Abstract
The phytosphere population densities of Pseudomonas fluorescens SBW25EeZY6KX (lacZX aph xylE) carrying pQBR103 (Hg(supr) Tra(sup+), 330 kbp) declined significantly relative to plasmid-free populations after seed inoculation. As the sugar beet plants matured, ca. 100 days after planting, simultaneous selections for plasmid-carrying hosts were observed in the 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(sup5) CFU/g in the rhizosphere) demonstrates that phytosphere-associated plasmids confer a specific fitness advantage to host bacteria.