Hydrophobins in the Life Cycle of the Ectomycorrhizal Basidiomycete <i>Tricholoma vaccinum</i>

<div><p>Hydrophobins—secreted small cysteine-rich, amphipathic proteins—foster interactions of fungal hyphae with hydrophobic surfaces, and are involved in the formation of aerial hyphae. Phylogenetic analyses of <i>Tricholoma vaccinum</i> hydrophobins showed a grouping with hydrophobins of other ectomycorrhizal fungi, which might be a result of co-evolution. Further analyses indicate angiosperms as likely host trees for the last common ancestor of the genus <i>Tricholoma</i>. The nine hydrophobin genes in the <i>T</i>. <i>vaccinum</i> genome were investigated to infer their individual roles in different stages of the life cycle, host interaction, asexual and sexual development, and with respect to different stresses. In aerial mycelium, <i>hyd8</i> was up-regulated. <i>In silico</i> analysis predicted three packing arrangements, i.e., ring-like, plus-like and sheet-like structure for Hyd8; the first two may assemble to rodlets of hydrophobin covering aerial hyphae, whereas the third is expected to be involved in forming a two-dimensional network of hydrophobins. Metal stress induced hydrophobin gene <i>hyd5</i>. In early steps of mycorrhization, induction of <i>hyd4</i> and <i>hyd5</i> by plant root exudates and root volatiles could be shown, followed by <i>hyd5</i> up-regulation during formation of mantle, Hartig’ net, and rhizomorphs with concomitant repression of <i>hyd8</i> and <i>hyd9</i>. During fruiting body formation, mainly <i>hyd3</i>, but also <i>hyd8</i> were induced. Host preference between the compatible host <i>Picea abies</i> and the low compatibility host <i>Pinus sylvestris</i> could be linked to a stronger induction of <i>hyd4</i> and <i>hyd5</i> by the preferred host and a stronger repression of <i>hyd8</i>, whereas the repression of <i>hyd9</i> was comparable between the two hosts.</p>