Characterising broad acting resistance to bacterial canker of cherry and elucidating tissue specific mechanisms of immunity

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Bacterial canker is a major disease of cherry globally. This perennial tree disease is highly complex, caused by several distinct Pseudomonas pathogens that occupy different tissues throughout the season. Almost nothing is known about the genetics of resistance to bacterial canker, despite the key role breeding could play in limiting this disease. Past and ongoing research has shown that broad resistance to all pathogens is exhibited by cherry cultivars such as Merton Glory and Colney. This resistance is predicted to be quantitative and likely involves a range of traits that exhibit differential expression between tissue types. This PhD would focus on characterising the genetic basis of resistance by genotyping and phenotyping wild and species-level cherry populations propagated at East Malling, with the aim to identify novel sources of resistance associated with canker resistance. This would lead to the development of markers for broad resistance, which would be highly useful in breeding programs and have already been implemented for other major diseases, such as fireblight in apple. In addition, the student would begin to study the biological basis of resistance responses in different host tissues and optimise screening assays for phenotyping. The relative importance of different tissue types in host resistance is unknown, and very little is understood about the immune responses that occur in woody tissues where the main disease occurs. Ongoing work on bacterial canker has shown that different tissues vary in their susceptibility to the different clades of Pseudomonas. Resistance seen in the field is not replicated fully in leaf and fruit assays in the laboratory. This has highlighted the importance of understanding the underlying mechanisms of resistance, particularly in woody tissues. The student will optimise assays to understand immune responses e.g. measurement of reactive oxygen species and study immune gene expression. They will use microscopy of fluorescently tagged strains to follow infection in resistant and susceptible plants and characterise the key differences in host response. The tools produced in this PhD will aid the development of rapid and accurate resistance screening as well as improve our understanding tissue-specific resistance responses to tree fruit pathogens generally.