Fungivore - Plants

Plants

Around 90% of land plants live in symbiosis with mycorrhizal fungi, where fungi gain sugars from plants and plants gain nutrients from the soil via the fungi. Some species of plant have evolved to manipulate this symbiosis, so that they no longer give fungi sugars that they produce and instead gain sugars from the fungi, a process called myco-heterotrophy. Some plants are only dependent on fungi as a source of sugars during the early stages of their development, these include most of the orchids as well as many ferns and lycopods. Others are dependent on this food source for their entire lifetime, including some orchids and Gentianaceae, and all species of Monotropaceae and Triuridaceae. Those that are dependent on fungi, but still photosynthesise are called mixotrophs since they gain nutrition in more than one way, by gaining a significant amount of sugars from fungi, they are able to grow in the deep shade of forests. Examples include the orchids Epipactis, Cephalanthera and Plantanthera and the Pyroleae tribe of the Ericaceae family. Others, such as Monotropastrum humile, no longer photosynthesise and are totally dependent on fungi for nutrients. Around 230 such species exist, and this trait is thought to have evolved independently on five occasions outside of the orchid family. Some individuals of the orchid species Cephalanthera damasonium are mixotrophs, but others do not photosynthesise. Because the fungi that myco-heterotrophic plants gain sugars from in turn gain them from plants that do photosynthesise, they are considered indirect parasites of other plants. The relationship between orchids and orchid mycorrhizae has been suggested to be somewhere between predation and parasitism.

The precise mechanisms by which these plants gain sugars from fungi are not known and has not been demonstrated scientifically. Two pathways have been proposed; they may either degrade fungal biomass, particularly the fungal hyphae which penetrate plant cells in a similar manner to in arbuscular mycorrhizae, or absorb sugars from the fungi by disrupting their cell membranes, through mass flow. To prevent the sugars returning to the fungi, they must compartmentalise the sugars or convert them into forms which the fungi cannot use.