Legume-Rhizobium Interactions that Maintain Mutualism

University Of California-Davis, Davis CA

Investigators

Abstract

The importance of legume plants to nitrogen budgets of many natural and agricultural ecosystems depends largely on their interactions with rhizobia, soil bacteria that can live inside root nodules and convert nitrogen gas into "fixed" nitrogen that their plant host can use. Rhizobia vary in costs and benefits to their hosts. Why do many, but not all, rhizobia use plant photosynthate (carbohydrates) to fix nitrogen that mainly benefits their hosts, rather than hoarding that photosynthate to support their own future survival and reproduction? This is especially puzzling since each plant typically hosts several competing strains of rhizobia. Strains that fix nitrogen may increase host photosynthesis, thereby increasing the pool of available photosynthate, but if all strains share in this collective benefit then selfish strains that hoard photosynthate should displace those that fix more nitrogen. Why hasn't this happened? Preliminary data appear to show that plants can monitor individual root nodules and cut off resources (including oxygen) to those that fix little or no nitrogen. Mathematical models show that such plant "sanctions" would be an effective deterrent to rhizobial "cheating," but additional research is needed on the mechanisms of plant sanctions and on their effects on survival and reproduction of rhizobia. Various experiments will expose some nodules to a nitrogen-free atmosphere, while allowing other nodules on the same plant to fix nitrogen. Noninvasive monitoring of nodule physiology and methods to count rhizobia are working well, and we also plan to measure resource levels in rhizobia. The importance to soil populations of rhizobial release from nodules, relative to increased reproduction of rhizobia already in the soil, will be determined using rhizobia expressing two different fluorescence genes. DNA fingerprinting of rhizobia collected from nodules in the field will test the key hypothesis that mixed nodules (containing two or more strains) are rare. Although details will differ, our research may be relevant to any system in which cooperation can break down, such as expulsion of algae by bleaching corals.