Effects of Insect Cuticular Compounds on Appressorium Formation and Metabolic Activity in Beauveria bassiana

The rate of appressoria formation following conidial germination in Beauveria bassiana is closely associated with its pathogenicity. This study investigated the effects of insect cuticular compounds on the formation and metabolism in B. bassiana through the addition of insect cuticle analogues. Results indicate that both the fatty acid compound carnitine C3:0 and the organic acid compound Thiamine Pyrophosphate (TPP) exert dose-dependent bell-shaped effects on B. bassiana spore germination and appressorium formation at different concentrations. Both low and high concentrations inhibit spore germination and appendage formation. At a concentration of 0.10 mg/mL, spore germination and appendage formation rates peaked at all time points, being significantly higher than the control (p ≤ 0.05). Compounds in the benzene and its derivatives class, such as enilconazole and disulfide bis(2-hydroxy-3-methylpropan-2-yl) (DSBA), significantly reduced spore germination and appressorium formation in B. bassiana (p ≤ 0.05), with stronger inhibition becoming more pronounced at higher concentrations. In contrast, amino acids and their metabolites (e.g., glycylmethionine) and glycerophospholipid compounds like 1,2-Dioleoyl-sn-Glycero-3-Phosphocholine (DOPC) had no significant effects on spore germination or appressorium formation at any tested concentration (p > 0.05). LC-MS analysis revealed that the insect cuticular fatty acyl compound carnitine C3:0 broadly modulated the secondary metabolism of B. bassiana. Following appressorium formation, 146 metabolites with significant changes in abundance were identified. Before appressorium formation, carnitine C3:0 promoted the activation of B. bassiana signaling pathways, such as Rap1, and stimulated antibiotic biosynthesis (penicillin and cephalosporin), thereby suppressing competing microorganisms and facilitating initial attachment. After appressorium formation, carnitine C3:0 activated pathways related to metabolite synthesis (e.g., arginine and nucleotides biosynthesis) and population regulation (ferroptosis), thereby enhancing appressorium function and structural stability. Thus, carnitine C3:0 enhances B. bassiana's ability to establish infection sites before appressorium formation through antibiotic clearance and signal activation, and maintain infection structures after formation via metabolic reinforcement and population regulation. This study lays a theoretical foundation for further investigations into B. bassiana infection mechanisms, pathogenicity, and the role of its conidiophores.