A bioinformatic examination of indole-3-acetic acid biosynthesis in insecta and hexapoda

Abstract

Indole-3-acetic acid (IAA) is the most common form of the phytohormone auxin found in plants. IAA has been previously reported from mammals, and has recently been observed to be widespread in arthropods, particularly insects. Several pathways for the biosynthesis of IAA from tryptophan (Trp) have been mapped and documented in plants, bacteria, and fungi, and a new pathway has been proposed from an insect. The widespread distribution of IAA in insects also argues that synthesis rather than consumption and sequestration accounts for its presence. We used exemplar enzymes from all known plant and bacterial Trp-based IAA biosynthesis pathways to search the 1KITE database of 670 hexapod and insect transcriptomes for evidence of a complete biosynthetic pathway for IAA. We first aligned exemplar enzymes to transcriptomes and then aligned transcripts with alignments against a database of well-annotated insect and hexapod genomes to ensure that we were not identifying paralogs. We found that none of the currently recognized IAA biosynthesis pathways were widespread in Hexapoda and Insecta (occur in > 80% species). However, transcripts encoding proteins homologous to enzymes in the pathway that converts Trp → TAM → IAAld → IAA via tryptamine (TAM) and indole–3-acetaldehyde (IAALD) were detected in the transcriptomes of most species within the Hexapoda, Palaeoptera, and Polyneoptera, but were also detected in genomes largely from the Holometabola. Transcripts encoding proteins homologous to two enzymes in the TAM pathway, aromatic-L-amino-acid/L-tryptophan decarboxylase (EC: 4.1.1.28 and EC: 4.1.1.105) and aldehyde dehydrogenase/indole-3-acetaldehyde oxidase (EC: 1.2.1.3, EC: 1.2.3.7) were found to be widespread in Hexapoda and Insecta. A newly proposed pathway for the biosynthesis of IAA in insects based on an examination of Euura sp. ‘Pontania’ and Bombyx mori, suggests that an aromatic aldehyde synthase (PonAAS2) is responsible for the conversion of Trp → IAAld and an aldehyde oxidase (BmIAO1) is responsible for the conversion of IAAld → IAA in Bombyx mori. We detected aromatic aldehyde synthases (AAS) and aldehyde oxidases (AO) in about 33% of our transcriptomes with RNA extracted largely from adults, which would imply that these enzymes are not widespread in Insects and Hexapoda. However, when we examined a sample of 167 insect genomes, we detected at least one aromatic aldehyde synthase and at least one aldehyde oxidase in 80.8 and 94% of the genomes, respectively, which suggest that these enzymes are more widespread in insect genomes even if undetected in transcriptomes of adult insects and hexapods. However, it is likely that some of these putative aromatic aldehyde synthase proteins are homologs of 3, 4-dihydroxyphenylacetaldehyde synthase, which is involved in cuticular hardening, rather than being homologs of the sole aromatic aldehyde synthase as yet identified to convert Trp to IAAld. So, both the TAM pathway and the newly proposed insect pathway based on Euura sp. ‘Pontania’ could account in part for the biosynthesis of IAA in insects and hexapods. Our results also suggest that an aldehyde dehydrogenase could contribute to the biosynthesis of IAA by the enzymatic conversion of IAAld to IAA. Further careful biochemical experiments and efforts to characterize additional AAS enzymes capable of converting Trp to IAAld might guide a refined assessment of the variety of enzymes involved and the breadth and distribution of IAA biosynthesis pathway in Hexapods and Insecta.