Mycorrhizae

Most folks don’t realize it, but the roots of plants are connected underground by a vast network of fungi that improve the function of plant roots. The fungi exist in what is known as a symbiosis—both the fungus and the plant benefit.  The fungus gets its energy from the plant, and the plant gains access to soil nutrients by the action of the fungus.

These plant-fungal relationships are known as mycorrhizae—translating into “fungus-root.”   There is some indication that land plants would not have evolved without the benefit of these fungi.  Even today, certain species of plants are unable to grow in certain soils unless they have mycorrhizal fungi with them. When you pick up a layer of decomposing leaves in a forest, the whitish strands are fungal hyphae.

Mycorrhizae are widely recognized to improve the supply of phosphorus to the host plant.  They release acids and enzymes, known as phosphatases, that mobilize phosphorus from decomposing materials and soil minerals.

Mycorrhizae come in two broad classes.  In ectotrophic mycorrhizae, the fungus occurs in a sheath around the root, extending hyphae into the soil to gather nutrients.   Most pines are colonized by ectotrophic mycorrhizae.  Ectotrophic mycorrhizae appear to be particularly effective in extracting nitrogen from decomposing materials.  Endotrophic mycorrhizae actually penetrate the cells of the root of the host, providing an efficient transfer of nutrients into the roots.  The network of mycorrhizae can even connect plants of different species.

The familiar mushrooms that pop up, especially after autumn rains, are the reproductive structures of mycorrhizal fungi. Beneath the cap of a mushroom are numerous gills or slits, which release spores.  The spores are dispersed by wind to new locations.  If you collect wild mushrooms to eat—something that should be done judiciously to avoid the poisonous varieties—you are consuming mycorrhizae.

The role of mycorrhizae has been overlooked in many environmental problems. The revegetation of mine spoils is often ineffective unless the mycorrhizae are established with the desired vegetation.  Remediation of these areas should focus on the successful growth of vegetation, not just the act of planting it, before the efforts are deemed successful.

The species composition of mycorrhizae are likely to change as vegetation is exposed to increasing concentrations of atmospheric CO2 and global warming.  Greater colonization of roots by mycorrhizae may allow trees to grow faster at high CO2.

Many home gardeners would see improved plant growth by ensuring that mycorrhizae are planted with their crops.  Buying sterilized topsoil is counter-productive; a shovel of native soil with obvious fungal hyphae can be helpful and reduce the need for phosphorus fertilizers.

 

References

Berliner, R., B.  Jacoby and E. Zamski. 1986.  Absence of Cistus incanus from basaltic soils in Israel—Effect of mycorrhizae.  Ecology 67: 1283-1288.

Chen, W., R.T. Koide, and D.M. Eissenstat. 2018.  Nutrient foraging by mycorrhizas: from species functional traits to ecosystem processes.  Functional  Ecology doi 10.1111/1365-2435.13041.

Parrent, J.L., W.F. Morris and R. Vilgalys. 2006.  CO2 enrichment and nutrient availability alter ecotmycorrhizal fungal communities.   Ecology 87: 2278-2287.

Phillips, R. R., E. Brzostek, and M.C. Midgley. 2013.  The mycorrhizal-associated nutrient economy: A new framework for predicting carbon-nutrient couplings in temperate forests.  New Phytologist 199: 41-51.

Simon, L., J. Bousquet, R.C. Levesque, and M. Lalonde. 1993.  Origin and diversification of endomycorrhizael fungi and coincidence with vascular land plants.  Nature 363: 67-69.

Terrer, C., S. Vicca, B.A. Hungate, R.P. Phillips, and I. Colin-Prentice. 2016.  Mycorrhizal association as a primary control of the CO2 fertilization effect.  Science 353: 72-74.

Treseder, K.K. and P. M. Vitousek. 2001.  Effects of soil nutrient availability on investment in acquisition of N and P in Hawaiian rain forests.  Ecology 82: 946-954.

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