MdBBX21, a B-Box Protein, Positively Regulates Light-Induced Anthocyanin Accumulation in Apple Peel

Abstract

The red coloration of apple (<i>Malus</i> × <i>domestica</i> Borkh.) is due to the accumulation of anthocyanins in the fruit peel. Light is essential for anthocyanin biosynthesis in apple. In this study, we performed a transcriptome sequencing (RNA-seq) analysis of apple fruit exposed to light after unbagging. The identified differentially expressed genes included <i>MdBBX21</i>, which is homologous to Arabidopsis <i>BBX21</i>, suggesting it may be involved in light-induced anthocyanin biosynthesis. Additionally, MdBBX21 was localized in the nucleus and its gene was expressed earlier than <i>MdMYB1</i> in apple peel treated with light. Overexpressing <i>MdBBX21</i> in Arabidopsis and apple calli under light increased anthocyanin accumulation. Dual-luciferase and yeast one-hybrid assays confirmed that MdBBX21 binds to the <i>MdHY5</i>, <i>MdBBX20</i>, and <i>MdBBX22</i>-<i>1</i>/<i>2</i> promoters and induces expression. At the same time, MdHY5 can also activate the expression of <i>MdBBX21</i>. Furthermore, bimolecular fluorescence complementation and yeast two-hybrid assays demonstrated that MdBBX21 can interact with MdHY5. This interaction can significantly enhance <i>MdMYB1</i> promoter activity. These findings clarify the molecular mechanism by which MdBBX21 positively regulates light-induced anthocyanin accumulation in apple.