Myco-heterotrophy

Symbiotism between certain parasitic plants and fungi

Myco heterotrophy (from Greek μύκης , "fungus", ἕτερος heteros, "another", "different" and τροφή trophe, "nutrition") is a symbiotic relationship between certain kinds of plants and fungi, in which the plant gets all or part of its food from parasitism upon fungi rather than from photosynthesis. A myco heterotroph is the parasitic plant partner in this relationship. Myco heterotrophy is considered a kind of cheating relationship and myco heterotrophs are sometimes informally referred to as "mycorrhizal cheaters". This relationship is sometimes referred to as mycotrophy, though this term is also used for plants that engage in mutualistic mycorrhizal relationships.

Full (or obligate) myco heterotrophy exists when a non photosynthetic plant (a plant largely lacking in chlorophyll or otherwise lacking a functional photosystem) gets all of its food from the fungi that it parasitizes. Partial (or facultative) myco heterotrophy exists when a plant is capable of photosynthesis, but parasitizes fungi as a supplementary food supply. There are also plants, such as some orchid species, that are non photosynthetic and obligately myco heterotrophic for part of their life cycle, and photosynthetic and facultatively myco heterotrophic or non myco heterotrophic for the rest of their life cycle. Not all non photosynthetic or "achlorophyllous" plants are myco heterotrophic – some non photosynthetic plants like dodder directly parasitize the vascular tissue of other plants. The partial or full loss of photosynthesis is reflected by extreme physical and functional reductions of plastid genomes in mycoheterophic plants, an ongoing evolutionary process. In the past, non photosynthetic plants were mistakenly thought to get food by breaking down organic matter in a manner similar to saprotrophic fungi. Such plants were therefore called "saprophytes". It is now known that these plants are not physiologically capable of directly breaking down organic matter and that in order to get food, non photosynthetic plants must engage in parasitism, either through myco heterotrophy or direct parasitism of other plants. The interface between the plant and fungal partners in this association is between the roots of the plant and the mycelium of the fungus. Myco heterotrophy therefore closely resembles mycorrhiza (and indeed is thought to have evolved from mycorrhiza),

Most myco heterotrophs can therefore be seen as ultimately being epiparasites, since they take energy from fungi that in turn get their energy from vascular plants. Indeed, much myco heterotrophy takes place in the context of common mycorrhizal networks, in which plants use mycorrhizal fungi to exchange carbon and nutrients with other plants. In congruence with older reports, it has been recently shown that some myco heterotrophic orchids can be supported by saprotrophic fungi, exploiting litter or wood decaying fungi. In addition, several green plants (evolutionarily close to myco heterotrophic species) have been shown to engage in partial myco heterotrophy, that is, they are able to take carbon from mycorrhizal fungi, in addition to their photosynthetic intake.

Myco heterotrophs are found among several plant groups, mainly flowering plants. All monotropes and non photosynthetic orchids are full myco heterotrophs, as is the non photosynthetic liverwort Aneura mirabilis (previously considered a species of Cryptothallus). Partial myco heterotrophy is common in the Gentian family, with a few genera such as Voyria being fully myco heterotrophic; in photosynthetic orchids; and in a number of other plant groups. Some ferns and clubmosses have myco heterotrophic gametophyte stages. The fungi that are parasitized by myco heterotrophs are typically fungi with large energy reserves to draw on, usually mycorrhizal fungi, though there is some evidence that they may also parasitize parasitic fungi that form extensive mycelial networks, such as Armillaria. The great diversity in unrelated plant families with myco heterotrophic members, as well as the diversity of fungi targeted by myco heterotrophs, suggests multiple parallel evolutions of myco heterotrophs from mycorrhizal ancestors.