Fungal hydroquinones contribute to brown rot of wood

The fungi that cause brown rot of wood initiate lignocellulose breakdown with an extracellular Fenton system in which Fe2+ and H2O2 react to produce hydroxyl radicals (.OH), which then oxidize and cleave the wood holocellulose. One such fungus, Gloeophyllum trabeum, drives Fenton chemistry on defined media by reducing Fe3+ and O-2 with two extracellular hydroquinones, 2,5-dimethoxyhydroquinone (2,5-DMHQ) and 4,5-dimethoxycatechol (4,5-DMC). However, it has never been shown that the hydroquinones contribute to brown rot of wood. We grew G. trabeum on spruce blocks and found that 2,5-DMHQ and 4,5-DMC were each present in the aqueous phase at concentrations near 20 mu M after 1 week. We determined rate constants for the reactions of 2,5-DMHQ and 4,5-DMC with the Fe3+-oxalate complexes that predominate in wood undergoing brown rot, finding them to be 43 l mol(-1) s(-1) and 65 l mol(-1) s(-1) respectively. Using these values, we estimated that the average amount of hydroquinone-driven .OH production during the first week of decay was 11.5 mu mol g(-1) dry weight of wood. Viscometry of the degraded wood holocellulose coupled with computer modelling showed that a number of the same general magnitude, 41.2 mu mol oxidations per gram, was required to account for the depolymerization that occurred in the first week. Moreover, the decrease in holocellulose viscosity was correlated with the measured concentrations of hydroquinones. Therefore, hydroquinone-driven Fenton chemistry is one component of the biodegradative arsenal that G. trabeum expresses on wood.