A. B. Frank’s observations and hypotheses about mycorrhizae in 1885 flew in the face of conventional thinking of the time. He reported that what we now term ectomycorrhizae were widespread on root systems of many woody plant species in a great diversity of habitats and soils. He hypothesized that mycorrhizae represent a pervasive mutualistic symbiosis in which fungus and host nutritionally rely on each other; that the fungus extracts nutrients from both mineral soil and humus and translocates them to the tree host; and that the tree, in turn, nourishes the fungus.
Initially opposed by much of the scientific community, nearly all of Frank`s major hypotheses have since been unequivocally demonstrated, although many decades were required to achieve conclusive evidence. Nonetheless, the revolution in thinking about plant and fungal evolution, ecology and physiology generated by Frank is still in the process of acceptance by much of the scientific community, 120 years and tens of thousands of scientific papers since he coined the term. – Trappe JM
A mycorrhiza (Greek: μυκός, mykós, “fungus”, and ρίζα, riza, “root”) is a symbiotic association composed of a fungus and roots of a vascular plant. In a mycorrhizal association, the fungus colonizes the host plant’s roots, either intracellularly as in arbuscular mycorrhizal fungi, or extracellularly as in ectomycorrhizal fungi. They are an important component of soil life and soil chemistry.
Fungi in mycorrhizae form a mutualistic relationship with the roots of most plant species. The roots in the relationship, and the plants themselves are referred to as mycorrhizal if mycorrhizae are formed. This mutualistic association provides the fungus with relatively constant and direct access to carbohydrates, such as glucose and sucrose. The carbohydrates are translocated from their source (usually leaves) to root tissue and on to the plant’s fungal partners. In return, the plant gains the benefits of the mycelium’s higher absorptive capacity for water and mineral nutrients due to the large surface area of fungal hyphae, which are much finer than plant roots, thus improving the plant’s mineral absorption capabilities.
Plant roots alone may be incapable of taking up phosphate ions that are demineralized in soils with a basic pH. The mycelium of the mycorrhizal fungus can, however, access these phosphorus sources, and make them available to the plants they colonise. Thus many plants are able to obtain phosphate, without using soil as a source. For example, in some dystrophic forests, large amounts of phosphate are taken up by mycorrhizal hyphae acting directly on leaf litter, bypassing the need for soil uptake. In some cases, the transport of water, carbon, and nutrients could be done directly from plant to plant through mycorrhizal networks that are underground hyphal networks created by mycorrhizal fungi that connect individual plants together.
The mechanisms of increased absorption are both physical and chemical. Mycorrhizal mycelia are much smaller in diameter than the smallest root, and thus can explore a greater volume of soil, providing a larger surface area for absorption. Also, the cell membrane chemistry of fungi is different from that of plants (including organic acid excretion which aids in ion displacement). Mycorrhizas are especially beneficial for the plant partner in nutrient-poor soils. Mycorrhizal plants are often more resistant to diseases, such as those caused by microbial soil-borne pathogens.
Bill Mollison »