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

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.