Genomic and Transcriptomic Analyses of Indole-3-Acetic Acid Biosynthesis in Diatoms

Indole-3-acetic acid (IAA) is a major plant growth hormone and a common mediator of plant-bacterial interactions. Recently, IAA has also been found to play a role in interactions between diatoms and bacteria, with IAA production by an associated Sulfitobacter leading to increased growth rates in the marine diatom Pseudo-nitzschia multiseries. It is unclear, however, if diatoms themselves are able to synthesize IAA and whether this capability is widespread throughout Bacillariophyta. Four major tryptophan-dependent IAA biosynthesis pathways have been identified in plants and bacteria, each denoted by the first intermediate downstream of tryptophan: the indole-3-pyruvate (IPyA), tryptamine (TAM), indole-3-acetaldoxime (IAOx) and indole-3-acetamide (IAM) pathways. To investigate the possibility of IAA biosynthesis in diatoms, we first analyzed publicly available genomes of raphid pennates P. multiseries, Phaeodactylum tricornutum, Fragilariopsis cylindrus and centric Thalassiosira pseudonana for potential homologs to plant and bacterial IAA biosynthesis genes. The P. multiseries, F. cylindrus and P. tricornutum genomes encode downstream enzymes for bacterial TAM and IAM and plant IPyA pathways. The more evolutionarily ancient T. pseudonana encodes one TAM enzyme in its genome. To investigate the potential distribution of these pathways more broadly, we surveyed the transcriptomes of 11 diatom species that include representatives from all four Bacillariophyta classes. Datasets used were sequenced as part of the Marine Microbial Eukaryote Transcriptome Sequencing Project (MMETSP) and obtained from cultures maintained axenically. Transcripts associated with the TAM pathway were most frequently detected, with potential homologs to required enzymes identified in 10 of the 11 species examined. Transcripts homologous to rate-limiting IPyA enzymes were detected in six species. Only two centric and araphid pennate species expressed transcripts associated with enzymes in the IAM and IAOx pathways. This pattern suggests multiple events of gene loss as the phylum expanded and diversified. Mass spectrometry analyses will be conducted to confirm the production of IAA in axenic cultures of P. pungens, P. multistriata, Skeletonema marinoi and F. cylindrus.