The Arabidopsis thaliana SOS2 gene encodes a protein kinase that is required for salt tolerance

In Arabidopsis thaliana, the Salt Overly Sensitive 2 (SOS2) gene is require d 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 positio nal cloning. SOS2 is predicted to encode a serine/threonine type protein ki nase 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-ter minal catalytic domain and the C-terminal regulatory domain of SOS2 are fun ctionally essential. The steady-state level of SOS2 transcript is up-regula ted by salt stress in the root. Autophosphorylation assays show that SOS2 i s an active protein kinase, In the recessive sos2-5 allele, a conserved gly cine residue in the kinase catalytic domain is changed to glutamate. This m utation abolishes SOS2 autophosphorylation, indicating that SOS2 protein ki nase activity is required for salt tolerance.