Responses to cadmium in leaves of transformed poplars overexpressing γ-glutamylcysteine synthetase

Poplars overexpressing a bacterial gamma-glutamylcysteine synthetase (gamma-ECS) in the cytosol (lines ggs11 and ggs28) had a 30-fold increase in foliar gamma-ECS activity relative to untransformed controls. Foliar gamma-glutamylcysteine (gamma-EC) was increased by 10-fold while foliar glutathione accumulation increased by up to 3.5-fold in the leaves of the transformants, Untransformed and transformed poplars were grown with different soil concentrations of cadmium (0-1100 mu g g(-1) soil) for 2 weeks. Cadmium accumulated in the leaves of both transformed and untransformed poplars and growth was inhibited. Growth inhibition and foliar cadmium accumulation were greatest at the highest soil cadmium concentrations in all lines. Exposure to cadmium enhanced the foliar cysteine, gamma-EC and glutathione pools in all lines but less glutathione was present in the leaves of the untransformed controls than the transformants under all growth conditions. Cadmium-induced changes in the activities of malic enzyme, isocitrate dehydrogenase and guaiacol peroxidase were less pronounced in the leaves of the transformed poplars overexpressing gamma-ECS than in the untransformed controls. Glutamate dehydrogenase and glutathione reductase activities were unchanged by exposure to cadmium. We conclude that overexpression of gamma-ECS activity and foliar glutathione accumulation in transformed poplar allows greater tissue cadmium accumulation but has only a marginal effect on cadmium tolerance in poplar.