The Strange and Wonderful Mycoheterotrophs

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Special Virtual Issue on Mycoheterotrophy

There are a number of flowering plants that have abandoned photosynthesis, and these fall into two categories: haustorial parasites and mycoheterotrophs.  Some choose to call all of these "parasitic plants", and in one sense this is true because both groups derive their nutrients from another plant.  The difference is that haustorial parasites feed directly on another plant via a modified root called the haustorium whereas mycoheterotrophs obtain their nutrition indirectly from the plant via a mycorrhizal fungus.  The mycorrhizal fungus, attached to the roots of a photosynthetic plant, thus acts as a bridge between that plant and the mycoheterotroph, such that nutrients (carbon) flow from plant root, to mycorrhizal fungus to the mycoheterotroph. One may see these plants called mycoheterotrophic epiparasites or ectomycorrhizal epiparasites. These epithets are quite descriptive (albeit cumbersome) in that the flowering plant could be considered an epiparasite of the fungus. Personally, to avoid confusion, I prefer to use the terms "parasite" or "parasitic plant" when referring to haustorial parasites, and "mycoheterotroph" or "mycoheterotrophic plant" when referring to the other nutritional mode.  Of course both groups are heterotrophs, but they obtain their nutrition in different manners.  And both groups have mixotrophic (combination of photosynthesis and heterotrophic feeding) and fully-heterotrophic (non-photosynthetic) representatives. Finally, mycoheterotrophs are sometimes 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 mycoheterotrophic 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 mycoheterotrophs 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 mycoheterotrophs include Thismia and Gymnosiphon (Burmanniaceae), Corsiaceae, Rhizanthella (Orchidaceae), Petrosaviaceae, Triuridaceae, Monotropeae,  Pterosporeae, and Voyria (Gentianaceae).  In some cases, the evolution of a nonphotosynthetic mycoheterotroph from a photosynthetic ancestor is clearly documented (e.g. Monotropeae) 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.  

The terminology associated with mycoheterotrophs has become burdonsome, so here's my take on the situation.

Autotroph(y, ic). A plant that obtains carbon via photosynthesis.

Heterotroph(y, ic). A plant that obtains carbon via a mechanism other than through photosynthesis.  Two categories: mycoheterotrophs and haustorial parasites.  

Holo-heterotroph(y, ic). A general term that refers (in angiosperms) to both holo-mycoheterotrophs and holoparasites.

Hemi-heterotroph(y, ic).  A general term that refers (in angiosperms) to both hemi-mycoheterotrophs and hemiparasites.

Full (or fully) heterotroph(y, ic).  Equivalent to holo-heterotrophy.

Part (or partial, partially) heterotroph(y, ic). Equivalent to hemi-heterotrophy.

Mycophyte.  A term I have seen used (e.g. Mabberley 2008 p. 330 with reference to Exacum) that I assume is synonymous with mycoheterotroph.

Mycotroph(y, ic).  Sometimes used as an equivalent to mycoheterotrophy. This can result in confusion because the term also refers to plants that have mycorrhizal roots, i.e. the "host" vascular plant in a mycoheterotrophic association. For this reason, I prefer to restrict the usage to the latter case.

Mycoheterotroph(y, ic). Term first used by Leake (1994).  Is also sometimes written hyphenated (myco-heterotroph, see Wikipedia page), but I prefer not to because this term often has further modifiers (prefixes) and this can become cumbersome. These plants obtain nutrients from mycorrhizal fungi that are attached to the roots of a vascular plant.

Full (or fully) mycoheterotroph(y, ic).  A mycoheterotroph that is dependent upon the fungus plus vascular plant association throughout its lifetime. Analagous to holoparasitism.

Holo-mycoheterotroph(y, ic).  Equivalent to full mycoheterotroph(y, ic). Term was used by Zhang and Saunders (2000), but not much since.  I prefer this term over full mycoheterotroph(y, ic) because it retains the linguistic pattern used with haustorial parasitic plants.

