A mycorrhiza (typically seen in the plural form mycorrhizae meaning "fungus roots") is a distinct type of root symbiosis in which individual hyphae extending from the mycelium of a fungus colonize the roots of a host plant. Different sorts of fungal structures are found in mycorrhizal trees and in roots of most herbaceous plants. These fungi belong to a group called ectomycorrhizae and endomycorrhizae respectively, forming mycorrhizal networks, which can be detected in the soil organic layers.

Mycorrhizae form a mutualistic symbiotic relationship with the roots of 95% of all vascular plants. Plant roots in nature are predominantly mycorrhizae. It is the non-mycorrhizal root that is the exception. Sterile soils and growth media often perform poorly without the addition of organic supplements that provide the spores and hyphae of mycorrhizal fungi to "infect" the plant roots to restore the natural state of affairs. The same organic supplements also populate the rhizoshpere (plant-soil interface) with beneficial bacteria such as Rhizobia (nitrogen-fixing bacteria) and other nitrogen or sulfur-releasing bacteria that make nutrients in the soil available for absorption by plants. The addition of biotic supplements (such as Rhizobia-treated legume seeds) is commonplace in modern agricultural practice to increase yields.

This symbiotic association provides the fungus with a renewable source of food by continuous access to plant assimilates that are mobilized from leaves to root tissues and then to the fungal partners, and allows the plant to make use of the mycelium's tremendous surface area to absorb nutrients from the soil. It is also believed that the mycelia of mycorrhizal networks have chemical absorption capabilities that are able to access some plant nutrients at lower concentrations than plant roots are capable of absorbing. An example of this is the manner in which phosphorus 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 cells and thus have a much larger surface area for absorption) and chemical (the cell membrane chemistry of fungi is different to that of plants).

Mycorrhizal fungi explore a larger volume of soil than root systems at a lower cost to the plant. Some of the earliest fossil plants show evidence of mycorrhizae associated with them. Mycorrhizal plants are generally more resistant to diseases e.g. caused by microbial soil-borne pathogens, and are also more resistant to the effects of drought.

Some mycorrhizal hyphae enter within the plant cell's wall and grow to envelop the cell. However, most mycorrhizae have a more advanced structure, in which the hypha lives inside an extensive invagination (inpocketing) of the cell membrane.

Vesicular-arbuscular mycorrhizae are an example of mycorrhizae that enter within plant cell walls to produce structures that are either balloon-like (Vesicles) or tree-like invaginations (Arbuscules). The structure of the arbuscules greatly increases the contact surface area between the hyphae and the cell cytoplasm to facilitate the transfer of nutrients between them.

The cytoplasmic streaming of the mycorrhizal hyphae 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 the same cytoplasmic streaming.

See also:

• soil life