The <i>Arabidopsis thaliana SOS2</i> gene encodes a protein kinase that is required for salt tolerance

In Arabidopsis thaliana , the Salt Overly Sensitive 2 ( SOS2 ) gene is required for intracellular Na + and K + homeostasis. Mutations in SOS2 cause Na + and K + imbalance and render plants more sensitive toward growth inhibition by high Na + and low K + environments. We isolated the SOS2 gene through positional cloning. SOS2 is predicted to encode a serine/threonine type protein kinase with an N-terminal catalytic domain similar to that of the yeast SNF1 kinase. Sequence analyses of sos2 mutant alleles reveal that both the N-terminal catalytic domain and the C-terminal regulatory domain of SOS2 are functionally essential. The steady-state level of SOS2 transcript is up-regulated by salt stress in the root. Autophosphorylation assays show that SOS2 is an active protein kinase. In the recessive sos2 - 5 allele, a conserved glycine residue in the kinase catalytic domain is changed to glutamate. This mutation abolishes SOS2 autophosphorylation, indicating that SOS2 protein kinase activity is required for salt tolerance.