Protection from Paraquat‐Mediated Photo‐Oxidative Stress by Glutathione in Poplar (Populus tremula×P. alba) Plants

Abstract: The sensitivity of hybrid poplar (Populus tremula×P. alba) to oxidative stress mediated by paraquat exposure was analysed with leaf discs from wild‐type plants and plants expressing the bacterial cDNA of the enzymes of glutathione synthesis, namely gshI, encoding γ‐glutamylcysteine synthetase (ECS), or gshII, encoding glutathione synthetase (GS), both in the cytosol. It was expected that leaf discs containing more than 2‐fold elevated glutathione concentrations due to over‐expression of ECS are less susceptible to paraquat exposure than wild‐type plants and transformants over‐expressing GS. However, neither over‐expression of GS nor of ECS improved paraquat tolerance of the leaves. This result was surprising, because in wild‐type plants reduced paraquat sensitivity of young compared with mature leaves coincided with ca. 30 % higher glutathione contents of the young leaves. Apparently, developmental changes in paraquat sensitivity of poplar leaves are controlled by factors different from glutathione contents. Feeding experiments with glutathione and its metabolic precursor γ‐glutamylcysteine (EC) plus gly showed that glutathione can provide protection from paraquat‐mediated photo‐oxidative stress; but at least ca. 5‐fold elevated glutathione levels are required for this effect in poplar leaves. Currently, such high glutathione levels have not been achieved by the application of plant molecular biology techniques. The significance of glutathione for the compensation of photo‐oxidative stress is discussed.