White rot fungi are the only known organisms that can completely break down lignin to CO2 and H2O (Kirk and Farrell 1987). However, lignin cannot be degraded as a sole source of carbon and energy by white rot fungi. Degradation of lignin enables them to gain access to cellulose and hemicellulose, which serve as their actual carbon and energy source. White rot fungi predominantly degrade wood from deciduous trees (angiosperms), containing hardwood. In a survey of 65 central European wood-decaying basidiomy-cetes: four were reported to only attack coniferous wood, 34 attacked angiosperms exclusively, and 27 attacked both (Rypacek 1977).
The many species that cause white rots are a heterogeneous group that may degrade greater or lesser amounts of a specific cell wall component. Some species preferentially remove lignin from wood, leaving pockets of white degraded cells that consist entirely of cellulose. This is referred to as selective delignification (Blanchette 1995; Eriksson et al. 1990). Other species degrade lignin and cellulose simultaneously which is referred to as nonselective delignification. Among the best-studied white rot fungi are Phanerochate chrysosporium and Phlebia radiata, which degrade lignin selectively, and Trametes versicolor which degrades lignin nonselectively. There are fungi such as Ganoderma applanatum and Heterobasidion annosum, which are capable of both forms of degradation (Blanchette 1995; Eriksson et al. 1990). The ratio lignin-hemicellulose-cellulose decayed by a selective fungus can differ enormously and even different strains of the same species, e.g., of P. chrysosporium and Ceriporiopsis subvermispora, may behave differently on the same kind of wood.
White rot fungi typically colonize the cell lumen and cause cell wall erosion. Degradation is usually localized to cells colonized by fungal hyphae and substantial amounts of undecayed wood remains. Progressive erosion of the cell wall occurs when components are degraded simultaneously during nonselective delignification and eroded zones coalesce as decay progresses forming large voids filled with mycelium. During selective delignification a diffuse attack of lignin occurs and white pocket or white-mottled type of rot results (Blanchette 1991; 2000).
Electron microscopy studies have revealed that lignin is degraded at some distance from the hyphae and is removed progressively from the lumen towards the middle lamella, which is also degraded (Blanchette 1984; Blanchette et al. 1987). Sheaths, often composed of p-1,3-glucans, appear to be produced during early stages of wood colonization and facilitate its degradation (Blanchette et al. 1989; Nicole et al. 1995; Ruel and Joseleau 1991). These hyphal sheaths may play an important role in transport and presentation of wood-degrading enzymes from the hyphae during the decay process, thus establishing a point of attachment to the site of degradation. The association of peroxidases to the glucan matrix is in favor of the role of the sheath as a supporting structure (Ruel and Joseleau 1991). Furthermore, the fact that the sheath was hydrolyzed during the attack demonstrated its active role both in providing the H2O2 necessary for peroxidase activity and in providing a mode of transport for fungal enzymes to their substrates at the surface of the wood cell wall.
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