Mycorrhiza

Beneficial organism

Mycorrhizae

Mycorrhization is symbiosis (the lasting and mutually beneficial association of two living beings) between a fungus and a plant. It is made possible when the hypha (the filamentous vegetative apparatus of the fungus) of a mycorrhizal fungus connects to the cells of a plant's root system.

Anestimated 95 % of plant species can mycorrhize with soil fungi. This symbiosis between the two kingdoms has many benefits for the growth and development of plants, even on a plot of arable land.

A number of factors must be taken into account if we are to avoid destroying the mycelium (cellular tissue made up of the hyphae of one or more fungi and forming a network for circulating nutrients in the soil) beneficial fungi and thus encourage the mycorrhisation of good fungi with cultivated plants.

Principle

Arbuscular mycorrhizal fungi (AMF), which account for approximately 80% of mycorrhizae in plant species, including agricultural and horticultural plants, use their hyphae to penetrate the cortical cells of the plant's root system - endomycorrhizae.

A plant's mycorrhization potential varies from one species to another. Grasses (maize, sorghum, millet, etc.), Fabaceae (peas, beans, crotalaria, etc.) and Alliaceae (leeks, onions, etc.) are the plant families that mycorrhizae the most, and are known as mycorhizotrophic species.

To date, it is accepted that Brassicaceae (cabbage, radish, turnip, rocket, etc.) and Chenopodiaceae (amaranth, beetroot) do not mycorrhize with CMA, but this consensus may be called into question by future research.

The plant will supply up to 30 % of these carbon compounds (glucose, fructose) from photosynthesis when the fungus supplies water and essential minerals (N, P, K) and trace elements (Cu, Zn, Mn).

The mycorrhiza of a flowering plant

Why protect and promote mycorrhization ?

The benefits of mycorrhization for crops and agrosystems are manifold :

• Soil phosphorus bioavailability Thanks to enzymes and associations with specific bacteria, mycorrhizal fungi are the only natural sources of phosphorus for plants. This key element in the constitution and functioning of living organisms (membranes, nucleic acids, basic metabolism) is essential for cell life and the good health of plants, animals and human beings. Moreover, it is not very mobile in soils, and without mycorrhiza, plants very quickly exhaust their supply sites (located between 1 and 2 mm around the absorbing hairs) and become dependent on phosphate fertilisers.
  

• Absorption of trace elements Absorption of trace elements is facilitated, which is good for the plant's immune system.

• Fixation of organic nitrogen by rhizobium Fixation of organic nitrogen by rhizobia: Legumes are particularly dependent on fungi because they need a lot of phosphorus for the bacteria in their nodules to fix nitrogen. Without mycorrhizae, nitrogen fixation by legumes would be very limited.

• Combating water stress Mushrooms are invaluable allies in adapting to drought and climate change. In fact, the mycelium has a capacity to explore the soil a thousand times greater than the roots of a plant (for every 1 cm of root in the soil, there are 10 m of mycelium), so fungi are able to absorb water further down and horizontally in the soil. What's more, the hypha of a fungus is smaller in diameter than that of a root (1/100 of a mm compared with 1/10 to 1 mm), enabling them to penetrate the microporosity of the soil and find the water that persists there during dry periods. Mycorrhizal fungi also help regulate the synthesis of abscisic acid (ABA), a plant hormone responsible for the closure of stomata during the day and therefore for evapotranspiration.

• Combating salinity stress Mycorrhizal fungi: A study showed that plants that had undergone mycorrhizal fungi were less sensitive to salinity than control plants. The mycorrhizal fungi used could adjust the physiology of the plants and consequently improve their growth and productivity.

• Biological control of pathogens MCAs play a major role in plant immunity to soil-borne bacterial pathogens and nematodes. Several cases have been observed :
  • Direct competition : By covering root tissues, the mycelium of CMAs prevents pathogenic fungal species from taking their place and parasitising the plant. However, its potential for competition is rather low, which is why CMAs need to be in symbiosis with the plant six months before the pathogens attack.
  • Favouring bacteria that compete with pathogens : CMAs activate the plant's synthesis of phytohormones, which increase the plant's root secretions, thereby serving as food for bacterial populations that compete with pathogenic bacteria.
  • Supply of defence molecules : CMAs synthesise molecules that are toxic to attackers (polyphenols, alkaloids), defence enzymes (proteases, chitinases) and components that strengthen the root cell wall (callose).

