Expression of Arabidopsis CAX1 in tobacco: Altered calcium homeostasis andincreased stress sensitivity

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 e xpression in response to various stimuli. CAX1 was highly expressed in resp onse to exogenous Ca2+. Transgenic tobacco plants expressing CAX1 displayed symptoms of Ca2+ deficiencies, including hypersensitivity to ion imbalance s, such as increased magnesium and potassium concentrations, and to cold sh ock, but increasing the Ca2+ in the media abrogated these sensitivities. To bacco plants expressing CAX1 also demonstrated increased Ca2+ accumulation and altered activity of the tonoplast-enriched Ca2+/H+ antiporter. These re sults emphasize that regulated expression of Ca2+/H+ antiport activity is c ritical for normal growth and adaptation to certain stresses.