Upstream kinases of plant Sn RK s are involved in salt stress tolerance

Summary Sucrose non‐fermenting 1‐related protein kinases (Sn RK s) are important for plant growth and stress responses. This family has three clades: Sn RK 1, Sn RK 2 and Sn RK 3. Although plant Sn RK s are thought to be activated by upstream kinases, the overall mechanism remains obscure. Geminivirus Rep‐Interacting Kinase ( GRIK )1 and GRIK 2 phosphorylate Sn RK 1s, which are involved in sugar/energy sensing, and the grik1‐1 grik2‐1 double mutant shows growth retardation under regular growth conditions. In this study, we established another Arabidopsis mutant line harbouring a different allele of gene GRIK 1 ( grik1‐2 grik2‐1 ) that grows similarly to the wild‐type, enabling us to evaluate the function of GRIK s under stress conditions. In the grik1‐2 grik2‐1 double mutant, phosphorylation of Sn RK 1.1 was reduced, but not eliminated, suggesting that the grik1‐2 mutation is a weak allele. In addition to high sensitivity to glucose, the grik1‐2 grik2‐1 mutant was sensitive to high salt, indicating that GRIK s are also involved in salinity signalling pathways. Salt Overly Sensitive ( SOS )2, a member of the Sn RK 3 subfamily, is a critical mediator of the response to salinity. GRIK 1 phosphorylated SOS 2 in vitro , resulting in elevated kinase activity of SOS 2. The salt tolerance of sos2 was restored to normal levels by wild‐type SOS 2, but not by a mutated form of SOS 2 lacking the T168 residue phosphorylated by GRIK 1. Activation of SOS 2 by GRIK 1 was also demonstrated in a reconstituted system in yeast. Our results indicate that GRIK s phosphorylate and activate Sn RK 1 and other members of the Sn RK 3 family, and that they play important roles in multiple signalling pathways in vivo .