Delignification by Wood-Decay Fungi

Abstract

Wood decay fungi are unique because of their capacity to decompose lignified cell walls. A few species are of special interest because they can selectively remove lignin from wood without extensive cellulose degradation. Lignin is a complex, heterogeneous phenylpropanoid structural polymer that occurs throughout the cell wall (71, 112). Spatially, lignin is intimately interspersed with hemicelluloses forming a matrix that surrounds cellulose microfibrils (67, 75), and provides a formidable physical and chemical barrier to biodeg­ radative systems. Although most saprophytic fungi produce some degradative enzymes, such as cellulases, xylanases, mannanases and others, these en­ zymes do not permeate and degrade effectively woody substrates unless lignin is unbound, modified, or removed. Investigations of decomposition processes in forest ecosystems have usual­ ly considered lignin to be the most recalcitrant component and the last degraded (l0, 88). The decomposition pathway for decay by some fungi in the Basidiomycotina, such as those that cause brown rots, involves the degradation of all wood carbohydrates, including crystalline cellulose. A residual lignin matrix, consisting of chemically modified lignin, is left to be gradually converted to humic substances by long-term processes involving other microbes (38, 41, 106). However, this is not always the sequence of events. Fungi in the Basidiomycotina that cause white rots of wood may simultaneously degrade lignin along with all cell wall carbohydrates, or lignin