Chloroplast envelope K + /H + antiporters are involved in cytosol pH regulation.

KEA1 and KEA2 are K + /H + antiporters in the chloroplast inner envelope that adjust stromal pH in light to dark transitions. We previously determined that stromal pH is higher in the kea1kea2 mutant cells. Given that cytosol and chloroplast volume are about equal in mesophyll cells, we wanted to test if this in turn could cause ionic imbalances in the cytosol as well. In this study we indeed find that the cytosol is more acidic in the kea1kea2 mutant. As cytosol pH is tightly controlled by the activity of the plasma membrane proton ATPase, we tested the activity of this enzyme. Acidic pH is expected to phosphorylate and activate the proton ATPase to restore pH. We could however not detect differences in plasma membrane H + -ATPase hydrolytic activity between Col-0 and the kea1kea2 mutant. No differences in the amount of plasma membrane H + -ATPase enzyme, regulatory 14-3-3 proteins and phosphorylation of H + -ATPase Thr932 between Col-0 and the kea1kea2 mutant were detected, indicating that cytosol acidification did not induce transcriptional regulation or post-translational phosphorylation of plasma membrane H + -ATPase in the kea1kea2 mutant. Curiously however, plasma membrane in the double mutant was depolarized, indicating a reduced electrogenic transport of protons by the ATPase, or an increased conductance to other ions. We determined that reconstituted membrane vesicles isolated from kea1kea2 mutants have slightly lower ATP dependent proton transport activity and appear especially more permeant to potassium, but also show increased proton conductance.