Biology Reference
In-Depth Information
C. WHITE-ROT FUNGI (WRF)
WRF, the most frequent wood-rotting organisms, have the ability to degrade
and mineralize all the major wood components (cellulose, hemicelluloses and
lignins;
Christman and Oglesby, 1971
). The common feature of white-rot is
the extensive degradation of lignins resulting in a bleached appearance of the
decayed wood. This ability of WRF is used in the process called biopulping.
White-rot decay is characterized by a moist, soft and spongy consistency
of rotten wood (
Fig. 1
B), with a strength loss at advanced stages (
Martinez
et al., 2005
). In some cases, WRF completely degrade wood to give residues
that are not stable components of forest soils (
Ryvarden, 1991
). Like soft-rot,
white-rot can be extended or variably restricted to certain zones of wood
(
Eriksson et al., 1990
).
WRF comprise numerous fungi, mostly Basidiomycota belonging to the
Polyporales and Agaricales orders (
Fig. 1
C and D), including Coriolus versi-
color, Dichomitus squalens, Lentinus edodes, Phlebia radiata, Panus tigrinus,
Pleurotus ostreatus and Pycnoporus cinnabarinus, and a few species of Asco-
mycota such as Xylaria spp.
The modes of action and wood-degrading patterns of WRF have been
extensively studied since the late 1970s, especially by Higuchi and co-workers
(
Higuchi, 1983; Kawai et al., 1988; Umezawa and Higuchi, 1985
), Kirk and
co-workers (
Faix et al., 1981
;
Kirk and Farrel, 1987; Kirk et al., 1975
), Gold
and co-workers (
Glenn et al., 1983; Gold et al., 1983
) and
Ander and
Eriksson (1977, 1978)
, using either wood samples or lignin model compounds
(model dimers with
b
-O-4,
b
-1,
b
-5, or
b
-
b
linkages) or
14
C-labelled synthetic
lignins. The fungus Phanerochaete chrysosporium (order Thelephorales) was
historically the first white-rot model to be studied. WRF rapidly colonize
wood, generally beginning by rays then invading all xylem cells. Both the
type of attack and the degradation order of the different components can
vary according to the fungus species and to the substrate (
Blanchette, 1995;
Eriksson et al., 1990
).
Two general white-rot patterns can be distinguished:
(i). The 'simultaneous' decay is characterized by the simultaneous degrada-
tion of cellulose, hemicelluloses and lignins. Cell walls are progressively
attacked from lumen towards the middle lamella, and show gradual
thinning. The formation of holes and the erosion of wood tissues occur
at the proximity of fungal hyphae. The causal agents include Basidio-
mycota such as Trametes versicolor, Irpex lacteus, P. chrysosporium and
Heterobasidion annosum, and Ascomycota such as Xylaria hypoxylon
(
Martinez et al., 2005
). Simultaneous white-rot generally takes place
on hardwoods, rarely on softwoods. The cell wall thinning caused by
