R. Ford Denison

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.