Decrease of pH Gradients in Tonoplast Vesicles by NO(3) and Cl: Evidence for H-Coupled Anion Transport

Chloride or nitrate decreased a pH gradient (measured as [(14)C]methylamine accumulation) in tonoplast-enriched vesicles. The DeltapH decrease was dependent on the anion concentration. These effects are independent of the anion-sensitive H(+)-ATPase of the tonoplast, since the pH gradient (acid inside) was imposed artificially using a pH jump or a K(+) gradient and nigericin. 4,4'-Diisothiocyano-2,2'-stilbene disulfonic acid partially prevented the decrease in pH gradient induced by Cl(-). Two possible models to account for this anion-dependent decrease of DeltapH are: (a) H(+) loss is accompanied by Cl(-) or NO(3) (-) efflux from the vesicles via H(+)/anion symport systems on the tonoplast and (b) H(+) loss is accompanied by Cl(-) or NO(3) (-) uptake into the vesicles via H(+)/anion antiport systems. Depending on the requirements and conditions of the cell, these two systems would serve to either mobilize Cl(-) and NO(3) (-) stored in the vacuole for use in the cytoplasm or to drive anions into the vacuole. Chloride or nitrate also decreased a pH gradient in fractions containing plasma membrane and Golgi, implying that these membranes may have similar H(+)-coupled anion transport systems.