Biology Reference
In-Depth Information
a higher proportion of H units (
Fukushima and Terashima, 1991
)and
condensed linkages (
Saito and Fukushima, 2005
).
Besides the classical p-hydroxyphenyl, guaiacyl and syringyl monomers,
other molecules may participate in the formation of the lignin polymer, the so-
called non-canonical subunits. These include ferulates, hydroxycinnamalde-
hydes (coniferaldehyde and sinapaldehyde), dihydrocinnamyl alcohols, and
variously acylated monolignols (
Ralph et al., 2004, 2008; Sederoff et al.,
1999
), the presence and quantity of which is usually related to different taxa.
The complexity of lignin's structure is not only determined by the mono-
lignol composition, but also by the bonds produced during lignification. The
main linkages are
b
-O-4,
b
-5, 5-5, 4-O-5,
b
-
b
and
b
-1 (
Ralph et al., 2004
). The
bond frequency varies among cell types, layers of the cell wall and plant
groups. Woody gymnosperms contain
b
-O-4 linkages as the major inter-unit
linkage, levels that are even higher in woody angiosperms. This implies that
lignin in gymnosperms contains more inter-unit C-C linkages than woody
angiosperm lignins. Sinapyl alcohol has two methoxy groups ortho to the
phenol; coupling therefore results in a more linear lignin polymer, with a
larger number of ether bonds (
b
-O-4). For the most part, the
b
-ether level
increases with increasing S/G.
II. LIGNIN OCCURRENCE IN THE DIFFERENT
PHYLOGENETIC GROUPS
This classical view of the distribution, composition and evolutionary history
of lignin is currently being reviewed, due to the new and more recent research
developed regarding diverse phylogenetic groups. These advances have been
enabled due to the development in the past decades of a number of analytical
techniques that are more accurate and sensitive, or more structurally reveal-
ing, than those available before:
.
Anti-lignin labelling has not been widely used to study the presence of
lignin in many samples. The reason lies in the difficulty of obtaining lignin
free from other cell wall components to develop antibodies. However, this
problem has been solved by using antibodies against synthetic dehydro-
genative polymers (
Joseleau et al., 2004; M
ยจ
sel et al., 1997; Ruel et al.,
1994
). This technique is usually accompanied by other lignin analyses
because it is not considered as determinant for lignin detection by itself.
.
Nuclear magnetic resonance (NMR) provides useful information about
the structure of the lignin polymer, but it is not as sensitive as other
procedures. Recently, solution-state 2D NMR has emerged as a powerful
tool to study lignin structure and composition, even without requiring its
