Environmental Engineering Reference
In-Depth Information
case sucrose metabolism-related genes were targeted. QTLs for sugar content (Brix and Pol) were
identified using composite interval mapping (CIM; Zeng 1994) and discriminant analysis.
Sequence information from the SUCEST database has been used to create a complementary
DNA (cDNA) microarray for sugarcane (Rocha et al. 2007). This technology allows for the direct
identification of the genes underlying important traits. Genes associated with hormone response
and environmental stress were identified using the microarray (Rocha et al. 2007). Sucrose
metabolism-associated transcripts were also identified in a later study using the same microarray
(Papini-Terzi et al. 2009). Molecular markers for use in breeding could be developed using this
genomic information.
1.2.2 E
thanol
from
g
rain
In the United States, ethanol is produced primarily from maize (
Zea mays
L.) grain, although
some is also produced from sorghum [
Sorghum bicolor
(L.) Moench] and other grains. Large-scale
production of grain ethanol for biofuel has increased dramatically in the last 2 decades, mostly
because of increased processing capacity as more ethanol plants were constructed in the Corn Belt.
High-yielding maize varieties have allowed grain producers to meet the demands of the ethanol
industry. The United States is the top maize producer in the world (FAO 2010), and much of that
is due to a long and successful history of maize breeding. The greatest leap in maize production
came with the introduction of hybrids in the 1930s. The first hybrids were double crosses, produced
by crossing two F
1
hybrids, resulting in a mixed population of mostly heterozygous individuals.
So-called three-way hybrids, produced by crossing an F
1
female parent to an elite inbred male
parent, gave slightly better uniformity and performance. Continued improvement of inbred lines
eventually allowed for the production of single cross hybrid (F
1
) seed on a larger scale. Single cross
hybrids offered better vigor and uniformity than double cross or three-way hybrids. The discov-
ery of cytoplasmic-genetic male sterility in sorghum also allowed for production of high-yielding
grain sorghum hybrids.
In addition to yield, grain composition and starch quality are important factors to consider in
selecting grain for ethanol production. For example, in sorghum, protein digestibility was found
to affect starch conversion because the proteins are believed to protect the starch granules from
amylase enzymes (Wu et al. 2007). Starch is a polymer of glucose molecules linked by α-glycosidic
bonds that consists of two forms: the mostly unbranched amylose and the highly branched amylo-
pectin. Wu et al. (2006) found that higher percentages of amylose in maize starch mixtures and in
other grains resulted in decreased conversion efficiency. A significant amount of breeding has been
conducted by the major seed companies (Pioneer and Monsanto) to produce maize hybrids with
grain qualities specifically designed for the fuel ethanol industry (Bothast and Schlicher 2005).
1.2.2.1 sweet sorghum
Some research has been done to investigate the use of sweet sorghum as a source of fermentable
sugars in places where sugarcane cannot be grown or to supplement the sugarcane crop in warm
temperate areas. Like sugarcane, sweet sorghums accumulate free sugars in their stems, which
can be easily fermented into ethanol. In the United States, most sweet sorghum cultivars in use
today are pure lines, developed by the U.S. Department of Agriculture (USDA) in Meridian, MS
(Murray et al. 2009). This is in contrast to grain sorghums, which are almost exclusively hybrids. In
general, sweet sorghums are very tall plants with limited seed production. Some research has been
conducted to investigate the potential of producing hybrid sweet sorghum seed using short-statured,
easily harvested seed parents (Corn and Rooney 2008; Makanda et al. 2009). Many genetic markers
have been developed for sorghum, including RFLP markers, some of which are derived from other
grasses (Bowers et al. 2003), several collections of SSR markers (Kong et al. 2000; Li et al. 2009;
Yonemaru et al. 2009), and recently chip-based Diversity Array Technology (DArT) markers (Mace
et al. 2008). Many linkage maps of sorghum have been published as well (Bhattramakki et al. 2000;
