Comparison of the effects of Erwinia amylovora effector proteins on pear cultivars in active and inactive chloroplastic electron transport chain conditions

Fire blight is the most destructive vascular necrosis disease of pome fruits. Furthermore, secretion of bacterial effector proteins including HrpN, HrpW and DspA/E to host cells is the most important virulence factor of the bacterium. In order to study the role of these effector proteins on hosts, three mutant strains of pathogen including hrpN-, hrpW-,dspA/E- and wild type strain Ea 273 were used for inoculation of various pear cultivars including, Bartlett, Harrow Sweet and Dargazi. Due to the chloroplasts role in this interaction, all in vitro tests performed in two active and inactive chloroplast electron transport chain conditions. Evaluation of necrosis development index in post inoculation periods showed that the lack of the HrpN and DspA/E effector proteins, in hrpN- and dspA/E- mutant strains respectively, postponed the first necrosis triggering and decreased necrosis progress rate in comparison with the control strain. However the effects of dspA/E- mutant were more considerable. Moreover mutation in hrpW gene had the least decline in bacterial pathogenicity in comparison with the control strain. The effects of mutant strains on severity of pathogenicity and its comparison in two active and inactive chloroplastic conditions revealed that DspA/E, HrpN and HrpW had the most impact on pathogenicity of the bacteria respectively; likewise the effect of HrpW is inconsiderable. The results also revealed that active chloroplasts are necessary for HrpN impact on hosts. It demonstrates that HrpN can be the main factor for interaction of pathogen with host cell chloroplasts which is in accordance with its role on activation of systemic acquired resistance.