Infection, carbohydrate utilization, and protein profiles of apple, pear, and raspberry isolates of Erwinia amylovora.
The bacterium Erwinia amylovora causes fire blight of about 200 species within the Rosaceae family. Several theories on why this pathogen is limited to this family have been proposed. The utilization by the pathogen of sorbitol, the primary photosynthate of rosaceous plants, has been one such theory. In this study, the carbohydrate contents of apple (Malus) and raspberry (Rubus) were quantified, and isolates of E. amylovora from these respective hosts were examined in relation to carbohydrate utilization, cross infection, and protein profiles to better understand possible reasons for host-range restrictions within the Rosaceae. Raspberry, unlike most rosaceous plants, contained sucrose as the primary photosynthate, while apple contained primarily sorbitol, except for the flower nectar, which contained primarily sucrose but no sorbitol. Apple isolates caused severe fire-blight symptoms on both apple and raspberry flowers and vegetative growing points, despite the differences in photosynthate content. Conversely, raspberry isolates failed to cause symptoms on apple shoots or flowers although they were easily recovered. Accordingly, there was no evidence that photosynthates play a role in delimiting the host range of E. amylovora. In contrast, two-dimensional sodium dodecyl sulphate-polyacrylamide gel electrophoresis (SDS-PAGE) profiles of proteins secreted in a hrp (hypersensitive response and pathogenicity)-inducing medium were distinctly different for the apple and raspberry isolates. Differences in flagellin proteins (known pathogenicity factors), in a major outer membrane protein (OmpA), in a periplasmic ABC transporter, and in a heat-shock protein (Hsp70) were observed. Thus, these differences in protein profiles are more likely to be responsible for delimiting the host range of E. amylovora than host carbohydrate content.