Abstract:
The protein phosphatases 2C ABI1 and ABI2 are negative regulators in signal transduction of the phytohormone abscisic acid (ABA). The aim of this work is to characterize two homologous proteins ABP7 and ABP9, which were identified as interacting partners of ABI2 in the yeast two-hybrid system.
In protoplasts, ABP7 and ABP9 interacted with both ABI1 and ABI2 in the nucleus and the cytosol. Overexpression of ABP7 and ABP9 resulted in dramatic inductions of ABA-induced gene expression in diverse wildtype and mutant Arabidopsis. The ABP7- and ABP9-mediated stimulation of ABA responses was antagonized by overexpression of ABI1 and ABI2. Arabidopsis thaliana with ectopic expression of ABP9 displayed ABA hypersensitivity in the root growth response, seed germination, and stomatal closure. The protein phosphatase assays demonstrated that ABP9 could inhibit the PP2C activities of ABI1 and ABI2 in an ABA-dependent manner.