The roots of almost all higher plants are known to form mutualistic symbioses with fungi. These are termed mycorrhizae (fungus roots, from the Greek: mykes = mushroom or fungus and rhiza = root). Allen (1991) defined a mycorrhiza as "a mutualistic symbiosis between plant and fungus localised in a root or root-like structure in which energy moves primarily from plant to fungus and inorganic resources move from fungus to plant".
Fungi of very diverse kinds, belonging to all the major groups, take part in mycorrhizae. Based on morphology and the species involved, several different types are recognised. These are described below. The types can be distinguished by their positions along two gradients, from a lack of penetration of cortical cells (ectomycorrhizae) to penetration (vesicular-arbuscular mycorrhizae) and from an enclosed (ectomycorrhizae) to open root (vesicular-arbuscular mycorrhizae). Any natural ecosystem normally contains a mixture of types of mycorrhizal associations
The main mycorrhizal response is one of increased efficiency of mineral uptake especially of poorly mobile ions and there is a voluminous literature dealing with the nutritional benefits that plants derive from mycorrhizal associations. Mycorrhizae are also involved in the transfer of nutrients from components of soil minerals and organic residues to solution, and in nutrient cycling in an ecosystem.
In all types of mycorrhiza, fungal hyphae permeate soil and litter beyond
the depletion zones developed around non-mycorrhizal roots. Thus,
mycorrhizal roots explore a larger soil volume, and have a greater absorptive
area, than non-mycorrhizal roots. The fungal partners in some mycorrhizae
may also have chemical mechanisms to liberate fixed inorganic ions or
minerals incorporated in organic matter in the soil.
In 1986, Laiho wrote "the physiology and nutrient uptake of the roots of forest trees have been mainly studied with non-mycorrhizal seedlings". This statement, in its generalised form which reflects that the vast majority of land plants are mycorrhizal, is critical to our understanding of how plants obtain their nutrients. Plant physiologists, however, have developed explanations and models of how non-mycorrhizal plants obtain nutrients from the soil. Despite the fact that these only apply to atypical situations, they are taught worldwide in university plant physiology courses.