The B-box transcription factor BBX5 enhances ultraviolet-B resistance by promoting anthocyanin biosynthesis in Arabidopsis

Abstract Ultraviolet-B (UV-B) light, a natural component of sunlight, plays a crucial role in the regulation of plant growth and development. B-box (BBX) proteins are zinc-finger transcription factors essential for plant growth, development, and responses to abiotic stress. The role of BBX5 in UV-B stress responses has not been previously identified. Here, we identify BBX5 as a novel regulator of UV-B stress tolerance in Arabidopsis. Firstly, UV-B treatment significantly induced the expression of BBX5. Transcriptome analyses revealed that BBX5 was associated with the anthocyanin biosynthesis pathway. Phenotypic analysis showed that bbx5 mutant exhibited heightened UV-B sensitivity and reduced anthocyanin levels under UV-B exposure, whereas BBX5-overexpressing lines displayed increased anthocyanin content and enhanced UV-B resilience. Mechanistically, BBX5 directly interacts with the promoter of anthocyanidin synthase (ANS), thereby stimulating its expression. HY5, a key regulator in UV-B signaling, enhances the expression of BBX5 by binding to its promoter region. Genetic analysis showed that BBX5 overexpression partially restored anthocyanin levels and improved UV-B resistance in hy5-215 mutant. Our study highlights the crucial role of the HY5–BBX5–ANS regulatory axis in modulating anthocyanin biosynthesis and UV-B acclimation in Arabidopsis.