Interaction between the signaling molecules hydrogen sulfide and hydrogen peroxide and their role in vacuolar H+-ATPase regulation in cadmium-stressed cucumber roots

Vacuolar H+-ATPase (V-ATPase; EC 3.6.3.14) is the main enzyme responsible for generating a proton gradient across the tonoplast. Under cadmium (Cd) stress conditions, V-ATPase activity is inhibited. In the present work, hydrogen sulfide (H2S) and hydrogen peroxide (H2O2) cross-talk was analyzed in cucumber (Cucumis sativus L.) seedlings exposed to Cd to explain the role of both signaling molecules in the control of V-ATPase. V-ATPase activity and gene expression as well as H2S and H2O2 content and endogenous production were determined in roots of plants treated with 100 mu M CdCl2 and different inhibitors or scavengers. It was found that H2S donor improved photosynthetic parameters in Cd-stressed cucumber seedlings. Cd-induced stimulation of H2S level was correlated with the increased activities of the H2S-generating desulfhydrases. Increased H2O2 and lowered H2S contents in roots were able to reduce V-ATPase activities similar to Cd. H2O2 and H2S-induced modulations in V-ATPase activities were not closely related to the transcript level of encoding genes, suggesting posttranslational modifications of enzyme protein. On the other hand, exogenous H2O2 raised H2S content in root tissues independently from the desulfhydrase activity. Although treatment of control plants with H2S significantly stimulated NADPH oxidase activity and gene expression, H2S did not affect H2O2 accumulation in roots exposed to Cd. The results suggest the existence of two pathways of H2S generation in Cd-stressed cucumber roots. One involves desulfhydrase activity, as was previously demonstrated in different plant species. The other, the desulfhydrase-independent pathway induced by H2O2/NADPH oxidase, may protect V-ATPase from inhibition by Cd.