Abscisic acid Induction of vacuolar H1-ATPase activity inmesembryanthemum crystallinum is developmentally regulated

Abscisic acid (ABA) has been implicated as a key component in water-deficit-induced responses, including those triggered by drought, NaCl, and low- temperature stress. In this study a role for ABA in mediating the NaCl-stress-induced increases in tonoplast H1-translocating ATPase (V-ATPase) and Na1/H1 antiport activity in Mesembryanthemum crystallinum, leading to vacuolar Na1 sequestration, were investigated. NaCl or ABA treatment of adult M. crystallinum plants induced V-ATPase H1 transport activity, and when applied in combination, an additive effect on V-ATPase stimulation was observed. In contrast, treatment of juvenile plants with ABA did not induce V-ATPase activity, whereas NaCl treatment resulted in a similar response to that observed in adult plants. Na1/H1 antiport activity was induced in both juvenile and adult plants by NaCl, but ABA had no effect at either developmental stage. Results indicate that ABA-induced changes in V-ATPase activity are dependent on the plant reaching its adult phase, whereas NaCl-induced increases in V-ATPase and Na1/H1 antiport activity are independent of plant age. This suggests that ABA-induced V-ATPase activity may be linked to the stress-induced, developmentally programmed switch from C3 metabolism to Crassulacean acid metabolism in adult plants, whereas, vacuolar Na1 sequestration, mediated by the V-ATPase and Na1//H1 antiport, is regulated through ABA-independent pathways.