CmBBX28-CmMYB9a Module Regulates Petal Anthocyanin Accumulation in Response to Light in Chrysanthemum

Light is one of the most important environmental factors that affect plant growth and development. It also stimulates anthocyanin biosynthesis in plants. However, the precise molecular mechanisms through which light regulates anthocyanin biosynthesis, particularly in non-model plant species, remain poorly understood. In this study, we discovered a CmBBX28-CmMYB9a molecular module that is responsive to light and regulates anthocyanin biosynthesis in chrysanthemums. Specifically, CmBBX28 interacts with CmMYB9a, interfering with its binding to the promoters of target genes and reducing the protein abundance of CmMYB9a. This interaction downregulates the transcription of CmMYB9a's downstream anthocyanin-associated genes, CmCHS, CmDFR, and CmUFGT. The expression of CmBBX28 was induced in the dark, and the accumulated CmBBX28 proteins interfered with the activation of CmMYB9a during anthocyanin biosynthesis. Concurrently, darkness also inhibited the expression of CmMYB9a to some extent. In contrast, light significantly induced the expression of CmMYB9a and suppressed the expression of CmBBX28, resulting in increased anthocyanin accumulation in chrysanthemum petals. Our findings reveal the mechanism by which light regulates anthocyanin biosynthesis in chrysanthemum flower petals.