Soybean - Bioenergy Crop Plants - page 513

population of N98-4445A × M23 showed oleic acid levels can be increased to more than 70% (Alt
et al. 2005b). Although N98-4445A, with more than 50% oleic acid, has been developed, yield
potential was less than a cultivar with seed oil containing a normal fatty acid profile (Burton et al.
2006). Like lines high or low in saturates, yield drag may be a significant factor in the development
of productive nongenetically modified lines with high oleic acid content.
Low heritability and limited variation have contributed to the slow development of high 18:1
cultivars by conventional breeding (Liu 1999). However, transgenic approaches to increase oleic
acid content in soybean oil show great promise. Genes for ω6 and ω3 desaturases have been cloned
in soybean (Liu 1999). This enabled scientists at E. I. du Pont de Nemours and Company (DuPont)
to suppress a ω6 desaturase gene, which resulted in a transgenic soybean with approximately 80%
oleic acid content. Genotypes with this transgene have shown excellent stability for 18:1 content
across a range of growing conditions. Knowlton et al. (1996) reported on a transgenic soybean with
very high levels of oleic (85.6%), low levels of linoleic (1.6%), and linolenic (2.2%) acids. This trans-
gene has shown no negative effect on grain yield, and levels of 18:1 were very stable across growing
environments. On the other hand, high oleic lines derived from nongenetically modified organism
sources such as N98-4445A and M23 have yielded less compared with commercial cultivars of sim-
ilar maturity, and 18:1 levels are influenced by growing environments (Oliva et al. 2006). Recently
there has been major progress in elevating oleic acid in soybean oil by conventional breeding with
marker-associated selection. Pham et al. (2010) found that when exiting Fad2-1A mutants were com-
bined with the novel mutant Fad2-1B alleles, high oleic acid (80%) was produced in soybean seed.
QTL for oleic acid content have been mapped to several positions in the soybean linkage group.
Diers and Shoemaker (1992) detected three QTL associated with variation for 18:1 levels. These
QTL were close to RFLP markers pA-82 ( R 2 = 28%) on Chro. 5 (LG A1) and pA-619 ( R 2 = 19%)
on Chro. 14 (LG B2) and were linked to the pb locus that determines the sharpness of pubescence
( R 2 = 21%) on Chro. 15 (LG E) (Pantalone et al. 2004). A large QTL ( R 2 = 35%) was mapped on
the position of 82.5 cM on Chro. 19 (LG L) (Hyten et al. 2004), and a single molecular marker
related to oleic acid content, Satt263, was mapped on Chro. 15 (LG E) ( R 2 = 10%) (Panthee et al.
2006). Bachlava et al. (2009) reported a QTL with a moderate effect on oleic acid concentration that
mapped to Chro. 20 (LG I) in close proximity to the Fad 2-1B locus.
Plant introductions with higher than average oleic acid concentration (30-50% vs. 24% from
most soybeans) are reported in the USDA soybean germplasm collection and are likely to have use-
ful genes for improving oleic acid content (Lee et al. 2009). However, the genetic basis for higher
oleic acid in these genotypes has not been reported.
20.7 lInoleIc acId
Linoleic acid (18:2) in soybean seeds is produced primarily by desaturation of oleic acid in phos-
phatidylcholine on the endoplasmic reticulum (Ohlrogge and Browse 1995). Linoleic acid is a pre-
dominant fatty acid that is typically approximately 53% in soybean oil (Wilson 2004). The genetic
control of linoleic acid has scarcely been studied, perhaps because there is no pressing need to
modify its levels in the oil. The accumulation of linoleic acid is the result of a balance of ω6 and ω3
fatty acid desaturase enzyme activity.
Molecular markers have been used to map the genes involved in linoleic acid production
(Table 20.2). Five RFLP markers—three on Chro. 5 (LG A1), one on Chro. 14 (LG B2), and one on
Chro. 15 (LG E)—and one morphological marker ( pb c ) were found in a mapping population (Diers
and Shoemaker 1992). Two large QTLs were mapped on the position of 93.7 cM ( R 2 = 10%) on
Chro. 13 (LG F) and on 74.5 cM ( R 2 = 51%) on Chro. 19 (LG L) (Hyten et al. 2004). A single molec-
ular marker, Satt185, was mapped on Chro. 15 (LG E) ( R 2 = 14%). Markers Satt185 and Satt263
(oleic acid marker) map within 0.6 cM of each other, suggesting that the QTL detected there may be
influencing an enzyme involved in the process of desaturating oleic acid to linoleic acid (Panthee
et al. 2006).