been suggested that LMW mediator compounds are involved with some enzymatic
attack, but this has not been well studied in
this type of decay. Brown rot fungi, and some
types of white rot fungi, employ LMW catalytic systems that allow small metabolites to
penetrate relatively deep into cell walls, and
even into the middle lamella, to catalyze reactions that depolymerize and solubilize cell wall
components and overcome the limitations
imposed by the bulkiness and size of enzymes
relative to the unmodified pore structure of
the wood secondary cell wall (Highley and
Illman 1991; Arantes et al. 2012). The importance of non-enzymatic radical-generating
mechanisms, and the lack of enzyme penetration into intact lignified cell walls, is sometimes ignored by pathologists and microbial
physiologists studying plant cell walls in the
scientific literature, leading to erroneous conclusions about how cell wall degradation and
even microbial pathogenesis involving cell
walls occurs.
B. Brown Rot Fungi
Brown rot fungi comprise only about 6% of all
known Basidiomycota species, yet they degrade
approximately 80% of wood, by mass, in the
northern hemisphere and other regions of the
world (Eastwood 2014). Although brown rot
fungi are present globally, in nature they tend
to attack softwoods preferentially or to be “generalists” (Krah et al. 2018), and therefore, they
have greater impact in the northern hemisphere
(primarily boreal forests) of the world and in
other regions where softwoods predominate.
Brown rot wood decay fungi are perhaps the
most destructive organisms of wood on earth,
and for this reason, considerable space in this
chapter will be devoted to their action.
Brown rot fungi are classified as such
because of the brown color of the wood residue
which remains after fungal degradation has
progressed to advanced stages. The wood in
this stage is also classically described as being
friable, crumbly, checked across the grain and
sometimes having a cross-hatched appearance
(Fig. 15.3). Early decay or incipient decay stages
of brown rot, defined as less than 10% mass
loss, often will appear visually unchanged
from undecayed wood other than appearing to
be wet in some areas. Because brown rot fungi
have a unique LMW catalytic mechanism for
initiating decay in wood, the LMW components
can diffuse through the wood cell wall rapidly
to depolymerize both lignin and holocellulose
components. It is this chelator-mediated Fenton (CMF) depolymerization, particularly of
the crystalline cellulose backbone of the elementary fibrils of wood, that dramatically
reduces the mechanical properties of wood
undergoing brown rot attack. In early work,
Wilcox (1978) reviewed literature demonstrating that 70% of both modulus of elasticity and
modulus of rupture could be lost with degradation by brown rot fungi at a level of only 10%
mass loss. Wood at this stage would not appear
brown and crumbly, and this is the key reason
why early brown rot decay stages are considered dangerous from a structural perspective,
because it can be impossible visually to tell how
much strength remains in a wood sample and
the extent that the wood has been attacked by
brown rot fungi is unknown. For this reason, it
is critically important to keep untreated wood
protected from moisture when wood is used in
either interior or exterior structural applications.
As reviewed above, even in initial stages of
brown rot degradation of wood, both holocellulose and lignin are depolymerized. The fact
that lignin is extensively depolymerized by
brown rot fungi has been recognized for some
time. In the 1990s, it was also recognized that
hydroxyl radicals would rapidly depolymerize
and then repolymerize lignin in modified form
(Barr and Aust 1994; Goodell et al. 1997).
Researchers also confirmed that lignin depolymerization occurred by the action of hydroxyl
radicals in brown rot fungi (Yelle et al. 2011)
and postulated further that repolymerization of
the modified lignin occurred: “via radical coupling of the phenolic units that become
enriched during attack on the aromatic rings
by
l
OH.” Because lignin is entangled with hemicellulose and encrusts the cellulose elementary
fibrils in wood, depolymerization of lignin is an
essential byproduct of hydroxyl radical attack
374
B. Goodell
attack, but this has not been well studied in
this type of decay. Brown rot fungi, and some
types of white rot fungi, employ LMW catalytic systems that allow small metabolites to
penetrate relatively deep into cell walls, and
even into the middle lamella, to catalyze reactions that depolymerize and solubilize cell wall
components and overcome the limitations
imposed by the bulkiness and size of enzymes
relative to the unmodified pore structure of
the wood secondary cell wall (Highley and
Illman 1991; Arantes et al. 2012). The importance of non-enzymatic radical-generating
mechanisms, and the lack of enzyme penetration into intact lignified cell walls, is sometimes ignored by pathologists and microbial
physiologists studying plant cell walls in the
scientific literature, leading to erroneous conclusions about how cell wall degradation and
even microbial pathogenesis involving cell
walls occurs.
B. Brown Rot Fungi
Brown rot fungi comprise only about 6% of all
known Basidiomycota species, yet they degrade
approximately 80% of wood, by mass, in the
northern hemisphere and other regions of the
world (Eastwood 2014). Although brown rot
fungi are present globally, in nature they tend
to attack softwoods preferentially or to be “generalists” (Krah et al. 2018), and therefore, they
have greater impact in the northern hemisphere
(primarily boreal forests) of the world and in
other regions where softwoods predominate.
Brown rot wood decay fungi are perhaps the
most destructive organisms of wood on earth,
and for this reason, considerable space in this
chapter will be devoted to their action.
Brown rot fungi are classified as such
because of the brown color of the wood residue
which remains after fungal degradation has
progressed to advanced stages. The wood in
this stage is also classically described as being
friable, crumbly, checked across the grain and
sometimes having a cross-hatched appearance
(Fig. 15.3). Early decay or incipient decay stages
of brown rot, defined as less than 10% mass
loss, often will appear visually unchanged
from undecayed wood other than appearing to
be wet in some areas. Because brown rot fungi
have a unique LMW catalytic mechanism for
initiating decay in wood, the LMW components
can diffuse through the wood cell wall rapidly
to depolymerize both lignin and holocellulose
components. It is this chelator-mediated Fenton (CMF) depolymerization, particularly of
the crystalline cellulose backbone of the elementary fibrils of wood, that dramatically
reduces the mechanical properties of wood
undergoing brown rot attack. In early work,
Wilcox (1978) reviewed literature demonstrating that 70% of both modulus of elasticity and
modulus of rupture could be lost with degradation by brown rot fungi at a level of only 10%
mass loss. Wood at this stage would not appear
brown and crumbly, and this is the key reason
why early brown rot decay stages are considered dangerous from a structural perspective,
because it can be impossible visually to tell how
much strength remains in a wood sample and
the extent that the wood has been attacked by
brown rot fungi is unknown. For this reason, it
is critically important to keep untreated wood
protected from moisture when wood is used in
either interior or exterior structural applications.
As reviewed above, even in initial stages of
brown rot degradation of wood, both holocellulose and lignin are depolymerized. The fact
that lignin is extensively depolymerized by
brown rot fungi has been recognized for some
time. In the 1990s, it was also recognized that
hydroxyl radicals would rapidly depolymerize
and then repolymerize lignin in modified form
(Barr and Aust 1994; Goodell et al. 1997).
Researchers also confirmed that lignin depolymerization occurred by the action of hydroxyl
radicals in brown rot fungi (Yelle et al. 2011)
and postulated further that repolymerization of
the modified lignin occurred: “via radical coupling of the phenolic units that become
enriched during attack on the aromatic rings
by
l
OH.” Because lignin is entangled with hemicellulose and encrusts the cellulose elementary
fibrils in wood, depolymerization of lignin is an
essential byproduct of hydroxyl radical attack
374
B. Goodell