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

Delk, Nikkí A., Johnson, Keith A., Chowdhury, Naweed I., Braam, Janet
Plant physiology 2005 v.139 no.1 pp. 240-253
Arabidopsis thaliana, plant proteins, calcium-binding proteins, calmodulin, calcium, signal transduction, messenger RNA, gene expression regulation, tissue distribution, abscisic acid, photoperiod, plant stress, abiotic stress, indole acetic acid, gibberellic acid, ethylene, seed germination, seedling growth, cations, magnesium, plant biochemistry, plant genetics, plant ecology, amino acid sequences, nucleotide sequences
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.