GENOMICS-ENABLED EXPLORATION OF INSECT TOXICITY IN THE PSEUDOMONAS FLUORESCENS GROUP
Pseudomonas is a diverse genus of Gamma proteobacteria that are ubiquitous in the natural environment, including soil, water, plant surfaces, and animals. The Pseudomonas fluorescens group is a diverse collection of seven subgroups and more than 50 named species. This group is known for their production of a variety of secondary metabolites and antimicrobial compounds that contribute to biological control of plant diseases. Recently, they also have been shown to relay both oral and injectable insect lethality by mechanisms that are unknown. The first objective of this study was to determine if ten fully sequenced strains in the P. fluorescens group are lethal to insects, using larvae of the tobacco hornworm (Manduca sexta) as a model system. A second objective was to characterize putative insect toxin complex (Tc) genes in the genomes of the ten strains. Additionally, genes encoding putative insect toxins (Tc) and possible modes of immunosuppression (the AprA metalloprotease and GacS/GacA regulatory system) were examined for their contribution to lethality of one strain of P. fluorescens. I established that six of ten strains representing diverse lineages of the P. fluorescens group exhibited injectable lethality to larvae of M. sexta, while four of the strains exhibited no significant lethality. Seven strains of the P. fluorescens group had Tc gene clusters. These clusters were categorized into six distinct types, based on the organization, genomic context, G+C content, and phylogeny of the Tc genes. One of the types included genes from strain A506, which was selected for further study because of its injectable lethality towards M. sexta, and its importance as a model strain in studies of phyllosphere microbial ecology and as a commercial biological control agent. Of the genes (aprA, gacS and tcaABC-tccC) evaluated for their roles in insect lethality, the tcaABC-tccC cluster had the greatest effect on insect lethality by A506. This study provides the first evidence for a significant effect of a Tc cluster on insect lethality by a Pseudomonas species. Nevertheless, none of the genes evaluated were fully responsible for the injectable lethality caused by A506, indicating that loci other than those evaluated here contribute to insect toxicity by this strain.
This research was published here.