COMPETITION BETWEEN A GENETICALLY-MODIFIED PSEUDOMONAS-FLUORESCENS AND ITS PARENT IN SUBTROPICAL SOIL MICROCOSMS

The ecological fitness of a Pseudomonas fluorescens strain (Br12) modi fied with a transposon Tn5::cryIVB chromosomal insertion in relation t o its parent strain (Br5) and to the indigenous microbiota nas assesse d in two subtropical soils of fine and course texture planted with mai ze. Both strains had similar growth rates when grown separately in ric h LB broth at 28 degrees C, but the modified strain was at a slight gr owth disadvantage during mixed growth. The introduced populations show ed a gradual decline in both kinds of unplanted soils. The decline rat es were affected by so it type and temperature. In unplanted clay soil , strain Br12 was less competitive than its cointroduced parent, its p roportion in the total surviving inoculant populations dropping from t he initial 50 to 23% after 30 days. A similar proportion of Br12 strai n (27%) was observed in the bulk and rhizosphere so ii of clay soil pl anted wi th maize, however a slightly higher one (32%) was found in th e rhizoplane. The percentage of the modified strain in unplanted sandy soil relative to total inoculant densities dropped slightly (from 50 to 37%) independently of temperature; in planted sand microcosms, its relative amount dropped to 37% - 30% in bulk soil, rhizosphere soil an d in the rhizoplane. In the introduced cell mixtures, both strains mad e up a progressively lower percentage of the total heterotrophic bacte ria and of the total fluorescent pseudomonads in the soil microcosms, with slight disadvantages for the modified strain as compared to its p arent. However, in clay soil at 25 degrees C, both strains kept up sta ble proportions of the total fluorescent pseudo monads, from day 5 to tile end of the experiments (day 25). The effect of the genetic modifi cation on ecological fitness was apparently minor compared with the ef fects of the soil environment acting on the introduced strains in the presence of the indigenous microbial soil communities.