Expression of Arabidopsis CAX1 in Tobacco: Altered Calcium Homeostasis and Increased Stress Sensitivity

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Abstract
Calcium (Ca2+) efflux from the cytosol modulates Ca2+ concentrations in the cytosol, loads Ca2+ into intracellular compartments, and supplies Ca2+ to organelles to support biochemical functions. The Ca2+/H+ antiporter CAX1 (for CALCIUM EXCHANGER 1) of Arabidopsis is thought to be a key mediator of these processes. To clarify the regulation of CAX1, we examined CAX1 RNA expression in response to various stimuli. CAX1 was highly expressed in response to exogenous Ca2+. Transgenic tobacco plants expressing CAX1 displayed symptoms of Ca2+ deficiencies, including hypersensitivity to ion imbalances, such as increased magnesium and potassium concentrations, and to cold shock, but increasing the Ca2+ in the media abrogated these sensitivities. Tobacco plants expressing CAX1 also demonstrated increased Ca2+ accumulation and altered activity of the tonoplast-enriched Ca2+/H+ antiporter. These results emphasize that regulated expression of Ca2+/H+ antiport activity is critical for normal growth and adaptation to certain stresses.