasmic form (Cren and Hirel, Plant Cell Physiol., 40: 1187-1193, 1999). Initial experiments indicated that the five cytoplasmic GS genes were differentially expressed in the roots, stems and leaves of maize (Sakakibara et al., Plant Cell Physiol., 33: 1193-1198, 1992; J. Biol. Chem., 271: 29561-29568, 1996; Li et al., Plant Mol. Biol., 23: 401-440, 1993). GS1-2, which is an important GS isoenzyme of the developing kernel, is abundant in the pedicel and pericarp, but has also been shown to be present in immature tassels, dehiscing anthers, kernel glumes, ear husks, cobs and stalks of maize plants (Muhitch et al., Plant Sci., 163: 865-872, 2002; Muhitch, J. Plant Physiol., 160: 601-605, 2003). Compared to the four other genes encoding GS1, Gln1-5 is expressed at a very low level in leaves, roots and stems (Sakakibara et al., aforementioned, 1992; Li et al., aforementioned, 1993). In a more recent study of maize leaves carried out by Hirel et al. (aforementioned, 2005b), it was shown that two of the five genes encoding GS1 (Gln1-3 and Gln1-4) were highly expressed regardless of the leaf age and the level of N fertilization, although there was an increase in Gln1-4 transcripts in the older leaves. It has been suggested that Gln1-4 encodes a GS isoform that is involved in the reassimilation of ammonium released during the remobilization of leaf proteins, whereas Gln1-3 encodes a GS isoform, which plays a house-keeping role during plant growth (Hirel et al., aforementioned, 2005a). The plastidic GS (GS2) encoded by Gln2 was only expressed in the early stages of plant development, presumably to reassimilate ammonium released during photorespiration, which is at a much lower rate in a C 4  plant compared with a C 3  plant (Ueno et al., Ann. Bot., 96: 863-869, 2005). 
         [0008]    Despite the information available, as outlined above, concerning the expression of the five cytosolic GS1 genes in maize and the evidence of their importance from the demonstration of QTLs, the precise effect of the individual GS1 isoenzymes in the plant phenotype and kernel production is not known. 
       SUMMARY OF THE INVENTION 
       [0009]    The inventors have now discovered the functional importance of the cytosolic GS isoenzyme products of the Gln1-3 and Gln1-4 genes in the control of kernel yield and its components in maize. 
         [0010]    They have found that Gln1-3 plays a major part in the control of the number of kernels, while Gln1-4 plays a major part in the control of the size of the kernels. Further, they have observed that these genes act specifically on grain production, without affecting vegetative biomass production. 
         [0011]    Gln1-3 encodes the cytosolic GS isoenzyme GS1-3. The cDNA sequence of Gln1-3 is available for instance in the GenBank database under the accession number X65928; the polypeptide sequence of GS1-3 is available for instance in the UniProtKB/Swiss-Prot database under the accession number P38561. A cDNA sequence of Gln1-3 is also represented herein as SEQ ID NO: 1, and the deduced polypeptide sequence is represented as SEQ ID NO: 2. 
         [0012]    Gln1-4 encodes the cytosolic GS isoenzyme GS1-4. The cDNA sequence of Gln1-4 is available for instance in the GenBank database under the accession number X65929; the polypeptide sequence of GS1-4 is available for instance in the UniProtKB/Swiss-Prot database under the accession number P38562. A cDNA sequence of Gln1-4 is also represented herein as SEQ ID NO: 3, and the deduced polypeptide sequence is represented as SEQ ID NO: 4. 
         [0013]    The invention provides a method for improving the kernel productivity of a maize plant, wherein said method comprises overexpressing in said plant a glutamine synthetase isoenzyme having at least 95% identity with the polypeptide SEQ ID NO: 2. 
         [0014]    Unless otherwise specified, the percents of identity between two sequences which are mentioned herein are calculated from an alignment of the two sequences over their whole length. 
         [0015]    According to a preferred embodiment of the invention, the kernel productivity is improved by increasing the number of kernels, and said glutamine synthetase is a GS1-3 isoenzyme having a serine at position 41 and an arginine at position 278. Preferably, said glutamine synthetase has the sequence SEQ ID NO: 2. 
         [0016]    According to another preferred embodiment the kernel productivity is improved by increasing the size of kernels, and said glutamine synthetase is a GS1-4 isoenzyme having a proline at position 41 and a lysine at position 278. Preferably, said glutamine synthetase has the sequence SEQ ID NO: 4. 
         [0017]    Advantageously, a method of the invention comprises overexpressing both the GS1-3 isoenzyme and the GS1-4 isoenzyme in a same plant, for increasing both the size and number of kernels. 
         [0018]    The term “overexpressing” a glutamine synthetase isoenzyme in a plant, herein refers to artificially increasing the quantity of said active GS isoenzyme produced in said plant compared to a reference plant. 
         [0019]    A preferred method for overexpressing a glutamine synthetase isoenzyme comprises introducing into the genome of said plant a DNA construct comprising a nucleotide sequence encoding said glutamine synthetase isoenzyme, placed under control of a promoter. 
         [0020]    The instant invention also provides means for carrying out said overexpression. 
         [0021]    This includes in particular recombinant DNA constructs for expressing a GS1-3 isoenzyme and/or a GS1-4 isoenzyme in a host-cell, or a host organism, in particular a plant cell or a plant. These DNA constructs can be obtained and introduced in said host cell or organism by the well-known techniques of recombinant DNA and genetic engineering. 
         [0022]    Recombinant DNA constructs of the invention include in particular expression cassettes, comprising a polynucleotide encoding a glutamine synthetase isoenzyme, as defined above, under control of an heterologous promoter. 
         [0023]    According to a particular embodiment, an expression cassette of the invention may comprise both a polynucleotide encoding a GS1-3 isoenzyme and a polynucleotide encoding a GS1-4 isoenzyme. 
         [0024]    The heterologous promoter of the invention is any promoter functional in a plant cell, i.e., capable of directing transcription of a polynucleotide encoding a glutamine synthetase isoenzyme, as defined above, in a plant cell. The choice of the more appropriate promoter may depend in particular on the organ(s) or tissue(s) targeted for expression, and on the type of expression (i.e. constitutive or inducible) that one wishes to obtain. 
         [0025]    A large choice of promoters suitable for expression of heterologous genes in plants, and in particular in maize, is available in the art. They can be obtained for instance from plants, plant viruses, or bacteria such as  Agrobacterium.  They include constitutive promoters, i.e. promoters which are active in most tissues and cells and under most environmental conditions, tissue or cell specific promoters which are active only or mainly in certain tissues or certain cell types, and inducible promoters that are activated by physical or chemical stimuli. 
         [0026]    Non-limitative examples of constitutive promoters that are commonly used are the cauliflower mosaic virus (CaMV) 35S promoter, the nopaline synthase (Nos) promoter, the Cassava vein Mosaic Virus (CsVMV) promoter (Verdaguer et al., Plant Mol. Biol., 6: 1129-39, 1996),the rice actin promoter followed by the rice actin intron (RAP-RAI) contained in the plasmid pAct1-F4 (McElroy et al., Mol. Gen. Genet., 231(1): 150-160, 1991). 
         [0027]    Non-limitative examples of organ or tissue specific promoters that can be used in the present invention include for instance High Molecular Weight (HMW) promoter which is kernel specific (Thomas and Flavell, Plant Cell., 2: 1171-80, 1990), or the leaf specific promoters as pPEPc promoter (Jeanneau et al., Biochimie, 84: 1127-1135, 2002), or the Rubisco small subunit promoter (rbcS) (Katayama et al., Plant Mol. Biol., 44: 99-106, 2000) which is specific of the bundle-sheath. 
         [0028]    Inducible promoters include for instance drought stress responsive promoters, such as the rd29A promoter which comprises a dehydration-responsive element (Kasuga et al., Nature Biotech., 17: 287-291, 1999; Narusaka et al., Plant J., 34: 137-148, 2003), or the senescence specific SAG12 promoter (Noh and Amasino, Plant Mol. Biol., 41(2): 181-194, 1999). 
         [0029]    The expression cassettes generally also include a transcriptional terminator, such as the 35S transcriptional terminator or Nos terminator (Depicker et al., J. Mol. Appl. Genet., 1: 561-73, 1982). They may also include other regulatory sequences, such as transcription enhancer sequences. 
         [0030]    Recombinant DNA constructs of the invention also include recombinant vectors containing an expression cassette comprising a polynucleotide encoding a glutamine synthetase isoenzyme, as defined above, under transcriptional control of a suitable promoter. Said expression cassette may be a recombinant expression cassette of the invention, or a cassette wherein the polynucleotide encoding a glutamine synthetase is under control of its endogenous promoter. 
