Environmental Engineering Reference
In-Depth Information
Hotta et al. 2010). As of 2005, sugarcane was genetically transformed with three genes conferring
resistance to herbicides, six genes offering resistance to diseases, and five genes each for impart-
ing resistance to pests and modifying the metabolomics of sugarcane. However, only a few of
the transgenic sugarcane were field tested (Lakshmanan et al. 2005). In South Africa, herbicide-
resistant sugarcanes (glufosinate ammonium and glyphosate resistant) have reached field trial stage.
Transgenic sugarcane resistant to Sugarcane Mosaic Potty virus with heterologous expression in
antisense and untranslatable form has reached field trial stage, whereas insect-resistant sugarcane
[Cry1A (c)] heterologous expression is still in the pot bioassay stage. Several transgenic crops with
modifications to sucrose metabolism have completed glasshouse trials or are being studied in the
field trial stage (Watt et al. 2010). Recent trends in the genetic engineering of sugarcane are directed
at the modification of sucrose metabolism to enhance sucrose production and accumulation, as well
as recovery of high-value products from sugarcane (McQualter et al. 2004; Petrasovits et al. 2007;
Wu and Birch 2007, 2010). Also, the range of transgenic sugarcane, especially those with modifica-
tion of sugar metabolism, has increased in recent years to enhance the sucrose content of sugarcane
(Ma et al. 2000; Wu and Birch 2007).
3.4.3 S
wEEt
S
orghum
Sweet sorghum is the fifth most important cereal crop worldwide. Although it can grow in harsh
environmental areas, sweet sorghum is mainly found in hot/dry tropical and subtropical areas.
Several factors make sorghum a good choice for the biofuel industry, including “yield potential
and composition, water-use efficiency and drought tolerance, established production systems, and
potential for genetic improvement” (Rooney et al. 2007). Several traits have been found to be con-
nected with drought tolerance, which have been enhanced by breeders to make sorghum variants
highly drought tolerant. These traits include “heat tolerance, osmotic adjustment, transpiration
efficiency, rooting depth, epicuticular wax, and stay green” (Rooney et al. 2007). Grain sorghum
provides starch and sweet sorghum produces sugar, and both types provide cellulose for biofuel
conversion. The average yields from grain sorghum are only slightly lower than those of corn, if
not the same as corn grain, with the potential of genetic improvement to increase yield. Hybrids
are being cultivated for regions where sugarcane production is limited. Biomass sorghum has
the potential to become a dedicated energy crop on the basis of high yield capability and broad
growth range.
One characteristic feature of sorghum useful in improving its yield is its height. There are four
genes known to influence this characteristic, the
Dwarfing
(
dw
)1-4 genes. These genes impart par-
tial dominance of the tallness trait to plants, the effects of which are additive in nature (meaning a
plant with
dw
1, 2, and 3 would be taller than a plant with
dw
1 only). Adapting sorghum to long days,
as found in temperate regions, led to the discovery of
Maturity
(
Ma
) genes. Specifically
Ma
1
gene
has
been shown to be involved in controlling the rate of maturity, making the plant carrying this
gene in its recessive form react to long days as it normally would to short days (photoperiod insensi-
tive). Increasing yield is highly dependent on changing the source/sink balance. Drought-resistant
sorghum keeps a higher photosynthetic rate during conditions of low water. One gene locus (
Alt
sb
)
is known to provide aluminum tolerance. Aluminum is found in acidic soils. These soils make up
about half of the possible arable land available. However, iron is the main problem for alkaline soils,
and some iron-tolerant lines have been found in sorghum (Saballos 2008).
Sweet sorghum is highly recalcitrant to in vitro manipulations, such as tissue culture and trans-
formation. Reports on tissue culture of sweet sorghum are limited to MacKinnon et al. (1986), Rao
et al. (1995), and Raghuwanshi and Birch (2010). There is only one published report on transforma-
tion of sweet sorghum. Raghuwanshi and Birch (2010) reported transformation of sweet sorghum
variety Ramada using microprojectile bombardment. They reported development of a transforma-
tion system for sweet sorghum and demonstrated production of transgenic sweet sorghum resistant
to the antibiotic hygromycin. Luciferase was used as the reporter gene in this case. Although the
