Auxin

Abstract

Auxin is an essential plant hormone that regulates growth and development of plants from embryo development to senescence. Auxin exerts its regulatory effects by controlling cell division, cell expansion and cell differentiation. According to the currently accepted model, auxin is mainly synthesised from tryptophan in the shoot apex and transported to other tissues, where auxin is perceived in the nucleus coordinately by two groups of co‐receptors, and controls growth and development by regulating the expression of auxin‐responsive genes. Auxin acts in association with many other growth regulatory factors and external cues to properly coordinate plant growth and development with its environment. While molecular mechanisms involved in auxin biosynthesis, transport and signalling have been unearthed to a great extent, a complete understanding of these intricate mechanisms is yet to be acquired.

Key Concepts

  • Auxins are small organic molecules with an essential aromatic ring structure and a carboxyl side group of variable length.
  • Auxin regulates almost every aspect of plant growth and development by affecting cell division, expansion and differentiation at all stages of life.
  • The predominant natural auxin in plants is indole‐3‐acetic acid, and many synthetic chemicals with auxinic activity are being widely used as plant growth regulators and selective herbicides.
  • Plants maintain the auxin homeostasis by coordinating biosynthesis, transport, conjugation/deconjugation and degradation of auxins.
  • Auxin is coordinately perceived by two groups of co‐receptor proteins in the nucleus, resulting in degradation of Aux/IAA repressors, thereby modulating the expression of auxin‐responsive genes.
  • There is evidence for additional auxin perception and signalling mechanisms, but further studies are necessary for a complete understanding of these pathways.

Keywords: auxin; indole‐3‐acetic acid; tryptophan; plant hormones; polar transport; protein degradation; SCF E3 ligase; growth and development

Figure 1. Structure of auxins. (a) IAA, Indole‐3‐acetic acid; 4Cl‐IAA, 4‐chloroindole‐3‐acetic acid; PAA, Phenylacetic acid are the natural active auxins in plants. IBA, Indole‐3‐butyric acid is considered as an inactive auxin precursor or storage form. (b) Structures of representative synthetic auxins form the three major groups: phenoxyacetic acids, pyridine carboxylic acids and benzoic acids. Abbreviations: 2,4‐D, 2,4‐Dichlorophenoxyacetic acid; 2,4,5‐T, 2,4,5‐Trichlorophynoxyacetic acid, 2,3,6‐TBA‐2,3,6‐Trichlorobenzoic acid.
Figure 2. Auxin homeostasis in plants. (a) Proposed tryptophan‐dependent (TRP‐D) and tryptophan‐independent (TRP‐I) pathways of auxin biosynthesis in plants. Tryptophan is synthesised in plastids from shikimate shunt and converted to IAA predominantly by a two‐step pathway via Indole‐3‐pyruvic acid (IPyA) intermediate in the cytoplasm, as illustrated by solid green arrows. Other less‐characterised pathways are shown either by solid blue arrows (where enzymes are known) or by dashed blue arrows (where enzymes are unknown). Suggested TRP‐I pathways are indicated by dashed black arrows. Dashed red arrows and solid blue arrows indicate conversion of IAA into inactive forms and its retrieval, respectively. Solid red arrows indicate irreversible steps leading to degradation of IAA. (b) Other hormones and environmental factors influence the biosynthesis of IAA by regulating the expression of TAAs and YUCCAs in IPyA pathway. Abbreviations: IGP, Indole‐3glycerol phosphate; TAAs, Tryptophan aminotransferase in Arabidopsis; YUCs, YUCCA flavin monooxygenases; IPyA, Indole‐3‐pyruvic acid.
Figure 3. Signalling events leading to auxin‐induced gene expression in plants. At low levels of auxin, Aux/IAA proteins along with co‐repressor, TPL (TOPLESS) represses the activity of ARFs (Auxin Response Factors), thus inhibiting the induction of transcription. At high levels of auxin, Aux/IAA proteins interact with TIR1/AFBs of the SCFTIR/AFBs–E3 ligase, in which auxin functions as a ‘molecular glue’. This interaction facilitates the ubiquitination of Aux/IAA proteins, which are then recognised and degraded by 26S proteasome. Removal of Aux/IAA repressors will allow ARFs to induce gene transcription.
Figure 4. Phenotypic characteristics of auxin‐insensitive axr1mutants of Arabidopsis, compared to wild‐type plants. The less‐severe axr1‐3 mutant shows reduced plant height and increased branching, while the more‐severe axr1‐12 mutant shows extremely high branching and stunted growth. The axr1 mutants have defects in auxin signalling pathway.
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Further Reading

Benjamins R and Scheres B (2008) Auxins: the looping star in plant development. Annual Review of Plant Biology 59: 443–465.

Kramer EM and Ackelsberg EM (2015) Auxin metabolism rates and implications for plant development. Frontiers in Plant Science 6: 150. DOI: 10.3389/fpts.2015.00150.

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Zazimalova E, Murphy AS, Yang H, et al. (2010) Auxin transporters. Why so many? Cold Spring Harbor Perspectives in Biology 2: a001552.

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Kathare, Praveen K, Cioffi, Timothy J, Dharmasiri, Nihal, and Dharmasiri, Sunethra(Sep 2017) Auxin. In: eLS. John Wiley & Sons Ltd, Chichester. http://www.els.net [doi: 10.1002/9780470015902.a0020090.pub2]