         [0031]    A recombinant vector of the invention may include both a polynucleotide encoding a GS1-3 isoenzyme and a polynucleotide encoding a GS1-4 isoenzyme. These polynucleotides will advantageously be placed in separate expression cassettes under control of different promoters; for instance, a polynucleotide encoding a GS1-3 isoenzyme can be placed under control of the CsVMV promoter, and a polynucleotide encoding a GS1-4 isoenzyme can be placed under control of the rbcS promoter. 
         [0032]    Recombinant vectors of the invention may also include other sequences of interest, such as, for instance, one or more marker genes, which allow for selection of transformed hosts. 
         [0033]    Advantageously, the selectable marker gene is comprised between two Ds elements (i.e., transposons) in order for its removal at a later stage by interacting with the Ac transposase. This elimination system is known from one skilled in the art. By way of example, it has been described in Goldsbrough et al. (Biotechnology, 11:1286-1292, 1993). 
         [0034]    The selection of suitable vectors and the methods for inserting DNA constructs therein are well known to persons of ordinary skill in the art. The choice of the vector depends on the intended host and on the intended method of transformation of said host. A variety of techniques for genetic transformation of plant cells or plants are available in the art for many plant species. By way of non-limitative examples, one can mention virus-mediated transformation, transformation by microinjection, by electroporation, microprojectile-mediated transformation,  Agrobacterium  mediated transformation (Ishida et al., Nat. Biotechnol., 14:745-750, 1996), and the like. 
         [0035]    The term “plant” as used herein, includes dicotyledons as well as monocotyledons, and in particular those of agronomical interest, as cereal crops, such as wheat plant or rice plant, preferably maize plant. 
         [0036]    The invention also comprises host cells containing a recombinant DNA construct of the invention. These host cells can be prokaryotic cells or eukaryotic cells, in particular plant cells, and preferably maize cells. 
         [0037]    The invention also provides a method for producing a transgenic plant, in particular a maize plant, having an improved kernel productivity. Said method comprises transforming a plant cell (e.g., a maize cell) by a DNA construct of the invention and regenerating from said plant cell (e.g., maize cell) a transgenic plant (e.g., maize plant) overexpressing a GS1-3 isoenzyme and/or a GS1-4 isoenzyme. 
         [0038]    According to a preferred embodiment or the method of the invention, it comprises transforming a plant cell, in particular a maize cell, by a recombinant vector of the invention comprising a polynucleotide encoding a GS1-3 isoenzyme, and by a recombinant vector of the invention comprising a polynucleotide encoding a GS1-4 isoenzyme, and regenerating from said plant cell (e.g., maize cell) a transgenic plant (e.g., maize plant) overexpressing a GS1-3 isoenzyme and GS1-4 isoenzyme. 
         [0039]    According to another preferred embodiment or the method of the invention, it comprises transforming a plant cell, in particular a maize cell, by a recombinant vector of the invention comprising a polynucleotide encoding a GS1-3 isoenzyme and a polynucleotide encoding a GS1-4 isoenzyme, and regenerating from said plant cell (e.g., maize cell) a transgenic plant (e.g., maize plant) overexpressing a GS1-3 isoenzyme and GS1-4 isoenzyme. 
         [0040]    The invention also comprises plants genetically transformed by a recombinant DNA construct of the invention, and overexpressing a GS1-3 isoenzyme and/or a GS1-4 isoenzyme. Preferably, said plants are transgenic maize plants, obtainable by a method of the invention, overexpressing a GS1-3 isoenzyme and/or a GS1-4 isoenzyme. In said transgenic plants a DNA construct of the invention is comprised in a transgene integrated (i.e., stably integrated) in the plant genome, so that it is passed onto successive plant generations. Thus the transgenic plants of the invention include not only the plants resulting from the initial transgenesis, but also their descendants, as far as they contain a recombinant DNA construct of the invention. The overexpression of a GS1-3 isoenzyme and/or a GS1-4 isoenzyme glutamine synthetase in said plants provides them an improved kernel productivity, when compared with a plant devoid of said transgene(s). 
         [0041]    Accordingly, the invention provides a transgenic plant or an isolated organ or tissue thereof comprising, stably integrated in its genome, a recombinant expression cassette comprising a polynucleotide encoding a GS1-3 isoenzyme as defined in any of claims  2  or  3 , and/or a polynucleotide encoding a GS1-4 isoenzyme as defined in any of claims  4  or  5 . 
         [0042]    The invention also encompasses isolated organs or tissues of said transgenic plants (such as seeds, leafs, flowers, roots, stems, ears) containing a recombinant expression cassette of the invention. 
         [0043]    Foregoing and other objects and advantages of the invention will become more apparent from the following detailed description and accompanying drawings. It is to be understood however that this foregoing detailed description is exemplary only and is not restrictive of the invention. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0044]      FIG. 1  depicts the structure of the maize Gln1-3 and Gln1-4 genes. Exons are shown as arrows and introns are shown as thin lines. Shaded lines located beneath the gene structure identify the relative positions of the GS-NH4+-binding site signature and the GS-ATP-binding region signature. Known Mutator (mu) insertion sites are represented with triangles. 
           [0045]      FIG. 2  depicts Western blot analysis of the GS subunit composition of: (A) leaves harvested at vegetative stage (VS), stained with tobacco GS antibodies; (B) of leaves harvested at VS, stained with  Phaseolus vulgaris  GS antibodies; (C) of leaves harvested at 55 days after silking (55DAS), stained with tobacco GS antibodies; and (D) of roots harvested at VS, stained with tobacco GS antibodies (D). Each blot includes samples from wild type (WT) maize plants and gln1-3, gln1-4, and gln1-3 and gln1-4 deficient maize plants. The upper band (molecular mass of 44 kD) corresponds to the plastidic GS (GS2) subunit, and the lower band (molecular mass of 39 kD) correspond to the cytosolic GS (GS1) subunit. The position of protein molecular weight markers is shown on the right. 
           [0046]      FIG. 3  depicts the GS activity of leaf extracts following separation by ion exchange chromatography. The leaf extracts were from (A) WT maize, (B) gln1-4 deficient maize, (C) gln1-3 deficient maize, and (D) gln1-3/gln1-4 deficient maize. The first peak of GS activity corresponds to cytosolic GS (GS1) and the second peak corresponds to plastidic GS (GS2). 
           [0047]      FIGS. 4A-C  depict results obtained from 2-D gel electrophoresis and liquid chromatography-tandem mass spectrometry (LC-MS/MS) analysis of protein extracts from WT and mutant maize plants.  FIG. 4A  is a 2-D gel of proteins extracted from the leaves of WT maize plants. The rectangular box identifies the region of the gel corresponding to the pI range and molecular mass range characteristic of GS proteins. Enlarged views of the 2-D gel region identified in  FIG. 4A  are shown in  FIG. 4B , with individual gels containing protein extracted from WT maize plants and gln1-4, gln1-3, and gln1-3/gln1-4 deficient maize plants. The arrows indicate protein spots corresponding to GS1-3, GS1-4, and GS2 proteins, as determined using liquid chromatography-tandem mass spectrometry (LC-MS/MS).  FIG. 4C  shows alignments of protein sequences from various GS proteins. The aligned regions reveal differences in the GLN1-3 and GLN1-4 protein sequences that can be used to distinguish the proteins. 
           [0048]      FIGS. 5A-B  are bargraphs that depict (A) the total dry weight of shoot vegetative parts and (B) total ear dry weight from WT maize plants and gln1-3, gln1-4 and gln1-3/gln1-4 deficient maize plants. Ord=Dry weight (DW) in gram (g). Values are the mean±SE of three individual plants. 
           [0049]      FIG. 6  depicts an image of ears of corn obtained from WT maize plants and gln1-4, gln1-3 and gln1-3/gln1-4 deficient maize plants. 
           [0050]      FIG. 7  depicts a schematic diagram of the pRec 445 plasmid used for  Agrobacterium -mediated transformation of plants. The pRec 445 plasmid includes a neomycin resistance cassette and an expression cassette containing the Gln1-3 cDNA. 
           [0051]      FIG. 8  depicts an image of ears of corn obtained from maize plants that overexpress the Gln1-3 cDNA (Line 1 and Line 9) and corresponding untransformed WT null segregants (WT1 and WT9). 
