Environmental Engineering Reference
In-Depth Information
vegetation after grain harvesting, for cellulosic ethanol. Traditionally, corn stover is left remaining
on the fields to help reduce soil erosion and replenish nutrients. It is estimated that in 2030, 256
million dry tons per year of corn stover (de Leon and Coors 2008) will become available in the
United States. The use of corn stover depends on its yield potential and carbohydrate composition
of the cell wall. Carbohydrate composition of corn stover is 37% cellulose, 28% hemicellulose, and
18% lignin. Corn will be very important in the immediate production of cellulosic ethanol (de Leon
and Coors 2008).
Cell wall composition in corn stover is being manipulated using various traits. The
brown midrib
mutations (
bm1, bm2, bm3, bm4
) are naturally occurring mutations found in corn that are known to
alter the “lignin concentration and/or composition of the plant” (de Leon and Coors 2008). Maize
bm1
is found to affect the expression of
CAD
, and
bm2
plants have lower lignin contents and significantly
diminished levels of Ferulic acid (FA) ether (Barriere et al. 2004). The
bm3
allele is the most efficient
at enhancing cell wall digestibility. Expressing the gene trait of
Lf y1
allows corn hybrids to gener-
ate more forage yields than other normal corn hybrids. These plants tend to have more nodes and
leaves on their main stalk. It is also thought that, through altered lateral branch formation, corn could
have increased biomass. The
grassy tiller 1 (gt1)
and
teosinte branched1 (tb1)
genes are connected
with activation of “lateral meristems and reduced apical dominance” (de Leon and Coors 2008).
Expansins (proteins) play a part in relaxing cell walls for growth and expansion. Although these
proteins have been found in corn stover, more research needs to be done to determine their ability to
reduce pretreatment cost through modifying cell walls (Sticklen 2008).
Enabling plants to survive in adverse environments would allow for the use of a broader range
of land, resulting in more biofuel production. Resistance to biotic and abiotic stresses is a key part
to making it possible. It has been suggested that reactive oxygen species (ROS) “act as interme-
diate signaling molecules to regulate gene expression … a central component of plant adaption”
(Schroder et al. 2008). ROS are very toxic in that they react with several cell components, such as
lipids, proteins, and/or nucleic acids. Higher production of ROS is induced by stresses, but plants
have some control by expressing different mechanisms, mainly “enzymatic and non-enzymatic
reactions” (Schroder et al. 2008). Such enzymes/proteins include “SOD, APOD, CAT, GST, GPOD,
enzymes of ascorbate-glutathione pathway dehydrin, actin, histone” (Schroder et al. 2008).
3.4.2 S
ugarcanE
Sugarcane is a large perennial C
4
grass found in tropical and subtropical regions. Mostly table
sugar (~70%) is produced from sugarcane (Matsuoka et al. 2009). Because sugarcane is efficient
at converting solar energy into chemical energy, this crop is important in ethanol fuel. The three
types of energy canes include sugarcane primarily grown for sugar production; type I energy cane,
which is grown for sugar and fiber production; and type II energy cane, which primarily yields
lingo cellulosic fiber (Tew and Cobill 2008). Bagasse, the residue left over from sugar mills, can be
used to generate heat and electricity. The separation of juice from the fiber is done through milling
or diffusion.
Brazil has the most advanced sugarcane-based ethanol industry, and ethanol replaces
a significant proportion of transportation fuel in Brazil (Goldemberg et al. 2008; Balat and Balat
2009; Matsuoka et al. 2009).
Genetic engineering of sugarcane focuses on several agronomic traits (e.g., resistance to dis-
ease and pests, herbicide resistance, increased sucrose content) because it is difficult and time-
consuming to develop sugarcane exhibiting such qualities through conventional breeding. Recent
reviews (Hotta et al. 2010; Watt et al. 2010) have discussed various methods used for transformation
of sugarcane and applications of genetic engineering for improvement of sugarcane to make it a
better biomass crop. Biolistic transformation using microprojectiles dominates sugarcane trans-
formation studies, but there are reports of
Agrobacterium
-mediated and electroporation-based
transformation of sugarcane (Arencibia et al. 1995, 1999; Enriquez et al. 2000; Manickavasagam
et al. 2004; Lakshmanan et al. 2005; Jain et al. 2007; Molinari et al. 2007; Wu and Birch 2007;
