IMPACT OF PLASMID PQBR103 ACQUISITION AND CARRIAGE ON THE PHYTOSPHEREFITNESS OF PSEUDOMONAS-FLUORESCENS SBW25 - BURDEN AND BENEFIT

The phytosphere population densities of Pseudomnas fluorescens SBW25Ee ZY6KX (lac ZX aph xylE) carrying pQBR103 (Hg-r Tra(+), 330 kbp) declin ed significantly relative to plasmid-free populations after seed inocu lation. As the sugar beet plants matured, ca. 100 days after planting, simultaneous selections for plasmid-carrying hosts were observed in t he phyllospheres and rhizospheres of field-grown plants. The recovery of these populations to densities indistinguishable from the densities of plasmid-free inocula (4 x 10(5) CFU/g in the rhizosphere) demonstr ates that phytosphere-associated plasmids confer a specific fitness ad vantage to host bacteria.