The plant hormone auxin is a key regulator of growth and development. Zhao et al. identified a compound, sirtinol, that stimulates biological responses similar to those initiated by auxin. They then screened for mutants of Arabidopsis that did not respond to sirtinol and identified a mutant they called sir1 that encodes a protein with similarity to a ubiquitin-activating E1-like protein. This result may help explain how auxin can rapidly degrade negative regulators as part of its mechanism of action.