Network of mycorrhizae inside a root -
© M. Durant / CC BY-SA 4.0
One gram of natural soil contains nearly 200 meters of symbiotic fungal filaments (i.e. mutually beneficial between plant and fungus), according to a study published by our partner The Conversation.
The most widespread are mycorrhizae and have a strong agroecological potential as a support for ecosystem “services” for crops.
The analysis of this phenomenon was carried out by Babacar Thioye, teacher-researcher in Agroecology / Mycorrhizal symbiosis / Microbial ecology of soils, Marie-Pierre Brouillard, teacher-researcher in Plant Sciences and Marc Legras, Director of Training (all three from the 'UniLaSalle Polytechnic Institute in Rouen).
In terrestrial ecosystems, plants are associated with very dense microbial communities around their roots.
If bacteria constitute by far the greatest diversity of species in these microbial communities, fungi are also very present.
Thus a gram of natural soil would contain nearly 200 meters of fungal hyphae (filaments).
Among these, several meters are made up of hyphae of symbiotic fungi of plants (a reciprocal association between the plant and the fungus).
These symbiotic fungi are associated with the roots of plants to form mycorrhizae (from the Greek "myco" for fungus and "rhize" for root).
Mycorrhizae are the most common plant symbioses in natural or cultivated ecosystems.
The symbiotic interaction results in the establishment of an extra-matrix hyphae network (around the root) which increases the water and nutrient absorption surface (eg phosphorus, nitrogen, etc.) of the roots.
Fungi receive sugars from photosynthesis by the root of the host plant, which they use for their own survival.
This is a bilateral resource sharing relationship between two species, and therefore a classic symbiotic mutualism.
There are several types of mycorrhizae, but the most studied are ectomycorrhizae and endomycorrhizae.
Structures of ectomycorrhizae and endomycorrhizae © B. Thioye, M.-P. Noisy, M. Legras / UniLaSalle 2020
Ectomycorrhizae (from the Greek ektos: outside) in which the fungi develop mainly around the root, forming a mycelial mantle from which hyphae start which are organized between the cortical cells of the root to form the network said de Hartig without ever crossing the wall of the latter.
Ectomycorrhizal fungi belong to the branches of Basidiomycetes (eg Boletes, Russules…) and Ascomycetes (eg Truffles).
About 5% of plants, mainly forest trees and shrubs (pine, hornbeam, oak, beech, etc.) form this type of symbiosis.
Endomycorrhizae or arbuscular mycorrhizae (from the Greek endon: inside) are characterized by the absence of an external mycelial sleeve and by the penetration of fungal hyphae into the periplasmic space of cortical cells.
The hyphae grow in the root intercellularly and intracellularly, forming vesicles and arbuscules.
Arbuscular mycorrhizae are the most common and oldest type of mycorrhizae dating back to the first appearance of land plants about 450 million years ago.
At least 85% of land plants form this type of symbiosis, including many important crops for agriculture (wheat, rice, corn, peas, beans, soybeans, etc.).
Belonging to the phylum Glomeromycetes, these mycorrhizal fungi are obligatory symbionts that cannot be cultivated in the absence of the host plant and they are ubiquitous in terrestrial ecosystems.
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In terrestrial plants, mycorrhization is the rule, non-mycorrhization the exception.
A very large number of species are capable of interacting with arbuscular mycorrhizal fungi (AMCs) in particular Bryophytes, Lycopods, Monilophytes, Gymnosperms and Angiosperms: the majority of fruit trees, herbaceous plants, vegetables, terrestrial ferns, mosses, flowering plants (asters ...), aquatic plants, some conifers, seed plants (Ginkgo ...).
There are, despite everything, families of plants incapable of achieving this symbiosis, such as the Brassicaceae.
If the arbuscular mycorrhizal symbiosis is so widespread in the plant world, it is because it is beneficial to the plant.
Indeed, CMAs promote the growth and development of plants thanks to an improvement in mineral nutrition and in particular bioavailable phosphorus.
They also allow plants to best explore water resources and resist abiotic stresses.
Underground highways for plant nutrition
The fungus colonizes the root by its mycelium (a thick tissue of filaments) forming reserve organs (vesicles), exchange organs (arbuscules) and hyphae and spores in the soil.
All these structures (spores, hyphae, vesicles, arbuscules, fragments of colonized roots) are called propagules.
Colonization of plant roots is therefore carried out from fungal propagules and in particular spores present in the soil.
