Cytosolic Sodium Influx in Mesophyll Protoplasts of Arabidopsis thaliana , wt, sos1:1 and nhx1 Differs and Induces Different Calcium Changes.

The sodium influx into the cytosol of mesophyll protoplasts from cv. Columbia, wild type, was compared with the influx into and genotypes, which lack the Na/H antiporter in the plasma membrane and tonoplast, respectively. Changes in cytosolic sodium and calcium concentrations upon a 100 mM NaCl addition were detected by use of epifluorescence microscopy and the sodium-specific fluorescent dye SBFI, AM, and calcium sensitive Fura 2, AM, respectively. There was a smaller and mainly transient influx of Na in the cytosol of the wild type compared with the and genotypes, in which the influx lasted for a longer time. Sodium chloride addition to the protoplasts' medium induced a significant increase in cytosolic calcium concentration in the wild type at 1.0 mM external calcium, and to a lesser extent in , however, it was negligible in the genotype. LiCl inhibited the cytosolic calcium elevation in the wild type. The results suggest that the salt-induced calcium elevation in the cytosol of mesophyll cells depends on an influx from both internal and external stores and occurs in the presence of an intact Na/H antiporter at the plasma membrane. The SOS1 more effectively regulates sodium homeostasis than NHX1.