Principal Investigator Shanghai Center for Plant Stress Biology
Osmoregulation is important for plant growth, development, and response to environmental changes. SNF1-related protein kinase 2s (SnRK2s) are quickly activated by osmotic stress and are central components in osmotic stress and abscisic acid (ABA) signaling pathways; however, the upstream components required for SnRK2 activation and early osmotic stress signaling are still unknown. Here, we report a critical role for Raf-like kinases, in early osmotic stress as well as ABA signaling in Arabidopsis thaliana. RAFs are quickly activated by osmotic stress and are required for phosphorylation and activation of SnRK2s. Analyses of high-order mutants of RAFs reveal critical roles of the RAFs in osmotic stress tolerance as well as in growth and development. Subgroup 4 RAFs specifically phosphorylate ABA-independent SnRK2s while subgroup 2 and 3 RAFs phosphorylate SnRK2.2, 2.3, and 2.6. The RAFs in the three subgroups work together to activate SnRK2s in osmotic stress signaling. Our findings uncover a kinase cascade mediating osmoregulation in higher plants. By generating an additional high-order mutant of subgroup 2 and 3 RAFs, we proved that subgroup 2 and 3 RAFs are essential for ABA signaling. The OK100-oct and OK100-nonu show a hyposensitivity to ABA. The seeds of OK100-nonu even germinate on medium containing 25 µM ABA. Transcriptome analysis revealed the expression of most ABA-responsive genes is abolished in the OK100-nonu mutant. In the OK100-nonu mutant, the ABA-induced SnRK2 activation is abolished. Subgroup 2 and 3 RAFs phosphorylate Ser171 and Ser175 in the activation loop of SnRK2s, and the phosphorylation interrupts the interaction between SnRK2s and PP2C. Thus, RAFs play a crucial role in ABA signaling by phosphorylating SnRK2.2, 2.3, and 2.6 in stressed conditions.
