Significance Understanding how hormones like auxin control plant growth and development has fascinated scientists since Darwin. The past two decades have seen breakthroughs in elucidating the molecular basis of auxin transport, perception, and response, but little is known about how auxin is metabolized or its homeostasis is controlled. We report that the DIOXYGENASE FOR AUXIN OXIDATION 1 ( AtDAO1 ) enzyme represents the major pathway for auxin oxidation in Arabidopsis . Disrupting AtDAO1 function elevates levels of auxin conjugates between ∼50- and 280-fold, but auxin levels remain close to the WT, helping explain why mutant phenotypes are relatively weak. We conclude that AtDAO1 and auxin conjugation pathways play highly redundant roles and reveal a level of regulation of auxin abundance in plants.