Patent Publication Number: US-2011061126-A1

Title: Plants having increased yield-related traits and a method for making the same

Description:
The present invention relates generally to the field of molecular biology and concerns a method for increasing various plant yield-related traits by increasing expression in a plant of a nucleic acid sequence encoding a high affinity nitrate transporter 2 (NRT2) polypeptide. The present invention also concerns plants having increased expression of a nucleic acid sequence encoding an NRT2 polypeptide, which plants have increased yield-related traits relative to control plants. The invention also provides constructs useful in the methods of the invention. 
     The ever-increasing world population and the dwindling supply of arable land available for agriculture fuels research towards increasing the efficiency of agriculture. Conventional means for crop and horticultural improvements utilise selective breeding techniques to identify plants having desirable characteristics. However, such selective breeding techniques have several drawbacks, namely that these techniques are typically labour intensive and result in plants that often contain heterogeneous genetic components that may not always result in the desirable trait being passed on from parent plants. Advances in molecular biology have allowed mankind to modify the germplasm of animals and plants. Genetic engineering of plants entails the isolation and manipulation of genetic material (typically in the form of DNA or RNA) and the subsequent introduction of that genetic material into a plant. Such technology has the capacity to deliver crops or plants having various improved economic, agronomic or horticultural traits. 
     A trait of particular economic interest is increased yield. Yield is normally defined as the measurable produce of economic value from a crop. This may be defined in terms of quantity and/or quality. Yield is directly dependent on several factors, for example, the number and size of the organs, plant architecture (for example, the number of branches), seed production, leaf senescence and more. Root development, nutrient uptake, stress tolerance and early vigour may also be important factors in determining yield. Optimizing the abovementioned factors may therefore contribute to increasing crop yield. 
     Seed yield is a particularly important trait, since the seeds of many plants are important for human and animal nutrition. Crops such as corn, rice, wheat, canola and soybean account for over half the total human caloric intake, whether through direct consumption of the seeds themselves or through consumption of meat products raised on processed seeds. They are also a source of sugars, oils and many kinds of metabolites used in industrial processes. Seeds contain an embryo (the source of new shoots and roots) and an endosperm (the source of nutrients for embryo growth during germination and during early growth of seedlings). The development of a seed involves many genes, and requires the transfer of metabolites from the roots, leaves and stems into the growing seed. The endosperm, in particular, assimilates the metabolic precursors of carbohydrates, oils and proteins and synthesizes them into storage macromolecules to fill out the grain. 
     Plant biomass is yield for forage crops like alfalfa, silage corn and hay. Many proxies for yield have been used in grain crops. Chief amongst these are estimates of plant size. Plant size can be measured in many ways depending on species and developmental stage, but include total plant dry weight, above-ground dry weight, above-ground fresh weight, leaf area, stem volume, plant height, rosette diameter, leaf length, root length, root mass, tiller number and leaf number. Many species maintain a conservative ratio between the size of different parts of the plant at a given developmental stage. These allometric relationships are used to extrapolate from one of these measures of size to another (e.g. Tittonell et al 2005 Agric Ecosys &amp; Environ 105: 213). Plant size at an early developmental stage will typically correlate with plant size later in development. A larger plant with a greater leaf area can typically absorb more light and carbon dioxide than a smaller plant and therefore will likely gain a greater weight during the same period (Fasoula &amp; Tollenaar 2005 Maydica 50:39). This is in addition to the potential continuation of the micro-environmental or genetic advantage that the plant had to achieve the larger size initially. There is a strong genetic component to plant size and growth rate (e.g. ter Steege et al 2005 Plant Physiology 139:1078), and so for a range of diverse genotypes plant size under one environmental condition is likely to correlate with size under another (Hittalmani et al 2003 Theoretical Applied Genetics 107:679). In this way a standard environment is used as a proxy for the diverse and dynamic environments encountered at different locations and times by crops in the field. 
     Another important trait for many crops is early vigour. Improving early vigour is an important objective of modern rice breeding programs in both temperate and tropical rice cultivars. Long roots are important for proper soil anchorage in water-seeded rice. Where rice is sown directly into flooded fields, and where plants must emerge rapidly through water, longer shoots are associated with vigour. Where drill-seeding is practiced, longer mesocotyls and coleoptiles are important for good seedling emergence. The ability to engineer early vigour into plants would be of great importance in agriculture. For example, poor early vigour has been a limitation to the introduction of maize ( Zea mays  L.) hybrids based on Corn Belt germplasm in the European Atlantic. 
     Harvest index, the ratio of seed yield to aboveground dry weight, is relatively stable under many environmental conditions and so a robust correlation between plant size and grain yield can often be obtained (e.g. Rebetzke et al 2002 Crop Science 42:739). These processes are intrinsically linked because the majority of grain biomass is dependent on current or stored photosynthetic productivity by the leaves and stem of the plant (Gardener et al 1985 Physiology of Crop Plants. Iowa State University Press, pp 68-73). Therefore, selecting for plant size, even at early stages of development, has been used as an indicator for future potential yield (e.g. Tittonell et al 2005 Agric Ecosys &amp; Environ 105: 213). When testing for the impact of genetic differences on stress tolerance, the ability to standardize soil properties, temperature, water and nutrient availability and light intensity is an intrinsic advantage of greenhouse or plant growth chamber environments compared to the field. However, artificial limitations on yield due to poor pollination due to the absence of wind or insects, or insufficient space for mature root or canopy growth, can restrict the use of these controlled environments for testing yield differences. Therefore, measurements of plant size in early development, under standardized conditions in a growth chamber or greenhouse, are standard practices to provide indication of potential genetic yield advantages. 
     Another trait of importance is that of improved abiotic stress tolerance. Abiotic stress is a primary cause of crop loss worldwide, reducing average yields for most major crop plants by more than 50% (Wang et al. (2003) Planta 218: 1-14). Abiotic stresses may be caused by drought, salinity, extremes of temperature, chemical toxicity, excess or deficiency of nutrients (macroelements and/or microelements), radiation and oxidative stress. The ability to increase plant tolerance to abiotic stress would be of great economic advantage to farmers worldwide and would allow for the cultivation of crops during adverse conditions and in territories where cultivation of crops may not otherwise be possible. 
     Crop yield may therefore be increased by optimising one of the above-mentioned factors. 
     Depending on the end use, the modification of certain yield traits may be favoured over others. For example for applications such as forage or wood production, or bio-fuel resource, an increase in the vegetative parts of a plant may be desirable, and for applications such as flour, starch or oil production, an increase in seed parameters may be particularly desirable. Even amongst the seed parameters, some may be favoured over others, depending on the application. Various mechanisms may contribute to increasing seed yield, whether that is in the form of increased seed size or increased seed number. 
     One approach to increase yield-related traits (seed yield and/or biomass) in plants may be through modification of the inherent growth mechanisms of a plant, such as the cell cycle or various signalling pathways involved in plant growth or in defense mechanisms. 
     It has now been found that various yield-related traits may be increased in plants relative to control plants, by increasing expression in a plant of a nucleic acid sequence encoding a high affinity  n it r ate  t ransporter 2 (NRT2) polypeptide. The increased yield-related traits comprise one or more of: increased early vigour, increased aboveground biomass, increased root biomass, increased total seed yield per plant, increased number of filled seeds, and increased total number of seeds. 
     BACKGROUND 
     Nitrate is a major primary source of nitrogen for crop plants and it is the nutrient that most frequently limits their growth and development. In the natural environment, nitrate concentrations may vary widely, by as much as 5 orders of magnitude. The first step in the assimilation of nitrate to ammonium, involves the influx of nitrate into cells, an active process depending on external nitrate concentration. 
     Two classes of nitrate transport systems, high- and low-affinity, have been identified. Nitrate transporter 2 (NRT2) are high-affinity nitrate transporters, whereas most NRT1 are low-affinity transporters. The first gene encoding a high-affinity nitrate transporter protein was cloned from Aspergillus nidulans nrtA (or crnA). This lead to the cloning and identity of orthologous genes from a number of species, including from seed plants (monocts and dicots), gymonosperms, moss, green algae, diatoms, and other fungi (for review, Tsay et al. (2007) FEBS Lett 581: 2290-2300, and references comprised therein). Nitrate transporters usually exist in a genome as gene families, for example at least seven have been identified in Arabidopsis (Wirth et al. (2007) J Biol Chem 282(32): 23541-23552). 
     NRT2 polypeptides are most typically characterized by at least 11, usually 12 hydrophobic transmembrane domains (Tms) in alpha-helical conformation (alpha-helices), passing through the membrane and connected by hydrophilic loops (although caution must be taken when predicting the exact locations of the Tms) (Unkles et al. (2004) Proc Natl Acd Sci 101(50): 17549-17554; Yin et al (2007) Plant Sci 172: 621-631). The NRT2 polypeptides belong to a distinct cluster of the largest secondary transporter family known, the major facilitator superfamily (MFS), which include a variety of functionally diverse transporters, each containing motifs characteristic of the MFS superfamily. However, within the high-affinity nitrate transporter subfamily of the nitrate/nitrite porter family, a specific motif (referred to as the nitrate signature) has been identified, comprising a number of highly conserved residues, (F/Y/K)-X 3 -(I/L/Q/R/K)-X-(G/A)-X-(V/A/S/K)-X-(G/A/S/N)-(L/I/V/F/Q)-X 1,2 -G-X-G-(N/I/M)-X-G-(G/V/T/A), which is not observed in any other classes of MFS proteins. This motif is usually present as two copies located in transmembrane domains Tm-5 and Tm-11. The MFS motifs may be important in promoting global conformational changes associated with transport in MFS members, in this case with nitrate transport. 
     In some species, NRT2 polypeptides can mediate nitrate transport on their own, such as the Aspergillus nidulans or  Chlorella sorokiniana  NRT2 transporters (CRNA or ChNRT2.1, respectively). However, in higher plants, similarly to NRT2 proteins of  Chlamydomonas reinhardtii , NRT2 polypeptides are not able to mediate transport on their own. The actual nitrate transport system actually requires a second polypeptide (called NAR2 or NRT3), and thus corresponds in fact to a dual component (NRT2/NAR2) transporter (Wirth et al. (2007) supra, and references comprised therein). 
     Transgenic tobacco plants overepressing the NpNRT2.1 nucleic acid sequence under the control of a constitutive or root-specific promoter were produced and studied by Fraisier et al. (2000; Plant J 23(4):489-496). The authors showed that deregulation of NRT2 alone can lead to an increase in NO3- influx mediated by HATS (high affinity transport system), indicating for the first time that nitrate influx could be increased in transgenic plants. 
     International patent application WO2007051866 describes methods for improving plant growth characteristics by modulating expression in a plant of nucleic acid sequences encoding NRT2 polypeptides from plants. Good et al. (US 20050044585) discloses transgenic plants with elevated levels of nitrogen utilisation proteins, and in particular aminotransferases, under control of a root specific promoter that may or may not be stress inducible. These plants showed improved nitrogen uptake efficiency, but no effects on seed yield were reported. In addition, this document also discloses that overexpression of nitrate transporter proteins in plants did not result in advantageous growth properties for these plants. 
     Surprisingly, it has now been found that increasing expression of a nucleic acid sequence encoding an NRT2 polypeptide gives plants having increased yield-related traits relative to control plants. 
     According to one embodiment, there is provided a method for increasing yield-related traits in plants relative to control plants, comprising increasing expression of a nucleic acid sequence encoding an NRT2 polypeptide as defined herein, in a plant. The increased yield-related traits comprise one or more of: increased early vigour, increased aboveground biomass, increased root biomass, increased total seed yield per plant, increased number of filled seeds, and increased total number of seeds. 
     DEFINITIONS 
     Polypeptide(s)/Protein(s) 
     The terms “polypeptide” and “protein” are used interchangeably herein and refer to amino acids in a polymeric form of any length, linked together by peptide bonds. 
     Polynucleotide(s)/Nucleic Acid(s)/Nucleic Acid Sequence (s)/Nucleotide Sequence(s) 
     The terms “polynucleotide(s)”, “nucleic acid sequence(s)”, “nucleotide sequence(s)”, “nucleic acid(s)” are used interchangeably herein and refer to nucleotides, either ribonucleotides or deoxyribonucleotides or a combination of both, in a polymeric unbranched form of any length. 
     Control Plant(s) 
     The choice of suitable control plants is a routine part of an experimental setup and may include corresponding wild type plants or corresponding plants without the gene of interest. The control plant is typically of the same plant species or even of the same variety as the plant to be assessed. The control plant may also be a nullizygote of the plant to be assessed. A “control plant” as used herein refers not only to whole plants, but also to plant parts, including seeds and seed parts. 
     Homologue(s) 
     “Homologues” of a protein encompass peptides, oligopeptides, polypeptides, proteins and enzymes having amino acid substitutions, deletions and/or insertions relative to the unmodified protein in question and having similar biological and functional activity as the unmodified protein from which they are derived. 
     A deletion refers to removal of one or more amino acids from a protein. 
     An insertion refers to one or more amino acid residues being introduced into a predetermined site in a protein. Insertions may comprise N-terminal and/or C-terminal fusions as well as intra-sequence insertions of single or multiple amino acids. Generally, insertions within the amino acid sequence will be smaller than N- or C-terminal fusions, of the order of about 1 to 10 residues. Examples of N- or C-terminal fusion proteins or peptides include the binding domain or activation domain of a transcriptional activator as used in the yeast two-hybrid system, phage coat proteins, (histidine)-6-tag, glutathione S-transferase-tag, protein A, maltose-binding protein, dihydrofolate reductase, Tag•100 epitope, c-myc epitope, FLAG®-epitope, lacZ, CMP (calmodulin-binding peptide), HA epitope, protein C epitope and VSV epitope. 
     A substitution refers to replacement of amino acids of the protein with other amino acids having similar properties (such as similar hydrophobicity, hydrophilicity, antigenicity, propensity to form or break α-helical structures or β-sheet structures). Amino acid substitutions are typically of single residues, but may be clustered depending upon functional constraints placed upon the polypeptide; insertions will usually be of the order of about 1 to 10 amino acid residues. The amino acid substitutions are preferably conservative amino acid substitutions. Conservative substitution tables are well known in the art (see for example Creighton (1984) Proteins. W.H. Freeman and Company (Eds) and Table 1 below). 
     
       
         
           
               
             
               
                 TABLE 1 
               
             
            
               
                   
               
               
                 Examples of conserved amino acid substitutions 
               
            
           
           
               
               
               
               
               
            
               
                   
                   
                 Conservative 
                   
                 Conservative 
               
               
                   
                 Residue 
                 Substitutions 
                 Residue 
                 Substitutions 
               
               
                   
                   
               
               
                   
                 Ala 
                 Ser 
                 Leu 
                 Ile; Val 
               
               
                   
                 Arg 
                 Lys 
                 Lys 
                 Arg; Gln 
               
               
                   
                 Asn 
                 Gln; His 
                 Met 
                 Leu; Ile 
               
               
                   
                 Asp 
                 Glu 
                 Phe 
                 Met; Leu; Tyr 
               
               
                   
                 Gln 
                 Asn 
                 Ser 
                 Thr; Gly 
               
               
                   
                 Cys 
                 Ser 
                 Thr 
                 Ser; Val 
               
               
                   
                 Glu 
                 Asp 
                 Trp 
                 Tyr 
               
               
                   
                 Gly 
                 Pro 
                 Tyr 
                 Trp; Phe 
               
               
                   
                 His 
                 Asn; Gln 
                 Val 
                 Ile; Leu 
               
               
                   
                 Ile 
                 Leu, Val 
               
               
                   
                   
               
            
           
         
       
     
     Amino acid substitutions, deletions and/or insertions may readily be made using peptide synthetic techniques well known in the art, such as solid phase peptide synthesis and the like, or by recombinant DNA manipulation. Methods for the manipulation of DNA sequences to produce substitution, insertion or deletion variants of a protein are well known in the art. For example, techniques for making substitution mutations at predetermined sites in DNA are well known to those skilled in the art and include M13 mutagenesis, T7-Gen in vitro mutagenesis (USB, Cleveland, Ohio), QuickChange Site Directed mutagenesis (Stratagene, San Diego, Calif.), PCR-mediated site-directed mutagenesis or other site-directed mutagenesis protocols. 
     Derivatives 
     “Derivatives” include peptides, oligopeptides, polypeptides which may, compared to the amino acid sequence of the naturally-occurring form of the protein, such as the protein of interest, comprise substitutions of amino acids with non-naturally occurring amino acid residues, or additions of non-naturally occurring amino acid residues. “Derivatives” of a protein also encompass peptides, oligopeptides, polypeptides which comprise naturally occurring altered (glycosylated, acylated, prenylated, phosphorylated, myristoylated, sulphated etc.) or non-naturally altered amino acid residues compared to the amino acid sequence of a naturally-occurring form of the polypeptide. A derivative may also comprise one or more non-amino acid substituents or additions compared to the amino acid sequence from which it is derived, for example a reporter molecule or other ligand, covalently or non-covalently bound to the amino acid sequence, such as a reporter molecule which is bound to facilitate its detection, and non-naturally occurring amino acid residues relative to the amino acid sequence of a naturally-occurring protein. 
     Orthologue(s)/Paralogue(s) 
     Orthologues and paralogues encompass evolutionary concepts used to describe the ancestral relationships of genes. Paralogues are genes within the same species that have originated through duplication of an ancestral gene; orthologues are genes from different organisms that have originated through specification, and are also derived from a common ancestral gene. 
     Domain 
     The term “domain” refers to a set of amino acids conserved at specific positions along an alignment of sequences of evolutionarily related proteins. While amino acids at other positions can vary between homologues, amino acids that are highly conserved at specific positions indicate amino acids that are likely essential in the structure, stability or function of a protein. Identified by their high degree of conservation in aligned sequences of a family of protein homologues, they can be used as identifiers to determine if any polypeptide in question belongs to a previously identified polypeptide family. 
     Motif/Consensus Sequence/Signature 
     The term “motif” or “consensus sequence” or “signature” refers to a short conserved region in the sequence of evolutionarily related proteins. Motifs are frequently highly conserved parts of domains, but may also include only part of the domain, or be located outside of conserved domain (if all of the amino acids of the motif fall outside of a defined domain). 
     Hybridisation 
     The term “hybridisation” as defined herein is a process wherein substantially homologous complementary nucleotide sequences anneal to each other. The hybridisation process can occur entirely in solution, i.e. both complementary nucleic acid molecules are in solution. The hybridisation process can also occur with one of the complementary nucleic acid molecules immobilised to a matrix such as magnetic beads, Sepharose beads or any other resin. The hybridisation process can furthermore occur with one of the complementary nucleic acid molecules immobilised to a solid support such as a nitro-cellulose or nylon membrane or immobilised by e.g. photolithography to, for example, a siliceous glass support (the latter known as nucleic acid sequence arrays or microarrays or as nucleic acid sequence chips). In order to allow hybridisation to occur, the nucleic acid molecules are generally thermally or chemically denatured to melt a double strand into two single strands and/or to remove hairpins or other secondary structures from single stranded nucleic acid molecules. 
     The term “stringency” refers to the conditions under which a hybridisation takes place. The stringency of hybridisation is influenced by conditions such as temperature, salt concentration, ionic strength and hybridisation buffer composition. Generally, low stringency conditions are selected to be about 30° C. lower than the thermal melting point (T m ) for the specific sequence at a defined ionic strength and pH. Medium stringency conditions are when the temperature is 20° C. below T m , and high stringency conditions are when the temperature is 10° C. below T m . High stringency hybridisation conditions are typically used for isolating hybridising sequences that have high sequence similarity to the target nucleic acid sequence. However, nucleic acid sequences may deviate in sequence and still encode a substantially identical polypeptide, due to the degeneracy of the genetic code. Therefore medium stringency hybridisation conditions may sometimes be needed to identify such nucleic acid sequence molecules. 
     The Tm is the temperature under defined ionic strength and pH, at which 50% of the target sequence hybridises to a perfectly matched probe. The T m  is dependent upon the solution conditions and the base composition and length of the probe. For example, longer sequences hybridise specifically at higher temperatures. The maximum rate of hybridisation is obtained from about 16° C. up to 32° C. below T m . The presence of monovalent cations in the hybridisation solution reduce the electrostatic repulsion between the two nucleic acid sequence strands thereby promoting hybrid formation; this effect is visible for sodium concentrations of up to 0.4M (for higher concentrations, this effect may be ignored). Formamide reduces the melting temperature of DNA-DNA and DNA-RNA duplexes with 0.6 to 0.7° C. for each percent formamide, and addition of 50% formamide allows hybridisation to be performed at 30 to 45° C., though the rate of hybridisation will be lowered. Base pair mismatches reduce the hybridisation rate and the thermal stability of the duplexes. On average and for large probes, the Tm decreases about 1° C. per % base mismatch. The Tm may be calculated using the following equations, depending on the types of hybrids:
     1) DNA-DNA hybrids (Meinkoth and Wahl, Anal. Biochem., 138: 267-284, 1984):
       T m =81.5° C.+16.6×log 10 [Na + ] a +0.41×%[G/C b ]−500×[ L   c ] −1 −0.61×% formamide   
       2) DNA-RNA or RNA-RNA hybrids:
       Tm=79.8+18.5 (log 10 [Na + ] a )+0.58 (% G/C b )+11.8 (% G/C b ) 2 −820/L c      
       3) oligo-DNA or oligo-RNA d  hybrids:
       For &lt;20 nucleotides: T m =2 (l n )   For 20-35 nucleotides: T m =22+1.46 (l n )     a  or for other monovalent cation, but only accurate in the 0.01-0.4 M range.     b  only accurate for % GC in the 30% to 75% range.     c L=length of duplex in base pairs.     d  oligo, oligonucleotide; l n, =effective length of primer=2×(no. of G/C)+(no. of NT).   
       

