Suppression subtractive hybridization identifies genes induced in response to UV-B irradiation in apple skin: isolation of a putative UDP-glucose 4-epimerase.

Suppression subtractive hybridization (SSH) successfully identified 11 cDNAs in apple skin with highly induced expression as a result of ultraviolet (UV)-B irradiation. Apart from three putative flavonoid biosynthetic genes, chalcone synthase (CHS; A5C), flavanone-3-hydroxylase (F3H; B5F), and flavonol synthase (FLS; D1F), five clones (A1H, A10E, B11G, D5F, and D11H) were induced by low temperature (17 degrees C) as well, which is also known to induce anthocyanin accumulation in apple skin. Moreover, four clones (A1H, A10E, B11G, and D11H), showing higher expression levels in the skin, accumulated higher anthocyanin concentrations than their counterparts. Of the four clones, only A10E, a putative UDP-glucose 4-epimerase (UGE), was deemed to play an important role in anthocyanin accumulation in apple skin based on the facts that: (i) its transcription level was higher in the deep red cultivar, 'Jonathan', than in the pale red cultivar, 'Tsugaru'; and (ii) it could reversibly catalyse UDP-glucose to UDP-galactose, and the latter molecule is a major sugar donor for cyanidin-glycoside in apple. Therefore, the full-length cDNA of A10E was isolated by rapid amplification of cDNA ends (RACE) and designated as MdUGE1. Further analysis demonstrated that UGE enzymatic activity was positively correlated with anthocyanin accumulation in apple skin. Thus, MdUGE1 isolated by SSH could play an important role in anthocyanin biosynthesis in apple skin in concert with other flavonoid biosynthetic genes.