CHX14 is a plasma membrane K-efflux transporter that regulates K+ redistribution in Arabidopsis thaliana

Potassium (K+) is essential for plant growth and development, yet the molecular identity of many K+ transporters remains elusive. Here we characterized cation/H+ exchanger (CHX) 14 as a plasma membrane K+ transporter. CHX14 expression was induced by elevated K+ and histochemical analysis of CHX14 promoter::GUS transgenic plants indicated that CHX14 was expressed in xylem parenchyma of root and shoot vascular tissues of seedlings. CHX14 knockout (chx14) and CHX14 overexpression seedlings displayed different growth phenotypes during K+ stress as compared with wild-type seedlings. Roots of mutant seedlings displayed higher K+ uptake rates than wild-type roots. CHX14 expression in yeast cells deficient in K+ uptake renders the mutant cells more sensitive to deficiencies of K+ in the medium. CHX14 mediates K+ efflux in yeast cells loaded with high K+. Uptake experiments using Rb-86(+) as a tracer for K+ with both yeast and plant mutants demonstrated that CHX14 expression in yeast and in planta mediated low-affinity K+ efflux. Functional green fluorescent protein (GFP)-tagged versions of CHX14 were localized to both the yeast and plant plasma membranes. Taken together, we suggest that CHX14 is a plasma membrane K+ efflux transporter involved in K+ homeostasis and K+ recirculation. K+ is an essential nutrient for plant growth and development; but K+ efflux mechanisms and K+ redistribution in plant are still poorly understood. Arabidopsis CHX14 is characterized as a plasma membrane-localized, low-affinity K+ efflux transporter. CHX14 is expressed in xylem parenchyma of root and shoot vascular tissues and induced by high concentrations of K+. It is involved in K+ recirculation and redistribution from roots to shoots to regulate K+ homeostasis and nutrition between plant tissues during K+ deficiency or under high concentration of K+.