     Non-specific binding may be controlled using any one of a number of known techniques such as, for example, blocking the membrane with protein containing solutions, additions of heterologous RNA, DNA, and SDS to the hybridisation buffer, and treatment with Rnase. For non-homologous probes, a series of hybridizations may be performed by varying one of (i) progressively lowering the annealing temperature (for example from 68° C. to 42° C.) or (ii) progressively lowering the formamide concentration (for example from 50% to 0%). The skilled artisan is aware of various parameters which may be altered during hybridisation and which will either maintain or change the stringency conditions. 
     Besides the hybridisation conditions, specificity of hybridisation typically also depends on the function of post-hybridisation washes. To remove background resulting from non-specific hybridisation, samples are washed with dilute salt solutions. Critical factors of such washes include the ionic strength and temperature of the final wash solution: the lower the salt concentration and the higher the wash temperature, the higher the stringency of the wash. Wash conditions are typically performed at or below hybridisation stringency. A positive hybridisation gives a signal that is at least twice of that of the background. Generally, suitable stringent conditions for nucleic acid sequence hybridisation assays or gene amplification detection procedures are as set forth above. More or less stringent conditions may also be selected. The skilled artisan is aware of various parameters which may be altered during washing and which will either maintain or change the stringency conditions. 
     For example, typical high stringency hybridisation conditions for DNA hybrids longer than 50 nucleotides encompass hybridisation at 65° C. in 1×SSC or at 42° C. in 1×SSC and 50% formamide, followed by washing at 65° C. in 0.3×SSC. Examples of medium stringency hybridisation conditions for DNA hybrids longer than 50 nucleotides encompass hybridisation at 50° C. in 4×SSC or at 40° C. in 6×SSC and 50% formamide, followed by washing at 50° C. in 2×SSC. The length of the hybrid is the anticipated length for the hybridising nucleic acid. When nucleic acid molecules of known sequence are hybridised, the hybrid length may be determined by aligning the sequences and identifying the conserved regions described herein. 1×SSC is 0.15M NaCl and 15 mM sodium citrate; the hybridisation solution and wash solutions may additionally include 5×Denhardt&#39;s reagent, 0.5-1.0% SDS, 100 μg/ml denatured, fragmented salmon sperm DNA, 0.5% sodium pyrophosphate. 
     For the purposes of defining the level of stringency, reference can be made to Sambrook et al. (2001) Molecular Cloning: a laboratory manual, 3 rd  Edition, Cold Spring Harbor Laboratory Press, CSH, New York or to Current Protocols in Molecular Biology, John Wiley &amp; Sons, N.Y. (1989 and yearly updates). 
     Splice Variant 
     The term “splice variant” as used herein encompasses variants of a nucleic acid sequence in which selected introns and/or exons have been excised, replaced, displaced or added, or in which introns have been shortened or lengthened. Such variants will be ones in which the biological activity of the protein is substantially retained; this may be achieved by selectively retaining functional segments of the protein. Such splice variants may be found in nature or may be manmade. Methods for predicting and isolating such splice variants are well known in the art (see for example Foissac and Schiex (2005) BMC Bioinformatics 6: 25). 
     Allelic Variant 
     Alleles or allelic variants are alternative forms of a given gene, located at the same chromosomal position. Allelic variants encompass Single Nucleotide Polymorphisms (SNPs), as well as Small Insertion/Deletion Polymorphisms (INDELs). The size of INDELs is usually less than 100 bp. SNPs and INDELs form the largest set of sequence variants in naturally occurring polymorphic strains of most organisms. 
     Gene Shuffling/Directed Evolution 
     Gene shuffling or directed evolution consists of iterations of DNA shuffling followed by appropriate screening and/or selection to generate variants of nucleic acid sequences or portions thereof encoding proteins having a modified biological activity (Castle et al., (2004) Science 304(5674): 1151-4; U.S. Pat. Nos. 5,811,238 and 6,395,547). 
     Regulatory Element/Control Sequence/Promoter 
     The terms “regulatory element”, “control sequence” and “promoter” are all used interchangeably herein and are to be taken in a broad context to refer to regulatory nucleic acid sequences capable of effecting expression of the sequences to which they are ligated. The term “promoter” typically refers to a nucleic acid sequence control sequence located upstream from the transcriptional start of a gene and which is involved in recognising and binding of RNA polymerase and other proteins, thereby directing transcription of an operably linked nucleic acid. Encompassed by the aforementioned terms are transcriptional regulatory sequences derived from a classical eukaryotic genomic gene (including the TATA box which is required for accurate transcription initiation, with or without a CCAAT box sequence) and additional regulatory elements (i.e. upstream activating sequences, increasers and silencers) which alter gene expression in response to developmental and/or external stimuli, or in a tissue-specific manner. Also included within the term is a transcriptional regulatory sequence of a classical prokaryotic gene, in which case it may include a −35 box sequence and/or −10 box transcriptional regulatory sequences. The term “regulatory element” also encompasses a synthetic fusion molecule or derivative that confers, activates or increases expression of a nucleic acid sequence molecule in a cell, tissue or organ. 
     A “plant promoter” comprises regulatory elements, which mediate the expression of a coding sequence segment in plant cells. The “plant promoter” preferably originates from a plant cell, e.g. from the plant which is transformed with the nucleic acid sequence to be expressed in the inventive process and described herein. This also applies to other “plant” regulatory signals, such as “plant” terminators. The promoters upstream of the nucleotide sequences useful in the methods of the present invention can be modified by one or more nucleotide substitution(s), insertion(s) and/or deletion(s) without interfering with the functionality or activity of either the promoters, the open reading frame (ORF) or the 3′-regulatory region such as terminators or other 3′ regulatory regions which are located away from the ORF. It is furthermore possible that the activity of the promoters is increased by modification of their sequence, or that they are replaced completely by more active promoters, even promoters from heterologous organisms. For expression in plants, the nucleic acid sequence molecule must, as described above, be linked operably to or comprise a suitable promoter which expresses the gene at the right point in time and with the required spatial expression pattern. 
     For the identification of functionally equivalent promoters, the promoter strength and/or expression pattern of a candidate promoter may be analysed for example by operably linking the promoter to a reporter gene and assaying the expression level and pattern of the reporter gene in various tissues of the plant. Suitable well-known reporter genes include for example beta-glucuronidase or beta-galactosidase. The promoter activity is assayed by measuring the enzymatic activity of the beta-glucuronidase or beta-galactosidase. The promoter strength and/or expression pattern may then be compared to that of a reference promoter (such as the one used in the methods of the present invention). Alternatively, promoter strength may be assayed by quantifying mRNA levels or by comparing mRNA levels of the nucleic acid sequence used in the methods of the present invention, with mRNA levels of housekeeping genes such as 18S rRNA, using methods known in the art, such as Northern blotting with densitometric analysis of autoradiograms, quantitative real-time PCR or RT-PCR (Heid et al., 1996 Genome Methods 6: 986-994). Generally by “weak promoter” is intended a promoter that drives expression of a coding sequence at a low level. By “low level” is intended at levels of about 1/10,000 transcripts to about 1/100,000 transcripts, to about 1/500,0000 transcripts per cell. Conversely, a “strong promoter” drives expression of a coding sequence at high level, or at about 1/10 transcripts to about 1/100 transcripts to about 1/1000 transcripts per cell. Generally, by “medium strength promoter” is intended a promoter that drives expression of a coding sequence at a level that is in all instances below that obtained under the control of a 35S CaMV promoter. 
     Operably Linked 
     The term “operably linked” as used herein refers to a functional linkage between the promoter sequence and the gene of interest, such that the promoter sequence is able to initiate transcription of the gene of interest. 
     Constitutive Promoter 
     A “constitutive promoter” refers to a promoter that is transcriptionally active during most, but not necessarily all, phases of growth and development and under most environmental conditions, in at least one cell, tissue or organ. Table 2a below gives examples of constitutive promoters. 
     
       
         
           
               
             
               
                 TABLE 2a 
               
             
            
               
                   
               
               
                 Examples of plant constitutive promoters 
               
            
           
           
               
               
            
               
                 Gene Source 
                 Reference 
               
               
                   
               
               
                 Actin 
                 McElroy et al, Plant Cell, 2: 163-171, 1990 
               
               
                 HMGB 
                 WO 2004/070039 
               
               
                 GOS2 
                 de Pater et al, Plant J Nov; 2(6): 837-44, 1992, 
               
               
                   
                 WO 2004/065596 
               
               
                 Ubiquitin 
                 Christensen et al, Plant Mol. Biol. 18: 675-689, 
               
               
                   
                 1992 
               
               
                 Rice cyclophilin 
                 Buchholz et al, Plant Mol Biol. 25(5): 837-43, 1994 
               
               
                 Maize H3 histone 
                 Lepetit et al, Mol. Gen. Genet. 231: 276-285, 1992 
               
               
                 Alfalfa H3 histone 
                 Wu et al. Plant Mol. Biol. 11: 641-649, 1988 
               
               
                 Actin 2 
                 An et al, Plant J. 10(1); 107-121, 1996 
               
               
                 Rubisco small 
                 U.S. Pat. No. 4,962,028 
               
               
                 subunit 
               
               
                 OCS 
                 Leisner (1988) Proc Natl Acad Sci USA 85(5): 2553 
               
               
                 SAD1 
                 Jain et al., Crop Science, 39 (6), 1999: 1696 
               
               
                 SAD2 
                 Jain et al., Crop Science, 39 (6), 1999: 1696 
               
               
                 V-ATPase 
                 WO 01/14572 
               
               
                 G-box proteins 
                 WO 94/12015 
               
               
                   
               
            
           
         
       
     
     Ubiquitous Promoter 
     A ubiquitous promoter is active in substantially all tissues or cells of an organism. 
     Developmentally-Regulated Promoter 
     A developmentally-regulated promoter is active during certain developmental stages or in parts of the plant that undergo developmental changes. 
     Inducible Promoter 
     An inducible promoter has induced or increased transcription initiation in response to a chemical (for a review see Gatz 1997, Annu. Rev. Plant Physiol. Plant Mol. Biol., 48:89-108), environmental or physical stimulus, or may be “stress-inducible”, i.e. activated when a plant is exposed to various stress conditions, or a “pathogen-inducible” i.e. activated when a plant is exposed to exposure to various pathogens. 
     Organ-Specific/Tissue-Specific Promoter 
     An organ-specific or tissue-specific promoter is one that is capable of preferentially initiating transcription in certain organs or tissues, such as the leaves, roots, seed tissue etc. For example, a “root-specific promoter” is a promoter that is transcriptionally active predominantly in plant roots, substantially to the exclusion of any other parts of a plant, whilst still allowing for any leaky expression in these other plant parts. Promoters able to initiate transcription in certain cells only are referred to herein as “cell-specific”. 
     Examples of root-specific promoters are listed in Table 2b below: 
     
       
         
           
               
             
               
                 TABLE 2b 
               
             
            
               
                   
               
               
                 Examples of root-specific promoters 
               
            
           
           
               
               
            
               
                 Gene Source 
                 Reference 
               
               
                   
               
               
                 Rice RCc3 
                 Xu et al (1995) Plant Mol Biol 27(2): 
               
               
                   
                 237-48 
               
               
                   Arabidopsis  phosphate 
                 Kovama et al., 2005 
               
               
                 transporter PHT1 
               
               
                   Medicago  phosphate transporter 
                 Xiao et al., 2006 
               
               
                   Arabidopsis  Pyk10 
                 Nitz et al. (2001) Plant Sci 161(2): 
               
               
                   
                 337-346 
               
               
                 Tobacco root-specific genes 
                 Conkling et al. (1990) Plant Phys 93(3): 
               
               
                 RB7, RD2, RD5, RH12 
                 1203-1211 
               
               
                 Barley root-specific lectin 
                 Lerner &amp; Raikhel (1989) Plant Phys 91: 
               
               
                   
                 124-129 
               
               
                 Root-specific hydroxy-proline 
                 Keller &amp; Lamb (1989) Genes &amp; Dev 3: 
               
               
                 rich protein 
                 1639-1646 
               
               
                   Arabidopsis  CDC27B/hobbit 
                 Blilou et al. (2002) Genes &amp; Dev 16: 
               
               
                   
                 2566-2575 
               
               
                   
               
            
           
         
       
     
     A seed-specific promoter is transcriptionally active predominantly in seed tissue, but not necessarily exclusively in seed tissue (in cases of leaky expression). The seed-specific promoter may be active during seed development and/or during germination. Examples of seed-specific promoters are shown in Table 2c below. Further examples of seed-specific promoters are given in Qing Qu and Takaiwa (Plant Biotechnol. J. 2, 113-125, 2004), which disclosure is incorporated by reference herein as if fully set forth. 
     
       
         
           
               
             
               
                 TABLE 2c 
               
             
            
               
                   
               
               
                 Examples of seed-specific promoters 
               
            
           
           
               
               
            
               
                 Gene source 
                 Reference 
               
               
                   
               
               
                 seed-specific genes 
                 Simon et al., Plant Mol. Biol. 5: 191, 1985; 
               
               
                   
                 Scofield et al., J. Biol. Chem. 262: 12202, 1987.; 
               
               
                   
                 Baszczynski et al., Plant Mol. Biol. 14: 633, 1990. 
               
               
                 Brazil Nut albumin 
                 Pearson et al., Plant Mol. Biol. 18: 235-245, 1992. 
               
               
                 Legumin 
                 Ellis et al., Plant Mol. Biol. 10: 203-214, 1988. 
               
               
                 glutelin (rice) 
                 Takaiwa et al., Mol. Gen. Genet. 208: 15-22, 1986; 
               
               
                   
                 Takaiwa et al., FEBS Letts. 221: 43-47, 1987. 
               
               
                 Zein 
                 Matzke et al Plant Mol Biol, 14(3): 323-32 1990 
               
               
                 NapA 
                 Stalberg et al, Planta 199: 515-519, 1996. 
               
               
                 Wheat LMW and HMW glutenin-1 
                 Mol Gen Genet 216: 81-90, 1989; NAR 17: 461-2, 1989 
               
               
                 Wheat SPA 
                 Albani et al, Plant Cell, 9: 171-184, 1997 
               
               
                 Wheat α, β, γ-gliadins 
                 EMBO J. 3: 1409-15, 1984 
               
               
                 Barley Itr1 promoter 
                 Diaz et al. (1995) Mol Gen Genet 248(5): 592-8 
               
               
                 Barley B1, C, D, hordein 
                 Theor Appl Gen 98: 1253-62, 1999; Plant J 4: 343-55, 
               
               
                   
                 1993; Mol Gen Genet 250: 750-60, 1996 
               
               
                 Barley DOF 
                 Mena et al, The Plant Journal, 116(1): 53-62, 1998 
               
               
                 blz2 
                 EP99106056.7 
               
               
                 Synthetic promoter 
                 Vicente-Carbajosa et al., Plant J. 13: 629-640, 1998. 
               
               
                 rice prolamin NRP33 
                 Wu et al, Plant Cell Physiology 39(8) 885-889, 1998 
               
               
                 rice a-globulin Glb-1 
                 Wu et al, Plant Cell Physiology 39(8) 885-889, 1998 
               
               
                 rice OSH1 
                 Sato et al, Proc. Natl. Acad. Sci. USA, 93: 8117-8122, 
               
               
                   
                 1996 
               
               
                 rice α-globulin REB/OHP-1 
                 Nakase et al. Plant Mol. Biol. 33: 513-522, 1997 
               
               
                 rice ADP-glucose pyrophosphorylase 
                 Trans Res 6: 157-68, 1997 
               
               
                 Maize ESR gene family 
                 Plant J 12: 235-46, 1997 
               
               
                 Sorghum α-kafirin 
                 DeRose et al., Plant Mol. Biol 32: 1029-35, 1996 
               
               
                 KNOX 
                 Postma-Haarsma et al, Plant Mol. Biol. 39: 257-71, 1999 
               
               
                 rice oleosin 
                 Wu et al, J. Biochem. 123: 386, 1998 
               
               
                 sunflower oleosin 
                 Cummins et al., Plant Mol. Biol. 19: 873-876, 1992 
               
               
                 PRO0117, putative rice 40S 
                 WO 2004/070039 
               
               
                 ribosomal protein 
               
               
                 PRO0136, rice alanine 
                 Unpublished 
               
               
                 aminotransferase 
               
               
                 PRO0147, trypsin inhibitor ITR1 
                 Unpublished 
               
               
                 (barley) 
               
               
                 PRO0151, rice WSI18 
                 WO 2004/070039 
               
               
                 PRO0175, rice RAB21 
                 WO 2004/070039 
               
               
                 PRO005 
                 WO 2004/070039 
               
               
                 PRO0095 
                 WO 2004/070039 
               
               
                 α-amylase (Amy32b) 
                 Lanahan et al, Plant Cell 4: 203-211, 1992; Skriver et al, 
               
               
                   
                 Proc Natl Acad Sci USA 88: 7266-7270, 1991 
               
               
                 Cathepsin β-like gene 
                 Cejudo et al, Plant Mol Biol 20: 849-856, 1992 
               
               
                 Barley Ltp2 
                 Kalla et al., Plant J. 6: 849-60, 1994 
               
               
                 Chi26 
                 Leah et al., Plant J. 4: 579-89, 1994 
               
               
                 Maize B-Peru 
                 Selinger et al., Genetics 149; 1125-38, 1998 
               
               
                   
               
            
           
         
       
     
     A green tissue-specific promoter as defined herein is a promoter that is transcriptionally active predominantly in green tissue, substantially to the exclusion of any other parts of a plant, whilst still allowing for any leaky expression in these other plant parts. 
     Examples of green tissue-specific promoters which may be used to perform the methods of the invention are shown in Table 2d below. 
     
