Increased planting densities have boosted maize yields. Upright plant architecture facilitates dense planting. Here, we cloned ( ) and , two quantitative trait loci conferring upright plant architecture. is controlled by a two-base sequence polymorphism regulating the expression of a B3-domain transcription factor ( ) located 9.5 kilobases downstream. exhibits differential binding by DRL1 (DROOPING LEAF1), and DRL1 physically interacts with LG1 (LIGULELESS1) and represses LG1 activation of regulates ( ), which underlies , altering endogenous brassinosteroid content and leaf angle. The allele that reduces leaf angle originated from teosinte, the wild ancestor of maize, and has been lost during maize domestication. Introgressing the wild allele into modern hybrids and editing enhance high-density maize yields.