Submitted to: Environmental Microbiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/12/2008
Publication Date: 9/15/2008
Citation: Pechy-Tarr, M., Bruck, D.J., Maurhofer, M., Fischer, E., Vogne, C., Henkels, M.D., Donahue, K.M., Grunder, J., Loper, J.E., Keel, C. 2008. Molecular analysis of a novel gene cluster encoding an insect toxin in plant-associated strains of Pseudomonas fluorescens. Environmental Microbiology. 10(9):2368-2386. Interpretive Summary: This manuscript shows that two well-known biological control agents that suppress plant diseases can also kill insects due to their production of an insect toxin, which was discovered in this study. By studying the newly-available genomic sequence of the biological control agent Pseudomonas fluorescens Pf-5, we found a gene that is related to an insecticidal toxin produced by another bacterium. After finding the toxin gene in the genome, we tested Pf-5 and the related bacterium Pseudomonas fluorescens CHA0 to see if they killed insects. Two types of insects, the tobacco hornworm (Manduca sexta) and the waxworm (Galleria mellonella), were injected with these bacteria. We found that even small numbers of bacteria killed the insects and that the toxin (which we call FitD) is responsible for some of the insecticidal properties of these bacteria. We also found that something else in these bacteria, besides the FitD toxin, also kill insects. This work is important because it provides the first example of a microorganism whose toxicity against both plant pathogens and insects has been identified at the molecular level. One subject of future work is to identify other insecticidal factors in these bacteria, as these could be tested for their usefulness in pest management .
Technical Abstract: Pseudomonas fluorescens CHA0 and the related strain Pf-5 are well-characterized representatives of rhizosphere bacteria that have the capacity to protect crop plants from fungal root diseases, mainly by releasing a variety of exoproducts that are toxic to plant pathogenic fungi. Here, we report that the two plant-beneficial pseudomonads also exhibit potent insecticidal activity. Anti-insect activity is linked to a novel genomic locus encoding a large protein toxin termed Fit (for P. fluorescens insecticidal toxin) that is related to the insect toxin Mcf (Makes caterpillars floppy) of the entomopathogen Photorhabdus luminescens, a mutualist of insect-invading nematodes. When injected into the hemocoel, even low doses of P. fluorescens CHA0 or Pf-5 killed larvae of the tobacco hornworm Manduca sexta and the greater wax moth Galleria mellonella. By contrast, mutants of CHA0 or Pf-5 with deletions in the Fit toxin gene were significantly less virulent to the larvae. When expressed from an inducible promoter in a non-toxic Escherichia coli host, the Fit toxin gene was sufficient to render the bacterium toxic to both insect hosts. Our findings establish the Fit gene products of P. fluorescens CHA0 and Pf-5 as potent insect toxins that define previously unappreciated anti-insect properties of these plant-colonizing bacteria.