       
         
           
               
             
               
                 TABLE 2d 
               
             
            
               
                   
               
               
                 Examples of green tissue-specific promoters 
               
            
           
           
               
               
               
            
               
                 Gene 
                 Expression 
                 Reference 
               
               
                   
               
               
                 Maize Orthophosphate dikinase 
                 Leaf specific 
                 Fukavama et al., 2001 
               
               
                 Maize Phosphoenolpyruvate 
                 Leaf specific 
                 Kausch et al., 2001 
               
               
                 carboxylase 
               
               
                 Rice Phosphoenolpyruvate 
                 Leaf specific 
                 Liu et al., 2003 
               
               
                 carboxylase 
               
               
                 Rice small subunit Rubisco 
                 Leaf specific 
                 Nomura et al., 2000 
               
               
                 rice beta expansin EXBP9 
                 Shoot specific 
                 WO 2004/070039 
               
               
                 Pigeonpea small subunit Rubisco 
                 Leaf specific 
                 Panguluri et al., 2005 
               
               
                 Pea RBCS3A 
                 Leaf specific 
               
               
                   
               
            
           
         
       
     
     Another example of a tissue-specific promoter is a meristem-specific promoter, which is transcriptionally active predominantly in meristematic tissue, substantially to the exclusion of any other parts of a plant, whilst still allowing for any leaky expression in these other plant parts. Examples of meristem-specific promoters which may be used to perform the methods of the invention are shown in Table 2e below. 
     
       
         
           
               
             
               
                 TABLE 2e 
               
             
            
               
                   
               
               
                 Examples of meristem-specific promoters 
               
            
           
           
               
               
               
            
               
                 Gene source 
                 Expression pattern 
                 Reference 
               
               
                   
               
               
                 rice OSH1 
                 Shoot apical meristem, 
                 Sato et al. (1996) Proc. 
               
               
                   
                 from embryo globular stage 
                 Natl. Acad. Sci. USA, 
               
               
                   
                 to seedling stage 
                 93: 8117-8122 
               
               
                 Rice metallothionein 
                 Meristem specific 
                 BAD87835.1 
               
               
                 WAK1 &amp; WAK 2 
                 Shoot and root apical 
                 Wagner &amp; Kohorn 
               
               
                   
                 meristems, and in 
                 (2001) Plant Cell 
               
               
                   
                 expanding leaves and 
                 13(2): 303-318 
               
               
                   
                 sepals 
               
               
                   
               
            
           
         
       
     
     Terminator 
     The term “terminator” encompasses a control sequence which is a DNA sequence at the end of a transcriptional unit which signals 3′ processing and polyadenylation of a primary transcript and termination of transcription. The terminator can be derived from the natural gene, from a variety of other plant genes, or from T-DNA. The terminator to be added may be derived from, for example, the nopaline synthase or octopine synthase genes, or alternatively from another plant gene, or less preferably from any other eukaryotic gene. 
     Modulation 
     The term “modulation” means in relation to expression or gene expression, a process in which the expression level is changed by said gene expression in comparison to the control plant, preferably the expression level is increased. The original, unmodulated expression may be of any kind of expression of a structural RNA (rRNA, tRNA) or mRNA with subsequent translation. The term “modulating the activity” shall mean any change of the expression of the inventive nucleic acid sequences or encoded proteins, which leads to increased yield and/or increased growth of the plants. 
     Increased Expression/Overexpression 
     The term “increased expression” or “overexpression” as used herein means any form of expression that is additional to the original wild-type expression level. 
     Methods for increasing expression of genes or gene products are well documented in the art and include, for example, overexpression driven by appropriate promoters, the use of transcription increasers or translation increasers. Isolated nucleic acid sequences which serve as promoter or increaser elements may be introduced in an appropriate position (typically upstream) of a non-heterologous form of a polynucleotide so as to upregulate expression of a nucleic acid sequence encoding the polypeptide of interest. For example, endogenous promoters may be altered in vivo by mutation, deletion, and/or substitution (see, Kmiec, U.S. Pat. No. 5,565,350; Zarling et al., WO9322443), or isolated promoters may be introduced into a plant cell in the proper orientation and distance from a gene of the present invention so as to control the expression of the gene. 
     If polypeptide expression is desired, it is generally desirable to include a polyadenylation region at the 3′-end of a polynucleotide coding region. The polyadenylation region can be derived from the natural gene, from a variety of other plant genes, or from T-DNA. The 3′ end sequence to be added may be derived from, for example, the nopaline synthase or octopine synthase genes, or alternatively from another plant gene, or less preferably from any other eukaryotic gene. 
     An intron sequence may also be added to the 5′ untranslated region (UTR) or the coding sequence of the partial coding sequence to increase the amount of the mature message that accumulates in the cytosol. Inclusion of a spliceable intron in the transcription unit in both plant and animal expression constructs has been shown to increase gene expression at both the mRNA and protein levels up to 1000-fold (Buchman and Berg (1988) Mol. Cell biol. 8: 4395-4405; Callis et al. (1987) Genes Dev 1:1183-1200). Such intron increasement of gene expression is typically greatest when placed near the 5′ end of the transcription unit. Use of the maize introns Adh1-5 intron 1, 2, and 6, the Bronze-1 intron are known in the art. For general information see: The Maize Handbook, Chapter 116, Freeling and Walbot, Eds., Springer, N.Y. (1994). 
     Endogenous Gene 
     Reference herein to an “endogenous” gene not only refers to the gene in question as found in a plant in its natural form (i.e., without there being any human intervention), but also refers to that same gene (or a substantially homologous nucleic acid/gene) in an isolated form subsequently (re)introduced into a plant (a transgene). For example, a transgenic plant containing such a transgene may encounter a substantial reduction of the transgene expression and/or substantial reduction of expression of the endogenous gene. 
     Decreased Expression 
     Reference herein to “decreased epression” or “reduction or substantial elimination” of expression is taken to mean a decrease in endogenous gene expression and/or polypeptide levels and/or polypeptide activity relative to control plants. The reduction or substantial elimination is in increasing order of preference at least 10%, 20%, 30%, 40% or 50%, 60%, 70%, 80%, 85%, 90%, or 95%, 96%, 97%, 98%, 99% or more reduced compared to that of control plants. 
     For the reduction or substantial elimination of expression an endogenous gene in a plant, a sufficient length of substantially contiguous nucleotides of a nucleic acid sequence is required. In order to perform gene silencing, this may be as little as 20, 19, 18, 17, 16, 15, 14, 13, 12, 11, 10 or fewer nucleotides, alternatively this may be as much as the entire gene (including the 5′ and/or 3′ UTR, either in part or in whole). The stretch of substantially contiguous nucleotides may be derived from the nucleic acid sequence encoding the protein of interest (target gene), or from any nucleic acid sequence capable of encoding an orthologue, paralogue or homologue of the protein of interest. Preferably, the stretch of substantially contiguous nucleotides is capable of forming hydrogen bonds with the target gene (either sense or antisense strand), more preferably, the stretch of substantially contiguous nucleotides has, in increasing order of preference, 50%, 60%, 70%, 80%, 85%, 90%, 95%, 96%, 97%, 98%, 99%, 100% sequence identity to the target gene (either sense or antisense strand). A nucleic acid sequence encoding a (functional) polypeptide is not a requirement for the various methods discussed herein for the reduction or substantial elimination of expression of an endogenous gene. 
     This reduction or substantial elimination of expression may be achieved using routine tools and techniques. A method for the reduction or substantial elimination of endogenous gene expression is by RNA-mediated silencing using an inverted repeat of a nucleic acid sequence or a part thereof (in this case a stretch of substantially contiguous nucleotides derived from the gene of interest, or from any nucleic acid sequence capable of encoding an orthologue, paralogue or homologue of the protein of interest), preferably capable of forming a hairpin structure. Another example of an RNA silencing method involves the introduction of nucleic acid sequences or parts thereof (in this case a stretch of substantially contiguous nucleotides derived from the gene of interest, or from any nucleic acid sequence capable of encoding an orthologue, paralogue or homologue of the protein of interest) in a sense orientation into a plant. Another example of an RNA silencing method involves the use of antisense nucleic acid sequences. Gene silencing may also be achieved by insertion mutagenesis (for example, T-DNA insertion or transposon insertion) or by strategies as described by, among others, Angell and Baulcombe ((1999) Plant J 20(3): 357-62), (Amplicon VIGS WO 98/36083), or Baulcombe (WO 99/15682). Other methods, such as the use of antibodies directed to an endogenous polypeptide for inhibiting its function in planta, or interference in the signalling pathway in which a polypeptide is involved, will be well known to the skilled man. Artificial and/or natural microRNAs (miRNAs) may be used to knock out gene expression and/or mRNA translation. Endogenous miRNAs are single stranded small RNAs of typically 19-24 nucleotides long. Artificial microRNAs (amiRNAs), which are typically 21 nucleotides in length, can be genetically engineered specifically to negatively regulate gene expression of single or multiple genes of interest. Determinants of plant microRNA target selection are well known in the art. Empirical parameters for target recognition have been defined and can be used to aid in the design of specific amiRNAs (Schwab et al., (2005) Dev Cell 8(4):517-27). Convenient tools for design and generation of amiRNAs and their precursors are also available to the public (Schwab et al., (2006) Plant Cell 18(5):1121-33). 
     For optimal performance, the gene silencing techniques used for reducing expression in a plant of an endogenous gene requires the use of nucleic acid sequences from monocotyledonous plants for transformation of monocotyledonous plants, and from dicotyledonous plants for transformation of dicotyledonous plants. Preferably, a nucleic acid sequence from any given plant species is introduced into that same species. For example, a nucleic acid sequence from rice is transformed into a rice plant. However, it is not an absolute requirement that the nucleic acid sequence to be introduced originates from the same plant species as the plant in which it will be introduced. It is sufficient that there is substantial homology between the endogenous target gene and the nucleic acid sequence to be introduced. 
     Described above are examples of various methods for the reduction or substantial elimination of expression in a plant of an endogenous gene. A person skilled in the art would readily be able to adapt the aforementioned methods for silencing so as to achieve reduction of expression of an endogenous gene in a whole plant or in parts thereof through the use of an appropriate promoter, for example. 
     Selectable Marker (Gene)/Reporter Gene 
     “Selectable marker”, “selectable marker gene” or “reporter gene” includes any gene that confers a phenotype on a cell in which it is expressed to facilitate the identification and/or selection of cells that are transfected or transformed with a nucleic acid sequence construct of the invention. These marker genes enable the identification of a successful transfer of the nucleic acid sequence molecules via a series of different principles. Suitable markers may be selected from markers that confer antibiotic or herbicide resistance, that introduce a new metabolic trait or that allow visual selection. Examples of selectable marker genes include genes conferring resistance to antibiotics (such as nptII that phosphorylates neomycin and kanamycin, or hpt, phosphorylating hygromycin, or genes conferring resistance to, for example, bleomycin, streptomycin, tetracyclin, chloramphenicol, ampicillin, gentamycin, geneticin (G418), spectinomycin or blasticidin), to herbicides (for example bar which provides resistance to Basta®; aroA or gox providing resistance against glyphosate, or the genes conferring resistance to, for example, imidazolinone, phosphinothricin or sulfonylurea), or genes that provide a metabolic trait (such as manA that allows plants to use mannose as sole carbon source or xylose isomerase for the utilisation of xylose, or antinutritive markers such as the resistance to 2-deoxyglucose). Expression of visual marker genes results in the formation of colour (for example β-glucuronidase, GUS or β-galactosidase with its coloured substrates, for example X-Gal), luminescence (such as the luciferin/luceferase system) or fluorescence (Green Fluorescent Protein, GFP, and derivatives thereof). This list represents only a small number of possible markers. The skilled worker is familiar with such markers. Different markers are preferred, depending on the organism and the selection method. 
     It is known that upon stable or transient integration of nucleic acid sequences into plant cells, only a minority of the cells takes up the foreign DNA and, if desired, integrates it into its genome, depending on the expression vector used and the transfection technique used. To identify and select these integrants, a gene coding for a selectable marker (such as the ones described above) is usually introduced into the host cells together with the gene of interest. These markers can for example be used in mutants in which these genes are not functional by, for example, deletion by conventional methods. Furthermore, nucleic acid sequence molecules encoding a selectable marker can be introduced into a host cell on the same vector that comprises the sequence encoding the polypeptides of the invention or used in the methods of the invention, or else in a separate vector. Cells which have been stably transfected with the introduced nucleic acid sequence can be identified for example by selection (for example, cells which have integrated the selectable marker survive whereas the other cells die). 
     Since the marker genes, particularly genes for resistance to antibiotics and herbicides, are no longer required or are undesired in the transgenic host cell once the nucleic acid sequences have been introduced successfully, the process according to the invention for introducing the nucleic acid sequences advantageously employs techniques which enable the removal or excision of these marker genes. One such a method is what is known as co-transformation. The co-transformation method employs two vectors simultaneously for the transformation, one vector bearing the nucleic acid sequence according to the invention and a second bearing the marker gene(s). A large proportion of transformants receives or, in the case of plants, comprises (up to 40% or more of the transformants), both vectors. In case of transformation with Agrobacteria, the transformants usually receive only a part of the vector, i.e. the sequence flanked by the T-DNA, which usually represents the expression cassette. The marker genes can subsequently be removed from the transformed plant by performing crosses. In another method, marker genes integrated into a transposon are used for the transformation together with desired nucleic acid sequence (known as the Ac/Ds technology). The transformants can be crossed with a transposase source or the transformants are transformed with a nucleic acid sequence construct conferring expression of a transposase, transiently or stable. In some cases (approx. 10%), the transposon jumps out of the genome of the host cell once transformation has taken place successfully and is lost. In a further number of cases, the transposon jumps to a different location. In these cases the marker gene must be eliminated by performing crosses. In microbiology, techniques were developed which make possible, or facilitate, the detection of such events. A further advantageous method relies on what is known as recombination systems; whose advantage is that elimination by crossing can be dispensed with. The best-known system of this type is what is known as the Cre/lox system. Cre1 is a recombinase that removes the sequences located between the loxP sequences. If the marker gene is integrated between the loxP sequences, it is removed once transformation has taken place successfully, by expression of the recombinase. Further recombination systems are the HIN/HIX, FLP/FRT and REP/STB system (Tribble et al., J. Biol. Chem., 275, 2000: 22255-22267; Velmurugan et al., J. Cell Biol., 149, 2000: 553-566). A site-specific integration into the plant genome of the nucleic acid sequences according to the invention is possible. Naturally, these methods can also be applied to microorganisms such as yeast, fungi or bacteria. 
     Transgenic/Transgene/Recombinant 
     For the purposes of the invention, “transgenic”, “transgene” or “recombinant” means with regard to, for example, a nucleic acid sequence, an expression cassette, gene construct or a vector comprising the nucleic acid sequence or an organism transformed with the nucleic acid sequences, expression cassettes or vectors according to the invention, all those constructions brought about by recombinant methods in which either
     (a) the nucleic acid sequences encoding proteins useful in the methods of the invention, or   (b) genetic control sequence(s) which is operably linked with the nucleic acid sequence according to the invention, for example a promoter, or   (c) a) and b)
 
are not located in their natural genetic environment or have been modified by recombinant methods, it being possible for the modification to take the form of, for example, a substitution, addition, deletion, inversion or insertion of one or more nucleotide residues. The natural genetic environment is understood as meaning the natural genomic or chromosomal locus in the original plant or the presence in a genomic library. In the case of a genomic library, the natural genetic environment of the nucleic acid sequence is preferably retained, at least in part. The environment flanks the nucleic acid sequence at least on one side and has a sequence length of at least 50 bp, preferably at least 500 bp, especially preferably at least 1000 bp, most preferably at least 5000 bp. A naturally occurring expression cassette—for example the naturally occurring combination of the natural promoter of the nucleic acid sequences with the corresponding nucleic acid sequence encoding a polypeptide useful in the methods of the present invention, as defined above—becomes a transgenic expression cassette when this expression cassette is modified by non-natural, synthetic (“artificial”) methods such as, for example, mutagenic treatment. Suitable methods are described, for example, in U.S. Pat. No. 5,565,350 or WO 00/15815.
   

