Transcriptome Analysis Reveals Intensity-Dependent Regulation of UV-B Radiation on Glucosinolate Biosynthesis in Rapeseed Leaves

Rapeseed (Brassica napus L.) is a globally important oilseed crop; however, its ‘double-low’ cultivars exhibit substantially reduced glucosinolate levels in vegetative tissues. To investigate whether UV-B radiation could be used to enhance glucosinolate accumulation, we systematically examined the modulation of glucosinolate profiles and associated biosynthetic pathways in leaves of the ‘double-low’ cultivar NY4 under white light (WL) supplemented with two UV-B intensities: low-intensity UV-B (UVBL, 0.1 W m−2) and high-intensity UV-B (UVBH, 0.4 W m−2). Rapeseed seedlings were treated for 21 days under a 16 h photoperiod, and leaf samples were collected at the end of the treatment period, with three biological replicates per condition. Compared with the WL control, UVBL significantly increased total glucosinolate content by 64.57%, driven predominantly by elevated accumulation of progoitrin and neoglucobrassicin. In contrast, UVBH reduced total glucosinolate levels but markedly elevated gluconasturtiin content. Transcriptome analysis revealed that UVBL upregulated key genes involved in glucosinolate biosynthesis (e.g., MAM, IPMDH, CYP79F1, and SOT17/18) and transcription factors (e.g., MYB28, MYB34, MYB51, and MYB122). Conversely, UVBH downregulated genes associated with side-chain elongation of aliphatic glucosinolates and secondary modification of indolic glucosinolate. Collectively, these results demonstrate that low-intensity UV-B radiation can effectively boost total glucosinolate content in rapeseed leaves via transcriptional reprogramming.