Are Chloroplasts Key Organelles for Determining Susceptibility of Apple and Pear Genotypes to Fire Blight?

Previous reports demonstrated the role of the electron transport chain (ETC) of chloroplasts during Erwinia amylovora/hosts interaction. This research was carried out confirm the role of chloroplast in this interaction and to determine the possible target site of pathogen effector protein(s). The effects of several inhibitors of chloroplasts ETC including methylviologen, glutaraldehyde and diuron were evaluated on in vitro shootlets of apple (Malus domestica) rootstocks MM-111 (resistant) and MM-106 (semi-susceptible), and pear (Pyrus communis) cultivars. Harrow Sweet' (resistant) and. Spadona' (semi-susceptible). Necroses caused by inhibitory effects of ETC inhibitors could be distinguished from necroses caused by the pathogen by a series of primary experiments in absence of the latter and by removing sucrose from the growth media used for ETC activation in chloroplasts. Effects of selected concentrations of inhibitors were evaluated on the progress of E. amylovora necrosis and it was observed that all inhibitors retarded necrosis progress in all tested genotypes. Interestingly, responses of resistant and susceptible genotypes were divergent with retardant effects that were more pronounced in susceptible genotypes. The results confirm previous reports and present more evidence for role of this ETC in host/E. amylovora interaction. According to the compared effects of inhibitors, the NADP reduction complex is the putative site of interaction between the effector proteins of pathogen and chloroplasts' ETC.