Ethylene inhibits ABA-induced stomatal closure via regulating NtMYB184-mediated flavonol biosynthesis in tobacco.

Stomatal movement could be regulated by ABA signaling through synthesis of reactive oxygen species (ROS) in guard cells. By contrast, ethylene triggers biosynthesis of antioxidant flavonol to suppress ROS accumulation and prevent ABA-induced stomatal closure, but the underlying mechanism remains largely unknown. We isolated and characterized tobacco R2R3-MYB NtMYB184, belonging to the flavonol-specific SG7 subgroup. The RNAi suppression (KD line) and knockout (myb184) of NtMYB184 caused a down-regulation of flavonol biosynthetic genes and decreased the concentration of flavonol in tobacco leaves. Yeast one-hybrid assay, transactivation assay, EMSA and ChIP-qPCR demonstrated that NtMYB184 specifically binds to the promoters of flavonol biosynthetic genes via the MYBPLANT motifs. In tobacco, NtMYB184 regulates flavonol biosynthesis in guard cells to modulate ROS homeostasis and stomatal aperture. ABA-induced ROS production was accompanied by the suppression of NtMYB184 and flavonol biosynthesis, which may accelerate ABA-induced stomatal closure. Furthermore, ethylene stimulated the NtMYB184 expression and flavonol biosynthesis to suppress ROS accumulation and curb ABA-induced stomatal closure. In myb184, however, neither the flavonol and ROS concentrations nor the stomatal aperture varied between the ABA and ABA+ethylene treatments, indicating that NtMYB184 was indispensable for the antagonism between ethylene and ABA via regulating flavonol and ROS concentrations in guard cells.