Soybean - Bioenergy Crop Plants - page 516

different sources may be less influenced by growing conditions. A transgene for high 18:1 was
reported to be very stable across growing environments (Knowlton et al. 1996), but to date it has
not been approved for use in commercially grown soybeans. Unless nontransgenic sources for 18:1
concentration can be found that are less influenced by temperature, special consideration will have
to be given to production in specific regions using cultural practices and earlier maturity groups
to produce desired levels of oleic and linolenic acid in soybean oil (Shannon and Sleper 2004). To
ensure the desired 18:1 and 18:3 levels, cultivars will need to be produced in warmer regions such as
the southern United States or during the warmest periods of the summer in cooler areas. This may
require adding high 18:1 and low 18:3 traits to earlier maturing cultivars within a region. Planting
early cultivars at early planting dates would increase the chance that temperatures are warmest
during reproductive growth, giving the highest probability of producing oil with high 18:1 and low
18:3. On the contrary, high oleic acid is negatively correlated with the polyunsaturated fatty acids
18:2 and 18:3. Thus, if high polyunsaturated fats are desired, cooler temperatures, later planting, and
later maturities would appear to favor this phenotype.
20.10 GenetIc enGIneerInG For BIoenerGy traIts In soyBean
Progress in the basic understanding of oil biosynthetic pathways and their regulation in plants,
coupled with the splicing of genes and the regulatory factors associated with the enzymes of fatty
acid modification and oil accumulation, have contributed toward the metabolic engineering of oil
seed crops with the agenda of “redesigning oil seeds” with improved total oil accumulation or fatty
acid components. Genetic engineering strategies for increased or modified oil content or composition
in crop plants have provided alternative routes to the classical plant breeding approaches to attain
the above goals. A combination of genetic engineering and classical plant breeding methods helped
to re-draw the progress in oil modification or accumulation in crop plants.
Oil storage in seeds constitutes reduced carbon with 1-60% variation between different spe-
cies. The fatty acid biosynthetic pathway is a primary metabolic pathway, and in plants major fatty
acids have a chain length of 16-18 carbons with 1-3 cis -double bonds (Ohlrogge and Browse 1995).
Biochemical pathways involved in the oil biosynthesis and the major genes associated with these
pathways are known in model plants including spp. However, the molecular and biochemical regu-
latory networks and signaling between sites of biosynthesis such as endoplasmic reticulum and
plastids are not completely elucidated. Above all, the environmental effects on these regulatory and
signaling pathways are not fully understood.
Although there is significant knowledge on fatty acids biosynthesis, considerably less progress
has been made on the improvement of soybeans with increased oil content through genetic engi-
neering. Conjugated double bonds in fatty acids increase their rate of oxidation when compared
with polyunsaturated fatty acids with methylene-interrupted double bonds. Cahoon et al. (1999) has
reported the production of fatty acid components of high-value drying oils in transgenic soybean
embryos. In this study, they have engineered full-length cDNAs for the Momordica (MomoFadX)
and Impatiens (ImpFadX) enzymes in somatic soybean embryos, and, as a result, α-eleostearic
and α-parinaric acids were accumulated in the transgenic somatic embryos, whereas none of these
enzymes were present in the nontransgenic soybean embryos. High oxidative reactivity of soybean
oil decreases the shelf life of soybean oil-derived biodiesel (Kinney and Clemente 2005). The oxi-
dative reaction of the fuel will result in the formation of a gel-like substance and solid crystals that
will clog the fuel filters and fuel lines (Durett et al. 2008). Seed-specific downregulation of FAD2-1
through genetic engineering has resulted in approximately 80% of 18:1 in seeds (Kinney 1997;
Kinney and Clemente 2005), which improved further up to 85% by downregulating the FATB gene
in FAD2-1-downregulated transgenic soybean (Buhr et al. 2002). Another important phenotypic
observation is the reduced palmitic acid content in the transgenic soybeans with FAD2-1 down-
regulation. These high oleic transgenic soybeans were used for biodiesel production, and they have
shown improved fuel characteristics such as cold temperature flow properties and oxides of nitrogen