Synergistic effect of fermented solution produced by transgenic Beauveria bassiana increasing the insecticidal efficacy of wild-type strain results from subtilisin-like activity
Abstract:We found that the ferment product of a transgenic Beauveria bassiana, overexpressing CDEP-1, a cuticle degrading subtilisin-like gene, obviously increased the insecticidal efficacy of wild type strain against aphids Myzus persicae. To investigate the relationship between synergistic effect and CDEP-1 activity in the fermented solution,we mixed the ferment product (44.3±0.6 U/μL of CDEP-1 activity) with wild type conidia. Bioassay indicated that adding with the ferment product led to a more than 4-fold decrease in the median lethal concentration (LC50) of conidia against aphids, with cotoxicity coefficient (CTC) of 404.6. However, after the solution was heated at 121℃ for 20 min to quench the protease activity, the synergistic effect almost lost, revealing that the synergistic material in the fermented solution was thermosensitive. When the fermented solution was precipitated by saturated (NH4)2SO4, and then the pellets and the supernatant were dialyzed respectively, and bioassay showed that the synergistic effect existed mainly in the protein part. At the spore concentration of 1×107 /mL, the median lethal time (LT50) of M. persicae was negatively correlated with CDEP-1 activity (the relative coefficient was-0.8946). The results suggested that the synergistic effect of the ferment product mostly resulted from subtilisin-like CDEP-1 activity.