Comparison of heavy metal effect on the proton pumps of plasma membrane and tonoplast in cucumber root cells

The effects of 10 μM cadmium, copper and nickel on the activities of vacuolar membrane and plasma membrane (PM) ATP-dependent proton pumps was investigated in Cucumis sativus L. root cells. It was demonstrated that vacuolar H+-ATPase (EC 3.6.3.14) and PM H+-ATPase (EC 3.6.3.6) differed in sensitivity to heavy metals. Exposure of cucumber seedlings to Cd, Cu and Ni had no significant effect on the activity of the vacuolar proton pump and, in the case of Ni, also on the activity of the PM proton pump. In contrast, Cd and Cu ions diminished both ATP hydrolysis and proton transport in plasma membranes. Transcript levels of genes encoding PM enzyme as well as the subunit A of tonoplast enzyme in roots stressed with heavy metals were similar to the control. Cd, Cu and Ni were accumulated in cucumber roots with similar efficiency, but their relative distribution between the symplast and apoplast differed. To explain the mechanism of heavy metal action on the plasma membranes of cucumber roots, the MDA content, as a lipid peroxidation product, and fatty acid composition were analyzed. It was shown that exposure of plants to Cd, Cu and Ni did not enhance the lipid peroxidation in the PM fraction. However, all metals caused an increase in the saturation of PM fatty acids and a decrease in the length of the fatty acid chain.