Organ specificity of a vacuolar Ca2 +-binding protein RVCaB in radish and its expression under Ca2 +-deficient conditions

Abstract

Radish vacuoles contain a new type of Ca²⁺-binding protein (RVCaB) with high capacity and low affinity for Ca²⁺. The protein is able to stimulate Ca²⁺ uptake into vacuoles, which is driven by Ca²⁺-ATPase and Ca²⁺/H⁺ antiporter. In the present study, we found that the level of RVCaB mRNA is high in seedling hypocotyls and mature taproots but low in young roots and mature leaves. The RVCaB protein was abundant in hypocotyls and taproots but absent in leaves. The levels of the transcript and protein of RVCaB in taproots were gradually increased during maturation. The level of RVCaB mRNA in seedling hypocotyls doubled within a few hours when the growth medium was changed from 10 mM CaCl₂ to water, although the level was strongly suppressed in 100 mM CaCl₂. This response of the RVCaB gene was specific to Ca²⁺ and did not occur with other ions including K⁺ and Mg²⁺. RVCaB functioning as a Ca²⁺-sequestering protein in taproot vacuoles to provide for the Ca²⁺ deficiency is discussed.