Differences in acidity of apples are probably mainly caused by a malic acid transporter gene on LG16

Acidity has profound effects on the taste of apples (Malus × domestica). Malic acid is the predominant organic acid in apples. Differences in malic acid content are caused by differences in accumulation of malic acid in the vacuole. This accumulation may be caused by a gene that is responsible for transport of malic acid from the cytosol into the vacuole. Here, we provide evidence that a malic acid transporter gene at the top of chromosome 16 caused significant differences in malic acid concentration and pH of apples. The pH of apples in a segregating F1 population was mapped and at the pH locus (named henceforth Ma locus for malic acid), two putative malic acid transporter genes were detected. These genes show high homology to AtALMT genes that code for malate channel proteins located in vacuolar membrane in Arabidopsis. The expression of one of the candidate genes (Ma1) cosegregated clearly with malic acid content. The inheritance of at least one dominant allele of this gene sufficed for an increased expression level that likely caused the observed threefold increase of the malic acid concentration and the reduction of the pH from 4 to 3 in mature apples, compared to the presence of the recessive, lowly expressed allele only. Our results show that differences in fruit acidity were probably caused by differences in expression levels of alleles of a malic acid transporter gene.