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UMNews: On the trail of a vital botanical skill


Spring is the season for matchmaking, and a university-led team of researchers will soon be introducing partners in what may rank as the single most productive union on the planet.

That would be the one between specialized bacteria and the root cells of legumes like lentils, soybeans, peas and alfalfa.

Such unions are actually symbiotic relationships that lead to the majority of the biological nitrogen production in agricultural systems, and from this, much of our plant-based protein. The researchers hope to find the best genetic matches between legumes and the bacteria that live in their roots; the better the match, the greater the protein production potential.

Better symbiotic relationships are important because they produce nitrogen that removes the need for millions of pounds of fertilizer each year. Moreover, more people around the world depend on legumes for protein than on meat, poultry, or fish. That’s why the National Science Foundation has awarded the research team, led by University plant pathology professor Nevin Young, a three-year, $5.7 million grant for the project.

“We want to find which genes are important in forming effective legume-bacteria relationships,” says Young. “Then we can ask which versions of those genes work best in different ecosystems.”

Instead of studying all types of legumes, the team is using a model legume as a stand-in. Called Medicago (from the Greek words for “root” and “life”), it grows in Mediterranean countries and is a close relative of alfalfa.

The other partner to the union is bacteria called Rhizobium. Denizens of the world’s soils, Rhizobium bacteria readily come into contact with legume roots. The bacteria and root cells recognize each other, after which the bacteria settle into the roots and form swellings called nodules.

Inside the nodules, atmospheric nitrogen is converted to ammonium, a form of nitrogen, in a process called nitrogen fixation. Plants can readily use ammonium to make protein; animals then eat the plants, and so protein passes up the food chain.

To find the best plant-bacteria matches, the team will draw on 400 samples of Medicago and about 150 samples of Rhizobium, both collected from around the Mediterranean. They will inoculate the plants with various strains of Rhizobium to see which combinations result in the best nodule formation.

Their results may show, for example, that one Medicago variety consistently forms better nodules than another variety. If so, the genes responsible for that difference must lie in regions of the genome that are different in the two plants.

To find those regions, the researchers will use state-of-the-art techniques to explore the DNA of all 400 Medicago samples. They then will be able to spot DNA sequences associated with superior ability to form nodules. Since genes are composed of DNA sequences, this will put the researchers in good position to identify which versions of which genes confer this desirable trait.

Once this is known, researchers can look for these genetic patterns in other legumes, especially those used as food crops, such as soybeans. The hope is eventually to supply farmers with the best combinations of legume varieties and Rhizobium strains to carry out nitrogen fixation.

Legumes supply protein not only to herbivores, but also to other plants by releasing their nitrogen to the soil when they die and decay. As legumes produce and supply more nitrogen, the need to apply this nutrient via fertilizer decreases.

Michael Sadowsky, a professor of soil science and co-investigator on the project, says this will yield multiple benefits.

“In an environment where energy costs are rising, a large part of the world is developing, and the cost of nitrogen fertilizer is going up, it’s good economics to try to reduce nitrogen inputs to the land,” he says. “In reducing the need to make fertilizer, we’ll reduce the negative impacts of fertilizers on lakes and rivers.”

Besides Young and Sadowsky, university associate plant biology professor Peter Tiffin and researchers from Cornell University, Hamline University and the National Center for Genome Resources are working on the project. The team includes other collaborators from around the world, and this summer three undergraduates apiece from Hamline and the University of Puerto Rico will work on the project at the University of Minnesota.

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