Differential gene expression patterns in two pear cultivars with differential susceptibility to the necrotrophic pathogen Erwinia amylovora

Pear and apple cultivars widely grown in Tunisia were tested for their susceptibility to Erwinia amylovora (Burrill). Based on a preliminary evaluation of 11 cultivars, three were classified as susceptible, six as moderately susceptible, and two as the least susceptible to E. amylovora. Investigation of physiological and molecular mechanisms modulating the host response in two pear cultivars with different susceptibility degree indicated that early positive regulation of SOD gene, together with a reduced CAT activity, leads to higher H2O2 burst, which was more sustained in the least susceptible cultivar. After inoculation, the genes involved in the salicylic acid (SA) pathway and coding for PR1 protein, PAL, ICS1, and ICS2 were repressed in both cultivars, much more so in the susceptible one. In addition, their expression levels were significantly higher in the least susceptible cultivar before inoculation, which suggested that triggering of the SA pathway is important for promoting the tolerance to E. amylovora. The genes coding for ANS, CHI, CHS, and FHT were repressed after inoculation, except that the degree of that repression was more prominent in the susceptible cultivar, which suggested that phenylpropanoid (PP) pathway could play a critical role in the tolerance of pear plants to E. amylovora. Furthermore, comparisons of the SA-related gene expression trends with that pursued by H2O2 indicated that higher SA and H2O2 accumulation together with PP metabolites are important factors of the tolerance of the cultivar 'Ambri'. Finally, our results provide important insights for genetic improvement programs aiming to develop cultivars resistant to E. amylovora.