Soybean Oil: Genetic Approaches for Modification of Functionality and Total Content
Year: 2009
Type: review
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 problem has been addressed for food and feed applications through partial hydrogenation. This typically will reduce the polyunsaturated fatty acids below 18% of the total fatty acids, with linolenic acid generally below 2%, and a concomitant rise in oleic and stearic acids. The negative aspect with partial hydrogenation is that it generates a preponderance of trans-fatty acids, which have been linked with cardiovascular disease (Danaei et al., 2009). Moreover, a partially hydrogenated soybean oil possesses poor cold flow properties, reduced lubricity, and increased viscosity, thus limiting its value as a biodiesel (Moser et al., 2007). Genetic strategies that reduce polyunsaturated fatty acid synthesis can be used to improve oxidative stability of soybean oil, without the production of trans-fatty acids, while maintaining functionality for end use in food, feed, and industrial applications. To this end, the primary fatty acid profiles targeted for soybean are (1) low linolenic acid oil, (2) high oleic acid oil, and (3) elevated stearic acid combined with high oleic acid oil. The production of novel fatty acids in seed oils for nutritional improvements for food and feed use or as a route for cost-effective/sustainable feedstock for industrial applications has been demonstrated (Kinney, 1998; Jaworski and Cahoon, 2003; Abbadi et al., 2004; Cahoon et al., 2007; Burgal et al., 2008). When considering feedstocks for the production of novel fatty acids for industrial applications, it is imperative to take into account the cropping system of the feedstock. Soybean as a feedstock for fatty acids with targeted industrial applications comes with distinct advantages. As a member of the Fabaceae, it possesses inherent reduced fertilization requirements due to its ability to fix nitrogen, and the infrastructure for production and transport is well established. However, if production of the novel fatty acid compromises the suitability of the protein component as a feed, the fatty acid would have to be of sufficient value to recoup the loss of the protein coproduct and the cost associated with identity preservation (Cahoon, 2003). Hence, the drawback of soybean as a feedstock for this purpose is the profit potential. Moreover, even if the profit margin of the novel fatty acid could overcome the potential reduced value of soybean meal, it would most likely be required in a relatively low volume, which in turn would mitigate the need for large production and transport capacity. For these reasons, if an industrial fatty acid is the target, and it is not compatible with feed usage, it may be prudent to use an alternative crop whose seeds are not used for animal feed as the host for this trait. Nutritional enhancement of soybean oil has emphasized improving the ω-3 fatty acid levels for food and feed applications (Kinney, 2003; Cahoon et al., 2007). Significant progress has been made in assembling pathways for heterologous production of targeted ω-3 fatty acids, such as stearidonic acid (SDA; 18:4Δ6,9,12,15; Eckert et al., 2006), and the very-long-chain polyunsaturated fatty acids eicosapentaenoic acid (EPA; 20:5 Δ5,8,11,14,17) and docosahexaenoic acid (DHA; 22:6Δ4,7,10,13,16,19) in plant systems (Abbadi et al., 2004; Damude and Kinney, 2007; Napier, 2007). Soybean oils with enhanced ω-3 fatty acid levels have value in both food and feed applications. Currently, the primary sources for and are or oil. as a feedstock for and are which significantly to However, with in systems as a feedstock for a for will be that will lead to the development of for and which in turn could be toward for the production of nutritional fatty acids. In addition, sources of the ω-3 fatty acid are to oils from plant such as that have potential for production et al., et al., 2007). a the for improved oil soybean have been on the for a of years, with plantings of increasing due to the and that percentage of trans-fats in food products be on While linolenic acid levels of soybean oil oxidative stability with soybean oil, the is not sufficient to be as a oil. The oxidative stability broadly as the of the of of an oil, is and for soybean oil, partially hydrogenated soybean oil, and high oleic acid oil, respectively 2009). with a high percentage of oleic acid a route to levels of trans-fats in food Moreover, oil with a high percentage of oleic acid will a source for applications such as and and products 1999). In soybean oleic acid is to linoleic acid in a by fatty acid by the et al., have been in soybean, and and are primarily during embryogenesis, while is et al., et al., Soybean with acid to have been through breeding by the variation found in the soybean germplasm and et al., et al., However, of acid germplasm has primarily due to yield associated with the et al., and 2008). acid levels in this germplasm are also influenced by and to the acid The environmental instability of this is likely due to the impact that has on et al., et al., The yield associated with the acid soybean is not due to the fatty acid profile of the seed per that targeted of in a results in seeds with greater than oleic acid that not have and et al., 2009). In the soybean the acid is associated with a in one of the et al., However, even environmental the genotype and the other acid of for of the acid et al., et al., 2008). While the the yield associated with soybean acid is not one is that the fatty acid profile of in is also This may impact in the ability to to environmental to reduced oleic acid soybean oil has the is not of sufficient value to for a significant reduction in of yield and environmental instability of oleic acid levels in acid have largely been through the use of This has been achieved by of and of et al., et al., et al., 2009). from this have oleic acid content of the total fatty acids, without of the fatty acid of Moreover, the is as a applications the functionality of fats. To this food have hydrogenated the requirements for both oxidative stability and relatively high oils with elevated fatty acids combined with high oleic acid and low polyunsaturated fatty acids can this need and as a for hydrogenated oil in applications. However, not fatty acids are with to on cardiovascular health. in impact cardiovascular stearic acid is cardiovascular as it to have minimal on levels and et al., Thus, a fatty acid profile that the of the for high and oxidative stability and not impact health would have elevated stearic acid and high oleic acid with low polyunsaturated fatty acids. have been for increasing stearic acid levels of seed (1) heterologous of a and 1998; et al., and (2) in et al., et al., et al., et al., or targeted of et al., et al., In the seeds of the stearic acid to amounts of the total fatty acids. A to this high stearic acid oil is a which possesses high for This stearic acid from fatty acid synthesis in the for into in the with this, of the in in a stearic acid content 20% of the total fatty acids and 1998; et al., 1999). 