A new protein kinase gene SSG1 is essential for adaptation of Arabidopsis to salt stress

Soil salinity significantly limits plant productivity on agricultural lands, and salt tolerance of plant requires a complex mechanism in which many genes are involved. From an activation-tagging mutant collection, we identified a loss-of-function Arabidopsis mutant which exhibits hypersensitivity to NaCl during seed germination and is designated ssg1 (salt sensitive during seed germination 1). Knocking down the expression of SSG1 by RNAi recapitulated the phenotype of ssg1, suggesting that functional SSG1 is necessary for normal seed germination in the presence of NaCl. The seed germination of ssg1 was not sensitive to K+, but was hypersensitive to osmotic stress. SSG1 encodes a protein kinase, possibly with alternatively spliced forms. In vitro kinase assay indicated that SSG1 possessed protein kinase activity which can both auto-phosphorylate and phosphorylate substrate. Quantitative real-time RT-PCR analysis showed that, in the mutants ssg1-1 and ssg1-2, the expression level of some salt-responsive marker genes, i.e., SOS1, SOS2, SOS3, SOS4, AtNHX and AtNKT, was down-regulated, whereas SSG1 expression level was not changed in sos mutants. These results suggest that SSG1 is possibly a component in the signaling pathway in response of plants to salt stress.

► ssg1 is a loss-of-function mutant hypersensitive to NaCl during seed germination. ► SSG1 is necessary for normal seed germination in the presence of NaCl. ► ssg1 seed germination is also hypersensitive to osmotic stress. ► SSG1 encodes a protein kinase with demonstrated phosphorylation activity. ► In ssg1-1 and ssg1-2, some salt-responsive marker genes are down-regulated.