Arabidopsis group C Raf-like protein kinases negatively regulate abscisic acid signaling and are direct substrates of SnRK2

Significance SNF1-related protein kinase 2s (SnRK2s) are central regulators in abscisic acid (ABA) signaling and stress responses in plants. However, SnRK2 substrates are largely unknown. Here, we identified Arabidopsis group C Raf-like kinases Raf36 and Raf22 as SnRK2 substrates. Reverse genetic analyses showed that Raf36 and Raf22 negatively regulate ABA signaling. Under optimal growth conditions, Raf36 accumulates in the cell and phosphorylates substrates to suppress ABA responses. Conversely, under abiotic stress conditions, ABA-activated SnRK2s phosphorylate Raf36 and promote Raf36 degradation, leading to stronger ABA responses. Taken together, this study provides insight into molecular and physiological functions of group C Raf kinases in ABA signaling.