     A transgenic plant for the purposes of the invention is thus understood as meaning, as above, that the nucleic acid sequences used in the method of the invention are not at their natural locus in the genome of said plant, it being possible for the nucleic acid sequences to be expressed homologously or heterologously. However, as mentioned, transgenic also means that, while the nucleic acid sequence according to the invention or used in the inventive method are at their natural position in the genome of a plant, the sequence has been modified with regard to the natural sequence, and/or that the regulatory sequences of the natural sequences have been modified. Transgenic is preferably understood as meaning the expression of the nucleic acid sequences according to the invention at an unnatural locus in the genome, i.e. homologous or, preferably, heterologous expression of the nucleic acid sequences takes place. Preferred transgenic plants are mentioned herein. 
     Transformation 
     The term “introduction” or “transformation” as referred to herein encompasses the transfer of an exogenous polynucleotide into a host cell, irrespective of the method used for transfer. Plant tissue capable of subsequent clonal propagation, whether by organogenesis or embryogenesis, may be transformed with a genetic construct of the present invention and a whole plant regenerated there from. The particular tissue chosen will vary depending on the clonal propagation systems available for, and best suited to, the particular species being transformed. Exemplary tissue targets include leaf disks, pollen, embryos, cotyledons, hypocotyls, megagametophytes, callus tissue, existing meristematic tissue (e.g., apical meristem, axillary buds, and root meristems), and induced meristem tissue (e.g., cotyledon meristem and hypocotyl meristem). The polynucleotide may be transiently or stably introduced into a host cell and may be maintained non-integrated, for example, as a plasmid. Alternatively, it may be integrated into the host genome. The resulting transformed plant cell may then be used to regenerate a transformed plant in a manner known to persons skilled in the art. 
     The transfer of foreign genes into the genome of a plant is called transformation. Transformation of plant species is now a fairly routine technique. Advantageously, any of several transformation methods may be used to introduce the gene of interest into a suitable ancestor cell. The methods described for the transformation and regeneration of plants from plant tissues or plant cells may be utilized for transient or for stable transformation. Transformation methods include the use of liposomes, electroporation, chemicals that increase free DNA uptake, injection of the DNA directly into the plant, particle gun bombardment, transformation using viruses or pollen and microprojection. Methods may be selected from the calcium/polyethylene glycol method for protoplasts (Krens, F. A. et al., (1982) Nature 296, 72-74; Negrutiu I et al. (1987) Plant Mol Biol 8: 363-373); electroporation of protoplasts (Shillito R. D. et al. (1985) Bio/Technol 3, 1099-1102); microinjection into plant material (Crossway A et al., (1986) Mol. Gen Genet 202: 179-185); DNA or RNA-coated particle bombardment (Klein T M et al., (1987) Nature 327: 70) infection with (non-integrative) viruses and the like. Transgenic plants, including transgenic crop plants, are preferably produced via Agrobacterium-mediated transformation. An advantageous transformation method is the transformation in planta. To this end, it is possible, for example, to allow the agrobacteria to act on plant seeds or to inoculate the plant meristem with agrobacteria. It has proved particularly expedient in accordance with the invention to allow a suspension of transformed agrobacteria to act on the intact plant or at least on the flower primordia. The plant is subsequently grown on until the seeds of the treated plant are obtained (Clough and Bent, Plant J. (1998) 16, 735-743). Methods for Agrobacterium-mediated transformation of rice include well known methods for rice transformation, such as those described in any of the following: European patent application EP 1198985 A1, Aldemita and Hodges (Planta 199: 612-617, 1996); Chan et al. (Plant Mol Biol 22 (3): 491-506, 1993), Hiei et al. (Plant J 6 (2): 271-282, 1994), which disclosures are incorporated by reference herein as if fully set forth. In the case of corn transformation, the preferred method is as described in either Ishida et al. (Nat. Biotechnol 14(6): 745-50, 1996) or Frame et al. (Plant Physiol 129(1): 13-22, 2002), which disclosures are incorporated by reference herein as if fully set forth. Said methods are further described by way of example in B. Jenes et al., Techniques for Gene Transfer, in: Transgenic Plants, Vol. 1, Engineering and Utilization, eds. S. D. Kung and R. Wu, Academic Press (1993) 128-143 and in Potrykus Annu. Rev. Plant Physiol. Plant Molec. Biol. 42 (1991) 205-225). The nucleic acid sequences or the construct to be expressed is preferably cloned into a vector, which is suitable for transforming Agrobacterium tumefaciens, for example pBin19 (Bevan et al., Nucl. Acids Res. 12 (1984) 8711). Agrobacteria transformed by such a vector can then be used in known manner for the transformation of plants, such as plants used as a model, like Arabidopsis (Arabidopsis thaliana is within the scope of the present invention not considered as a crop plant), or crop plants such as, by way of example, tobacco plants, for example by immersing bruised leaves or chopped leaves in an agrobacterial solution and then culturing them in suitable media. The transformation of plants by means of Agrobacterium tumefaciens is described, for example, by Hofgen and Willmitzer in Nucl. Acid Res. (1988) 16, 9877 or is known inter alia from F. F. White, Vectors for Gene Transfer in Higher Plants; in Transgenic Plants, Vol. 1, Engineering and Utilization, eds. S. D. Kung and R. Wu, Academic Press, 1993, pp. 15-38. 
     In addition to the transformation of somatic cells, which then have to be regenerated into intact plants, it is also possible to transform the cells of plant meristems and in particular those cells which develop into gametes. In this case, the transformed gametes follow the natural plant development, giving rise to transgenic plants. Thus, for example, seeds of Arabidopsis are treated with agrobacteria and seeds are obtained from the developing plants of which a certain proportion is transformed and thus transgenic [Feldman, K A and Marks M D (1987). Mol Gen Genet 208:274-289; Feldmann K (1992). In: C Koncz, N-H Chua and J Shell, eds, Methods in Arabidopsis Research. Word Scientific, Singapore, pp. 274-289]. Alternative methods are based on the repeated removal of the inflorescences and incubation of the excision site in the center of the rosette with transformed agrobacteria, whereby transformed seeds can likewise be obtained at a later point in time (Chang (1994). Plant J. 5: 551-558; Katavic (1994). Mol Gen Genet, 245: 363-370). However, an especially effective method is the vacuum infiltration method with its modifications such as the “floral dip” method. In the case of vacuum infiltration of Arabidopsis, intact plants under reduced pressure are treated with an agrobacterial suspension [Bechthold, N (1993). C R Acad Sci Paris Life Sci, 316: 1194-1199], while in the case of the “floral dip” method the developing floral tissue is incubated briefly with a surfactant-treated agrobacterial suspension [Clough, S J and Bent A F (1998) The Plant J. 16, 735-743]. A certain proportion of transgenic seeds are harvested in both cases, and these seeds can be distinguished from non-transgenic seeds by growing under the above-described selective conditions. In addition the stable transformation of plastids is of advantages because plastids are inherited maternally is most crops reducing or eliminating the risk of transgene flow through pollen. The transformation of the chloroplast genome is generally achieved by a process which has been schematically displayed in Klaus et al., 2004 [Nature Biotechnology 22 (2), 225-229]. Briefly the sequences to be transformed are cloned together with a selectable marker gene between flanking sequences homologous to the chloroplast genome. These homologous flanking sequences direct site specific integration into the plastome. Plastidal transformation has been described for many different plant species and an overview is given in Bock (2001) Transgenic plastids in basic research and plant biotechnology. J Mol Biol. 2001 Sep. 21; 312 (3):425-38 or Maliga, P (2003) Progress towards commercialization of plastid transformation technology. Trends Biotechnol. 21, 20-28. Further biotechnological progress has recently been reported in form of marker free plastid transformants, which can be produced by a transient co-integrated maker gene (Klaus et al., 2004, Nature Biotechnology 22(2), 225-229). 
     T-DNA Activation Tagging 
     T-DNA activation tagging (Hayashi et al. Science (1992) 1350-1353), involves insertion of T-DNA, usually containing a promoter (may also be a translation increaser or an intron), in the genomic region of the gene of interest or 10 kb up- or downstream of the coding region of a gene in a configuration such that the promoter directs expression of the targeted gene. Typically, regulation of expression of the targeted gene by its natural promoter is disrupted and the gene falls under the control of the newly introduced promoter. The promoter is typically embedded in a T-DNA. This T-DNA is randomly inserted into the plant genome, for example, through Agrobacterium infection and leads to modified expression of genes near the inserted T-DNA. The resulting transgenic plants show dominant phenotypes due to modified expression of genes close to the introduced promoter. 
     TILLING 
     The term “TILLING” is an abbreviation of “Targeted Induced Local Lesions In Genomes” and refers to a mutagenesis technology useful to generate and/or identify nucleic acid sequences encoding proteins with modified expression and/or activity. TILLING also allows selection of plants carrying such mutant variants. These mutant variants may exhibit modified expression, either in strength or in location or in timing (if the mutations affect the promoter for example). These mutant variants may exhibit higher activity than that exhibited by the gene in its natural form. TILLING combines high-density mutagenesis with high-throughput screening methods. The steps typically followed in TILLING are: (a) EMS mutagenesis (Redei GP and Koncz C (1992) In Methods in Arabidopsis Research, Koncz C, Chua N H, Schell J, eds. Singapore, World Scientific Publishing Co, pp. 16-82; Feldmann et al., (1994) In Meyerowitz E M, Somerville C R, eds, Arabidopsis. Cold Spring Harbor Laboratory Press, Cold Spring Harbor, N.Y., pp 137-172; Lightner J and Caspar T (1998) In J Martinez-Zapater, J Salinas, eds, Methods on Molecular Biology, Vol. 82. Humana Press, Totowa, N.J., pp 91-104); (b) DNA preparation and pooling of individuals; (c) PCR amplification of a region of interest; (d) denaturation and annealing to allow formation of heteroduplexes; (e) DHPLC, where the presence of a heteroduplex in a pool is detected as an extra peak in the chromatogram; (f) identification of the mutant individual; and (g) sequencing of the mutant PCR product. Methods for TILLING are well known in the art (McCallum et al., (2000) Nat Biotechnol 18: 455-457; reviewed by Stemple (2004) Nat Rev Genet 5(2): 145-50). 
     Homologous Recombination 
     Homologous recombination allows introduction in a genome of a selected nucleic acid sequence at a defined selected position. Homologous recombination is a standard technology used routinely in biological sciences for lower organisms such as yeast or the moss Physcomitrella. Methods for performing homologous recombination in plants have been described not only for model plants (Offring a et al. (1990) EMBO J 9(10): 3077-84) but also for crop plants, for example rice (Terada et al. (2002) Nat Biotech 20(10): 1030-4; lida and Terada (2004) Curr Opin Biotech 15(2): 132-8). 
     Yield 
     The term “yield” in general means a measurable produce of economic value, typically related to a specified crop, to an area, and to a period of time. Individual plant parts directly contribute to yield based on their number, size and/or weight, or the actual yield is the yield per acre for a crop and year, which is determined by dividing total production (includes both harvested and appraised production) by planted acres. The term “yield” of a plant may relate to vegetative biomass, to reproductive organs, and/or to propagules (such as seeds) of that plant. 
     Early Vigour 
     “Early vigour” refers to active healthy well-balanced growth especially during early stages of plant growth, and may result from increased plant fitness due to, for example, the plants being better adapted to their environment (i.e. optimizing the use of energy resources and partitioning between shoot and root). Plants having early vigour also show increased seedling survival and a better establishment of the crop, which often results in highly uniform fields (with the crop growing in uniform manner, i.e. with the majority of plants reaching the various stages of development at substantially the same time), and often better and higher yield. Therefore, early vigour may be determined by measuring various factors, such as thousand kernel weight, percentage germination, percentage emergence, seedling growth, seedling height, root length, root and shoot biomass and many more. 
     Increase/Improve/Increase 
     The terms “increase”, “improve” or “increase” are interchangeable and shall mean in the sense of the application at least a 5%, 6%, 7%, 8%, 9% or 10%, preferably at least 15% or 20%, more preferably 25%, 30%, 35% or 40% more yield and/or growth in comparison to control plants as defined herein. 
     Seed Yield 
     Increased seed yield may manifest itself as one or more of the following: a) an increase in seed biomass (total seed weight) which may be on an individual seed basis and/or per plant and/or per hectare or acre; b) increased number of flowers per panicle and/or per plant; c) increased number of (filled) seeds; d) increased seed filling rate (which is expressed as the ratio between the number of filled seeds divided by the total number of seeds); e) increased harvest index, which is expressed as a ratio of the yield of harvestable parts, such as seeds, divided by the total biomass; f) increased number of primary panicles; (g) increased thousand kernel weight (TKW), which is extrapolated from the number of filled seeds counted and their total weight. An increased TKW may result from an increased seed size and/or seed weight, and may also result from an increase in embryo and/or endosperm size. 
     An increase in seed yield may also be manifested as an increase in seed size and/or seed volume. Furthermore, an increase in yield may also manifest itself as an increase in seed area and/or seed length and/or seed width and/or seed perimeter. Increased seed yield may also result in modified architecture, or may occur because of modified architecture. 
     Greenness Index 
     The “greenness index” as used herein is calculated from digital images of plants. For each pixel belonging to the plant object on the image, the ratio of the green value versus the red value (in the RGB model for encoding color) is calculated. The greenness index is expressed as the percentage of pixels for which the green-to-red ratio exceeds a given threshold. Under normal growth conditions, under salt stress growth conditions, and under reduced nutrient availability growth conditions, the greenness index of plants is measured in the last imaging before flowering. In contrast, under drought stress growth conditions, the greenness index of plants is measured in the first imaging after drought. 
     Plant 
     The term “plant” as used herein encompasses whole plants, ancestors and progeny of the plants and plant parts, including seeds, shoots, stems, leaves, roots (including tubers), flowers, and tissues and organs, wherein each of the aforementioned comprise the gene/nucleic acid sequence of interest. The term “plant” also encompasses plant cells, suspension cultures, callus tissue, embryos, meristematic regions, gametophytes, sporophytes, pollen and microspores, again wherein each of the aforementioned comprises the gene/nucleic acid sequence of interest. 
     Plants that are particularly useful in the methods of the invention include all plants which belong to the superfamily Viridiplantae, in particular monocotyledonous and dicotyledonous plants including fodder or forage legumes, ornamental plants, food crops, trees or shrubs selected from the list comprising  Acer  spp.,  Actinidia  spp.,  Abelmoschus  spp.,  Agave sisalana, Agropyron  spp.,  Agrostis stolonifera, Allium  spp.,  Amaranthus  spp.,  Ammophila arenaria, Ananas comosus, Annona  spp.,  Apium graveolens, Arachis  spp,  Artocarpus  spp.,  Asparagus officinalis, Avena  spp. (e.g.  Avena sativa, Avena fatua, Avena byzantina, Avena fatua  var.  sativa, Avena hybrida ),  Averrhoa carambola, Bambusa  sp.,  Benincasa hispida, Bertholletia excelsea, Beta vulgaris, Brassica  spp. (e.g.  Brassica napus, Brassica rapa  ssp. [canola, oilseed rape, turnip rape]),  Cadaba farinosa, Camellia sinensis, Canna indica, Cannabis sativa, Capsicum  spp.,  Carex elata, Carica papaya, Carissa macrocarpa, Carya  spp.,  Carthamus tinctorius, Castanea  spp.,  Ceiba pentandra, Cichorium endivia, Cinnamomum  spp.,  Citrullus lanatus, Citrus  spp.,  Cocos  spp.,  Coffea  spp.,  Colocasia esculenta, Cola  spp.,  Corchorus  sp.,  Coriandrum sativum, Corylus  spp.,  Crataegus  spp.,  Crocus sativus, Cucurbita  spp.,  Cucumis  spp.,  Cynara  spp.,  Daucus carota, Desmodium  spp.,  Dimocarpus longan, Dioscorea  spp.,  Diospyros  spp.,  Echinochloa  spp.,  Elaeis  (e.g.  Elaeis guineensis, Elaeis oleifera ),  Eleusine coracana, Erianthus  sp.,  Eriobotrya japonica, Eucalyptus  sp.,  Eugenia uniflora, Fagopyrum  spp.,  Fagus  spp.,  Festuca arundinacea, Ficus carica, Fortunella  spp.,  Fragaria  spp.,  Ginkgo biloba, Glycine  spp. (e.g.  Glycine max, Soja hispida  or  Soja max ),  Gossypium hirsutum, Helianthus  spp. (e.g.  Helianthus annuus ),  Hemerocallis fulva, Hibiscus  spp.,  Hordeum  spp. (e.g.  Hordeum vulgare ),  Ipomoea batatas, Juglans  spp.,  Lactuca sativa, Lathyrus  spp.,  Lens culinaris, Linum usitatissimum, Litchi chinensis, Lotus  spp.,  Luffa acutangula, Lupinus  spp.,  Luzula sylvatica, Lycopersicon  spp. (e.g.  Lycopersicon esculentum, Lycopersicon lycopersicum, Lycopersicon pyriforme ),  Macrotyloma  spp.,  Malus  spp.,  Malpighia emarginata, Mammea americana, Mangifera indica, Manihot  spp.,  Manilkara zapota, Medicago sativa, Melilotus  spp.,  Mentha  spp.,  Miscanthus sinensis, Momordica  spp.,  Morus nigra, Musa  spp.,  Nicotiana  spp.,  Olea  spp.,  Opuntia  spp.,  Ornithopus  spp.,  Oryza  spp. (e.g.  Oryza sativa, Oryza latifolia ),  Panicum miliaceum, Panicum virgatum, Passiflora edulis, Pastinaca sativa, Pennisetum  sp.,  Persea  spp.,  Petroselinum crispum, Phalaris arundinacea, Phaseolus  spp.,  Phleum pratense, Phoenix  spp.,  Phragmites australis, Physalis  spp.,  Pinus  spp.,  Pistacia vera, Pisum  spp.,  Poa  spp.,  Populus  spp.,  Prosopis  spp.,  Prunus  spp.,  Psidium  spp.,  Punica granatum, Pyrus communis, Quercus  spp.,  Raphanus sativus, Rheum rhabarbarum, Ribes  spp.,  Ricinus communis, Rubus  spp.,  Saccharum  spp.,  Salix  sp.,  Sambucus  spp.,  Secale cereale, Sesamum  spp.,  Sinapis  sp.,  Solanum  spp. (e.g.  Solanum tuberosum, Solanum integrifolium  or  Solanum lycopersicum ),  Sorghum bicolor, Spinacia  spp.,  Syzygium  spp.,  Tagetes  spp.,  Tamarindus indica, Theobroma cacao, Trifolium  spp.,  Triticale  sp.,  Triticosecale rimpaui, Triticum  spp. (e.g.  Triticum aestivum, Triticum durum, Triticum turgidum, Triticum hybernum, Triticum macha, Triticum sativum  or  Triticum vulgare ),  Tropaeolum minus, Tropaeolum majus, Vaccinium  spp.,  Vicia  spp.,  Vigna  spp.,  Viola odorata, Vitis  spp.,  Zea mays, Zizania palustris, Ziziphus  spp., amongst others. 
     DETAILED DESCRIPTION OF THE INVENTION 
     Surprisingly, it has now been found that increasing expression in a plant of a nucleic acid sequence encoding an NRT2 polypeptide as defined herein, gives plants having increased yield-related traits relative to control plants. According to a first embodiment, the present invention provides a method for increasing yield-related traits in plants relative to control plants, comprising increasing expression in a plant of a nucleic acid sequence encoding an NRT2 polypeptide. 
     A preferred method for increasing expression of a nucleic acid sequence encoding an NRT2 polypeptide is by introducing and expressing in a plant a nucleic acid sequence encoding an NRT2 polypeptide. 
     Any reference hereinafter to a “protein useful in the methods of the invention” is taken to mean a NRT2 polypeptide as defined herein. Any reference hereinafter to a “nucleic acid sequence useful in the methods of the invention” is taken to mean a nucleic acid sequence capable of encoding such an NRT2 polypeptide. The nucleic acid sequence to be introduced into a plant (and therefore useful in performing the methods of the invention) is any nucleic acid sequence encoding the type of polypeptide, which will now be described, hereafter also named “NRT2 nucleic acid sequence” or “NRT2 gene”. 
     An “NRT2 polypeptide” as defined herein refers to any polypeptide having in increasing order of preference at least 50%, 55%, 60%, 65%, 70%, 75%, 80%, 85%, 90%, 95%, 98%, 99% or more amino acid sequence identity to an NRT2 polypeptide as represented by SEQ ID NO: 2. 
     Alternatively or additionally, an “NRT2 polypeptide” as defined herein refers to any polypeptide comprising: (i) a nitrate transporter family domain with an InterPro accession IPR0004737; and/or (ii) a major facilitator superfamily domain with an InterPro accession IPR007114; and/or a major facilitator superfamily MSF-1 domain with an InterPro accession IPR011701; and (ii) at least 11 transmembrane spanning helices. 
     Alternatively or additionally, an “NRT2 polypeptide” as defined herein refers to any polypeptide sequence which when used in the construction of an NRT2 phylogenetic tree, such as the one depicted in  FIGS. 3 and 4 , clusters with the Glade of NRT2 polypeptides from diatoms rather than with any other NRT2 Glade. 
     Alternatively or additionally, an “NRT2 polypeptide” as defined herein refers to any polypeptide having in increasing order of preference at least 50%, 55%, 60%, 65%, 70%, 75%, 80%, 85%, 90%, 95%, 98%, 99% or more amino acid sequence identity to any of the polypeptide sequences given in Table A herein. 
     The term “domain” and “motif” is defined in the “definitions” section herein. Specialist databases exist for the identification of domains, for example, SMART (Schultz et al. (1998) Proc. Natl. Acad. Sci. USA 95, 5857-5864; Letunic et al. (2002) Nucleic Acids Res 30, 242-244), InterPro (Mulder et al., (2003) Nucl. Acids. Res. 31, 315-318), Prosite (Bucher and Bairoch (1994), A generalized profile syntax for biomolecular sequences motifs and its function in automatic sequence interpretation. (In) ISMB-94; Proceedings 2nd International Conference on Intelligent Systems for Molecular Biology. Altman R., Brutlag D., Karp P., Lathrop R., Searls D., Eds., pp 53-61, AAAI Press, Menlo Park; Hulo et al., Nucl. Acids. Res. 32: D134-D137, (2004)), or Pfam (Bateman et al., Nucleic Acids Research 30(1): 276-280 (2002). A set of tools for in silico analysis of protein sequences is available on the ExPASy proteomics server (Swiss Institute of Bioinformatics (Gasteiger et al., ExPASy: the proteomics server for in-depth protein knowledge and analysis, Nucleic Acids Res. 31:3784-3788 (2003)). Analysis of the polypeptide sequence of SEQ ID NO: 2 is presented below in Example 4 herein. For example, an NRT2 polypeptide as represented by SEQ ID NO: 2 comprises a nitrate transporter family domain with an InterPro accession IPR0004737, amongst others. Domains may also be identified using routine techniques, such as by sequence alignment. An alignment of the polypeptides of Tables A and A1 herein, is shown in  FIG. 5 . Such alignments are useful for identifying the most conserved domains between the NRT2 polypeptides, such as the nitrate/nitrite porter (NPP motif) or the MSF I motif. 
     Methods for the alignment of sequences for comparison are well known in the art, such methods include GAP, BESTFIT, BLAST, FASTA and TFASTA. GAP uses the algorithm of Needleman and Wunsch ((1970) J Mol Biol 48: 443-453) to find the global (i.e. spanning the complete sequences) alignment of two sequences that maximizes the number of matches and minimizes the number of gaps. The BLAST algorithm (Altschul et al. (1990) J Mol Biol 215: 403-10) calculates percent sequence identity and performs a statistical analysis of the similarity between the two sequences. The software for performing BLAST analysis is publicly available through the National Centre for Biotechnology Information (NCBI). Homologues may readily be identified using, for example, the ClustalW multiple sequence alignment algorithm (version 1.83), with the default pairwise alignment parameters, and a scoring method in percentage. Global percentages of similarity and identity may also be determined using one of the methods available in the MatGAT software package (Campanella et al., (2003) BMC Bioinformatics, 10: 29. MatGAT: an application that generates similarity/identity matrices using protein or DNA sequences). Minor manual editing may be performed to optimise alignment between conserved motifs, as would be apparent to a person skilled in the art. Furthermore, instead of using full-length sequences for the identification of homologues, specific domains may also be used. The sequence identity values may be determined over the entire nucleic acid sequence or polypeptide sequence or over selected domains or conserved motif(s), using the programs mentioned above using the default parameters. Example 3 herein describes in Table B the percentage identity between the NRT2 polypeptide as represented by SEQ ID NO: 2 and the NRT2 polypeptides listed in Tables A and A1. The percentage identity between the NRT2 polypeptide as represented by SEQ ID NO: 2 and the NRT2 polypeptides listed in Table A (diatom NRT2 polypeptides) amino acid sequence identity is of at least 60%. The percentage identity is decreased down to 25%, if the identity calculation is performed between the NRT2 polypeptide as represented by SEQ ID NO: 2 and the NRT2 polypeptides listed in Table A1 (non-diatom NRT2 polypeptides). 
     The task of protein subcellular localisation prediction is important and well studied. Knowing a protein&#39;s localisation helps elucidate its function. Experimental methods for protein localization range from immunolocalization to tagging of proteins using green fluorescent protein (GFP) or beta-glucuronidase (GUS). Such methods are accurate although labor-intensive compared with computational methods. Recently much progress has been made in computational prediction of protein localisation from sequence data. Among algorithms well known to a person skilled in the art are available at the ExPASy Proteomics tools hosted by the Swiss Institute for Bioinformatics, for example, PSort, TargetP, ChloroP, LocTree, Predotar, LipoP, MITOPROT, PATS, PTS1, SignalP, TMHMM, and others. The prediction of the subcellular localisation and topology of an NRT2 polypeptide as represented by SEQ ID NO: 2 is described in Example 5 of the present application. 
     The present invention is illustrated by transforming plants with the nucleic acid sequence represented by SEQ ID NO: 1, encoding the NRT2 polypeptide sequence of 2. However, performance of the invention is not restricted to these sequences; the methods of the invention may advantageously be performed using any nucleic acid sequence encoding an NRT2 polypeptide as defined herein. 
     Examples of nucleic acid sequences encoding NRT2 polypeptides are given in Table A of Example 1 herein. Such nucleic acid sequences are useful in performing the methods of the invention. The polypeptide sequences given in Table A of Example 1 are example sequences of orthologues and paralogues of the NRT2 polypeptide represented by SEQ ID NO: 2, the terms “orthologues” and “paralogues” being as defined herein. Further orthologues and paralogues may readily be identified by performing a so-called reciprocal blast search. Typically, this involves a first BLAST involving BLASTing a query sequence (for example using any of the sequences listed in Table A of Example 1) against any sequence database, such as the publicly available NCBI database. BLASTN or TBLASTX (using standard default values) are generally used when starting from a nucleotide sequence, and BLASTP or TBLASTN (using standard default values) when starting from a protein sequence. The BLAST results may optionally be filtered. The full-length sequences of either the filtered results or non-filtered results are then BLASTed back (second BLAST) against sequences from the organism from which the query sequence is derived (where the query sequence is SEQ ID NO: 1 or SEQ ID NO: 2, the second BLAST would therefore be against  Phaeodactylum tricornutum  sequences). The results of the first and second BLASTs are then compared. A paralogue is identified if a high-ranking hit from the first blast is from the same species as from which the query sequence is derived, a BLAST back then ideally results in the query sequence amongst the highest hits; an orthologue is identified if a high-ranking hit in the first BLAST is not from the same species as from which the query sequence is derived, and preferably results upon BLAST back in the query sequence being among the highest hits. 
     High-ranking hits are those having a low E-value. The lower the E-value, the more significant the score (or in other words the lower the chance that the hit was found by chance). Computation of the E-value is well known in the art. In addition to E-values, comparisons are also scored by percentage identity. Percentage identity refers to the number of identical nucleotides (or amino acids) between the two compared nucleic acid (or polypeptide) sequences over a particular length. In the case of large families, ClustalW may be used, followed by a neighbour joining tree, to help visualize clustering of related genes and to identify orthologues and paralogues. 
     Nucleic acid variants may also be useful in practising the methods of the invention. Examples of such variants include nucleic acid sequences encoding homologues and derivatives of any one of the polypeptide sequences given in Table A of Example 1, the terms “homologue” and “derivative” being as defined herein. Also useful in the methods of the invention are nucleic acid sequences encoding homologues and derivatives of orthologues or paralogues of any one of the polypeptide sequences given in Table A of Example 1. Homologues and derivatives useful in the methods of the present invention have substantially the same biological and functional activity as the unmodified protein from which they are derived. 
     Further nucleic acid variants useful in practising the methods of the invention include portions of nucleic acid sequences encoding NRT2 polypeptides, nucleic acid sequences hybridising to nucleic acid sequences encoding NRT2 polypeptides, splice variants of nucleic acid sequences encoding NRT2 polypeptides, allelic variants of nucleic acid sequences encoding NRT2 polypeptides and variants of nucleic acid sequences encoding NRT2 polypeptides obtained by gene shuffling. The terms hybridising sequence, splice variant, allelic variant and gene shuffling are as described herein. 
     Nucleic acid sequences encoding NRT2 polypeptides need not be full-length nucleic acid sequences, since performance of the methods of the invention does not rely on the use of full-length nucleic acid sequences. According to the present invention, there is provided a method for increasing yield-related traits, in plants, comprising introducing and expressing in a plant a portion of any one of the nucleic acid sequences given in Table A of Example 1, or a portion of a nucleic acid sequence encoding an orthologue, paralogue or homologue of any of the polypeptide sequences given in Table A of Example 1. 
     A portion of a nucleic acid sequence may be prepared, for example, by making one or more deletions to the nucleic acid sequence. The portions may be used in isolated form or they may be fused to other coding (or non-coding) sequences in order to, for example, produce a protein that combines several activities. When fused to other coding sequences, the resultant polypeptide produced upon translation may be bigger than that predicted for the protein portion. 
     Portions useful in the methods of the invention, encode an NRT2 polypeptide as defined herein, and have substantially the same biological activity as the polypeptide sequences given in Table A of Example 1. Preferably, the portion is a portion of any one of the nucleic acid sequences given in Table A of Example 1, or is a portion of a nucleic acid sequence encoding an orthologue or paralogue of any one of the polypeptide sequences given in Table A of Example 1. Preferably the portion is, in increasing order of preference at least 400, 450, 500, 550, 600, 650, 700, 750, 800, 850, 900, 950, 1000, 1050, 1100, 1150, 1200, 1250, 1300, 1350, 1400 or more consecutive nucleotides in length, the consecutive nucleotides being of any one of the nucleic acid sequences given in Table A of Example 1, or of a nucleic acid sequence encoding an orthologue or paralogue of any one of the polypeptide sequences given in Table A of Example 1. Preferably, the portion is a portion of a nucleic sequence encoding a polypeptide sequence having in increasing order of preference at least 50%, 55%, 60%, 65%, 70%, 75%, 80%, 85%, 90%, 95%, 98%, 99% or more amino acid sequence identity to an NRT2 polypeptide as represented by SEQ ID NO: 2. More preferably, the portion is a portion of the nucleic acid sequence of SEQ ID NO: 3. Most preferably, the portion is as represented by SEQ ID NO: 1. 
     Another nucleic acid sequence variant useful in the methods of the invention is a nucleic acid sequence capable of hybridising, under reduced stringency conditions, preferably under stringent conditions, with a nucleic acid sequence encoding an NRT2 polypeptide as defined herein, or with a portion as defined herein. 
     According to the present invention, there is provided a method for increasing yield-related traits in plants, comprising introducing and expressing in a plant a nucleic acid sequence capable of hybridizing to any one of the nucleic acid sequences given in Table A of Example 1, or comprising introducing and expressing in a plant a nucleic acid sequence capable of hybridising to a nucleic acid sequence encoding an orthologue, paralogue or homologue of any of the nucleic acid sequences given in Table A of Example 1. 
     Hybridising sequences useful in the methods of the invention encode an NRT2 polypeptide as defined herein, and have substantially the same biological activity as the polypeptide sequences given in Table A of Example 1. Preferably, the hybridising sequence is capable of hybridising to any one of the nucleic acid sequences given in Table A of Example 1, or to a portion of any of these sequences, a portion being as defined above, or wherein the hybridising sequence is capable of hybridising to a nucleic acid sequence encoding an orthologue or paralogue of any one of the polypeptide sequences given in Table A of Example 1. Preferably, the hybridising sequence is capable of hybridising to a nucleic acid sequence encoding a polypeptide sequence having in increasing order of preference at least 50%, 55%, 60%, 65%, 70%, 75%, 80%, 85%, 90%, 95%, 98%, 99% or more amino acid sequence identity to an NRT2 polypeptide as represented by SEQ ID NO: 2. Most preferably, the hybridising sequence is capable of hybridising to a nucleic acid sequence as represented by SEQ ID NO: 1 or to a portion thereof. 
     Another nucleic acid sequence variant useful in the methods of the invention is a splice variant encoding an NRT2 polypeptide as defined hereinabove, a splice variant being as defined herein. 
     According to the present invention, there is provided a method for increasing yield-related traits, comprising introducing and expressing in a plant a splice variant of any one of the nucleic acid sequences given in Table A of Example 1, or a splice variant of a nucleic acid sequence encoding an orthologue, paralogue or homologue of any of the polypeptide sequences given in Table A of Example 1. 
     Preferred splice variants are splice variants of a nucleic acid sequence represented by SEQ ID NO: 1, or a splice variant of a nucleic acid sequence encoding an orthologue or paralogue of SEQ ID NO: 2. Preferably, the splice variant is a splice variant of a nucleic acid sequence encoding a polypeptide sequence having in increasing order of preference at least 50%, 55%, 60%, 65%, 70%, 75%, 80%, 85%, 90%, 95%, 98%, 99% or more amino acid sequence identity to an NRT2 polypeptide as represented by SEQ ID NO: 2. 
     Another nucleic acid sequence variant useful in performing the methods of the invention is an allelic variant of a nucleic acid sequence encoding an NRT2 polypeptide as defined hereinabove, an allelic variant being as defined herein. 
     According to the present invention, there is provided a method for increasing yield-related traits, comprising introducing and expressing in a plant an allelic variant of any one of the nucleic acid sequences given in Table A of Example 1, or comprising introducing and expressing in a plant an allelic variant of a nucleic acid sequence encoding an orthologue, paralogue or homologue of any of the polypeptide sequences given in Table A of Example 1. 
     The allelic variants useful in the methods of the present invention have substantially the same biological activity as the NRT2 polypeptide of SEQ ID NO: 2 and any of the polypeptide sequences depicted in Table A of Example 1. Allelic variants exist in nature, and encompassed within the methods of the present invention is the use of these natural alleles. Preferably, the allelic variant is an allelic variant of SEQ ID NO: 1 or an allelic variant of a nucleic acid sequence encoding an orthologue or paralogue of SEQ ID NO: 2. Preferably, the allelic variant is an allelic variant of a polypeptide sequence having in increasing order of preference at least 50%, 55%, 60%, 65%, 70%, 75%, 80%, 85%, 90%, 95%, 98%, 99% or more amino acid sequence identity to an NRT2 polypeptide as represented by SEQ ID NO: 2. 
     Gene shuffling or directed evolution may also be used to generate variants of nucleic acid sequences encoding NRT2 polypeptides as defined above, the term “gene shuffling” being as defined herein. 
     According to the present invention, there is provided a method for increasing yield-related traits, comprising introducing and expressing in a plant a variant of any one of the nucleic acid sequences given in Table A of Example 1, or comprising introducing and expressing in a plant a variant of a nucleic acid sequence encoding an orthologue, paralogue or homologue of any of the polypeptide sequences given in Table A of Example 1, which variant nucleic acid sequence is obtained by gene shuffling. 
     Preferably, the variant nucleic acid sequence obtained by gene shuffling encodes a polypeptide sequence having in increasing order of preference at least 50%, 55%, 60%, 65%, 70%, 75%, 80%, 85%, 90%, 95%, 98%, 99% or more amino acid sequence identity to an NRT2 polypeptide as represented by SEQ ID NO: 2. 
     Furthermore, nucleic acid sequence variants may also be obtained by site-directed mutagenesis. Several methods are available to achieve site-directed mutagenesis, the most common being PCR based methods (Current Protocols in Molecular Biology. Wiley Eds). 
     Nucleic acid sequences encoding NRT2 polypeptides may be derived from any natural or artificial source. The nucleic acid sequence may be modified from its native form in composition and/or genomic environment through deliberate human manipulation. The nucleic acid sequence encoding an NRT2 polypeptide is from the Eukaryota domain, preferably from the Chromalveolata kingdom, further preferably from the  Heterokontophyta phylum . More preferably, the nucleic acid sequence encoding an NRT2 polypeptide is from the Bacillariophyceae (diatoms) class, and for example, from the following orders: Achnanthales, Bacillariales, Centrales (such as  Thalassiosira pseudonana ), Cymbellales, Eunotiales, Mastogloiales, Naviculales, Pennales (such as  Pheaodactylum tricornutum ), Rhopalodiales, Surirellales, or Thalassiophysales. Most preferably, the nucleic acid sequence is encoding an NRT2 polypeptide is from  Pheaodactylum tricornutum.    
     Performance of the methods of the invention gives plants having increased yield-related traits relative to control plants. The terms “yield” and “seed yield” are described in more detail in the “definitions” section herein. 
     Taking corn as an example, a yield increase may be manifested as one or more of the following: increase in the number of plants established per hectare or acre, an increase in the number of ears per plant, an increase in the number of rows, number of kernels per row, kernel weight, thousand kernel weight, ear length/diameter, increase in the seed filling rate (which is the number of filled seeds divided by the total number of seeds and multiplied by 100), among others. Taking rice as an example, a yield increase may manifest itself as an increase in one or more of the following: number of plants per hectare or acre, number of panicles per plant, number of spikelets per panicle, number of flowers (florets) per panicle (which is expressed as a ratio of the number of filled seeds over the number of primary panicles), increase in the seed filling rate (which is the number of filled seeds divided by the total number of seeds and multiplied by 100), increase in thousand kernel weight, among others. 
     The present invention provides a method for increasing yield-related traits of plants relative to control plants, which method comprises increasing expression in a plant of a nucleic acid sequence encoding an NRT2 polypeptide as defined herein. 
     Since the transgenic plants according to the present invention have increased yield-related traits, it is likely that these plants exhibit an increased growth rate (during at least part of their life cycle), relative to the growth rate of control plants at a corresponding stage in their life cycle. 
     The increased growth rate may be specific to one or more parts of a plant (including seeds), or may be throughout substantially the whole plant. Plants having an increased growth rate may have a shorter life cycle. The life cycle of a plant may be taken to mean the time needed to grow from a dry mature seed up to the stage where the plant has produced dry mature seeds, similar to the starting material. This life cycle may be influenced by factors such as early vigour, growth rate, greenness index, flowering time and speed of seed maturation. The increase in growth rate may take place at one or more stages in the life cycle of a plant or during substantially the whole plant life cycle. Increased growth rate during the early stages in the life cycle of a plant may reflect increased (early) vigour. The increase in growth rate may alter the harvest cycle of a plant allowing plants to be sown later and/or harvested sooner than would otherwise be possible (a similar effect may be obtained with earlier flowering time; delayed flowering is usually not a desirede trait in crops). If the growth rate is sufficiently increased, it may allow for the further sowing of seeds of the same plant species (for example sowing and harvesting of rice plants followed by sowing and harvesting of further rice plants all within one conventional growing period). Similarly, if the growth rate is sufficiently increased, it may allow for the further sowing of seeds of different plants species (for example the sowing and harvesting of corn plants followed by, for example, the sowing and optional harvesting of soybean, potato or any other suitable plant). Harvesting additional times from the same rootstock in the case of some crop plants may also be possible. Altering the harvest cycle of a plant may lead to an increase in annual biomass production per acre (due to an increase in the number of times (say in a year) that any particular plant may be grown and harvested). An increase in growth rate may also allow for the cultivation of transgenic plants in a wider geographical area than their wild-type counterparts, since the territorial limitations for growing a crop are often determined by adverse environmental conditions either at the time of planting (early season) or at the time of harvesting (late season). Such adverse conditions may be avoided if the harvest cycle is shortened. The growth rate may be determined by deriving various parameters from growth curves, such parameters may be: T-Mid (the time taken for plants to reach 50% of their maximal size) and T-90 (time taken for plants to reach 90% of their maximal size), amongst others. 
     According to a preferred feature of the present invention, performance of the methods of the invention gives plants having an increased growth rate relative to control plants. Therefore, according to the present invention, there is provided a method for increasing the growth rate of plants, which method comprises increasing expression in a plant of a nucleic acid sequence encoding an NRT2 polypeptide as defined herein. 
     Increased yield-related traits occur whether the plant is under non-stress conditions or whether the plant is exposed to various stresses compared to control plants grown under comparable conditions. Plants typically respond to exposure to stress by growing more slowly. In conditions of severe stress, the plant may even stop growing altogether. Mild stress on the other hand is defined herein as being any stress to which a plant is exposed which does not result in the plant ceasing to grow altogether without the capacity to resume growth. Mild stress in the sense of the invention leads to a reduction in the growth of the stressed plants of less than 40%, 35% or 30%, preferably less than 25%, 20% or 15%, more preferably less than 14%, 13%, 12%, 11% or 10% or less in comparison to the control plant under non-stress conditions. Due to advances in agricultural practices (irrigation, fertilization, pesticide treatments) severe stresses are not often encountered in cultivated crop plants. As a consequence, the compromised growth induced by mild stress is often an undesirable feature for agriculture. Mild stresses are the everyday biotic and/or abiotic (environmental) stresses to which a plant is exposed. Abiotic stresses may be due to drought or excess water, anaerobic stress, salt stress, chemical toxicity, oxidative stress and hot, cold or freezing temperatures. The abiotic stress may be an osmotic stress caused by a water stress (particularly due to drought), salt stress, oxidative stress or an ionic stress. Biotic stresses are typically those stresses caused by pathogens, such as bacteria, viruses, fungi, nematodes, and insects. The term “non-stress” conditions as used herein are those environmental conditions that allow optimal growth of plants. Persons skilled in the art are aware of normal soil conditions and climatic conditions for a given location. 
     Performance of the methods of the invention gives plants grown under non-stress conditions or under mild stress conditions having increased yield-related traits, relative to control plants grown under comparable conditions. Therefore, according to the present invention, there is provided a method for increasing yield-related traits in plants grown under non-stress conditions or under mild stress conditions, which method comprises increasing expression in a plant of a nucleic acid sequence encoding an NRT2 polypeptide. 
     Performance of the methods according to the present invention results in plants grown under abiotic stress conditions having increased yield-related traits relative to control plants grown under comparable stress conditions. As reported in Wang et al. (Planta (2003) 218: 1-14), abiotic stress leads to a series of morphological, physiological, biochemical and molecular changes that adversely affect plant growth and productivity. Drought, salinity, extreme temperatures and oxidative stress are known to be interconnected and may induce growth and cellular damage through similar mechanisms. Rabbani et al. (Plant Physiol (2003) 133: 1755-1767) describes a particularly high degree of “cross talk” between drought stress and high-salinity stress. For example, drought and/or salinisation are manifested primarily as osmotic stress, resulting in the disruption of homeostasis and ion distribution in the cell. Oxidative stress, which frequently accompanies high or low temperature, salinity or drought stress, may cause denaturing of functional and structural proteins. As a consequence, these diverse environmental stresses often activate similar cell signalling pathways and cellular responses, such as the production of stress proteins, up-regulation of anti-oxidants, accumulation of compatible solutes and growth arrest. Since diverse environmental stresses activate similar pathways, the exemplification of the present invention with drought stress should not be seen as a limitation to drought stress, but more as a screen to indicate the involvement of NRT2 polypeptides as defined above, in increasing yield-related traits relative to control plants grown in comparable stress conditions, in abiotic stresses in general. 
     The term “abiotic stress” as defined herein is taken to mean any one or more of: water stress (due to drought or excess water), anaerobic stress, salt stress, temperature stress (due to hot, cold or freezing temperatures), chemical toxicity stress and oxidative stress. According to one aspect of the invention, the abiotic stress is an osmotic stress, selected from water stress, salt stress, oxidative stress and ionic stress. Preferably, the water stress is drought stress. The term salt stress is not restricted to common salt (NaCl), but may be any stress caused by one or more of: NaCl, KCl, LiCl, MgCl 2 , CaCl 2 , amongst others. 
     Performance of the methods of the invention gives plants having increased yield-related traits, under abiotic stress conditions relative to control plants grown in comparable stress conditions. Therefore, according to the present invention, there is provided a method for increasing yield-related traits, in plants grown under abiotic stress conditions, which method comprises increasing expression in a plant of a nucleic acid sequence encoding an NRT2 polypeptide. According to one aspect of the invention, the abiotic stress is an osmotic stress, selected from one or more of the following: water stress, salt stress, oxidative stress and ionic stress. 
     Another example of abiotic environmental stress is the reduced availability of one or more nutrients that need to be assimilated by the plants for growth and development. Because of the strong influence of nutrition utilization efficiency on plant yield and product quality, a huge amount of fertilizer is poured onto fields to optimize plant growth and quality. Productivity of plants ordinarily is limited by three primary nutrients, phosphorous, potassium and nitrogen, which is usually the rate-limiting element in plant growth of these three. Therefore the major nutritional element required for plant growth is nitrogen (N). It is a constituent of numerous important compounds found in living cells, including amino acids, proteins (enzymes), nucleic acids, and chlorophyll. 1.5% to 2% of plant dry matter is nitrogen and approximately 16% of total plant protein. Thus, nitrogen availability is a major limiting factor for crop plant growth and production (Frink et al. (1999) Proc Natl Acad Sci USA 96(4): 1175-1180), and has as well a major impact on protein accumulation and amino acid composition. Therefore, of great interest are crop plants with increased yield-related traits, when grown under nitrogen-limiting conditions. 
     Performance of the methods of the invention gives plants grown under conditions of reduced nutrient availability, particularly under conditions of reduced nitrogen availability, having increased yield-related traits relative to control plants grown under comparable conditions. Therefore, according to the present invention, there is provided a method for increasing yield-related traits in plants grown under conditions of reduced nutrient availability, preferably reduced nitrogen availability, which method comprises increasing expression in a plant of a nucleic acid sequence encoding an NRT2 polypeptide. Reduced nutrient availability may result from a deficiency or excess of nutrients such as nitrogen, phosphates and other phosphorous-containing compounds, potassium, calcium, cadmium, magnesium, manganese, iron and boron, amongst others. Preferably, reduced nutrient availability is reduced nitrogen availability. 
     The present invention encompasses plants or parts thereof (including seeds) or cells thereof obtainable by the methods according to the present invention. The plants or parts thereof or cells thereof comprise a nucleic acid transgene encoding an NRT2 polypeptide as defined above. 
     The invention also provides genetic constructs and vectors to facilitate introduction and/or increased expression in plants of nucleic acid sequences encoding NRT2 polypeptides as defined herein. The gene constructs may be inserted into vectors, which may be commercially available, suitable for transforming into plants and for expression of the gene of interest in the transformed cells. The invention also provides use of a gene construct as defined herein in the methods of the invention. 
     More specifically, the present invention provides a construct comprising:
     (a) a nucleic acid sequence encoding an NRT2 polypeptide as defined above;   (b) one or more control sequences capable of increasing expression of the nucleic acid sequence of (a); and optionally   (c) a transcription termination sequence.   

