Environmental Engineering Reference
In-Depth Information
cellulose microfibrils. One of the major challenges faced by researchers in the biofuel industry
is removing these cross-linking xylans, thereby exposing the cellulose fibers to bioconverting
enzymes. On the other hand, xylans themselves can be considered a key raw material for bioeth-
anol production through d-xylose fermentation (Lachke 2002). During the past decade, several
researchers, using
A. thaliana
as a model, have gained new insights into the biosynthesis and
chemistry of xylans.
Recent work on several
Arabidopsis
genes has revealed in more detail the mechanisms of xylan
biosynthesis. Two important genes,
IRX7/FRA8
(At2g28110) and
GAUT12/IRX8
(At5g54690),
were discovered that are involved in xylan biosynthesis in
A. thaliana
plants (Zhong et al. 2005;
Peña et al. 2007; Persson et al. 2007a). The above studies also reported that
fragile fiber8
(
fra8
or
irx7
) and
irx8
mutants of
Arabidopsis
plants show defects in their xylan structures. Xylan poly-
saccharides isolated from these mutants lack the complex oligosaccharide sequence normally
found at the reducing ends, suggesting that IRX7 and IRX8 proteins might have important roles
in synthesizing this reducing end oligosaccharide of xylan (York and O'Neill 2008). Also, reduced
levels of glucuronoxylan and HG were also noted in these two mutants (Persson et al. 2007a).
Specifically, the
fra8
mutation causes a notable reduction in fiber wall thickness and decreased
stem strength. However,
irx8
mutants were dwarf mutants with impaired secondary cell wall integ-
rity. The Golgi-complex-localized FRA8 (IRX7) and IRX8 share identical expression patterns in
Arabidopsis
tissues (Peña et al. 2007). In brief, all of the above studies show that
IRX7
and
IRX8
genes can be used for identifying potential candidate genes in biofuel crops to develop xylan-defi-
cient plants with less recalcitrance.
Simultaneous research efforts (Brown et al. 2007; Peña et al. 2007) discovered other impor-
tant genes,
IRX9
(At2g37090) and
IR X14
(At4g36890), that are required for xylan synthesis in
Arabidopsis
. Research using respective
Arabidopsis
mutants clearly demonstrates that IRX9 and
IRX14 proteins, although not functionally redundant, are involved in the xylan backbone elongation
process (York and O'Neill 2008). Recent studies reported yet another couple of
Arabidopsis
genes
that are critical in xylan biosynthesis,
IRX10
and
IRX10-like
(Brown et al. 2009; Wu et al. 2009).
Like IRX9 and IRX14, IRX10 and IRX10-like proteins also contribute to xylan backbone elonga-
tion. However, in contrast to IRX9 and IRX14, a functional redundancy exists between IRX10 and
IRX10-like proteins (Brown et al. 2009; Wu et al. 2009).
Another important gene that contributes to xylan biosynthesis is the
PARVUS/GLZ1
gene, which
encodes a putative family 8 glycosyl transferase (Lao et al. 2003; Shao et al. 2004). Its gene expres-
sion is clearly demonstrated to be linked with secondary cell wall synthesis, and
parvus
mutants
exhibited thinner cell walls (Lee et al. 2007). Further studies showed that PARVUS/GLZ1, also
known as GATL1, has putative galacturonosyltransferase activity and is again speculated to be
involved in the formation of the reducing end primer sequence of xylan similar to IRX7 and IRX8
(Lee et al. 2007; York and O'Neill 2008). As mentioned earlier, studies by Brown et al. (2007)
described the identification of a xylan-deficient mutant
irx14
. This study (Brown et al 2007) revealed
more details on the intricate mechanism of xylan biosynthesis. Mutants of five xylan biosynthesis-
associated genes (
irx7
,
irx8
,
irx9
,
parvus
, and
irx14
) were similar in that they showed a significantly
reduced amount of xylan and drastically diminished GlcUA to Me-GlcUA side chains ratio (Brown
et al. 2007). Only
irx7
,
irx8
, and
parvus
showed the absence of complex xylan oligosaccharide,
unlike
irx14
plants, which did possess this structure (Brown et al. 2007). Taken together, these
studies give insight into the specific physiological contributions of distinct biosynthetic genes in
Arabidopsis
xylan biosynthesis. Ongoing works such as those mentioned above will be instrumental
to altering cell wall xylan networks to create more suitable biofuel crops in the future.
In addition to the above-mentioned genes,
ATCSLD5
, a member of the large family of cellulose
synthase-like genes, has been recently characterized (Bernal et al. 2007). The knockout
atcsld5
mutants show reduced xylan content, which causes a negative growth effect (Bernal et al. 2007).
Although these results hint that ATCSLD5 might be playing a role in xylan synthesis, the detailed
physiological functions of this protein remain unknown.
