ABI1 Protein Phosphatase 2C Is a Negative Regulator of Abscisic Acid Signaling
Résumé
The plant hormone abscisic acid (ABA) is a key regulator of seed maturation and germination and mediates adaptive responses
to environmental stress. In Arabidopsis, the ABI1 gene encodes a member of the 2C class of protein serine/
threonine phosphatases (PP2C), and the abi1-1 mutation markedly reduces ABA responsiveness in both seeds and vegetative
tissues. However, this mutation is dominant and has been the only mutant allele available for the ABI1 gene.
Hence, it remained unclear whether ABI1 contributes to ABA signaling, and in case ABI1 does regulate ABA responsiveness,
whether it is a positive 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 mutant. In contrast to the ABA-resistant abi1-1 mutant, these revertants
were more sensitive than the wild type to the inhibition of seed germination and seedling root growth by applied
ABA. They also displayed increases in seed dormancy and drought adaptive responses that are indicative of a higher
responsiveness to endogenous ABA. The revertant alleles were recessive to the wild-type ABI1 allele in enhancing ABA
sensitivity, indicating that this ABA-supersensitive phenotype results from a loss of function in ABI1. The seven suppressor
mutations are missense mutations in conserved regions of the PP2C domain of ABI1, and each of the corresponding
revertant alleles encodes an ABI1 protein that lacked any detectable PP2C activity in an in vitro enzymatic
assay. These results indicate that a loss of ABI1 PP2C activity leads to an enhanced responsiveness to ABA. Thus, the
wild-type ABI1 phosphatase is a negative regulator of ABA responses.