Calcium Regulation of Sodium Hypersensitivities of sos3 and athkt1 Mutants

T-DNA disruption mutations in the AtHKT1 gene have previously been shown to suppress the salt sensitivity of the sos3 mutant. However, both sos3 and athktl single mutants show sodium (Na + ) hypersensitivity. In the present study we further analyzed the underlying mechanisms for these non-additive and counteracting Na + sensitivities by characterizing athktl-1 sos3 and athktl-2 sos3 double mutant plants. Unexpectedly, mature double mutant plants grown in soil clearly showed an increased Na + hypersensitivity compared with wild-type plants when plants were subjected to salinity stress. The salt sensitive phenotype of athktl sos3 double mutant plants was similar to that of athktl plants, which showed chlorosis in leaves and stems. The Na + content in xylem sap samples of soil-grown athktl sos3 double and athktl single mutant plants showed dramatic Na + overaccumulation in response to salinity stress. Salinity stress analyses using basic minimal nutrient medium and Murashige-Skoog (MS) medium revealed that athktl sos3 double mutant plants show a more athktl single mutant-like phenotype in the presence of 3 mM external Ca 2 + , but show a more sos3 single mutant-like phenotype in the presence of 1 mM external Ca 2 + . Taken together multiple analyses demonstrate that the external Ca 2 + concentration strongly impacts the Na + stress response of athktl sos3 double mutants. Furthermore, the presented findings show that SOS3 and AtHKT1 are physiologically distinct major determinants of salinity resistance such that sos3 more strongly causes Na + overaccumulation in roots, whereas athktl causes an increase in Na + levels in the xylem sap and shoots and a concomitant Na + reduction in roots.