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Abstract Background Desirable apple varieties are clonally propagated by grafting vegetative scions onto rootstocks. Rootstocks influence many phenotypic traits of the scion, including resistance to pathogens such as Erwinia amylovora, which causes fire blight, the most serious bacterial disease of apple. The purpose of the present study was to quantify rootstock-mediated differences in scion fire blight susceptibility and to identify transcripts in the scion whose expression levels correlated with this response. Results Rootstock influence on scion fire blight resistance was quantified by inoculating three-year old, orchard-grown apple trees, consisting of 'Gala' scions grafted to a range of rootstocks, with E. amylovora. Disease severity was measured by the extent of shoot necrosis over time. 'Gala' scions grafted to G.30 or MM.111 rootstocks showed the lowest rates of necrosis, while 'Gala' on M.27 and B.9 showed the highest rates of necrosis. 'Gala' scions on M.7, S.4 or M.9F56 had intermediate necrosis rates. Using an apple DNA microarray representing 55,230 unique transcripts, gene expression patterns were compared in healthy, un-inoculated, greenhouse-grown 'Gala' scions on the same series of rootstocks. We identified 690 transcripts whose steady-state expression levels correlated with the degree of fire blight susceptibility of the scion/rootstock combinations. Transcripts known to be differentially expressed during E. amylovora infection were disproportionately represented among these transcripts. A second-generation apple microarray representing 26,000 transcripts was developed and was used to test these correlations in an orchard-grown population of trees segregating for fire blight resistance. Of the 690 transcripts originally identified using the first-generation array, 39 had expression levels that correlated with fire blight resistance in the breeding population. Conclusions Rootstocks had significant effects on the fire blight susceptibility of 'Gala' scions, and rootstock-regulated gene expression patterns could be correlated with differences in susceptibility. The results suggest a relationship between rootstock-regulated fire blight susceptibility and sorbitol dehydrogenase, phenylpropanoid metabolism, protein processing in the endoplasmic reticulum, and endocytosis, among others. This study illustrates the utility of our rootstock-regulated gene expression data sets for candidate trait-associated gene data mining.

RpoN is a σ54 factor regulating essential virulence gene expression in several plant pathogenic bacteria, including Pseudomonas syringae and Pectobacterium carotovorum. In this study, we found that mutation of rpo N in the fire blight pathogen Erwinia amylovora caused a nonpathogenic phenotype. The E. amylovora rpo N Tn 5 transposon mutant rpo N1250:: Tn 5 did not cause fire blight disease symptoms on shoots of mature apple trees. In detached immature apple fruits, the rpo N1250:: Tn 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 N1250:: Tn 5 mutant failed to elicit a hypersensitive response when infiltrated into nonhost tobacco plant leaves, and rpo N1250::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 N1250::Tn 5 mutant phenotypes tested. The rpo N1250::Tn 5 mutant was prototrophic on minimal solid and liquid media, indicating that the rpo N1250:: Tn 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 Pe. carotovorum. [ABSTRACT FROM AUTHOR]

The gram-negative bacterium Erwinia amylovora is the causal agent of fire blight, the most destructive bacterial disease of rosaceous plants, including apple and pear. Here, we compared the virulence levels of six E. amylovora strains (Ea273, CFBP1367, Ea581a, E2002a, E4001a, and HKN06P1) on apple trees and seedlings. The strains produced a range of disease severity, with HKN06P1 producing the greatest disease severity in every assay. We then compared virulence characteristic expression among the six strains, including growth rates in immature apple fruit, amylovoran production, levansucrase activity, biofilm formation, carbohydrate utilization, hypersensitive cell death elicitation in tobacco leaves, and protein secretion profiles. Multiple regression analysis indicated that three of the virulence characteristics (amylovoran production, biofilm formation, and growth in immature apple fruit) accounted for >70% of the variation in disease severity on apple seedlings. Furthermore, in greenhouse-grown 'Gala' trees, >75% of the variation in disease severity was accounted for by five of the virulence characteristics: amylovoran production, biofilm formation, growth in immature apple fruit, hypersensitive cell death elicitation, and sorbitol utilization. This study demonstrates that virulence factor expression levels account for differences in disease severity caused by wild isolates of E. amylovora on apple trees. [ABSTRACT FROM AUTHOR]