AtNHX3 is a vacuolar K+/H+ antiporter required for low‐potassium tolerance in Arabidopsis thaliana

Three vacuolar cation/H + antiporters, AtNHX1 (At5g27150), 2 (At3g05030) and 5 (At1g54370), have been characterized as functional Na + /H + antiporters in Arabidopsis. However, the physiological functions of AtNHX3 (At5g55470) still remain unclear. In this study, the physiological functions of AtNHX3 were studied using T-DNA insertion mutant and 35S-driven AtNHX3 over-expression Arabidopsis plants. RT-PCR analyses revealed that AtNHX3 is highly expressed in germinating seeds, flowers and siliques. Experiments with AtNHX3::YFP fusion protein in tobacco protoplasts indicated that AtNHX3 is mainly localized to vacuolar membrane, with a minor localization to pre-vacuolar compartments (PVCs) and endoplasmic reticulum (ER). Seedlings of null nhx3 mutants were hypersensitive to K + -deficient conditions. Expression of AtNHX3 complemented the sensitivity to K + deficiency in nhx3 seedlings. Tonoplast vesicles isolated from transgenic plants over-expressing AtNHX3 displayed significantly higher K + /H + exchange rates than those isolated from wild-type plants. Furthermore, nhx3 seeds accumulated less K + and more Na + when both wild-type and nhx3 were grown under normal growth condition. The overall results indicate that AtNHX3 encodes a K + /H + antiporter required for low-potassium tolerance during germination and early seedling development, and may function in K + utilization and ion homeostasis in Arabidopsis.