Biology Reference
In-Depth Information
P. chrysosporium can be the result of various modes of action depending
on the strain involved. The strain ME446 provokes thinning all around
the cell circumference (
Ruel et al., 1994
). The strain K3 provokes holes in
the inner part of the S2 layer, which can join at advanced stages of decay,
leaving S3 intact (
Ruel et al., 1994
).
(ii). The 'selective' (i.e. sequential and preferential) white-rot is characterized
by the preferential degradation of lignins and hemicelluloses. Cellulose
is generally not attacked (
Eriksson et al., 1990; Liese, 1970
). Decayed
wood has a fibrous aspect, with a cell loosening. Lignin degradation
occurs in middle lamella and secondary walls, and is apparent at con-
siderable distance from the colonizing hyphae (diffusion mechanism).
Selective white-rot takes place both on hardwood and softwood. The
causal agents include Basidiomycota such as Ceriporiopsis subvermis-
pora, Cerrena unicolor, D. squalens, Ganoderma australe, Phlebia tremel-
losa, Pleurotus spp., P. cinnabarinus and Phellinus pini (
Buswell, 1991;
Martinez et al., 2005
).
Both simultaneous and selective decays can be produced by the same WRF
depending on the stage of decay. Moreover, on some woods, WRF can
selectively attack certain cell types without altering others. For instance,
Blanchette et al. (1988)
have shown that Phellinus kawakamii specifically
degrades the fibres and parenchyma cells of Acacia koa wood without affect-
ing the vessels. Phlebia chrysocrea and G. australe, which similarly degrade
lignins, cause a simultaneous rot in Laurelia philippiana, but a selective
delignification in Eucryphia cordifolia (
Barrasa et al., 1992
).
In wood cell walls, the decomposition of lignins is possible due to lignin-
modifying enzymes which are extracellular metallo-oxidoreductases, mainly
heme-containing peroxidases and laccases (as discussed in the next section).
Besides these enzymes, the degradation reactions also involve secreted meta-
bolites such as phenolic and other aromatic compounds, small peptides,
organic acids, metal ions and reactive oxygen species (
Kundell et al., 2010
).
II. MAIN ENZYMES OF LIGNIN DEGRADATION:
TYPE, STRUCTURE AND REACTIONS
Although lignin units are relatively simple molecules provided with a phenyl-
propane backbone, the tri-dimensional structure of the resulting polymer is
very complex and non-repeating, a feature with important consequences
for biological delignification. First, only extracellular enzymes can access
the highly variable polymer surface. In addition, the diversity of the lignin
interunit bonds prevents their specific cleavage by enzymes. Hence, lignins