Obligate mycoheterotroph(y, ic). In Wikipedia this is give as equivalent to full or holo-mycoheterotroph(y, ic).  I disagree with this usage because some partial or hemi-mycoheterotrophs may require the fungal association during one phase of their life cycle but not others (as in some orchids where the fungus is needed at germination but maybe not later).  The converse term is facultative mycoheterotroph.

Part (or partial, partially) mycoheterotroph(y, ic).  A mycoheterotroph that obtains some nutrition from the fungus / vascular plant association but is also photosynthetic. Analagous to hemiparasitism.

Hemi-mycoheterotroph(y, ic).  Equivalent to partial mycoheterotroph(y, ic). I have not seen this term used, but as above, I prefer it because it preserves the linguist analogy to haustorial parasites.

Mixotroph(y, ic).  An organism that can use a mix of different sources of energy, for example, photosynthesis (autotrophy) and fungal feeding (mycoheterotrophy).  Equivalent to partial or hemi-mycoheterotrophs and to hemiparasites.

Facultative mycoheterotroph(y, ic).  Equivalent to partial or hemi-mycoheterotroph(y, ic).  I disagree with the Wikipedia definition of this term where they say an orchid is an obligate mycoheterotroph during the early phase of its life and facultative later.  To me, the orchid is either obligate or facultative and this status does not change throughout its life.  If it required the fungus early in its life, it is an obligate association.  If the orchid can be grown from seed to adult status with or without the fungus, this would be facultative (again paralleling the usage of this term in haustorial parasites). 

Hemiparasite.  A plant that forms haustorial connections to another vascular plant but retains at least some photosynthetic ability. There may be facultative or obligate hemiparasites. The former do not require a host to complete their life cycle whereas the latter do. Sometimes the term "semiparasite" or "partial parasite" is used, but I do not like these because these plants are not "half way to parasitism", they are fully parasitic!

Holoparasite.   A nonphotosynthetic plant that obtains all its water and nutrients from the host.  Sometimes referred to as "fully parasitic".  I don't prefer this terminology because the hemiparasites as just as parasitic as the holoparasites.

Links

Naturalis Biodiversity Center. Vincent Merckx page.

Mycoheterotrophy: The Mysterious World of Plants Living on Fungi HERE.

Mycoheterotrophic Plants - How many of them are there?  A nice "Scratchpad" tabulation of all the mycoheterotrophic species by Stephan Imhof. He, along with input from Hiltje and Paul Maas, formerly maintained a web site called The Mycoheterotroph Gallery. A number of photos were borrowed from this site and are credited below. From what I can determine, this site is no longer functional. You can visit the "Spezielle Botanik" webpage at Philipps Universität Marburg for contact information.

Martin Bidartondo works on the ecology and evolution of mycorrhizas. His professional web page at Imperial College London is HERE.

Ericaceae Homepage.  Blueberries, Heathers, and Rhododendrons.  Maintained by Kathy Kron, Wake Forest University. Her professional page is HERE.

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

Gentian Research Network.  Maintained by Lena Struwe at Rutgers University. Her professional page is HERE.

The mycoheterotrophic habit has evolved several times independently.  Listed below are the angiosperm families that have genera exhibiting this syndrome, with links to photographs of representative species.

Monocots

Burmanniaceae (Dioscoreales)

Thismiaceae (Dioscoreales - sometimes merged into Burmanniaceae)

Corsiaceae (Liliales)


Iridaceae [formerly Geosiridaceae] (Asparagales)

Orchidaceae (Asparagales)

Many orchids are mycoheterotrophs at the seedling stage. This list focuses on those that are also leafless and loosing photosynthetic function.
 Peterosaviaceae [also called Melanthiaceae] (Petrosaviales)
Triuridaceae (Pandanales) including Lacandoniaceae

Eudicots

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 partially mycoheterotrophic members (tribe Pyroleae) and fully mycoheterotrophic members (tribes Monotropeae and Pterosporeae).

Pyroleae

Monotropeae

Pterosporeae Gentianaceae (Gentianales)
Polygalaceae (Fabales)

References to Myco-heterotrophs



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