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. 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. 

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.

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 IAM, a compound proven to be endogenous to Arabidopsis and several other plant species. In the past, we succeeded in identifying and characterizing the first plant IAM hydrolase (AMI1) from Arabidopsis, capable of catalyzing the conversion of IAM to IAA (for review see: Pollmann et al. 2006; Lehmann et al. 2010). AMI1 is located in the cytoplasm, assumed to be the main locus of IAA biosynthesis. Judged by its primary amino acid composition and homology to other well-characterized enzymes, AMI1 is considered as a member of the amidase signature family that comprises enzymes that can be found widespread in nature, catalyzing a diverse range of different reactions. To date, more than twenty AMI1-like proteins from both monocot and dicot plant species have been identified (Mano et al. 2010; Lehmann et al. 2010, Sánchez-Parra et al. 2014), suggesting a conserved and likely important function of AMI1-like enzymes. Currently, our focus is on the elucidation of the role of AMI1-mediated IAA formation and on how AMI1 integrates into the already deciphered framework of auxin biosynthesis in plants. Furthermore, we are interested in the regulation of auxin biosynthesis. To tackle these objectives, we use a combination of genetic, molecular biological, protein biochemical, cell biological, and mass spectrometric techniques.