Brassinosteroid-Activated BRI1-EMS-SUPPRESSOR 1 Inhibits Flavonoid Biosynthesis and Coordinates Growth and UV-B Stress Responses in Plants

UV-B light is a potential stress factor in plants, but how plants coordinate growth and UV-B stress responses is not well understood. Here, we report that brassinosteroid (BR) signaling inhibits UV-B stress responses in Arabidopsis (Arabidopsis thaliana) and various crops by controlling flavonol biosynthesis. We further demonstrate that BRI1-EMS-SUPPRESSOR 1 (BES1) mediates the tradeoff between plant growth and UV-B defense responses. BES1, a master transcription factor involved in BR signaling, represses the expression of transcription factor genes MYB11, MYB12, and MYB111, which activate flavonol biosynthesis. BES1 directly binds to the promoters of these MYBs in a BR-enhanced manner to repress their expression, thereby reducing flavonol accumulation. However, exposure to broadband UV-B down-regulates BES1 expression, thus promoting flavonol accumulation. These findings demonstrate that BR-activated BES1 not only promotes growth but also inhibits flavonoid biosynthesis. UV-B stress suppresses the expression of BES1 to allocate energy to flavonoid biosynthesis and UV-B stress responses, allowing plants to switch from growth to UV-B stress responses in a timely manner.