CML 24 , Regulated in Expression by Diverse Stimuli , Encodes a Potential Ca 2 1 Sensor That Functions in Responses to Abscisic Acid , Daylength , and Ion Stress 1

Changes in intracellular calcium (Ca) levels serve to signal responses to diverse stimuli. Ca signals are likely perceived through proteins that bind Ca, undergo conformation changes following Ca binding, and interact with target proteins. The 50-member calmodulin-like (CML) Arabidopsis (Arabidopsis thaliana) family encodes proteins containing the predicted Ca-binding EF-hand motif. The functions of virtually all these proteins are unknown. CML24, also known as TCH2, shares over 40% amino acid sequence identity with calmodulin, has four EF hands, and undergoes Ca-dependent changes in hydrophobic interaction chromatography and migration rate through denaturing gel electrophoresis, indicating that CML24 binds Ca and, as a consequence, undergoes conformational changes. CML24 expression occurs in all major organs, and transcript levels are increased from 2to 15-fold in plants subjected to touch, darkness, heat, cold, hydrogen peroxide, abscisic acid (ABA), and indole-3-acetic acid. However, CML24 protein accumulation changes were not detectable. The putative CML24 regulatory region confers reporter expression at sites of predicted mechanical stress; in regions undergoing growth; in vascular tissues and various floral organs; and in stomata, trichomes, and hydathodes. CML24-underexpressing transgenics are resistant to ABA inhibition of germination and seedling growth, are defective in long-day induction of flowering, and have enhanced tolerance to CoCl2, molybdic acid, ZnSO4, and MgCl2. MgCl2 tolerance is not due to reduced uptake or to elevated Ca accumulation. Together, these data present evidence that CML24, a gene expressed in diverse organs and responsive to diverse stimuli, encodes a potential Ca sensor that may function to enable responses to ABA, daylength, and presence of various salts.