Fig. 2
Hypothesis of the symbiotic interface establishment in Usnea hakonensis. a The hydrophilic cell wall layer and the hydrophobic layer produce passive fluxes of water and nutritional solutes from the thallus exterior to interior algal cells, securing gas-filled space for efficient CO2 diffusion. b Fungal glycoside hydrolases degrade the fungal and algal cell walls, let a hyphal tip grow into the algal cell wall and form a haustorium. c The hydrophilic layer containing polysaccharides produced by fungal glycosyltransferases overlies the surface of the fungal cell wall. d The hydrophobic layer consisting of mycobiont-derived hydrophobins, secondary metabolites, lipids, and fatty acids, seals the surface of fungal and algal cell walls. e The lipid-derived communication tools are used by the mycobiont to communicate with the photobiont. f Fungal lipases degrade lipids and release glycerol that is recycled by the mycobiont