Environmental Engineering Reference
In-Depth Information
immunological studies have shown that there is a large variation in RG-I structures found in differ-
ent groups of plants and even in different tissues of a single plant.
The most complex pectic-polysaccharide is RG-II. Its structure is highly conserved in all vascu-
lar plants and it comprises approximately 10% of total pectin (O'Neill et al. 2004). RG-II consists of
an HG backbone with side branches consisting of at least 12 different monosaccharides with more
than 20 different types of linkages. The monosaccharides found in RG-II include some that are
rarely found in other polysaccharides such as 2Me-Fuc, 2Me-Xyl, Dha, and Kdo (2-keto-3-deoxy-
d-manno-octulosonic acid), d-apiose, l-aceric acid (3-C-carboxy 5-deoxy-l-xylose) and d-Dha
(3-deoxy-d-lyxo-2-heptulosaric acid). RG-II in plant cell walls exists predominantly as dimers that
are cross-linked by borate diesters at a 1:2 ratio. Despite its relatively low abundance, RG-II appears
to be very important for cell wall structure and function, given that it is highly conserved through-
out the plant kingdom (O'Neill et al. 2004) and plants having mutations affecting RG-II structure
show growth abnormalities (O'Neill et al. 2001, 2004).
The other substituted galacturonan is xylogalacturonan (XGA). XGA is an HG substituted at O-3
with a β-1,3-linked xylose and this xylose is further substituted at O-4 with an additional β-1,3-
linked xylose in
A. thaliana
(Zandleven et al. 2006). XGA has also been detected in cell walls from
all
Arabidopsis
tissues with the most predominant expression in flowers (Zandleven et al. 2007).
Recently, Henrik Scheller and his group identified a mutant with much lower levels of XGA, called
xgd1
(xylogalacturonan deficient 1), indicating a role of At5g33290 in XGA biosynthesis (Jensen
et al. 2008).
5.2.5.2 localization, Function, and mechanism of Pectin Biosynthesis
There are several reports suggesting that pectin is biosynthesized in the Golgi vesicles (Staehelin
and Moore 1995; Willats et al. 2001) and is then transported to the wall via membrane vesicles.
This localization study is now supported by studies showing that RGXT1/2 (Egelund et al. 2006),
ARAD1 (arabinan deficient 1; Harholt et al. 2006), XGD1 (xylogalacturonan deficient 1, Jensen
et al. 2008) and galacturonosyltransferase 1 (GAUT1; Dunkley et al. 2004) are located in the Golgi
vesicles. All published data to date suggest that pectin is synthesized in the Golgi lumen by glyco-
syltransferases (GTs).
Pectin polysaccharide synthesis occurs in different Golgi cisternae as pectin moves from the
cis-, through the medial-, then to the trans-Golgi. Some antibody-based studies suggest that HG
and RG-I synthesis begins in the cis-Golgi and moves into the trans-Golgi through the medial
Golgi (Staehelin and Moore 1995). It is believed that HG is transported to the plasma membrane
and inserted into the wall as a highly methylesterified polymer, where it then goes through vari-
ous degrees of de-esterification by pectin methylesterases, which convert HG to a more negatively
charged form (Pelloux et al. 2007).
5.2.5.3 Genes Involved in hG Biosynthesis
Mohnen and coworkers have successfully proven enzymatically that a HG α-1,4-galacturo-
nosyltransferase (HG:α1,4GalAT) called
GAUT1
(galacturonosyltransferase 1) and found in
Arabidopsis
(Sterling et al. 2006) is involved in HG synthesis. They also showed that
GAUT1
belongs to the GT8 family in the CAZY classification system (www.cazy.org). In
Arabidopsis
, the
GAUT1
-related gene family is made up of 15
GAUT
genes with 56-100% sequence similarity to
GAUT1
and 10 galacturonosyltransferase-like (
GATL
) genes with 43-53% identity with
GAUT1
(Sterling et al. 2006). The exact role of GAUT1 in pectin synthesis is not clear and is currently
under investigation.
Several mutations in genes related to
GAUT1
that affect HG and/or xylan synthesis have been
identified in
Arabidopsis
. Although the proteins encoded by these genes are putative pectin bio-
synthetic glycosyltransferases on the basis of their sequence similarity to
GAUT1
, proof of their
enzyme activity is still needed to confirm their role in pectin synthesis. Bouton et al. (2002) isolated
two allelic mutants, named
quasimodo1
(
qua1-1
and
qua1-2
). Both of these mutants are dwarfed
