Auxin in action: signalling, transport and the control of plant growth and development

Auxins form a class of small indolic plant growth regulators that was initially identified as the motile stimulus that causes plants to bend towards the light. Auxins have profound effects on many aspects of plant development — they affect cell division, elongation and differentiation — but their modes of action are complicated.The most well-characterized auxin signalling events are mediated by auxin-responsive elements (AREs) in the promoters of auxin primary-response genes. ARE-mediated transcription is initiated through the action of auxin response factors (ARFs) to AREs. ARE-mediated transcription is inhibited by the binding of Aux/IAA proteins to ARFs.Auxin functions by stimulating the ubiquitin-mediated proteolysis of Aux/IAAs. Aux/IAAs are marked for proteolysis by the action of the SCFTIR1 E3 ubiquitin ligase. The association between TIR1 and Aux/IAAs is enhanced by the direct binding of auxin to TIR1. TIR1 is one of a family of four similar F-box-domain-containing proteins, which all mediate an auxin response and represent the first confirmed auxin receptors.These F-box proteins probably do not represent the only mechanism of auxin perception. Other auxin-binding proteins such as AUXIN-BINDING PROTEIN-1 (ABP1) also confer auxin sensitivity and might also represent auxin receptors. These other pathways are likely to be extremely rapid, and certain auxin responses are seen almost instantaneously.The phytohormone that best fits the classical definition of a hormone is auxin as it is transported between its site of synthesis and site of action. This transport is mediated by several transporters and signalling events that are separate from ARE-mediated gene transcription.Although not traditionally considered together, it is becoming increasingly clear that auxin signalling and auxin transport are linked. The involvement of an increasing number of proteins that were previously considered as components of signalling networks that are also involved in auxin transport is currently opening a new chapter in auxin biology.