Rice B2 and B3 subgroup RAF kinases play central roles in both ABA and osmotic stress signaling
Description
Protein kinases in the mitogen-activated protein kinase kinase kinase (MAPKKK) supper family are known to be central actors in orchestrating cell differentiation, growth, and responses to extracellular stimuli in eukaryotes. However, the rice (Oryza sativa L.) MAPKKKs, especially the RAF subgroup ones have rarely been studied yet. In this study, we generated high-order mutants of B1, B2, and B3 RAF subgroup MAPKKKs. The resulting raf-b2, raf-b3q, and raf-b2/3q mutants are hyposensitive to ABA but show dramatic sensitivity to various osmotic stresses. The raf-b1 mutant only shows reduced growth but responds to ABA and osmotic stress normally. The rice B2 and B3 RAFs could phosphorylate SAPKs and rescue the ABA-induced expression of RD29B-LUC in mesophyll protoplasts of Arabidopsis OK100-nonu mutant. In the rice raf-b2/3q, the ABA-triggered activation of SAPKs is largely impaired, and the expression of stress-response genes is decreased compared to that in the wild type. We also compared the ABA and osmotic stress phenotypes among B2/3 RAF mutants in Arabidopsis, P. patens, and rice. Our study reveals a crucial role of B2 and B3 subgroup MAPKKKs in ABA signaling and osmotic stresses and sheds light on the functional divergence of RAF-SAPK cascade in environmental stress responses in different plant species.