Biology Reference
In-Depth Information
reactions, such as rearomatization of the quinone methide following
b
-
b
0
coupling and, if the
g
-OH group is acetylated, this reaction is no longer
possible resulting in the production of other novel
b
-
b
0
-coupled units in the
polymer. The physiological role of acetylated lignins remains unclear, al-
though their involvement in drought tolerance has been suggested, as the
acetylated lignin polymer is more hydrophobic than normal lignin, which
could make vascular tissues more hydrophobic reducing water loss. This
hypothesis is supported by the presence of acetylated lignin in some succulent
agaves (
del R´o et al., 2007
). However, the latter role is not widely accepted,
as there are plants with highly acetylated lignin that are not drought tolerant.
Monocotyledonous angiosperms. Usually, monocots show the most com-
plex lignin composition among the phylogenetic clades. Besides H, G and S
units, some monocotyledons contain important quantities of p-coumarate
esters (
Ralph et al., 1994
). For example, grasses have high concentrations of
ferulates and p-coumarates acylating cell wall polymers (
Grabber and Lu,
2007
), and in maize samples ferulate represents 4% of the lignin (even though
it solely acylates polysaccharides), whereas p-coumarate (which acylates both
polysaccharides and lignins) represents 3% (
Hatfield et al., 1999; Saulnier
et al., 1999
). Ferulate and diferulate cross-links contribute to cell wall stiff-
ening, the cessation of plant growth and reduced enzymatic hydrolysis
(
Grabber et al., 1998; Schopfer, 1996
) and may act as nucleation sites for
lignin formation (
Grabber et al., 2002
). Most p-coumarates acylate the side
chains of mainly S units in lignin and could reach up to 20% of lignin in some
C4 grasses (
Ralph et al., 1994
). p-Coumarate concentrations vary consider-
ably among tissues, with extremely low levels in pith parenchyma, scleren-
chyma and vascular tissues (
Hatfield et al., 1999
). Most p-coumarates remain
as terminal units with an unsaturated side chain and a free phenolic group
(
Ralph et al., 1994
), and are supposed to play a role in oxidative cross-linking
of cell walls in response to pathogen attack (
Grabber and Lu, 2007
).
The deposition of p-coumaroylated lignins is associated with growth ces-
sation in grasses (
M
¨
sel et al., 1997
), but the role of p-coumarate in lignifica-
tion has not been fully ascertained (
Grabber and Lu, 2007
). Catalytic
amounts of p-hydroxycinnamic acids or their esters are reported to enhance
sinapyl alcohol oxidation by apoplastic peroxidases (
Hatfield et al., 2008;
Takahama et al., 1996
).
In maize, an 8% increase in syringyl lignin has been
reported after adding catalytic amounts of methyl p-coumarate (
Grabber
and Lu, 2007
).
All the monocots studied have shown the presence of G and S units. In
some cases, the presence of lignin is of evolutionary importance. For exam-
ple, Posidonia oceanica is found in marine habitats, where it forms large
