This award is funded under the American Recovery and Reinvestment Act of 2009 (Public Law 111-5).
Rhizobia are bacteria that can live in soil, but also symbiotically, inside root nodules on plants like soybean or alfalfa. Although many rhizobia provide their host plants with nitrogen, saving farmers billions in fertilizer costs, less beneficial strains cause problems in some areas. Some hosts, including alfalfa and pea, make rhizobia swell up as they start to provide nitrogen. Unlike the nonswollen rhizobia from soybean or cowpea nodules, swollen rhizobia apparently lose the ability to reproduce, but does rhizobial swelling somehow benefit the plant? To find out, the investigators will map this trait on the family tree for crops and wild plants that host rhizobia, to see if causing swelling evolved more than once, suggesting a positive benefit to the plants. Three dual-host rhizobia (plus mutants that differ in their ability to hoard resources) will be used to measure effects of rhizobial swelling on costs and benefits to the plants. Plant defenses against rhizobia that provide little or no nitrogen, already demonstrated in soybean, will be tested in species that impose bacterial swelling. This research will increase understanding of a symbiosis that supplies nitrogen to agricultural and natural ecosystems, with implications for other important symbioses. Results could guide the development of crops that selectively enrich soils with the best rhizobia, decreasing future fertilizer requirements. Educational opportunities will be provided for undergraduates, at least one graduate student, and a postdoctoral researcher. Two female high school students have already won trips to the International Science Fair for research done in the principal investigator's laboratory, where such mentoring will continue to be a priority.
