Save your TIRs - more to auxin than meets the eye.

Auxin has long been known as an important regulator of plant growth and development. Classical studies in auxin biology have uncovered a 'canonical' transcriptional auxin-signalling pathway involving the TRANSPORT INHIBITOR RESPONSE1/AUXIN SIGNALING F-BOX (TIR1/AFB) receptors. TIR1/AFB perception of auxin triggers the degradation of repressors and the de-repression of auxin-responsive genes. Nevertheless, the canonical pathway cannot account for all aspects of auxin biology, such as physiological responses that are too rapid for transcriptional regulation. This Tansley review will explore several 'non-canonical' pathways that have been described in recent years mediating fast auxin responses. We focus on the interplay between a non-transcriptional branch of TIR1/AFB signalling and a TRANSMEMBRANE KINASE1 (TMK1)-mediated pathway in root acid growth. Other developmental aspects involving the TMKs and their association with the controversial AUXIN-BINDING PROTEIN 1 (ABP1) will be discussed. Finally, we provide an updated overview of the ETTIN (ETT)-mediated pathway in contexts outside of gynoecium development.