Oxidative Degradation of Wood by Brown-Rot Fungi

Brown-rot fungi are Basidiomycetes that remove cellulose and other polysaccharides from wood, leaving an amorphous, brown, crumbly residue that is composed largely of lignin, hence the name brown-rot. Decay by brown-rot fungi is by far the most serious type of damage to wood in-service. These fungi cause structural failure before losses in total wood substance are detected. The effect of brown-rot fungi on wood strength properties is due to cellulose depolymerization. Worldwide losses of millions of dollars annually result from fungal deterioration of untreated or inadequately-treated wood (11). Several species of brown-rot fungi decay heartwood in living frees. These tree pathogens add to the losses in wood product utilization and threaten forest and plantation stands (9). Brown-rot fungi represent an untapped resource for industrial bioconversion of lignocelluloses. With the unique ability to circumvent the lignin barrier in cellulose utilization, brown-rot fungi offer a potential cost-effective pretreatment process for saccharification of cellulose. Relatively little research attention has been given to identifying the mechanism of wood cellulose depolymerization by brown-rot fungi. The actual biochemical system is not known, but evidence has accumulated in support of an oxidative mechanism of depolymerization. A model of cellulose depolymerization is based on fungal production of extracellular reduced oxygen species.