Environmental Engineering Reference
In-Depth Information
This increased knowledge about plant cell walls is of direct benefit to the biofuel industry because
detailed understanding of wall structure and biosynthesis will be necessary to optimize procedures
for cell wall deconstruction into fermentable sugars. The next section of this review will provide an
overview of these advancements by individually covering the major components of plant cell walls.
5.2.3 l
lignin
Lignin is a plant cell-wall-associated complex polymer composed of aromatic subunits. It is usu-
ally derived from phenylalanine (Whetten and Sederoff 1995). Lignin is present in most vascular
plants and plays significant roles in mechanical support, transportation of water, and defense against
pathogens (Lewis and Yamamoto 1990; Whetten and Sederoff 1995; Douglas 1996). Lignin biosyn-
thesis regulation is dependent on the plant's growth and development. Thus, deposition of lignin is
usually limited to specific plant cell types. Some good examples for these cell types are tracheary
elements in the xylem and sclerenchyma, where lignin is usually deposited in the secondary thick-
ened walls. Lignin biosynthesis is significantly scaled up in woody plants, where secondary xylem
constitutes most of the plant body.
There are three monomeric lignin subunits, namely
p
-hydroxyphenyl (H), guaiacyl (G), and
syringyl (S) subunits. The relative amounts these monomeric subunits can vary depending on the
plant species and cell type. Lignin composition varies with plant types, with the G and S monomers
usually predominating in most lignin. For example, most monomer subunits constituting lignin in
dicotyledonous angiosperms are G and S, whereas in conifers, lignin consists almost entirely of G
units (Lee et al. 1997). G and S monomer composition also varies according to the cell types and
the developmental stage of the tissue (Whetten and Sederoff 1995). Tracheary elements in vascular
bundles
of
A. thaliana
contain lignin that is mainly composed of G subunits, whereas the bordering
heavily lignified sclerenchyma cells contain substantial amounts of S subunits (Chapple et al. 1992).
Thus, one of the major challenges in lignin biosynthesis research is unraveling the mechanisms
that regulate differential carbon flow during the synthesis of these different lignin precursors and
subsequent synthesis of lignin polymers of differing subunit composition.
Cloning and biochemical characterization of most major lignin biosynthetic enzymes have been
reported previously by several groups (Lewis and Yamamoto 1990; Whetten and Sederoff 1995;
Boudet and Grima-Pettenati 1996; Campbell and Sederoff 1996; Douglas 1996). A detailed scheme
for lignin biosynthesis by these enzymes has been previously reported (Whetten and Sederoff 1995).
Table 5.1 lists the major lignin biosynthetic enzymes that current research has focused on and their
proposed functions.
taBle 5.1
major lignin Biosynthetic enzymes Focused in
Arabidopsis
thaliana
research
enzyme
abbreviation
Function
Cinnamate-4-hydroxylase
C4H
Hydroxylation
Cinnamate-3-hydroxylase
C3H
Hydroxylation
Ferulate-5-hydroxylase
F5H
Hydroxylation
Caffeic acid/5-hydroxyferulic acid
O
-methyl-transferase
COMT
Methylation
4-Coumarate:coenzymeA ligase
4CL
Ferulic acid/Sinapic acid
reduction step
Cinnamyl-CoA reductase
CCR
Reduction
Cinnamyl alcohol dehydrogenase
CAD
Reduction
Caffeoyl-CoA-
O
-methyl transferase
CCoOMT
Methylation
Coumaroyl-CoA-3-hydroxylase
CCo3H
Hydroxylation