     Preferably, the nucleic acid sequence encoding an NRT2 polypeptide is as defined above. The term “control sequence” and “termination sequence” are as defined herein. 
     Preferably, one of the control sequences of a construct is a constitutive promoter isolated from a plant genome. An example of a plant constitutive promoter is a GOS2 promoter, preferably a rice GOS2 promoter, more preferably a GOS2 promoter as represented by SEQ ID NO: 31. 
     Plants are transformed with a vector comprising any of the nucleic acid sequences described above. The skilled artisan is well aware of the genetic elements that must be present on the vector in order to successfully transform, select and propagate host cells containing the sequence of interest. The sequence of interest is operably linked to one or more control sequences (at least to a promoter). 
     Advantageously, any type of promoter, whether natural or synthetic, may be used to increase expression of the nucleic acid sequence. A constitutive promoter is particularly useful in the methods, preferably a constitutive promoter isolated from a plant genome. The plant constitutive promoter drives expression of a coding sequence at a level that is in all instances below that obtained under the control of a 35S CaMV viral promoter. 
     Other organ-specific promoters, for example for preferred expression in leaves, stems, tubers, meristems, seeds (embryo and/or endosperm), are useful in performing the methods of the invention. See the “Definitions” section herein for definitions of the various promoter types. 
     It should be clear that the applicability of the present invention is not restricted to a nucleic acid sequence encoding the NRT2 polypeptide, as represented by SEQ ID NO: 1 or by SEQ ID N: 3, nor is the applicability of the invention restricted to expression of an NRT2 polypeptide-encoding nucleic acid sequence when driven by a constitutive promoter. 
     Optionally, one or more terminator sequences may be used in the construct introduced into a plant. Additional regulatory elements may include transcriptional as well as translational increasers. Those skilled in the art will be aware of terminator and increaser sequences that may be suitable for use in performing the invention. An intron sequence may also be added to the 5′ untranslated region (UTR) or in the coding sequence to increase the amount of the mature message that accumulates in the cytosol, as described in the definitions section. Other control sequences (besides promoter, increaser, silencer, intron sequences, 3′UTR and/or 5′UTR regions) may be protein and/or RNA stabilizing elements. Such sequences would be known or may readily be obtained by a person skilled in the art. 
     The genetic constructs of the invention may further include an origin of replication sequence that is required for maintenance and/or replication in a specific cell type. One example is when a genetic construct is required to be maintained in a bacterial cell as an episomal genetic element (e.g. plasmid or cosmid molecule). Preferred origins of replication include, but are not limited to, the f1-ori and colE1. 
     For the detection of the successful transfer of the nucleic acid sequences as used in the methods of the invention and/or selection of transgenic plants comprising these nucleic acid sequences, it is advantageous to use marker genes (or reporter genes). Therefore, the genetic construct may optionally comprise a selectable marker gene. Selectable markers are described in more detail in the “definitions” section herein. 
     It is known that upon stable or transient integration of nucleic acid sequences into plant cells, only a minority of the cells takes up the foreign DNA and, if desired, integrates it into its genome, depending on the expression vector used and the transfection technique used. To identify and select these integrants, a gene coding for a selectable marker (such as the ones described above) is usually introduced into the host cells together with the gene of interest. These markers can for example be used in mutants in which these genes are not functional by, for example, deletion by conventional methods. Furthermore, nucleic acid sequence molecules encoding a selectable marker can be introduced into a host cell on the same vector that comprises the sequence encoding the polypeptides of the invention or used in the methods of the invention, or else in a separate vector. Cells which have been stably transfected with the introduced nucleic acid sequence can be identified for example by selection (for example, cells which have integrated the selectable marker survive whereas the other cells die). The marker genes may be removed or excised from the transgenic cell once they are no longer needed. Techniques for marker gene removal are known in the art, useful techniques are described above in the definitions section. 
     The invention also provides a method for the production of transgenic plants having increased yield-related traits relative to control plants, comprising introduction and expression in a plant of any nucleic acid sequence encoding an NRT2 polypeptide as defined hereinabove. 
     More specifically, the present invention provides a method for the production of transgenic plants having increased yield-related traits relative to control plants, which method comprises:
     (i) introducing and expressing in a plant, plant part, or plant cell a nucleic acid sequence encoding an NRT2 polypeptide; and   (ii) cultivating the plant cell, plant part or plant under conditions promoting plant growth and development.   

