Anthocyanin accumulation and related gene family expression in the skin of dark-grown red and non-red apples (Malus domestica Borkh.) in response to sunlight

- Anthocyanin biosynthesis in apple skin must be induced by light.
- Transcript levels of related genes in apple skin were enhanced by light.
- UFGT family showed differences between red and non-red cultivars.
- Mechanisms of anthocyanin regulation in apple skin of different colors are different.

We investigated anthocyanin concentrations by HPLC (high-performance liquid chromatography) and expression of MYB, bHLH and UFGT gene families by quantitative real time PCR in apple skin of red cultivar 'Starkrimson' and non-red cultivars 'Golden Delicious' and 'Granny Smith' in response to light. Young fruits were bagged at 40 DAFB (days after full bloom), and fruits of 'Golden Delicious' and 'Starkrimon' were debagged at 120 DAFB, while fruits of 'Granny Smith' at 160 DAFB. In bagged fruit, anthocyanin synthesis and related genes expression were both depressed, whereas these genes expression was enhanced by light, accompanied by anthocyanin accumulation in apple skin. 'Starkrimson' and 'Granny Smith' accumulated relatively higher anthocyanin concentrations than 'Golden Deliicous'. Transcript levels of most selected regulatory genes, including MdMYB1-1, MdMYB1-2, MdbHLH3-1 and MdbHLH33-1, were up-regulated in apple skin of all the three cultivars, although transcript levels of these genes in 'Starkrimson' and 'Granny Smith' increased before outer layered bag removal, and increased after outer layered in 'Golden Delicious'. However, UFGT family showed differences between red and non-red cultivars. MdUFGT2 was up-regulated only in non-red cultivars, while MdUFGT4 was up-regulated only in red skin cultivar, suggesting that UFGT is also important for anthocyanin accumulation in apple skin of different colors. Our results indicated that anthocyanin regulation in different colored apple cultivars were accomplished through different mechanisms, which will be valuable for further research of anthocyanin regulation in apple skin and for apple breeding programs that take genetic tools to improve the consistency of color in apples.