| Summary: | <i>Tricholoma vaccinum</i> is an ectomycorrhizal basidiomycete with high host specificity. The slow-growing fungus is able to produce twenty sesquiterpenes, including α-barbatene, sativene, isocaryophyllene, α-cuprenene, β-cedrene, ß-copaene, 4-epi-α-acoradiene, and chamigrene in axenic culture. For the three major compounds, Δ<sup>6</sup>-protoilludene, β-barbatene, and an unidentified oxygenated sesquiterpene (<i>m/z</i> 218.18), changed production during co-cultivation with the ectomycorrhizal partner tree, <i>Picea abies</i>, could be shown with distinct dynamics. During the mycorrhizal growth of <i>T. vaccinum</i>–<i>P. abies</i>, Δ<sup>6</sup>-protoilludene and the oxygenated sesquiterpene appeared at similar times, which warranted further studies of potential biosynthesis genes. In silico analyses identified a putative protoilludene synthesis gene, <i>pie1</i>, as being up-regulated in the mycorrhizal stage, in addition to the previously identified, co-regulated geosmin synthase, <i>ges1</i>. We therefore hypothesize that the sesquiterpene synthase <i>pie1</i> has an important role during mycorrhization, through Δ<sup>6</sup>-protoilludene and/or its accompanied oxygenated sesquiterpene production.
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