Convergent roles of six fungal specific transcription factors in insecticidal process and environmental fitness of Beauveria bassiana

BACKGROUND: binuclear cluster DNA-binding (C6) domain alone or both C6 and FsTF domains in Beauveria bassiana, a wide-spectrum mycoinsecticide. FsTFs and C6 TFs are often confused with each other and are generally underexplored, making it necessary to explore their regulatory roles in the fungal insect-pathogenic lifecycle. Here, six FsTFs (TF1-TF6) with different (representative) molecular sizes and unique size-different FsTF domain located in different sequence regions are functionally characterized. RESULTS: The deletion mutants of tf1-tf6 were much more compromised in virulence via normal cuticle infection (NCI) than cuticle-bypassing infection (intrahemocoel injection). The markedly attenuated virulence via NCI was ascribed to severely blocked secretion of cuticle-degrading enzymes required for hyphal invasion into host hemocoel through cuticular penetration. All the mutants also exhibited increased sensitivities to hyperosmotic, oxidative and cell wall perturbing stresses, impaired conidial quality, and sharp increases in submerged blastospore production in vitro despite negligible changes in aerial conidiation and radial growth on different carbon or nitrogen sources. The altered phenotypes were well restored by targeted gene complementation. CONCLUSION: TF1-TF6 play essential roles in the positive regulation of insect-pathogenic lifecycle and serve as negative regulators of submerged blastospore production in vitro in B. bassiana. These findings add knowledge to the family of FsTFs and unveil convergent evolution of TF1-TF6 in supporting the insecticidal activity and environmental fitness of B. bassiana.