A protein found in plant cells that binds auxins and thus acts as the initial receptor of the signal transduction pathway for these plant hormones. A receptor for auxin has been identified as the protein TIR1 (transport inhibitor response 1), found in the nucleus. Binding of auxin to TIR1 leads to the degradation of certain repressors of gene transcription, which allows genes to be ‘switched on’. Another, cytosolic, auxin-binding protein called ABP1 has been proposed as a key component of the auxin signalling pathway, although recent research has refuted this role in Arabidopsis. See also acid growth theory.