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.