SURVIVAL OF GENETICALLY-MODIFIED PSEUDOMONAS-FLUORESCENS INTRODUCED INTO SUBTROPICAL SOIL MICROCOSMS

A genetically modified strain of Psertdomonas fluorescens and its parent showed grossly similar decline rates following introduction into subtropical clay and sandy soils. In unplanted clay soil at pH 6.9 and 25 degrees C, population densities declined progressively from about 10(8) to 10(3) colony forming units (cfu) g(-1) dry soil over 75 days, but in unplanted sandy soil the introduced populations could not be detected after 25 days. In clay soil at pH 8.7 or 4.7, or at environmental temperature, decay rates were enhanced as compared to those at pH 6.9 and 25 degrees C. Counts of introduced strains in clay bulk soil and in the rhizosphere and rhizoplane of maize suggested that the introduced bacteria competed well with the native bacteria, and colonized the roots at about 10(6) cfu g(-1) dry root at 25 degrees C, over 20 days. However, rhizoplane colonization was lower at environmental temperature. The decay rate of both strains was slower in planted than in unplanted sandy soil. The population densities in the rhizosphere and rhizoplane in the sandy soil were significantly lower than those in the clay soil. Both introduced strains colonized the maize roots in both soils, using seeds coated with bacteria in 1% carboxymethyl cellulose. Introduced cells were localized at different sites along the roots of plants developing in clay soil, with higher densities in the original (near the seeds) and root hair zones as compared to the intermediate zones. No significant difference was observed between the extent of root colonization of the genetically modified strain and its parent.