ARABIDOPSIS ABA RESPONSE GENE ABI1 - FEATURES OF A CALCIUM-MODULATED PROTEIN PHOSPHATASE

The Arabidopsis ABI1 locus is essential for a wide spectrum of abscisi c acid (ABA) responses throughout plant development. Here, ABI1 was sh own to regulate stomatal aperture in leaves and mitotic activity in ro ot meristems. The ABI1 gene was cloned and predicted to encode a signa ling protein. Although its carboxyl-terminal domain is related to seri ne-threonine phosphatase 2C, the ABI1 protein has a unique amino-termi nal extension containing an EF hand calcium-binding site. These result s suggest that the ABI1 protein is a Ca2+-modulated phosphatase and fu nctions to integrate ABA and Ca2+ signals with phosphorylation-depende nt response pathways.