     The nucleic acid sequence of (i) may be any of the nucleic acid sequences capable of encoding an NRT2 polypeptide as defined herein. 
     The nucleic acid sequence may be introduced directly into a plant cell or into the plant itself (including introduction into a tissue, organ or any other part of a plant). According to a preferred feature of the present invention, the nucleic acid sequence is preferably introduced into a plant by transformation. The term “transformation” is described in more detail in the “definitions” section herein. 
     The genetically modified plant cells can be regenerated via all methods with which the skilled worker is familiar. Suitable methods can be found in the above-mentioned publications by S. D. Kung and R. Wu, Potrykus or Hofgen and Willmitzer. 
     Generally after transformation, plant cells or cell groupings are selected for the presence of one or more markers which are encoded by plant-expressible genes co-transferred with the gene of interest, following which the transformed material is regenerated into a whole plant. To select transformed plants, the plant material obtained in the transformation is, as a rule, subjected to selective conditions so that transformed plants can be distinguished from untransformed plants. For example, the seeds obtained in the above-described manner can be planted and, after an initial growing period, subjected to a suitable selection by spraying. A further possibility consists in growing the seeds, if appropriate after sterilization, on agar plates using a suitable selection agent so that only the transformed seeds can grow into plants. Alternatively, the transformed plants are screened for the presence of a selectable marker such as the ones described above. 
     Following DNA transfer and regeneration, putatively transformed plants may also be evaluated, for instance using Southern analysis, for the presence of the gene of interest, copy number and/or genomic organisation. Alternatively or additionally, expression levels of the newly introduced DNA may be monitored using Northern and/or Western analysis, both techniques being well known to persons having ordinary skill in the art. 
     The generated transformed plants may be propagated by a variety of means, such as by clonal propagation or classical breeding techniques. For example, a first generation (or T1) transformed plant may be selfed and homozygous second-generation (or T2) transformants selected, and the T2 plants may then further be propagated through classical breeding techniques. The generated transformed organisms may take a variety of forms. For example, they may be chimeras of transformed cells and non-transformed cells; clonal transformants (e.g., all cells transformed to contain the expression cassette); grafts of transformed and untransformed tissues (e.g., in plants, a transformed rootstock grafted to an untransformed scion). 
     The present invention clearly extends to any plant cell or plant produced by any of the methods described herein, and to all plant parts and propagules thereof. The present invention extends further to encompass the progeny of a primary transformed or transfected cell, tissue, organ or whole plant that has been produced by any of the aforementioned methods, the only requirement being that progeny exhibit the same genotypic and/or phenotypic characteristic(s) as those produced by the parent in the methods according to the invention. 
     The invention also includes host cells containing an isolated nucleic acid sequence encoding an NRT2 polypeptide as defined hereinabove, operably linked to a plant constitutive promoter. Preferred host cells according to the invention are plant cells. Host plants for the nucleic acid sequences or the vector used in the method according to the invention, the expression cassette or construct or vector are, in principle, advantageously all plants, which are capable of synthesizing the polypeptides used in the inventive method. 
     The methods of the invention are advantageously applicable to any plant. Plants that are particularly useful in the methods of the invention include all plants, which belong to the superfamily Viridiplantae, in particular monocotyledonous and dicotyledonous plants including fodder or forage legumes, ornamental plants, food crops, trees or shrubs. According to a preferred embodiment of the present invention, the plant is a crop plant. Examples of crop plants include soybean, sunflower, canola, alfalfa, rapeseed, cotton, tomato, potato and tobacco. Further preferably, the plant is a monocotyledonous plant. Examples of monocotyledonous plants include sugarcane. More preferably the plant is a cereal. Examples of cereals include rice, maize, wheat, barley, millet, rye, triticale, sorghum and oats. 
     The invention also extends to harvestable parts of a plant comprising an isolated nucleic acid sequence encoding an NRT2 (as defined hereinabove) operably linked to a plant constitutive promoter, such as, but not limited to seeds, leaves, fruits, flowers, stems, rhizomes, tubers and bulbs. The invention furthermore relates to products derived, preferably directly derived, from a harvestable part of such a plant, such as dry pellets or powders, oil, fat and fatty acids, starch or proteins. 
     Methods for increasing expression of nucleic acid sequences or genes, or gene products, are well documented in the art and examples are provided in the definitions section. 
     As mentioned above, a preferred method for increasing expression of a nucleic acid sequence encoding an NRT2 polypeptide is by introducing and expressing in a plant a nucleic acid sequence encoding an NRT2 polypeptide; however the effects of performing the method, i.e. increasing yield-related traits, may also be achieved using other well known techniques, including but not limited to T-DNA activation tagging, TILLING, homologous recombination. A description of these techniques is provided in the definitions section. 
     The present invention also encompasses use of nucleic acid sequences encoding NRT2 polypeptides as described herein and use of these NRT2 polypeptides in increasing any of the aforementioned yield-related traits in plants, under normal growth conditions, under abiotic stress growth (preferably osmotic stress growth conditions) conditions, and under growth conditions of reduced nutrient availability, preferably under conditions of reduced nitrogen availability. 
     Nucleic acid sequences encoding NRT2 polypeptides described herein, or the NRT2 polypeptides themselves, may find use in breeding programmes in which a DNA marker is identified that may be genetically linked to an NRT2 polypeptide-encoding gene. The genes/nucleic acid sequences, or the NRT2 polypeptides themselves may be used to define a molecular marker. This DNA or protein marker may then be used in breeding programmes to select plants having increased yield-related traits, as defined hereinabove in the methods of the invention. 
     Allelic variants of a gene/nucleic acid sequence encoding an NRT2 polypeptide may also find use in marker-assisted breeding programmes. Such breeding programmes sometimes require introduction of allelic variation by mutagenic treatment of the plants, using for example EMS mutagenesis; alternatively, the programme may start with a collection of allelic variants of so called “natural” origin caused unintentionally. Identification of allelic variants then takes place, for example, by PCR. This is followed by a step for selection of superior allelic variants of the sequence in question and which give increased yield-related traits. Selection is typically carried out by monitoring growth performance of plants containing different allelic variants of the sequence in question. Growth performance may be monitored in a greenhouse or in the field. Further optional steps include crossing plants in which the superior allelic variant was identified with another plant. This could be used, for example, to make a combination of interesting phenotypic features. 
     Nucleic acid sequences encoding NRT2 polypeptides may also be used as probes for genetically and physically mapping the genes that they are a part of, and as markers for traits linked to those genes. Such information may be useful in plant breeding in order to develop lines with desired phenotypes. Such use of nucleic acid sequences encoding an NRT2 polypeptide requires only a nucleic acid sequence of at least 15 nucleotides in length. The nucleic acid sequences encoding an NRT2 polypeptide may be used as restriction fragment length polymorphism (RFLP) markers. Southern blots (Sambrook J, Fritsch EF and Maniatis T (1989) Molecular Cloning, A Laboratory Manual) of restriction-digested plant genomic DNA may be probed with the nucleic acid sequences encoding an NRT2 polypeptide. The resulting banding patterns may then be subjected to genetic analyses using computer programs such as MapMaker (Lander et al. (1987) Genomics 1: 174-181) in order to construct a genetic map. In addition, the nucleic acid sequences may be used to probe Southern blots containing restriction endonuclease-treated genomic DNAs of a set of individuals representing parent and progeny of a defined genetic cross. Segregation of the DNA polymorphisms is noted and used to calculate the position of the nucleic acid sequence encoding an NRT2 polypeptide in the genetic map previously obtained using this population (Botstein et al. (1980) Am. J. Hum. Genet. 32: 314-331). 
     The production and use of plant gene-derived probes for use in genetic mapping is described in Bernatzky and Tanksley (1986) Plant Mol. Biol. Reporter 4: 37-41. Numerous publications describe genetic mapping of specific cDNA clones using the methodology outlined above or variations thereof. For example, F2 intercross populations, backcross populations, randomly mated populations, near isogenic lines, and other sets of individuals may be used for mapping. Such methodologies are well known to those skilled in the art. 
     The nucleic acid sequence probes may also be used for physical mapping (i.e., placement of sequences on physical maps; see Hoheisel et al. In: Non-mammalian Genomic Analysis: A Practical Guide, Academic press 1996, pp. 319-346, and references cited therein). 
     In another embodiment, the nucleic acid sequence probes may be used in direct fluorescence in situ hybridisation (FISH) mapping (Trask (1991) Trends Genet. 7:149-154). Although current methods of FISH mapping favour use of large clones (several kb to several hundred kb; see Laan et al. (1995) Genome Res. 5:13-20), improvements in sensitivity may allow performance of FISH mapping using shorter probes. 
     A variety of nucleic acid sequence amplification-based methods for genetic and physical mapping may be carried out using the nucleic acid sequences. Examples include allele-specific amplification (Kazazian (1989) J. Lab. Clin. Med 11:95-96), polymorphism of PCR-amplified fragments (CAPS; Sheffield et al. (1993) Genomics 16:325-332), allele-specific ligation (Landegren et al. (1988) Science 241:1077-1080), nucleotide extension reactions (Sokolov (1990) Nucleic acid sequence Res. 18:3671), Radiation Hybrid Mapping (Walter et al. (1997) Nat. Genet. 7:22-28) and Happy Mapping (Dear and Cook (1989) Nucleic acid sequence Res. 17:6795-6807). For these methods, the sequence of a nucleic acid sequence is used to design and produce primer pairs for use in the amplification reaction or in primer extension reactions. The design of such primers is well known to those skilled in the art. In methods employing PCR-based genetic mapping, it may be necessary to identify DNA sequence differences between the parents of the mapping cross in the region corresponding to the instant nucleic acid sequence. This, however, is generally not necessary for mapping methods. 
     The methods according to the present invention result in plants having increased yield-related traits, as described hereinbefore. These traits may also be combined with other economically advantageous traits, such as further yield-increasing traits, tolerance to abiotic and biotic stresses, tolerance to herbicides, insectides, traits modifying various architectural features and/or biochemical and/or physiological features. 
    
    
     
       DESCRIPTION OF FIGURES 
       The present invention will now be described with reference to the following figures in which: 
         FIG. 1  represents the graphical output of the algorithm TMHMM2.0 for SEQ ID NO: 2. Using this algorithm, at least 11 transmembrane spanning domains can be identified. 
         FIG. 2  represents the graphical output of the algorithm SignalP for SEQ ID NO: 2. The signal peptide is necessary to target the NRT2 polypeptide to the secretory pathway, then from there to a membrane of the cell. 
         FIG. 3  is a phylogenetic tree from Yin et al. (2007; Plant Science 172: 621-631). It shows that plant NRT2 polypeptides occur within a single group, the other major eukaryote taxa also forming robust monophyletic groups, including diatom NRT2 polypeptides. 
         FIG. 4  shows a phylogenetic tree of NRT2 polypeptides from Tables A and A1, after an AlignX (from Vector NTI 10.3, Invitrogen Corporation) multiple sequence alignment (and default values). The diatom NRT2 polypeptides represent as expected a monophyletic group, the circle representing the branching point in the tree between the polypeptides useful in performing the methods of the invention (from diatoms, Table A), and the other NRT2 polypeptides (from non-diatoms, Table A1). 
         FIG. 5  shows an AlignX (from Vector NTI 10.3, Invitrogen Corporation) multiple sequence alignment of the NRT2 polypeptides from Tables A and A1. The diatom NRT2 polypeptides are separated from the non-diatom NRT2 polypeptides by a horizontal line. The predicted transmembrane domains as predicted by TMHMM (Example 5) are indicated by X under the consensus sequence, the predicted transmemebrane domains according to Hildebrand and Dahlin (2000) J Phycol 36: 702-713) are indicated by H under the consensus sequence. Two motifs are boxed: (1) the MSF I motif (major facilitator superfamily); and (2) the NNP motif (nitrate/nitrite porter). The predicted cleavage site by SignalP (Example 5) is shown by a vertical bar. 
         FIG. 6  shows the binary vector for increased expression in  Oryza sativa  of a nucleic acid sequence encoding an NRT2 polypeptide under the control of a GOS2 promoter (pGOS2) from rice. 
         FIG. 7  details examples of sequences useful in performing the methods according to the present invention. 
     
