Isolation and Functional Analysis of a MYB Transcription Factor Gene that is a Key Regulator for the Development of Red Coloration in Apple Skin

Red coloration of apple (Malus domestica) skin is an important determinant of consumer preference and marketability. Anthocyanins are responsible for this coloration, and their accumulation is positively correlated with the expression level of anthocyanin biosynthetic genes. Regulation of expression of these genes is believed to be controlled by MYB transcription factors, and the MYB transcription factors involved in the activation of anthocyanin biosynthetic genes have been isolated in various plants. In the present study, we isolated and characterized a MYB transcription factor gene (MdMYBA) from apple skin. Characterization of MdMYBA demonstrated that (i) MdMYBA expression was specifically regulated depending on the tissue and cultivar/ species; (ii) its expression level was much higher in a deep-red cultivar ('Jonathan') than in a pale-red cultivar ('Tsugaru'); (iii) when cauliflower mosaic virus 35S::MdMYBA was introduced into the cotyledons of apple seedlings by means of a transient assay, reddish-purple spots were induced, and MdMYBA also induced anthocyanin accumulation in reproductive tissues of transgenic tobacco; (iv) the expression of MdMYBA was induced by UV-B irradiation and lowtemperature treatment, both of which are known to be important in the promotion of anthocyanin accumulation in apple skin; (v) MdMYBA bound specifically to an anthocyanidin synthase (MdANS) promoter region in a gel-shift assay; and (vi) MdMYBA was mapped to the near region of the BC226-STS (a 1 ) marker for the red skin color locus (R f ). These results suggest that MdMYBA is a key regulatory gene in anthocyanin biosynthesis in apple skin.