Mycorrhiza

A "fungus root" or mycorrhiza (plural: mycorrhizae) is a fungus living in a mutually beneficial symbiosis (or mutualism) with the roots of a vascular plant. In this intimate relationship, the fungus benefits from access to energy-containing carbohydrates, proteins, and other organic nutrients excreted by, or contained in, the roots while the host plant benefits from an enhanced supply of inorganic nutrients, especially phosphorus.

The fungi carry out this function largely by increasing the rate of decomposition of organic matter in the immediate vicinity of the plant root, and by efficiently absorbing the inorganic nutrients that are liberated by this process. From the perspective of the plant, the most important of the mineral nutrients supplied by the fungus are compounds of phosphorus, and to a lesser degree, of nitrogen.

Mycorrhizae are a common type of mutualism; about 90% of the families of vascular plants live in this sort of beneficial relationship with fungi. Only a few economically important plant families do not develop mycorrhizae, among them the mustards (family Brassicaceae) and knotweeds (Polygonaceae).