Screening of UV-B-induced genes from apple peels by SSH: possible involvement of MdCOP1-mediated signaling cascade genes in anthocyanin accumulation.

Suppression subtractive hybridization (SSH) was employed to identify candidate genes involved in red coloration in apple peel with the ultraviolet (UV)-B-treated 'Mutsu'. After reverse Northern blotting verification, nearly 80 clones were successfully sequenced. Large portions of the expressed sequence tags (ESTs) are well characterized anthocyanin biosynthesis-related genes, such as chalcone synthase (11A5), flavonol synthase (12F3), anthocyanidin synthase (11H5) and UDP-glycosyl transferase (14A12) whose presence proved the success of SSH. Eight ESTs were selected for quantitative real-time polymerase chain reaction analysis and their expressions were all elevated in 'Induction', further confirming the reliability of the SSH library. One EST, 11F4 (CONSTITUTIVE PHOTOMORPHOGENIC 1: COP1) with putative function in light signal relay was further analyzed in 'Mutsu' and 'Tsugaru', along with MdHY5 (ELONGATED HYPOCOTYL 5: the downstream target of COP1), MdMYB22 (a possible flavonol-specific activator under the regulation of HY5, belonging to the SG7/PRODUCTION OF FLAVONOL GLYCOSIDES family) and MdMYBA. Results showed that MdCOP1, MdHY5, MdMYB22 and MdMYBA were all UV-B inducible genes and anthocyanin accumulation occurred after their increased expressions. Moreover, their expressions and anthocyanin content were enhanced under UV-B plus 17°C treatment. The presence of G box, a known consensus binding site of HY5, in the MdMYBA promoter region implicated that it could be regulated by MdHY5, which was verified by the result of the yeast one-hybrid analysis. Our data suggested that UV-B irradiation would induce the utmost upstream light signaling factor, MdCOP1, which activates MdHY5 signaling by binding to the promoter regions of MdMYBs, and finally leads to the red coloration of apple peels.