Virulence Characteristics Accounting for Fire Blight Disease Severity in Apple Trees and Seedlings

Abstract

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.

Host Exopolysaccharide Quantity and Composition Impact Erwinia amylovora Bacteriophage Pathogenesis

“…phages is amylovoran, the obvious way for the cell to become phage resistant is to change the polymer's composition or abolish its production. However, amylovoran produced by E. amylovora is correlated with pathogenicity, as deficient mutants are avirulent (4,5,9,10,17,21,63,64,65). If amylovoran composition were altered by mutation, it would likely also reduce virulence of the bacterium to the plant host.…”

“…Functions include key roles in bacterial virulence and pathogenesis (20,21,22,23,24), surface adhesion (18), as a major constituent of the biofilm glycocalyx (18), and as a component that renders the cell susceptible (25) or resistant (26,27,28) to bacteriophage (phage) attack. EPSs prevent cellular desiccation (3,25,29,30) by keeping nutrients and water in close proximity to the bacterial cell (29,30).…”

“…The protective nature of the EPSs of E. amylovora is exhibited in the ability of the pathogen to "hide" from the plant host defenses (22). Amylovoran mutants are avirulent, producing no disease symptoms (4,11), and thus EPS is an essential contributor to E. amylovora pathogenicity (4,23) and virulence (2,6,7,15,20,21). Mutants deficient for levansucrase produce disease symptoms on immature pears and therefore remain pathogenic (10,17).…”

Mutation of the Erwinia amylovora argD Gene Causes Arginine Auxotrophy, Nonpathogenicity in Apples, and Reduced Virulence in Pears

“…Then, 20 l of this cell suspension was introduced onto wounds on the skin side of halved immature 'Gala' apple and 'Bosc' pear fruits (46). Bacterial populations in the immature fruits were quantified at 0 and 7 dpi by macerating the apples in 10 mM MgCl 2 solution, followed by serial dilution plating, and expressed as CFU per gram of plant tissue (36).…”

“…In order to discover additional genetic aspects critical for E. amylovora's pathogenicity, we mutagenized E. amylovora strain HKN06P1 (Table 1) (36) with an engineered Tn5 transposon. The E. amylovora mutants were screened for loss of virulence on immature apple fruits.…”