Maize - Bioenergy Crop Plants - page 425

tolerable. Once the molecular components for acetylation of GAX are identified, they could
similarly be manipulated to potentially reduce the acetyl content of the dry matter for improved
fermentation.
Lignin has been down regulated by the introgression of spontaneous bmr mutations into elite
germplasm and by transgenically silencing a number of genes from the monolignol biosynthetic
pathway (Anterola and Lewis 2002; Burk and Ye 2002; Zhong et al. 2004, 2005). Reduction in lignin
is generally accompanied by a concomitant reduction in biomass (Pedersen et al. 2005). In addition,
plants with lowered lignin are generally more susceptible to insects and diseases that likely result
from a weakening of the physical barrier that lignin poses in accessing the wall polysaccharides
that the pathogens need for growth (Pedersen et al. 2005). Variable expression of the bmr mutations
in different genetic backgrounds suggests that it may be possible to reduce lignin to some extent
without a remarkable effect on total biomass (Grabber et al. 2004; Pedersen et al. 2005; Pichon et
al. 2006). For further details, see Section 16.7.
Lignin-rich residue remaining after biomass hydrolysis in the current cellulosic ethanol protocols
is projected to be burnt to generate electricity and heat (Wright et al. 2006). Various technologies
to generate high-value products from this residue are being explored, which, if commercially
successful, might help boost the value of stover (Hahn-Hagerdal et al. 2006).
An increase in cellulose concentration in lignin-downregulated aspen was reported to be
accompanied by increased growth rate (Hu et al. 1999). This conclusion has been questioned for
various reasons, the main ones being (1) a lack of relationship between the degree of lignin reduction
and growth rates of various transgenic events and (2) non-normalization of the proportions of
different wall constituents to compensate for reduced lignin, a major constituent itself (Anterola
and Lewis 2002).
The residue from the grain remaining after ethanol distillation is referred to as distiller's dried
grains with solubles (DDGS), which is rich in oil, protein, and fiber (Belyea et al. 2004). Currently,
DDGS is sold mainly as cattle feed. Because of the high amount of the pentose-rich polysaccharide
fraction, which adversely affects digestibility in monogastric animals, it is blended only in small
amounts with poultry and swine feed (Fastinger et al. 2006; Amezcua and Parsons 2007). Lowering
the amount of GAX in the grain and replacing it with cellulose will expand the uses of DDGS and
increase its suitability as a cellulosic feedstock for ethanol production.
Hydrolytic enzymes have been successfully expressed in plants, apparently with the objective
of reducing the cost of their production (Dai et al. 2005). The idea of these enzymes eliminating
or reducing the addition of exogenous enzyme cocktails is unlikely to be valid under the currently
used protocols to process biomass for ethanol production because these enzymes are unlikely to
withstand the harshness of the pretreatment steps. Whether production of these enzymes in plants
proves to be more economical than the microbially produced enzymes remains to be seen.
Exogenous addition of an expansin protein at a level of 10 mg/g (or, in practical terms, 10 kg/Mg)
of isolated maize cell walls caused significantly greater swelling than the control walls (Yennawar
et al. 2006). Apparently, this occurs because of facilitated access of the wall polysaccharides to
water. Whether increased swelling improves digestibility of the cell walls by the hydrolytic enzymes
is not yet known. Also, the cell walls were prepared from the silk tissue that hardly has any lignin. It
would be interesting to see if the expansin proteins can be accumulated in plant to such high levels
as used in this study without affecting plant biomass. Secondary walls may be the preferred sites
for the in planta expression of these proteins because their deposition begins as the cell expansion
begins to cease.
As is obvious from the aforementioned discussion, genetic engineering of polysaccharide
biosynthetic pathways for cell wall formation is still in its infancy. Complete understanding of these
pathways will greatly expand the tool kit needed for biotechnological manipulation of the cell wall.
Enough critical information is already available for exploring the use of a combination of genetic
and transgenic methods to alter stover and grain composition for improved stover digestibility and
increased ethanol yield.