The protein phosphatase 2C clade A protein OsPP2C51 positively regulates seed germination by directly inactivating OsbZIP10

Protein phosphatase 2C clade A members are major signaling components in the ABA-dependent signaling cascade that regulates seed germination. To elucidate the role of PP2CA genes in germination of rice seed, we selected OsPP2C51, which shows highly specific expression in the embryo compared with other protein phosphatases based on microarray data. GUS histochemical assay confirmed that OsPP2C51 is expressed in the seed embryo and that this expression pattern is unique compared with those of other OsPP2CA genes. Data obtained from germination assays and alpha-amylase assays of OsPP2C51 knockout and overexpression lines suggest that OsPP2C51 positively regulates seed germination in rice. The expression of alpha-amylase synthesizing genes was high in OsPP2C51 overexpressing plants, suggesting that elevated levels of OsPP2C51 might enhance gene expression related to higher rates of seed germination. Analysis of protein interactions between ABA signaling components showed that OsPP2C51 interacts with OsPYL/RCAR5 in an ABA-dependent manner. Furthermore, interactions were observed between OsPP2C51 and SAPK2, and between OsPP2C51 and OsbZIP10 and we found out that OsPP2C51 can dephosphorylates OsbZIP10. These findings suggest the existence of a new branch in ABA signaling pathway consisting of OsPYL/RCAR-OsPP2C-bZIP apart from the previously reported OsPYL/RCAR-OsPP2C-SAPK-bZIP. Overall, our result suggests that OsPP2C51 is a positive regulator of seed germination by directly suppressing active phosphorylated OsbZIP10.