Abscisic acid induction of vacuolar H+-ATPase activity in Mesembryanthemumcrystallinum 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- te mperature stress. In this study a role for ABA in mediating the NaCl-stress -induced increases in tonoplast H+-translocating ATPase (V-ATPase) and Na+/ H+ antiport activity in Mesembryanthemum crystallinum, leading to vacuolar Na+ sequestration, were investigated. NaCl or ABA treatment of adult M. cry stallinum plants induced V-ATPase H+ transport activity, and when applied i n combination, an additive effect on V-ATPase stimulation was observed. In contrast, treatment of juvenile plants with ABA did not induce V-ATPase act ivity, whereas NaCl treatment resulted in a similar response to that observ ed in adult plants. Na+/H+ antiport activity was induced in both juvenile a nd adult plants by NaCl, but ABA had no effect at either developmental stag e. Results indicate that ABA-induced changes in V-ATPase activity are depen dent on the plant reaching its adult phase, whereas NaCl-induced increases in V-ATPase and Na+/H+ antiport activity are independent of plant age. This suggests that ABA-induced V-ATPase activity may be linked to the stress-in duced, developmentally programmed switch from C-3 metabolism to Crassulacea n acid metabolism in adult plants, whereas, vacuolar Na+ sequestration, med iated by the V-ATPase and Na+/H+ antiport, is regulated through ABA-indepen dent pathways.