Analysis of Appressorium Formation in Metarhizium anisopliae and Its Impact on the Defense Metabolism of Opisina arenosella Larvae Based on LC-MS

The appressorium is a specialized infection structure formed by Metarhizium anisopliae during host invasion. To investigate the correlation between appressorium formation and fungal pathogenicity, as well as its impact on insect cuticular metabolism, different concentrations of sulforaphane were used to inhibit the appressorium formation rate of M. anisopliae. The relationship between appressorium formation rate and pathogenicity against Opisina arenosella larvae was evaluated, and LC-MS-based metabolomics was employed to characterize changes in cuticular compounds during the appressorium formation stage, thereby elucidating the chemical responses of the insect cuticle to appressorium formation. The appressorium formation rate of M. anisopliae was significantly and positively correlated with its pathogenicity (p ≤ 0.05). As the appressorium formation rate increased, pathogenicity against O. arenosella larvae increased and the killing speed accelerated. LC-MS metabolomics revealed that after appressorium formation, 102 differential cuticular compounds unique to O. arenosella larvae were identified, mainly including benzenes and substituted derivatives, amino acids and derivatives, and heterocyclic compounds. In addition, metabolic pathways associated with immune defense (tyrosine metabolism), antifungal defense (histidine metabolism), and toxin degradation (flavonoid degradation) in the larval cuticle were activated. These results demonstrate that the appressorium plays an important role in host infection by M. anisopliae, markedly alters cuticular metabolism, and activates defense- and detoxification-related metabolic pathways in the host. This study provides a theoretical basis for further investigations into fungus–insect cuticle interactions.