Competition for nodulation of Pisum sativum cv. Afghanistan requires live rhibozia and a plant component

Nodulation of Pisum sativum cv. Afghanistan by Rhizobium leguminosarum strain Tom can be blocked by R. leguminosarum strain PF2 (isolated from P. sativum cv. Rondo) which does not form nodules of Afghanistan peas. We tested PF2 for its ability to produce bacteriocins and other compounds inhibitory to the growth of Tom. Neither strain was antagonistic toward the other. Similarly, there was no evidence for the production of inhibitors as the rhizobia grew in the plant rhizosphere. Apart from an already noted (Broughton et al. 1980. Can. J. Microbiol. 26: 562–565) ability of PF2 to accumulate on the root surfaces two to three times faster than Tom, we could not find a reason to explain the complete blocking of nodulation. In other experiments using PF2 (and Tom) killed by exposure to massive doses of gamma irradiation, dead cells still bound to the roots but they were incapable of blocking nodulation by living Tom. Finally, when both rhizobia were used to inoculate P. sativum cv. Rondo (they both form nodules on this plant), roughly one third of the nodules contained Tom, one third contained PF2, and one third contained both strains (i.e.,there was no evidence for competition). We conclude, therefore, that competition in this system is dependent upon live bacteria and requires cooperation from the plant.