Functional Involvement of Highly Abscisic Acid-Induced Clade A Protein Phosphatase 2Cs in Delayed Seed Germination Under Cold Stress

Abstract

Seed germination represents a vital stage in the life cycle of plants, in which the phytohormone abscisic acid (ABA) plays important modulatory roles, including the promotion of seed dormancy and inhibition of germination. The Arabidopsis genome contains nine clade A protein phosphatases (PP2Cs), which in conjunction with PYR1/PYLs/RCARs, function as ABA co-receptors. However, the precise role of these PP2Cs in seeds is yet to be ascertained. In this study, we report the functional involvement of HIGHLY ABA-INDUCED (HAI) PP2Cs, including AIP1/HAI2, HAI1, and HAI3, in cold stress-induced inhibition of germination. The expression of HAI PP2C genes in dry seeds was found to be dependent on ABA, and the expression levels decreased as the seeds germinated. Under cold stress conditions, germination of the seeds of HAI PP2C knockout mutants was observed to be significantly slower than that of wild-type seeds, with the aip1-1 mutant showing a notably stronger inhibition of germination than other PP2C mutants. This distinct phenotype of aip1-1 was determined at the pre-germinative stage and demonstrated to be compromised by the inhibition of ABA synthesis. Our data indicate that AIP1 could be the main PP2C involved in ABA-mediated inhibition of germination under cold stress conditions. We speculate that this protein functions downstream of RCAR5/PYL11 and contributes to the regulation of seed germination by interacting with RCAR5/PYL11 in the presence of ABA.