Paris (AFP)

Subterranean fungal networks have been an essential partner of plants since their conquest of emerged lands 450 million years ago, confirmed a recently published scientific study.

An international team of researchers, led by those from the Toulouse-III Plant Science Research Laboratory (LRSV), has found the missing link in a theory dating back to the 1980s.

She posits that the ancestor of all existing terrestrial plants, probably descended from a freshwater alga and emerged from the waters around 450 million years ago, lived in symbiosis with tiny fungi to develop on Earth. .

Today, around 80% of land plants use this symbiosis, in which the underground fungus "is really an extension of the plant," Pierre-Marc Delaux told AFP.

CNRS researcher at LRSV, he is the main co-author of the study published in Science last week and signed by his colleague and post-doctoral fellow Mélanie Rich.

The mycelium of the fungus, its vegetative apparatus, consists of an innumerable quantity of tiny white filaments, which network in the subsoil.

Its microscopic ends, intimately linked to the roots of the plant, provide it mainly with water, nitrogen and phosphates.

In exchange, the plant supplies the fungus with lipids, a fat essential for its development.

- Sprawling hair -

"If one of the two partners stops feeding the other, the exchanges stop in both directions", and everyone suffers, explains Mr. Delaux: the mushrooms, which depend "100% on the plant for their development ", as well as the plant, which can survive in a rich ecosystem but" will suffer much more in very poor soil ".

The consequences of stopping the symbiosis go far beyond, as the mycelium of the fungus expands like a sprawling hair.

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"Fungi are connected to hundreds, if not thousands of plants at the same time," says Delaux, who reports "quite convincing work" on the role they would play in distributing resources in this ecosystem.

The researchers' study showed that the same "symbiotic" gene, known to play an essential role in the transfer of lipid from the plant to the fungus, was at work in the two large branches of terrestrial plants.

We can therefore conclude "that their common ancestor who lived 450 million years ago also had these genes", according to the researcher.

The mechanism was already well identified for vascular plants, with stems and roots.

It has been found in non-vascular plants, such as mosses, called bryophytes, this "other great line of land plants".

- Mutant moss -

Scientists have confirmed the role of the famous gene by depriving a "mutant" of a moss, Marchantia paleacea.

With the direct consequence of the failure of the symbiosis, and the stopping of the development of the fungus.

To achieve its goals, the LRSV team worked with a host of European researchers, from the Universities of Cologne, Zurich, Leiden and Cambridge, among others, and Japanese, from the University of Sendai.

LRSV research is now moving towards a different kind of symbiosis, explains Mélanie Rich.

That which is exerted between plants and "nitrogen fixing bacteria, which make it possible to recover atmospheric nitrogen and to fertilize the plants with which they coexist".

This symbiosis exists in legumes such as lentils.

The researchers hope to "recreate it with plants of agronomic interest such as wheat, corn, rice", and "to contribute to the transfer of intensive agriculture which impoverishes the soils towards more sustainable agriculture", adds the researcher.

Because mastering this symbiosis would limit the massive use of nitrogen fertilizers in rich countries, and make up for their absence in poorer countries, Africa and Southeast Asia.

pcl / juc / fmp / nm

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