Role for the Vacuolar H^+-ATPase in Regulating the Cytoplasmic pH: An in Vivo Study Carried out in chl1, an Arabidopsis thaliana Mutant Impaired in NO^-_3 Transport

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The effects of the growth in a medium containing NH_4NO_3 as nitrogen source were studied on cell sap pH, cytoplasmic pH and malate content in chl1, an Arabidopsis thaliana mutant impaired in chlorate and nitrate transport. In all the conditions tested the pH of the cytoplasm in chl1 was more alkaline, and that of the vacuole was more acidic as compared with those measured in wt. Treatment with bafilomycin A1, a specific inhibitor of the vacuolar H^+-ATPase, induced a small alkalinization of the vacuole, and a significant acidification of the cytoplasm, these effects being greater in chl1 than in wt. The greater response of the mutant to bafilomycin A1 suggests that, in the absence of the inhibitor, the activity of the tonoplast H^+-ATPase in chl1 is higher than in wt, this diversity being a possible reason for the differences in intracellular pH detected between the two strains. A possible role for the vacuolar H^+-ATPase in regulating the cytoplasmic pH is discussed.
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