           [0052]      FIG. 9  depicts Western blot analysis of vegetative stage (VS) leaves obtained from maize plants that overexpress the Gln1-3 cDNA (Line 1 and Line 9) and corresponding untransformed WT null segregants (WT1 and WT9). The blot was stained with tobacco GS antibodies. The upper band (molecular mass of 44 kD) corresponds to the plastidic GS (GS2) subunit, and the lower band (molecular mass of 39 kD) correspond to the cytosolic GS (GS1) subunit. 
           [0053]      FIGS. 10A-D  depict graphs showing the relationship between GS activity and shoot and grain yield in maize plants that overexpress the Gln1-3 cDNA (Line 1 and Line 9) and corresponding untransformed WT null segregants (WT1 and WT9).  FIG. 10A  is a bar graph showing the total leaf GS activity of each line.  FIG. 10B  is a bar graph showing kernel yield of each line.  FIG. 10C  is a bar graph showing the total dry weight of shoot vegetative parts of each line.  FIG. 10D  is a scatter plot showing the relationship between leaf GS activity (abscissa) and kernel yield (ordinate) for the lines. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     EXAMPLE 1 
     Characteristics of the Gln1-3:: and Gln1-4::l Mu-Insertion Events 
       [0054]    Knock out mutants of Gln1-3 and Gln1-4 genes have been isolated using the maize Mutator (Mu) system (Hanley et al., Plant J., 23: 557-566, 2000). The insertion lines of Gln1-3::Mu and Gln1-4::Mu have undergone extensive backcrossing to the wild type non-Mu line and homozygous, heterozygous and null mutant lines have been obtained. In addition, a double mutant (gln1-3/gln1-4) of the Gln1-3 and Gln1-4 genes has been produced. 
         [0055]    The position of the Mutator elements inserted within the Gln1-3 and Gln1-4 genes is shown in  FIG. 1   
         [0056]    For clarity and as the Gln1-3 and Gln1-4 genes show high sequence similarity, they are shown as a single structure. The Gln1-3 and Gln1-4 gene structure and exon sizes were determined by sequencing genomic DNA PCR products using primers designed from the corresponding cDNA sequences (Li et al., aforementioned, 1993); the Mu-insertion sequences and the rice genomic clone AC105364. 
         [0057]    The maize Gln1-3 and Gln1-4 genes consist of 10 exons (black arrows), ranging in size from 40 bp to 252 bp, and 9 introns (black lines). The size of the last exon was 150 bp in Gln1-3 and 147 bp in Gln1-4. The introns and exon structure is drawn to scale. The gene numbering starts with the ATG codon (1 bp) and continues to the stop codon (position 3857 bp). 
         [0058]    The two Mu-insertion events are indicated by triangles (not to scale). 
         [0059]    Also shown is the relative position of the GS-NH 4   + -binding site signature and the GS-ATP-binding region signature. 
         [0060]    Sequence analysis of the flanking regions surrounding the Mu-elements indicated that in the case of Gln1-3::Mu1 the insertion had occurred within exon 8 and in the case of Gln1-4::Mu5 the insertion had occurred within the intron separating exon 7 and exon 8. 
       EXAMPLE 2 
     Glutamine Synthetase Expression and Activity in Gln1-3, Gln1-4 and Double Mutants 
     Plant Material 
       [0061]    Seeds of the three GS1-deficient mutants, or the corresponding WT were first sown on coarse sand and after 1 week, when 2 to 3 leaves had emerged, were transferred either to hydroponic culture for root harvesting, or to soil for leaf harvesting. For the hydroponic culture, 12 plants (3 for the WT and 3 for each mutant) were randomly placed on a 130 L aerated culture unit. The experiment was performed in triplicate for each line and plants were grown for 18 days in a growth chamber with a 16/8 light/dark period. A photosynthetic photon flux density of 400 μmol m −2  s −1  was provided by metal halide lamps. The relative humidity was maintained at 60% of saturation. Plants were harvested at the 10-11 leaf stage between 9 to 12 am and separated into young leaves (3 youngest leaves) and roots. The root samples were immediately placed in liquid N 2  and then stored at −80° C. until further analysis. 
         [0062]    Three plants of the three mutants were transferred to pots (diameter and height of 30cm) containing clay loam soil and at the 10-11 leaf stage, the 3 youngest fully expanded leaves were harvested and pooled for the vegetative stage (VS) samples. The leaf below the ear was harvested at later stages of plant development including 15 days after silking (15DAS) and 55 days after silking (55DAS). The leaf, below the ear, was selected since it has been shown to provide a good indication of the source sink transition during grain filling (Hirel et al., aforementioned, 2005a,b; Martin et al., New Phytol., 167: 483-492, 2005). No major variations in N metabolite content and enzyme activity within a single leaf blade have been observed until the latest stages of leaf development. Therefore, it has been concluded that the entire leaf blade can be used for measuring physiological traits related to N metabolism (Hirel et al., aforementioned, 2005b). Leaf samples were harvested between 9 am to 12 am and frozen in liquid N 2 , ground to a homogenous powder and stored at −80° C. for use in subsequent RNA, protein and metabolite analyses. 
         [0063]    In the hydroponic culture, plants were grown on a complete nutrient solution containing 10 mM NO 3   −  as the sole N source (Coïc and Lesaint, Hortic. Française, 8: 11-14, 1971). The nutrient solution was replaced daily. In the glasshouse, plants were watered daily with a complete nutrient solution containing 10 mM NO 3   −  as the sole N source (Coïc and Lesaint, aforementioned, 1971). For both growth methods, the complete nutrient solution contained 1.25 mM K + , 0.25 mM Ca 2+ , 0.25 mM Mg 2+ , 1.25 mM H 2 PO 4   − , 0.75 mM SO 4   2− , 21.5 μM Fe 2+  (Sequestrene; Ciba-Geigy, Basel, Switzerland), 23 μM B 3+ , 9 μM Mn 2+ , 0.3 μM Mo 2+ , 0.95 μM Cu 2+  and 3.5 μM Zn 2+ . 
       Glutamine Synthetase Protein Expression 
       [0064]    The GS isoenzyme protein content of roots and leaves of the WT and three mutants was examined following one dimensional polyacrylamide gel electrophoresis and Western blot analysis, using antibodies raised against GS2 and GS1. This technique provides a reliable method of estimating the relative amounts of both GS 1 and GS2 in a crude protein extract of maize leaves and roots (Becker et al., Planta, 211: 800-806, 2000). 
         [0065]    Proteins were extracted from frozen leaves and roots powder obtained as described above, in cold extraction buffer containing 50 mM Tris-HCl pH 7.5, 1 mM EDTA, 1 mM MgCl2, 0.5% (w/v) PVP, 0.1% (v/v) 2-mercaptoethanol and 4 mM leupeptin, and separated by SDS-PAGE (Laemmli, Nature, 227 : 680-685, 1970). The percentage of polyacrylamide in the gels was 10% and equal amounts of protein (10 μg) were loaded onto each track. Proteins were electrophoretically transferred to nitrocellulose membranes for Western blot analysis. GS1 and GS2 polypeptides were detected using polyclonal antisera raised either against GS2 of tobacco (Hirel et al., Plant Physiol., 74: 448-450, 1984), or against GS1 from  Phaseolus vulgaris  root nodules (Cullimore and Miflin, J. Exp. Bot., 35: 581-587, 1984). Soluble protein was determined using a commercially available kit (Coomassie Protein assay reagent, Biorad, München, Germany) using bovine serum albumin as a standard. 
         [0066]    The results are shown in  FIG. 2 . 
         [0067]    Legend of  FIG. 2 : 
         [0068]    Western blot analysis of the GS subunit composition of leaves harvested at VS, using tobacco GS antibodies (A), of leaves harvested at VS, using  Phaseolus vulgaris  GS antibodies (B), of leaves harvested at 55DAS, using tobacco GS antibodies (C), and of roots harvested at VS, using tobacco GS antibodies (D). 
         [0069]    On the right side of the panels is shown the position of the protein molecular mass markers. 
         [0070]    WT=wild type 
         [0071]    gln1-3=gln1-3 deficient mutant 
         [0072]    gln1-4=gln1-4 deficient mutant 
         [0073]    gln1-3/1-4=gln1-3 and gln1-4 deficient mutant 
         [0074]    In the leaves of WT plants at VS, two polypeptides of similar relative abundance were detected, using the tobacco GS antibodies. The 44 kD polypeptide corresponds to the plastidic form of GS (GS2) whereas the 40 kD polypeptide corresponds to the cytosolic form of GS (GS1). In the gln1-3 and gln1-4 mutants, a decrease in the amount of GS1 protein was observed, which was more pronounced in the former. In the gln1-3/gln1-4, the GS1 protein was barely detectable Similar amounts of GS2 protein were visible in the WT and in all three mutants ( FIG. 2A ). The decrease in GS1 protein contents of the gln1-3, gln1-4 and gln1-3/gln1-4 mutants at the VS, was confirmed using antibodies raised against  Phaseolus vulgaris  root nodule GS. However, since this antibody preparation is apparently more specific for cytosolic GS (GS1), GS2 protein was less readily visible on the Western blot ( FIG. 2B ). When the GS proteins were analyzed in the leaves at 55DAS, using the tobacco antibodies, a similar pattern of decrease in GS1 protein content was obtained in the mutants. However, at this later stage of plant development, the amount of GS2 protein isolated from all four lines was much lower ( FIG. 2C ). 