The microscopic fungal filaments greatly expand the root system with their incredible ability to connect to the roots of plants for many miles in the ground.
Establishment of mycorrhizal symbiosis © B. Thioye, M.-P. Noisy, M. Legras / UniLaSalle 2020
They draw water and nutrients from a large volume of surrounding soil, and supply them to the plant, improving its nutrition and growth.
But one of the most important abilities of mycorrhizal fungi is that they stay attached to the roots and support the plant throughout its life.
Many researchers have shown that most plants depend on mycorrhizae for their development.
Consequently, each plant species associates preferentially with the MACs which are the most favorable to it in terms of growth stimulation.
Plants with a strong mycorrhizal network tend to be healthier and to grow and defend themselves better.
They therefore require fewer resources to grow, which is good for farmers and the environment.
An application in tropical areas
Mycorrhizal fungi are a key component in plant-soil relationships.
In forest and agroforestry systems, it is well known that trees with high rates of mycorrhizal colonization can be used to replenish the mycorrhizal potential of soils.
This is why in tropical areas, one of the main ecological engineering strategies that can be used to allow mycorrhizal symbiosis to develop its properties for the benefit of plant development is controlled mycorrhization.
It is a set of techniques that allows to optimize the symbiosis from a process of isolation, culture, selection, multiplication, inoculation and monitoring of a fungus in the soil in order to to produce "biologically improved" plants.
In these ecosystems, researchers have shown that plants inoculated with mycorrhizal fungi mobilize phosphorus better and have a higher survival rate in the field than uninoculated plants.
Mycorrhizae of a 13-month-old jujube tree (Sahelian forest fruit tree) along the route of the Great Green Wall (Senegal), and spores of the arbuscular mycorrhizal fungus Rhizophagus irregularis_ © B. Thioye, M.-P. Noisy, M. Legras / UniLaSalle 2020
The most commonly accepted hypothesis to explain this result is that mycorrhizal plants have a greater capacity to absorb phosphorus than non-mycorrhizal plants thanks to the network of extra-matrix hyphae that they develop and which allow them to explore a larger volume of soil.
The use of this technology is therefore particularly suited to operations to rehabilitate degraded soils which generally have strong deficiencies in mineral elements and more particularly in assimilable phosphorus.
Application in temperate zones
Mycorrhizae can be considered as an agroecological potential and recognized as supporting ecosystem services for crops.
However, they are sensitive to certain factors unfavorable to mycelial development such as fungicides, intensive tillage, over-fertilization of nitrogen and phosphorus (mineral or organic), herbicides and sometimes crop systems and agricultural practices without vegetation cover between two crops.
It is well established that a diversified living plant cover constitutes a real relay for mycorrhizae.
Mycorrhizae of walnut (fruit tree) and faba bean (Legumes) in a walnut grove in Dordogne (CASDAR MycoAgra Project 2017-2020) © B. Thioye, M.-P. Noisy, M. Legras / UniLaSalle 2020
This is why the presence of plant cover such as legumes in intercropping constitutes an innovative practice that can act as a relay for mycorrhization as long as the introduced species are favorable to it.
This is the case with the cultivation of vetch, faba bean, etc.
In such an ecosystem, vegetation covers will provide a host for MACs during fall and early spring, during which time they could lose their viability.
The fungal mycelium will form an underground network interconnecting the roots of plants between them, thus allowing a direct exchange between them.
This shows the importance of the integrity of the fungal mycelium for plant health, and it is evident that tillage can lead to destruction of this mycelial network, further reducing the ecosystem services provided.
The beneficial effects of AMCs contribute to many ecosystem services crucial for the sustainability of agroecosystems.
For these reasons, CMAs are key players in the formulation of biofertilizers for the development of sustainable agriculture.
Thus, fostering specific communities of mycorrhizal fungi could represent an important contribution towards a cropping system guaranteeing efficient absorption of water and nutrients.
Although mycorrhizae have been the subject of numerous studies, a lack of in-depth knowledge remains among farmers.
Likewise, the potentialities really offered by mycorrhizal symbiosis in open field cultivation conditions, and the environmental and cultural factors influencing it still require a great deal of attention.
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This analysis was written by Babacar Thioye, teacher-researcher in Agroecology / Mycorrhizal symbiosis / Microbial ecology of soils, Marie-Pierre Brouillard, teacher-researcher in Plant Sciences and Marc Legras, Director of Training (all three from the UniLaSalle Polytechnic Institute from Rouen). The original article was published on The Conversation website.
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