K Efflux Antiporters 4, 5, and 6 Mediate PH and K Homeostasis in Endomembrane Compartments

KEA4, KEA5, and KEA6 are members of the Arabidopsis () K efflux antiporter (KEA) family that share high sequence similarity but whose function remains unknown. Here, we show their gene expression pattern, subcellular localization, and physiological function in Arabidopsis. , , and had similar tissue expression patterns, and the three KEA proteins localized to the Golgi, the trans-Golgi network, and the prevacuolar compartment/multivesicular bodies, suggesting overlapping roles of these proteins in the endomembrane system. Phenotypic analyses of single, double, and triple mutants confirmed functional redundancy. The triple mutant had small rosettes, short seedlings, and was sensitive to low K availability and to the sodicity imposed by high salinity. Also, the mutant plants had a reduced luminal pH in the Golgi, trans-Golgi network, prevacuolar compartment, and vacuole, in accordance with the K/H exchange activity of KEA proteins. Genetic analysis indicated that KEA4, KEA5, and KEA6 as well as endosomal Na/Hexchanger5 (NHX5) and NHX6 acted coordinately to facilitate endosomal pH homeostasis and salt tolerance. Neither cancelling nor overexpressing the vacuolar antiporters and in the mutant background altered the salt-sensitive phenotype. The NHX1 and NHX2 proteins in the mutant background could not suppress the acidity of the endomembrane system but brought the vacuolar pH close to wild-type values. Together, these data signify that KEA4, KEA5, and KEA6 are endosomal K transporters functioning in maintaining pH and ion homeostasis in the endomembrane network.