Upregulation of MdMYB110a is responsible for ABA-mediated colouration of type 2 red-fleshed apples

Type 2 red-fleshed apples are attractive to consumers due to their sweet taste, which is not present in type 1 apples, in addition to their vivid colour and the accumulation of health-promoting polyphenol compounds to a greater degree than in white apples. Although the red pigmentation of type 2 apples is caused by the expression of MdMYB110a at colouring initiation, the endogenous factors that induce the expression of MdMYB110a are poorly understood. In this study, we investigated the effect of two phytohormones, ethylene and abscisic acid (ABA), on MdMYB110a expression and flesh pigmentation in type 2 red-fleshed apples. Although induction of the expression of the ethylene biosynthetic gene MdACO1 coincided with the onset of flesh pigmentation, no obvious correlation was found between treatment with ethylene or its inhibitor 1-methylcyclopropene and the expression pattern of MdMYB110a. However, homologues of ABA-induced genes in apple were differentially expressed between pigmented and nonpigmented flesh derived from a single fruit, and ABA treatment in type 2 red-fleshed fruits led to the upregulation of MdMYB110a expression and the enhancement of anthocyanin production. These results strongly suggest that ABA, not ethylene, induced upregulation of MdMYB110a, the gene responsible for the type 2 red-flesh trait.