A cuticle-degrading protease (CDEP-1) of Beauveria bassiana enhances virulence

In order to investigate virulence enhancement of entomopathogenic fungi, a Beauveria bassiana-sourced Pr1 protease (CDEP-1) was expressed by a methylotrophic yeast Pichia pastoris and then used as an additive to three gradient sprays of B. bassiana strain (Bb0062) onto apterous green peach aphid Myzus persicae adults in six bioassays. The resultant data fit well to a time-concentration-mortality model. Generally, the LC50 estimates of the fungal pathogen against the aphid species decreased with increasing CDEP-1 concentrations from 0 to 100 mu g mL(-1). The LC(50)s on days 5-7 after spray were reduced by 1.5-2.5-fold at the concentrations of 20-100 mu g mL(-1). However, sprays of 20-100 mu g CDEP-1 mL(-1) aqueous solution alone had no significant effect on aphid mortality compared to water spray only. Neither did inclusion of inactivated CDEP-1 at a concentration of 50 mu g mL(-1) affect significantly the fungal virulence to aphids. Our results confirm for the first time that the cuticle-degrading protease CDEP-1 enhanced fungal virulence due to acceleration of conidial germination and cuticle penetration. This suggests a new approach to utilising the protease in microbial control.