Journal Number 89
December 2003
EDITORIAL
Getting Close And Then Cheating
By Ian St George
Two close relationships in the orchid world are that between the orchid and its pollinator,
and that between the orchid and its mycorrhizal fungus. It has been said that stable
mutualisms must prevent cheating by a partner (getting benefit, bearing no cost), but
some orchids do cheat.
Cheating the pollinator: many flowering plants are brightly-coloured and use a food reward like nectar to entice pollinators. Many orchids, however, cheat their pollinators - they are brightly-coloured but contain no reward. Gigord and co-workers found that "experienced bees that had already learned to visit nectar-producers did prefer cheats of similar colour. Fraudsters in some plant communities could therefore be mimics, even if they are similar to nectar-producers only in colour" [1].
Equally of course, some animals cheat: they may take nectar but do not carry pollen.
Cheating the fungus: green plants use the chlorophyll in their leaves to manufacture carbon compounds by photosynthesis, and they seek and retrieve water, minerals and other substances from the soil with fine rootlets. Terrestrial orchids store nutrients in their thick, fleshy bulbs or rhizomes.
They very often do not have an extensive network of rootlets.
Soil fungi do have an immense, spreading network of fine threadlike hyphae, but they lack chlorophyll,
so cannot produce their own carbon compounds.
Seems like a partnership made in heaven, and indeed 95% of flowering plants grow well only with this fungal partnership. The fungal hyphae act as extra roots for the plant. The fungi transfer carbon from the plant to themselves, and in return they transfer phosphorus, nitrogen, and other nutrients to the plants, as well as producing antibiotics and other protective devices. This living arrangement is called mycorrhiza. Some plants can live without it, but cannot thrive.
Taylor and Bruns found that two nongreen orchids associated exclusively with two distinct ectomycorrhizal fungi [2]. Yet both orchids retained the internal mycorrhizal structure typical of photosynthetic orchids that do not associate with ectomycorrhizal fungi. This is proof of ectomycorrhizal epiparasitism in nature by orchids. Taylor and Bruns argue that "these orchids are cheaters because they do not provide fixed carbon to associated fungi..Mycorrhizae, like other ancient mutualisms, are susceptible to cheating".
Selosse and co-workers concluded the nongreen European "birds-nest orchid" Neottia nidus-avis "is likely to derive its resources from surrounding trees, a mycorrhizal cheating strategy" [3].
Our nongreen orchids (the Gastrodia, Molloybas and Danhatchia) are cheats, as the late
Dame Ella Campbell described so eruditely. Lacking chlorophyll, unable to provide carbon compounds for themselves or for the fungus, they nonetheless form "mutualistic" relationships with soil fungi, and through the fungi, derive carbon from nearby green plants.
Furthermore, this may be true even of green terrestrial orchids that have dormant, underground years, when they may depend entirely on soil fungi.
If the world of close relationships is made up of givers and takers, in this one the fungus is
the giver, and the orchid is the taker.
Did you know? Forest seedlings can get as much as a third of their food from trees nearby.
The mycorrhizal fungi form an underground network to take nutrients from older trees.
So adult trees may feed their young, just as animals do.
References
1. Gigord LDB, MR Macnair, M Stritesky and A Smithson. The potential for floral mimicry in a rewardless
orchids: an experimental study. Proc B 2002 7 July 1389
2. Taylor DL and TD Bruns . Independent, specialized invasions of ectomycorrhizal mutualism by two
nonphotosynthetic orchids. Proc. Natl. Acad. Sci. USA 1997. 94: 4510-4515.
3. Selosse. Communities and populations of sebacinoid basidiomycetes associated with the
achlorophyllous orchid Neottia nidus-avis (L.) LCM Rich. and neighbouring tree ectomycorrhizae.
Molecular Ecology. 11(9):1831-1844.
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