Abstract
Protein phosphorylation has pivotal roles in ABA signaling in higher plants. As one of the major signaling molecules function in protein phosphorylation, we focused on Arabidopsis SnRK2 protein kinase family (SRK2A-SRK2J), and found that SRK2E/OST1 have essential roles in ABA signaling. To gain more insight into a molecular nature of SRK2E at cellular levels, we overexpressed its gene in Arabidopsis T87 cells and analyzed its activation profiles. Using this system, SRK2E/OST1 was found to be activated not only by ABA but also by osmotic stress such as sorbitol or NaCl. abi1-1 mutation strongly inhibited ABA-dependent SRK2E/OST1 activity, but did not affect osmotic stress-dependent activity. This osmotic stress-dependent activity was also observed in ABA-deficient aba2-1 or NCED3 knockout mutant. We also found that a part of C-terminal region is involved in the regulation of ABA-dependent SRK2E/OST1 activity. These results indicate that ABA and osmotic stress may independently activate SRK2E/OST1, respectively.
