A Putative Chloroplast-Localized Ca2+/H+ Antiporter CCHA1 Is Involved in Calcium and pH Homeostasis and Required for PSII Function in Arabidopsis

Calciumis important for chloroplast, not only in its photosynthetic but also nonphotosynthetic functions. Multiple Ca2+/H+ transporters and channels have been described and studied in the plasma membrane and organelle membranes of plant cells; however, the molecular identity and physiological roles of chloroplast Ca2+/H+ antiporters have remained unknown. Here we report the identification and characterization of a member of the UPF0016 family, CCHA1 (a chloroplast-localized potential Ca2+/H+ antiporter), in Arabidopsis thaliana. We observed that the ccha1 mutant plants developed pale green leaves and showed severely stunted growth along with impaired photosystem II (PSII) function. CCHA1 localizes to the chloroplasts, and the levels of the PSII core subunits and the oxygen-evolving complex were significantly decreased in the ccha1 mutants compared with the wild type. In high Ca2+ concentrations, Arabidopsis CCHA1 partially rescued the growth defect of yeast gdt1 Delta null mutant, which is defective in a Ca2+/H+ antiporter. The ccha1 mutant plants also showed significant sensitivity to high concentrations of CaCl2 and MnCl2, as well as variation in pH. Taken these results together, we propose that CCHA1 might encode a putative chloroplast-localized Ca2+/H+ antiporter with critical functions in the regulation of PSII and in chloroplast Ca2+ and pH homeostasis in Arabidopsis.