The Symbiotic Secret
In the woodlands of the Mount Lofty Ranges, a hidden relationship supports the survival of some of the region's most beautiful and rare terrestrial orchids. Unlike most plants, orchid seeds are microscopic, like dust, and contain virtually no food reserves (endosperm). To germinate, they must form a symbiotic, and initially parasitic, relationship with a specific type of soil fungus. This relationship is called a mycorrhiza. The fungus infects the orchid seed, and the developing plantlet, called a protocorm, draws all of its required carbon and nutrients directly from the fungal hyphae.
The fungi involved are often from the genus Tulasnella. Many orchid species, such as those in the Caladenia genus (spider orchids), have evolved to rely on just one or a very small number of fungal species. This specificity is a high-stakes evolutionary risk. The orchid can only grow where its fungal partner is present in the soil. DNA sequencing is now the primary tool for identifying these cryptic fungi, as many are difficult or impossible to identify through physical characteristics alone. In the Mount Lofty Ranges, this specificity is a conservation challenge: the orchid might be rare, but its fungal partner can be even rarer.
A Conservation Conundrum
The extreme dependence of orchids on their fungal partners makes conservation uniquely complex. An area might seem like a perfect habitat for a threatened orchid, but if the specific mycorrhizal fungus is absent, any effort to reintroduce the orchid will fail. In the Mount Lofty Ranges, habitat fragmentation has reduced populations of both orchids and fungi, creating a precarious situation for species like the Pink-lipped Spider-orchid (Caladenia behrii) and the Bayonet Spider-orchid (Caladenia gladiolata).
To combat this, conservation scientists at institutions like the South Australian Seed Conservation Centre are pioneering propagation techniques. The process, called in vitro symbiotic germination, is painstaking. First, the specific fungus must be isolated from the roots of a wild orchid and cultured in a sterile laboratory environment. Then, orchid seeds are introduced to the fungus on a nutrient agar. If the pairing is successful, the seeds germinate and develop into small seedlings over several months or even years. These lab-grown orchids can then be reintroduced to protected sites in the wild where the fungus is known to exist. This work, supported by programs like the "Back from the Brink" project, has led to the reintroduction of hundreds of plants, pulling some species away from the immediate threat of extinction.