Soybean - Bioenergy Crop Plants - page 511

chemical mutagenesis. Various combinations of these alleles can increase palmitic acid content to
35% of crude oil (Fehr et al. 1991b).
Some agronomic traits were affected in the genotypes with altered palmitic acid genes reported
above. Soybean yield was significantly less in genotypes with both the reduced and elevated levels
of 16:0 (Wilcox and Cavins 1990; Ndzana et al. 1994; Rebetzke et al. 1998; Hayes et al. 2002). In
comparison to related lines with normal 16:0 levels, high-palmitic acid BC 1 F 2:4 lines averaged across
each of three different populations typically had a shorter height, smaller seed size, higher protein
levels, lower oil levels, and reduced oleic and linoleic acids (Hayes et al. 2002). These associations
suggest that pleiotropy and/or linkage drag could hinder efforts to develop competitive cultivars
with altered palmitic acid phenotypes (Pantalone et al. 2004).
Molecular markers closely associated with palmitic acid alleles have been discovered
(Table 20.2). The fap2 allele elevated 16:0 in C1727 was mapped to Chro. 17 (LG D2) (Nickell et al.
1994). An elevated palmitic acid soybean line containing the fap 2 mutation was characterized with
a candidate gene approach. The underlying mutation in a KAS II gene, which is responsible for the
elevated palmitic acid phenotype, was discovered and led to the development of a perfect molecu-
lar marker assay (Aghoram et al. 2006). The major low palmitic acid allele fap nc ( R 2 = 31-38%) in
NS79-2077-12 was mapped near Satt684 on Chro. 5 (LG A1) (Li et al. 2002). A low 16:0 soybean
line with the fap nc allele was characterized and found to contain a deletion in one of the four FATB
genes catalyzing a thioesterase function in the fatty acid pathway (Cardinal et al. 2007). Again, a
molecular marker assay was described that is specific for the mutant allele and could be used in a
breeding program.
QTL was associated with reduced palmitic acid in C1726 mapped to Chros. 14 and 12 (LGs B2 and
H) using simple sequence repeat (SSR) markers in a BC 1 F 2 population from a cross Cook × C1726.
The lowest 16:0 contents were observed where the QTL on Chro. 14 (LG B2) was homozygous for
the allele from normal soybean Cook, and the QTL on Chro. 12 (LG H) was homozygous for the
C1726 allele (Pantalone et al. 2004).
Several QTLs associated with palmitic acid content were mapped in populations in which the
parents had normal palmitic acid content. Diers and Shoemaker (1992) used an F 2 -derived popula-
tion of A81-356022 ( G. max ) × PI468916 ( G. soja ) to map the restriction fragment length polymor-
phism (RFLP) markers associated with 16:0 levels and found three markers: pA-343a ( R 2 = 2 4%)
and pA-18 ( R 2 = 19%) on Chro. 14 (LG B2) and pK-375 on Chro. 16 (LG J). The low palmitic
acid alleles at the Chro. 14 (LG B2) QTL came from G. soja , whereas the one at the Chro. 16
(LG J) QTL was from the G. max line. A major QTL for reduced 16:0 from an Essex × Williams
population was found in an 89.1-cM region on Chro. 19 (LG L) ( R 2 = 13%) and was derived from
Williams (Hyten et al. 2004). Recently two QTL that reduce palmitic acid—Satt133 on Chro. 8
(LG A2) ( R 2 = 12%), located approximately 7 cM upstream from a QTL affecting oil concentra-
tion (Brummer et al. 1997), and Satt537 on Chro. 2 (LG D1b) ( R 2 = 20%)—were detected from
mapping population N87-984-16 × TN93-99, in which both parents had normal fatty acid profiles
(Panthee et al. 2006).
20.5 stearIc acId
Stearic acid (18:0) content in soybeans averages approximately 3% of the crude oil. Soybean oil
higher in the saturated fats 16:0 and 18:0 has improved functional properties for certain food appli-
cations, such as solid fats, margarines and shortenings, and confectionery uses. Genotypes high in
stearic acid are preferred because, unlike palmitic acid, 18:0 has been shown to be neutral in raising
blood serum cholesterol, which is associated with heart disease (Yadav 1996). Soybean genotypes
with elevated stearic acid have been developed by chemical or X-ray mutagenesis and from natural
mutations. However, high 18:0 genotypes developed from these techniques appear to be associated
with poor yield potential (Hayes et al. 2002), so development of productive high stearic cultivars
may be difficult.