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Myco-heterotrophy

2007 Schools Wikipedia Selection. Related subjects: General Biology

   Monotropastrum humile, an obligate myco-heterotroph.
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   Monotropastrum humile, an obligate myco-heterotroph.

   Myco-heterotrophy 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.

Relationship between myco-heterotrophs and host fungi

   Myco-heterotrophic roots of Monotropa uniflora with Russula brevipes
   mycelium.
   Myco-heterotrophic roots of Monotropa uniflora with Russula brevipes
   mycelium.

   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 (Leake 1994). (It is important to note, however, that not all
   non-photosynthetic or " achlorophyllous" plants are myco-heterotrophic
   – some non-photosynthetic plants like dodder directly parasitize the
   vascular tissue of other plants (Dawson et al. 1994).)

   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 no plant is physiologically capable of direct
   breakdown of 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 (Bidartondo et
   al. 2005, Leake 2005).

   The interface between the plant and fungal partners in this association
   is between the roots of the plant and the mycelium of the fungus. It
   therefore closely resembles mycorrhiza (and indeed is thought to have
   evolved from mycorrhiza) (Bidartondo et al. 2005), except that in
   myco-heterotrophy, the flow of carbon is from the fungus to the plant,
   rather than vice versa (Trudell et al. 2003, Bidartondo et al. 2004).

   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 (Bidartondo et al. 2005, Leake 2005).
   Indeed, much myco-heterotrophy takes place in the context of a common
   mycorrhizal network, in which plants use mycorrhizal fungi to exchange
   carbon and nutrients with other plants (Leake 2005). In these systems,
   myco-heterotrophs play the role of "mycorrhizal cheaters", taking
   carbon from the common network, but giving nothing in return
   (Bidartondo et al. 2005).

Species diversity of myco-heterotrophs and host fungi

   Myco-heterotrophs are found among a number of plant groups. All
   monotropes and non-photosynthetic orchids are full myco-heterotrophs,
   as is the non-photosynthetic liverwort Cryptothallus. Partial
   myco-heterotrophy is common in the Gentian family, in photosynthetic
   orchids, and a number of other plant groups. Some ferns and clubmosses
   have myco-heterotrophic gametophyte stages (Leake 1994, 2005, Taylor et
   al. 2002). 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 (Leake 2005).
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