Abstract
Abstract Bitter pit (BP) is a physiological disorder of calcium deficiency in apple fruit and has not been fully characterized. The Ca2+/H+ exchanger (CAX) is a kind of Ca2+ transport protein. In this study, we analysed mineral contents in different parts of fruit, the expression levels of genes related to Ca transport and the functional roles of MdCAXs. The results showed lower water-soluble Ca content and significantly higher ratios of water-soluble P/Ca and (K + Mg)/Ca in the calyx-end flesh of BP and normal fruit than the peduncle-end flesh of them. The expression level of MdCAX11 increased as the severity of BP increased, and the expression profile was negatively correlated with the water-soluble Ca content. Yeast growth assays showed that both MdCAX11 and MdCAX5 have the function of transporting Ca to vacuoles. Subcellular localization analysis showed that MdCAX11-GFP and MdCAX5-GFP were colocalized on the vacuole membrane with a tonoplast marker. Taken together, the results of this study indicate that high expression levels of MdCAX11 and MdCAX5 may cause an influx of Ca from the cytosol into vacuoles, which may be related to the occurrence of BP.
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