Identification and functional characterisation of glycoside hydrolases from the kauri dieback pathogen, Phytophthora agathidicida : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy (PhD) in Plant Science at Massey University, Manawatū, New Zealand. EMBARGOED until 4 October 2024.
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Date
2022
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Massey University
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Abstract
The survival of kauri, an ancient conifer species endemic to New Zealand, is currently threatened by
kauri dieback disease, caused by the oomycete plant pathogen Phytophthora agathidicida. As P.
agathidicida continues to spread throughout kauri forests in the northern North Island of New Zealand,
encouraging research has indicated there may be natural tolerance to the disease within the kauri
population. This resistance is likely governed, in part, by the plant immune system, which is activated
upon recognition of pathogen invasion patterns such as microbe-associated molecular patterns
(MAMPs), damage-associated molecular patterns (DAMPs), and effectors (virulence factors required
for host colonisation), which are recognised at the plant cell surface by plant immune receptors. To
better understand how P. agathidicida interacts with its plant host on a molecular level, pathogenproduced proteinaceous invasion patterns need to be identified and characterized to aid in the
identification of cognate immune receptors in the plant host, which may be involved in activation of the
plant immune system. As the role of glycoside hydrolase (GH) proteins in virulence and pathogenicity
of fungal and oomycete plant pathogens is well established (Chapter two), an effectoromics approach
was used to identify six P. agathidicida GH12 proteins that appear to act as MAMPs in both Nicotiana
benthamiana and Nicotiana tabacum (Chapter three). Furthermore, nuclear magnetic resonance was
used to identify considerable changes in kauri leaf apoplastic wash fluid of approximately 17
metabolites, including sucrose and glucose, in response to P. agathidicida inoculation (Chapter four),
thus suggesting a role for GH proteins in the hydrolysis of some of these metabolites. Finally, proteomic
analysis of P. agathidicida culture filtrates via liquid chromatography-mass spectrometry (LC–MS) was
used to validate the expression of predicted P. agathidicida proteins and to investigate the capacity of
this method to identify candidate invasion patterns for future analysis. Chapter five established that
LC–MS analysis of Phytophthora culture filtrate was an effective method for the identification of
putative apoplastic invasion pattern candidates and confirmed the production of all six P. agathidicida
GH12 cell death elicitors in culture. Collectively, this thesis has advanced our understanding of the
molecular mechanisms underpinning the interaction of P. agathidicida with its host and has contributed
to the identification of candidate apoplastic effectors.
Description
Embargoed until 4 October 2024
Listed in 2023 Dean's List of Exceptional Theses
Listed in 2023 Dean's List of Exceptional Theses
Keywords
Kauri, Diseases and pests, Dieback, Phytophthora diseases, New Zealand, Glycosidases, Dean's List of Exceptional Theses