Two B-Box Proteins, GhBBX21 and GhBBX24, Antagonistically Modulate Anthocyanin Biosynthesis in Cotton.

The red plant phenotype of cotton is a genetic marker produced by light-induced anthocyanin accumulation. controls this trait. There are two 228 bp tandem repeats upstream of in cotton. In this study, GUS staining assays in transgenic (L.) Heynh. demonstrated that tandem repeats in the promoter-enhanced transcriptional activity. is a homolog of . expression is regulated by photomorphogenesis-related transcription factors such as AtHY5 and AtBBXs. We identified the homologs of , , and in cotton, designated as , , and , respectively. Y1H assays confirmed that GhHY5, GhBBX21, and GhBBX24 each bound to the promoter. Dual-luciferase reporter assays revealed that GhHY5 weakly activated the promoter activity of . Heterologous expression assays in indicated that promoted anthocyanin accumulation, whereas had the opposite effect. Dual-luciferase assays showed GhBBX21 activated transcription, while GhBBX24 repressed it. Further study indicated that GhHY5 did not enhance GhBBX21-mediated transcriptional activation of but alleviates GhBBX24-induced repression. Together, our results demonstrate that GhBBX21 and GhBBX24 antagonistically regulate anthocyanin accumulation in 1 cotton under GhHY5 mediation, providing insights into light-responsive anthocyanin biosynthesis in cotton.