Distinct Growth and Secretome Strategies for Two Taxonomically Divergent Brown Rot Fungi

<jats:title>ABSTRACT</jats:title> <jats:p> Brown rot fungi are wood-degrading fungi that employ both oxidative and hydrolytic mechanisms to degrade wood. Hydroxyl radicals that facilitate the oxidative component are powerful nonselective oxidants and are incompatible with hydrolytic enzymes unless they are spatially segregated in wood. Differential gene expression has been implicated in the segregation of these reactions in <jats:named-content content-type="genus-species">Postia placenta</jats:named-content> , but it is unclear if this two-step mechanism varies in other brown rot fungi with different traits and life history strategies that occupy different niches in nature. We employed proteomics to analyze a progression of wood decay on thin wafers, using brown rot fungi with significant taxonomic and niche distances: <jats:named-content content-type="genus-species">Serpula lacrymans</jats:named-content> (Boletales; “dry rot” lumber decay) and <jats:named-content content-type="genus-species">Gloeophyllum trabeum</jats:named-content> (order Gloeophyllales; slash, downed wood). Both fungi produced greater oxidoreductase diversity upon wood colonization and greater glycoside hydrolase activity later, consistent with a two-step mechanism. The two fungi invested very differently, however, in terms of growth (infrastructure) versus protein secretion (resource capture), with the ergosterol/extracted protein ratio being 7-fold higher with <jats:named-content content-type="genus-species">S. lacrymans</jats:named-content> than with <jats:named-content content-type="genus-species">G. trabeum</jats:named-content> . In line with the native substrate associations of these fungi, hemicellulase-specific activities were dominated by mannanase in <jats:named-content content-type="genus-species">S. lacrymans</jats:named-content> and by xylanase in <jats:named-content content-type="genus-species">G. trabeum</jats:named-content> . Consistent with previous observations, <jats:named-content content-type="genus-species">S. lacrymans</jats:named-content> did not produce glycoside hydrolase 6 (GH6) cellobiohydrolases (CBHs) in this study, despite taxonomically belonging to the order Boletales, which is distinguished among brown rot fungi by having CBH genes. This work suggests that distantly related brown rot fungi employ staggered mechanisms to degrade wood, but the underlying strategies vary among taxa. </jats:p> <jats:p> <jats:bold>IMPORTANCE</jats:bold> Wood-degrading fungi are important in forest nutrient cycling and offer promise in biotechnological applications. Brown rot fungi are unique among these fungi in that they use a nonenzymatic oxidative pretreatment before enzymatic carbohydrate hydrolysis, enabling selective removal of carbohydrates from lignin. This capacity has independently evolved multiple times, but it is unclear if different mechanisms underpin similar outcomes. Here, we grew fungi directionally on wood wafers and we found similar two-step mechanisms in taxonomically divergent brown rot fungi. The results, however, revealed strikingly different growth strategies, with <jats:named-content content-type="genus-species">S. lacrymans</jats:named-content> investing more in biomass production than secretion of proteins and <jats:named-content content-type="genus-species">G. trabeum</jats:named-content> showing the opposite pattern, with a high diversity of uncharacterized proteins. The “simplified” <jats:named-content content-type="genus-species">S. lacrymans</jats:named-content> secretomic system could help narrow gene targets central to oxidative brown rot pretreatments, and a comparison of its distinctions with <jats:named-content content-type="genus-species">G. trabeum</jats:named-content> and other brown rot fungi (e.g., <jats:named-content content-type="genus-species">Postia placenta</jats:named-content> ) might offer similar traction in noncatabolic genes. </jats:p>