Are phenolic compounds involved in the differential responses of apple genotypes to Erwinia amylovora?

Profiles of methanol-extractable leaf phenolics of MM106 and Evereste, a susceptible and a resistant genotype of apple to fire blight, were analysed by HPLC after infection. On the basis of these profiles, principal component analysis discriminated plants challenged with a virulent wild-type strain of E. amylovora from plants challenged with a non-pathogenic TTSS-defective mutant or with water. This demonstrated that the phenylpropanoid metabolism of apple is altered by TTSS effectors. Patterns of changes in the level of the phenolics subgroups were similar in the two genotypes, with a decrease of dihydrochalcones and an increase of hydroxycinnamate derivatives. Phloretin (an aglycone dihydrochalcone) was the only direct antibacterial constitutive compound found in the leaves at a concentration close to lethal for E. amylovora in both genotypes, but it was observed that the bacterium exhibited the ability to stabilize its concentration to a sub-lethal level in the susceptible as well as in the resistant genotype, which tends to deny its role in resistance to fire blight.