Screening of effector proteins from the Kauri dieback pathogen Phytophthora agathidicida : a thesis presented in partial fulfilment of the requirements for the degree of Master of Science (MSc) in Biological Sciences at Massey University, Manawatū, New Zealand
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2024
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Massey University
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Phytophthora agathidicida is the causative agent of kauri dieback, a destructive disease threatening kauri that are endemic to the Northernmost regions of New Zealand. Relatively little is understood of this pathogen and its interaction with kauri at the molecular level. However, advances in the understanding of other Phytophthora pathogens, and the completion of a chromosome-level P. agathidicida genome sequence, now allow for this interaction to be studied in greater detail. Core components of any pathogen-plant pathosystem are effectors, which are proteins released by the pathogen during infection that promote virulence. A well characterised type of effector from Phytophthora are the glycoside hydrolases (GHs), a broad group of extracellular effectors that include XEG1 from the soybean pathogen Phytophthora sojae. In the model host Nicotiana benthamiana, XEG1 is recognised by an extracellular, membrane-bound receptor-like protein (RLP) that associates with the co-receptor SOBIR1 to begin the signalling cascade needed for plant immunity. In line with this, the presence of SOBIR1 strongly impedes the ability of P. agathidicida to grow on N. benthamiana. However, it is unclear what role RLPs may have in the immunity of kauri against kauri dieback disease via recognition of the XEG1 homolog from P. agathidicida. In this study, we aimed to characterise the role of XEG1 and a novel kauri dieback effector, Pa8011, to determine if the immunity observed in N. benthamiana could also be found in kauri and how this influences the host-pathogen interaction. Kauri from families with differing levels of tolerance to kauri dieback were kindly provided by Te Roroa, with the requirement that XEG1 testing in kauri leaves was performed using purified protein in place of transient expression used in N. benthamiana. Experiments were first conducted in N. benthamiana to confirm that the P. agathidicida XEG1 protein elicits a plant defence response in the form of localised cell death and that this response depends on the SOBIR1 co-receptor. The purified proteins were then infiltrated into kauri leaves where neither XEG1 or Pa8011 caused localised cell death, despite the response seen with XEG1 in N. benthamiana. To assess how the effectors influence the ability of P. agathidicida to infect kauri, protein-infiltrated leaf tissue was then inoculated with the pathogen, but neither effector caused a significant increase or decrease in the size of pathogen lesions formed. The results obtained indicate a lack of XEG1/Pa8011-specific RLP immune receptors in kauri and opens up new questions about what form this system takes in kauri and how comparable it is to the model host.
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Figure 1.2 is reproduced under a Creative Commons CC BY license.