Expression analysis and functional characterization of apple MdVHP1 gene reveals its involvement in Na+, malate and soluble sugar accumulation

► MdVHP1 expression was consistent with VHP activities in apple fruits. ► MdVHP1 expression was significantly correlated with MdNHX1 and MdtDT. ► MdVHP1 overexpression noticeably increases Na+ and malate content in apple callus. ► MdVHP1 overexpression significantly increases Na+ and malate content in tomato fruit.