Hungry for fungi: green mycoheterotrophic plants are far more common than previously thought

If you’ve been led to believe your entire life that plants are always green and only get their food from photosynthesis, you might be in for a shock. Turns out lots of plants get their nutrition in weird and wonderful ways, from the carnivorous plants which consume animal prey, to the parasitic plants which steal nutrients from other plants, to the mycoheterotrophic plants whose roots form a symbiotic relationship with fungi belowground. These kinds of plants are often thought of as rare or unusual oddities, but in a recent paper by Giesemann and colleagues have found that, actually, there are way more mycoheterotrophic plants out there than we’ve ever thought before. They say that “prior knowledge on mycoheterotrophy appears to just graze the surface of something much bigger…”

Before we get into the details, it’s important to know that mycoheterotrophic plants can have varying levels of reliance on their fungal partners for nutrition. Some plants are fully reliant on mycoheterotrophy: these plants are achlorophyllous, which means they don’t produce the green pigment chlorophyll and therefore cannot photosynthesise. If you’ve ever seen a ghostly completely-white-leafed plant, then you’ve likely encountered a fully mycoheterotrophic plant. On the other hand, some green (chlorophyll-producing) plants can be a bit greedy: they can get nutrition both from photosynthesis AND from their fungal friends, making them “partially mycoheterotrophic”. Partial mycoheterotrophy is common within the Orchidaceae and Ericaceae with their orchid- and ericoid-mycorrhiza, but, until now, had only been recorded for very few species with arbuscular mycorrhiza.

Giesemann and colleagues suspected there may be many more partially mycoheterotrophic plants than we’ve been led to believe. They compiled published data on the stable isotope natural abundances of more than 130 plant species to figure out whether any of them were gaining nutrition from their fungal partners. Giesemann explained: “A plant’s uptake of fungal nutrients, such as carbon, can be revealed by a significant isotopic enrichment in ¹³C compared with ‘normal’ photosynthetic plants. In recent years, more and more plants have been identified as ‘partial mycoheterotrophs’ because their stable isotope composition values lie between non-mycoheterotrophic neighboring plants and full mycoheterotrophs.”

Shedding new light on the nutrition of a huge number of green-leaved plants, the authors demonstrated partial mycoheterotrophy in most of the pteridophytes they studied (i.e. ferns and horsetails) as well as in ten families of seed plants where mycoheterotrophy had not been recorded, or even suspected, previously: Apiaceae, Asphodelaceae, Dioscoreaceae, Euphorbiaceae, Geraniaceae, Oxalidaceae, Poaceae, Primulaceae, Rosaceae, and Smilacaceae. Add onto this list the two species and families that the authors found evidence of mycoheterotrophy for in another recent paper (Paris quadrifolia in Melanthiaceae and Anemone nemorosa in Ranunculaceae), and it turns out we’ve got a LOT of partially mycoheterotrophic green plants.

For the fungi enthusiasts out there, one of Giesemann and colleague’s key findings relates to the morphological structure of the fungi partnered with these partially mycoheterotrophic plants. Arbuscular mycorrhiza usually take one of two structural forms: Paris-type or Arum-type, each originally named after the fungal structures found within the root cells of Paris quadrifolia and Arum maculatum respectively. “In the Arum-morphotype, the hyphae grow along the intercellular spaces between root cells and form branching structures called arbuscules within the root cells,” Giesemann explained. “By contrast, hyphae of the Paris-morphotype grow in dense coils within root cells and rarely appear in the intercellular spaces.”

All of the plants found to be partially mycoheterotrophic in their study were associated with Paris-type fungi, which has huge implications for our understanding of plant nutrition. “We think this coiling structure within root cells may be an important prerequisite for the evolution of mycoheterotrophy,” Giesemann said. “About 85 % of all terrestrial plants form symbiotic associations with arbuscular mycorrhizal fungi, and about half of these have the Paris-type structure. We’ve found that significant nutrient acquisition from fungi is quite common and widespread amongst plants associated with the Paris-type fungi, so this could mean that a lot more plants may be partially mycoheterotrophic than we ever thought possible previously.”