In the early spring of 1928, an Australian farmer named Jack Trott was plowing his land in preparation for the upcoming growing season. As he glanced backward, he noticed pale like flower structures being tossed into the air. He stopped his tractor to examine the specimens and found these amazing little plants, with no green pigments at all. Ahead of the tractor, he walked on the cracked, dry soil surface. He started to smell a sweet fragrance and as he moved closer to the soil’s surface, the intensity of the smell increased. He removed the top layers of the dry earth, and to his astonishment found these flowers in a really high density. Jack had found the first subterranean flowering plant.
Flowers are like billboards that say, “Look here! Sweet nectar!” These showy sexual organs this widely successful plant lineage produce, entice a diverse array of pollinators to come and suck the sweet sugary solution these plants synthesize. In return, pollen, the male gametophyte in the plant’s life cycle, gets a free ride to another individual with a female gametophyte waiting to be fertilized. Thanks to pollinators like insects, birds and mammals, flowering plants in a relatively short time have completely taken over every ecosystem Earth has to offer. With this in mind, one might ponder a bit and question how good is an underground billboard? What’s the point of a showy flower if it remains hidden beneath the soils surface? Without knowing what he was looking at, Jack brought some of these unearthed specimens to universities in Western Australia where botanists studied the plant.
These plant specialists even before the use of genetic sequencing confirmed that this plant was actually an orchid. This plants physiology is awesome to say the least. Remember, the vast majority of plants fix carbon into sugars through photosynthesis. Green pigments absorb incoming solar radiation and this light energy becomes utilized in the first series of reactions the plant carries out. So, when you do indeed find a pale looking plant without green pigments, you know that it’s not acquiring energy like most plants. And this is where our fungus comes in.
This rare orchid is a myco-heterotroph, which is even a more unique form of parasitism only a handful of plants carry out. Many plant parasites that receive some or all of their energy from other organisms do so through the parasitism of plants. They have specialized structures known as haustoria, tentacle-like structures that penetrate and suck both sugar and water from their host plant. Rhizanthella gardneri and other myco-heterotrophs actually parasitize fungi. This plant has a unique ecology involving a relationship that three organisms are involved in.
Interestingly, Rhizanthella gardneri is still receiving sugars from a specific plant, but this time it is indirectly doing so. A shrub called broombush (Melaleuca uncinate) is never too far away from patches of this rare orchid. Broombrush is a plant that requires a fungal symbiont to find rare soil nutrients in this ecologically demanding region of the world. This plant pairs with more than one type of fungus to tap into the rare supplies of limiting resources. Thanatephorus gardneri and certain Ceratobasidium species are mycorrhizal fungi that have been isolated from both broombush and Rhizanthella gardneri roots. So even though this orchid was found more than 90 years ago we are just now uncovering how it functions.
This tripartite ecology is quite fascinating and we can thank researchers Jeremy Bougoure, Mark Brundrett and Pauline Grierson for their work uncovering the underlying biology of this amazing plant. In a trophic dynamic study, they radiolabeled carbon dioxide pumping a known amount of this labeled Co2 directly into leaf surfaces. As the broombush photosynthesized, it fixed this radiolabeled carbon into sugar and that sugar could then be traced throughout the plant and other organisms living in the rhizosphere. A radiolabeled amino acid (13C-15N glycine) was then fed to the mycorrhizal fungus, in this caseCeratobasidium species. By tracing these radiolabeled substances through biological structures, this study revealed that the shrub sends sugars down to its fungal symbiote, where the orchid then steals carbon and nitrogen from the fungus.
Tripartite relationships are insanely cool, but many times, these three species don’t align as they have slightly different niche requirements. This is a bit of a problem. On the other hand, a hardy plant species with no known symbiote depends solely on itself. Here, Rhizanthella gardneri needs both an autotrophic shrub that is colonized by a compatible mycorrhizal fungus for this critically endangered plant to successfully reproduce. The study mentioned above also found that the plant does sequester nutrients directly from the soil, but the plant simply cannot do it alone.
With only six known populations, this orchid is critically endangered. It really is a fascinating plant that escapes the extreme heat present in Western Australia by having its subterranean ecology. Termites and gnats have no problem following the fragrances escaping soil cracks which lead to these underground flower chambers. But its seed dispersal proposes another limitation. Another explanation for its low abundance is that its marsupial seed dispersers are being replaced by invasive placental mammals from other parts of the world. These invasive mammals compete, and reduce the numbers of the native mammals that could potentially disperse this amazing orchid’s seeds. All in all, a ton of interactions must go right for the success of this species.