THE ROLE OF GLYCINE IN DETERMINING THE RATE OF GLUTATHIONE SYNTHESIS IN POPLAR - POSSIBLE IMPLICATIONS FOR GLUTATHIONE PRODUCTION DURING STRESS

The terminal step of glutathione (GSH) synthesis is the condensation o f gamma-glutamylcysteine (gamma-EC) with glycine. Relatively little in formation exists concerning the importance of photorespiratory glycine in determining the rate of conversion of gamma-EC to GSH. Consequentl y, the effect of exogenous glycine and of illumination on foliar conte nts of gamma-EC and GSH was studied in excised leaves and leaf discs f rom untransformed poplar (Populus tremula x P. alba) and poplar overex pressing gamma-glutamylcysteine synthetase (gamma-ECS; EC 6.3.2.2). Po plars strongly overexpressing gamma-ECS (ggs28) had enhanced levels of gamma-EC and GSH compared to untransformed poplars. The relationship between gamma-EC and GSH contents in ggs28 was light dependent. In ill uminated leaves, GSH contents were up to 50-fold higher than gamma-EC. On darkening, gamma-EC accumulated markedly and GSH declined, so that the GSH: gamma-EC ratio was close to 1. These dark-induced changes we re prevented by supplying glycine through the petiole or by incubation of leaf discs on glycine. Dark accumulation of B-EC in leaf discs fro m untransformed poplar was also prevented by supplying glycine. Supply ing cysteine in the dark to discs from untransformed poplar and ggs28 increased gamma-EC levels markedly but GSH levels only slightly. Subse quent illumination caused gamma-EC to decrease and GSH to increase. Su pplying glycine in concert with cysteine had similar effects to illumi nation. The data suggest that photorespiratory glycine is essential fo r GSH synthesis, especially under stress conditions, where increased a mounts of GSH are required.