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Title: Characterization of toxin complex gene clusters and insect toxicity of bacteria representing four subgroups of Pseudomonas fluorescens

item RANGEL, LORENA - Oregon State University
item Henkels, Marcella
item Shaffer, Brenda
item WALKER, FRANCESCA - Oregon State University
item DAVIS II, EDWARD - Oregon State University
item Stockwell, Virginia
item Bruck, Denny
item TAYLOR, BARBARA - Oregon State University
item Loper, Joyce

Submitted to: PLOS ONE
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/15/2016
Publication Date: 8/31/2016
Citation: Rangel, L.I., Henkels, M.D., Shaffer, B.T., Walker, F.L., Davis Ii, E.W., Stockwell, V.O., Bruck, D.J., Taylor, B.J., Loper, J.E. 2016. Characterization of toxin complex gene clusters and insect toxicity of bacteria representing four subgroups of Pseudomonas fluorescens. PLoS One. 11(8):e0161120. doi: 10.1371/journal.pone.0161120.

Interpretive Summary: Many bacteria known for their biological control of plant disease are members of the Pseudomonas fluorescens group. Recently, we and others discovered that of one member of this group, Pseudomonas protegens strain Pf-5, can also kill certain insects, opening up the possibility that it could be used for biological control of insect pests of importance in agriculture, as well as plant diseases. Here, we tested Pf-5 and nine other strains in the Pseudomonas fluorescens group for toxicity to two insects: the tobacco hornworm Manduca sexta and the fruit fly Drosophila melanogaster. We discovered that seven of the ten strains exhibited toxicity to at least one of these two insects. We also characterized the presence and type of gene clusters for known insect toxins in the genomes of the ten strains. Only strains having the fitD gene, which encodes the previously-described FitD insect toxin, exhibited oral toxicity to M. sexta. A fitD mutant of one strain did not show oral toxicity to M. sexta, providing convincing evidence that FitD is required for oral toxicity of the Pseudomonas spp. to this lepidopteran insect. In contrast, of the seven strains that were toxic to M. sexta when injected into larvae of this insect, four strains lacking fitD. These results indicate that factors other than the known FitD toxin are involved in injectable toxicity. Furthermore, six of the ten strains exhibited oral toxicity to the dipteran insect D. melanogaster, and only two of these strains have fitD. This study showed that many strains within the P. fluorescens group are toxic to insects and could be further evaluated for their capacities to suppress insect pests as well as plant diseases of importance in agriculture. Furthermore, we identified a number of strains that exhibit insect toxicity but lack the previously described FitD toxin, and therefore have novel mechanisms of insect toxicity that could be identified in future studies.

Technical Abstract: Ten strains representing four lineages of Pseudomonas (P. chlororaphis, P. corrugata, P. koreensis, and P. fluorescens subgroups) were evaluated for toxicity to the tobacco hornworm Manduca sexta and the fruit fly Drosophila melanogaster. The three strains within the P. chlororaphis subgroup exhibited both oral and injectable toxicity to the lepidopteran M. sexta and several lines of evidence supported a role for the FitD insect toxin in this toxicity. Only one of the three strains, P. protegens Pf-5, exhibited substantial levels of oral toxicity against the dipteran D. melanogaster. Three strains in the P. fluorescens subgroup, which lack fitD, showed significant levels of injectable toxicity against M. sexta and oral toxicity against D. melanogaster. Tc gene clusters were identified in the genomes of these three strains along with four other strains evaluated in this study. Within those seven genomes, six types of Tc gene clusters were identified, distinguished by gene content, organization and genomic location, but we observed no correlation between the presence of Tc genes and insect toxicity of the strains evaluated in this study. Our results demonstrate that members of the P. chlororaphis and P. fluorescens subgroups have the capacity to kill insects by both FitD-dependent and independent mechanisms.