Glutamine synthetase (GS) plays a central role in nitrogen metabolism in plants. GS catalyzes the ATP dependent condensation of ammonia with glutamate, to yield glutamine. Plant GS is an octamer and occurs as a number of isoenzyme forms and these GS isoforms are located wither in the cytosol (GS1) or chloroplast/plastid (GS2). The GS1 in leaves and stem is localized in the phloem and functions to generate glutamine for nitrogen transport while GS2 is found only in the photosynthetic cells and serves to assimilate ammonia produced by the production of nitrate and NH2 released during photorespiration. Our preliminary data shows that over-expression of GS1 in a constitutive manner in Lotus japonicus results in an overall increase in GS activity. In all organs, plant growth, and protein content per gram fresh weight, suggesting an overall increase in nitrogen assimilation. Since photosynthesis drives nitrogen assimilation by the provision of assimilatory power and the C skeletons, it also follows that increased nitrogen assimilation is accompanied by increased photosynthesis. Our working hypothesis is that GS1 when synthesized in the photosynthetic cell functions in assimilating the NH3 produced by photorespiration and that is more efficient to re-assimilate photorespiratory NH3 in the cytoplasm than in the chloroplast. Here we propose to test our hypothesis by comparing the N assimilating capabilities of plants over-expressing either Gs1 or to test our hypothesis by comparing the N assimilating capabilities of plants over-expressing either GS1 or GS2 in the photosynthetic cells. A second objective is to test if increased GS activity is accompanied by an increase in the synthesis of other key enzymes in N and C metabolism. Our long term goal is to improve crop productivity by manipulating primary metabolism and the proposed research is to add to our understanding of how N and C assimilation are coordinated.
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