| abstract
| - A mycorrhiza (typically seen in the plural forms mycorrhizae or mycorrhizas, Greek for fungus roots) is the result of a symbiotic association between a fungus and a plant. This symbiosis takes place at the root level, where individual hyphae extending from the mycelium of a fungus colonize the roots of a host plant. This symbiotic association provides the fungus with a renewable source of food through access to fixed carbon (sugars) from plant photosynthate. These are translocated to the root tissues from their source location (usually leaves), and then to the fungal partners. In return, the plant gains the use of the mycelium's tremendous surface area to absorb mineral nutrients from the soil. It is believed that the mycelia of mycorrhizal networks have better mineral absorption capabilities compared to plant roots. An example of this is the manner in which phosphate ions are tightly bound to iron oxides in many soils. Plant roots are generally incapable of accessing these phosphorus sources (which can be large and are termed sinks), yet mycorrhizal mycelia can access these forms of phosphorus. The mechanisms of increased absorption are both physical—mycorrhizal mycelia are much smaller in diameter than the smallest root hair and thus are able to explore a greater volume of soil and have a much larger surface area for absorption—and chemical—the cell membrane chemistry of fungi is different from that of plants. Mycorrhizae are especially beneficial for the plant partner in nutrient poor soils. Furthermore, mycorrhizal plants are generally more resistant to diseases, such as those caused by microbial soil-borne pathogens, and are also more resistant to the effects of drought. The cytoplasmic streaming of the mycorrhizal hypha is a mechanism that facilitates the transfer of nutrients from the soil, at relatively remote distances from the root, to the root at rates far exceeding those that would be possible by osmotic flow alone. This has an energy cost to the fungus. In return, the mycorrhizal fungus is rewarded by the "payment" of nutrients in the form of sugars, starches, proteins and lipids from the plant roots. These nutrients, in turn, flow to the whole mycelial network through cytoplasmic streaming. Mycorrhizas form a mutualistic symbiotic relationship with the roots of most plant species (95% of all plant families are predominantly mycorrhizal). Plants grown in sterile soils and growth media often perform poorly without the addition of spores or hyphae of mycorrhizal fungi to "infect" the plant roots and aid in the uptake of soil mineral nutrients. The absence of mycorrhizal fungi can also slow plant growth in early succession or on degraded landscapes.
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