High salt-treatment-induced Na⁺ extrusion and low salt-treatment-induced Na⁺ accumulation in suspension-cultured cells of the mangrove plant, Bruguiera sexangula

A suspension-cultured cell strain of the mangrove plant (Bruguiera sexangula) was established from a callus culture and maintained in an amino acid medium in the absence of NaCl. NaCl non-adapted cells were transferred to media containing 0-200 mM NaCl. The initial growth rate decreased gradually with increasing salt concentrations. However, at up to 150 mM NaCl, cell number growth at the highest point was almost the same as that at lower salt concentrations. Cells even continued to grow in the presence of 200 mM NaCl. Cells incubated in a medium containing 50 mM NaCl for 3 weeks accumulated Na⁺, while those incubated in 150 mM NaCl for 2 d showed only a transient increase in Na⁺ and Cl^- concentrations. In the latter treatment, the intracellular concentration of Na⁺ returned to the original low level within 2 weeks. It took a longer time for Cl^- to return to its original level. As a result, the Na⁺ and Cl^- concentrations in cells cultured with 50 mM NaCl were much larger than those in cells cultured with 150 mM NaCl. The intracellular distribution of ions after transfer to the medium containing 150 mM NaCl was analysed by isolating the vacuoles. Treatment with amiloride, an inhibitor of the Na⁺/H⁺ antiporter, suppressed the recovery of Na⁺ to the original level in the cells. Treatment with 150 mM NaCl for 3 d stimulated the activities of both the vanadate-dependent H⁺-ATPase and the Na⁺/H⁺ antiporter in the plasma membrane fraction.