         [0075]    In the root tissue of WT plants, two polypeptides of 38 and 40 kD corresponding to two cytosolic forms of GS (GSr and GS1, respectively), were detected ( FIG. 2D ). As already described by Sakakibara et al. (aforementioned, 1992), the upper band corresponds to a GS1 protein of a similar size to that found in the leaves, whereas the lower band is a root-specific cytosolic GS (GSr). A small decrease in the amount of the GS1 (40 kD) polypeptide was just visible in the gln1-4 mutant, whilst a considerable decrease was clearly detectable in the gln1-3 mutant. In the gln1-3/gln1-4 double mutant, the GS1 (40 kD) polypeptide was not detected. Similar amounts of the GSr (38 kD) polypeptide were detected in the WT and in all three mutants ( FIG. 2D ). 
       Glutamine Synthetase Protein Activity 
       [0076]    The GS isoenzyme activity content (the relative proportions of plastidic and cytosolic GS activities) of the WT and the three mutants was examined following ion exchange chromatography and enzymatic assays, using plants at the VS. 
         [0077]    Proteins were extracted from frozen leaves and roots powder obtained as described in Example 1, in cold extraction buffer containing 100 mM TEA, 1 mM EDTA, 10 mM MgSO 4 , 5 mM glutamic acid, 10% (v/v) ethylene glycol, 6 mM DTT, pH 7.6. Extracts were then centrifuged at 10 000 g for 15 min at 4° C. Separation on a Mono Q anion exchange column (Amersham Pharmacia Biotech) attached to an HPLC (DX 500; Dionex (UK) Ltd. Camberley, Surrey, UK) was performed as described by Habash et al. (Ann Appl. Biol., 138: 83-89, 2001), except that a 0.1-0.7 M NaCl linear gradient was used for elution and 1 ml fractions were collected. 
         [0078]    Glutamine synthetase (GS) activity was measured according to the method of Lea et al. (1999) for the transferase reaction and O&#39;Neal and Joy (Archiv. Biochem. Biophys., 159: 113-122, 1973) for the synthetase reaction. 
         [0079]    The results obtained from leaf material with three independent extractions are shown in  FIG. 3 . 
         [0080]    Legend of  FIG. 3 : 
         [0081]    Ion exchange chromatography of GS activity in leaf extracts from WT (A), gln1-4 (B), gln1-3 (C) and gln1-3/gln1-4 (D). The first peak of GS activity corresponds to cytosolic GS (GS1) and the second peak to plastidic GS (GS2). 
         [0082]    The relative amounts of GS1 and GS2 activity indicated in brackets was calculated using as a maximum, the value measured for the amount of GS 1 and GS2 activities in the WT. 
         [0083]    Compared to the WT ( FIG. 3A ), a 16% decrease in GS1 activity was observed in the gln1-4 mutant ( FIG. 3B ). The decrease in GS1 activity was higher in the gln1-3 mutant (45%) ( FIG. 3C ). Only 26% of the WT GS1 activity remained in the gln1-3/gln1-4 mutant ( FIG. 3D ). Interestingly an increase in GS2 activity ranging from 11 to 47% was observed in all three mutants ( FIG. 3B-D ). 
         [0084]    Compared to the WT, root GS activities in the three mutants measured using the synthetase reaction at the VS were similar (3.2±0.06 μmol. min-1 g-1 DW for the WT, 3.4±0.3 for the gln1-4 mutant, 2.8±0.05 for the gln1-3 mutant and 3.1±0.05 for the gln1-3/gln1-4 mutant). This result indicates that root GS activity is mostly represented by the GSr protein which is encoded by the Gln1-1 gene, the mRNA of which is most abundantly expressed in roots. 
       Identification of GS Protein Sequences 
       [0085]    Using plants harvested at the VS, leaf proteins extracted from the WT, the gln1-3, the gln1-4 and the gln1-3/gln1-4 mutant as above described, were separated by 2-D gel electrophoresis. 
         [0086]    The 2-D gel of proteins extracted from the leaves of the WT is shown in  FIG. 4A . 
         [0087]    To unambiguously demonstrate that the corresponding GS protein was lacking in the gln1-3, gln1-4 and gln1-3/gln1-4 mutants, thirty-six protein spots in the range of the pI and molecular mass (MM) of GS from said region, were analyzed by liquid chromatography-tandem mass spectrometry (LC-MS/MS). The results are shown in  FIG. 4B . 
         [0088]    Protein identification revealed that out of these 36 spots, four of them (spots 8, 9, 14 and 38) contained a GS protein. 
         [0089]    By comparison with the protein profile of the WT, it was observed that the gln1-3 mutant and the gln1-4 mutant were lacking spots 8 and 9 respectively and that they were both absent in the gln1-3/gln1-4 mutant. Spots 8 and 9 were very close to each other and exhibited a partial overlap. They had the same apparent pI but slightly differed by their apparent MM. 
         [0090]    This finding is not surprising, because according to the sequences of the genes isolated by Li et al (aforementioned, 1993), GS1-3 and GS1-4 proteins have the same pI and exhibit only a 188 D difference in their mass. Both products of the allelic forms isolated by Sakakibara et al. (aforementioned, 1992), (respectively GS112 and GS107 for GS1-3 and GS1-4), have one more basic amino acid, and thus also exhibit similar pI. 
         [0091]    Two sites of sequence polymorphism were used to distinguish between the GS1-3 and GS1-4 proteins from each other. The results are shown in  FIG. 4C . 
         [0092]    Legend of  FIG. 4C : 
         [0093]    Entry names of the sequence in the UNIPROT Database are given. Ref 2 of GS1-3 and GS1-4 refer to the allelic form sequenced by Sakakibara et al. (aforementioned, 1992). Positions are relative to the first methionine for all GS except for GS2, where it is relative to the first amino acid after the signal peptide. Shaded background shows the differences between GS1-3 and GS1-4 and between GS1-3 allelic forms. Differences between GS1-3 and GS1-4 on one hand and the rest of GS proteins on the other hand are underlined. 
         [0094]    The sequences alignment shows that amino acid at position 41 is a serine in GS1-3 and a proline in GS1-4, and the amino acid at position 278 is an arginine in GS1-3 and a lysine in GS1-4. Peptides containing these sites of polymorphism were identified: TLSGPVTDPSK (SEQ ID NO : 5) (and the mis-cleaved TLSGPVTDPSKLPK (SEQ ID NO: 6)) allowed the identification of GS1-3, while TLPGPVTDPSK (SEQ ID NO: 7) (and TLPGPVTDPSKLPK (SEQ ID NO: 8)) allowed the identification of GS1-4 according to the S/P polymorphism at position 41, and HREHIAAYGEGNER (SEQ ID NO: 9) and HKEHIAAYGEGNER (SEQ ID NO: 10) allowed the identification of GS1-3 and GS1-4 proteins respectively according to the R/K polymorphism at position 278. It should be noted that the peptide HREHIAAYGEGNER (SEQ ID NO : 9) is similar to the allelic form of GS1-3 sequenced by Sakakibara et al. (aforementioned, 1992), but not to that sequenced by Li et al. (aforementioned, 1993), in which the two last amino acids of the peptide, ER, are substituted by DG. 
       EXAMPLE 3 
     Phenotype and Kernal Production of the GS1 Deficient-Mutants 
       [0095]    Kernel yield, its components, and the N content of different parts of the plant at stages of development from silking to maturity were determined according to the method described by Martin et al. (aforementioned, 2005). 
         [0096]    To determine the impact of the mutations on plant phenotype and kernel production, plants were grown in a glasshouse on soil and watered daily with a nutrient solution containing 10 mM NO 3   −  (N suboptimal conditions) described by Coïc and Lesaint (aforementioned, 1971). Plants were harvested at maturity, and total N content was measured using the Dumas combustion method. 
       Shoot and Ear Dry Weight (DW) Accumulation in GS1-Deficient Mutants 
       [0097]    Total dry weight of shoot vegetative parts (A) and total ear dry weight (B) of WT, gln1-3, gln1-4 and gln1-3/gln1-4 are shown in  FIG. 5 . 
         [0098]    Legend of  FIG. 5 : 
         [0099]    Ord=Dry weight (DW) in gram (g) 
         [0100]    Values are the mean±SE of three individual plants. 
         [0101]    No significant differences were seen in the dry matter production of the vegetative parts of the shoot ( FIG. 5A ). In contrast, a major decrease in the dry matter content of the ear was observed, which when compared to the WT was 68% for gln1-3, 48% for gln1-4 mutant and 84% for the gln1-3/gln1-4 mutant. A reduction of ear size was observed in gln1-3 and gln1-4 mutants, and was even more severe in the gln1-3/gln1-4 mutant ( FIG. 5B ). 
       Phenotype of the Ear in GS1-Deficient Mutants 
       [0102]    Ears of WT, gln1-4, gln1-3 and gln1-3/gln1-4 of maize plants (line B73) are shown in  FIG. 6 . 
         [0103]    In both gln1-3 and gln1-4 mutants, a strong reduction in kernel yield was observed, which was more important in the former, thus confirming the phenotype shown in  FIG. 6 . 
         [0104]    The main components of kernel yield for the three GS mutants grown under N suboptimal conditions are presented in Table 1. 
         [0000]    
       
