Engineering bacterial symbionts to improve the biological control potential of entomopathogenic nematodes against an important agricultural pest

Bacterial symbionts play a crucial role for entomopathogenic nematodes (EPNs) by enhancing their ability to kill insects. I report that introducing engineered bacterial symbionts into two commercially used nematode strains results in high infectivity of these new nematode–symbiont pairs against western corn rootworm (WCR) larvae (Diabrotica virgifera virgifera LeConte), a major maize pest. WCR larvae are able to sequester benzoxazinoid secondary metabolites that are produced by maize and use them to increase their resistance to the nematodes and their symbionts. To counter this, I isolated 27 Photorhabdus symbionts from different nematodes and increased their benzoxazinoid resistance through experimental evolution. Benzoxazinoid resistance in Photorhabdus bacteria evolved through multiple mechanisms, including a mutation in a multidrug efflux pump. I reintroduced benzoxazinoid-resistant Photorhabdus strains into two strains of Heterorhabditis bacteriophora EPNs and identified four nematode–symbiont pairs that were able to kill benzoxazinoid-sequestering WCR larvae as efficiently as the commercially available nematodes under laboratory conditions. Tested in two field trials, these pairs similarly controlled the pest as did commercial nematode strains. This suggests that the selection processes did not lead to any major trade off, but also that other pathogenicity factors (rather than benzoxazinoid resistance) may be more relevant for successful biocontrol under field conditions. My results suggest that modifying bacterial symbionts and targeting candidate genes to engineer better biocontrol agents provides a successful and time-efficient strategy to improve the pathogenicity of entomopathogenic nematodes, which could be expanded to combat other agricultural pests in the future.

2024

dissertation

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