Hydrophobins are small amphiphilic proteins that confer filamentous fungal hydrophobicity needed for hyphal growth, development, dispersal and adhesion to host and substrata. In insect-pathogenic Beauveria bassiana, nine hydrophobins (class I Hyd1A-F and class II Hyd2A-C) were proven to localize on the cell walls of aerial hyphae and conidia but accumulate in the vacuoles and vesicles of submerged hyphae and blastospores, respectively. Conidial hydrophobicity, adhesion to insect cuticle, virulence via normal cuticle infection and dispersal potential were significantly more reduced by the hyd1A deletion leading to complete ablation of slender rodlets on conidial coat than the hyd1B deletion, which caused a failure to assemble morphologically irregular rodlets into orderly bundles. Aerial conidiation and submerged blastospore production were compromised in Δhyd2A and Δhyd2C. The deletion of hyd1D stimulated conidial germination and virulence via insect hemocoel colonization, which was accelerated in Δhyd2A but decelerated in Δhyd2B. However, these deletion mutants were unaffected in radial growth on rich/minimal media and responses to osmotic, oxidative, cell wall-perturbing and heat-shock stresses except for an increase in conidial thermotolerance of Δhyd1A or cell sensitivity of Δhyd1B to Congo red-induced stress. None of examined phenotypes was altered in Δhyd1C, Δhyd1E and Δhyd1F. Conclusively, Hyd1A and Hyd1B co-regulate the formation, morphology and orderly assembly of rodlet bundles required for conidial hydrophobicity and infectivity, which are independent of Hyd1C-F and Hyd2A-C in B. bassiana. These results unveil a necessity to distinguish major, minor and dispensable roles among multiple class I/II hydrophobin genes in an ascomycetous pathogen.
Keywords: Asexual development; Fungal insect pathogens; Hydrophobicity and adhesion; Hydrophobin assembly; Pathogenicity and virulence; Rodlet bundles.
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