A MaBBX21-MaGCN5 Histone Acetyltransferase Module Regulates Flavonoid Biosynthesis to Improve Heat and UV-B Stress Tolerance in Banana ( Musa acuminata )

ABSTRACT Flavonoids are important specialized metabolites that contribute to plant pigmentation, stress adaptation, and nutritional value. In banana, a major global staple crop, their accumulation is highly tissue-specific, with very low levels in the edible pulp, and the mechanisms underlying this spatial distribution remain unclear. Here, integrative transcriptomic and metabolomic analyses across vegetative and reproductive tissues reveal that light-responsive regulatory networks control tissue-specific flavonoid biosynthesis. We identified a B-box transcription factor MaBBX21 as a key positive regulator of flavonoid biosynthesis. Its overexpression enhances flavonoid accumulation, whereas knockdown leads to a reduction in flavonoid levels in banana. Mechanistically, MaBBX21 interacts with MaHY5 and directly activates anthocyanin biosynthesis genes ( MaDFR2 and MaANS ). It also regulates metabolic flux by binding to the MaWRKY23 promoter, promoting flavonol biosynthesis through activation of MaFLS1 while partially repressing the anthocyanin branch. In addition, MaBBX21 introduces an epigenetic layer by activating the histone acetyltransferase MaGCN5, increasing H3K9 acetylation at target promoters, including MaDFR2, MaANS, and MaBBX21 itself, thereby forming a positive feedback loop. Functionally, MaBBX21 overexpression enhances flavonoid accumulation, ROS scavenging, and tolerance to heat and UV-B stress, whereas knockdown lines show reduced metabolite levels and increased stress sensitivity. Collectively, these results define a MaBBX21–MaWRKY23/MaGCN5 regulatory axis that integrates transcriptional regulation, and chromatin modification to control flavonoid biosynthesis, providing a foundation for improving nutritional quality and stress resilience in banana.