Figure 1. Natural and synthetic auxins. IAA, indole-3-acetic acid; IBA, indole-3-butyric acid; 2,4-D, 2,4-dichlorophenoxyacetic acid; 1-NAA, 1-naphthaleneacetic acid; picloram, an auxinic herbicide.

Figure 2. IAA biosynthesis. Tryptophan-independent pathways of IAA synthesis might begin with either indole or indole-3-glycerol phosphate. Tryptophan-dependent pathways might use a variety of intermediates. For details and discussion, see references in Further Reading.

Figure 3. Auxin transport. IAA is protonated (IAAH) in the acidic cell wall (pH ~5), enabling it to enter cells by diffusion or by coimport through auxin importers with protons (the presumed auxin import proteins, AUX1/LAXs and MDR/PGPs, are shown). In the neutral cell cytoplasm IAAH ionizes, becoming IAA^–. As its ability to diffuse across membranes is greatly reduced, it primarily exits the cell through basally localized auxin exporters (PINs and MDR/PGPs), using the energy of the chemiosmotic gradient. Auxin can then enter the next cell in series. The cell shown, with oppositely localized importers and exporters, could be in the root protophloem. Other cell types show different patterns of expression and localization of auxin transport proteins.

Figure 4. Auxin signalling. (a) With basal levels of auxin (low or none), Aux/IAA proteins dimerize with ARFs and repress the transcriptional-activating activity of ARFs at promoters containing Auxin Response Elements (AuxREs). (b) When auxin levels increase, auxin (IAA) binds to TIR1 (on the right of SCF^TIR1), causing SCF^TIR1 to bind to and, in collaboration with the E2 enzyme (on the left of SCF^TIR1) that carries activated ubiquitin, attach Ubiquitin to the Aux/IAA proteins. (c) Continued ubiquitination of the Aux/IAA protein gives a polyubiquitin chain on this protein, leading to recognition and degradation of the protein by the proteasome. (d) The ARF protein is released from inhibition by the Aux/IAA protein and can therefore stimulate transcription of the auxin-induced genes. Not shown are the pathways described in the text, of RUB modification of cullin and the feedback loop in which some of the auxin-induced genes are Aux/IAA proteins that inhibit further synthesis of auxin-induced genes.

Figure 5a. Phenotypes of transgenic and mutant plants altered in auxin levels and response. (a) Transgenic tobacco plants deficient in auxin due to the auxin-inactivating IAA-Lys gene from Pseudomonas syringae (right) show decreased root growth compared to control plants (left). From Romano et al. (1991), reproduced by permission of Cold Spring Harbor Laboratory Press.

Figure 5b. Phenotypes of transgenic and mutant plants altered in auxin levels and response. (b) Transgenic Arabidopsis thaliana plants overproducing auxin but not overproducing ethylene (carrying both the bacterial iaaM and ACCase genes; right) show reduced shoot branching compared to control plants (left). From Romano et al. (1993), reproduced by permission of American Society of Plant Biologists.

Figure 5c. Phenotypes of transgenic and mutant plants altered in auxin levels and response. (c) Compared to control wild-type plants (left), the auxin-resistant1 mutants of Arabidopsis thaliana – the strong axr1-12 allele (middle) and the weak axr1-3 allele (right) – show increased shoot branching. Plants are 7 weeks old. Bar, is 3 cm long. From Lincoln et al. (1990), reproduced by permission of American Society of Plant Biologists.