The AtNHX1 exchanger mediates potassium compartmentation in vacuoles of transgenic tomato

NHX-type antiporters in the tonoplast have been reported to increase the salt tolerance of various plants species, and are thought to mediate the compartmentation of Na+ in vacuoles. However, all isoforms characterized so far catalyze both Na+/H+ and K+/H+ exchange. Here, we show that AtNHX1 has a critical involvement in the subcellular partitioning of K+, which in turn affects plant K+ nutrition and Na+ tolerance. Transgenic tomato plants overexpressing AtNHX1 had larger K+ vacuolar pools in all growth conditions tested, but no consistent enhancement of Na+ accumulation was observed under salt stress. Plants overexpressing AtNHX1 have a greater capacity to retain intracellular K+ and to withstand salt-shock. Under K+-limiting conditions, greater K+ compartmentation in the vacuole occurred at the expense of the cytosolic K+ pool, which was lower in transgenic plants. This caused the early activation of the high-affinity K+ uptake system, enhanced K+ uptake by roots, and increased the K+ content in plant tissues and the xylem sap of transformed plants. Our results strongly suggest that NHX proteins are likely candidates for the H+-linked K+ transport that is thought to facilitate active K+ uptake at the tonoplast, and the partitioning of K+ between vacuole and cytosol.