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 (<i>Lactuca sativa</i>) 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 <i>LsHY5a</i>/<i>b</i> showed positive correlations with ten <i>LsBBXs</i> and flavonoid/anthocyanin-related genes. UV-B treatment rapidly induced the transcription of <i>LsHY5a</i>/<i>b</i> and <i>LsBBX22-1</i>. 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 <i>LsCHS1</i>, <i>LsCHI3</i>, and <i>LsF3H</i>. 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.