ABI1 protein phosphatase 2C is a negative regulator of abscisic acid signaling

The plant hormone abscisic acid (ABA) is a key regulator of seed maturation and germination and mediates adaptive responses to environmental stress. I n Arabidopsis, the ABI1 gene encodes a member of the 2C class of protein se rine/threonine phosphatases (PP2C), and the abi1-1 mutation markedly reduce s ABA responsiveness in both seeds and vegetative tissues. However, this mu tation is dominant and has been the only mutant allele available for the AB I1 gene. Hence, it remained unclear whether ABI1 contributes to ABA signali ng, and in case ABI1 does regulate ABA responsiveness, whether it is a posi tive or negative regulator of ABA action. In this study, we isolated seven novel alleles of the ABI1 gene as intragenic revertants of the abi1-1 mutan t. In contrast to the ABA-resistant abi1-1 mutant, these revertants were mo re sensitive than the wild type to the inhibition of seed germination and s eedling root growth by applied ABA. They also displayed increases in seed d ormancy and drought adaptive responses that are indicative of a higher resp onsiveness to endogenous ABA. The revertant alleles were recessive to the w ild-type ABI1 allele in enhancing ABA sensitivity, indicating that this ABA -supersensitive phenotype results from a loss of function in ABI1. The seve n suppressor mutations are missense mutations in conserved regions of the P P2C domain of ABI1, and each of the corresponding revertant alleles encodes an ABI1 protein that lacked any detectable PP2C activity in an in vitro en zymatic assay. These results indicate that a loss of ABI1 PP2C activity lea ds to an enhanced responsiveness to ABA. Thus, the wild-type ABI1 phosphata se is a negative regulator of ABA responses.