You cannot be much closer to extinction than the swamp helmet orchid (Corybas carsei), a tiny terrestrial orchid that is found in a single wetland in the North Island of Aotearoa New Zealand. Fortunately, recently published studies, part of Te Papa/VUW student Jennifer Alderton-Moss’s thesis, are helping to understand how mycorrhizal fungi can be used to save one of our most threatened orchids. Jennifer Alderton-Moss and Botany Curator Carlos Lehnebach describe the work.

The swamp helmet orchid is currently known from a single population with less than 500 individuals and is classified as Threatened – Nationally Critical. Although other populations were previously known around the North Island, draining and conversion of their wetland habitat has wiped them out.

Now, funding from the Department of Conservation and collaboration of scientists from Te Papa, Ōtari-Wilton’s Bush, and Victoria University of Wellington, are helping to improve the chances of survival of this species.

Desperately seeking saprotrophs: the love affair between orchids and decomposing fungi

All orchids form relationships with fungi. Orchid seeds are small – so small that they are not able to germinate without getting some external nutrition. This is where fungi come in – they can provide carbon and other nutrients to orchid seeds and stimulate germination.

Not just any fungi will do – they typically associate with the saprotrophic Rhizoctonia-like fungi. This group, which includes fungi in the families Ceratobasidiaceae, Sebacinaceae, Serendipitaceae, and Tulasnellaceae, are unified by their feeding habits (they all get their energy by decomposing plant material) and appearance. Some orchids can be incredibly choosy – forming partnerships with a single species of fungus.

The fungus, which is also capable of free-living, is hosted inside the orchid root where it forms coil-like structures called pelotons. Fungi can be grown in the lab by extracting these pelotons from inside the root cells and placing them on special nutrient media.

Understanding this relationship can really help with the management of orchids! If you can grow the fungal partner of an orchid, it can be introduced to seeds and promote their growth. Seedlings can then form ex situ, or offsite collections or be returned to support wild populations.

Former Te Papa postgraduate student, Jonathan Frericks, isolated a Tulasnella fungus from the Aotearoa New Zealand wetland orchid Spiranthes australis and showed that it could promote germination and seedling development. After only two years these plants were mature enough to flower, and one of the plants now forms part of the collection at Ōtari Native Botanic Garden!

Inspired by this success, we have been aiming to understand which fungi form connections with Corybas carsei and use them to germinate seeds.

Corybas carsei and Tulasnella: a match made in… Wilton?

In a recently published study, we isolated and grew fungi from Corybas carsei. One surprising result was that, although we cultured 18 fungi, none of these were the Rhizoctonia-like fungi that typically associate with orchids, including many other Corybas species.

For instance, at the Lions Ōtari Plant conservation laboratory in Wilton (Wellington), we have recently isolated Rhizoctonia-like fungi from Corybas cheesemanii, C. dienemus, C. hatchii, C. iridescens and C. oblongus.

Under lab conditions we have seen that these fungi are able to support germination and – although it is still early – we are confident we have formed some successful partnerships as illustrated below.

So, what’s up with the fussy swamp helmet orchid?

To understand what’s happening with C. carsei, we first tried introducing the non-Rhizoctonia fungi that we cultured to seeds. These were not a match. Six months passed with no signs of life from the seeds, so we switched our approach. We are now testing if there is a match between Rhizoctonia-like fungi isolated from other Corybas species with seeds of C. carsei.

Amazingly, we are seeing some of the early stages of germination with a Tulasnella fungus!

This Tulasnella fungus, found in orchids growing in Wellington, may not naturally encounter Corybas carsei, but has shown a remarkable ability to form a partnership when we bring them together.

Although the germination process is just beginning, it is the first step towards Corybas carsei plants that can improve conservation outcomes.

Happy ever after? – Future planning  

If we are successful in propagating the swamp helmet orchids from seeds, we will be able to support the only wild population of this orchid.

This may involve taking seedlings grown in the laboratory/nursery back to the wetland, or direct sowing of seed and fungi. The latter approach is something we are currently trialling with our threatened epiphytic orchid, Drymoanthus flavus.

This may be more difficult with Corybas carsei – one concern is that the fungus they are currently germinating with did not come from the same part of the country as Corybas carsei – what are the risks of introducing a new fungus to the Corybas carsei site?

While conservation of Corybas carsei is an ongoing process, by understanding their love affair with fungi, we are in a better place than ever to save this species.