Biology Reference
In-Depth Information
have been reported to affect the degree of lignification. Reduced lignification
in the middle lamella occurs in Cu-deficient Pinus radiata, although the
change in lignification is not thought to be directly attributable to the
physiological effects of Cu deficiency (
Downes et al., 1991
). Severely reduced
lignification of both middle lamella and secondary wall is observed in
drought-stressed radiata pine trees where it leads to shelling (tangential
fracture) of logs (
Donaldson, 2002
).
It should be recognised that wall thickness, and hence wood density, has an
effect on lignin content as measured on bulk wood samples, due to the
heterogeneous nature of lignin distribution in tracheid cell walls. Thin tra-
cheid walls associated with low wood density have a higher proportion of
highly lignified middle lamella and hence have a higher apparent lignin
content compared to wood with thicker walls (
Siddiqui, 1976
).
C. LIGNIN POLYMERISATION AND STRUCTURE
It is well established that lignin polymerisation in conifers begins in the
middle lamella/primary wall region at cell corners. The precise molecular
mechanism of lignin initiation remains unknown, but ferulate could play a
role in nucleating lignification in conifers (
Carnachan and Harris, 2000
). In
angiosperms, monolignols are polymerised within the cell wall in the pres-
ence of peroxidases and/or laccases (
Berthet et al., 2011; Czaninski et al.,
1993; Deighton et al., 1999; Terashima et al., 1995; Tsutsumi et al., 1998
)and
the same can be expected to apply to conifers. It has been speculated that
peroxidases and laccases fulfil different functions in lignin polymerisation.
For example, it has been proposed that laccases are associated with the
earliest stages of lignification and peroxidase with the later stages, thus
avoiding the phytotoxic effects of H
2
O
2
until apoptosis has commenced
(
Sterjiades et al., 1993
). Laccases have also been proposed to have a role in
late stages of lignification where H
2
O
2
is essentially excluded from
penetrating the hydrophobic, lignified cell wall environment (
Gavnholt and
Larsen, 2002
). These speculations make it quite obvious that we lack detailed
knowledge of how peroxidases and laccases share their role in lignin poly-
merisation, a situation complicated by the differing substrate preferences
that laccases and peroxidases can have (
Deighton et al., 1999; G
ยด
mez
Ros et al., 2007; Marjamaa et al., 2006; Tsutsumi et al., 1998
). Recent
observations have shown that laccases are among the most abundant
proteins in lignin-rich compression wood (
Mast et al., 2010
), which suggests
an important role for laccases in lignin polymerisation in conifers.
p-Coumaryl alcohol and coniferyl alcohol are the principal building blocks
of conifer lignin, which are complemented by minor constituents such as
