REGULATION OF GLUTATHIONE SYNTHESIS IN LEAVES OF TRANSGENIC POPLAR (POPULUS-TREMULA X POPULUS-ALBA) OVEREXPRESSING GLUTATHIONE SYNTHETASE

The poplar hybrid Popolus tremula x P. alba was transformed with the Escherichia coli gene for glutathione synthetase (gsh II) targetted to the cytosol. Leaves of five lines of transgenic plants exhibited glutathione synthetase activities 15- to 60-fold higher than those of wild-type plants. Total glutathione levels and GSH/GSSG ratios were similar in transgenic and wild-type plants. Precursor feeding experiments with cysteine and gamma-glutamylcysteine suggest that glutathione synthesis in the cytoplasm is controlled by a multistep procedure that includes (i) the availability of cysteine, (ii) the availability of gamma-glutamylcysteine, and (iii) regulation of the activities of both gamma-glutamylcysteine synthetase and glutathione synthetase. However step (ii) may set an upper limit for the cellular glutathione content.