Introduction to Fungi, Third Edition

IMPORTANCE OF HOMOBASIDIOMYCETES
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with various sugars or uronic acids added as side
chains. Since pectin, cellulose and hemicelluloses
are all polymerized by enzymes in a regular
fashion, they can also be degraded by hydrolytic
Fig19.12 Wood rot symptoms. (a) White-rot of beech caused
byTrametes hirsuta (left). Note the bleached appearance of the
branch interior at the broken surface. Beech wood attacked
by a brown-rot is also shown for comparison (right). (b) Trunk
segments of Picea abies attacked by Fomitopsis pinicola.The
brown-rot had caused a hollowing of the standing tree prior
to felling. (c) Log of Picea abies colonized by Fomitopsis pinicola,
showing brown rot symptoms.The wood has cracked into
cube-like fragments which are easily ground into a powder.
enzymes, although accessibility problems may
prevent this from happening in situ (see below).
The architecture of secondary plant walls is
radically different. Apart from hemicelluloses
as a minor component, the principal building
material is lignin, a polymer of aromatic
alcohols which are cross-linked in a random
fashion by free radical reactions. Lignin therefore
has a complex, non-repetitive three-dimensional
structure resistant to direct attack by
enzymes. Only few basidiomycetes are able to
mineralize lignin to H 2 O and CO 2 , and this is
achieved by oxidative rather than hydrolytic
enzymes. The result is that wood undergoes
white-rot (see Plate 10a; Fig. 19.12a), i.e. it
appears bleached because all its components
are degraded more or less simultaneously, or
lignin degradation precedes attack on cellulose.
In contrast, many fungi which degrade cellulose
leave behind the lignin component of wood
which turns brown upon oxidation, and this type
of decay is therefore called brown-rot. The
removal of cellulose destroys the structural
integrity of wood so that the lignin cracks into
cubes (Fig. 19.12c) and ultimately breaks up into
a powder which becomes incorporated into
humus. Living trees may survive infection by
brown-rot fungi if this is confined to the heartwood
in the core of the trunk, leaving a
sufficiently strong cylinder of sound wood
(Fig. 19.12b). Prolonged or repeated attacks by
such wood-rotting basidiomycetes are responsible
for the hollowing of trunks observed in
many historic trees.
Brown-rot
The hyphae of brown-rot fungi colonizing wood
through its lignin-encased tubular cavities face
the major problem of gaining access to degradable
substrates. One point of attack is the middle
lamella, which is exposed by simple or bordered
pits in the secondary cell walls of adjacent cells,
and polygalacturonases are indeed produced
by many brown-rot fungi (Green & Clausen,
1999, 2003). A typical feature of brown-rots is
the production of oxalic acid, which may be
important in chelating calcium ions released at
potentially toxic concentrations by the degradation
of calcium pectate. Calcium oxalate crystals