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

• Published in: Nature reviews. Molecular cell biology Vol. 7; no. 11; pp. 847 - 859
• Main Authors: Teale, William D., Paponov, Ivan A., Palme, Klaus
• Format: Journal Article
• Language: English
• Published: London Nature Publishing Group UK 01.11.2006; Nature Publishing Group
• Subjects: ADP-Ribosylation Factors - metabolism; Arabidopsis - genetics; Arabidopsis - growth & development; Arabidopsis - metabolism; Arabidopsis Proteins - genetics; Arabidopsis Proteins - metabolism; Biochemistry; Biological Transport; Biomedical and Life Sciences; Cancer Research; Cell Biology; Cell division; Cluster Analysis; Developmental Biology; F-Box Proteins - metabolism; Gene expression; Gene Expression Regulation, Plant; Hormones; Indoleacetic Acids - chemistry; Indoleacetic Acids - metabolism; Influence; Life Sciences; Molecular Structure; Physiology; Plant growth; Plant physiology; Proteins; Receptors, Cell Surface - metabolism; review-article; Signal Transduction; Stem Cells
• ISSN: 1471-0072; 1471-0080
• DOI: 10.1038/nrm2020

Key Points 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 SCF TIR1 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. Auxins are a class of plant hormones that affect various aspects of plant development. Recent studies have identified certain F-box-domain proteins as auxin receptors that control auxin-regulated gene expression through ubiquitin-mediated proteolysis. Other modes of auxin perception and signalling are also thought to exist. Hormones have been at the centre of plant physiology research for more than a century. Research into plant hormones (phytohormones) has at times been considered as a rather vague subject, but the systematic application of genetic and molecular techniques has led to key insights that have revitalized the field. In this review, we will focus on the plant hormone auxin and its action. We will highlight recent mutagenesis and molecular studies, which have delineated the pathways of auxin transport, perception and signal transduction, and which together define the roles of auxin in controlling growth and patterning.