Critical role of divalent cations and Na+ efflux in Arabidopsis thaliana salt tolerance

Detrimental effects of salinity on plants are known to be partially allevia ted by external Ca2+. Previous work demonstrated that the Arabidopsis SOS3 locus encodes a Ca2+-binding protein with similarities to CnB, the regulato ry subunit of protein phosphatase 2B (calcineurin), In this study, we furth er characterized the role of SOS3 in salt tolerance. We found that reduced root elongation of sos3 mutants in the presence of high concentrations of e ither NaCl or LiCl is specifically rescued by Ca2+ and not Mg2+, whereas ro ot growth is rescued by both Ca2+ and Mg2+ in the presence of high concentr ations of KCl, Phenocopies of sos3 mutants were obtained in wild-type plant s by the application of calmodulin and calcineurin inhibitors. These data p rovide further evidence that SOS3 is a calcineurin-like protein and that ca lmodulin plays an important role in the signalling pathways involved in pla nt salt tolerance. The origin of the elevated Na : K ratio in sos3 mutants was investigated by comparing Na+ efflux and influx in both mutant and wild type. No difference in Na+ influx was recorded between wild type and sos3; however, sos3 plants showed a markedly lower Na+ efflux, a property that w ould contribute to the salt-oversensitive phenotype of sos3 plants.