Effect of inoculation of a TOL plasmid containing mycorrhizosphere bacterium on development of Scots pine seedlings, their mycorrhizosphere and the microbial flora in m-toluate-amended soil

The purpose of this study was to evaluate the influence of introduced bacte ria containing a contaminant degrading plasmid on the growth and survival o f pine seedlings and mycorrhizosphere microbial flora in contaminated soil. The Pseudomonas fluorescens strain OS81, originally isolated from fungal h yphae in contaminated soil, was supplied with the TOL plasmid pWWO::Km (to generate OS81(pWW0::Km)) and inoculated in humus-soil microcosms with and w ithout pine seedlings mycorrhized with Suillus bovinus. After 3 months of r egular treatment with m-toluate (mTA) solutions, the introduced catabolic p lasmid was found to be disseminated in the indigenous bacterial population of both mycorrhizosphere and soil uncolonized by the fungus. Transconjugant s were represented by bacteria of the genera Pseudomonas and Burkholderia a nd their number correlated positively with the concentration of mTA applied . Indigenous mTA degrading bacteria with low similarity to Burkholderia spe cies were also enriched in microcosms. They were mostly associated with myc orrhizal soil or fungal structures and virtually absent in microcosms witho ut pines. The total number of Tol(+) bacteria was higher in mycorrhizospher ic soil compared with bulk soil. Inoculation with P. fluorescens OS81(pWW0: :Km) had a positive effect on the development of roots and fungus in contam inated soil. Both inoculation with the P. fluorescens OS81(pWW0::Km) and mT A contamination as well as the presence of mycorrhized pine roots and funga l hyphae had an effect on the microbial community structure of soil as meas ured by carbon source oxidation patterns. However, the impact of mTA on the microbial community was more prominent. The study indicates that an effect on plant and fungal development can be obtained by manipulating the mycorr hizosphere. Both introduction of the bacterium carrying the degradative pla smid and the plasmid itself are likely to have a positive effect not only o n the organisms involved, but also on bioremediation of contaminated soil, a factor that was not directly monitored here. (C) 2000 Federation of Europ ean Microbiological Societies. Published by Elsevier Science B.V. All right s reserved.