The plant hormone auxin regulates diverse aspects of plant growth and development. Despite its importance, the mechanisms of auxin action remain poorly understood. In particular, the identities of the auxin receptor and other signaling proteins are unknown. Recent studies have shown that auxin acts by promoting the degradation of a family of transcriptional regulators called the Aux/IAA proteins. These proteins interact with another large family of plant-specific transcription factors called Auxin Response Factors (ARF) and negatively regulate their activity. Auxin stimulates Aux/IAA degradation by promoting the interaction between a ubiquitin protein ligase (E3) called SCF(TIR1) and the Aux/IAA protein. In this report, we demonstrate that auxin promotes the interaction between the Aux/IAA proteins and SCF(TIR1) in a soluble extract free of membranes, indicating that this auxin response is mediated by a soluble receptor. In addition, we show that the response is not dependent on protein phosphorylation or dephosphorylation but rather is prevented by an inhibitor of peptidyl-prolyl isomerases.