UV-B application enhances the aroma quality of oolong tea; however, the underlying regulatory mechanism remains unclear. This study investigates the regulatory role of UV-B in the biosynthesis of α-farnesene, an important floral and fruity characteristic aroma. UV-B treatment significantly improved the aroma quality of ‘Foshou’ and ‘Yuquan’ oolong teas, increasing α-farnesene levels by 1.8- and 1.4-fold, respectively. The α-farnesene synthase (CsAFS), ELONGATED HYPOCOTYL 5 (CsHY5), and myelocytomatosis protein 2 (CsMYC2) exhibited a highly correlated expression pattern closely associated with α-farnesene accumulation. Single-factor treatment revealed that CsAFS expression was induced by both UV-B and mechanical wounding, with CsHY5 predominantly responding to UV-B radiation, while CsMYC2 primarily responded to tumbling-induced mechanical wounding signal. Transient suppression of CsHY5 in tea leaves reduced the expression of both CsAFS and CsMYC2 whereas CsMYC2 suppression decreased CsAFS expression. G-box motifs were identified in promoters of CsMYC2 and CsAFS, and the dual-luciferase reporter assay (LUC) and electrophoretic mobility shift assays (EMSA) demonstrated direct binding functions of CsHY5 to CsAFS and CsMYC2 promoters, as well as CsMYC2 to the CsAFS promoter. Based on sensory evaluation, odourant quantification, gene expression, and molecular functional analysis, we propose that UV-B radiation and tumbling-induced wounding signals synergistically regulate α-farnesene biosynthesis through a coordinated interaction of CsHY5 and CsMYC2 during oolong tea processing. These findings improve our understanding of flavour formation during oolong tea production and also provide novel insights into artificial light application in tea manufacturing.

Flavonoids are critical secondary metabolites that determine the health benefits and flavor of tea, while chlorophylls are important contributors to the appearance of tea. However, transcription factors (TFs) that can integrate both chlorophyll biosynthesis and flavonoid accumulation in response to specific light signals are rarely identified. In this study, we report that the GOLDEN 2-LIKE TF pair, CsGLK1 and CsGLK2, orchestrate UV-B-induced responses in the chlorophyll biosynthesis and flavonoid accumulation of tea leaves. The absence of solar UV-B reduced the transcriptional expression of CsGLKs in the tea leaves and was highly correlated with a decrease in flavonoid levels (especially flavonol glycosides) and the expression of genes and TFs involved in chlorophyll biosynthesis and flavonoid accumulation. In vivo and in vitro molecular analyses showed that CsGLKs could be regulated by the UV-B signal mediator CsHY5, and could directly bind to the promoters of gene and TF involved in light-harvesting (CsLhcb), chlorophyll biosynthesis (CsCHLH, CsHEMA1, and CsPORA), and flavonoid accumulation (CsMYB12, CsFLSa, CsDFRa, and CsLARa), eventually leading to UV-B-induced responses in the chlorophylls and flavonoids of tea leaves. Furthermore, UV-B exposure increased the levels of total flavonoids, CsGLK1 protein, and expression of CsGLKs and target genes in the tea leaves. These results indicate that CsGLKs may modulate tea leaf characteristics by regulating chlorophyll biosynthesis and flavonoid accumulation in response to solar UV-B. As the first report on UV-B-induced changes in flavonoid and chlorophyll regulation mediated by CsGLKs, this study improves our understanding of the environmental regulations regarding tea quality and sheds new light on UV-B-induced flavonoid responses in higher plants.