PbPH5, an H+ P-ATPase on the tonoplast, is related to malic acid accumulation in pear fruit

Organic acid content is one of the most important factors influencing fruit flavors. The predominant organic acid in most pear cultivars is malic acid, but the mechanism controlling its accumulation remains unclear. In this study, by comparing gene expression levels and organic acid contents, we found that the expression of PbPH5, which encodes a P3A-ATPase, is highly correlated with malic acid accumulation in four different pear species, with correlation coefficients of 0.932**, 0.656*, 0.900**, and 0.518* (*, P<0.05; **, P<0.01) for Pyrus bretschneideri Rehd., P. communis Linn., P. pyrifolia Nakai., and P. ussuriensis Maxim., respectively. Moreover, the overexpression of PbPH5 in pear significantly increased the malic acid content. In contrast, silencing PbPH5 via RNA interference significantly decreased both its transcript level and the pear fruit malic acid content. A subcellular localization analysis indicated that PbPH5 is located in the tonoplast. Additionally, a phylogenetic analysis indicated that PbPH5 is a PH5 homolog gene that is clustered with the Petunia hybrida, Malus domestica, and Citrus reticulata genes. Considered together, these findings suggest that PbPH5 is a functionally conserved gene. Furthermore, the accumulation of malic acid in pear fruit is at least partly related to changes in the PbPH5 transcription levels.