AUTHOR=Zhang Bo , Zhu Zhen-Zhen , Qu Dong , Wang Bo-Chen , Hao Ni-Ni , Yang Ya-Zhou , Yang Hui-Juan , Zhao Zheng-Yang 

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

JOURNAL=Frontiers in Plant Science

VOLUME=12

YEAR=2021

URL=https://www.frontiersin.org/journals/plant-science/articles/10.3389/fpls.2021.774446

DOI=10.3389/fpls.2021.774446

ISSN=1664-462X

ABSTRACT=<p>The red coloration of apple (<italic>Malus</italic> × <italic>domestica</italic> 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 <italic>MdBBX21</italic>, which is homologous to Arabidopsis <italic>BBX21</italic>, suggesting it may be involved in light-induced anthocyanin biosynthesis. Additionally, MdBBX21 was localized in the nucleus and its gene was expressed earlier than <italic>MdMYB1</italic> in apple peel treated with light. Overexpressing <italic>MdBBX21</italic> in Arabidopsis and apple calli under light increased anthocyanin accumulation. Dual-luciferase and yeast one-hybrid assays confirmed that MdBBX21 binds to the <italic>MdHY5</italic>, <italic>MdBBX20</italic>, and <italic>MdBBX22</italic>-<italic>1</italic>/<italic>2</italic> promoters and induces expression. At the same time, MdHY5 can also activate the expression of <italic>MdBBX21</italic>. Furthermore, bimolecular fluorescence complementation and yeast two-hybrid assays demonstrated that MdBBX21 can interact with MdHY5. This interaction can significantly enhance <italic>MdMYB1</italic> promoter activity. These findings clarify the molecular mechanism by which MdBBX21 positively regulates light-induced anthocyanin accumulation in apple.</p>