R. Ford
Denison
Adjunct Professor
Ecology, Evolution, and Behavior
I study the evolution of cooperation between species, with an applied focus on increasing the efficiency of nitrogen fixation and perhaps other benefits that soybeans get from their root-nodule symbionts.
Research statement
Soybeans need little nitrogen fertilizer, thanks to nitrogen-fixing bacterial symbionts (rhizobia) in their root nodules. However, rhizobia strains differ in how much nitrogen they give the plant and in their cost to the plant. Because there are many strains per plant, no strain will sacrifice its own reproduction just to benefit the host – and its competitors on the same plant. So legumes like soybeans evolved to limit losses to “cheaters” by shutting down nodules that provide no nitrogen, limiting reproduction of the bacteria inside. Our field data suggest that these host-imposed “sanctions” improve the symbiont legacy for future soybean crops. However, soybeans do not consistently impose sanctions on mediocre symbiont strains. Therefore, most root nodules in the field are occupied by less-beneficial strains. My students hope to improve the symbiont-legacy effect. They are measuring differences in net benefits among strains (including hormonal effects) and screening soybean varieties for stricter sanctions. This research has also contributed to our basic understanding of how evolution can maintain cooperation between species.
Selected publications
Porter, S., Dupin, S., Denison, R., Kiers, E., & Sach, J. (2024) Host-imposed Control Mechanisms in the Legume-Rhizobia Symbiosis. Nature Microbiology https://www.nature.com/articles/s41564-024-01762-2:
Muller, K., & Denison, R. (2024) Prolonged soybean absence in the field selects for rhizobia that accumulate more polyhydroxybutyrate during symbiosis. Agronomy Journal 116: 1882-1893.
Monnens, D., Denison, R.F., & Sadok, W. (2023) Rising vapor-pressure deficit increases nitrogen fixation in a legume crop. New Phytologist 239: 54-65.
Denison, R., & Muller, K. (2022) An evolutionary perspective on increasing net benefits to crops from symbiotic microbes. Evolutionary Applications 15: 1490-1504.
Denison, R.F. (2021) Legume-imposed selection for more-efficient symbiotic rhizobia. Proceedings of the National Academy of Sciences USA 118: e2107033118
Denison, R.F. (2012) Darwinian agriculture: how understanding evolution can improve agriculture. Princeton University Press, Princeton.
Denison, R.F. (2011) Past evolutionary tradeoffs represent opportunities for crop genetic improvement and increased human lifespan. Evolutionary Applications 4: 216-224.
Kiers, E.T., Rousseau, R.A., West, S.A., & Denison, R.F. (2003) Host sanctions and the legume-rhizobium mutualism. Nature 425: 78-81. Denison, R.F. (2000) Legume sanctions and the evolution of symbiotic cooperation by rhizobia. American Naturalist 156: 567-576.