Mycorrhizae
Mycorrhizae are mutualistic associations of plant roots united with the hyphae (filaments) of
fungi (Campbell, 755). In some associations the mycorrhizal fungi surround the
living root cells, while others penetrate the root cells. The extensions of the
hyphae from the mycelium forming the mycorrhizae greatly increase the absorptive
surface of the plant roots (Campbell, 628). Each centimeter of root length may
have roughly 3 m of hyphae extended into the soil thereby reaching a far greater
volume of soil than could be possible by the root alone (Campbell, 756). This
association allows the fungus to exchange minerals (phosphate) it has accrued
from the soil for organic nutrients (sugar) that have been synthesized by the
plant (Campbell, 628) (Campbell, 778). The fungi also stimulate roots to grow
and branch by secreting growth factors. In addition, the fungi possibly protect
the plant from pathogenic bacteria and pathogenic fungi in the soil by producing
antibiotics (Campbell, 778).
Nearly all vascular plants have mycorrhizae forming permanent associations with their fungal partners (Campbell, 628). Likewise, it has been estimated that more than 80% of all plants have a mycorrhizal partner (Tree of Life, http://tolweb.org/tree?group=fungi). Roughly half of all basidiomycetes that form mushrooms live as mycorrhizae with birch, oak, and pine trees. Evidence of this unique symbiotic relationship is usually only seen when mushrooms sprout around the bases of these trees (Campbell, 629).
There are two types of mycorrhizae: ectomycorrhizae and endomycorrhizae. In ectomycorrhizae the mycelium forms a dense sheath, or mantle, over the surface of the root. The fungal hyphae grow into the root cortex but do not penetrate the root cells. This type of mycorrhizae is particularly common in woody plants such as pine, spruce, oak, walnut, birch, willow, and eucalyptus trees. In contrast, endomycorrhizae do not have a dense sheath, or mantle over the surface of the root. In addition to hyphae extending from the root into the soil, hyphae also extend inward by digesting small patches of the root cell walls. Endomycorrhizae are much more prevalent than ectomycorrhizae being found in over 90% of all plant species(Campbell, 778).
Works Cited
Campbell, Neil A. Biology. 4th ed. Menlo Park: The Benjamin/Cummings Publishing
Company, 1996.
Tree of Life Web Project. 2006. The University of Arizona College of Agriculture
and Life Sciences and The University of Arizona Library. 21 April 2006.
<http://tolweb.org/tree/>