Reactive oxygen species play a dual role in the resistance and susceptibility of Camellia to flower blight disease : a thesis presented in partial fulfilment of the requirements for the degree of Master of Science in Plant Biology at Massey University, Manawatū, New Zealand

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Reactive oxygen species (ROS), a group of highly reactive biomolecules, are known to rapidly accumulate in plant tissue as an early defence response to pathogen invasion. However, ROS can also contribute to pathogen virulence. Currently, little is known about the activity of these compounds during the interaction between Camellia and the Camellia flower blight (CFB) necrotrophic fungal pathogen, Ciborinia camelliae L. M. Kohn (Sclerotiniaceae). It has been shown that there is a spectrum of resistance and susceptibility to the disease within the Camellia genus. This study aimed to elucidate the role that ROS play during C. camelliae interactions with Camellia on this spectrum of resistance. To achieve this, hydrogen peroxide accumulation was first visualised and compared between the CFB resistant Camellia lutchuensis and the susceptible Camellia ‘Nicky Crisp’ in response to C. camelliae. Following the inoculation of flower petals with C. camelliae ascospores, widespread apoplastic hydrogen peroxide accumulation and upregulation of genes encoding NADPH oxidase and cell wall peroxidase began 12 hours earlier in the resistant C. lutchuensis species than the susceptible C. ‘Nicky Crisp’, which showed very little observable accumulation. In addition, the quantity of hydrogen peroxide significantly increased in the resistant C. lutchuensis petals, but no change was observed in the susceptible C. ‘Nicky Crisp’ within the same timeframe. The application of exogenous antioxidants to scavenge the hydrogen peroxide accumulation resulted in disease development in the normally resistant C. lutchuensis, while the incidence of disease was significantly reduced in the susceptible C. ‘Nicky Crisp’. Therefore, it was hypothesised that early ROS accumulation contributes to CFB resistance and that late ROS accumulation contributes to CFB susceptibility. This work further expands knowledge of plant interactions with necrotrophic fungal pathogens from the Sclerotiniaceae family by demonstrating that ROS both positively and negatively regulates CFB development based on temporal accumulation, thereby discovering a dual role for ROS accumulation during this interaction.