GOLDEN 2-LIKE transcription factors regulate chlorophyll biosynthesis and flavonoid accumulation in response to UV-B in tea plants

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.