SYMBIOTIC PLASMID TRANSFER IN RHIZOBIUM-LEGUMINOSARUM BIOVAR TRIFOLII AND COMPETITION BETWEEN THE INOCULANT STRAIN ICMP2163 AND TRANSCONJUGANT SOIL BACTERIA

In vitro self-transmission of symbiotic plasmids was demonstrated from three Rhizobium leguminosarum bv. trifolii strains used as clover seed inoculants and from two other strains to a cured derivative of R. leguminosarum bv. trifolii. Tn5-marked symbiotic plasmids from strain ICMP2163 and ICMP2668, transferred to three strains of native soil bacteria at frequencies of 10(-4). Plasmid transfer to one of the native soil bacterial isolates was also demonstrated in a soil microcosm containing white clover seedlings. Insertion of Tn5 into the symbiotic plasmid of the strain ICMP2163 impaired nitrogen fixation in some Tn5-labelled derivatives and effective, ineffective or partly effective Rhizobium=Tn5 derivatives were identified. When representative derivatives were crossed with the non-nodulating native soil bacterium NR40 and crosses were inoculated on white clover seedlings, all transconjugants formed nodules. Electron micrographs showed that ineffective or partly effective associations invaded fewer plant cells. Some of the bacteroids formed appeared damaged in comparison to the bacteroids of effective transconjugants which formed nodules and invaded cortical cells in a similar manner to ICMP2163. Competition experiments between the inoculant Rhizobium strain ICMP2]63 and strains of transconjugant soil bacteria showed that the transconjugant soil bacteria outcompeted the inoculant strain to form nodules on clover roots. The implications of lateral transfer of symbiotic plasmids from inoculants to other bacteria in virgin native soils and the biogenesis of indigenous rhizobial populations after the introduction of inoculants in such agro-ecosystems is discussed.