Isolation, characterization, and transfer of cryptic gene-mobilizing plasmids in the wheat rhizosphere

A set of self-transmissible plasmids with IncQ plasmid-mobilizing capacity was isolated by triparental exogenous isolation from the wheat rhizosphere with an Escherichia coli IncQ plasmid host and a Ralstonia eutropha recipient. Three plasmids of 38 to 45 kb, denoted pIPO1, pIPO2, and pIPO3, were selected for further study. No selectable traits (antibiotic or heavy-metal resistance) were identified in these plasmids. The plasmids were characterized by replicon typing via PCR and hybridization with replicon-specific probes and other hybridizations. pIPO1 and pIPO3 were similar to each other, whereas pIPO2 was different. None of these plasmids belonged to any known incompatibility group. pIPO2 was selected for further work, and a mini-Tn5-tet transposon was inserted to confer selectability. Plasmid pIPO2 had a broad IncQ plasmid mobilization and self-transfer range among the alpha, beta, and gamma subclasses of the Proteobacteria but did not show productive transfer to gram-positive bacteria. Plasmid pIPO2 mobilized IncQ plasmid pIE723 from Pseudomonas fluorescence to diverse indigenous proteobacteria in the rhizosphere of field-grown wheat. Transfer of pIE723 to indigenous bacteria was not observed in the absence of added pIPO2. A specific PCR primer system and a probe were developed for the detection of pIPO2-type plasmids in soil and rhizosphere. Analysis of soil DNA provided evidence for the presence of pIPO2 in inoculated wheat rhizosphere soil in the field study, as well as in the rhizosphere of uninoculated wheat plants growing in soil microcosms. The system failed to identify major reservoirs of pIPO2 in a variety of other soils.