Na+/H+ exchange in the halophyte Mesembryanthemum crystallinum is associated with cellular sites of Na+ storage

The tonoplast Na+/H+ exchanger is involved in sequestering Na+ in plant vacuoles, providing solutes for osmotic adjustment while avoiding cytoplasmic Na+ toxicity. As such it is assumed to be one of the key mechanisms involved in salt-tolerance in plants. In this study, we measured tonoplast Na+/H+ exchange in roots and different leaf tissues of adult Mesembryanthemum crystallinum L. plants to determine if activity of the exchanger follows the gradient from roots to leaves previously observed for Na+ and pinitol accumulation. Na+/H+ exchange was absent from roots of control and NaCl-treated plants. In contrast, leaves showed constitutive Na+/H+ exchange that was enhanced by growth of the plants in NaCl. Highest activity was measured in the epidermal bladder cells in agreement with the highest concentrations of Na+ found in this tissue. Tonoplast H+-translocating ATPase activity was also greatest in this tissue, as were protein levels for myo-inositol-O-methyltransferase, a key enzyme in the pinitol biosynthesis pathway. The strong correlation between Na+/H+ exchange and Na+ accumulation confirms the role of this transporter in vacuolar sequestration of Na+ and plant salt tolerance.