Genetic and plasmid diversity within natural populations of Pseudomonas syringae with various exposures to copper and streptomycin bactericides

Abstract

We examined the genetic and plasmid diversity within natural populations of Pseudomonas syringae isolated from three ornamental pear nurseries in eastern Oklahoma. The bactericide spray regimen differed at each nursery; copper and streptomycin, only copper, and no bactericides were applied at nurseries I, II, and III, respectively. Resistance to copper (Cu(r)) and resistance to streptomycin (Sm(r)) were determined for 1,938 isolates of P. syringae; isolates from nurseries I and II were generally Cu(r) Sm(s); whereas most isolates from nursery III were Cu(s) Sm(s). The plasmid profiles of 362 isolates were determined, and six, one, seven, and four plasmid profiles were obtained for Cu(r), Sm(r), Cu(r) Sm(r), and Cu(s) Sm(s) isolates, respectively. All Sm(r) plasmids contained sequences homologous to the strA and strB Sm(r) genes from broad-host-range plasmid RSF1010 and were associated with Sm(r) transposon Tn5393. Plasmids were placed into two groups on the basis of hybridization to the oriV and par sequences from pOSU900, a cryptic plasmid in P. syringae pv. syringae. A total of 100 randomly chosen P. syringae isolates from nurseries I and III were analyzed for genetic diversity by using the arbitrarily primed PCR (AP-PCR) technique. An analysis of chromosomal genotypes by AP-PCR revealed a high degree of genetic diversity among the isolates, and the results of this analysis indicated that the isolates could be clustered into two distinct groups. The plasmid profiles were specific to isolates belonging to particular AP-PCR groups. Within each AP-PCR group, identical plasmid profiles were produced by isolates that had different chromosomal genotypes, implying that plasmid transfer has played an important role in the dissemination of Cu(r) and Sm(r) within the populations studied.