    
    
     EXAMPLES 
     The present invention will now be described with reference to the following examples, which are by way of illustration alone. The following examples are not intended to completely define or otherwise limit the scope of the invention. 
     DNA manipulation: unless otherwise stated, recombinant DNA techniques are performed according to standard protocols described in (Sambrook (2001) Molecular Cloning: a laboratory manual, 3rd Edition Cold Spring Harbor Laboratory Press, CSH, New York) or in Volumes 1 and 2 of Ausubel et al. (1994), Current Protocols in Molecular Biology, Current Protocols. Standard materials and methods for plant molecular work are described in Plant Molecular Biology Labfax (1993) by R. D. D. Croy, published by BIOS Scientific Publications Ltd (UK) and Blackwell Scientific Publications (UK). 
     Example 1 
     Identification of Sequences Related to the Nucleic Acid Sequence Used in the Methods of the Invention 
     Sequences (full length cDNA, ESTs or genomic) related to the nucleic acid sequence used in the methods of the present invention were identified amongst those maintained in the Entrez Nucleotides database at the National Center for Biotechnology Information (NCBI) using database sequence search tools, such as the Basic Local Alignment Tool (BLAST) (Altschul et al. (1990) J. Mol. Biol. 215:403-410; and Altschul et al. (1997) Nucleic Acids Res. 25:3389-3402). The program is used to find regions of local similarity between sequences by comparing nucleic acid sequence or polypeptide sequences to sequence databases and by calculating the statistical significance of matches. For example, the polypeptide encoded by the nucleic acid sequence of the present invention was used for the TBLASTN algorithm, with default settings and the filter to ignore low complexity sequences set off. The output of the analysis was viewed by pairwise comparison, and ranked according to the probability score (E-value), where the score reflect the probability that a particular alignment occurs by chance (the lower the E-value, the more significant the hit). In addition to E-values, comparisons were also scored by percentage identity. Percentage identity refers to the number of identical nucleotides (or amino acids) between the two compared nucleic acid sequence (or polypeptide) sequences over a particular length. In some instances, the default parameters may be adjusted to modify the stringency of the search. For example the E-value may be increased to show less stringent matches. This way, short nearly exact matches may be identified. 
     Table A provides a list of nucleic acid sequences related to the nucleic acid sequence used in the methods of the present invention. 
     
       
         
           
               
             
               
                 TABLE A 
               
             
            
               
                   
               
               
                 Examples of NRT2 polypeptide sequences, and encoding nucleic acid sequences 
               
               
                 from diatoms: 
               
            
           
           
               
               
               
               
               
            
               
                   
                   
                 Public database 
                 Nucleic acid 
                 Polypeptide 
               
               
                   
                 Source 
                 accession 
                 sequence 
                 sequence 
               
               
                 Name 
                 organism 
                 number 
                 SEQ ID NO: 
                 SEQ ID NO: 
               
               
                   
               
            
           
           
               
               
               
               
               
            
               
                 Phatr_NRT2_short 
                 
                   Phaeodactylum 
                 
                 jgi_Phatr2_26029_estExt_Genewise1.C_chr_40071 
                 1 
                 2 
               
               
                   
                 
                   tricornutum 
                 
               
               
                 Phatr_NRT2_long 
                 
                   Phaeodactylum 
                 
                 jgi_Phatr2_26029_estExt_Genewise1.C_chr_40071 
                 3 
                 4 
               
               
                   
                 
                   tricornutum 
                 
               
               
                 Phatr_NRT2.2 
                 
                   Phaeodactylum 
                 
                 jgi_Phatr2_54101_estExt_Phatr1_ua_kg.C_chr_20030 
                 5 
                 6 
               
               
                   
                 
                   tricornutum 
                 
               
               
                 Cylfu_NAT1_short 
                 
                   Cylindrotheca 
                 
                 AF135038 
                 7 
                 8 
               
               
                   
                 
                   fusiformis 
                 
               
               
                 Cylfu_NAT2_short 
                 
                   Cylindrotheca 
                 
                 AF135039 
                 9 
                 10 
               
               
                   
                 
                   fusiformis 
                 
               
               
                 Thawe_NRT2_short 
                 
                   Thalassiosira 
                 
                 AY078281 
                 11 
                 12 
               
               
                   
                 
                   weissflogii 
                 
               
               
                 Thaps_NRT2 
                 
                   Thalassiosira 
                 
                 jgi_Thaps3_265650_estExt_thaps1_ua_pm.C_chr_30190 
                 13 
                 14 
               
               
                   
                 
                   pseudonana 
                 
               
               
                 Thaps_NRT2.2 
                 Thalassiosira 
                 jgi_Thaps3_28986_estExt_Genewise1.C_chr_70378 
                 15 
                 16 
               
               
                   
                 
                   pseudonana 
                 
               
               
                 Skeco_NRT2 
                 Skeletonema 
                 AY078280 
                 17 
                 18 
               
               
                   
                 
                   costatum 
                 
               
               
                   
               
            
           
         
       
     
     
       
         
           
               
             
               
                 TABLE A1 
               
             
            
               
                   
               
               
                 Examples of non-diatom NRT2 polypeptide sequences, and encoding 
               
               
                 nucleic acid sequence 
               
            
           
           
               
               
               
               
               
            
               
                   
                   
                   
                   
                 Poly- 
               
               
                   
                   
                   
                 Nucleic 
                 peptide 
               
               
                   
                   
                 Public 
                 acid 
                 se- 
               
               
                   
                   
                 database 
                 sequence 
                 quence 
               
               
                   
                   
                 accession 
                 SEQ 
                 SEQ 
               
               
                 Name 
                 Source organism 
                 number 
                 ID NO: 
                 ID NO: 
               
               
                   
               
            
           
           
               
               
               
               
               
            
               
                 ArathNRT2.4 
                 
                   Arabidopsis 
                 
                 NM_125470 
                 19 
                 20 
               
               
                   
                 
                   thaliana 
                 
               
               
                 Chlso_NRT2 
                 
                   Chlorella 
                 
                 AY026523 
                 21 
                 22 
               
               
                   
                 
                   sorokiniana 
                 
               
               
                 Cyame_NRT2 
                 
                   Cyanidioschyzon 
                 
                 AP006489 
                 23 
                 24 
               
               
                   
                 
                   merolae 
                 
               
               
                 Ostlu_NRT2 
                 
                   Ostreococcus 
                 
                 CP000590 
                 25 
                 26 
               
               
                   
                 
                   lucimarinus 
                 
               
               
                   
                 CCE9901 
               
               
                 Phypa_NRT2.2 
                 
                   Physcomitrella 
                 
                   
                 27 
                 28 
               
               
                   
                 
                   patens 
                 
               
               
                 Prupe_NRT2.1 
                 
                   Prunus persica 
                 
                 AB097402 
                 29 
                 30 
               
               
                   
               
            
           
         
       
     
     In some instances, related sequences have tentatively been assembled and publicly disclosed by research institutions, such as The Institute for Genomic Research (TIGR). The Eukaryotic Gene Orthologs (EGO) database may be used to identify such related sequences, either by keyword search or by using the BLAST algorithm with the nucleic acid sequence or polypeptide sequence of interest. On other instances, special nucleic acid sequence databases have been created for particular organisms, such as by the Joint Genome Institute, for example for  Thalassiosira pseudonana  and  Phaeodactylum tricornutum.    
     Example 2 
     Alignment of NRT2 Polypeptide Sequences 
     Multiple sequence alignment of all the NRT2 polypeptide sequences in Tables A and A1 was performed using the AlignX algorithm (from Vector NTI 10.3, Invitrogen Corporation). The diatom NRT2 polypeptides are separated from the non-diatom NRT2 polypeptides by a horizontal line. The predicted transmembrane domains as predicted by TMHMM (Example 5) are indicated by X under the consensus sequence, the predicted transmembrane domains according to Hildebrand and Dahlin (2000) J Phycol 36: 702-713) are indicated by H under the consensus sequence. Two motifs are boxed: (1) the MSF I motif (major facilitator superfamily); and (2) the NNP motif (nitrate/nitrite porter). The predicted cleavage site by SignalP (Example 5) is shown by a vertical bar. 
     Example 3 
     Calculation of Global Percentage Identity Between Polypeptide Sequences Useful in Performing the Methods of the Invention 
     Global percentages of similarity and identity between full length polypeptide sequences useful in performing the methods of the invention were determined using one of the methods available in the art, the MatGAT (Matrix Global Alignment Tool) software (BMC Bioinformatics. 2003 4:29. MatGAT: an application that generates similarity/identity matrices using protein or DNA sequences. Campanella J J, Bitincka L, Smalley J; software hosted by Ledion Bitincka). MatGAT software generates similarity/identity matrices for DNA or protein sequences without needing pre-alignment of the data. The program performs a series of pair-wise alignments using the Myers and Miller global alignment algorithm (with a gap opening penalty of 12, and a gap extension penalty of 2), calculates similarity and identity using for example Blosum 62 (for polypeptides), and then places the results in a distance matrix. Sequence similarity is shown in the bottom half of the dividing line and sequence identity is shown in the top half of the diagonal dividing line. 
     Parameters used in the comparison were:
         Scoring matrix: Blosum62   First Gap: 12   Extending gap: 2       

     Results of the software analysis are shown in Table B for the global similarity and identity over the full length of the polypeptide sequences (excluding the partial polypeptide sequences). 
     The percentage identity between an NRT2 full length polypeptide sequence as represented by SEQ ID NO: 2 and other NRT2 polypeptide sequences from diatoms, compiled in Table A, is 60% or more. The percentage identity between an NRT2 full length polypeptide sequence as represented by SEQ ID NO: 2 and NRT2 polypeptide sequences from non diatoms can as low as 26% amino acid identity. 
     
       
         
           
               
             
               
                 TABLE B 
               
             
            
               
                   
               
               
                 MatGAT results for global similarity and identity over the full length of 
               
               
                 the polypeptide sequences of Tables A (diatoms) and A1 (non diatoms). 
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
            
               
                   
                 1 
                 2 
                 3 
                 4 
                 5 
                 6 
                 7 
                 8 
                 9 
                 10 
                 11 
                 12 
                 13 
                 14 
                 15 
               
               
                   
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
            
               
                  1. Phatr_NRT2_short 
                   
                 89 
                 79 
                 72 
                 72 
                 61 
                 63 
                 62 
                 68 
                 30 
                 26 
                 32 
                 35 
                 31 
                 29 
               
               
                  2. Phatr_NRT2 
                 89 
                   
                 86 
                 65 
                 65 
                 65 
                 68 
                 67 
                 62 
                 33 
                 28 
                 34 
                 34 
                 34 
                 31 
               
               
                  3. Phatr_NRT2.2 
                 84 
                 92 
                   
                 61 
                 61 
                 63 
                 66 
                 65 
                 60 
                 33 
                 27 
                 33 
                 33 
                 35 
                 32 
               
               
                  4. Cylfu_NAT1_short 
                 83 
                 76 
                 74 
                   
                 99 
                 62 
                 60 
                 60 
                 66 
                 29 
                 26 
                 32 
                 35 
                 33 
                 28 
               
               
                  5. Cylfu_NAT2_short 
                 83 
                 76 
                 74 
                 100 
                   
                 62 
                 60 
                 60 
                 66 
                 29 
                 26 
                 32 
                 35 
                 33 
                 28 
               
               
                  6. Skeco_NRT2 
                 73 
                 80 
                 78 
                 73 
                 72 
                   
                 79 
                 79 
                 72 
                 32 
                 29 
                 35 
                 34 
                 35 
                 33 
               
               
                  7. Thaps_NRT2 
                 74 
                 81 
                 78 
                 70 
                 70 
                 88 
                   
                 94 
                 75 
                 31 
                 27 
                 33 
                 32 
                 33 
                 31 
               
               
                  8. Thaps_NRT2.2 
                 72 
                 79 
                 78 
                 69 
                 69 
                 88 
                 95 
                   
                 74 
                 32 
                 28 
                 32 
                 34 
                 34 
                 32 
               
               
                  9. Thawe_NRT2_short 
                 79 
                 74 
                 73 
                 77 
                 77 
                 81 
                 81 
                 80 
                   
                 29 
                 26 
                 31 
                 34 
                 32 
                 29 
               
               
                 10. Arath_NRT2.4 
                 41 
                 45 
                 45 
                 43 
                 42 
                 48 
                 45 
                 46 
                 42 
                   
                 40 
                 34 
                 42 
                 58 
                 81 
               
               
                 11. Chlso_NRT2 
                 37 
                 40 
                 40 
                 37 
                 37 
                 42 
                 40 
                 41 
                 37 
                 57 
                   
                 29 
                 38 
                 41 
                 41 
               
               
                 12. Cyame_NRT2 
                 44 
                 47 
                 47 
                 44 
                 44 
                 49 
                 48 
                 48 
                 44 
                 49 
                 47 
                   
                 28 
                 32 
                 32 
               
               
                 13. Ostlu_NRT2 
                 50 
                 51 
                 52 
                 49 
                 48 
                 50 
                 50 
                 51 
                 49 
                 60 
                 51 
                 43 
                   
                 44 
                 42 
               
               
                 14. Phypa_NRT2.2 
                 43 
                 49 
                 49 
                 45 
                 44 
                 51 
                 47 
                 49 
                 44 
                 75 
                 59 
                 50 
                 61 
                   
                 59 
               
               
                 15. Prupe_NRT2.1 
                 41 
                 46 
                 45 
                 41 
                 41 
                 49 
                 45 
                 45 
                 41 
                 90 
                 57 
                 47 
                 58 
                 75 
               
               
                   
               
            
           
         
       
     
     Example 4 
     Identification of Domains Comprised in Polypeptide Sequences Useful in Performing the Methods of the Invention 
     The Integrated Resource of Protein Families, Domains and Sites (InterPro) database is an integrated interface for the commonly used signature databases for text- and sequence-based searches. The InterPro database combines these databases, which use different methodologies and varying degrees of biological information about well-characterized proteins to derive protein signatures. Collaborating databases include SWISS-PROT, PROSITE, TrEMBL, PRINTS, Panther, ProDom and Pfam, Smart and TIGRFAMs. Interpro is hosted at the European Bioinformatics Institute in the United Kingdom. 
     The results of the InterPro scan of the polypeptide sequence as represented by SEQ ID NO: 2 are presented in Table C. 
     
       
         
           
               
             
               
                 TABLE C 
               
             
            
               
                   
               
               
                 InterPro scan results of the polypeptide sequence as represented 
               
               
                 by SEQ ID NO: 2 
               
            
           
           
               
               
               
               
            
               
                 InterPro accession 
                 Integrated 
                 Integrated 
                   
               
               
                 number 
                 database 
                 database 
                 Integrated database 
               
               
                 and name 
                 name 
                 accession number 
                 accession name 
               
               
                   
               
               
                 IPR0004737 
                 TIGR 
                 TIGR00886 
                 Nitrate transporter 
               
               
                 Nitrate transporter 
               
               
                 family 
               
               
                 IPR007114 
                 PS 
                 PS50850 
                 MFS (Major facilitator 
               
               
                 Major facilitator 
                   
                   
                 superfamily) 
               
               
                 superfamily 
               
               
                 IPR011701 
                 PF 
                 PF07690 
                 MFS_1 (Major 
               
               
                 Major facilitator 
                   
                   
                 facilitator superfamily) 
               
               
                 superfamily 
               
               
                 MSF-1 
               
               
                 Not integrated in 
                 Panther 
                 PTHR10074 
                 MFS Major facilitator 
               
               
                 InterPro 
                   
                   
                 superfamily member 
               
               
                 Not integrated in 
                 Panther 
                 PTHR10074:SF2 
                 Nitrate transporter 
               
               
                 InterPro 
               
               
                 Not integrated in 
                 signalp 
                   
                 Signal-peptide 
               
               
                 InterPro 
               
               
                 Not integrated in 
                 tmhmm 
                   
                 Transmembrane 
               
               
                 InterPro 
                   
                   
                 regions 
               
               
                   
               
            
           
         
       
     
     Example 5 
     Subcellular Localisation Prediction of the Polypeptide Sequences Useful in Performing the Methods of the Invention 
     Experimental methods for protein localization range from immunolocalization to tagging of proteins using green fluorescent protein (GFP) or beta-glucuronidase (GUS). For example, the  Arabidopsis thaliana  NRT2.1 polypeptide has been found to mainly localized in the plasma membrane of root cortical and epidermal cells, using a GFP-based approach combined with an immunological approach (Wirth et al. (2007) supra). 
     Computational prediction of protein localisation from sequence data was also performed. Among algorithms well known to a person skilled in the art are available at the ExPASy Proteomics tools hosted by the Swiss Institute for Bioinformatics, for example, PSort, TargetP, ChloroP, LocTree, Predotar, LipoP, MITOPROT, PATS, PTS1, SignalP, TMHMM and others. 
     A transmembrane domain usually denotes a single transmembrane alpha helix of a transmembrane protein. It is called “domain” because an alpha-helix in membrane can be folded independently on the rest of the protein. More broadly, a transmembrane domain is any three-dimensional protein structure which is thermodynamically stable in membrane. This may be a single alpha helix, a stable complex of several transmembrane alpha helices, a transmembrane beta barrel, a beta-helix of gramicidin A, or any other structure. 
     Transmembrane helices are usually about 20 amino acids in length, although they may be much longer or shorter. TMHMM2.0 is an algorithm that can predict transmembrane spanning helices in proteins. The algorithm is hosted on the server of Technical University of Denmark. Table D below shows the output of TMHMM2.0 using the polypeptide sequence information of SEQ ID NO: 2.  FIG. 1  is a graphical representation of the output as in Table D. From the prediction, at least 11 transmembrane spanning helices are identified in an NRT2 polypeptide as represented by SEQ ID NO: 2. 
     
       
         
           
               
             
               
                 TABLE D 
               
             
            
               
                   
               
               
                 Output of TMHMM2.0 using the polypeptide sequence information 
               
               
                 of SEQ ID NO: 2. 
               
