Doubling Nitrogen Fixation in Soybeans

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A Washington State University plant scientist may have discovered how to double nitrogen fixation in 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. Mechthild Tegeder, a Washington State University biologist, designed a novel way to increase the flow of nitrogen 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.

“The biggest implication of our research is that by ramping up the natural nitrogen allocation process, we can increase the amount of food we produce,” Tegeder says.

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 says. “Some evidence we have suggests they might also be highly efficient under stressful conditions like drought.” Tegeder thinks her research can eventually be applied to varieties of legumes suited for a diverse array of climates. One major benefit of growing legumes 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.

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