Exchangers man the pumps: functional interplay between proton pumps and proton-coupled Ca2+ exchangers

Tonoplast-localized proton-coupled Ca²⁺ transporters encoded by cation/H⁺ exchanger (CAX) genes play a critical role in sequestering Ca²⁺ into the vacuole. These transporters may function in coordination with Ca²⁺ release channels, to shape stimulus-induced cytosolic Ca²⁺ elevations. Recent analysis of Arabidopsis CAX knockout mutants, particularly cax1 and cax3, identified a variety of phenotypes including sensitivity to abiotic stresses, which indicated that these transporters might play a role in mediating the plant’s stress response. A common feature of these mutants was the perturbation of H⁺-ATPase activity at both the tonoplast and the plasma membrane, suggesting a tight interplay between the Ca²⁺/H⁺ exchangers and H⁺ pumps. We speculate that indirect regulation of proton flux by the exchangers may be as important as the direct regulation of Ca²⁺ flux. These results suggest cautious interpretation of mutant Ca²⁺/H⁺ exchanger phenotypes that may be due to either perturbed Ca²⁺ or H⁺ transport.