Conjugal transfer of a Sinorhizobium meliloti cryptic plasmid evaluated during a field release and in soil microcosms

► The conjugal transfer in the field of a S. meliloti cryptic plasmid was analyzed. ► After seed inoculation no transconjugants were detected in field nodules (<0.1%). ► After soil inoculation plasmid transfer was evaluated in microcosms systems. ► Plasmid transfer in the rhizosphere occurred at ca. 10−6 events/recipient. ► Such frequency was 102-103 times lower than the transfer frequency in rich media.