Conjugative transfer of a derivative of the IncP-1alpha plasmid RP4 and establishment of transconjugants in the indigenous bacterial community of poplar plants

The persistence of traits introduced into the indigenous bacterial community of poplar plants was investigated using bioluminescence mediated by the luc gene. Three endophytic bacterial strains provided with the IncP-1a plasmid RP4-Tn-luc were used to inoculate poplar cuttings at different phenological stages. Screening of isolates by bioluminescence and real-time PCR detection of the luc gene revealed stable persistence for at least 10 weeks. Although the inoculated strains became established with a high population density after inoculation at leaf development (April) and senescence (October), the strains were suppressed by the indigenous bacteria at stem elongation (June). Transconjugants could be detected only at this phenological stage. Indigenous bacteria harbouring RP4-Tn-luc became established with densities ranging from 2 × 105 to 9 × 106 CFU g-1 fresh weight 3 and 10 weeks after inoculation. The increased colonization of the cuttings by indigenous bacteria at stem elongation seemed to strongly compete with the introduced strains. Otherwise, the phenological stage of the plants as well as the density of the indigenous recipients could serve as the driver for a more frequent conjugative plasmid transfer. A phylogenetic assignment of transconjugants indicated the transfer of RP4-Tn-luc into six genera of Proteobacteria, mainly Sphingomonas, Stenotrophomonas and Xanthomonas.