Biology Reference
In-Depth Information
walls is due to an increased need for more easily digestible forages and for the
production of bioenergy resources, or feedstocks, through plant breeding
including lignin engineering (
Li et al., 2008; Vanholme et al., 2008; Weng
et al., 2008
; reviewed in
Xu et al., 2011
).
A.
INITIATION OF LIGNIFICATION MAY VARY
BETWEEN GRASSES AND DICOTS
Previous studies have demonstrated that lignin deposition in the primary wall
begins after the formation of the secondary wall (
Donaldson 1991, 1992;
Hepler et al., 1970; Kutscha and Schwarzmann, 1875; Saka and Thomas,
1982; Takabe and Akazawa, 1981; Takabe et al., 1986; Wardrop, 1957,
1976
). Therefore, on the molecular level, the underlying mechanisms of lignin
deposition in both cell layers appear to be under the control of the same genes/
enzymes. Indeed, now it is not clear that genes encoding for 'lignin enzymes'
are specifically involved in primary or in secondary cell wall as evidenced for
cellulose synthases, CESA (
Kotake et al., 2011; Wang et al., 2010
). Neverthe-
less, since S units are mainly found in secondary wall, it is therefore possible to
speculate that ferulate-5-hydroxylase for instance (a limitant enzyme for S
production) may be considered as a secondary cell wall gene.
In grasses, the start of lignification is accompanied by the detection of lignin
units, H (p-hydroxyphenyl), G (guaiacyl) and S (syringyl). Lignin polymer is
characterized by the bonding of these units by
-O-4 ether bonds and by
resistant carbon-carbon and biphenyl-ether linkages (or condensed bonds;
Higuchi, 1990; Terashima et al., 1993; Yamamoto et al., 1989
). Initially, the
ratio of lignin units in the cell wall of grasses and cereals was thought to be in
the order of H
b
S(
Lewis and Yamamoto, 1990
). However, more recent
studies show a range of variability in the cell walls of these plants, yet the S and
G units are found in abundance as compared to H units (
Barri`re et al., 2007;
Lapierre, 1993
). This is important because although H units comprise less than
a few percent of total lignin monomer content in the wall, a signature of grass
cell walls is the relatively high amounts of H units found in grass lignin as
compared to the trace amounts observed in dicotyledons (reviewed in
Barri
`
re
et al., 2007; Dixon et al., 2001
).
Unique to grasses is the high amount of p-hydroxycinnamic acids, particu-
larly pCA and FA found in cell walls. Previous studies have shown that these
acids play an important functional role in the incorporation of lignin into the
cell wall. It was suggested that the major role of these compounds is to aid in
establishing ester or/and ether linkages to cell wall polymers (
Grabber et al.,
2004
). pCA is mainly esterified to the phenylpropane side chain of S lignin units
and its quantity is thus related to the total amount of S lignin deposited into
G
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