Panicle architecture is an agronomic determinant of crop yield and a target for cereal crop improvement. To investigate its molecular mechanisms in rice, we performed map-based cloning and characterization of OPEN PANICLE 1 (OP1), a gain-of-function allele of LIGULELESS 1 (LG1), controlling the spread-panicle phenotype. This allele results from a 48-bp deletion in the LG1 upstream region and promotes pulvinus development at the base of the primary branch. Increased OP1 expression and altered panicle phenotype in chimeric transgenic plants and upstream-region knockout mutants indicated that the deletion regulates spread-panicle architecture in the mutant spread panicle 1 (sp1). Knocking out BRASSINOSTEROID UPREGULATED1 (BU1) gene in the background of OP1 complementary plants resulted in compact panicles, suggesting OP1 may regulate inflorescence architecture via the brassinosteroid signaling pathway. We regard that manipulating the upstream regulatory region of OP1 or genes involved in BR signal pathway could be an efficient way to improve rice inflorescence architecture.