Environmental Engineering Reference
In-Depth Information
Irregular xylem mutants
irx1
,
irx3
, and
irx5
, corresponding to genes
AtCesA8
,
AtCesA7
, and
AtCesA4
, respectively, showed collapsed xylem phenotypes and approximately 30% cellulose reduc-
tion in secondary walls (Taylor et al. 1999, 2000, 2003). In these mutants, primary walls remained
unaffected. All of these single gene mutations resulted in severe phenotypes with defects in the
secondary cell wall structures. This indicates that these are nonredundant genes with independent
functions for secondary wall synthesis.
Korrigan is a β-1,4-glucanase required for cellulose synthesis in primary and secondary cell
walls. It is predicted to be a membrane-bound protein, but the exact localization of the protein is
unknown. Several mutants show dwarfism, swelling of root tips, and collapse of xylem vessels
(Lane et al. 2001; Sato et al. 2001; Szyjanowicz et al. 2004). In summary, studies to date suggest
that Korrigan has a role in the processing of growing microfibrils or the release of cellulose syn-
thase complexes (Szyjanowicz et al. 2004). Null mutants of Cobra, a glycosyl-phosphatidylino-
sitol (GPI)-anchored protein, showed disorganization in the orientation of cellulose microfibrils
and reduction in the crystalline cellulose, which makes Cobra a possible regulator of cellulose
biosynthesis during cell expansion (Roudier et al. 2005). Kobito mutants (
kob1
) are dwarfed and
deficient in cellulose during the elongation phase of the cell. Kobito is a plasma membrane-bound
protein, and although overexpression did not result in any phenotype, mutants showed altered
microfibril orientation. Pagant et al. (2002) stated that KOB1 may be part of the cellulose syn-
thesis machinery in elongating cells and/or may play a role in the coordination between cellulose
synthesis and cell expansion.
In summary, cellulose is considered to be a significant source of biomass raw material for bio-
fuel production because it is rich in carbon. Because cellulose synthesis is highly conserved among
plants, using the knowledge obtained from
Arabidopsis
research will likely be applicable to other
plant systems. Better understanding of the regulation of cellulose synthesis in
Arabidopsis
will help
us recognize one of the key steps in cell wall biosynthesis, which will eventually allow scientists to
modify the cell walls for sustainable biofuel production. The knowledge from
Arabidopsis
cellulose
biosynthesis research will also help us to focus on nonfood crops instead of food crops (e.g., corn)
as target raw materials for bioenergy because many nonfood biomass energy crops can be produced
with lower energy input than food crops.
5.2.5 p
Ectin
5.2.5.1 Pectin structure
Pectin is a complex group of polysaccharides that contain covalently linked galacturonic acid
residues. Homogalacturonan (HG) is the most abundant pectic polysaccharide and is a homo-
polymer of α-1,4-linked galacturonic acid (GalA). HG comprises approximately 65% of pectin
in the primary walls of dicots and nongraminaceous monocots (Mohnen 2008). HG can be
acetylated on C
2
or C
3
of the GalA residues; however, the degree of acetylation varies from
species to species.
RG-I, another component of pectin, consists of a backbone of alternating GalA and Rha (-α-1,4 -
GalA-α-1,2-Rha). It represents approximately 20-35% of pectin, and l-rhamnose residues have
side chains, which are linear or branched and largely composed of β-d-galactose and α-l-arabinose
residues (Mohnen 2008). The galactan branches are mostly linear chains of β-1,4 -linked d-galac-
tose residues, whereas arabinan chains include α-1,5 linked l-arabinan residues that are frequently
branched with O-2 and O-3 (Nakamura et al. 2002). Nakamura et al. (2002) showed for the first time
that pectic polysaccharides RG-I and HG are linked via their backbones. RG-I is usually decorated
with arabinogalactan side chains that are thought to be of two basic types. Arabinogalactan I side
chains are β-1,4 -linked d-galactans chains with β-3-linked l-arabinose or arabinan branches, and
highly branched arabinogalactan II side chains are β-1,3-linked d-galactan chains with β-6-linked
galactan or arabinogalactan branches (Mohnen 2002). In some species, ferulic acid esters substi-
tute the position of arabinose and galactose residues in RG-I side chains (Fry 1982). Chemical and
