Environmental Engineering Reference
In-Depth Information
to degrade and loosen cell walls, the research on these cell-wall-associated proteins could be con-
sidered a potential area of interest.
The cell walls of higher plants contain a family of HRGPs called extensins (Showalter 1993). An
excellent review is available focusing on structural details and distribution of extensins (Showalter
1993). Extensins contain considerable amounts of hydroxyproline, serine, and various proportions of
amino acids like valine, tyrosine, lysine, and hystidine (Showalter 1993). The presence of repeating
pentapeptide motifs, like Ser-Hyp
4,
is a characteristic of HRGPs (Showalter 1993). ATEXT1 proteins
from
Arabidopsis
have plant extensin characteristics and are well characterized (Merkouropoulos
et al. 1999; Roberts and Shirsat 2006). However, not much work has been conducted in this area.
One additional report in this area of research is on the characterization of the second prolyl 4-hdrox-
ylase enzyme (At-P4H-2) from
Arabidopsis
(Tiainen et al. 2005).
Experimental results showing
enhanced growth rates in plant cells, when exposed to acidic solutions, led to the identification of
a class of proteins that were later called expansins (Rayle and Cleland 1992). Expansins were first
defined based on their pH dependency and their unique action on the rheological behavior of isolated
cell walls (Cosgrove 1999). Current understanding about these proteins is mostly based on their
amino acid sequences and not on their biological activities (Cosgrove 1999). The α- and β-expansin
families, which share approximately 25% amino acid similarity, are the only ones recognized so
far (Cosgrove 1999). Although these classes are identical in their effect on cell wall properties,
the exact mechanism of their action is not fully known (Cosgrove 1999). Detailed comparative
genome analysis studies of
Arabidopsis
identified 26 putative α-expansins and 5 β-expansin genes
(Lee et al. 2001). The gene structural regions shared by expansins contain three exon regions that
encode three potential functional domains of the expansin protein, such as a signal peptide and the
N-terminus of the mature protein, an endoglucanase-like core region, and a domain with structural
resemblance to microbial cellulose binding domains (CBDs) (Cosgrove 1999). Several other studies
using
Arabidopsis
as a model system additionally prove that expansins have significant roles in cell
wall growth and development (Cho and Cosgrove 2000; Cosgrove 2000; Choi et al. 2006). These
outcomes recommend considering expansins as target genes in biomass enhancement research.
5.2.6.4 arabinogalactan Proteins
Arabinogalactan proteins (AGPs) are extracellular-wall-associated hydroxyproline-rich proteogly-
cans. Showalter (2001) reviewed the important roles of AGPs in plant growth and development.
Some of the important physiological roles of AGPs include their functions in vegetative/repro-
ductive growth, xylem development, programmed cell death and signaling (Showalter 2001). The
protein backbone of these molecules is rich in proline/hydroxyproline, serine, alanine and threonine
(Sommer-Knudsen et al. 1998). The family members of most AGPs share less than 40% amino acid
sequence similarity (Schultz et al. 2002). Taking advantage of the completed
Arabidopsis
genome,
Schultz et al. (2002) identified several classes of AGP molecules that included 21 fasciclin-like
AGPs, 13 AGPs, 10 AG peptides, and 3 basic AGPs with short lysine-abundant regions. Recent stud-
ies have also shown that all classical genes of arabinogalactan backbone proteins, especially
AGP6
and
AGP11
, are expressed in pollen tubes of
Arabidopsis
plants (Pereira et al. 2006; Coimbra et al.
2009). Therefore, considering the above information and the vital function of AGPs in plant growth
and development, further research on AGPs in bioenergy crops are equally important to biomass
enhancement for biofuel production.
5.3 conclusIons
For a future sustainable and reliable biofuel industry, it is necessary to understand the ins and outs
of plant cell walls, which are the most abundant component of the natural biomass raw material.
The most challenging property of plant cell walls is the integrity of its structures, which are mainly
composed of cellulose microfibrils cross-liked with hemicelluloses that bind with lignin. This prop-
erty of cell walls, which inhibits its use in the biofuel industry, is now popularly termed “cell wall
