Significance C ₄ cereals in subfamily Panicoideae typically produce large and long leaf blades for efficient capture of light and photosynthesis but the leaves droop downward, particularly at the adult stage, thus, adversely affecting canopy structure and grain yield. Identification of key regulators that control leaf droopiness is crucial to improve plant architecture in these crops. We showed that DPY1, a regulator of SiBRI1–SiBAK1 interaction, prevents BR signaling from overactivation in response to high doses of BRs to ensure that the long leaf blades grow upward in Setaria . Overexpressing DPY1 improves plant architecture with upright leaves. This study provides cellular and molecular insights into plant architecture control for cereal breeding.