Figure 1. The fungal hypha as an efficient tunnelling machine. At the extreme apex, plastic wall components are synthesized at the plasma membrane and pushed into the extensible apical wall. These wall components are gradually crosslinked (increased yellow colour) as the apex pushes forward and the wall matures. The force for penetration is the hydrostatic pressure caused by the turgor (osmotic uptake of water), while proteins delivered in vesicles (red) are extruded in the apical wall by fusion of vesicles with the plasma membrane (red) and transported over the wall by the flow of plastic wall components (blue arrows). Among these proteins are degradative enzymes that dissolve solid polymers and thus ease the way for hyphal penetration and hydrophobins that attach the hyphal wall to hydrophobic solids. Small products generated in the degradative process are taken up by the hypha by means of the proton-motive force produced by ATPases in the plasma membrane (red arrows) and are used for metabolic purposes to generate cell components and energy.

Figure 2. Models showing the role of hydrophobins in the emergence of aerial hyphae (a and b) and the role of hydrophobins in adherence of fungal structures to hydrophobic surfaces such as plant leaves. In (a) hydrophobin molecules are secreted at the tips of hyphae into the medium and assemble at the water–air interface into a membrane that lowers the water surface tension so that hyphae can grow through the surface into the air. Once in the air (b) the hydrophobins assemble at the surface of the hyphae providing them with a hydrophobic coating. In (c) a hypha growing in the air adheres to a hydrophobic surface. In (d) it grows in an aqueous environment over a hydrophobic surface. Secreted hydrophobins assemble at the hydrophobic–hydrophilic interface making the surface hydrophilic (wettable) so that proteins or polysaccharides in wall or secreted capsular material (dark blue) can now glue the hypha to the surface. Chains of hydrophobin monomers are folded in such a way that hydrophobic amino acids (red) are located at the inside, hydrophilic amino acids (blue) at the outside of the molecule where they can interact with water. When assembling at a hydrophobic–hydrophilic interface the hydrophobins undergo a conformational change exposing hydrophobic amino acids to one side and amino acids to the other, forming an amphipathic membrane (from Wessels, 2000).