Research HighlightsBreeding Research Lays Foundation to Investigate Links Between Fixing Nitrogen and Protein Concentration
By John Lovett
Breeding a modern soybean that does not “fix” its nitrogen may initially seem counterintuitive. In typical soybean plants, root nodules interact with microbes in the soil to transform nitrogen from the air into a form that plants can use, eliminating the need to apply nitrogen fertilizer.
However, breeding soybeans that don’t fix nitrogen is part of the grand plan to assess nitrogen fixation in new high-yielding varieties and potentially reverse the decline in soybean protein content seen over the past 20 years. Although U.S. soybean yield increased by about 14 bushels per acre between 1990 and 2019, average grain protein concentration decreased by about 2%.
With support from the soy checkoff through the United Soybean Board, researchers with the Arkansas Agricultural Experiment Station in collaboration with scientists at Kansas State University and the University of Georgia have bred new lines of high-yielding soybeans in Maturity Groups 4 and 5. These soybeans do not create root nodules that typically interact with microbes to source nitrogen from the atmosphere.
The new “non-nod” soybeans will be released to the USDA Germplasm collection this winter and will be available to researchers soon after.
“Having non-nodulating soybean varieties with similar yield potential when fertilized with nitrogen as nodulating cultivars is key to understanding potential crop nitrogen limitations, developing management strategies to improve grain protein concentration and identifying soybean varieties with high rates of nitrogen fixation,” says Larry Purcell, Distinguished Professor of crop physiology and Altheimer Chair for Soybean Research at the University of Arkansas.
He notes that about 35 to 40% of the bean is protein and about 20% is oil. Soybean plants take up mineral nitrogen from the soil when available to build high protein levels, but they also form nodules on roots in symbiosis with rhizobium bacteria to obtain atmospheric nitrogen, especially when soil nitrogen is inadequate.
Purcell has worked with nitrogen fixation for most of his career, but he began research on breeding a modern “non-nod” soybean about five years ago with Kansas State University professor and soybean breeder William Schapaugh Jr. The USB-supported program to breed the modern, high-yielding “non-nod” soybean in all maturity groups is in its third year.
“The United Soybean Board saw that the lack of high-yielding non-nods was a real limitation for advancing genetic research that looks to improve the protein concentration in grain,” Purcell said. “They have generously funded this national program to develop ‘non-nod’ lines along with their nodulating isolines.”
Bradyrhyizobium japonicum is a rhizobium, a soil bacterium that converts atmospheric nitrogen into organic forms the plant can use.
“From strictly a biological perspective, it’s a fascinating symbiosis,” Purcell says. “You have the soybean that needs the nitrogen, the Bradyrhyizobium japonicum that needs the sugars and everything else the plant provides, in addition to an enclosed environment in that nodule where the magic takes place. The close coordination is a beautiful symbiosis that really caught my attention.”
Non-nod varieties have been around since the 1950s, but prior to this research, there were no non-nodulating varieties of modern, high-yielding soybeans. For example, an existing non-nodulating MG 6 variety, when fertilized heavily with nitrogen, has a yield potential of about 50 bushels per acre, Purcell notes. A modern, high-yielding genotype under optimum growing conditions has a yield potential of 75 to 80 bushels per acre.
Most of the soybeans grown in Arkansas are in MG 4. Agricultural research stations in Georgia, Kansas, Minnesota and Nebraska have worked to breed non-nodulating soybeans and their nodulating counterparts in MG 0, 1, 2, 3, 6, 7, 8 and 9.
Purcell said that these soybean isolines, or nearly identical twin lines, differ primarily by just one gene that is essential for fixing nitrogen. The isoline that is unable to fix nitrogen allows researchers to determine the fraction of nitrogen that comes from interactions with rhizobia compared to the nitrogen from the soil.
“We knew the potential was not there genetically for these old non-nods to be able to adequately represent high-yielding genotypes that we grow today,” Purcell says.
Nitrogen fixation potential differs among soybean varieties, and nitrogen is critical to both yield and protein concentration, Purcell says. He adds that the key to maximum nitrogen uptake is having the “proper genetics and proper environment” to take advantage of the symbiosis between the plant and rhizobium.
The University of Arkansas Division of Agriculture’s Pine Tree Research Station hosted field trials on the “non-nod” soybeans and their high-yielding isolines. To see if the “non-nod” varieties could produce yields as high as their nodulating twin if fertilized, researchers applied the equivalent of 300 pounds of nitrogen fertilizer to “non-nod” plots. They applied no nitrogen on other “non-nod” plots of the same variety to serve as a control for the trials.
“If genetics only differ for that nodulating trait, we would expect the yields to be similar when plenty of nitrogen was provided. And that’s been pretty much what we have seen,” Purcell says.
Last year at Pine Tree Research Station, the modern “non-nod” plants with nitrogen fertilizer added produced about 65 bushels per acre, the same as their nodulating twins. Experiments in Kansas and Georgia had similar results.
These non-nodulating soybeans are not bred for farmers to plant, because they bring the added expense of nitrogen fertilizer.
“This is strictly a tool for being able to do large-scale field research to evaluate the contribution of nitrogen fixation to soybeans and indirectly determine the importance of nitrogen fixation to grain protein concentration,” Purcell says.
Purcell’s research colleagues on the non-nodulating soybean project include Aaron Lorenz at the University of Minnesota, George Graef at the University of Nebraska, William Schapaugh Jr. at Kansas State University, Zenglu Li at the University of Georgia and Andrea Acuna-Galindo with the Arkansas Agricultural Experiment Station.
Published: Nov 7, 2022
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.