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ENVIRONMENTAL STRESS AND ADAPTATION

CML24, Regulated in Expression by Diverse Stimuli, Encodes a Potential Ca²⁺ Sensor That Functions in Responses to Abscisic Acid, Daylength, and Ion Stress¹

Department of Biochemistry and Cell Biology, Rice University, Houston, Texas 77005–1892 (N.A.D., N.I.C., J.B.); and Department of Biology, Bradley University, Peoria, Illinois 61625 (K.A.J.)

Changes in intracellular calcium (Ca²⁺) levels serve to signal responses to diverse stimuli. Ca²⁺ signals are likely perceived through proteins that bind Ca²⁺, undergo conformation changes following Ca²⁺ binding, and interact with target proteins. The 50-member calmodulin-like (CML) Arabidopsis (Arabidopsis thaliana) family encodes proteins containing the predicted Ca²⁺-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 Ca²⁺-dependent changes in hydrophobic interaction chromatography and migration rate through denaturing gel electrophoresis, indicating that CML24 binds Ca²⁺ and, as a consequence, undergoes conformational changes. CML24 expression occurs in all major organs, and transcript levels are increased from 2- to 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 Ca²⁺ accumulation. Together, these data present evidence that CML24, a gene expressed in diverse organs and responsive to diverse stimuli, encodes a potential Ca²⁺ sensor that may function to enable responses to ABA, daylength, and presence of various salts.

Article, publication date, and citation information can be found at www.plantphysiol.org/cgi/doi/10.1104/pp.105.062612.

^* Corresponding author; e-mail braam{at}bioc.rice.edu; fax 713–348–5154.

Received March 11, 2005; returned for revision May 10, 2005; accepted May 15, 2005.

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