Ally, adversary or something else : do co-occurring Phytophthora pathogens influence each other in culture? : a thesis presented in partial fulfilment of the requirements for the degree of Master of Science (MSc) in Biological Science at Massey University, Manawatu, New Zealand. EMBARGOED until 21st August 2025
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Date
2023
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Massey University
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Abstract
Kauri (Agathis australis), a New Zealand native and taonga tree species, is currently under threat from the devasting disease, kauri dieback, caused by the oomycete Phytophthora agathidicida. Recent research highlighted that, along with P. agathidicida, two other Phytophthora species, Phytophthora cinnamomi and Phytophthora multivora, can co-occur in the soil surrounding kauri trees, and that both P. cinnamomi and P. multivora cause lesions on kauri seedlings in glasshouse trials. Whether all three Phytophthora species interact, whether cooperatively or antagonistically, remains unknown. Such information is needed because it may mean that kauri dieback disease control strategies need to target all three species. In an attempt to gain such information, macroscopic inter- and intra-species colony interaction assays were first carried out on solid growth media; however all colonies formed a zone of growth inhibition around themselves irrespective of the media type or species tested. However, it was anticipated that interactions between the three species could occur at the molecular level through secreted proteins, a proteomic analysis was then carried out to determine how the repertoire of secreted proteins changed when multiple species were grown together in liquid culture, relative to grown alone. In total, 20 putatively secreted proteins, termed effector candidates (most of which had predicted enzymatic functions), were identified that were only produced, or that significantly increased in abundance, during co-culturing. All but one of these were from P. agathidicida and most were encoded by genes that were upregulated during infection of kauri leaves and/or roots, suggesting that they may play an important role during host colonisation. As a starting point for future experiments aiming to investigate Phytophthora–Phytophthora as well as Phytophthora–candidate effector interactions in planta, each Phytophthora species was tested for its ability to infect the model host plant, Nicotiana benthamiana. This test revealed large differences in the consistency and rate of infection between the Phytophthora species, meaning that interaction assays in this host would likely be difficult. But interestingly, P. agathidicida and P. multivora more readily infected plants lacking SOBIR1, a major component of extracellular immunity-related signalling, suggesting that extracellular immunity plays a significant role in slowing infection of N. benthamiana by these species. Taken together, the results of this study have considerably advanced our understanding of how Phytophthora species interact at the molecular level and provide a solid foundation for determining whether Phytophthora species work together to cause kauri dieback disease.
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Embargoed until 21st August 2025