Abstract
The phytohormone abscisic acid (ABA) triggers an oscillation in the cytosolic Ca(2+) concentration, which is then perceived by unknown Ca(2+) binding proteins to initiate a series of signaling cascades that control many physiological processes, including adaptation to environmental stress. We report here that a Ca(2+) binding protein, SCaBP5, and its interacting protein kinase, PKS3, function as global regulators of ABA responses. Arabidopsis mutants with silenced SCaBP5 or PKS3 are hypersensitive to ABA in seed germination, seedling growth, stomatal closing, and gene expression. PKS3 physically interacts with the 2C-type protein phosphatase ABI2 (ABA-insensitive 2) and to a lesser extent with the homologous ABI1 (ABA-insensitive 1) protein. Thus, SCaBP5 and PKS3 are part of a calcium-responsive negative regulatory loop controlling ABA sensitivity.