Spider orchid conservation: the importance of serendipitous fungi

Did you know that if you would like to germinate orchids from seeds, you can increase your chances by using some mycorrhizal fungi?  As orchids lack true roots, mycorrhizal fungi are their main source of nutrition during germination and throughout their development. 

Out of 327 species of spider orchids from genus Caladenia, 71 are nationally threatened in Australia. A total of 58 Caladenia species belong to subgenus Calonema which have been associated with mycorrhizal fungi from the genus Serendipita. Considering the distribution and effect on orchid propagation of specific mycorrhizal fungi before the reintroducing endangered orchids back into the wild could lead to more successful orchid conservation programs (example). 

Dr Noushka Reiter at the Royal Botanic Gardens Victoria (Australia) and colleagues isolated and sequenced mycorrhizal fungi, Serendipita spp. from 135 spider orchid species. The researchers found that most of the endangered orchid species were associated with one operational taxonomic unit (OTU) of Serendipita spp. whilst a few orchid species associated with specific OTUs. Germination trials showed that using fungi isolates from different species and sites is possible and so, the potential translocation sites are not limited by the widely distributed mycorrhizal fungi. Dr Reiter previously investigated the pollination of an endangered Caladenia orchid and discussed the importance of considering the distribution of pollinators, use of symbiotic organisms for orchid propagation and reintroductions in Australia. 

In the most recent study, the scientists asked:

1. How specific are the Serendipita mycorrhizal associations of threatened species of Caladenia subgenus Calonema?
2. Do threatened Caladenia from subgenus Calonema share Serendipita orchid mycorrhizal fungi with common Caladenia?
3. How geographically widespread are the Serendipita mycorrhizal fungi associated with Caladenia?

Dr Reiter and colleagues investigated the specificity of mycorrhizal fungi from threatened Caladenia species and common species at different sites. The research team sampled 356 plants from 127 Caladenia species and after combining previous data from GenBank, they analysed 597 Serendipita samples from 135 orchid species. They produced a phylogenetic tree of the mycorrhizal fungi and mapped their geographical distribution. For three types of germination trials, the researchers collected seeds from each orchid species and tested propagation efficiency of different combinations of Serendipita operational taxonomic units (OTUs) from threatened and common orchid species and from the same or different sites.

In total, 10 OTUs of Serendipita spp. associated with the spider orchids and different orchid subgenera strongly associated with specific types of OTUs. All orchid species in genus Caladenia propagated when the same OTUs from the same species and same sites were used. The main OTUs, called “A”, also helped the germination of common and threatened Caladenia species until the adult stage. The OTU “A” was geographically widespread around Australia whilst some OTUs were specific to Tasmania and Western Australia. 

“For the majority of species in Caladenia subgenus Calonema fungal associations involve just a single OTU, meaning that these orchids are reliant on a particular fungus for maintaining populations and establishing at a site”, the researchers revealed.

“Such high specificity could potentially constrain the availability of suitable colonisation sites, though at least if orchids are grown symbiotically, they have the potential to inoculate the site with suitable fungi following planting of the orchid. However, the combination of high specificity and sharing in subgenus Calonema is important from a conservation perspective. For our target threatened species, fungal isolates for symbiotic propagation, for both ex situ conservation and conservation translocations, can now be sourced from other orchid species”, they added.

“This means that undertaking conservation translocations outside an orchid’s geographic range are possible, a potentially important option given the small geographic range of some Caladenia species and the shifts in environmental suitability in southern Australia already occurring under climate change”, they concluded. 

Besides climate change and habitat destruction, commercial harvesting and illegal trade are the biggest threats of wild orchids. Between 1996-2015, over 1.1 billion live orchid plants were in trade. Whilst orchids are the best-protected plant taxa globally, 99.5% of a recently discovered Vietnamese species, Paphiopedilum canhii, was quickly commercially harvested for its high value. The study by Dr Reiker and colleagues revealed how investigating the role of mycorrhizal fungi, sampling across Australia and using germination trials can help conservationists to better protect these unique orchids.