HFB8, the orphan hydrophobin of Trichoderma guizhouense, is involved in mycoparasitism, surface growth and protects hyphae from fungicides
Hydrophobins (HFBs) are surface-active proteins secreted by filamentous fungi. They can self-assembly in single molecular layers at hydrophobic-hydrophilic interfaces and can, therefore, be directly involved in the establishment of fungi in their habitat. In this work, we have studied the function of hydrophobin 8 (HFB8) that is encoded in genomes of T. guizhouense and T. afroharzianum from the Harzianum clade of Trichoderma (Hypocreales, Ascomycota). hfb8-overexpressing mutants were constructed for T. guizhouense under a promoter of a constitutively expressed cdna1 gene of T. reesei with an mRFP reporter. Results showed that HFB8 was predominantly secreted and can be readily found in the fungal culture filtrates and also on the fungal cell walls especially on germinated spore surfaces, growing hyphal tips, and near septae. The hfb8-overexpressing mutants had substantially increased capacity to colonize the hydrophilic glass surface in water, but their ability to colonize tomato roots was reduced compared to the wild-type strain. Confrontation experiments confirmed the involvement of HFB8 in the interactions between Trichoderma and other fungi, which shows reduced antagonism ability of the hfb8-expressing mutants. More remarkable, overexpression of hfb8 showed improved growth of Trichoderma cells in the cultivations containing ROS-producing compounds (i.e., menadion) or fungicides (i.e., amphotericin B) demonstrating a “raincoat” effect of this particular HFBs. We conclude that HFB8 plays a role in Trichoderma mycoparasitism, attachment to hydrophilic surfaces and it is also required for the physical protection of the hyphal surface against fungicides.