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

A genetically modified strain of Psertdomonas fluorescens and its pare nt showed grossly similar decline rates following introduction into su btropical clay and sandy soils. In unplanted clay soil at pH 6.9 and 2 5 degrees C, population densities declined progressively from about 10 (8) to 10(3) colony forming units (cfu) g(-1) dry soil over 75 days, b ut in unplanted sandy soil the introduced populations could not be det ected after 25 days. In clay soil at pH 8.7 or 4.7, or at environmenta l 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 introdu ced 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 temperat ure. The decay rate of both strains was slower in planted than in unpl anted sandy soil. The population densities in the rhizosphere and rhiz oplane in the sandy soil were significantly lower than those in the cl ay soil. Both introduced strains colonized the maize roots in both soi ls, using seeds coated with bacteria in 1% carboxymethyl cellulose. In troduced cells were localized at different sites along the roots of pl ants developing in clay soil, with higher densities in the original (n ear the seeds) and root hair zones as compared to the intermediate zon es. No significant difference was observed between the extent of root colonization of the genetically modified strain and its parent.