The pleiotropic functions of intracellular hydrophobins in aerial hyphae and fungal spores

Abstract

Higher fungi can rapidly produce large numbers of spores suitable for aerial dispersal. The efficiency of the dispersal and spore resilience to abiotic stresses correlate with their hydrophobicity provided by the unique amphiphilic and superior surface-active proteins–hydrophobins (HFBs)–that self-assemble at hydrophobic/hydrophilic interfaces and thus modulate surface properties. Using the HFB-enriched mold Trichoderma (Hypocreales, Ascomycota) and the HFB-free yeast Pichia pastoris (Saccharomycetales, Ascomycota), we revealed that the rapid release of HFBs by aerial hyphae shortly prior to conidiation is associated with their intracellular accumulation in vacuoles and/or lipid-enriched organelles. The occasional internalization of the latter organelles in vacuoles can provide the hydrophobic/hydrophilic interface for the assembly of HFB layers and thus result in the formation of HFB-enriched vesicles and vacuolar multicisternal structures (VMSs) putatively lined up by HFBs. These HFB-enriched vesicles and VMSs can become fused in large tonoplast-like organelles or move to the periplasm for secretion. The tonoplast-like structures can contribute to the maintenance of turgor pressure in aerial hyphae supporting the erection of sporogenic structures (e.g., conidiophores) and provide intracellular force to squeeze out HFB-enriched vesicles and VMSs from the periplasm through the cell wall. We also show that the secretion of HFBs occurs prior to the conidiation and reveal that the even spore coating of HFBs deposited in the extracellular matrix requires microscopic water droplets that can be either guttated by the hyphae or obtained from the environment. Furthermore, we demonstrate that at least one HFB, HFB4 in T. guizhouense, is produced and secreted by wetted spores. We show that this protein possibly controls spore dormancy and contributes to the water sensing mechanism required for the detection of germination conditions. Thus, intracellular HFBs have a range of pleiotropic functions in aerial hyphae and spores and are essential for fungal development and fitness.

Fungal Cell Factories for Efficient and Sustainable Production of Proteins and Peptides

“…Class II HFBs have a role in signaling the moisture conditions on the spore surface, which has been described for Trichoderma [134]. If the water concentration decreases, the HFB concentration increases, which signals that the germination is not favorable, whereas the HFBs in wet conditions are low, which signals favorable germination conditions [134].…”

“…Class I HFBs are involved in production of fibrillar structures (rodlets) that help fungal conidia bind to surfaces resulting in better resistance to the environment [130]. Class II HFBs have a role in signaling the moisture conditions on the spore surface, which has been described for Trichoderma [134]. If the water concentration decreases, the HFB concentration increases, which signals that the germination is not favorable, whereas the HFBs in wet conditions are low, which signals favorable germination conditions [134].…”

“…Fungi produce a vast number of peptides with different activities, of which the most wellknown and studied peptides are those that exhibit cytotoxicity or antimicrobial functions (AMPs) [139]. Several studies in recent decades have shown that many of these harmful HFBs are produced in the cells, transported to the periplasm in lipid-enriched HFB vacuoles, and released to the exterior through the cell wall [134]. In connection to conidia-tion, HFBs are highly expressed so that HFBs can coat the spores, thereby supporting the attachment to different surfaces and protecting the fungi from stress conditions [134].…”

Aspergillus Hydrophobins: Physicochemical Properties, Biochemical Properties, and Functions in Solid Polymer Degradation

“…Hydrophobins have β-barrel structures that are similar to each other [ 2 , 3 , 4 , 5 , 6 ]. Some filamentous fungi such as Aspergillus , Penicillium , Trichoderma , extremophilic species, or mycorrhizal fungi have several to over 10 hydrophobin-encoding genes, whereas many filamentous fungi have only a few such genes [ 7 , 8 , 9 , 10 , 11 ]. The expression profiles of multiple hydrophobin genes depend on the growth stage of filamentous fungi and culture conditions, and cellular localization varies among hydrophobins [ 12 , 13 , 14 ].…”

“…Because formation of such membranes reduces interfacial surface tension, hydrophobins contribute to the formation of aerial hyphae and conidia [ 18 , 19 , 20 ]. Hydrophobins are specifically accumulated inside aerial hyphae, where they associate with lipid-enriched organelles and may affect the structure and increase longevity of aerial hyphae [ 7 ]. Secretion of hydrophobins becomes highest at the sporulation phase, when they form a protective coating of rapidly produced spores [ 7 ].…”