         
               
               
               
               
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                   
                   
                   
                   
                 Thousand  
                 Grain N  
               
               
                   
                   
                 Grain 
                 Grain 
                 Kernels 
                 content 
               
               
                   
                   
                 Yield (g) 
                 Number 
                 Weight (g) 
                 (% DW) 
               
               
                   
               
             
             
               
                   
                 WT (line 
                 117 ±  
                 464 ±  
                 252 ± 
                 2.10 ±  
               
               
                   
                 B73) 
                 15 (100) 
                 63 (100) 
                 12 (100) 
                 0.12 (100) 
               
               
                   
                 gln1-4 
                 59 ±  
                 331 ±  
                 178 ±  
                 2.49 ±  
               
               
                   
                   
                 4.5 (50) a   
                 25 (71) a   
                 11 (70) a   
                 0.04 (118) a   
               
               
                   
                 gln1-3 
                 27 ±  
                 120 ±  
                 220 ±  
                 2.66 ± 
               
               
                   
                   
                 9.5 (23) a   
                 43 (26) a   
                 5 (87) a   
                 0.07 (126) a   
               
               
                   
                 gln1-3/ 
                 14 ±  
                 89 ±  
                 158 ±  
                 2.48 ±  
               
               
                   
                 1-4 
                 8 (12) a   
                 51 (19) a   
                 4 (63) a   
                 0.08 (118) a   
               
               
                   
               
               
                 Each value is the mean ± SE obtained from three individual plants. 
               
               
                 The values indicated in parentheses are expressed as percentage of the value in the WT. 
               
               
                   a Significantly different from the WT at 0.05 probability level. 
               
             
          
         
       
     
         [0105]    In the gln1-3/gln1-4 mutant, the kernel yield was reduced to only 12% of the wild type. In gln1-4, there was a greater reduction in kernel weight but a larger kernel number, when compared to gln1-3. In the gln1-3/gln1-4 mutant, both yield components were strongly reduced compared to the WT and the two single mutants. Compared to the WT, an approximately 20% increase in N kernel content was observed in the three mutants. 
       EXAMPLE 4 
     Physiology of the GS1 Deficient-Mutants 
       [0106]    The effect of decreased levels of GS1 activity on representative markers of leaf N and C metabolism (Hirel et al., aforementioned, 2005b), was examined in the three maize mutants. 
       Metabolite Extraction and Analyses 
       [0107]    Metabolites were quantified in maize plants at VS. Lyophilized plant material was used for metabolite extraction. NH 4   +  and amino acids were extracted with 2% 5-sulfosalicylic acid (10 mg DW, ml −1 ; Ferrario-Méry et al., Plant Physiol., 117: 293-302, 1998). Total amino acid content and individual amino acid composition was determined by ion-exchange chromatography on pooled samples extracted from equal dry weights. Total free amino acids were determined by the Rosen colorimetric method, using leucine as a standard (Rosen, Arch. Biochem. Biophys., 67(1): 10-15, 1957). The composition of individual amino acids was performed by ion-exchange chromatography followed by detection with ninhydrin using the AminoTac JLC-500/V amino acid analyzer according to the instructions of the manufacturer (JEOL [Europe], Croissy-sur-Seine, France). Free NH 4   +  was determined by the phenol hypochlorite assay (Berthelot reaction), which provides reliable data for comparative studies when the concentration of NH 4   +  is low, although a more precise quantification can be obtained by other methods (Husted et al., Physiol. Plant, 109: 167-179, 2000). Sucrose, glucose, fructose, starch were extracted with 1 M HClO 4  (1 ml per 5 to 10 mg DW of plant material) as described by Ferrario-Méry et al. (aforementioned, 1998). The soluble sugars (glucose, fructose and sucrose) were measured enzymatically using a commercially available kit assay (Boehringer Mannheim, Germany). 
         [0108]    The results are shown in Table 2. 
         [0000]    
       
         
               
             
               
               
               
               
               
             
           
               
                 TABLE 2 
               
             
             
               
                   
               
               
                 Metabolite Concentration (μmol g DW −1 ) and Total N (%) 
               
             
          
           
               
                 Meta- 
                   
                   
                   
                   
               
               
                 bolites 
                 WT 
                 gln1-4 
                 gln1-3 
                 gln1-3/1-4 
               
               
                   
               
               
                 NH 4   +   
                 24.22 ±  
                 41.5 ±  
                 43.6 ±  
                 99.15 ±  
               
               
                   
                 1.9 
                 3.1 a   
                 2.3 a   
                 11.4 a   
               
               
                 NO 3   −   
                 64 ±  
                 57.3 ±  
                 61.5 ± 
                 64 ±  
               
               
                   
                 2.5 
                 11.4 
                 6.7 
                 2.1 
               
               
                 Soluble  
                 2350 ±  
                 1930 ±  
                 1980 ±  
                 2030 ±  
               
               
                 Sugars 
                 110 
                 220 
                 410 
                 306 
               
               
                 Amino  
                 73.20 ±  
                 93.03 ±  
                 83.1 ±  
                 98.9 ±  
               
               
                 Acids 
                 17 
                 6.5 
                 3 
                 30 
               
               
                 Total N 
                 4.36 ±  
                 4.31 ±  
                 4.28 ±  
                 4.42 ±  
               
               
                   
                 0.08 
                 0.1 
                 0.1 
                 0.3 
               
               
                   
               
               
                   a Values are the mean of three plants ± SD. For Soluble Sugars, sucrose represented 98% of the total. 
               
