Upstream kinases of plant SnRKs 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 .