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The ability to or into a plant for the of A of fatty acids can be found within the although the fatty acids in most plant are and linolenic acids, at on and of the relatively fatty acids found in have applications, the relatively high cost of these oils are for to the due to the low content of fatty acids in sources and/or the ability to the source In some plant a has been to have on both linoleic and linolenic acids, to the production of acid and respectively et al., 2003). an fatty has been to have both and an ω-3 fatty is an in the biosynthesis of the very-long-chain polyunsaturated ω-3 fatty acids and and has nutritional value in food and feed applications. of a from the in in the in of and et al., with the levels of over and in et al., 1999). et a and a from the in This to the of to over 40% and of approximately 2% in seeds. 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However, significant potential for enhanced soybean oil may also be found in feed applications, and In the latter it is that approximately of the is used as feed for and 2003). of the oil and of the used in is from 2003). used for both and indirect consumption are in of and environmental This has to a in with an of by from production et al., a potential to the on its impact will be if need to be with and oil to and the nutritional quality of the et al., 2003). To a of have oils in These have in these For example, of oil with oil in with to the of the in the and a reduction in as with feed et al., 2006). In a with et oil and oil and fatty acid profiles in the The significant variation in or oil reduced levels of both and observed in the feed with the oil et al., 2006). 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These results that the of oils and in the greater nutritional than can be achieved with the use of have for the development of soybean seeds that oils with and is that these oils can be at a lower cost in soybean seeds with and the sources of and In addition, soybean a more production than The likely use of soybean oil in and is the of food and feed and feed for a of strategies could be used to produce and in soybean a route that off of that for the of a or fatty acid to the of to acid and a to The production of through this to the acid and a not produce and and for the synthesis of these fatty acids from potential sources of and and that very-long-chain polyunsaturated fatty acids et al., Napier, 2007; Damude and Kinney, 2008). to pathways in soybean these have levels 20% of the total seed fatty acids et al., 2004). 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As U.S. soybean production has during this Soybean the increased demand for soybean oil has in to large in the of soybean oil. The high demand for soybean oil has an in increasing the relative content of oil per for 18% to 20% of the of soybean which is low relative to most other of for example, approximately oil per seed The of the of soybean seeds to yield such high content of oil are the of both breeding and efforts has been on increasing the oil content of soybean seeds without protein content. This the need for more oil yield from soybean without the of this A of have been for increasing seed oil content. some of this has been with most of these efforts have used as a for oils are composed of amounts of other such as and of fatty acid to a As metabolic efforts for seed oil enhancement have on increasing the synthesis of fatty acids or increasing the In the former this has to enhance the of carbon toward fatty acid synthesis and to increase the of for fatty acid A of have been the of such as and 2004; et al., and metabolic such as et al., to increase oil content by fatty acid the have on This the last of the of a fatty of and in and other et al., et al., The reported to for enhancement of soybean oil content was achieved by the of a for a from the et al., 2008). In these the oil content was increased from approximately 20% of the seed to approximately This increase was over in two the increased oil content or impact on the protein levels of seeds and the seed yield per was that at 2008 soybean oil prices U.S. per this increase in oil content would into a increase in the value of the U.S. soybean crop et al., 2008). soybean oil prices per this would be a million increase in the value of the soybean is that other metabolic strategies will be from to soybean to yield or more in seed oil content. of soybean in on ha and over ha in 2007). the of a high oleic acid soybean generated through of and for and has soybean oil in with by The of the other novel soybean is influenced by two (1) the ability to the in high-yielding genotypes, and (2) the value of the trait. The latter both the potential of the and associated with and the of traits. The in a will be by the of tools for soybean et al., 2003; et al., 2006), the recent of the These combined with efforts et al., et al., and flux et al., both within and efforts will to into carbon flow during in will for the of that of the metabolic will the relative of carbon between seed storage and the of fatty acid in soybean seeds. This will the for the soybean of the
Source: PLANT PHYSIOLOGY
Authors: Tom E. Clemente, Edgar B. Cahoon
Institution: University of Nebraska–Lincoln
Language: English
Subfield: Plant Science
Domain: Life Sciences
SDG: Zero hunger
Open Access status: green
OpenAlex ID: W2114735139