Cryptic plasmid and rifampin resistance in Rhizobium meliloti influencing nodulation competitiveness.

An assessment was made of the relative contributions of a spontaneous mutation to rifampin resistance and a cryptic plasmid, pTA2, to competitive nodulation of Medicago sativa by a strain of Rhizobium meliloti. This was facilitated by use of rifampin-resistant derivatives of this strain in which pTA2 was originally present, cured, or reintroduced. Both curing of pTA2 and spontaneous mutation to rifampin resistance significantly influenced nodulating competitiveness, but the effect of rifampin resistance was greater and such that the contribution of pTA2 was evident only in cases in which paired competitors had the common rifampin resistance background. The data suggest that rifampin-resistant derivatives contain an altered RNA polymerase insensitive to the action of rifampin. All R. meliloti derivatives had symbiotic characteristics and phage susceptibility patterns similar to those of the wild type. Plasmid pTA2 transfer or other genetic interchange was not detected in nodules of M. sativa inoculated with paired competitors.