Submitted to: Book Chapter
Publication Type: Review Article
Publication Acceptance Date: March 21, 2000
Publication Date: N/A
Technical Abstract: The assimilation of symbiotically derived ammonia into amides in root nodules of temperate legumes occurs through the concerted action of four enzymes: glutamine synthetase (GS) and glutamate synthase (GOGAT) are the initial enzymes that convert ammonia into glutamine and glutamate. Subsequent synthesis of fixed nitrogen into aspartate and asparagine is catalyzed by aspartate aminotransferase (AAT) and asparagine synthetase (AS). These reactions, juxtaposed with phosphoenolpyruvate carboxylase (PEPC) and malate dehydrogenase (MDH), interconnect nitrogen and carbon metabolism in a complex framework that channels resources equitably to both symbiotic partners. Due to the fact that nodule enhanced forms of GOGAT, AAT, and MDH are localized in amyloplasts, this organelle in nodules must play a pivotal role in channeling of nitrogen and carbon. Nodule amyloplasts are important sites for the synthesis of aspartate, glutamate, alpha-ketoglutarate, and oxaloacetate. Glutamine and asparagine synthesis occurs in the cytosol. Two cycles linked through GOGAT can explain how amino acids of primary nitrogen assimilation are channeled in amide transporting legumes. Glutamate synthase (GOGAT) is a good candidate as the control point for nodule assimilation of nitrogen into amides. It is the common link in the nitrogen channeling cycles, occurs in low abundance as a single isoform, requires effective nitrogen fixation for maximum transcription, and results in impaired nitrogen assimilation when reduced in expression.