Indole-3-acetic acid (IAA) is the major naturally occurring auxin. In the past decades, evidence has accumulated both emphasizing the physiological importance of auxin in the context of coordinating plant development and describing the molecular mode of auxin action.IAA is one of the major growth factors in plants. It is recognized to be involved in virtually all aspects of plant growth and development. However, although IAA constitutes a rather simple molecule, sharing major structural features with the proteinogenic amino acid L-tryptophan, the biosynthesis of IAA remains elusive. Over the past, experimental proof has been provided that auxin biosynthesis in plants is realized by a small number of alternative pathways, each of them designated for an intermediate that is a hallmark of the pathway (Fig. 1).
Figure 1. Proposed pathways of IAA biogenesis in plants. The IAOx pathway that is seemingly restricted to IG-producing plant species is given in the green box. In the middle, the TPA pathway is shown (dark blue), followed by the the TAM pathway (dark red) and the IAM pathway (orange), respectively, further left. Dashed lines indicate assumed reaction steps for which the corresponding enzymes have yet to be identified. Enzymes are abbreviated as follows: AAO, arabidopsis aldehyde oxidase 1; AMI1, amidase 1; CYP71A13, cytochrome P450 monooxygenase 71A13; CYP79B2/B3, cytochrome P450 monooxygenase 79B2/B3; NIT, nitrilase; TAA1, tryptophan aminotransferase of Arabidopsis 1; TAR2, tryptophan aminotransferase related 2; TDC, tryptophan decarboxylase; YUC, YUCCA.
Thus far, four Trp-dependent pathways for auxin biosynthesis have been proposed. These are the indole-3-acetaldoxime (IAOx)-pathway, the tryptamine (TAM)-pathway, the indole-3-pyruvic acid (IPA)-pathway, and the indole-3-acetamide (IAM)-pathway. Some people also consider a Trp-independent pathway for the formation of IAA, but genetic and molecular evidence for its existence is, so far, missing. As yet, only one of the proposed pathways, the IPA route, is fully disclosed with respect to the catalyzed reaction steps and the enzymes involved. Due to the obvious gaps in the pathways, the functional redundancy, and the tissue and plant specific variations in expression patterns of the identified components, the relevance of each of these pathways is difficult to assess.
One major research line of our lab concerns auxin biosynthesis and its regulation in the model plant Arabidopsis thaliana. Our previous work provided evidence that led us to suggest that one route of auxin formation takes its course via the intermediate