Functional and structural characterisation of the Eop1 effector from Erwinia amylovora and related species : a thesis submitted to Massey University for the degree of Master of Science in Plant Breeding, School of Agriculture and Environment

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Massey University
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Erwinia amylovora is the etiologic agent of the fire blight disease of apples (Malus species), pears (Pyrus species), and many other members of the Rosaceae family. Fire blight affects virtually all pome species and encompasses major pome fruit-producing countries worldwide. Moreover, it is gradually progressing into the far east Asian countries, which hold a prime position in the pome fruits production. Management of the fire blight disease is challenging due to the lack of effective control measures capable of suppressing its necrogenic effects in the diseased plants or restricting the pathogen's spread. Using resistant cultivars for pome fruit production is one of the most environmentally friendly and sustainable methods. However, it is not a permanent solution due to the pathogen's ability to evolve and overcome host resistance by employing proteinaceous virulence factors termed 'effectors' and restoring host susceptibility. This dynamism of host-pathogen molecular interactions and their role in disease development in the host plant and eliciting an immune response in the non-host plant necessitates a thorough understanding of pathogen-delivered effectors, their mechanisms, and host targets. In this study, non-host resistance was used to decipher the activity, molecular mechanism, and potential host targets of Eop1, one of the effectors secreted by E. amylovora and many other related species during pathogenesis. Firstly, it is demonstrated that Eop1 and its sequence homologs function as a 'putative' avirulence factor in the non-host plant Nicotiana tabacum. Following that, evidence was produced to show that the effector utilises an enzymatic mechanism for its activity; additionally, Eop1s' tertiary structure and catalytic motif were also examined using in-silico protein modelling. Moreover, it was discovered that RIN4, a plant immune regulator, and an R-protein, RPA1, are crucial for the Eop1s' recognition in the tobacco plant. Finally, by connecting all the aforementioned pieces of evidence, a model for Eop1s' activity and its recognition in the host and non-host plants is proposed, which follows the 'guard' paradigm of effector recognition. A thorough understanding of the Eop1 effectors, including their structure, activity mechanism and host targets, would contribute to a better understanding of host-pathogen interaction in the Erwinia-Rosaceae pathosystem, ultimately assisting in the delivery of elite cultivars with durable resistance.