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

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

doi:10.3389/fpls.2021.774446

Abstract

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

Highlights

Apples are one of the most widely cultivated fruits worldwide
MdBBX21 Responds to Light and Induces Anthocyanin Biosynthesis in Apple
Light is required for anthocyanin biosynthesis in the apple fruit peel (Saure, 1990)

Summary

Introduction

Apples are one of the most widely cultivated fruits worldwide. Red-skinned apples are more popular with consumers than green-skinned or yellow-skinned apples. The redness of the peel is mainly determined by anthocyanins, which are natural water-soluble pigments (Kim et al, 2003; Zhang Y. et al, 2014). Anthocyanins are stored in plant vacuoles in the form of glycosides (Passeri et al, 2016). The diversity in the colors of flowers, stems, leaves, fruits, and other plant organs and tissues is due to the type, content, and distribution of anthocyanins (Lawrence et al, 1939; Zhang H. et al, 2014)

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