Regulation of glutathione synthesis in leaves of transgenic poplar (Populus tremula X P. alba) overexpressing glutathione synthetase

The poplar hybrid Populus 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 γ‐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 γ‐glutamylcysteine, and (iii) regulation of the activities of both γ‐glutamylcysteine synthetase and glutathione synthetase. However step (ii) may set an upper limit for the cellular glutathione content.