The symbiosis between legume plants and the bacteria that inhabit their roots seems sweet. These bacteria –called rhizobia– turn nitrogen molecules from the air into a form that the plant can use and, in return, feast on carbohydrates the plant provides. But conflicting interests lurk beneath the surface and can undermine such cooperation, as both parties might prefer to benefit without paying their dues. A new study shows that the process determining the exchange rate of nitrogen and carbohydrates has profound effects on how cooperation can be maintained between legumes and rhizobia.
NIMBioS postdoctoral fellow Erol Akçay and Ellen Simms, a professor at the University of California, Berkeley, developed a mathematical model that envisions the symbiosis as being similar to a manufacturer firm negotiating with different suppliers. For natural selection to favor beneficial rhizobia, the plant must negotiate a deal that preferentially rewards more cooperative rhizobia, a phenomenon called partner choice.
Akçay and Simms show that partner choice can happen only if cooperative strains also provide good outside options to the plant. "You can play hardball against a bad business partner only if you have a good outside option," Akçay says. But even if partner choice is effective for an individual plant, less cooperative strains still have an advantage at the population level. This occurs because the plant can play the cooperative strains that provide good outside options against each other and depress their average fitness.
"It's similar to a price war between two suppliers, which drives them both out of business," says Simms. However, this effect can be countered by some mechanism that provides all rhizobia an automatic benefit from the plant’s growth, like firms that share profits with workers. Termed "partner fidelity feedback," such mechanisms have been dismissed in the past because they are vulnerable to the "tragedy of the commons," or exploitation by free riders. Negotiations prevent such exploitation.
The study's results show how the mechanisms that govern the distribution of benefits from an interaction strongly affect its outcome and underscore the importance of determining how these mechanisms operate and what patterns of genetic variation they exhibit in natural populations.
Citation: Akçay E, Simms E. 2011. Negotiation, sanctions and context dependency in the legume-rhizobium mutualism. The American Naturalist. Published [online] June 2, 2011.
The National Institute for Mathematical and Biological Synthesis (NIMBioS) brings together researchers from around the world to collaborate across disciplinary boundaries to investigate solutions to basic and applied problems in the life sciences. NIMBioS is sponsored by the National Science Foundation, the U.S. Department of Homeland Security, and the U.S. Department of Agriculture with additional support from The University of Tennessee, Knoxville.