Environmental Engineering Reference
In-Depth Information
and show reduced cell adhesion, particularly between epidermal cells in seedlings and young
leaves, and show a approximately 25% reduction in GalA content in isolated cell walls, suggesting
a possible role for QUA1/GAUT8 in pectin biosynthesis. Independently, Henrik Scheller's group
further investigated QUA1 and found for the first time that protein extracts from
qua1
stems have
reduced HG:α-1,4-GalAT (~33%) and β-1,4-xylosyltransferase activities (~40%) relative to wild
type, suggesting a connection between pectin HG and xylan synthesis (Orfila et al. 2005). The
only putative HG-methyltransferase characterized so far is Quasimodo2 (Qua2) (At1g78240) in
Arabidopsis
, which encodes a Golgi-localized protein. The
qua2
mutants showed a reduced cell
adhesion, a 50% reduction in HG levels without affecting other cell wall polysaccharides, and are
dwarf phenotype-like common pectin mutants. The QUA2 protein contains a putative methyltrans-
ferase domain and may function as a HG-methyltransferase (Mouille et al. 2007). However, enzy-
matic evidence of function is still needed to confirm the role of QUA2 in HG biosynthesis.
5.2.5.4 Genes Involved in rG-I Biosynthesis
Henerik Scheller's group first identified a mutant called
arad1
(At2g35100;
ARABINAN
DEFICIENT 1
) through screening of a T-DNA insertion population of
Arabidopsis
(Harholt et
al. 2006). The
arad1
mutant showed a 25% reduction in Ara in leaves and 54% less Ara in stems
compared with its wild-type counterpart, which suggests that
arad1
affects the incorporation of
Ara into plant cell wall glycans. RG-I isolated from the
arad1
mutant has a 68% reduction in Ara
content compared with that isolated from wild-type plants, suggesting a role for ARAD1 in RG-1
biosynthesis. However, the enzymatic function of ARAD1 has yet to be proven experimentally.
5.2.5.5 Genes Involved in xGa Biosynthesis
Recently an insertion mutation in the At5g33290 locus of the
Arabidopsis
genome was identified
and named
xylogalacturonan deficient 1
(
xgd1
) (Jensen et al. 2008). In the
xgd1
mutant, XGA was
almost completely absent except at the root tip and septa of siliques. Transient expression of
XGD1
in
Nicotiana benthamiana
yielded a protein that catalyzes the transfer of xylose from UDP-α-d-
xylose onto HG oligosaccharides, confirming that XGD1 is a xylosyltransferase. Expression of a
fluorescently tagged fusion protein in
N. benthamiana
confirmed that XGD1 is a Golgi-localized
type II membrane protein (Jensen et al. 2008). XGD1 belongs to CAZy family 47.
5.2.5.6 Genes Involved in rG-II Biosynthesis
The Geshi group (Egelund et al. 2006) identified two homologous plant-specific
A. thaliana
genes,
RGXT1
(At4g01770) and
RGXT2
(At4g01750), that belong to a new family of glycosyltransferases
(CAZy GT-family-77) and encode cell wall RG-II-α-d-1,3-xylosyltransferases (RG-II-α-1, 3XylTs).
Heterologously expressed RGXT1 and RGXT2 proteins catalyze the transfer of d-xylose from
UDP-d-xylose to l-fucose with an α-glycosidic linkage (Egelund et al. 2006). The product of the
reaction was confirmed by biochemical analysis using specific xylosidases and NMR spectroscopy.
On the basis of these results, the authors hypothesized that RGXT1 and RGXT2 function in the
synthesis of RG-II side chain A, which contains 2-
O
-methyl-d-Xyl attached in a α-1,3 linkage to
α-l fuc. RG-II isolated from the
rgxt1
and
rgxt2
mutants serves as an acceptor for the enzyme, thus
providing strong evidence that RGXT1 and RGXT2 function in RG-II synthesis. To our knowledge
this is the first identification of enzymatically proven RG-II biosynthetic genes.
5.2.6 c
roSS
-l
inking
g
lycanS
(h
EmicElluloSES
)
Hemicelluloses are branched cell wall polysaccharides with a backbone that is composed of neutral
sugars. As an integral part of complex plant cell wall make up, hemicelluloses cross-link cellulose
microfibrils by forming hydrogen bonds to their surface. Approximately, one-third of the lignocel-
lulosic biomass is estimated to be composed of various hemicelluloses (Ragauskas et al. 2006).
Both hemicelluloses and cellulose microfibrils do share a structural resemblance that in turn creates
