The aboveground body of higher plants has a modular structure of repeating units, or phytomers. As such, the position, size, and shape of the individual phytomer dictate the plant architecture. The Arabidopsis (Arabidopsis thaliana) ERECTA (ER) gene regulates the inflorescence architecture by affecting elongation of the internode and pedicels, as well as the shape of lateral organs. A large-scale activation-tagging genetic screen was conducted in Arabidopsis to identify novel genes and pathways that interact with the ER locus. A dominant mutant, super1-D, was isolated as a nearly complete suppressor of a partial loss-of-function allele er-103. We found that SUPER1 encodes YUCCA5, a novel member of the YUCCA family of flavin monooxygenases. The activation tagging of YUCCA5 conferred increased levels of free indole acetic acid, increased auxin response, and mild phenotypic characteristics of auxin overproducers, such as elongated hypocotyls, epinastic cotyledons, and narrow leaves. Both genetic and cellular analyses indicate that auxin and the ER pathway regulate cell division and cell expansion in a largely independent but overlapping manner during elaboration of inflorescence architecture.