Expression of the vacuolar Ca2+/H+ exchanger, OsCAX1a, in rice:: Cell and age specificity of expression, and enhancement by Ca2+

Calcium is an essential macronutrient for plants and functions in signal transduction. Regulation of the cytosolic calcium concentration is required for normal cell growth. In calcium homeostasis in plant cells, Ca2+/H+ exchangers are involved in Ca2+ compartmentalization into intracellular compartments. Here, we examine the intracellular localization of a rice Ca2+/H+ exchanger, OsCAX1a, fused to a green fluorescent protein and transiently expressed in onion epidermis and rice protoplasts. Green fluorescence was observed in the vacuolar membrane. After sucrose gradient centrifugation of the homogenate of rice plants, OsCAX1a was detected in the same fraction as the vacuolar membrane aquaporin gamma-TIP. We then quantified the mRNA and protein of OsCAX1a in plants grown with metal ions. OsCAX1a mRNA was induced in roots by high concentrations of Ca2+. The protein level in shoots was also increased in the presence of high concentrations of Ca2+. Furthermore, transgenic rice plants transformed with the OsCAX1a promoter fused to beta-glucuronidase showed reporter expression in vascular bundles, stomata, trichomes, steles, flowers, embryos and aleurone layers. In the case of stomata and trichomes, transcription of OsCAX1a was particularly high in aged organs. These results suggest that OsCAX1a transports Ca2+ into vacuoles and is involved in Ca2+ homeostasis in cells that suffer from high concentrations of Ca2+.