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

Abstract

We are using molecular, biochemical, and genetic approaches to study the structural and regulatory genes controlling the assimilation of inorganic nitrogen into the amino acids glutamine, glutamate, aspartate and asparagine. These amino acids serve as the principal nitrogentransport amino acids in most crop and higher plants including Arabidopsis thaliana. We have begun to investigate the regulatory mechanisms controlling nitrogen assimilation into these amino acids in plants using molecular and genetic approaches in Arabidopsis. The synthesis of the amide amino acids glutamine and asparagine is subject to tight regulation in response to environmental factors such as light and to metabolic factors such as sucrose and amino acids. For instance, light induces the expression of glutamine synthetase (GLN2) and represses expression of asparagine synthetase (ASN1) genes. This reciprocal regulation of GLN2 and ASN1 genes by light is reflected at the level of transcription and at the level of glutamine and asparagine biosynthesis. Moreover, we have shown that the regulation of these genes is also reciprocally controlled by both organic nitrogen and carbon metabolites. We have recently used a reverse genetic approach to study putative components of such metabolic sensing mechanisms in plants that may be conserved in evolution. These components include an Arabidopsis homolog for a glutamate receptor gene originally 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 biology of both structural and regulatory genes of the nitrogen assimilatory pathway, we have developed a model for metabolic control of the genes involved in the nitrogen assimilatory pathway in plants. Key words Light and metabolic regulation of genes involved in nitrogen metabolismIn non-legume plants, nitrogen assimilation begins with the uptake of inorganic nitrogen, nitrate or ammonium, from the soil.Nitrate is subsequently reduced to ammonium (NH 4 + ) by the sequential action of nitrate reductase and nitrite reductase (1,2). The resulting ammonium is then assimilated into an organic form as glutamate and glutamine. These amino acids are the nitrogen donors in the biosynthesis of essentially all

Systems approach identifies an organic nitrogen-responsive gene network that is regulated by the master clock control gene CCA1

“…6A, compare Control to NmsϩMSXϩGlu and NmsϩMSXϩGln). These results indicate that the induction of ASN1 is due to Glu or a downstream metabolite, as shown previously (10,11). In addition, the control treatments showed that MSX alone does not induce expression of ASN1 or ASN2 (SI Fig.…”

Primary N-assimilation into Amino Acids in Arabidopsis

“…3B). This analysis suggests that N-assimilation into Gln is active when levels of carbon backbones are high (in the light), and is repressed when levels of Gln are high (Oliveira and Coruzzi, 1999;Oliveira et al, 2001). This negative regulation of GS expression by levels of Glu/Gln is reminiscent of the Ntr system of GS in bacteria (Magasanik, 1994).…”

Growth Platform-Dependent and -Independent Phenotypic and Metabolic Responses of Arabidopsis and Its Halophytic Relative, Eutrema salsugineum, to Salt Stress

“…Although we did not grow E. salsugineum under low-nitrogen conditions, our in vitro growth medium did contain Suc, which may have increased the carbon:nitrogen ratio in a similar manner to reducing nitrogen levels. Such changes in carbon:nitrogen ratio can have profound effects upon the reciprocal regulation of carbon and nitrogen metabolism by carbon and nitrogen signals (Oliveira et al, 2001;Price et al, 2004). For instance, Suc and Glc have been shown to repress expression of genes involved in nitrogen metabolism, including biosynthesis of amino acids (Lam et al, 1994;Melo-Oliveira et al, 1996;Lam et al, 1998;Oliveira et al, 2001;Price et al, 2004;Sahrawy et al, 2004).…”