Soybean - Bioenergy Crop Plants - page 508

phosphatidylethanolamine, and phosphatidylinositol have structural functions in cell membranes
and may be metabolically involved in triacylglycerol synthesis. Each glycerolipid class is composed
of molecular species formed by various combinations of the five fatty acids; 16:0, 18:0, 18:1, 18:2,
and 18:3; or polar groups that are esterified at the sn -1, sn -2, and sn -3 stereospecific positions of the
glycerol molecule.
The increased demand for renewable fuels such as biodiesel has increased the emphasis on breed-
ing soybeans with higher oil content. Genetic sources are very important for improving soybean oil
content. There is a wide range (8-25% on a dry weight basis) in seed oil content among accessions
in the U.S. Department of Education (USDA) soybean ( Glycine max ) germplasm collection (USDA,
ARS 2007), with most soybean cultivars averaging approximately 20%. However, genes affecting
oil content and oil biosynthesis can be affected by the environmental conditions such as temperature
and rainfall. Also, oil and protein content in soybean seed are negatively correlated. Thus, it will be
difficult to develop soybeans with simultaneously very high oil content and a 40% protein content
(dry weight basis) through conventional breeding approaches.
Soybean breeders are interested in increasing soybean oil content using major oil loci for marker-
assisted selection (MAS). Soybeans with stable oil content across environments will be developed
by combining confirmed quantitative trait loci (QTL). Many genes have been reported to affect oil
concentration. Recent genetic maps show 68 QTL affecting soybean oil content have been found
on all chromosomes [Chros. (linkage groups (LGs)] except Chro. 3 (LG N). These QTL have been
reported in the Breeder's Toolbox in Soybase (http://www.soybase.org) and are shown in Table 20.2.
Diacylglycerol transferase (DGAT) is also a potential candidate gene for increasing oil content in
soybean. In developing seeds, DGAT catalyzes the acyl-coenzyme A (acyl-CoA)-dependent acylation
of sn -1,2-diacylglycerol (DAG) to generate triacylglycerol (TAG) (Weselake 2005). There are two
distinct family genes of DGAT: DGAT1 and DGAT2. Since the first identification of the DGAT1 gene
from Arabidopsis spp. (Routaboul et al. 1999), the influence of DGAT1 in increasing oil and fatty acid
production was reported in several crops including soybean (Wang et al. 2006) DGAT1 has resulted
in an 11-28% increase in seed oil content in homozygous napin (Jako et al., 2001); 47% in DGAT1-2
transformed maize (Zheng et al. 2008) and 14% in DGAT1-transformed canola (Weselake et al.
2005). In soybean, several reports indicate a role for DGAT in oil accumulation in developing seeds
(Kwanyuen and Wilson 1990; Settlage et al. 1998). Sequence and expression levels of a DGAT1 gene
were characterized in cultivated and wild soybean (Wang et al. 2006). However, the expression level
of this gene was similar in different types of tissues, including mature leaves, flower, and seeds 20 and
30 days after flowering. Therefore, this gene may not be closely involved in oil production in soybean
seeds. Two isoforms of DGAT1—DGAT1a (AB257589) with 7575 bp plus DGAT1b (AB257590) with
8164 bp—were reported, and these two proteins differ 4% in amino acid sequence but both have 14
introns and 15 exons. Although DGAT1b has greater activity compared with DGAT1a, TAG biosyn-
thesis activity with DGAT1 from soybeans is 5-fold less than that of a DGAT from Vernonia galamen-
sis (Hildebrand et al. 2008). It is not clear which isoform is more critical to oil biosynthesis in soybean.
The DGAT2 gene was discovered after DGAT1, and its influence on oil production in some crops
was examined. DGAT2 was first identified in the fungus Mortierella ramanniana with two homo-
logs DGAT2A and DGAT2B (Lardizabal et al. 2001). The expression of M. ramanniana DGAT2A
in soybean led to a 1.5% increase (by weight) in seed oil with no significant impact on yield or pro-
tein content (Lardizabal et al. 2008). To date, the role of DGAT2 in oil accumulation in Arabidopsis
spp. and common oilseed crops such as soybean has not been investigated.
20.4 PalmItIc acId
Palmitic acid is the predominant saturated fatty acid in soybean oil. Generally, common soybean
cultivars contain approximately 12% palmitic acid. Because of health risks associated with the
cholesterogenic properties of saturated fatty acids and the poor cold flow properties they cause in
biodiesel, reduced levels of palmitic acid are a goal in breeding for improved soybean oil quality.