Plant Genome III Conference Plant Genome III Conference
Our long-term goals are to understand the regulation of nitrogen assimilation in plants. As nitrogen is a rate-limiting element in plant growth, we are ..interested in using a genetic model system to test which enzyme(s) may be rate limiting for nitrogen assimilation. Our most recent studies have focused on the regulation of genes for amide amino acid metabolism in Arabidopsis thaliana, an organism in which we may use mutants to define rate-limiting steps in nitrogen assimilation. We have shown by HPLC analysis of amino acids that the principle nitrogen transport amino acids in Arabidopsis are glutamate, glutamine, aspartate and asparagine. Accordingly, we have been studying the regulation of genes for glutamine synthetase (GS), glutamate synthase (GOGAT), glutamate dehydrogenase (GDH), aspartate aminotransferase (AspAT) and asparagine synthetase (AS). These studies have shown that phytochrome-related light signaling and metabolic control play an important roles in relating the gene expression of some of these enzymes. Our studies on AS gene expression further indicate that nitrogen assimilated into glutamine is transported as asparagine under conditions of carbon limitation or nitrogen excess. We found that each of the amide amino acid metabolic enzymes is encoded by multiple distinct genes. To define the role of each isoenzyme and the rate-limiting isoenzymes in the process of nitrogen assimilation, we have begun to identify Arabidopsis mutants with specific defects in an isoenzyme. We have also begun to alter the cell-specific expression patterns of individual enzymes in transgenic plants and test the effects on nitrogen use and plant growth. Thusfar, these studies demonstrate that the ectopic expression of particular GS genes leads to an increased growth rate in transgenic plants. These studies suggest that it should be possible to alter nitrogen use in transgenic crop plants by the ectopic expression of individual transgenes.