Forests contain 90% of terrestial biomass and are a significant contributor to carbon flux. Much of this biomass is lignocellulose, which is decomposed by basidiomycetous fungi. Brown rot fungi are unique in that they have evolved several times to develop non-enzymatic free radical systems which has permitted them to reduce the physiologically-expensive process of producing large extracellular enzymes for plant cell wall deconstruction. The first non-enzymatic system to be identified in Gloeophyllum trabeum (Pers.) Murrill, and now found in several other brown rot species, is the chelator mediated Fenton system (CMFS). The mechanism involves low molecular weight compounds, typically catecholates (hydroxybenzene derivatives) such as 2, 5-dimethoxyhydroquinone, that bind and reduce iron in the wood cell wall. This mechanism has been demonstrated to be dependent on pH and oxalate concentration. Iron is bound by oxalate, found in the low pH, immediate environment of the fungal hyphae. Iron-oxalate and free fungal catecholate diffuse into the highly buffered environment of the wood cell wall, where iron from the oxalate will transfer to the catecholate chelator. Iron in the catecholate-bound form will react with peroxide within the cell wall to generate short-lived, but powerful, hydroxyl radicals via Fenton chemistry. This mechanism explains how reactive oxygen species can be generated at a distance from the fungal hyphae to protect the fungus from damage. Further, it provides a basis to explain how both cellulose and lignin are modified by brown rot fungi, with the cellulosic components being metabolized by the fungus and the lignin components re-polymerized to produce a highly recalcitrant 'brown rot modified lignin' that has been reported to persist for centuries in the environment. The role of low molecular weight fungal metabolites in association with iron will be discussed in the context wood cell structure, cellulose crystallinity, and carbon cycling.

Non-enzymatic decomposition of lignocellulose by brown rot fungi and the role it plays in carbon sequestration and cycling

247th National Meeting (2014)

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