Cooperation and Conflict in the Legume-Rhizobium Symbiosis

University Of California-Davis, Davis CA

Investigators

Abstract

Legumes, such as clover, alfalfa, and soybean, are important in agriculture and in nature. Soil bacteria known as rhizobia can benefit legumes when they infect their roots, because they can take up atmospheric nitrogen and supply it to the plant. This extra nitrogen increases plant photosynthesis, thereby indirectly increasing the energy supply to the rhizobia. Thus, symbiosis between legume and rhizobium seems beneficial to both. But what if, as often happens, two or more different strains of rhizobia infect the same plant? An initially rare strain that uses plant resources for its own reproduction, rather than for nitrogen fixation, could increase in numbers at the expense of both the plant and the more cooperative rhizobial strain. Although most rhizobia seem to be predominantly beneficial, such "parasitic" strains do exist. This research will determine whether plant responses to rhizobia that fail to fix nitrogen can select for more beneficial rhizobia. Previous research shows that legumes can control the oxygen supply to rhizobia inside their root nodules. The PIs will determine how this oxygen supply changes with changes in nitrogen fixation by rhizobia, and how this legume response affects the survival and reproduction of the rhizobia. Most of the research will be conducted under controlled laboratory conditions, but a field test will determine whether legumes alter the frequency of cooperative vs. parasitic rhizobia in soil. In addition to possible practical applications in agriculture, the research will contribute to overall understanding of the general problem of cooperation between species. Such cooperation is critical to the pollination of flowers, the survival and growth of coral reefs, plant growth in phosphorus-poor soils, and many other natural systems.