Lifestyle alternatives for rhizobia: mutualism, parasitism, and forgoing symbiosis.
Strains of rhizobia within a single species can have three different genetically determined strategies. Mutualistic rhizobia provide their legume hosts with nitrogen. Parasitic rhizobia infect legumes, but fix little or no nitrogen. Nonsymbiotic strains are unable to infect legumes at all. Why have rhizobium strains with one of these three strategies not displaced the others? A symbiotic (mutualistic or parasitic) rhizobium that succeeds in founding a nodule may produce many millions of descendants. The chances of success can be so low, however, that nonsymbiotic rhizobia can have greater reproductive success. Legume sanctions against nodules that fix little or no nitrogen favor more mutualistic strains, but parasitic strains that use plant resources only for their own reproduction may do well when they share nodules with mutualistic strains.
Denison
R Ford
RF
Department of Agronomy and Range Science, University of California at Davis, 1 Shields Avenue, Davis, CA 95616, USA. rfdenison@ucdavis.edu
Kiers
E Toby
ET
D020561:Alphaproteobacteria / Q000254:growth & development / Q000502:physiology; D007887:Fabaceae / Q000382:microbiology; D006790:Host-Parasite Interactions; D009586:Nitrogen Fixation; D012231:Rhizobium / Q000254:growth & development / Q000502:physiology*; D013559:Symbiosis