The diversity and environmental plasticity of plant growth resultsfrom variations of repetitive modules, such as the basic shoot unitsmade of a leaf, axillary bud, and internode. Internode elongationis regulated both developmentally and in response to environmentalconditions, such as light quality, but the integration of internal andenvironmental signals is poorly understood. Here, we show that thecompressed rosette growth habit of Arabidopsis is maintained by theconvergent activities of the organ boundary gene ARABIDOPSISTHALIANA HOMEOBOX GENE 1 (ATH1) and of the gibberellinsignalingDELLA genes. Combined loss of ATH1 and DELLA functionactivated stem development during the vegetative phase andchanged the growth habit from rosette to caulescent. Chromatinimmunoprecipitation high-throughput sequencing and genetic analysisindicated that ATH1 and the DELLA gene REPRESSOR OF GA1-3(RGA) converge on the regulation of light responses, including thePHYTOCHROME INTERACTING FACTORS (PIF) pathway, and showedthat the ATH1 input is mediated in part by direct activation ofBLADE ON PETIOLE (BOP1 and BOP2) genes, whose products destabilizePIF proteins. We conclude that an organ-patterning gene convergeswith hormone signaling to spatially restrict environmentalresponses and establish a widespread type of plant architecture.