Soybean Oil: Genetic Approaches for Modification of Functionality and Total Content

Abstract

World consumption of soybean (Glycine max) in 2008 was over 221 million metric tons, with approximately 50% of this supply coming from U.S. production, where soybean plantings on an annual basis are over 77 million ha. Soybeans are desired on the marketplace as a valuable source of protein and oil. The former is primarily used as feed, with some food applications, while the latter is more broadly incorporated into food, feed, and some industrial applications (e.g. biodiesel). Protein and oil percentages in soybean, while influenced by both genotype and environmental cues, average approximately 40% and 20%, respectively. A strong indirect phenotypic correlation exists between these traits. In addition, variation in soybean germplasm for protein content is significantly greater than that observed for total oil content. Historically, soybean breeders have used total protein content as a selection criterion for germplasm development. However, recently, both oil content and quality have drawn much attention in soybean genetics and breeding programs, due to the increased demand for vegetable oils and increased consumer awareness of health issues around dietary fats. To this end, significant efforts have been made to increase oxidative stability of soybean oil as a means to avoid trans-fats generated through the hydrogenation process and to enhance ω-3 fatty acid content of the oil for use in both food and feed applications and increase the total oil content of seeds. Commodity soybean prices have risen over 65% during the last decade, from $158.3 per metric ton in June 1999 to $445.2 per metric ton in June 2009. The world demand for soybean is driven by its highly valued protein and oil for use in food, feed, and industrial applications. During embryogenesis, carbon flux in soybean is primarily partitioned between protein and oil, such that at maturity approximately 40% and 20% of the dry matter is in one of these two respective carbon reserves. The remainder of the seed dry matter is largely carbohydrate, which possesses negligible economic value. While some starch accumulates early in embryogenesis, minimal amounts remain at maturity. The inverse relationship between total oil and protein content in soybean is well documented, where typically a 1% reduction in total oil content will lead to a 2% increase in total protein content. Thus, the regulation of carbon flux during embryogenesis will be shifted toward one or the other, which is impacted by both genetics and environment, although strong metabolic links between oil and storage protein synthesis are not apparent (Schwender et al., 2003). The phenotypic variation in protein and oil within the U.S. Department of Agriculture soybean germplasm collection has been reported to range from 34.1% to 56.8% and from 8.1% to 27.9% for protein and oil, respectively (Wilson, 2004). Interestingly, the “microenvironment” (i.e. the location of seeds on the plant) can also impact carbon flux during embryogenesis, with pods positioned at the top of the plant having seeds with a higher percentage of protein and lower oil content than in those positioned at the bottom of the plant (Bennett et al., 2003). Soybean breeders have made significant progress in improving the overall yield of soybean, which translates into more protein and oil on a per ha basis. Despite this, minimal advancements have been made in the selection of high-yielding genotypes, with major shifts in carbon flux for improvements in total oil or protein content (Mahmoud et al., 2006). On the other hand, implementing the tools of molecular biology and biotechnology has opened the door to the development of improved end-use quality of the oil for food, feed, and industrial applications. These have been achieved by directed modification of fatty acid biosynthesis to alter relative amounts of fatty acids naturally found in soybean or to produce novel fatty acids (Jaworski and Cahoon, 2003; Damude and Kinney, 2008). Modulating endogenous levels and/or production of novel fatty acids of oils has gained significant attention in recent years, due to the increasing awareness of consumers of the impact dietary lipids have on health. Commodity soybean oil is composed of five fatty acids: palmitic acid (16:0), stearic acid (18:0), oleic acid (18:1), linoleic acid (18:2), and linolenic acid (18:3). The percentage of these five fatty acids in soybean oil averages 10%, 4%, 18%, 55%, and 13%, respectively. This fatty acid profile results in low oxidative stability that limits the uses of soybean oil in food products and industrial applications. Oxidative breakdown of soybean oil, for example, results in rancidity and off flavors in food products and the buildup of viscous materials in soybean-derived biodiesel that clogs oil filters (Canakci et al., 1999). Oxidative instability of soybean oil results from the relatively high percentage of the polyunsaturated fatty acids linoleic acid and linolenic acid. Historically, this prob