               
                   a Significant changes at the 0.05 probability level compared to the WT. 
               
             
          
         
       
     
         [0109]    Table 2 shows that the reduction in GS1 activity in gln1-3 and gln1-4 mutants resulted in an almost 2-fold increase in the quantity of free NH 4   +  in the leaves, whilst in the gln1-3/gln1-4 mutant, this amount was approximately four times higher compared to the wild type. At the VS, no significant differences in the NO 3   − , soluble sugars, free amino acids (both qualitatively and quantitatively) and total N contents of the leaves were observed between the WT and the three mutants. Although less marked, an increase in the quantity of free NH 4   +  was still observed 15DAS in the three GS mutants. At this stage of plant development, the relative concentrations of NO 3   − , amino acids and soluble sugars were also reduced but no significant changes between the WT and the mutants were detected (data not shown). As already reported earlier (Hirel et al., aforementioned, 2005b), the amounts of NO 3   − , NH 4   + , and soluble sugars in the leaves 55DAS were at least four times lower in both the WT and the three GS mutants. However, no marked differences were observed between the WT and the three mutants. 
         [0110]    At the 55DAS stage, the effect of the reduction in GS1 activity on the levels of amino acids and N were investigated. The results are shown in Table 3. 
         [0000]    
       
         
               
             
               
               
               
               
               
             
           
               
                 TABLE 3 
               
             
             
               
                   
               
               
                 Amino Acid Concentration (μmol g DW −1 ) and Proportions (%) a   
               
             
          
           
               
                 Amino 
                   
                   
                   
                 gln 1-3/ 
               
               
                 Acid 
                 WT 
                 gln1-4 
                 gln1-3 
                 1-4 
               
               
                   
               
               
                 Asp 
                 1.85 ± 
                 4.19 ± 
                 1.61 ± 
                 1.07 ± 
               
               
                   
                 0.52 (11) 
                 1.24 (10) 
                 0.31 (6) 
                 0.24 (3) 
               
               
                 Asn 
                 0.58 ± 
                 6.18 ± 
                 1.58 ± 
                 11.91 ± 
               
               
                   
                 0.07 (3) 
                 1.61 (15) b   
                 0.10 (6) b   
                 1.60 (29) b   
               
               
                 Glu 
                 1.03 ± 
                 2.27 ± 
                 3.56 ± 
                 9.57 ± 
               
               
                   
                 0.15 (6) 
                 0.22 (5) b   
                 0.55 (14) b   
                 3.88 (23) b   
               
               
                 Gln 
                 3.56 ± 
                 6.85 ± 
                 3.57 ± 
                 1.96 ± 
               
               
                   
                 1.01 (21) 
                 1.63 (16) c   
                 0.10 (14) c   
                 0.44 (5) c   
               
               
                 Ala 
                 4.80 ± 
                 10.40 ± 
                 4.02 ± 
                 4.61 ± 
               
               
                   
                 1.41 (29) 
                 1.58 (25) 
                 0.12 (16) 
                 1.59 (11) 
               
               
                 GABA 
                 0.11 ± 
                 0.15 ± 
                 0.21 ± 
                 0.30 ± 
               
               
                   
                 0.02 (1) 
                 0.04 (1) 
                 0.02 (1) 
                 0.08 (1) 
               
               
                 Pro 
                 0.86 ± 
                 1.25 ± 
                 1.17 ± 
                 0.64 ± 
               
               
                   
                 0.32 (5) 
                 0.46 (3) 
                 0.22 (5) 
                 0.09 (2) 
               
               
                 Others 
                 3.88 ± 
                 10.63 ± 
                 9.50 ± 
                 10.90 ±  
               
               
                   
                 0.67 (23) 
                 1.61 (25) 
                 0.51 (37) 
                 (27) 
               
               
                 Total 
                 16.67 ± 
                 41.92 ± 
                 25.22 ± 
                 40.96 ± 
               
               
                   
                 3.66 
                 7.20  
                 0.6  
                 4.73  
               
               
                   
                 (100) 
                 (100) b   
                 (100) b   
                 (100) b   
               
               
                 Total N  
                 2.43 ± 
                 2.84 ± 
                 2.80 ± 
                 2.77 ± 
               
               
                 (%) 
                 0.16 
                 0.18 b   
                 0.16 b   
                 0.14 b   
               
               
                   
               
               
                   a Values are the mean of three plants ± SD. Relative amino acid proportions are given in parentheses. 
               
               
                   b Significant changes in the amino acid content and N content compared to the WT at the 0.05 probability level. 
               
               
                   c No significant changes compared to the WT. 
               
             
          
         
       
     
         [0111]    In contrast with the VS, at the 55DAS stage of plant development, there was a marked increase in the leaf total amino acid content of the leaves of the three mutants. This increase was due in part to an increase in Asn and Glu contents, which were up to 20- and 10-fold higher respectively in the gln1-3/gln1-4 mutant. 
         [0112]    An increase in the amount of total N was also observed in the leaves of the three mutants. 
       Xylem and Phloem sap Collection 
       [0113]    Phloem exudates were obtained using the technique described by King and Zeevaart (Plant Physiol., 53: 96-103, 1974). The leaves were cut off and re-cut under water before rapid immersion in the collection buffer. For each experiment, fully expanded leaves of three individual plants (WT and the three mutants) were placed separately in a solution of 10 mM Hepes, 10 mM EDTA (adjusted to pH 7.5 with NaOH), in a humid chamber (relative humidity &gt;90%) and in the dark. Exudates were collected during 6 h from 10 am to 4 pm. The fresh weights of the leaves were then measured and the exudates stored at −80° C. Phloem exudates (in the EDTA solution) were adjusted to pH 2.1 and centrifuged to remove debris and EDTA, which precipitate at that pH. 
         [0114]    The results are shown in Table 4. 
         [0000]    
       
         
               
             
               
               
               
               
               
             
           
               
                 TABLE 4 
               
             
             
               
                   
               
               
                 Amino Acid Concentration (μmol μL −1 ) and Proportions (%) a   
               
             
          
           
               
                 Amino 
                   
                   
                   
                   
               
               
                 Acid 
                 WT 
                 gln1-4 
                 gln1-3 
                 gln1-3/1-4 
               
               
                   
               
               
                 Asp 
                 81.1 ±  
                 43.9 ±  
                 60.5 ±  
                 46.9 ±  
               
               
                   
                 10.1(10) 
                 8.4(13) b   
                 6.8(16) b   
                 4.6(14) b   
               
               
                 Asn 
                 21.2 ±  
                 5.2 ±  
                 2.6 ±  
                 4.3 ±  
               
               
                   
                 10.5(3) 
                 1.3(2) b   
                 0.4(1) b   
                 0.4(1) b   
               
               
                 Glu 
                 79.5 ±  
                 46.8 ±  
                 79.5 ±  
                 56.3 ±  
               
               
                   
                 15.1(10) c   
                 9.5(14) c   
                 13(21) c   
                 6.2(17) c   
               
               
                 Gln 
                 159.8 ±  
                 70.5 ±  
                 66 ±  
                 67.1 ±  
               
               
                   
                 42.5(19) 
                 19.4(21) b   
                 32(18) b   
                 6.6(20) b   
               
               
                 Ala 
                 85.6 ±  
                 22.1 ±  
                 10.9 ±  
                 18.3 ±  
               
               
                   
                 1.4(4.6) 
                 4.5(7) b   
                 12(3) b   
                 1.2(5) b   
               
               
                 GABA 
                 48.0 ±  
                 9.2 ±  
                 4.9 ±  
                 4.5 ±  
               
               
                   
                 29.1(6) 
                 5.3(3) 
                 2.4(1) 
                 1.1 (1) 
               
               
                 Pro 
                 9.9 ±  
                 5.2 ±  
                 2.8 ±  
                 2.8 ±  
               
               
                   
                 2.1(1) 
                 0.9(2) b   
                 0.6(2) b   
                 2.8(1) b   
               
               
                 Others 
                 289.7 ±  
                 116.3 ±  
                 136.4. ±  
                 126.8 ±  
               
               
                   
                 24.7(35) 
                 9.3(35) 
                 10.8(38) 
                 26(37) 
               
               
                 Total 
                 825 ±  
                 329 ±  
                 367 ±  
                 335 ±  
               
               
                   
                 28.2(100) 
                 64 b   
                 54(100) b   
                 24(100) b   
               
               
                   
               
               
                   a Values are the mean of three plants ± SD. Relative amino acid proportions are given in parentheses. 
               
