The ABI1 and ABI2 protein phosphatases 2C act in a negative feedback regulatory loop of the abscisic acid signalling pathway

The Arabidopsis ABI1 and ABI2 genes encode two protein serine/threonine pho sphatases 2C (PP2C). These genes have been originally identified by the dom inant mutations abi1-1 and abi2-1, which reduce the plant's responsiveness to the hormone abscisic acid (ABA). However, recessive mutants of ABI1 were recently shown to be supersensitive to ABA, which demonstrated that the AB I1 phosphatase is a negative regulator of ABA signalling. We report here th e isolation and characterisation of the first reduction-of-function allele of ABI2, abi2-1R1. The in vitro phosphatase activity of the abi2-1R1 protei n is approximately 100-fold lower than that of the wild-type ABI2 protein. Abi2-1R1 plants displayed a wild-type ABA sensitivity. However, doubly muta nt plants combining the abi2-1R1 allele and a loss-of-function allele at th e ABI1 locus were more responsive to ABA than each of the parental single m utants. These data indicate that the wild-type ABI2 phosphatase is a negati ve regulator of ABA signalling, and that the ABI1 and ABI2 phosphatases hav e overlapping roles in controlling ABA action. Measurements of PP2C activit y in plant extracts showed that the phosphatase activity of ABI1 and ABI2 i ncreases in response to ABA. These results suggest that ABI1 and ABI2 act i n a negative feedback regulatory loop of the ABA signalling pathway.