Physiological function of hydrophobin Hydph16 in cell wall formation in agaricomycete Pleurotus ostreatus

Hydrophobins are small-secreted proteins with both hydrophobic and hydrophilic regions, enabling the mycelium to break through the air-medium interface by reducing the medium surface tension. Over 20 putative hydrophobin-encoding genes have been predicted in the agaricomycete Pleurotus ostreatus. Three hydrophobin-encoding genes, vmh2, vmh3, and hydph16, were predominantly expressed in the vegetative mycelium. Despite these common properties, we have previously demonstrated the distinct functions of Vmh2 and Vmh3 in environmental stress resistance. In this study, we focused on hydph16 and found that Δhydph16 strains had sparser aerial mycelium than control strains. The cell wall thickness of Δhydph16 strains reduced by 40 % compared to that of control strains, but no significant differences were found in the relative chitin and glucan percentages or relative putative cell wall synthesis-related gene expression levels. Furthermore, unlike vmh2 and vmh3, hydph16 deletion did not change the hydrophobicity of the aerial mycelium. This study is the first to report that the lack of hydrophobin can lead to a significant change in aerial hyphae cell wall formation without altering the major cell wall polysaccharide composition. Additionally, this study revealed multiple roles for Hydph16, distinct from those of other highly expressed hydrophobins, Vmh2 and Vmh3. These results suggested that agaricomycetes, including P. ostreatus, have evolved to possess multiple hydrophobins with different functions.