RELATIVE FITNESS IN-VITRO AND IN PLANTA OF PSEUDOMONAS-SYRINGAE STRAINS CONTAINING COPPER AND STREPTOMYCIN RESISTANCE PLASMIDS

The effect of resistance plasmids encoding copper resistance (Cu-r), s treptomycin resistance (Sm-r), and both Cu-r and Sm-r on competitive f itness of Pseudomonas syringae pv. syringae was studied in vitro and i n planta. The Cu-r Sm-r plasmid pPSR1 provided a selective advantage t o its bacterial host (Pseudomonas syringae pv. syringae FF5.1) on Pyru s calleryana leaves that were treated weekly with copper and (or) stre ptomycin bactericides. However, populations of the plasmid free Cu-s S m-s FF5.1 were reduced 10- to 1000-fold over a 12-week period on trees treated with bactericides. The resistance plasmids pPSR4 (Cu-r), pPSR S (Smr), and pPSR4::Tn5393 (Cu-r Sm-r) were highly stable for over 200 generations of growth in glucose-limited batch culture. Results of co mpetition experiments in vitro indicated that Pseudomonas syringae pv. syringae FF5 containing pPSR4, pPSRS, or pPSR4::Tn5393 was reduced to less than 5% of the total culture in competition with wild-type FF5. In growth chamber studies, the resistance plasmids studied did not hav e an impact on epiphytic fitness of Pseudomonas syringae pv. syringae. Our data suggest that resistance plasmids will persist in populations of Pseudomonas syringae pv. syringae following their initial selectio n regardless of the bactericidal spray regime.