The Arabidopsis cax1 mutant exhibits impaired ion homeostasis, development, and hormonal responses and reveals interplay among vacuolar transporters

The Arabidopsis Ca²⁺/H⁺ transporter CAX1 (Cation Exchanger1) may be an important regulator of intracellular Ca²⁺ levels. Here, we describe the preliminary localization of CAX1 to the tonoplast and the molecular and biochemical characterization of cax1 mutants. We show that these mutants exhibit a 50% reduction in tonoplast Ca²⁺/H⁺ antiport activity, a 40% reduction in tonoplast V-type H⁺-translocating ATPase activity, a 36% increase in tonoplast Ca²⁺-ATPase activity, and increased expression of the putative vacuolar Ca²⁺/H⁺ antiportersCAX3 and CAX4. Enhanced growth was displayed by the cax1 lines under Mn²⁺and Mg²⁺ stress conditions. The mutants exhibited altered plant development, perturbed hormone sensitivities, and altered expression of an auxin-regulated promoter-reporter gene fusion. We propose that CAX1 regulates myriad plant processes and discuss the observed phenotypes with regard to the compensatory alterations in other transporters.