Colour accumulation patterns and the anthocyanin biosynthetic pathway in 'Red Delicious' apple variants

Colour accumulation and expression of the anthocyanin biosynthetic pathway were analysed in four variants of 'Red Delicious' apple (Malus x domestica) that differed in their levels of red colouration. Colour characteristics were assessed by analysing the percentage red colour skin coverage, colour quality, colour intensity, and anthocyanin concentration during maturation and fruit ripening in two seasons: a typical one, and a high-colouration season. Anthocyanins and their precursors were analysed by HPLC, while expression of structural genes in the anthocyanin biosynthetic pathway was followed by quantitative real-time RT-PCR. Colour characteristics varied between the apple variants. In one variant, a large fraction of the fruit skin turned red at an early stage, while in others the red colour accumulated only close to ripening. Red variants showed earlier appearance and higher levels of anthocyanins and several quercetin derivatives as well as higher expression of genes encoding most of the anthocyanin biosynthetic enzymes. Our results suggest that in the redder variants, the entire anthocyanin biosynthetic pathway, rather than any specific gene, is highly induced in the early stages of fruit maturation. Differences in overall expression patterns of genes that regulate anthocyanin biosynthesis are probably the cause of the different colouration patterns observed in these variants.