Only one of three hydrophobins (Hyd1–3) contributes to conidial hydrophobicity and insect pathogenicity of Metarhizium robertsii

Abstract

Class I/II hydrophobins constitute a family of small amphiphilic proteins that mediate cell hydrophobicity and adhesion to host or substrata and have pleiotropic effects in filamentous fungi. Here we report that only class I Hyd1 is essential for conidial hydrophobicity and insect pathogenicity among three hydrophobins (Hyd1–3) characterized in Metarhizium robertsii, an insect-pathogenic fungus. Aerial conidiation levels of three Δhyd1 mutants were much more reduced in 5-day-old cultures than in 7-day-old cultures, which were wettable (hydrophilic), but restored to a wild-type level in 15-day-old cultures. The Δhyd1 mutants were compromised in conidial quality, including significant decreases in hydrophobicity (58%), adhesion to insect cuticle (36%), insect pathogenicity via normal cuticle infection (37%), UVB resistance (20%), and heat tolerance (10%). In contrast, none of all examined phenotypes were affected in the null mutants of hyd2 and hyd3. Intriguingly, micromorphology and integrity of hydrophobin rodlet bundles on conidial coat were not affected in all mutant and wild-type strains, but the rodlet bundles were disordered in the absence of hyd1, suggesting a link of the disorder to the decreased hydrophobicity. Therefore, Hyd1 mediates the fungal hydrophobicity and plays an important role in conidial quality control and insect-pathogenic lifecycle. Class I Hyd2 and class II Hyd3 seem functionally redundant in M. robertsii.