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Research Highlights
Modifying Soluble Carbohydrates in Soybean Seed for Enhanced Nutritional Energy Meal

By Lais Andrade Pereira, Purdue University

Most of the soybeans produced in the U.S. go to animal agriculture. Therefore, growing soybeans that produce a more valuable meal for animal feed would benefit both soybean farmers and the poultry and swine industries. 

One major factor that affects soybean meal quality is the composition of soluble carbohydrates. Carbohydrates make up 35% of soybean meal, and of that fraction, 10% are the soluble free sugars: sucrose, stachyose, raffinose and galactinol. Sucrose is a nutritionally useful component for both human and animal diets, but the remaining soluble sugars are classified as raffinose family oligosaccharides (RFOs). In fact, RFOs are known to reduce metabolizable energy (ME) for animals and are considered anti-nutritional. Earlier poultry feeding studies conducted as part of this project indicated that the feed conversion ratio in chickens could be improved by reducing RFOs and increasing sucrose content in soybean seeds. Therefore, the meal derived from these soybeans is termed enhanced nutritional energy meal (ENEM). In addition, these studies showed the soybeans with lower RFO and higher sucrose had no negative agronomic impacts from these modifications. Therefore, modifying the soluble carbohydrate composition of soybean seeds continues to be one of the most promising soybean meal improvement targets, and the United Soybean Board has invested $688,206 in this project in 2019. 

Specifically, this project involves a multi-institutional team with broad expertise, including plant breeders, molecular geneticists and nutritionists, among other specialties. The main goal is to develop soybean varieties with more than 7% sucrose and lower levels of RFOs (1-2%), which will result in an increase of ME up to of 100 kcal per pound of meal. The team is developing soybeans and conducting research to support the use of this value-enhanced soybean that can produce ENEM meal. 

One of the first steps taken in this project was to understand the genes that are involved in controlling the production of sucrose and RFO in soybean seeds. It was determined that sucrose content is multigenic — that is, it is an inherited trait controlled by a number of different genes, and can be increased quantitatively through selection for sucrose levels. This team has discovered new gene variants (alleles) that encode soybean seeds with ultra-low RFOs and at least 7% sucrose. Several varieties with altered carbohydrate profiles are being characterized that may be the result of new genes. This ongoing project has also developed soybean lines that differ only in their RFO genes. These lines are called near-isogenic lines, or NILs, and are important controls needed to understand the effect of the RFO genes conferring higher sucrose in agronomic and feeding studies, with greater precision and accuracy. 

All the breeders and geneticists on this project are working to incorporate these valuable new genes into high-yielding lines, to provide farmers with soybean varieties that are both high yielding and contain desirable levels of both sucrose and RFOs. In addition, they are stacking the higher sucrose soybeans producing ENEM with high oleic/low linolenic acid (HOLL) oil traits in order to obtain NILs with contrasting low and ultra-low raffinose/stachyose in a high oleic/low linolenic background.

There are several teams, each of which focuses on its own environmental factors, such as temperature, light, soil and rain, to investigate how those affect composition of soybean seeds with different genomic compositions. In addition, each team is developing soybean seed varieties that can be adapted to grow successfully in a range of environments while especially focusing on early and later maturity groups. Some of their specific goals are to understand how the sucrose and RFO content respond to different environments, and to identify germplasm that produces valuable meal in a wide range of environments. To date, they have identified genetic combinations that are responsible for a stable, increased energy meal soybean that produces at least 7% seed sucrose and has either low or ultra-low RFO across multiple environments, and studies are ongoing.

Figure 1. Sucrose content (%) averaged across three years and four locations. 

Future work will continue to identify and characterize new alternatives and innovative technologies and partners in order to reach even more exciting results. One new direction is gene editing, which has recently been incorporated into this project, to help more directly guide the evolution of new lines. In addition, the team is working with animal nutritionists at the USB Animal Nutrition Working Group, and talking to farmers and other stakeholders in order to understand and address their needs and goals. By continuing to communicate results to stakeholders and incorporate the needs and priorities of stakeholders in future plans, this project continues to produce new seed lines that can meet the current needs of soybean growers and ultimately the animal end users. 

In summary, modified carbohydrate lines developed by breeders on this project have met both RFO and sucrose composition goals and are high yielding. A total of 23 lines were selected for the USDA Uniform Trials and were evaluated in the Preliminary Traited Material Test for 2019; results will be reported in early 2020. This is the first year of evaluations that will lead to the release of soybean lines in the next few years capable of producing ENEM. In the longer term, this project will provide valuable information for USB’s strategic objective to capture the full value of U.S. soybean meal.

Figure 2. Mutant rs3G75E alleles combine with rs2W331-for high-energy meal. Data from 2018 field experiment in Columbia, Missouri.

This project was funded by the soybean checkoff. To find research related to this research highlight or to see other checkoff research projects, please visit the National Soybean Checkoff Research Database.