• Maintaining soil structure Fungi help to maintain the structure of the mineral and organic parts of the soil thanks to their mycelium and the secretion of glomalin, a glycoprotein that has a "glue" effect on the components of aggregates .

• Human health : Observations have shown that associations with CMAs lead to earlier fruiting of fruit trees (apple trees) and improved quality of market garden produce (strawberries, artichokes, melons, tomatoes, etc.) due to their higher concentration of antioxidants and sugars.

How can mycorrhization be encouraged in the plot ?

• The mycelium is a fragile cellular tissue that extends for the most part 10 cm below the surface. Farmers wishing to preserve mycorrhizae should adopt practices that have as little impact as possible on the physical and biological state of the soil, and favour shallow cultivation.

• Organic rather than mineralfertilisation stimulates the development of the bacteria associated with the fungi and therefore encourages their development. Conversely, mineral fertilisers that can be directly absorbed by plants will limit the appearance of mycorrhizae. Phosphate fertilisers should be avoided, as they completely block mycorrhization, slow down the expansion of CMAs in the soil and deprive plants of the essential benefits they would gain from mycorrhizae.

• Ideally, the best way to preserve mycorrhizae in the soil is to leave as much soil cover as possible using plant cover. The greater the diversity of plant species on the plot (both agricultural crops and weeds), the greater the mycorrhization. It is also possible to leave unproductive plots fallow, as long as the root system is left in place when the plot is mown or destroyed.

• Finally, one of the keys to promoting mycorrhizae in the fields is to introduce mychorhizotrophic species into the rotation and diversify it as much as possible. The legumes are particularly well suited to this task, as well as providing nitrogen for the following crop.

Favourable practices	Unfavourable practices
Shallow tillage. Maximum cover / plant cover. Fallow without grubbing up at the end of the cycle. Longer rotations. Introduction of mycorrhizotrophic species (grasses, legumes or Alliaceae). Use of organic fertilisers(compost, manure, RCW, etc.). Little or no fungicides or herbicides.	Deep tillage / Ploughing. Bare soil or removal of fallow land. Application of phosphate and other mineral fertilisers. Monoculture. Introduction of non-mycorrhizal species (Brassicaceae or Chenopodiaceae). Heavy use of fungicides and herbicides.

Multiplying and inoculating mycorrhizal fungi

If a soil has undergone deep tillage or regular fungal treatments, it may be worth inoculating a new population of CMA to get it off to a good start. It is essential to avoid, as far as possible, practices that are unfavourable to the presence of mycorrhizal fungi (see table above) so that the new population has a chance to settle in the plot and carry out mycorrhization with the plants.

Next, it is necessary to choose the MCA population to be inoculated and multiply it. A multitude of strains exist and can be propagated industrially or by hand. There are 3 different origins of mycorrhizal fungi strains, which can be propagated in a variety of ways :

• Standardised : These can be produced industrially in vivo (in open soil, on inert substrate, in hydroponics or aeroponics) or in vitro (on transformed roots). They have a broad colonisation spectrum, a high multiplication rate and are marketed in association with liquid biofertilisers (fertirrigation), substrates such as potting soil, clay for seed coating and to be mixed with the sowing substrate (see results of experimentation on vegetable crops by CA 56).
• Locals selected (listed) : Dedicated to their territory of origin, they can be multiplied within regional production units, on different types of soil to be adapted to specific pedoclimatic conditions.
• Indigenous (not listed) : Dedicated to a small agricultural region, they are propagated in a traditional way by regional production units or directly on the farm. Production takes place in the open ground or on a substrate (gravel, pumice stone, perlite).

Sources

• This article is partly based on the GECO fact sheet : https: //geco.ecophytopic.fr/geco/Concept/Mycorhize
• This article is partly taken from the document Mycorrhizae - allies in plant nutrition and protection