Arabidopsis SnRK2.3/SRK2I plays a positive role in seed germination under cold stress conditions

Abscisic acid (ABA) plays a negative role in seed germination by inducing and maintaining seed dormancy. As core components of ABA signaling, the three subclass III SnRK2 protein kinases SnRK2.2, SnRK2.3, and SnRK2.6 play essential roles in the regulation of seed dormancy, seed germination, and seedling growth. However, their precise role in seed germination under cold stress conditions remains unclear. In Arabidopsis seeds, SnRK2.2 and SnRK2.3 genes were highly abundant compared to SnRK2.6, and their expression levels significantly declined during seed germination at 4 °C. Among knockout mutants of these SnRK2s, only snrk2.3 seeds showed significantly delayed germination under cold stress conditions, compared to wild-type seeds. Although this phenotypic abnormality of snrk2.3 was different from those caused by mutations in other ABA core components, such as RCAR5/PYL11 and AIP1/HAI2, it was partially rescued by blocking ABA biosynthesis. When germinated snrk2.3 seeds were grown vertically at 4 °C, their root growth was also severely inhibited compared to that of wild-type plants. In contrast to snrk2.3, SnRK2.3-overexpressing plants exhibited enhanced seed germination under cold stress conditions, and this SnRK2.3-mediated regulation of seed germination was accompanied by altered expression of genes involved in the response to ABA and GA, as well as genes involved in ABA and GA biosynthesis/catabolism. Taken together, the results of our study suggest that SnRK2.3 acts as a positive regulator of seed germination under cold stress conditions, and that this role may be mediated through ABA-dependent signaling pathways.