Soybean - Bioenergy Crop Plants - page 517

(NO x ) emissions (Tat et al. 2007). The high 18:1 property in seed oil is a desirable property because
of their oxidative stability. Also, engineering of FAD3 in soybean through the seed-specific over-
expression helped enhance the 18:3 content up to 50% when compared with the nontransgenic wild
type (Cahoon 2003).
Another strategy to improve the oxidative stability of oil content and enhanced fuel charac-
teristics is the increase of stearic acid component in the seeds. Seed-specific downregulation of
stearoyal-ACP desaturase in soybean has demonstrated the increased stearic acid content in the
seed storage lipids. The desaturation reaction of the enzyme stearoyal-ACP desaturase converts
stearic acid to oleic acid. So, the seed-specific downregulation of this protein through genetic engi-
neering will reduce the carbon flow by desaturation and increase the stearic acid content in seeds
(Kinney 1998; Kinney and Clemente 2005). It is reported that 20:1 is more oxidatively stable than
18:1 in plants (Isabell et al. 1999). To produce the more oxidatively stable oil component in soybean,
Cahoon et al. (2000) has partially reconstructed the biosynthetic pathway producing meadowfoam-
type seed oil in transgenic soybean. They have engineered cDNAs for the Limnanthes douglasii
acyl-CoA desaturase and fatty acid elongase 1 (FAE1) in somatic embryos, and the phenotypic
analysis of the transgenic embryos has shown that the 20:1 and 5-docosenoic acid composed up to
12% of the total fatty acids. Genetic engineering of oil biosynthetic pathways in soybean shows the
enhanced fuel properties of soybean oil. Also, other biochemical methods such as catalytic crack-
ing of soybean oil clearly demonstrate the acceptable fuel properties when compared with those of
petroleum-based fuel (Xu et al. 2010).
20.11 conclusIons and Future PersPectIves
Soybean is one of the major legumes rich in protein and oil and is a potential crop for designing
for future bioenergy needs. The plant oil commonly called TAG is chemically the most similar to
fossil oil and has the greatest potential to be used as one of the major bioenergy resources. Soybean
oil has been broadly studied as a raw material for fatty acid methyl esters-based biodiesel produc-
tion by transesterification. Recent advances in the dissection of oil biosynthesis pathways in model
species Arabidopsis and the investigation of oil crops such as canola, sunflower, etc. have increased
the possibility of deep understanding of oil biosynthetic machinery in soybeans. Also, the avail-
ability of genomics technologies and the recent release of soybean genome sequence information
offer a unique opportunity to investigate the regulatory networks and signaling pathways associated
with oil biosynthesis and modification. Forward and reverse genetic approaches are being used to
alter the seed oil content and composition in soybean. Genetic mapping of oil traits in soybean has
increased the possibility of identifying the genomic regions associated with fatty acid biosynthesis
and has also helped to develop markers (including the SNP markers) for future molecular breeding
applications. Also, the classical plant breeding approaches (including mutational breeding) have
been successful in generating soybean varieties with improved oil content and composition. On the
other hand, metabolic engineering approaches (including the redesigning of the specific steps in
the pathway) are also contributing toward enhanced seed oil content and composition in soybean.
A major challenge in redesigning oil crops, especially soybean for enhanced and modified oil
content and composition, is the limited knowledge of transcriptional regulatory networks associated
with lipid metabolism pathways. Also, the understanding of the tissue-specific or organelle-specific
signaling pathways are in the early stages. Utilization of next-generation sequencing technologies
coupled with the targeted metabolomic analysis focusing on subcellular compartments will help to
advance the construction of these networks and also help to understand the genotypic variation and
develop more SNP markers for molecular-assisted breeding purposes.
Another major challenge is the stability of soybean oil content and composition under various
environmental conditions. Designing enzymes and specific catalytic reactions controlling the flux
of fatty acids between phospholipid and acyl-CoA pools will help improve the production of vari-
ous fatty acids and storage of total oil content in plants. Several molecular modeling tools can be