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Abstract

HydrophobinsÐsecreted small cysteine-rich, amphipathic proteinsÐfoster interactions of
fungal hyphae with hydrophobic surfaces, and are involved in the formation of aerial hyphae.
Phylogenetic analyses of Tricholoma vaccinum 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 Tricholoma. The nine hydrophobin genes in the T. vaccinum 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, hyd8
was up-regulated. In silico 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 hyd5. In
early steps of mycorrhization, induction of hyd4 and hyd5 by plant root exudates and root
volatiles could be shown, followed by hyd5 up-regulation during formation of mantle, Hartig'
net, and rhizomorphs with concomitant repression of hyd8 and hyd9. During fruiting body
formation, mainly hyd3, but also hyd8 were induced. Host preference between the compatible
host Picea abies and the low compatibility host Pinus sylvestris could be linked to a
stronger induction of hyd4 and hyd5 by the preferred host and a stronger repression of
hyd8, whereas the repression of hyd9 was comparable between the two hosts.