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

The vacuolar H+-pyrophosphatase (VHP) is a proton pump, which energizes transport across the tonoplast. The contributions of VHP to ion, organic acid and sugar storage are unclear in fruit. Here we characterized the role of an apple vacuolar H+-pyrophosphatase gene (MdVHP1) in Na+, malate and soluble sugar accumulation. MdVHP1 expression was consistent with VHP activities in apple fruits at most developmental stages, and highly significantly correlated with Na+ concentration during fruit development. In apple fruits treated in vitro (i.e., fruit-bearing branches were cultured in Hoagland solution containing each of salt, malate and sucrose, respectively, or irradiated by blue light), MdVHP1 expression was significantly correlated with Na+ and malate transporter genes MdNHX1 and MdtDT, while subunit A of the vacuolar H+-ATPase (MdVHA-A) was significantly correlated with MdtDT and sucrose transporter gene (MdSUT1). In addition, MdVHP1 overexpression noticeably promoted Na+ and malate accumulation, but slightly increased soluble sugar accumulation in transgenic apple callus and tomato fruit, partially by regulating transporter genes MdNHX1, MdtDT and MdSUT1. Taken together, it appears that MdVHP1 favorably contributes to Na+, malate and soluble sugar accumulation in apple fruit.