The B-box zinc finger protein MdBBX20 integrates anthocyanin accumulation in response to ultraviolet radiation and low temperature.

Ultraviolet-B (UV-B) radiation and low temperature promote the accumulation of anthocyanins, which give apple skins their red colour. Although many transcription regulators have been characterized in the UV-B and low-temperature pathways, their interregulation and synergistic effects are not well understood. Here, a B-box transcription factor gene, MdBBX20, was characterized in apple and identified to promote anthocyanin biosynthesis under UV-B conditions in field experiments and when overexpressed in transgenic apple calli. The transcript level of MdBBX20 was significantly induced by UV-B. Specific G-box elements in the promoters of target genes were identified as interaction sites for MdBBX20. Further experimental interrogation of the UV-B signalling pathways showed that MdBBX20 could interact with MdHY5 in vitro and in vivo and that this interaction was required to significantly enhance the promoter activity of MdMYB1. MdBBX20 also responded to low temperature (14°C) with the participation of MdbHLH3, which directly bound a low temperature-response cis elements in the MdBBX20 promoter. These findings demonstrate the molecular mechanism by which MdBBX20 integrates low-temperature- and UV-B-induced anthocyanin accumulation in apple skin.