Nodulation of soybeans carrying the nodulation-restrictive gene, rj1, by an incompatible Rhizobium japonicum strain upon mixed inoculation with a compatible strain.

The rj1 gene in soybeans prevents nodulation by most strains of Rhizobium japonicum. Several strains, however, are known to nodulate rj1 plants in vermiculite or sand culture. Pure broth cultures of one of these strains (61 NalR) and a strain producing the typical non-nodulating response with rj1 (I-110 ARS) were mixed and used as inoculum on Clark rj1 soybeans in a growth chamber experiment. Both strains carried drug resistance markers and were identified using selective media. Analysis of the nodules formed indicated that 32% of the nodules contained both strains, 36% contained only the usually non-nodulating strain I-110 ARS, and 32% contained the usually infective strain (61 NalR). These results indicate that under conditions of high inoculum density the roots of Clark rj1 plants did not distinguish between Rhizobium strains 61 NalR and I-110 ARS. Subsequent tests with Rhizobium isolates from the nodules containing only strain I-110 ARS indicated that these rhizobia had not undergone a permanent genetic change in nodulation potential but were infective only because of temporary association with strain 61 NalR.