In vivo treatments that modulate PP_i-dependent H⁺ transport activity of tonoplast-enriched membrane vesicles from barley roots

The PP_i-dependent H⁺ transport activity of tonoplast-enriched membrane vesicles prepared from barley roots was greatly reduced when the plants were grown for 4 or 5 days with an additional 3 raM KC1 in growth medium that contained only 0.1 mM CaCl₂ in water. To characterize the mechanism of this reduction in activity, we attempted to treat barley roots with K⁺ ions, Cl^-ions (or acetate), and A23187 (with or without Ca²⁺ ions), which might be expected to cause alkalization, acidification and mobilization of Ca²⁺ ions in the cytoplasm, respectively. One-day treatment of barley roots with K⁺ ions significantly decreased PP_i--dependent H⁺ transport activity of prepared tonoplast-enriched membrane vesicles, while treatment with Cl^- ions or acetate significantly increased the activity. A similar increase in the activity also occurred by treatment with Ca²⁺ ions alone or in combination with A23187. Determination of the PP_i-hydrolyzing activity of membrane vesicles showed that changes in this activity by the various treatments were similar to those in the PP_i-dependent H⁺ transport activity. The changes in ATP-dependent H⁺ transport activity of membrane vesicles caused by these treatments were small. These results indicate that the in vivo treatments had significant effects on the H⁺ transport activity of H⁺-PP_i-ase, one of the two active vacuolar H⁺-pumps (H⁺-PP_iase and H⁺-ATPase). In addition, these results suggest the possibility that changes in levels of cytoplasmic H⁺ or Ca²⁺ ions may be involved in modulation of the H⁺ transport activity of the vacuolar H⁺-PP_iase during plant growth.