MANIPULATION OF GLUTATHIONE AND AMINO-ACID BIOSYNTHESIS IN THE CHLOROPLAST

Poplars (Populus tremula x Populus alba) were transformed to overexpre ss Escherichia coli gamma-glutamylcysteine synthetase (gamma-ECS) or g lutathione synthetase in the chloroplast. Five independent lines of ea ch transformant strongly expressed the introduced gene and possessed m arkedly enhanced activity of the gene product. Glutathione (GSH) conte nts were unaffected by high chloroplastic glutathione synthetase activ ity. Enhanced chloroplastic gamma-ECS activity markedly increased gamm a-glutamylcysteine and GSH levels. These effects are similar to those previously observed in poplars overexpressing these enzymes in the cyt osol. Similar to cytosolic gamma-ECS overexpression, chloroplastic ove rexpression did not deplete foliar cysteine or methionine pools and di d not lead to morphological changes. Light was required for maximal ac cumulation of GSH in poplars overexpressing gamma-ECS in the chloropla st. High chloroplastic, but not cytosolic, gamma-ECS activities were a ccompanied by increases in amino acids synthesized in the chloroplast. We conclude that (a) GSH synthesis can occur in the chloroplast and t he cytosol and may be up-regulated in both compartments by increased g amma-ECS activity, (b) interactions between GSH synthesis and the path ways supplying the necessary substrates are similar in both compartmen ts, and (c) chloroplastic up-regulation of GSH synthesis is associated with an activating effect on the synthesis of specific amino acids fo rmed in the chloroplast.