The Strange and Wonderful Myco-heterotrophs

There are a number of flowering plants that have abandoned photosynthesis, but not all of these are haustorial parasites.  These plants are frequently mistaken for parasitic plants but are more accurately called myco-heterotrophs (also sometimes mycotrophs).  These plants can live without photosynthesis because they have established a coevolutionary relationship with a mycorrhizal fungus that is attached to the root of a photosynthetic, woody plant.  So, there is a three way association such that nutrients (carbon) flow from plant root, to mycorrhizal fungus to the myco-heterotroph.

One may see these plants called myco-heterotrophic epiparasites or ectomycorrhizal epiparasites. These epithets are quite descriptive (albeit cumbersome) in that the flowering plant could be considered a parasite of the fungus.  But personally, I prefer avoiding the term "parasite" when describing these organisms because they do not obtain their nutrition in the same manner as haustorial parasites that actually invade host stems or roots by means of a modified root called the haustorium.  The mycotrophs don't directly invade the photosynthetic "host" roots but indirectly obtain nutrients via the intermediate fungus.  And by the way, myco-heterotrophs are frequently mistakenly called saprophytes.  There are no true saprophytes in the angiosperms.  Only fungi can directly utilize dead organic material.  

As reviewed in Bidartondo (2005), many plants, e.g. bryophtyes, lycophytes, ferns, gymnosperms (such as Parasitaxus), monocots and dicots, are myco-heterotrophic during some phase of their life cycle, especially during early establishment phases (seeds, gametophytes).  Some of these (e.g. many orchids) continue this association throughout their life cycles.  In this latter category, some myco-heterotrophs are photosynthetic (e.g. Burmannia, Galeola, and Pyrola) whereas others lose their photosynthetic capacity to some degree or another. There are even mixed trophic modes in the orchid Cephalanthera damasonium where some individuals are photosynthetic and others are not (Julou et al. 2005).  Fully nonphotosynthetic myco-heterotrophs include Thismia and Gymnosiphon (Burmanniaceae), Corsiaceae, Rhizanthella (Orchidaceae), Petrosaviaceae, Triuridaceae, Monotropeae,  Pterosporeae, and Voyria (Gentianaceae).  In some cases, the evolution of a nonphotosynthetic myco-heterotroph from a photosynthetic ancestor is clearly documented (e.g. monotropes) whereas in other cases the situation is not as clear (e.g. Arachnitis in Corsiaceae or Voyria in Gentianaceae).  These scenarios very much parallel the situations with haustorial parasitic angiosperms where there exists several cases of the evolution of nonphotosynthetic parasites (holoparasites) from photosynthetic relatives (hemiparasites).  For example, holoparasitism has evolved at least five times independently in Orobanchaceae.  Maybe we should start using the words "hemimyco-heterotrophic" and "holomyco-heterotrophic" to describe these (talk about cumbersome terminology!).

The myco-heterotrophic habit has evolved several times in unrelated angiosperm families.  Listed below are the angiosperm families that have genera exhibiting this syndrome, with photographs of representative species.

Monocots


Burmanniaceae (Dioscoreales)
Corsiaceae (Liliales)

Iridaceae [formerly Geosiridaceae] (Asparagales)

Orchidaceae (Asparagales)

Given that nearly all orchids are myco-heterotrophs initially, this listing focuses on those that are also leafless, i.e. are loosing photosynthetic function.  A number of these link to the Internet Orchid Encyclopedia.
Peterosaviaceae [also called Melanthiaceae] (Petrosaviales)

Triuridaceae (Pandanales). Including Lacandoniaceae.

Dicots


Diapensiaceae (Ericales)

Ericaceae (Ericales)

Ericaceae s. lat. (in the broad sense) now includes Empetraceae, Epacridaceae (Styphelioideae), Monotropaceae, and Pyrolaceae].   Mycotrophic plants likely occur in all of these groups.  For this page, I will present only the mycotrophs that are now considered members of subfamily Monotropoideae of Ericaceae. Within that subfamily are "hemimycotrophic" members (tribe Pyroleae) and "holomycotrophic" members (tribes Monotropeae and Pterosporeae).

Pyroleae

Monotropeae

Pterosporeae Gentianaceae (Gentianales)
Polygalaceae (Fabales)

Links

The Mycoheterotroph Gallery, by Stephan Imhof (Marburg University, Germany) with input from Hiltje and Paul Maas.  See also "Spezielle Botanik." Parasitic and mycotrophic flowering plants: structural, ecological, and phylogenetical and Afrothismia winkleri (Engl.) Schlecht, also by Stefan Imhof.

Martin's Treasure Chest.  Martin Bidartondo (w/ Tom Bruns, currently working at Kew Gardens in London England).

Thismia americana: A mystery that still haunts — and helps — the Calumet region.  Chicago Wilderness Magazine.

Article in Coastweek.com about Afrothismia baerae: "A  Rare  Plant 'Find' In Shimba Hills Reserve".  Kenya.

Ericaceae Homepage.  Blueberries, Heathers, and Rhododendrons.  Kathy Kron, Wake Forest University. Go to Monotropoideae.

Fungus flowers: Flowering plants that resemble fungi.  Wayne's Word.

Gentian Research Network.  Rutgers University and Cook College.



References to Myco-heterotrophs


SIUC / College of Science / Parasitic Plant Connection / Mycotrophs
URL: http://www.parasiticplants.siu.edu/Mycotrophs.html
Last updated: 31-Dec-08 / dln