Colocalizing incipient reactions in wood degraded by the brown rot fungus Postia placenta

Abstract

Brown rot fungi are theorized to use both free radicals and enzymes to degrade wood. If these incompatible agents are employed in sequence (enzymatic after oxidative) in order to avoid interaction, this should be resolvable spatially in rotting wood. To assess this, we used thin spruce wafers as substrates, with the largest face the transverse plane. Propped wafers were colonized from the bottom (tangential to grain) by Postia placenta, using wood cell orientation and gravity to slow fungal egress and accentuate spatial gradients. After brief colonization, wafers were cut into 1-mm strips progressing up the wafer, and subsectioned for complementary analyses. Analyses included fungal growth, pH, cellulase activity, and wood modifications attributable to non-enzymatic mechanisms. Hyphae were imaged using confocal microscopy of fluorophore-tagged chitin. Dilute alkali solubility and lignin demethylation were measured as proxies (consequences) of carbohydrate depolymerization and lignin oxidation, respectively. Because P. placenta lacks genes for cellobiohydrolases, endoglucanase (EG) activity was measured. In composites of reassembled sections, hyphal fronts and apparent depolymerization preceded EG and lignin demethylation fronts by more than 6 mm. Although detection limits are a caveat when implicating novel fungal metabolites, results encourage and provide methodology for targeting this interesting leading edge of cellulolysis.