MrBBX22 induces a flavonol biosynthetic brake MrMYB95 under UV-B radiation in Morella rubra

Flavonols are important bioactive compounds and plant sunscreens in response to UV-B radiation. However, regulatory mechanism of flavonol biosynthetic homeostasis under UV-B radiation remains unclear. In this study, exposure to UV-B radiation resulted in a significant elevation of flavonol content in Morella rubra fruit. MrMYB95 was identified as an induced repressor under UV-B radiation that could constrain effect of activator MrMYB12 and target key structural gene MrFLS2. Transient overexpression of MrMYB95 decreased flavonol level, while virus-induced gene silencing (VIGS) of MrMYB95 increased flavonol accumulation in M. rubra leaves. MrBBX22 was further identified as an activator to induce expression of MrMYB12, MrMYB95, and MrFLS2 upon UV-B exposure. Transient overexpression and VIGS analysis verified the activation effect of MrBBX22 on flavonol accumulation in M. rubra leaves. Thus, MrBBX22 induced both activator and repressor to regulate flavonol biosynthetic homeostasis under UV-B radiation. The presence of the induced repressor in response to UV-B signal indicates a brake mechanism is initiated as well as the induction of flavonol biosynthesis upon UV-B exposure in plants.