No need for orchids: mycorrhizal fungi do not depend on their hosts in Australia

Did you know that there are almost 28,500 species of orchids around the world? And did you know that orchids rely on their symbiotic mycorrhizal fungi for nutrition? This incredible diversity of host plants and their symbiotic partners raise many questions about their co-evolution and dependency on each other.

Australian scientists teamed up to explore the distribution and co-occurrences of over 500 mycorrhizal fungi samples (belonging to the Ceratobasidium genus) isolated from 111 Australian orchid species. These fungi were scattered around the continent, associated with particular orchid species but could also be found where their host plants did not grow. 

The lead researcher is Marc Freestone, a PhD student at the Australian National University in Dr Celeste Linde’s research group. Freestone is an orchid expert who is extremely passionate about saving the endangered leek orchids in Australia by understanding the fungi-plant relationships.

Between 2007 and 2016, the researchers isolated 217 strains of mycorrhizal fungi belonging to the Ceratobasidium genus from 70 orchid species from 74 sites in Southern Australia. The scientists sequenced the isolates’ DNA and added 311 earlier sequences from 55 orchid species to their taxonomic analyses. 

Whilst the data mentioned above came from directly isolating mycorrhizal fungi from orchids, Freestone and colleagues also wanted to check the presence of these fungi in soil samples. The Australian Microbiome project is a publicly accessible database of microbial diversity across the continent, including soil samples from 902 locations. These samples were analysed for the presence of Ceratobasidium species. 

Based upon the distribution of the mycorrhizal fungi, the researchers used a species distribution model to investigate if 35 environmental variables (e.g., temperature, rainfall, soil moisture) could explain the distribution of a few fungi across the continent. 

To double-check if these mycorrhizal fungi aid orchid germination, Freestone and colleagues set up germination trials for 89 isolates and used previous germination data for other strains.

The mycorrhizal fungi formed 23 groups (called Operational Taxonomic Units; OTUs) based on their genetic diversity. The botanists found these groups associated with particular orchid species and nine groups both on the eastern and western sides of Australia. According to the germination trials, 16 out of the 17 groups were confirmed to be mycorrhizal. 

The species distribution modelling showed that the mean soil moisture index and rainfall explained fungi occurrences and highlighted where different groups could be found around Australia. 

When considering where the fungi and their host could be found, the scientists found that the fungi were often in places where the orchids were not. 

“The presence of over half of our OTUs in a continent-wide soil sequencing project suggests that the Ceratobasidium fungi identified in this study are not constrained in their distribution by their host orchids”, Freestone and colleagues wrote. 

The fact that most fungi groups were associated with specific orchid species is a sign of co-evolution. For example, the fungi group, OTU C, associated exclusively with three species of underground orchids, Rhizanthella, whilst the group OTU O were associated with Pterostylis orchid species. Despite their associations, the fungi had wider distributions than their partners’. 

“[T]he distribution of OTU O extends well beyond the range occupied by its host orchids, into generally more arid, inland parts of the continent and into northern Australia, where its host orchids do not occur,” the researchers explained. 

Whilst Freestone and colleagues only had enough occurrence data to model the distribution of two fungi groups, future studies could estimate the distribution of other mycorrhizal fungi. The researchers also found a few interesting fungi species known to cause disease in plants. For example, the OTU S fungi group is weakly pathogenic on strawberry seedlings. 

“[W]e confirmed that Ceratobasidium OMF [orchid mycorrhizal fungi] detected in orchids are closely related to pathogenic [disease-causing] taxa, and there are some tantalising potential applications of a better understanding of the mechanisms by which orchids harness these near-pathogenic fungi for their benefit.”

Previous research focused on a single orchid or mycorrhizal fungus species, but this study paves the way for future studies to understand orchid-fungi relationships better.