            
           
           
               
               
               
            
               
                   
                   
                 Amino acid 
               
               
                   
                 Location 
                 coordinates 
               
               
                   
                   
               
               
                   
                 TMhelix 
                  7-24 
               
               
                   
                 TMhelix 
                 39-58 
               
               
                   
                 TMhelix 
                 65-87 
               
               
                   
                 TMhelix 
                  93-115 
               
               
                   
                 TMhelix 
                 122-144 
               
               
                   
                 TMhelix 
                 164-186 
               
               
                   
                 TMhelix 
                 223-245 
               
               
                   
                 TMhelix 
                 255-277 
               
               
                   
                 TMhelix 
                 298-320 
               
               
                   
                 TMhelix 
                 349-371 
               
               
                   
                 TMhelix 
                 384-406 
               
               
                   
                   
               
            
           
         
       
     
     The NRT2 polypeptide as represented by SEQ ID NO: 2 was also submitted to the SignalP algorithm (using default values; Version 2.0). SignalP predicts the presence and location of signal peptide cleavage sites in proteins, using neural networks (NN) and hidden Markov models (HMM) trained on eukaryotes. Using the NN, a predicted cleavage is found in SEQ ID NO: 2 between amino acid coordinates 26 and 27 (when counting from the N-terminus end of the polypeptide): LLS*EI, where the * is the cleavage site. The result of this analysis is also shown in  FIG. 2 . 
     Example 6 
     Assay Related to the Polypeptide Sequences Useful in Performing the Methods of the Invention 
     NRT2 polypeptides are capable of transporting nitrate across membranes. Many assays exist to measure such uptake activity, including measuring activity in heterologous expression system such  Xenopus laevis oocytes  (Zhou et al. (2000) FEBS Lett. 466: 225-227; Chopin et al. (2007) The Plant Cell 19: 1590-1602). Root influx of  15 NO 3  assays and plant mutant complementation assays have also been reported when characterizing the capacity of a given polypeptide to transport nitrate (Chopin et al. (2007) supra). A person skilled in the art is well aware of such experimental procedures to measure NRT2 activity, including NRT2 activity of an NRT2 polypeptide as represented by SEQ ID NO: 2. 
     Example 7 
     Cloning of Nucleic Acid Sequence as Represented by SEQ ID No: 1 
     Unless otherwise stated, recombinant DNA techniques are performed according to standard protocols described in (Sambrook (2001) Molecular Cloning: a laboratory manual, 3rd Edition Cold Spring Harbor Laboratory Press, CSH, New York) or in Volumes 1 and 2 of Ausubel et al. (1994), Current Protocols in Molecular Biology, Current Protocols. Standard materials and methods for plant molecular work are described in Plant Molecular Biology Labfax (1993) by R. D. D. Croy, published by BIOS Scientific Publications Ltd (UK) and Blackwell Scientific Publications (UK). 
     The  Phaeodactylum tricornutum  cDNA encoding an NRT2 polypeptide sequence as represented by SEQ ID NO: 2 was amplified by PCR using as template cDNA synthesized from mRNA extracted from  Phaeodactylum tricornutum  at different stages of multiplication, and under different growth conditions. The following primers, which include the AttB sites for Gateway recombination, were used for PCR amplification: 
     
       
         
           
               
            
               
                 1) Prm 09462 (SEQ ID NO: 32, sense): 
               
               
                 5′- ggggacaagtttgtacaaaaaagcaggcttaaacaatgcgggcctt 
               
               
                   
               
               
                 ccatt -3′ 
               
               
                   
               
               
                 2) Prm 09463 (SEQ ID NO: 33, reverse, 
               
               
                 complementary): 
               
               
                 5′- ggggaccactttgtacaagaaagctgggtttcaagctcaggcttc 
               
               
                   
               
               
                 aattt-3′ 
               
            
           
         
       
     
     PCR was performed using Hifi Taq DNA polymerase in standard conditions. A PCR fragment of the expected length (including attB sites) was amplified and purified also using standard methods. The first step of the Gateway procedure, the BP reaction, was then performed, during which the PCR fragment recombined in vivo with the pDONR201 plasmid to produce, according to the Gateway terminology, an “entry clone”. Plasmid pDONR201 was purchased from Invitrogen, as part of the Gateway® technology. 
     Example 8 
     Expression Vector Construction Using the Nucleic Acid Sequence as Represented by SEQ ID NO: 1 
     The entry clone comprising SEQ ID NO: 1 was subsequently used in an LR reaction with a destination vector used for Oryza sativa transformation. This vector contained as functional elements within the T-DNA borders: a plant selectable marker; a screenable marker expression cassette; and a Gateway cassette intended for LR in vivo recombination with the nucleic acid sequence of interest already cloned in the entry clone. A rice GOS2 promoter (SEQ ID NO: 31) for constitutive expression was located upstream of this Gateway cassette. 
     After the LR recombination step, the resulting expression vector pGOS2::NRT2 ( FIG. 6 ) was transformed into Agrobacterium strain LBA4044 according to methods well known in the art. 
     Example 9 
     Plant Transformation 
     Rice Transformation 
     The Agrobacterium containing the expression vector was used to transform Oryza sativa plants. Mature dry seeds of the rice japonica cultivar Nipponbare were dehusked. Sterilization was carried out by incubating for one minute in 70% ethanol, followed by 30 minutes in 0.2% HgCl 2 , followed by a 6 times 15 minutes wash with sterile distilled water. The sterile seeds were then germinated on a medium containing 2,4-D (callus induction medium). After incubation in the dark for four weeks, embryogenic, scutellum-derived calli were excised and propagated on the same medium. After two weeks, the calli were multiplied or propagated by subculture on the same medium for another 2 weeks. Embryogenic callus pieces were sub-cultured on fresh medium 3 days before co-cultivation (to boost cell division activity). 
     Agrobacterium strain LBA4404 containing each individual expression vector was used independently for co-cultivation. Agrobacterium was inoculated on AB medium with the appropriate antibiotics and cultured for 3 days at 28° C. The bacteria were then collected and suspended in liquid co-cultivation medium to a density (OD 600 ) of about 1. The suspension was then transferred to a Petri dish and the calli immersed in the suspension for 15 minutes. The callus tissues were then blotted dry on a filter paper and transferred to solidified, co-cultivation medium and incubated for 3 days in the dark at 25° C. Co-cultivated calli were grown on 2,4-D-containing medium for 4 weeks in the dark at 28° C. in the presence of a selection agent. During this period, rapidly growing resistant callus islands developed. After transfer of this material to a regeneration medium and incubation in the light, the embryogenic potential was released and shoots developed in the next four to five weeks. Shoots were excised from the calli and incubated for 2 to 3 weeks on an auxin-containing medium from which they were transferred to soil. Hardened shoots were grown under high humidity and short days in a greenhouse. 
     Approximately 35 independent T0 rice transformants were generated for each construct. The primary transformants were transferred from a tissue culture chamber to a greenhouse. After a quantitative PCR analysis to verify copy number of the T-DNA insert, only single copy transgenic plants that exhibit tolerance to the selection agent were kept for harvest of T1 seed. Seeds were then harvested three to five months after transplanting. The method yielded single locus transformants at a rate of over 50% (Aldemita and Hodges1996, Chan et al. 1993, Hiei et al. 1994). 
     Example 10 
     Phenotypic Evaluation Procedure 
     10.1 Evaluation Setup 
     Approximately 35 independent T0 rice transformants were generated. The primary transformants were transferred from a tissue culture chamber to a greenhouse for growing and harvest of T1 seed. Six events, of which the T1 progeny segregated 3:1 for presence/absence of the transgene, were retained. For each of these events, approximately 10 T1 seedlings containing the transgene (hetero- and homo-zygotes) and approximately 10 T1 seedlings lacking the transgene (nullizygotes) were selected by monitoring visual marker expression. The transgenic plants and the corresponding nullizygotes were grown side-by-side at random positions. Greenhouse conditions were of shorts days (12 hours light), 28° C. in the light and 22° C. in the dark, and a relative humidity of 70%. 
     Four T1 events were further evaluated in the T2 generation following the same evaluation procedure as for the T1 generation but with more individuals per event. From the stage of sowing until the stage of maturity the plants were passed several times through a digital imaging cabinet. At each time point digital images (2048×1536 pixels, 16 million colours) were taken of each plant from at least 6 different angles. 
     Reduced Nutrient (Nitrogen) Availability Screen 
     Plants from six events (T2 seeds) were grown in potting soil under normal conditions except for the nutrient solution. The pots were watered from transplantation to maturation with a specific nutrient solution containing reduced N nitrogen (N) content, usually between 7 to 8 times less. The rest of the cultivation (plant maturation, seed harvest) was the same as for plants not grown under abiotic stress. Growth and yield parameters were recorded as detailed for growth under normal conditions. 
     10.2 Statistical Analysis: F-Test 
     A two factor ANOVA (analysis of variants) was used as a statistical model for the overall evaluation of plant phenotypic characteristics. An F-test was carried out on all the parameters measured of all the plants of all the events transformed with the gene of the present invention. The F-test was carried out to check for an effect of the gene over all the transformation events and to verify for an overall effect of the gene, also known as a global gene effect. The threshold for significance for a true global gene effect was set at a 5% probability level for the F-test. A significant F-test value points to a gene effect, meaning that it is not only the mere presence or position of the gene that is causing the differences in phenotype. 
     10.3 Parameters Measured 
     Biomass-Related Parameter Measurement 
     From the stage of sowing until the stage of maturity the plants were passed several times through a digital imaging cabinet. At each time point digital images (2048×1536 pixels, 16 million colours) were taken of each plant from at least 6 different angles. 
     The plant aboveground area (or leafy biomass) was determined by counting the total number of pixels on the digital images from aboveground plant parts discriminated from the background. This value was averaged for the pictures taken on the same time point from the different angles and was converted to a physical surface value expressed in square mm by calibration. Experiments show that the aboveground plant area measured this way correlates with the biomass of plant parts above ground. The above ground area is the area measured at the time point at which the plant had reached its maximal leafy biomass. The early vigour is the plant (seedling) aboveground area three weeks post-germination. Increase in root biomass is expressed as an increase in total root biomass (measured as maximum biomass of roots observed during the lifespan of a plant); or as an increase in the root/shoot index (measured as the ratio between root mass and shoot mass in the period of active growth of root and shoot). 
     Seed-Related Parameter Measurements 
     The mature primary panicles were harvested, counted, bagged, barcode-labelled and then dried for three days in an oven at 37° C. The panicles were then threshed and all the seeds were collected and counted. The filled husks were separated from the empty ones using an air-blowing device. The empty husks were discarded and the remaining fraction was counted again. The filled husks were weighed on an analytical balance. The number of filled seeds was determined by counting the number of filled husks that remained after the separation step. The total seed weight per plant was measured by weighing all filled husks harvested from one plant. Total seed number per plant was measured by counting the number of husks harvested from a plant. Thousand Kernel Weight (TKW) is extrapolated from the number of filled seeds counted and their total weight. The Harvest Index (HI) in the present invention is defined as the ratio between the total seed weight per plant and the above ground area (mm 2 ), multiplied by a factor 10 6 . The total number of flowers per panicle as defined in the present invention is the ratio between the total number of seeds and the number of mature primary panicles. The seed fill rate as defined in the present invention is the proportion (expressed as a %) of the number of filled seeds over the total number of seeds (or florets). 
     Example 11 
     Results of the Phenotypic Evaluation of the Transgenic Rice Plants Expressing the Nucleic Acid Sequence Encoding an NRT2 Polypeptide as Represented by SEQ ID NO: 2, Grown Under Normal Growth Conditions 
     The results of the evaluation of T2 generation transgenic rice plants expressing a nucleic acid sequence encoding an NRT2 polypeptide as represented by SEQ ID NO: 2, under the control of the GOS2 promoter for constitutive expression, and grown under normal growth conditions, are presented below. 
     There was an increase in the early vigor, in the aboveground biomass, in root biomass, in the total seed yield per plant, in the number of filled seeds, and in the total number of seeds of the transgenic plants compared to corresponding nullizygotes (controls), as shown in Table E. 
     
       
         
           
               
             
               
                 TABLE E 
               
             
            
               
                   
               
               
                 Results of the evaluation of T2 generation transgenic rice plants 
               
               
                 expressing a nucleic acid sequence encoding an NRT2 
               
               
                 polypeptide as represented by SEQ ID NO: 2, under 
               
               
                 the control of the GOS2 promoter for constitutive expression. 
               
            
           
           
               
               
               
            
               
                   
                   
                 Overall average % increase in 
               
               
                   
                 Trait 
                 4 events in the T2 generation 
               
               
                   
                   
               
            
           
           
               
               
               
            
               
                   
                 Early vigor 
                 21% 
               
               
                   
                 Aboveground biomass 
                 10% 
               
               
                   
                 Root biomass 
                 6% 
               
               
                   
                 Total seed yield per plant 
                 10% 
               
               
                   
                 Number of filled seeds 
                 10% 
               
               
                   
                 Total number of seeds 
                 13% 
               
               
                   
                   
               
            
           
         
       
     
     Example 12 
     Results of the Phenotypic Evaluation of the Transgenic Rice Plants Expressing the Nucleic Acid Sequence Encoding an NRT2 Polypeptide as Represented by SEQ ID NO: 2, Grown Under Reduced Nutrient (Nitrogen) Availability Growth Conditions 
     Transgenic rice plants expressing a nucleic acid sequence encoding an NRT2 polypeptide as represented by SEQ ID NO: 2, under the control of the GOS2 promoter for constitutive expression, and grown under reduced nutrient (nitrogen) availability growth conditions, showed a positive tendency for the following yield-related traits: total seed yield per plant, number of filled seeds, total number of seeds, number of flowers per panicle, and harvest index. 
     Example 13 
     Examples of Transformation of Other Crops Corn Transformation 
     Transformation of maize ( Zea mays ) is performed with a modification of the method described by Ishida et al. (1996) Nature Biotech 14(6): 745-50. Transformation is genotype-dependent in corn and only specific genotypes are amenable to transformation and regeneration. The inbred line A188 (University of Minnesota) or hybrids with A188 as a parent are good sources of donor material for transformation, but other genotypes can be used successfully as well. Ears are harvested from corn plant approximately 11 days after pollination (DAP) when the length of the immature embryo is about 1 to 1.2 mm. Immature embryos are cocultivated with Agrobacterium tumefaciens containing the expression vector, and transgenic plants are recovered through organogenesis. Excised embryos are grown on callus induction medium, then maize regeneration medium, containing the selection agent (for example imidazolinone but various selection markers can be used). The Petri plates are incubated in the light at 25° C. for 2-3 weeks, or until shoots develop. The green shoots are transferred from each embryo to maize rooting medium and incubated at 25° C. for 2-3 weeks, until roots develop. The rooted shoots are transplanted to soil in the greenhouse. T1 seeds are produced from plants that exhibit tolerance to the selection agent and that contain a single copy of the T-DNA insert. 
     Wheat Transformation 
     Transformation of wheat is performed with the method described by Ishida et al. (1996) Nature Biotech 14(6): 745-50. The cultivar Bobwhite (available from CIMMYT, Mexico) is commonly used in transformation. Immature embryos are co-cultivated with Agrobacterium tumefaciens containing the expression vector, and transgenic plants are recovered through organogenesis. After incubation with Agrobacterium, the embryos are grown in vitro on callus induction medium, then regeneration medium, containing the selection agent (for example imidazolinone but various selection markers can be used). The Petri plates are incubated in the light at 25° C. for 2-3 weeks, or until shoots develop. The green shoots are transferred from each embryo to rooting medium and incubated at 25° C. for 2-3 weeks, until roots develop. The rooted shoots are transplanted to soil in the greenhouse. T1 seeds are produced from plants that exhibit tolerance to the selection agent and that contain a single copy of the T-DNA insert. 
     Soybean Transformation 
     Soybean is transformed according to a modification of the method described in the Texas A&amp;M patent U.S. Pat. No. 5,164,310. Several commercial soybean varieties are amenable to transformation by this method. The cultivar Jack (available from the Illinois Seed foundation) is commonly used for transformation. Soybean seeds are sterilised for in vitro sowing. The hypocotyl, the radicle and one cotyledon are excised from seven-day old young seedlings. The epicotyl and the remaining cotyledon are further grown to develop axillary nodes. These axillary nodes are excised and incubated with Agrobacterium tumefaciens containing the expression vector. After the cocultivation treatment, the explants are washed and transferred to selection media. Regenerated shoots are excised and placed on a shoot elongation medium. Shoots no longer than 1 cm are placed on rooting medium until roots develop. The rooted shoots are transplanted to soil in the greenhouse. T1 seeds are produced from plants that exhibit tolerance to the selection agent and that contain a single copy of the T-DNA insert. 
     Rapeseed/Canola Transformation 
     Cotyledonary petioles and hypocotyls of 5-6 day old young seedling are used as explants for tissue culture and transformed according to Babic et al. (1998, Plant Cell Rep 17: 183-188). The commercial cultivar Westar (Agriculture Canada) is the standard variety used for transformation, but other varieties can also be used. Canola seeds are surface-sterilized for in vitro sowing. The cotyledon petiole explants with the cotyledon attached are excised from the in vitro seedlings, and inoculated with Agrobacterium (containing the expression vector) by dipping the cut end of the petiole explant into the bacterial suspension. The explants are then cultured for 2 days on MSBAP-3 medium containing 3 mg/l BAP, 3% sucrose, 0.7% Phytagar at 23° C., 16 hr light. After two days of co-cultivation with Agrobacterium, the petiole explants are transferred to MSBAP-3 medium containing 3 mg/l BAP, cefotaxime, carbenicillin, or timentin (300 mg/l) for 7 days, and then cultured on MSBAP-3 medium with cefotaxime, carbenicillin, or timentin and selection agent until shoot regeneration. When the shoots are 5-10 mm in length, they are cut and transferred to shoot elongation medium (MSBAP-0.5, containing 0.5 mg/l BAP). Shoots of about 2 cm in length are transferred to the rooting medium (MS 0 ) for root induction. The rooted shoots are transplanted to soil in the greenhouse. T1 seeds are produced from plants that exhibit tolerance to the selection agent and that contain a single copy of the T-DNA insert. 
     Alfalfa Transformation 
     A regenerating clone of alfalfa (Medicago sativa) is transformed using the method of (McKersie et al., 1999 Plant Physiol 119: 839-847). Regeneration and transformation of alfalfa is genotype dependent and therefore a regenerating plant is required. Methods to obtain regenerating plants have been described. For example, these can be selected from the cultivar Rangelander (Agriculture Canada) or any other commercial alfalfa variety as described by Brown DCW and A Atanassov (1985. Plant Cell Tissue Organ Culture 4: 111-112). Alternatively, the RA3 variety (University of Wisconsin) has been selected for use in tissue culture (Walker et al., 1978 Am J Bot 65:654-659). Petiole explants are cocultivated with an overnight culture of  Agrobacterium tumefaciens  C58C1 pMP90 (McKersie et al., 1999 Plant Physiol 119: 839-847) or LBA4404 containing the expression vector. The explants are cocultivated for 3 d in the dark on SH induction medium containing 288 mg/L Pro, 53 mg/L thioproline, 4.35 g/L K2SO4, and 100 μm acetosyringinone. The explants are washed in half-strength Murashige-Skoog medium (Murashige and Skoog, 1962) and plated on the same SH induction medium without acetosyringinone but with a suitable selection agent and suitable antibiotic to inhibit Agrobacterium growth. After several weeks, somatic embryos are transferred to BOi2Y development medium containing no growth regulators, no antibiotics, and 50 g/L sucrose. Somatic embryos are subsequently germinated on half-strength Murashige-Skoog medium. Rooted seedlings were transplanted into pots and grown in a greenhouse. T1 seeds are produced from plants that exhibit tolerance to the selection agent and that contain a single copy of the T-DNA insert. 
     Cotton Transformation 
     Cotton ( Gossypium hirsutum  L.) transformation is performed using Agrobacterium tumefaciens, on hypocotyls explants. The commercial cultivars such as Coker 130 or Coker 312 (SeedCo, Lubbock, Tex.) are standard varieties used for transformation, but other varieties can also be used. The seeds are surface sterilized and germinated in the dark. Hypocotyl explants are cut from the germinated seedlings to lengths of about 1-1.5 centimeter. The hypotocyl explant is submersed in the Agrobacterium tumefaciens inoculum containing the expression vector, for 5 minutes then co-cultivated for about 48 hours on MS+1.8 mg/l KNO3+2% glucose at 24° C., in the dark. The explants are transferred the same medium containing appropriate bacterial and plant selectable markers (renewed several times), until embryogenic calli is seen. The calli are separated and subcultured until somatic embryos appear. Plantlets derived from the somatic embryos are matured on rooting medium until roots develop. The rooted shoots are transplanted to potting soil in the greenhouse. T1 seeds are produced from plants that exhibit tolerance to the selection agent and that contain a single copy of the T-DNA insert. 
     Example 14 
     Examples of Abiotic Stress Screens 
     Drought Screen 
     Plants from a selected number of events are grown in potting soil under normal conditions until they approached the heading stage. They are then transferred to a “dry” section where irrigation is withheld. Humidity probes are inserted in randomly chosen pots to monitor the soil water content (SWC). When SWC go below certain thresholds, the plants are automatically re-watered continuously until a normal level is reached again. The plants are then re-transferred to normal conditions. The rest of the cultivation (plant maturation, seed harvest) is the same as for plants not grown under abiotic stress conditions. Growth and yield parameters are recorded as detailed for growth under normal conditions. 
     Salt Stress Screen 
     Plants are grown on a substrate made of coco fibers and argex (3 to 1 ratio). A normal nutrient solution is used during the first two weeks after transplanting the plantlets in the greenhouse. After the first two weeks, 25 mM of salt (NaCl) is added to the nutrient solution, until the plants were harvested. Growth and yield parameters are recorded as detailed for growth under normal conditions.