Mycorrhiza

A mycorrhiza is a symbiotic association between a fungus and a plant root system. The term refers to the role of the fungus within the rhizosphere—encompassing the root and its surrounding soil environment. Mycorrhizal relationships are widespread across vascular plants and contribute substantially to plant nutrition, soil ecology and chemical processes. Although the association is usually mutualistic, certain plant–fungus pairings may shift towards parasitism under specific ecological conditions.

Definition

In a mycorrhizal symbiosis, the plant provides the fungus with organic carbon compounds derived from photosynthesis, typically sugars or lipids. In exchange, the fungus facilitates the uptake of water and essential mineral nutrients, notably phosphorus, from the soil. Most vascular plants exhibit some form of mycorrhizal association, and similar relationships also appear in bryophytes. Fossil evidence suggests that early land plants—long before the evolution of true roots—developed arbuscular mycorrhizal partnerships to meet their nutrient requirements.
Some plant families, including the Brassicaceae and Chenopodiaceae, generally do not form mycorrhizae. Among the many forms of mycorrhizal associations, arbuscular mycorrhizae are the most common, occurring in around seventy per cent of plant species and including many important agricultural crops.

Evolution

Mycorrhizal symbioses are ancient, with fossil and genetic data placing their origin at least as far back as the Ordovician. Genetic studies indicate that all land plants descend from a common ancestor that rapidly adopted mycorrhizal associations. Protomycorrhizal fungi appear to have facilitated plant colonisation of terrestrial environments by enhancing nutrient acquisition in nutrient-poor soils.
The Rhynie chert—about 400 million years old—contains fossils in which arbuscular mycorrhizae are clearly visible in early land plants such as Aglaophyton. Ectomycorrhizal relationships emerged later, during the Jurassic, whereas orchid and ericoid mycorrhizae are linked to the diversification of flowering plants in the Cretaceous. There is evidence that the nitrogen-fixing symbiosis found in modern legumes evolved as an extension of earlier mycorrhizal mechanisms. As angiosperms rose to ecological dominance in the Cenozoic, increasingly complex root structures shaped the modern distribution of mycorrhizal fungi.
Mycorrhizal lifestyles evolved independently numerous times from saprotrophic ancestors. These repeated origins produced distinct forms of nutrient exchange across different fungal lineages, with three major types arising independently: arbuscular mycorrhizae, ectomycorrhizae and ericoid mycorrhizae.

Arbuscular Mycorrhizae

Arbuscular mycorrhizae (AM) are the most ancient and widespread form. They involve the penetration of fungal hyphae into the root cortical cells of the host plant, making them a form of endomycorrhiza. Within these cells the fungus forms arbuscules—finely branched, tree-like structures that increase the surface area for nutrient exchange.
AM associations occur in most angiosperms, in some gymnosperms and pteridophytes, and even in nonvascular plants. They likely evolved alongside the first terrestrial plants, roughly 450–500 million years ago, at a time when plants lacked true roots and soils were extremely nutrient-poor. Fossil evidence from the Rhynie chert and clubmoss rootlets supports this deep antiquity.
This form of mycorrhiza is associated with fungi in the Glomeromycota, and their wide host range suggests minimal specificity. Arbuscular mycorrhizae often appear even in seedlings of plant species that later develop other forms of mycorrhiza, reinforcing the hypothesis that AM symbiosis underpinned the earliest stages of land plant evolution.

Ectomycorrhizae

Ectomycorrhizae (ECM) form when fungal hyphae surround the root surface without penetrating root cells. Instead, they develop a hyphal sheath around the root and a Hartig net occupying spaces between epidermal and cortical cells. These relationships are common among woody trees and shrubs in temperate and boreal ecosystems.
The earliest confirmed ECM fossils date to the Eocene, around 48 million years ago, though evolutionary evidence suggests that ECM arose earlier in the Pinaceae stem group during the mid-Jurassic. Ectomycorrhizal partnerships evolved independently many times in both plants and fungi. In angiosperms alone, ECM symbiosis appears to have arisen at least eighteen times, while fungal lineages show dozens of independent origins from saprotrophic ancestors.
Phylogenomic studies demonstrate that ECM fungi evolved convergently from white-rot and brown-rot fungi, as well as from soil saprotrophs, and that some ECM lineages retain enzymes for lignin degradation. Rare reversals back to saprotrophy appear possible, further illustrating ecological flexibility.

Ericoid Mycorrhizae

Ericoid mycorrhizae evolved about 140 million years ago from saprotrophic ascomycetes. They are specialised associations occurring exclusively in members of the Ericales among plants and in the Leotiales among fungi. This highly specific partnership likely reflects parallel evolution in response to shared environmental pressures rather than direct reciprocal selection.
Ericoid mycorrhizae are characteristic of nutrient-poor, acidic soils in both hemispheres, where plants develop sclerophyllous traits—high lignin content and low nitrogen and phosphorus. The resulting slow-decomposing litter creates a challenging environment. Ericoid fungi have adapted by retaining saprotrophic capabilities, enabling them to extract nutrients directly from organic material and to tolerate toxic ions. They play a crucial role in nutrient conservation in these ecosystems.

Types of Mycorrhizal Associations

Mycorrhizal symbioses can be broadly categorised into two groups based on how fungal hyphae interact with plant root cells:

• Endomycorrhizae, where hyphae penetrate the plant cell wall and invaginate the cell membrane. Arbuscular mycorrhizae fall into this category.
• Ectomycorrhizae, where hyphae remain outside plant cells and form a sheath around the root.

These functional distinctions align with independently evolved strategies for nutrient exchange. Both types play major roles in ecological processes across terrestrial ecosystems.

Ecological and Evolutionary Significance

Mycorrhizal associations are fundamental to plant nutrition, affecting water uptake, phosphorus and nitrogen cycling and plant resilience in nutrient-poor environments. They shape community structure, influence soil chemistry and underpin the success of terrestrial flora. The deep evolutionary history of mycorrhizae reflects their importance in enabling the colonisation of land and their ongoing role in plant adaptation to diverse environments.