Arabidopsis Raf-like kinases act as positive regulators of subclass III SnRK2 in osmostress signaling.

Given their sessile nature, land plants must use various mechanisms to manage dehydration under water-deficit conditions. Osmostress-induced activation of the SNF1-related protein kinase 2 (SnRK2) family elicits physiological responses such as stomatal closure to protect plants during drought conditions. With the plant hormone abscisic acid (ABA) receptors (PYR/PYL/RCAR proteins) and Group A protein phosphatases (PP2CAs), subclass III SnRK2 also constitutes a core signaling module for ABA, and osmostress triggers ABA accumulation. How SnRK2 is activated through ABA has been clarified, but how it is activated through osmostress is unclear. Here we show that Arabidopsis Raf-like kinases (AtARKs) of the B3 clade of the mitogen-activated kinase kinase kinase (MAPKKK) family are crucial in SnRK2-mediated osmostress responses. Disruption of AtARKs in Arabidopsis results in increased water loss from detached leaves because of impaired stomatal closure in response to osmostress. Our findings in vitro and in planta have shown that AtARKs interact physically with SRK2E, a core factor for stomatal closure in response to drought. Furthermore, we show that AtARK phosphorylates S171 and S175 in the activation loop of SRK2E in vitro and that Atark mutants have defects in osmostress-induced subclass III SnRK2 activity. Our findings identify a specific type of B3-MAPKKKs as upstream kinases of subclass III SnRK2 in Arabidopsis. Taken together with earlier reports that ARK is an upstream kinase of SnRK2 in moss, an existing member of a basal land plant lineage, we propose that ARK/SnRK2 module is evolutionarily conserved across 400 million years of land plant evolution for conferring protection against drought.