Wood decay by basidiomycetes: Extracellular tripartite membranous structures

Decay of pine-wood by the brown-rot fungi Poria placenta, Fomitopsis pinicola, Serpula lacrimans, Paxillus panuoides and the white-rot fungi Trametes versicolor, Trametes hirsuta, Sporotrichum pulverulentum was investigated with the transmission electron microscope. Material stained with ruthenium red showed extracellular, tripartite structures associated with all fungi throughout all stages of wood decay. These structures surrounded the intensely stained fungal cell wall, were partly embedded in an extracellular slime and were contiguous with the wood cell wall. Besides brown-rot fungi with filter paper activity S. pulverulentum showed similar tripartite structures, however these were branched and extended from the fungal cell wall into the lumen of the wood cell. The origin, formation and function of these extracellular structures are discussed.