Protection from paraquat-mediated photo-oxidative stress by glutathione inpoplar (Populus tremula x P-alba) plants

The sensitivity of hybrid poplar (Populus tremula x P. alba) to oxidative s tress mediated by paraquat exposure was analysed with leaf discs from wild- type plants and plants expressing the bacterial cDNA of the enzymes of glut athione synthesis, namely gshI, encoding gamma -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 glut athione concentrations due to over-expression of ECS are less susceptible t o paraquat exposure than wildtype plants and transformants over-expressing GS. However, neither over-expression of GS nor of ECS improved paraquat tol erance of the leaves. This result was surprising, because in wildtype plant s reduced paraquat sensitivity of young compared with mature leaves coincid ed with co. 30% higher glutathione contents of the young leaves. Apparently , developmental changes in paraquat sensitivity of poplar leaves are contro lled by factors different from glutathione contents. Feeding experiments wi th glutathione and its metabolic precursor gamma -glutamylcysteine (EC) plu s gly showed that glutathione can provide protection from paraquat-mediated photo-oxidative stress; but at least co. 5-fold elevated glutathione level s are required for this effect in poplar leaves. Currently, such high gluta thione 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.