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 (H 2 S) and hydrogen peroxide (H 2 O 2 ) 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 H 2 S and H 2 O 2 content and endogenous production were determined in roots of plants treated with 100 μM CdCl 2 and different inhibitors or scavengers. It was found that H 2 S donor improved photosynthetic parameters in Cd‐stressed cucumber seedlings. Cd‐induced stimulation of H 2 S level was correlated with the increased activities of the H 2 S‐generating desulfhydrases. Increased H 2 O 2 and lowered H 2 S contents in roots were able to reduce V‐ATPase activities similar to Cd. H 2 O 2 and H 2 S‐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 H 2 O 2 raised H 2 S content in root tissues independently from the desulfhydrase activity. Although treatment of control plants with H 2 S significantly stimulated NADPH oxidase activity and gene expression, H 2 S did not affect H 2 O 2 accumulation in roots exposed to Cd. The results suggest the existence of two pathways of H 2 S 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 H 2 O 2 /NADPH oxidase, may protect V‐ATPase from inhibition by Cd.