The wheat NHX antiporter gene TaNHX2 confers salt tolerance in transgenic alfalfa by increasing the retention capacity of intracellular potassium

Previous studies have shown that TaNHX2 transgenic alfalfa (Medicago sativa L.) accumulated more K+ and less Na+ in leaves than did the wild-type plants. To investigate whether the increased K+ accumulation in transgenic plants is attributed to TaNHX2 gene expression and whether the compartmentalization of Na+ into vacuoles or the intracellular compartmentalization of potassium is the critical mechanism for TaNHX2-dependent salt tolerance in transgenic alfalfa, aerated hydroponic culture was performed under three different stress conditions: control condition (0.1 mM Na+ and 6 mM K+ inside culture solution), K+-sufficient salt stress (100 mM NaCl and 6 mM K+) and K+-insufficient salt stress (100 mM NaCl and 0.1 mM K+). The transgenic alfalfa plants had lower K+ efflux through specific K+ channels and higher K+ absorption through high-affinity K+ transporters than did the wild-type plants. Therefore, the transgenic plants had greater K+ contents and [K+]/[Na+] ratios in leaf tissue and cell sap. The intracellular compartmentalization of potassium is critical for TaNHX2-induced salt tolerance in transgenic alfalfa.