               
                   b Significant changes in the amino acid content compared to the WT at the 0.05 probability level. 
               
               
                   c No significant changes compared to the WT. 
               
             
          
         
       
     
         [0115]    Analysis of the phloem sap composition at the VS revealed that in the three mutants there was a general decrease in the concentration of all the main amino acids except for Glu. This decrease was approximately 2-fold for Asp, Gln, Pro and at least 4-fold for Asn and Ala. However, the relative proportions of the amino acids were not markedly modified except for Glu, for which a slight increase was observed. No marked changes were observed for the free amino acid content of the kernel, except a 2-fold decrease in Asn in the mutants, which represented about 14% of the total in the WT (data not shown). 
         [0116]    In order to determine if the roots were able to provide enough N assimilates to the shoots in the three mutants, the amino acid composition of the xylem sap was analyzed. For xylem sap collection, stems of plants were cut 2 cm above the root system, and the cut stem was rinsed with water and blotted dry. Root pressure bleeding sap (about 200 to 400 μL per plant) was collected with a micropipette and the samples were immediately stored at −80° C. Amino acid analysis was performed as described previously. The results are shown in Table 5. 
         [0000]    
       
         
               
             
               
               
               
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                 TABLE 5 
               
             
             
               
                   
               
               
                 Amino Acid Concentration (μmol μL −1 )  
               
               
                 and Proportions (%) a   
               
             
          
           
               
                 Amino 
                   
                   
                   
                   
                   
                   
                   
               
             
          
           
               
                 Acid 
                 WT 
                 gln1-4 
                 gln1-3 
                 gln1-3/1-4 
               
               
                   
               
               
                 Asp 
                 13.8 ±  
                 na c   
                 19.8 ±  
                 13.6 ±  
               
               
                   
                 7.8(7.8) 
                   
                 2.8(17.1) 
                 7.0(9.1) 
               
               
                 Asn 
                 57.0 ±  
                   
                 18.0 ±  
                 38 ±  
               
               
                   
                 18.5(32.1) 
                   
                 4.2(15.5) b   
                 13.6(25.5) b   
               
               
                 Glu 
                 5.6 ±  
                   
                 2.2 ±  
                 4.6 ±  
               
               
                   
                 2.5(3.1) 
                   
                 0.4(1.8) b   
                 1.8(3.0) b   
               
               
                 Gln 
                 42.4 ±  
                   
                 32.6 ±  
                 35.6 ±  
               
               
                   
                 8.5(23.9) 
                   
                 3.2 (28.1) b   
                 1.6(23.8) b   
               
               
                 Ala 
                 8.2 ±  
                   
                 1.0 ±  
                 9.2 ±  
               
               
                   
                 1.4(4.6) 
                   
                 2(0.8) 
                 2.8(6.1) 
               
               
                 GABA 
                 1.0 ±  
                   
                 0.2 ±  
                 1.4 ± 
               
               
                   
                 0.6(0.5) 
                   
                 0.4(0.2) 
                 1.0 (0.9) 
               
               
                 Pro 
                 0 ± (0) 
                   
                 0 ± (0) 
                 0 ± (0) 
               
               
                 Others 
                 49.4 ±  
                   
                 42.4 ±  
                 46.6 ±  
               
               
                   
                 11.4(28) 
                   
                 4.4(36.5) 
                 (31.6) 
               
               
                 Total 
                 177.4 ±  
                   
                 116.2 ±  
                 149.0 ±  
               
               
                   
                 8.9(100) 
                   
                 2.0(100) 
                 7.3(100) 
               
               
                   
               
               
                   a Values are the mean of three plants ± SD. Relative amino acid proportions are given in parentheses. 
               
               
                   b No significant changes in the amino acid content compared to the WT at the 0.05 probability level. 
               
               
                   c Not available 
               
             
          
         
       
