THE ACQUISITION OF INDIGENOUS PLASMIDS BY A GENETICALLY MARKED PSEUDOMONAD POPULATION COLONIZING THE SUGAR-BEET PHYTOSPHERE IS RELATED TO LOCAL ENVIRONMENTAL-CONDITIONS

The transfer of naturally occurring conjugative plasmids from the indi genous microflora to a genetically modified population of bacteria col onizing the phytospheres of plants has been observed. The marked strai n (Pseudomonas fluorescens SBW25EeZY6KX) was introduced as a seed dres sing to sugar beets (Beta vulgaris var, Amethyst) as part of a field e xperiment to assess the ecology and genetic stability of deliberately released bacterial inocula. The sustained populations of the introduce d strain, which colonized the phytosphere, were assessed throughout th e growing season for the acquisition of plasmids conferring mercury re sistance (Hg-r). Transconjugants were isolated only from root and leaf samples collected within a narrow temporal window coincident with the midseason maturation of the crop, Conjugal-transfer events were recor ded during this defined period in two separate field release experimen ts conducted over consecutive Sears. On one occasion seven of nine ind ividual plants sampled supported transconjugant P. fluorescens SBW25Ee ZY6KX, demonstrating that conjugative gene transfer between bacterial populations in the phytosphere may be a common event under specific en vironmental conditions. The plasmids acquired in situ by the colonizin g inocula were identified as natural variants of restriction digest pa ttern group I, III, or TV plasmids from five genetically distinct grou ps of large, conjugative mercury resistance plasmids known to persist in the phytospheres of sugar beets at the field site, These data demon strate not only that gene transfer may be a common event but also that the genetic and phenotypic stability of inocula released into the nat ural environment cannot be predicted.