Metagenomic characterisation of AMF and other mycorrhizal communities associated with healthy versus diseased Agathis australis (kauri) roots.

Abstract

Agathis australis (kauri) is both an ecologically and culturally important (taonga) species in New Zealand and the only endemic member of the Araucariaceae found in New Zealand. Arbuscular mycorrhizal fungi inhabit A. australis roots and root nodules. Arbuscular mycorrhizal fungi (AMF) are obligate, biotrophic organisms that form symbiotic associations with the roots of more than 80% of all land plants. They perform a range of beneficial functions for host plants such as enhanced nutrient access, stress tolerance, and pathogen resistance. Currently, a pathogen of A. australis, Phytophthora agathidicida (once known as Phytophthora ‘taxon Agathis’, PTA), is destroying A. australis populations over parts of its range in the northern part of New Zealand. Recent advances in next generation sequencing have established that host roots contain complex communities of AMF species, but it is unclear whether these communities vary spatially and/or temporally, or the extent of host specificity of AMF. Although the community of AMF occurring in A. australis has been characterised previously at one site, I aimed to determine AMF community similarity between distant sites and across seasons, and seek evidence of host specificity of AMF in A. australis roots. I also sought to determine the effect of P. agathidicida on A. australis root AMF communities. Root samples were collected from the Hunua ranges (asymptomatic site) and the Waitakere ranges (symptomatic site). Root staining techniques confirmed the presence of AMF within A. australis roots and their distribution within the roots. Putative P. agathidicida oospores and Glomus spores were also observed for the first time in field-collected roots from symptomatic A. australis trees. Fluorescence in situ hybridisation (FISH) assay confirmed the presence of P. agathidicida within the roots. Next generation 454 sequencing was used to characterise root AMF communities and other non-AMF endophytes. Non-metric multidimensional scaling (nMDS) plots showed statistically significant differences in biogeographical and seasonal patterns within the AMF communities as well as differences in community structure between symptomatic and asymptomatic trees. ITS region showed a number of dark septate endophytes within A. australis roots. Future studies should investigate further the possible interactions of DSE and P. agathidicida within A. australis.