Chemotaxis and inhibition of the kauri killer, Phytophthora agathidicida

Abstract

Phytophthora are devastating pathogens which cause diseases in thousands of economically and ecologically important plants worldwide. Often referred to as 'fungus-like', Phytophthora are a genus of eukaryotic oomycetes. There are over 100 species of Phytophthora and all are pathogenic to plants. Phytophthora agathidicida is one species which is of great concern here in New Zealand, as it infects native Agathis australis (kauri) causing kauri dieback disease.

Kauri forests have been greatly reduced over the past 200 years, resulting in low and fragmented genetic diversity of kauri. The remaining kauri are now under serious threat from P. agathidicida and could be extinct if intervention is not taken. Phytophthora have several life cycle stages, some of which are not found in most true fungi. In addition to a mycelial growth phase (similar to fungi), Phytophthora produce zoospores, which are key to the epidemic spread of disease, facilitating host-to-host transmission. In this thesis, a variety of approaches have been used to explore the different lifecycle stages of P. agathidicida. An improved method for zoospore production was developed, which facilitated further study of this emerging pathogen.

The chemotactic behaviour of P. agathidicida zoospores was studied for the first time in the research described in this thesis. Initial chemotaxis assays showed P. agathidicida zoospores were attracted to a variety of amino acids and sugars. P. agathidicida also exhibited increased chemotaxis to amino acids at pH 3.0, a pH level similar to the soil conditions around mature kauri.

Currently there is no cure for kauri dieback; therefore a chemical treatment for P. agathidicida is vital for the survival of kauri. In the research conducted in this thesis a high throughput screen was developed for anti-oomycete compounds as a potential new tool for finding a cure to kauri dieback. Over a hundred compounds were efficiently screened for mycelium inhibition and any resulting compounds were then tested on multiple P. agathidicida life cycle stages, including zoospore motility and germination. Copper salts and a quaternary ammonium salt were the most effective across all three life stages, indicating their potential application to help manage the disease. Overall, the improved methodologies, and fundamental insights from this work will help to inform further efforts to control the disease.