Salt modulation of vacuolar H +-ATPase and H +-Pyrophosphatase activities in Vigna unguiculata

Salt modulation of the tonoplast H +-pumping V-ATPase and H +-PPase was evaluated in hypocotyls of Vigna unguiculata seedlings after 3 and 7 days of treatment. In 3-day-old seedlings, treatment with 100 mmol/L NaCl decreased the proton transport and hydrolytic activities of both the V-ATPase and the H +-PPase. After 7 days, the proton transport and hydrolysis activities of the V-ATPase were higher, while the H +-PPase activities were lower in seedlings. Western blot analysis of A- and B-subunits of V-ATPase revealed that the protein content of the two subunits varied in parallel with their activities, i.e. to a higher activity corresponded a higher protein content of the subunits and vice versa. Contrarily, Western blot analysis of H +-PPase levels failed to show any correlation with PPase activity, suggesting a partial enzyme inactivation. The results indicate that salt stress induces V-ATPase expression in V. unguiculata with concomitant enhancement of its activity as a homeostatic mechanism to cope with salt stress. Under the same conditions PPase is inhibited.