Transposition and copy number of insertion sequence ISRm1 are not correlated with symbiotic performance of Rhizobium meliloti from two field sites

The insertion sequence ISRm1 frequently occurs in Rhizobium meliloti and is a potential mutagen. Data for the frequency of ISRm1 transposition in the commercial inoculant strain SU47 (containing eight copies of IsRm1) indicate that this insertion sequence does not significantly affect the ability of SU47 to occupy nodules of alfalfa (Medicago sativa L.) grown at two field sites. Thirty representative isolates from the indigenous population of R. meliloti at each of these sites contained from 0 to 10 copies of IsRm1; there was no significant correlation between insertion sequence copy number and nodulating competitiveness, symbiotic effectiveness, or frequency of occurrence of indigenous rhizobia in nodules of plants grown at these sites. Collectively, these data suggest that ISRm1 has little impact on the stability of agriculturally important properties of R. meliloti from the two field sites, despite its potential as a mutagen.