Arabidopsis RAV 1 transcription factor, phosphorylated by S n RK 2 kinases, regulates the expression of ABI 3 , ABI 4 , and ABI 5 during seed germination and early seedling development

Summary The phytohormone abscisic acid ( ABA ) modulates a number of processes during plant growth and development. In this study, the molecular mechanism of A rabidopsis RAV (Related to ABI 3/ VP 1) transcription factor RAV 1 involving ABA signaling was investigated. RAV 1 ‐underexpressing lines were more sensitive to ABA than wild‐type plants during seed germination and early seedling development, whereas RAV 1 ‐overexpressing lines showed strong ABA ‐insensitive phenotypes. Overexpression of RAV 1 repressed ABI 3 , ABI 4 , and ABI 5 expression, and RAV 1 bound to the ABI 3 , ABI 4 , and ABI 5 promoters in vitro and in vivo , indicating that RAV 1 directly down‐regulates the expression of ABI 3 , ABI 4 , and ABI 5 . The interruption of ABI 5 function in RAV 1‐ U abi5 plants abolished the ABA ‐hypersensitive phenotype of RAV 1‐ U plants, demonstrating that ABI 5 is epistatic to RAV 1 . RAV 1 interacted with SNF 1‐ RELATED PROTEIN KINASE S n RK 2.2, S n RK 2.3 and S n RK 2.6 in the nucleus. In vitro kinase assays showed that S n RK 2.2, S n RK 2.3 and S n RK 2.6 phosphorylated RAV 1. Transient expression assays revealed that S n RK 2.2, S n RK 2.3 and S n RK 2.6 reduced the RAV 1‐dependent repression of ABI 5 , and the ABA ‐insensitive phenotype of the RAV 1 ‐overexpressing line was impaired by overexpression of S n RK 2.3 in the RAV 1 OE 3 plants. Together, these results demonstrated that the A rabidopsis RAV 1 transcription factor plays an important role in ABA signaling by modulating the expression of ABI 3 , ABI 4 , and ABI 5 , and that its activity is negatively affected by S n RK 2s.