October 12, 2016

Soybean nitrogen breakthrough could nearly double the production of soybeans and legumes which make up 30% of the world's crops

Washington State University biologist Mechthild Tegeder has developed a way to dramatically increase the yield and quality of soybeans.

Her greenhouse-grown soybean plants fix twice as much nitrogen from the atmosphere as their natural counterparts, grow larger and produce up to 36 percent more seeds.

Tegeder designed a novel way to increase the flow of nitrogen, an essential nutrient, from specialized bacteria in soybean root nodules to the seed-producing organs. She and Amanda Carter, a biological sciences graduate student, found the increased rate of nitrogen transport kicked the plants into overdrive.

Current Biology- Increasing Nitrogen Fixation and Seed Development in Soybean Requires Complex Adjustments of Nodule Nitrogen Metabolism and Partitioning Processes



Highlights
• Increasing nodule ureide export improves nitrogen fixation and shoot nutrition
• UPS1 function is coupled with nodule metabolic and transport pathways
• Nitrogen partitioning processes and nodulation are linked
• Organic nitrogen transporters can be used in plant breeding and seed production

Summary

Legumes are able to access atmospheric di-nitrogen (N2) through a symbiotic relationship with rhizobia that reside within root nodules. In soybean, following N2 fixation by the bacteroids, ammonia is finally reduced in uninfected cells to allantoin and allantoic acid. These ureides present the primary long-distance transport forms of nitrogen (N), and are exported from nodules via the xylem for shoot N supply. Transport of allantoin and allantoic acid out of nodules requires the function of ureide permeases (UPS1) located in cells adjacent to the vasculature [2 and 3]. We expressed a common bean UPS1 transporter in cortex and endodermis cells of soybean nodules and found that delivery of N from nodules to shoot, as well as seed set, was significantly increased. In addition, the number of transgenic nodules was increased and symbiotic N2 fixation per nodule was elevated, indicating that transporter function in nodule N export is a limiting step in bacterial N acquisition. Further, the transgenic nodules showed considerable increases in nodule N assimilation, ureide synthesis, and metabolite levels. This suggests complex adjustments of nodule N metabolism and partitioning processes in support of symbiotic N2 fixation. We propose that the transgenic UPS1 plants display metabolic and allocation plasticity to overcome N2 fixation and seed yield limitations. Overall, it is demonstrated that transporter function in N export from nodules is a key step for enhancing atmospheric N2 fixation and nodule function and for improving shoot N nutrition and seed development in legumes.



Improving grain yields

Legumes account for around 30 percent of the world’s agricultural production. They consist of plants like soybeans, alfalfa, peas, beans and lentils, among others.
Unlike crops that rely on naturally occurring and artificially made nitrogen from the soil, legumes contain rhizobia bacterioids in their root nodules that have the unique capability of converting or “fixing” nitrogen gas from the atmosphere.

For years, scientists have tried to increase the rate of nitrogen fixation in legumes by altering rhizobia bacterioid function or interactions that take place between the bacterioid and the root nodule cells.

Tegeder took a different approach: She increased the number of proteins that help move nitrogen from the rhizobia bacteria to the plant’s leaves, seed-producing organs and other areas where it is needed.

The additional transport proteins sped up the overall export of nitrogen from the root nodules. This initiated a feedback loop that caused the rhizobia to start fixing more atmospheric nitrogen, which the plant then used to produce more seeds.

“They are bigger, grow faster and generally look better than natural soybean plants,” Tegeder said. “Some evidence we have suggests they might also be highly efficient under stressful conditions like drought.”

Protecting the environment

Nitrogen is a macronutrient essential for plant growth. Large amounts of synthetic nitrogen fertilizer are applied around the world to ensure high plant productivity.
Application is an environmental issue in industrialized countries like the United States because of high energy input, increased greenhouse gas emissions, water pollution and other adverse effects on ecosystems and human health.

In developing countries, where nitrogen fertilizer is scarce, insufficient plant nitrogen results in low crop yields and limited food supplies.
Tegeder thinks her soybean-focused research can eventually be applied to varieties of legumes suited for a diverse array of climates. One major benefit of growing legumes such as chickpeas, common beans, peas and soybeans is that they not only can use atmospheric nitrogen for their own growth but also leave residual nitrogen in the soil for subsequent crops.

Hence, increasing nitrogen fixation could improve overall plant productivity for farmers who grow legumes in both industrial and developing countries while diminishing or eliminating the need for nitrogen fertilizers.

“Legumes with higher yields have huge implications for agriculture and food production around the world,” Tegeder said. “Our research also has the potential to be transferred to other crop plants that don’t fix nitrogen from the atmosphere but would benefit from being able to uptake nitrogen more efficiently from the soil.”

SOURCES- Washington State University, Current Biology

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