CELL SPECIFIC MODULATION--CHLOROPLAST/CYTOSOL GLUTAMINE

谷氨酰胺合成酶（GS）在植物氮代谢中起核心作用。GS催化ATP依赖的氨与谷氨酸的缩合，产生谷氨酰胺。植物GS是一种八聚体，以多种同工酶形式存在，这些同工酶形式位于细胞质（GS1）或叶绿体/质体（GS2）中。叶片和茎中的GS1定位于韧皮部，产生谷氨酰胺用于氮运输，而GS2仅存在于光合细胞中，用于吸收光呼吸过程中释放的硝酸盐和NH2产生的氨。我们的初步数据表明，在日本莲中，GS1以组成性的方式过表达导致GS活性的整体增加。在所有器官、植物生长和每克鲜重蛋白质含量方面，表明氮素同化总体增加。由于光合作用通过提供同化能力和C骨架来驱动氮同化，因此氮同化的增加也伴随着光合作用的增加。我们的工作假设是，GS1在光合细胞中合成时具有同化光呼吸产生的NH3的功能，并且在细胞质中比在叶绿体中更有效地再同化光呼吸NH3。在这里，我们建议通过比较过表达Gs1的植物的氮同化能力来验证我们的假设，或者通过比较过表达Gs1或GS2的植物在光合细胞中的氮同化能力来验证我们的假设。第二个目的是测试GS活性的增加是否伴随着氮和碳代谢中其他关键酶合成的增加。我们的长期目标是通过控制初级代谢来提高作物产量，而拟议的研究是为了增加我们对氮和碳同化如何协调的理解。

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.