mycoheterotroph

mycoheterotroph, plant that relies on fungal networks for either all or some of its nutrition. Mycoheterotrophic plants may or may not photosynthesize. They form a one-sided, nonreciprocal relationship with fungi, extracting carbon-based nutrients from their fungal partners without giving anything in return.

Mycoheterotrophic taxa

Almost all major groups of land plants—including liverworts, hornworts, mosses, lycophytes, ferns, gymnosperms, and angiosperms—contain at least one mycoheterotrophic species. Among them, about 550 plant species qualify as full, or obligate, mycoheterotrophs, meaning they have lost their ability to photosynthesize and depend entirely on their fungal partners for nutrition.

The number of partial mycoheterotrophic plants is even greater. Also called “mixotrophs,” these plants have the ability to absorb nutrition through both photosynthesis and mycoheterotrophy during the same stage of life, favoring one method when the other yields insufficient food. Orchids are a relatively well-known variety of partial mycoheterotrophs, and all of the more than 26,000 orchid species show mycoheterotrophy at least in the initial phases of their development (“initial mycoheterotrophs”). The dustlike seeds of orchids lack sufficient endosperm to fuel germination and require fungal nutrition to sprout. Most species then transition during their life cycle from mycoheterotrophs to autotrophs, eventually gaining the majority of their energy from photosynthesis instead of leaching it from fungal networks. However, at least 210 species of orchids continue life as full mycoheterotrophic plants.

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Mechanics of mycoheterotrophy

Most mycoheterotrophic plants exploit a common relationship between plants and fungi called mycorrhizal symbiosis, in which both partners usually engage in bidirectional nutrient exchange. Mycorrhizal symbiosis is a widespread mutualistic relationship: Photosynthetic (autotrophic) plants provide carbohydrates from photosynthesis to the fungi, and the mycorrhizal fungi supply usable (fixed) nitrogen from the soil to the plants.

Mycoheterotrophs, on the other hand, form root attachments to mycorrhizal fungi and only imitate the mutualistic tendencies of other plants. They leach the carbohydrates that the fungi obtained from symbiotic plant partners and provide no reciprocal benefits. This interaction creates a tripartite relationship involving the autotrophic plant, the fungus, and the mycoheterotrophic plant, with the mycoheterotroph serving as the ultimate sink for the carbon fixed by the autotrophic plant.

Although such nutritional larceny may seem comparable to parasitism, in which the parasite benefits at the cost of its host, whether and to what extent mycoheterotrophy adversely affects the host fungi is not clear. Some experts have even suggested that mycoheterotrophy may stimulate the growth of the fungal host. Because the carbon received by the mycoheterotroph originated from an autotrophic plant, some experts suggest calling this type of symbiosis “epiparasitism.”

One major exception to this complicated arrangement is found in certain mycoheterotrophic plants of the orchid family. These plants derive some or all of their carbon-based nutrition directly from saprotrophic fungi, which feed on decaying organic matter and do not involve a third party.

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Evolution of mycoheterotrophs

Mycoheterotrophic plants did not all come from a single ancestor with this trait. Instead, they evolved independently from different groups of photosynthetic plants. Over time these separate lineages developed the same ability to draw nutrients from fungi. As relying on fungi proved effective, the ancestors of fully mycoheterotrophic plants gradually lost their ability to photosynthesize and became completely dependent on fungi for food. This transition from photosynthesis to full mycoheterotrophy happened step by step—through partial dependence on fungi, or partial mycoheterotrophy—mainly by losing genes and reprogramming metabolism, not by gaining new genes or pathways.

The diverse array of extant mycoheterotrophs has evolved in tandem with other species. Evidence has suggested that mycoheterotrophs coevolved with fungi, adapting over time to preserve and promote their relationships with these energy sources. This coevolution enabled the specialization of many mycoheterotrophs to undergo symbiosis with particular types of fungus. These specialized mycoheterotrophs developed the ability to leach nutrients from particular fungal species with unique efficacy, but they lost the capacity to undergo symbiosis with other types of fungus too physically or genetically different from their ideal host. Therefore, although mycorrhizal fungi associate with multiple plants and one autotrophic plant may harbor multiple mycorrhizal fungi, a fully mycoheterotrophic plant partners only with select fungal lineages.

Mycoheterotroph features

Mycoheterotrophic plants are most commonly found in habitats where photosynthesis is restricted by lack of sunlight, such as forest environments with high tree cover. Some grow entirely underground. Although some mycoheterotrophs may share similarities in appearance with closely related photosynthetic plants, many mycoheterotrophic plants are characterized by short statures, threadlike stems, scalelike leaves, and almost no root hairs. The color of their stems and leaves is not green, instead ranging in color from crimson and purple to blue and brown. A characteristic pure-white hue often marks obligate mycoheterotrophs such as Indian pipe (Monotropa uniflora) and Voyriella parviflora; these nonphotosynthetic plants have no need for chlorophyll, the pigment that makes many plants appear green to the human eye, as chlorophyll is only necessary for photosynthesis.

Mycoheterotroph research

Given that mycoheterotrophic plants are relatively rare, tend to be short-lived, and are found in remote locations, mycoheterotrophy is challenging to study. One evaluation suggested that early studies of mycoheterotrophs focused on temperate species, skewing research on the plants. As a result, information regarding their classification, genetic composition, and metabolism remains incomplete. Scientists have also achieved limited success in culturing the fungi associated with mycoheterotrophs in the laboratory. Nevertheless, such mycorrhizal species provide vital insights into the role of underground fungal networks in linking terrestrial plants and supporting their functions.