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Research Highlights

Research Highlights
Gene Editing May Improve Digestibility of Soy Protein

Photo: United Soybean Board

By Laura Temple

Soybean meal is widely accepted as an excellent source of protein for animal feed. But valid concerns exist about its drawbacks, as well.

“I have heard many opinions that soybeans are excellent for animal feed, but I have heard many disagree and express opinions that soybeans are not good for feed,” says Dr. Bo Zhang, an assistant professor in the School of Plant and Environmental Sciences at Virginia Tech University. “And both opinions are correct. Soybeans provide high-quality protein in feed, but they also contain anti-nutritional factors that inhibit digestion of that protein.”

One of Zhang’s research goals is to improve the quality of soybean composition, minimizing the presence of anti-nutritional factors to develop soybeans with highly digestible protein. Gene editing is proving to be a valuable tool to help her team meet this goal, and the Virginia Soybean Board is investing in this effort to maintain or increase the feed value of soybean meal.

Zhang’s research addresses several anti-nutritional factors, including trypsin inhibitors, low test weights, the raffinose family of oligosaccharides and phytic acid. Zhang is collaborating with Dr. Bingyu Zhao, an associate professor at the School of Plant and Environmental Sciences at Virginia Tech University who specializes in molecular biology. Other team members include a Ph.D. student, two research associates and a postdoctoral associate. Together, this team is focusing on reducing the concentration of trypsin inhibitors in soybean seeds.

“Right now, soybeans are heated as the first step of the crushing process to inactivate trypsin inhibitors,” she explains. “However, that requires a lot of energy, and the temperature is hard to control and maintain during processing. If temperatures are too low, the trypsin inhibitors are not completely inactivated. However, temperatures that are too high can degrade the quality of the amino acids in the soybeans, so they don’t provide as much high-quality protein in feed.”

Photo: United Soybean Board

Developing soybean varieties with a low concentration of trypsin inhibitors will improve soybean meal quality and reduce energy needs of soybean processing.

Precise Management of Trypsin Inhibitors

Trypsin is an enzyme that breaks large protein molecules into smaller pieces, making it easier to digest. However, soybeans naturally contain trypsin inhibitors, which prevent that breakdown process.

“Trypsin inhibitors serve as part of the natural defense system for soybeans,” Zhang says. “The inability to break down proteins from soybeans make them somewhat less desirable to deer and some insects. We want to reduce trypsin-inhibitor content in soybean meal without compromising plant defenses.”

Genetic research revealed that different genes control the concentration of trypsin inhibitors in plant tissues and soybean seeds.

“Pests like deer and insects feed mostly on soybean leaves,” Zhang says. “We are working to keep trypsin inhibitors in soybean leaves, stems and flowers to maintain poor digestion for pests. At the same time, we want to minimize trypsin inhibitors in the soybean seeds that will be harvested and processed into soybean meal, so the meal can easily be digested in animal feed.”

Her team used CRISPR-Cas9 technology to edit a gene named Kunitz Trypsin Inhibitor 3, or KTI 3. This gene impacts the concentration of trypsin inhibitors just in developing soybean seeds, without impacting the rest of the plant.

“This precision would not be possible with traditional cross-breeding,” she adds.

To date, the gene edits have been applied to a soybean accession, or related and identified plant material, commonly used for genomic research. The team has collected the first generation of seeds following the gene editing. These seeds will be grown in 2021, and by the end of the season, the team will determine if this approach is successful. Any individual plants from these soybean seeds that demonstrate noticeably different concentrations of trypsin inhibitors in plant tissues and seeds will be further purified over generations to create germplasm with more digestible protein. That work would become available to other public breeding programs.

Stacking Protein Digestibility Traits

Zhang’s research has also developed advanced genetically engineered soybean lines with low phytate and low raffinose family of oligosaccharides, addressing other anti-nutritional factors. Their next steps include developing soybean varieties that combine these and other traits to address multiple anti-nutritional factors.

For example, Virginia Tech has released a soybean variety, ‘Barrack,’ that produces high protein content, high sucrose content, which provides high-quality energy and low trypsin inhibitor concentration. This low trypsin inhibitor trait impacts the whole plant, meaning the soybean meal digests more easily, but the variety does not have a strong natural defense system.

As soybean varieties and germplasm are developed that reduce the anti-nutritional factors, the quality — and value — of soybean meal will improve. This research supports the ability of soybean farmers to deliver even higher-quality protein for animal feed.

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.