Production of heat-resistant blastospores by Cordyceps javanica IF-1106 through optimizing metal ions composition

Heat tolerance of fungi is critical for field stability and efficacy of fungal insecticides. Components of the medium, such as carbon source, nitrogen source and metal ions, contribute to the thermotolerance of fungi. In this study, several metal ions were detected to establish their effects on the sporulation and thermotolerance of the blastospores of Cordyceps javanica IF-1106 when cultured in liquid medium. The results showed that metal ions play different roles in sporulation and thermotolerance of the fungi. Mn²⁺ greatly increased the thermotolerance of the blastospores, and Fe²⁺ increased the thermotolerance slightly. However, both has no or negative effects on sporulation of the fungi. In the case of Cu²⁺ and Fe³⁺, sporulation yield and thermotolerance of the blastospores increased significantly. In addition, intracellular trehalose and mannitol involved in thermotolerance of fungi were extracted and then quantified by ion chromatography. We find that the trehalose and mannitol are higher in the presence of Mn²⁺, implying that Mn²⁺ could adjust the intracellular trehalose and mannitol level confers the blastospores with higher tolerance to thermal stress. Further, Response Surface Methodology (RSM) was applied to screen the optimal composition of metal ions. When the formula was MnSO₄·H₂O 8 mg/L, CuSO₄·5H₂O 5 mg/L, FeSO₄·7H₂O 11 mg/L, FeCl₃·6H₂O 8 mg/L, maximum sporulation yield and GT₅₀ were obtained. Lastly, the virulence of the blastospores to Acyrthosiphon pisum was validated, a higher virulence is achieved with decreased LT₅₀ (2 d) and LC₅₀ (2.37 × 10⁴ blastospores mL⁻¹). In conclusion, thermotolerant blastospores of C. javanica IF-1106 and higher sporulation yield could be achieved by optimizing the metal ions composition, which helps to determine the formula for larger scale fermentation process.