ECTOMYCORRHIZAE AND ENDOMYCORRHIZAE: MAIN CHARACTERISTICS - BIOLOGY - 2025

The ectomicorrizas and endomicorrizas are symbiotic associations established between the roots of vascular plants and fungi in the soil. About 80% of vascular plants present these associations, which are mutualistic, since the two species involved benefit from it.

In ectomycorrhizae, the fungus does not penetrate the interior of the plant, but rather produces a highly branched network of hyphae which will surround the root. This covering that surrounds the root is called a mantle.

Ectomycorrhizal mycelium (white) associated with roots of Picea glauca (brown). Taken and edited from: André-Ph. D. Picard.

In endomycorrhizae, on the other hand, there is a penetration into the root of the plant by the fungus. In this case, a mantle is not produced, but branched structures called arbuscules.

Characteristics of ectomycorrhizae

Mutualistic associations of the ectomycorrhiza type involve fewer vascular plants than those of the endomycorrhiza type. Currently it is estimated that only about 2-3% of vascular plants are involved in this type of association.

In ectomycorrhizae, the hyphae of the fungus do not penetrate the cells of the root epithelium of the plant, instead they form a dense mantle around the roots and penetrate between their cortical cells, forming a structure called Hartig's network.

The hyphal mantle can reach 40 µm thick and project the hyphae several centimeters. This mantle helps the plant in the absorption of water and minerals.

Species involved

The plant species colonized by fungi are all arboreal or shrub-like. As mentioned before, only about 3% of vascular plants are colonized by ectomycorrhizae, however, these species have a wide distribution worldwide.

Ectomycorrhizal symbiotic relationships are more frequent in temperate zones than in tropical zones and to date this association has been observed in about 43 families and 140 genera. These genera include, for example, Pinus, Picea, Abies, Eucalyptus and Northofagus.

Among the fungi, for their part, at least 65 genera have been identified, of which more than 70% belong to the Basidiomycota. Ascomycota and, to a lesser extent, representatives of the Zygomycota have also been identified. Additionally, there are numerous species that have not yet been classified.

Ectomycorrhizae do not show great specificity in their relationships, neither by the fungi nor by their hosts. For example, plants of the genus Picea can be colonized by more than 100 species of ectomycorrhizal fungi, while the Amanita muscaria fungus can colonize at least five species of plants.

Development of ectomycorrhizae

The development of ectomycorrhizae begins when the hyphae colonize the secondary or tertiary roots of the plants. The hyphae of the fungus begin to grow from the root forming a network or sheath that can completely surround it.

The hyphae will also grow towards the interior of the root, between the epidermal cells and the cortical cells, without penetrating them; neither do they penetrate the wake. This inward growth is achieved by mechanical forces that separate cells and through the action of pectinase enzymes. In this way the Hartig network is formed.

Hartig's network will surround each cell and will allow the exchange of water, nutrients and other substances between the fungus and the plant.

Due to the colonization of the root by the fungus, it will grow less in length, but more in thickness, than non-colonized roots. Additionally, the root will present less hair development. The fungus, for its part, will develop the pod to completely cover the root and prevent colonization by other fungi.

Endomycorrhiza characteristics

Endomycorrhizae are much more frequent than ectomycorrhizae, they can occur in more than three quarters of vascular plants, although they mainly involve grasses and grasses.

In endomycorrhizae, the hyphae of the fungus initially penetrate between the cells of the root cortex but then enter inside them. In this case, the fungus does not form a Hartig mantle or net. Rather, they grow to form structures called vesicles and arbuscules.

Arbuscular mycorrhiza. Taken and edited from: Arbuscular_mycorrhiza_cross-section.png: mederivative work: Edward the Confessor.

The arbuscules facilitate the exchange of nutrients between the fungus and the plant, while the vesicles are used mainly as reserve organs.

Species involved

80% of vascular plants are colonizable by endomycorrhizae, however, fungi seem to show a preference for grasses and grasses. On the other hand, the fungi that form endomycorrhizae belong to the phylum Glomeromycota. The association is mandatory for fungi but not for plants.

Scientists believe that the development of this type of symbiotic relationship was essential for vascular plants to be able to colonize the terrestrial environment from aquatic environments, as well as for their subsequent evolution.

Endomycorrhizae are abundant in low-quality soils such as grasslands, mountains, and in tropical forests.

Development of endomycorrhizae

The association is established when hyphae present in the soil colonize the roots of the plant. At the beginning of colonization, the hyphae of the fungus only penetrate between the cells that enter the interior of these without breaking the cell membrane, which is invaginated by the pressure of the fungus.

Later the fungus can develop two types of structure; in the first, a hypha will undergo successive dichotomous ramifications near the vascular cylinder of the plant to form an arbuscle. This structure has the function of allowing the exchange of water and nutrients between the two organisms involved in the association.

The second structure that can develop, although it is not always present, is the vesicle, and it can grow externally or internally to root cells. Its shape is oval or spherical and serves as a place for food storage.

Benefits of mycorrhizae

The ecto and endomycorrhizal associations constitute a mutualistic symbiosis, in which the two species involved benefit. The main benefit of the association is the exchange of substances.

On the one hand the fungus provides water and mineral nutrients, and on the other the plant supplies the fungus with processed organic nutrients, mainly carbohydrates. The contribution of nutrients to the host plant by the endomycorrhizal fungus is so important that it is vital for many plants during their early stages of growth.

The growth and dispersal of ectomycorrhizal hyphae, on the other hand, not only increases the absorbent surface area of ​​the root, but also its potential reach, transporting nutrients from distant sites.

Additionally, the fungus is capable of capturing nutrients, for example phosphate and ammonium ions that are not available to the root, thus achieving greater absorption of minerals for the plant.

Ectomycorrhizal fungi, for their part, are mostly incapable of using lignin and cellulose as a carbon source, so they depend entirely on the plant to obtain the carbohydrates that it can metabolize.

Additionally, the ectomycorrhizal sheaths that surround the roots prevent their colonization by other fungi and pathogenic microorganisms.

References

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6. Mycorrhizae. Recovered from ecured.cu.
7. MF Allen (1996). The ecology of mycorrhizae, Cambridge University Press.
8. Arbuscular mycorrhiza. On Wikipedia. Recovered from es.wikipedia.org.