Plant hormones play a major role in plant growth and development. They affect similar processes but, paradoxically, their signaling pathways act nonredundantly. Hormone signals are integrated at the gene-network level rather than by cross-talk during signal transduction. In contrast to hormone-hormone integration, recent data suggest that light and plant hormone pathways share common signaling components, which allows photoreceptors to influence the growth program. We propose a role for the plant hormone auxin as an integrator of the activities of multiple plant hormones to control plant growth in response to the environment.