Environmental Engineering Reference
In-Depth Information
the formation of linkages among diverse cell wall polysaccharides, potentially augmenting the
cell wall network (Hrmova et al. 2007). However, occurrence of many of these interpolymer
linkages in the cell wall remains to be demonstrated. Feruloyl transferase and transglycosylase/
hydrolase are attractive targets for potentially altering the degree of cell wall cross-linking for
improved digestibility. A candidate gene for feruloyl transferase from rice was proposed using
a bioinformatics approach (Mitchell et al. 2007). However, the protein encoded by this gene is
devoid of a transmembrane domain. Golgi transferases identified thus far are type II membrane
proteins with a transmembrane domain near the N-terminus (Edwards et al. 1999; Perrin et al.
1999; Faik et al. 2002).
The effect of lignin on increasing mechanical strength of cellulose is indirect and is achieved
by keeping the latter relatively dry. Because of the plasticizing effect of water, moist cellulose
is mechanically weaker than dry cellulose (Tolstogurov 2000). In agreement, the amount of
cellulose in a unit length of maize stalk nearly completely explained the mechanical strength of the
internodes below the ear (Appenzeller et al. 2004). In an independent study involving a mutation
in the maize
bk2
gene that specifically and dramatically reduced cellulose accumulation in the
secondary walls, lignin and hemicellulose amounts were unaltered but mechanical strength was
dramatically reduced, further indicating that cellulose is the primary determinant of mechanical
strength and that any contribution of lignin is through alteration of the aqueous environment
around cellulose in the cell wall (Figure 16.10) (Ching et al. 2006). Selection for reduced lignin
and increased cellulose in stover can increase mechanical strength and ethanol yield (Appenzeller
et al. 2004; Ching et al. 2006).
16.6 BIoenerGetIc consIderatIons For BIomass InterconversIon
Ethanol production from renewable resources is a direct function of the available carbohydrate content.
The grain in maize is the richest source of carbohydrates in the plant (Earle et al. 1946). Protein and oil
content in the grain can be reduced to further increase carbohydrate content, but because of the reduced
calorific content in these plants they will be suitable only for ethanol production and not animal feed.
Stover-only corn, after elimination of the ear, is another option to increase carbohydrate content
(Dhugga 2007). Although technically feasible, this avenue is not very attractive because (1) grain,
40
35
30
25
Wildtype
bk2 mutant
20
15
10
5
0
0
4
8
12
17
Displacement (mm)
FIGure 16.10
Difference in internodal mechanical strength between the brittle stalk (
bk2
) mutant and
its wild-type sibling. Internodal flexural strength was measured below the ear 1 week after flowering. The
seeds from the same selfed ear segregating for
bk2
were grown in the greenhouse. These data are for the fifth
internode below the ear node.
Brittle stalk 2
encodes a putative glycosylphosphatidylinositol-anchored protein
that affects the mechanical strength of maize tissues by altering the composition and structure of secondary
cell walls. (Figure from Ching A., et al.
Planta
, 224, 1174-1184, 2006. With kind permission from Springer
Science+Business Media B.V.)
