Ninety percent of vascular plants form symbiotic associations with soil-dwelling fungi, according to the New York Botanical Garden. Known as mycorrhizal fungi, these organisms have ancient relationships with plants that benefit both the plant and the fungus, allowing plants to survive in environments ordinarily not conducive to plant life.
Mycorrhizal fungi live in the soil surrounding plant roots and form close associations with those roots. Once colonized, the fungi grow and extend from the roots, drawing in water and nutrients that exist out of reach of the plant's root system. The fungus delivers water and minerals to the plant's roots, and in exchange, the plant gives the fungus sugars that it has produced from photosynthesis, which the fungus uses for energy.
The hypha is the primary structure found on mycorrhizal fungi. Just as plant roots grow down into the soil to maximize nutrient and water absorption, the hyphae form threads that extend beyond the plant roots and absorb water and minerals. In some species, the fungus also possesses an arbuscule, a branched structure that forms inside of the root cell and assists in the transfer of nutrients between the fungus and the plant.
There are two primary types of mycorrhizal fungi, defined by the type of relationship they form with the plant root. Ectotrophic fungi enshroud the roots of the plant without penetrating the root cells. Endotrophic fungi pierce the root cells with their hyphae, facilitating the exchange of nutrients with the plant. Today, mycologists recognize additional divisions based on the types of plant the fungus inhabits, but those two basic distinctions remain.
Mycorrhizal fungi allow plants to survive in nutrient-poor soil that ordinarily could not sustain plant life. In biomes like the rainforest, competition for nutrients quickly depletes the soil of minerals, and mycorrhizal fungi help plants to compete. Because the hyphae can extend beyond the root zone, they can access nutrients and water that would ordinarily be out of reach to the plant. In addition, mycorrhizal fungi offer protection to plant roots from certain pathogens.
Given the benefits of mycorrhizal fungi to plants, they are naturally of interest to gardeners and farmers. According to a study by the University of California Cooperative Extension, the fungi grow best on land with minimal agricultural inputs. Organic fields produced a greater abundance of mycorrhizal fungi than conventionally farmed fields, and uncultivated fields produced the most of all.