Enhanced colonization and pathogenicity of Erwinia amylovora strains transformed with the near-ubiquitous pEA29 plasmid on pear and apple

Three plasmid-free strains of Erwinia amylovora, the causal agent of fire blight disease of pome trees, one from Iran, one from Egypt and one from Spain, were transformed with the near-ubiquitous nonconjugative pEA29 plasmid from a wildtype strain and characterized. The plasmid-deficient strains were levan- and slime-positive, motile, chemotaxis-positive, induced HR on Nicotiana tabacum var. xanthi but produced several-fold less amylovoran and were weakly pathogenic on pear slices and apple seedlings compared to plasmid-bearing wild-type strains. When inoculated onto wounded young apple (cv. Royal Gala) leaves, the plasmid-free strains labelled with green fluorescent protein gene (gfp) were mainly restricted to the inoculation site at the leaf tips, in contrast to the plasmid-carrying wild-type strains that moved into the midrib xylem vessel and colonized the adjacent parenchyma cells. Upon introduction of the transposon-labelled pEA29 plasmid, amylovoran production, degree of oozing and tissue necrosis on pear slices were significantly elevated in all three strains, whilst the levels of levan and levansucrase declined. Only the strains from Iran and Egypt gained the ability to invade and colonize the young apple leaves following the introduction of pEA29. It is concluded that acquisition of the nonconjugative near-ubiquitous plasmid may not necessarily confer increasing pathogenicity in all bacterial strains.