     
         [0117]    A slight decrease in the total amino acid concentration of the xylem sap at the VS was observed, particularly in gln1-3, due mainly to a decrease in Asn, the most abundant amino acid in the xylem sap. However this decrease was not significant. Interestingly, the amount of Gln, which is almost as abundant as Asn, was not significantly modified in the mutants compared to the WT. 
       EXAMPLE 5 
     Overexpression of GS1  
     Plant Transformation, Regeneration and Characterization 
       [0118]    Maize transformation of the inbred line A188 with  Agrobacterium tumefaciens  strain LBA4404 harbouring a super-binary plasmid was performed essentially as described by Ishida et al. (Nat. Biotechnol., 14: 745-750, 1996). In particular, the composition of all media cited hereafter is detailed in this reference. The protocol was slightly modified concerning the selective marker, which was the NPTII gene instead of the bar gene. 
         [0000]    Super-Binary Plasmid pRec 445 
         [0119]    The super-binary plasmid used for transformation was the result of a recombination between plasmid pBIOS 445 and the plasmid pSB1 (harbouring the virB and virG genes isolated from the super-virulent strain A281) within the  agrobacterium  strain LBA4404 (pSB1) (Komari et al., Plant J., 10: 165-74, 1996) forming the plasmid pRec 445. pBIOS 445 is a derivative of pSB11 (Komari et al., aforementioned, 1996) harbouring between the T-DNA borders a neomycin resistance cassette (NPTII gene) (Bevan et al., Biotechnol., 24: 367-70, 1992; Berg and Berg, Biotechnol., 1: 417-435, 1983) flanked by an actin promoter (McElroy et al., Plant Cell, 2(2): 163, 1990), and 3′Nos terminator (Depicker et al., J. Mol. Appl. Genet., 1: 561-73, 1982) and the Gln1-3 cDNA (Sakakibara et al., aforementioned, 1992) flanked by the Cassava vein mosaic virus promoter (pCsVMV) (Verdaguer et al., Plant Mol. Biol., 6: 1129-39, 1996) linked to an actin intron (McElroy et al., aforementioned, 1990), and 3′Nos terminator. The resulting agrobacterial strain used for transformation was LB4404 (pRec 445). 
         [0120]    pRec 445 is schematized in  FIG. 7 : 
         [0121]    RB and LB represent the right and left borders of the T-DNA. 
         [0122]    Ds (3′Ac) and Ds (5′Ac) represent the Ds transposable element used to further remove the selection marker conferring resistance to kanamycin. 
         [0123]    NptII is the neomycin phosphotransferase gene with the Actin promoter and the Actin intron and nopaline synthetase terminator (3 ′Nos) conferring kanamycin resistance. 
         [0124]    An Actin intron was placed between the maize Gln1-3 cDNA and the CsVMV promoter. The LB and RB extra borders, the 3′Nos, the Gln1-3 and the intron actin were used as probes to determine the pattern of the construct insertion events in the transgenic maize lines. 
       Transgenic Plants 
       [0125]    Plant transformation was conducted using immature maize embryos isolated at 10 days after pollination. Immature embryos were incubated for 5 min with  A. tumefaciens  and cultured for 3 days on a LSAs medium without antibiotic selection in the dark at 25° C. Upon transfer to the LSD5 medium,  A. tumefaciens  was counter-selected by the presence of 250 mg L −1  cefotaxime, and the transformed calli were selected by the presence of 50 mg μL −1  kanamycin. After 2 weeks of culture, developing calli were transferred to LSD10 medium containing 50 mg μL −1  kanamycin and grown for 3 weeks. Type I calli were excised and cultured for another 3 weeks on kanamycin. For regeneration, well-developed type I calli were cultured on LSZ medium at 22° C. under continuous selective pressure and on kanamycin. After 2 weeks, calli bearing shoots were transferred to RMG2 medium and cultured another 2 weeks to allow the development of roots before the transfer of the plantlets to soil and gradual acclimatization to ambient humidity. Plants were then cultivated in a glasshouse (18° C.-24° C.) and selfed or pollinated with line A188 to produce seeds. 
         [0126]    A number of transgenic lines were selected and tested for the pattern of insertion of the pCsVMV-Gln1-3 construct. Following digestion of the genomic DNA with NcoI, hybridization was performed with: Gln1-3 cDNA, actin intron and 3′Nos probes in order to determine T-DNA copy number. In addition, two probes designed on the plasmid pRec 445 each overlapping the two RB and LB T-DNA borders in order to check the eventual presence of plasmids sequence external to the T-DNA (See  FIG. 7  for the position of the probes). In two primary transformants (Line 1 and Line 9), a single insertion event was detected without the presence of additional T-DNA sequences, apart from those flanking the insert containing the pCsVMV-Gln1-3 and the pActin-NPTII chimeric constructs (data not shown). These two primary transformants were crossed to the line FV2, which contained the unfavourable allele for Gln1-3 (Hirel et al., aforementioned, 2001). T1 transformants were then selected and back-crossed three times with the line FV2. In the T4 generation containing 50% transgenics, the 50% null segregants for Line 1 and Line 9 were used as untransformed control plants and named WT1 and WT9 respectively. 
         [0127]    The cDNA encoding Gln1-3 was made constitutive by fusing it with the CsVMV promoter. After selection and regeneration, followed by three back-crosses with the line FV2, two T4 transgenic plants (Line 1 and Line 9) overexpressing Gln1-3 and the corresponding null segregants (WT1 and WT9) were selected for further analysis. 
         [0128]    These maize plants were grown under N suboptimal conditions on a nutrient solution containing 10 mM NO 3   −  and harvested at maturity. Ears of WT untransformed null segregants (WT1 and WT9) and T4 transgenic lines (Line 1 and Line 9) overexpressing the Gln1-3 cDNA are shown in  FIG. 8 . 
         [0129]    It is to be noted that the ear in the untransformed WT plants was smaller compared to that of the untransformed null segregants (WT) shown in  FIG. 6 , to compare the phenotype of the mutants due to the fact that the European line FV2 produces fewer kernels than the North-American line B73, used for generating the transgenic maize plants. 
       Characterization of the Transgenic Plants 
       [0130]    The impact of Gln1-3 overexpression on plant phenotype and kernel production was determined on plants grown in a glasshouse until maturity under suboptimal N feeding conditions. 
       GS Subunit Composition 
       [0131]    The GS subunit composition of leaves at the VS of the two untransformed null segregants (WT1 and WT9) and the two transgenic lines (Line 1 and Line 9) was examined by Western blot analysis using the tobacco antibodies recognizing GS1 and GS2. The results are shown in  FIG. 9 . 
         [0132]    Legend of  FIG. 9 : 
         [0133]    The upper band (molecular mass of 44 kD) corresponds to the plastidic GS (GS2) subunit, and the lower band (molecular mass of 39 kD) correspond to the cytosolic GS (GS1) subunit. 
         [0134]    An increase in the amount of GS1 protein was clearly visible in the protein extracts of the two transformed plants, that of Line 9 being noticeably higher. When the GS proteins were analyzed in the leaves 55DAS, a similar pattern of increase in GS1 protein content was obtained (data not shown). 
       Shoot and Grain Yield in Relation to GS Activity 
       [0135]    Shoot and grain yield in relation to GS activity of the two untransformed null segregants (WT1 and WT9) and the two transgenic lines (Line 1 and Line 9) was determined. The results are shown in  FIG. 10 . 
         [0136]    Legend of  FIG. 10 : 
         [0137]    A=total leaf GS activity 
         [0138]    B=kernel yield 
         [0139]    C=total dry weight of shoot vegetative parts 
         [0140]    D=scatter plots between leaf GS activity (abscissa) and kernel yield (ordinate). 
         [0141]    Compared to the two untransformed null segregants, a two and three fold increase in total leaf GS activity was observed in Lines 1 and 9 respectively ( FIG. 10A ). 
         [0142]    A significant increase in kernel yield was observed, which when compared to two corresponding WT control plants was approximately 30% for both Line 1 and Line 9 ( FIG. 10B ), thus confirming the phenotype shown in  FIG. 8 . Kernel number was the yield component that was mostly responsible for the increase in kernel production, as shown below in Table 6. 
         [0000]    
       
         
               
               
               
               
               
             
           
               
                 TABLE 6 
               
               
                   
               
               
                   
                   
                   
                 Thousand  
                 Grain N  
               
               
                   
                 Grain 
                 Grain 
                 Kernels 
                 content 
               
               
                   
                 Yield (g) 
                 Number 
                 Weight (g) 
                 (% DW) 
               
               
                   
               
             
             
               
                 WT(line F2) 
                 26.05 ±  
                 102.5 ±  
                 259.9 ±  
                 2.16 ±  
               
               
                   
                 3.5(100) 
                 17.4(100) 
                 8.2(100) 
                 0.02 (100) 
               
               
                 CsVMV-Gln1-3 
                 36.9 ±  
                 127.9 ±  
                 276.9 ±  
                 2.19 ±  
               
               
                   
                 3.8(142) a   
                 13.6(125) a   
                 6.7(106) 
                 0.06 (101) 
               
               
                   
               
               
                 Each value is the mean ± SE obtained from six individual plants. 
               
               
                 The values indicated in parentheses are expressed as percentage of the value in the WT (the mean of the two transgenic lines and of the corresponding null segregants) 
               
             
          
         
       
     
         [0143]    Linear regression and the resulting correlation coefficient were calculated for the level of leaf GS activity versus grain yield in the two untransformed null segregants and the two transgenic lines.  FIG. 10D , shows that there is a strong relationship between leaf GS activity and grain yield (r 2  0.91), thus indicating that the increase in yield is proportional to the increase in enzyme activity. 
         [0144]    In contrast, no significant differences were seen in shoot dry matter production between the two WT control plants and the two transgenic lines ( FIG. 10C ). 
       EXAMPLE 6 
     Overexpression of GS1 in Leaf Mesophyll and Bundle-Sheath  Cells 
       [0145]    As described in example 5, expression of GS1-3 from the pCsVMV promoter results in significantly increased GS activity in transformed plants and improves agronomic performance. However expression of pCsVMV is largely confined to mesophyll cells, thus expression of GS1 in transformed maize plant leaves can be further optimised by the use of promoters that direct high level expression both in the mesophyll and bundle sheath cells. An alternative strategy, described below, is to transform maize with a T-DNA expressing GS1-3 under control of the pCsVMV promoter, together with GS1-4 under the control of a bundle-sheath cell specific promoter such as maize rbcS promoter (Katayama et al., aforementioned, 2000), in order to obtain transgenic maize plants expressing GS1-3 in the mesophyll and GS1-4 in the bundle sheath cells. It is expected that grain yields are improved in said transgenic plants, both in N-limiting and non-limiting conditions. 
       Plant Transformation, Regeneration and Characterization 
       [0146]    Maize transformation of the inbred line A188 with  Agrobacterium tumefaciens  strain LBA4404 harbouring a super-binary plasmid was performed essentially as described by Ishida et al. (aforementioned, 1996). In particular, the composition of all media cited hereafter is detailed in this reference. The only modification concerns the selective marker, which is the NPTII gene in the present example, instead of the bar gene used by Ishida et al. 
         [0000]    Super-Binary Plasmid pRec 445+ 
         [0147]    The super-binary plasmid used for transformation is the result of a recombination between the plasmid pBIOS 445+ and the plasmid pSB1 (harbouring the virB and virG genes isolated from the super-virulent strain A281) within the  Agrobacterium  strain LBA4404 (pSB1) (Komari et al., aforementioned, 1996) forming the plasmid pRec 445. (pBIOS 445+ is identical to pBIOS 445 (example 5) except that it also contains between the T-DNA borders, a maize rbcS promoter (Katayama et al., aforementioned, 2000) linked to Gln1-4 cDNA and a nos terminator. The resulting agrobacterial strain used for transformation was LB4404 (pRec 445+).