Multiomics Analysis Revealed the Role of LsBBX22–LsHY5a/b Module in Promoting UV–B-Mediated Anthocyanin Accumulation in Lettuce

Ultraviolet-B (UV–B) radiation profoundly affects plant photomorphogenesis and secondary metabolism, yet its regulatory role in lettuce (Lactuca sativa) remains poorly understood. We conducted metabolome and transcriptome analyses to reveal the molecular basis of UV–B adaptation in highland-grown lettuce. Flavonoid accumulation was enhanced as a key adaptive strategy for UV–B acclimation. The key UV–B signaling components LsHY5a/b showed positive correlations with ten LsBBXs and flavonoid/anthocyanin-related genes. UV–B treatment rapidly induced the transcription of LsHY5a/b and LsBBX22-1. LsBBX22-1 physically interacted with LsHY5a/b. While LsBBX22-1 activated the expression of anthocyanin-related genes, it could not bind directly to their promoters. By contrast, LsHY5a/b directly bound to the promoters of LsCHS1, LsCHI3, and LsF3H. Thus, LsBBX22 and LsHY5a/b acted synergistically to regulate UV–B-induced anthocyanin biosynthesis. This study clarified the molecular mechanism of the LsBBX22-1–LsHY5a/b module in promoting UV–B-induced anthocyanin accumulation in lettuce, providing a theoretical basis for cultivating high-quality vegetables in highland regions.