From perception to attenuation: auxin signalling and responses

• TIR1/AFB-AUX/IAA auxin co-receptors show auxin-dependent specificities for each pair.
• The ABP1 auxin receptor mediates auxin signalling at the PM, and perhaps at the ER.
• The SKP2a auxin receptor is a positive regulator of cell division.
• Attenuation of auxin signalling is ROS-dependent.

The plant hormone auxin is essential for growth, development, and responses to environmental factors. Recently, Auxin Binding Protein 1 was shown to mediate non-transcriptional auxin signalling at the cell periphery. This has provoked reexamination of the paradigm that all auxin perception is intracellular and is mediated by the TIR1/AFB-Aux/IAA co-receptors for which auxin functions as a concentration-dependent molecular glue. Further, another F-box protein, SKP2a, was shown to bind auxin in the same way as TIR1/AFB, which provides a link to the role of auxin in the cell cycle. New work on auxin signalling and homeostasis include D6 PROTEIN KINASE activation of PINFORMED (PIN) auxin carriers, ROP-GTPase mediation of PIN localization, endoplasmic reticulum localization PIN and PIN-LIKES auxin carriers, and auxin biosynthesis and metabolism.