Expression of a phenoloxidase cascade inhibitor enhances the virulence of the fungus Beauveria bassiana against the insect Helicoverpa armigera

Abstract

Entomopathogenic fungi have high potential for controlling insect pests, although the slow killing speed has blocked their widespread application. To increase the virulence of entomopathogenic fungi, genetic modification can be employed. Egf1.0 is an immunosuppressive protein encoded by polydnavirus, carried by parasitoid wasp Microplitis demolitor, which blocks the prophenoloxidase (PPO) activation response of host insects. In this study, we explored the feasibility of genetically modifying entomopathogenic fungi with increased virulence by expressing Egf1.0. In comparison with the wild-type parents, the median lethal concentration (LC50) of Beauveria bassiana expressing Egf1.0 against Helicoverpa armigera was reduced by 2.7-fold, and the median lethal time (LT50) was reduced by 22.8%. In vitro assay showed that recombinant Egf1.0 was able to inhibit the PPO activation response of H. armigera. In vivo assay revealed that the expression of Egf1.0 in B. bassiana caused a higher degree of suppression to PPO activation response of H. armigera. These assays suggested that the increased virulence of the transgenic fungi is due to the increased ability to suppress the host insect's immune response. Moreover, colony growth, conidia yield, and germination assays revealed that the expression of Egf1.0 in B. bassiana had no effect on its growth and development. In conclusion, the expression of Egf1.0 can significantly enhance the pathogenicity of B. bassiana against host insects.