LAUSR

Under high light conditions or UV radiation, tea plant leaves produce more flavonols, which contribute to the bitter taste of tea; however, neither the flavonol biosynthesis pathways nor the regulation of their production are well understood. Intriguingly, tea leaf flavonols are enhanced by UV-B but reduced by shading treatment. CsFLS , CsUGT78A14, CsMYB12 , and CsbZIP1 were upregulated by UV-B radiation and downregulated by shading. CsMYB12 and CsbZIP1 bound to the promoters of CsFLS and CsUGT78A14 , respectively, and activated their expression individually. CsbZIP1 positively regulated CsMYB12 and interacted with CsMYB12, which specifically activated flavonol biosynthesis. Meanwhile, CsPIF3 and two MYB repressor genes, CsMYB4 and CsMYB7 , displayed expression patterns opposite to that of CsMYB12 . CsMYB4 and CsMYB7 bound to CsFLS and CsUGT78A14 and repressed their CsMYB12-activated expression. While CsbZIP1 and CsMYB12 regulated neither CsMYB4 nor CsMYB7 , CsMYB12 interacted with CsbZIP1, CsMYB4, and CsMYB7, but CsbZIP1 did not physically interact with CsMYB4 or CsMYB7. Finally, CsPIF3 bound to and activated CsMYB7 under shading to repress flavonol biosynthesis. These combined results suggest that UV activation and shading repression of flavonol biosynthesis in tea leaves are coordinated through a complex network involving CsbZIP1 and CsPIF3 as positive MYB activators and negative MYB repressors, respectively. The study thus provides insight into the regulatory mechanism underlying the production of bitter-tasting flavonols in tea plants.