Biology Reference
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the wall. FA is ester-linked to arabinoxylans and can also be ether-linked to G
units ( Jacquet et al., 1995 ). It was demonstrated that ferulate esters act as
initiation or nucleation sites of lignin deposition in grasses ( Ralph et al., 1995 ),
and this phenomenon would make the lignification process start differently in
grasses and dicotyledons. Ferulate molecules connect lignin to arabinoxylans
primarily through ester-ether bonds and form dimeric structures cross-linking
arabinoxylan chains to polysaccharides ( Ralph et al., 1994, 1998 ). It was pro-
posed that development of ferulate cross-linked structures between arabinoxy-
lans and lignins is the mechanismwhereby grass cells end their elongation process
and shift from primary to secondary wall development ( MacAdam et al.,
1992a,b ). Jung (2003) showed that ferulate-mediated cross-linking of lignins to
cell wall carbohydrates occurs in significant amounts during secondary wall
formation. In comparison, only a very small fraction of pCAwas found esterified
to hemicelluloses in immature tissues. The primary function of S-acylated mole-
cules could be to act as a precursor in monolignol polymerization, influencing the
binding ability of S lignin units and their incorporation into lignins (reviewed in
Barri`re et al., 2007; Grabber et al., 2004; Lu and Ralph, 1999 ).
Together, these results demonstrate that lignin biosynthesis and deposition
into the cell wall of grasses displays unique characteristics not found in
dicotyledons or/and other non-grass species and that hydroxycinnamic
acids play a crucial role in this process. Many unanswered questions remain
particularly in reference to how the lignin polymer is incorporated into the
cell wall (i.e., the role of laccases and peroxidases as well as the transport of
monolignols) and the molecular regulators of this process. In addition,
further studies are needed in the area of plant cell engineering through the
alteration of lignin composition, using a systems biology approach instead of
the single gene reduction method that is typically carried out, towards the
goal of biomass feedstock production for biofuels.
B. LIGNIN COMPOSITION VARIES ACCORDING TO CELL TYPES
It is well documented that lignin deposition varies between species and cell
types. A positive correlation between lignin levels and plant maturation is
thus often observed across species. For instance, in most angiosperms,
progressively higher amounts of S and G unit lignins are found in cell walls
as plants age ( Buxton and Redfearn, 1997; Buxton and Russel, 1988 ). In the
following paragraphs, the tissue type specific changes associated with lignin
deposition on the cellular and molecular level are discussed focusing specifi-
cally on the root, stems and leaves (when possible).
Lignification of the root is a necessary step of early plant development.
Lignin is deposited in the root protecting the plant against the soil environment
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