Abstract
Arabidopsis SnRK2 protein kinase, SRK2E/OST1, plays important roles in the response to ABA-induced stomatal closure. We previously showed that SRK2E/OST1 is not only activated by ABA, but also by osmotic stress (OS) in Arabidopsis. ABA-dependent activation of SRK2E/OST1 was strongly inhibited in ABA-insensitive abi1-1 mutant. However, OS normally activated SRK2E/OST1 under abi1-1 background. Deletion analysis provided that C-terminus 319-357 amino acids were sufficient for the ABA response but were not for the OS response. Full length SRK2E completely complemented the wilty phenotype of srk2e, but the C-terminus truncated SRK2E/OST1 did not. SRK2E was shown to bind to ABI1 by yeast two-hybrid assay. Furthermore, ABI1 was specifically bind to the C-terminus 319-357 amino acids. Our results demonstrated the essential roles of the C-terminal fragment on both the activation and physiological function of SRK2E/OST1, and also strongly support the presence of two independent pathways, namely ABA and OS pathways, in the signaling.
