Session 7: Metabolic Regulation
Chair: Gloria Coruzzi, New York University, USA
email: coruzg01@mcrcr6.med.nyu.edu
The ability of metabolites to regulate metabolic and developmental processes is well documented in microorganisms. Both carbon and nitrogen metabolites have also been shown to regulate growth/development and the expression of numerous genes in plants.Understanding the mechanisms by which metabolic sensing and signalling occurs in plants, has both evolutionary and practical implications. Session 7, entitled Metabolic regulation highlighted research on metabolic regulation in Arabidopsis, focusing on gene regulation and the identification of components of the metabolic sensing/signal transduction pathway using mutant and gene identification approaches.Gloria Coruzzi showed that carbon and nitrogen metabolites act in a reciprocal fashion to regulate the expression of a set of nitrogen assimilatory genes in Arabidopsis. She described an Arabidopsis homologue of PII, a component of a carbon:nitrogen-sensing/signalling system in bacteria.
Jen Sheen's studies concern the role of hexokinase, a known sugar sensor in yeast and mammals, as a sugar sensor in plants using a transgenic antisense approach. She has also used a mutant approach to isolate gin (glucose insensitive) mutants that define components downstream of the sugar sensor in the glucose signal transduction pathway and reveal cross-talk between sugar and plant hormone (e.g. ethylene) signalling.
Ian Graham presented work on their carbohydrate insensitive (cai) mutants isolated by their ability to form green cotyledons under high sucrose low nitrate conditions. Several of these mutants also show increased resistance to mannose which halts wild-type seedling growth apparently due to sequestration of Pi. In one such mutant, cai10, mannose resistance appears to be due to an increased flux of carbon out of the hexose phosphate pool, thus highlighting the possibility that mutants altered in carbohydrate metabolism may give a sugar sensing phenotype even though they are not necessarily disrupted in signal transduction.
David Lightfoot presented a gsr1 (glutamine sensing regulatory) mutant selected on media containing high concentrations of methylamine and glutamine. The gsr1 mutant showed a complex pattern of aberrant regulation of nitrogen assimilation and chloroplast function. The failure of the mRNA abundance of GS2, AS1, NR1 and GDH1 to respond to exogenous glutamine treatments in gsr1 suggest the mutant phenotype reflects the pleiotrophic effects of the failure to sense glutamine concentration within the cell.
Eric Ward's presentation focused onadenylosuccinate synthetase, an enzyme in de novo purine biosynthesis, the target for hydantocidin, a natural product with potent herbicidal activity. Based on the crystal structure of the enzyme/inhibitor complex, a site-directed mutation was made that decreases sensitivity to inhibitor by almost 100-fold. Preliminary results indicate that expression of the mutant gene in Arabidopsis results in tolerance to hydantocidin.