The fire blight pathogen E rwinia amylovora requires the rpo N gene for pathogenicity in apple

RpoN is a σ 54 factor regulating essential virulence gene expression in several plant pathogenic bacteria, including P seudomonas syringae and P ectobacterium carotovorum . In this study, we found that mutation of rpo N in the fire blight pathogen E rwinia amylovora caused a nonpathogenic phenotype. The E . amylovora rpo N Tn 5 transposon mutant rpo N 1250:: T n 5 did not cause fire blight disease symptoms on shoots of mature apple trees. In detached immature apple fruits, the rpo N 1250:: T n 5 mutant failed to cause fire blight disease symptoms and grew to population levels 12 orders of magnitude lower than the wild‐type. In addition, the rpo N 1250:: T n 5 mutant failed to elicit a hypersensitive response when infiltrated into nonhost tobacco plant leaves, and rpo N 1250::Tn 5 cells failed to express HrpN protein when grown in hrp ( hypersensitive response and pathogenicity )‐inducing liquid medium. A plasmid‐borne copy of the wild‐type rpo N gene complemented all the rpo N 1250::Tn 5 mutant phenotypes tested. The rpo N 1250::Tn 5 mutant was prototrophic on minimal solid and liquid media, indicating that the rpo N 1250:: T n 5 nonpathogenic phenotype was not caused by a defect in basic metabolism or growth. This study provides clear genetic evidence that rpo N is an essential virulence gene of E . amylovora , suggesting that rpo N has the same function in E . amylovora as in P . syringae and P e. carotovorum .