Studies of symbiotic plasmids in Rhizobium trifolii and fast‐growing bacteria that nodulate soybeans

The Rhizobium trifolii symbiotic plasmid pRt5a was transferred to the fast‐growing soybean strain USDA 194. Transconjugants carrying pRt5a were not able to nodulate clovers and one of the transconjugants had lost its smallest resident plasmid and did not fix nitrogen in soybean. Transconjugants of USDA 194 carrying pRt5a were able to transfer pRt5a back to a non‐nodulating R. trifolii which inherited the symbiotic properties of the R. trifolii strain from which the plasmid was derived.