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 of gamma-glutamylcysteine (gamma-EC) with glycine. Relatively little information exists concerning the importance of photorespiratory glycine in determining the rate of conversion of gamma-EC to GSH. Consequently, the effect of exogenous glycine and of illumination on foliar contents of gamma-EC and GSH was studied in excised leaves and leaf discs from untransformed poplar (Populus tremula x P. alba) and poplar overexpressing gamma-glutamylcysteine synthetase (gamma-ECS; EC 6.3.2.2). Poplars 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 illuminated 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 were prevented by supplying glycine through the petiole or by incubation of leaf discs on glycine. Dark accumulation of B-EC in leaf discs from untransformed poplar was also prevented by supplying glycine. Supplying cysteine in the dark to discs from untransformed poplar and ggs28 increased gamma-EC levels markedly but GSH levels only slightly. Subsequent illumination caused gamma-EC to decrease and GSH to increase. Supplying glycine in concert with cysteine had similar effects to illumination. The data suggest that photorespiratory glycine is essential for GSH synthesis, especially under stress conditions, where increased amounts of GSH are required.