A CYTOCHEMICAL STUDY OF EXTRACELLULAR SHEATHS ASSOCIATED WITH RIGIDOPORUS-LIGNOSUS DURING WOOD DECAY

An ultrastructural and cytochemical investigation of the development o f Rigidoporus lignosus, a white-rot fungus inoculated into wood blocks , was carried out to gain better insight into the structure and role o f the extracellular sheaths produced by this fungus during wood degrad ation. Fungal sheaths had a dense or loose fibrillar appearance and we re differentiated from the fungal cell wall early after wood inoculati on. Close association between extracellular fibrils and wood cell wall s was observed at both early and advanced stages of wood alteration. F ungal sheaths were often seen deep in host cell walls, sometimes enclo sing residual wood fragments. Specific gold probes were used to invest igate the chemical nature of R. lignosus sheaths. While labeling of ch itin, pectin, beta-1,4- and beta-1,3-glucans, beta-glucosides, galacto samine, mannose, sialic acid, RNA, fucose, and fimbrial proteins over fungal sheaths did not succeed, galactose residues and laccase (a fung al phenoloxidase) were found to be present. The positive reaction of s heaths with the PATAg test indicates that polysaccharides such as beta -1,6-glucans are important components. Our data suggest that extracell ular sheaths produced by R. lignosus during host cell colonization pla y an important role in wood degradation. Transportation of lignin-degr ading enzymes by extracellular fibrils indicates that alteration of pl ant polymers may occur within fungal sheaths. It is also proposed that R. lignosus sheaths may be involved in recognition mechanisms in fung al cell-wood surface interactions.