Molecular markers and genes associated with low palmitic and low linolenic acid content in N97-3681-11 and N97-3708-13 soybean lines.
Soybeans (Glycine max L. Merr.) are a very important source of edible oil in the U.S. Palmitic acid (16:0) is the major saturated fatty acid present in soybean oil and it has been linked to increased risk of coronary diseases. Soybean oil typically contains 110g kg-1 of palmitic acid and it is desirable to decrease its content to 40 g kg-1. To accomplish this objective it is important to study the target genes that control the production of fatty acids in the biosynthetic pathway, involved in de novo fatty acid biosynthesis and triacylglycerol assembly. It is known that at least five recessive alleles condition the low palmitic acid contents of soybean seeds: fap1, fap3, fap*, fapnc and sop1. The fapnc mutation affects the normal function of the enzyme 16:0-ACP thioesterase encoded by the gene FATB1. There are four FATB1 isoforms in soybean: GmFATB1a, GmFATB1b, GmFATB2a, and GmFATB2b. GmFATB1a is deleted in N97-3681-11 and N97-3708-13 lines homozygous for the fapnc mutation, and specific markers were developed for this isoform. The GmFATB1a specific marker accounted 62 to 70% of the genotypic variation in palmitic acid content in two F4 derived populations. It is 100% linked to the locus of interest and should be very useful in marker-assisted selection programs designed to lower the palmitic acid levels of soybean oil.
Linolenic acid (18:3) contains three double bonds and is particularly susceptible to oxidation. There are at least three major loci responsible for low 18:3 content in soybean seed oil (fan, fan2 and fan3), and possibly 2 more loci if the fanx allele and the low 18:3 allele from N78-2245 are also independent loci. By decreasing the 18:3 content of soybean oil from its typical level to a level of 30 g kg-1, it may be possible to eliminate or reduce the need for hydrogenation of the oil. The FAD3 gene encodes an omega-3 desaturase that converts 18:2 to 18:3 in the endoplasmic reticulum. It has four highly similar microsomal isoforms: GmFAD3A and GmFAD3B (93% DNA identity shared), FAD3-2a and FAD3-2b. A single polymorphism was confirmed for the GmFAD3A gene when cDNA sequences from 6 soybean parental lines used in our segregating populations were compared. A random SNP in intron 2 of the GmFAD3A gene isoform was discovered in Boggs, PI123440, and the two low linolenic parental lines in our populations. We have developed a CAPS marker that distinguishes the GmFAD3A allele inherited from PI123440. The segregation at this locus explained 77.5 to 89.2% of the genotypic variation in linolenic acid content in two F4 derived populations.
The root and leaf tissue fatty acid composition of two lines carrying the fapnc, fap1, and fan(PI123440) alleles and of a genetically related high yielding normal line, Brim was examined to determine if these mutations were also expressed in those tissues (pleiotropy). We concluded that fapnc or fap1 or both mutations have pleiotropic effects on the leaf fatty acid composition. Therefore, leaf membranes such as chloroplast membranes could be affected and those changes could affect their physiological behavior, representing a likely explanation for the yield drag observed in lines carrying the fapnc mutation. The fan mutation also has pleiotropic effects on the leaf and root fatty acid composition but has not been negatively correlated with yield. Our future efforts will be to map this GmFAD3A gene in these populations and compare their genomic location with the fan locus and QTL associated with linoleic acid content.
Advisor:Dr. Andrea J. Cardinal; Dr. Ralph E. Dewey; Dr. Joseph W. Burton
School:North Carolina State University
School Location:USA - North Carolina
Source Type:Master's Thesis
Date of Publication:05/17/2006