Contribution of calystegine catabolic plasmid to competitive colonization of the rhizosphere of calystegine-producing plants by Sinorhizobium meliloti Rm41

Calystegines are plant secondary metabolites produced by the roots of a few plant species, and the ability to catabolize calystegines is infrequent in rhizosphere bacteria. In Sinorhizobium meliloti Rm41, the endosymbiont of the legume Medicago sativa, this ability results from the presence of the g enes cac (for calystegine catabolism) located on the nonsymbiotic plasmid p Rme41a. The effect of the cac catabolic plasmid pRme41a on the ability of R m41 to colonize the rhizosphere of calystegine-positive plants was studied using derivatives of Rm41 with or without cac catabolic plasmid. When strai ns were inoculated alone, the presence of a cac catabolic plasmid had no ef fect on their colonization of the rhizosphere, regardless of whether plants produced calystegines or not. However, a spontaneous rifampicin-resistant mutant of Rm41 containing a cac catabolic plasmid reached population levels in the rhizosphere of calystegine-positive plants that were several orders of magnitude higher than those of the same strain without the plasmid, whe n each Tvas co-inoculated with a derivative of Rm41 cured of pRme41a. In co ntrast, the cac catabolic plasmid provided little or no selective advantage in the rhizosphere of calystegine-negative plants. In conclusion, the cac catabolic plasmid pRme41a can contribute to the ability of S. meliloti Rm41 to colonize the rhizosphere of alternative, nonlegume plant hosts producin g calystegines.