Transfer of plasmid RP4 between pseudomonads after introduction into soil; influence of spatial and temporal aspects of inoculation

Transfer of plasmid RP4 between introduced strains of Pseudomonas fluorescens was studied in 2 soils, Ede loamy sand and Guelph loam, in non-rhizosphere and rhizosphere soil using soil chambers and microcosm systems. Short-term organism survival was generally at high levels (> 106/g dry soil), in both soils, whereas long-term survival was poorer, particularly in the loamy sand. Amendment of this soil with bentonite clay improved bacterial survival. Plasmid transfer between donor and recipient strains freshly introduced into separate portions of Ede loamy sand, which were subsequently mixed, was only detected in the vicinity of growing wheat roots, suggesting roots stimulate bacterial migration and/or growth. However, no transfer was detected between resident donor and recipient cell populations (introduced 48 days previously), due to poor organism survival. Plasmid transfer was detected in the rhizosphere between established, resident donor cell populations, and newly-introduced recipients, and vice-versa, in both soils. These data suggested that plant roots enhance the frequency of bacterial matings not only between organisms present in the same niches, but also between organisms from different niches, or in different conditions of stress, probably by stimulating bacterial migration and/or growth, or by providing additional surfaces for cell-to-cell contact.