Metabolite and light regulation of metabolism in plants: lessons from the study of a single biochemical pathway

We are using molecular, biochemical, and genetic approaches to study the st ructural and regulatory genes controlling the assimilation of inorganic nit rogen into the amino acids glutamine, glutamate, aspartate and asparagine. These amino acids serve as the principal nitrogen-transport amino acids in most crop and higher plants including Arabidopsis thaliana. We have begun t o investigate the regulatory mechanisms controlling nitrogen assimilation i nto these amino acids in plants using molecular and genetic approaches in A rabidopsis. The synthesis of the amide amino acids glutamine and asparagine is subject to tight regulation in response to environmental factors such a s light and to metabolic factors such as sucrose and amino acids. For insta nce, light induces the expression of glutamine synthetase (GLN2) and repres ses expression of asparagine synthetase (ASN1) genes, This reciprocal regul ation of GLN2 and ASN1 genes by light is reflected at the level of transcri ption and at the level of glutamine and asparagine biosynthesis. Moreover, we have shown that the regulation of these genes is also reciprocally contr olled by both organic nitrogen and carbon metabolites, We have recently use d a reverse genetic approach to study putative components of such metabolic sensing mechanisms in plants that may be conserved in evolution. These com ponents include an Arabidopsis homolog for a glutamate receptor gene origin ally found in animal systems and a plant PII gene, which is a homolog of a component of the bacterial Ntr system. Based on our observations on the bio logy of both structural and regulatory genes of the nitrogen assimilatory p athway, we have developed a model for metabolic control of the genes involv ed in the nitrogen assimilatory pathway in plants.