RA3 REPORT: Effect of soil pH on Phytophthora ...

Investigating the growth, reproduction and survival of Phytophthora agathidicida under different soil pH

2033

Davies B. 2023. Investigating the growth, reproduction and survival of Phytophthora agathidicida under different soil pH. Unpublished MSc Report, Massey University.

NOTE

This Master's report is not publicly available online. For further information please contact supervisor, Dr. Nari Williams at Plant and Food Research: Nari.Williams@plantandfood.co.nz

ABSTRACT

In order to better understand the biosystem around kauri and P. agathidicida, this research focuses on elucidating environmental and genetic parameters that affect P. agathidicida, particularly in connection to the poorly studied aspect of pH. In this study, the pH levels at two P. agathidicida-containing kauriland sites were surveyed and a soil pH range of 3.42 – 7.02 detected, with site-specific averages of 4.36 (Ngāti Rua) and 4.96 (Waitākere Ranges). Detection of P. agathidicida was limited to the lower 89% of the total pH range, at 3.42 – 6.64, but this was not statistically significant. Another species Phytophthora cinnamomi was also surveyed, and contrastingly found to be present in the upper 88% of total pH range, at 3.84 – 7.02. This was a statistically significant result, and suggests that the two Phytophthora species have different pH limitations. The presence of P. agathidicida in these soils appeared to be driven by factors other than pH, with P. agathidicida-containing soils at an average pH of 5.49 for the Waitākere Ranges and an average of 4.52 for Ngāti Rua. When looking at growth parameters, rather than presence alone, P. agathidicida was able to grow in minimal media across the assayed range of pH 5.1 – 6.9, but displayed significantly higher optical density (OD) increases in two distinct peaks of pH 5.1 and 6.8. Looking at sporulation, no pH-dependency was seen across an assayed range of 3.5 – 8, though formation of immature sporangia, mature sporangia and oospores all had significant isolate-dependent variation. One further step in creating a deeper understanding of this biosystem lies in exploring the genetic elements of P. agathidicida, through pathogenicity-related genes from other genetically similar microorganisms. While pH-modulated pathogenicity genes are of particular interest, it would be remiss to limit exploration to this spectrum, hence this delimitation was not applied. From a total of 22 pathogenicity-related genes in selected from other Phytophthora spp., 18 potentially orthologous coding sequences were identified in the P. agathidicida genome, five of which showed differential expression during infection. Three of these five putative proteins had upregulation during infection, and likely correspond to an aspartyl protease (AP), an NLP effector, and a PDR-like ABC transporter. Additionally, five pathogenicity-related genes from non-Phytophthora microorganisms, with pH-modulated behaviour, were investigated. Four of these proteins corresponded to P. agathidicida coding sequences, although with high divergence. One of these sequences, which may be distantly related to the fungal pH-responsive transcription factor PacC, showed a significant increase in expression during infection. It is hoped that the findings in this report will increase the understanding of pH parameters in P. agathidicida growth, and provide direction for further research prospects within the host-pathogen relationship responsible for kauri dieback.