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Series of photographs for the book -The Art of acclimating plants, the garden of the Villa Thuret - by Catherine Ducatillion and Landy Blanc-Chabaud, published by Editions Quae. © INRA, SLAGMULDER Christian

Plants also feel, move and communicate!

External communication: the relationships between plants and fungi

The interactions between plant roots and soil fungi are determined by a molecular dialogue that involves a wide variety of communication compounds.

By Pascale Mollier, translated by Vicky Hawken
Updated on 09/05/2014
Published on 07/30/2014

Rhizophagus irregularis (alias Glomus intraradices) colonising the roots of carrot.. © Université Toulouse III - Paul Sabatier, Guillaume Bécard
Rhizophagus irregularis (alias Glomus intraradices) colonising the roots of carrot. © Université Toulouse III - Paul Sabatier, Guillaume Bécard

 

The roots of a plant extend into the soil and are in contact with thousands of bacteria, fungi and microscopic animals that compete for the carbonaceous resources released by the plant.  Among these micro-organisms, some fungi establish privileged contacts with the plant; their minuscule but tight mycelial networks communicate with the plant roots.  This is a symbiotic relationship, or a win-win exchange in which the fungus favours the mineral nutrition of the plant (water, phosphates, nitrates, micro-elements) while in return being supplied with organic compounds resulting from photosynthesis (glucose, fructose, etc.).

Different strategies to establish contact

A recent comparative analysis of the genomes of two symbiotic fungi (laccaria and truffle) highlighted similarities in the molecular mechanisms governing symbiotic interactions; in particular, these fungi have lost their ability to degrade plant cell walls.  The objective of the fungal mycelium is to install itself between the root cells of its host without causing any damage, in order to avoid the induction of defence reactions.  In order to facilitate the development of this symbiotic interface, the laccaria injects the roots with a panoply of messenger proteins that are responsible for establishing the dialogue, some of which control the immune defences of the tree to prevent rejection of the fungus.  The truffle is more brutal, forcing its entry between the root cells.  Indeed, it even seems that after exploiting them, the truffle may even digest the roots that have become its home.

Plant-fungus recognition: a molecular dialogue

How do symbiotic fungi recognise their plant partners in the underground tangle of roots?  And reciprocally, how do host plants distinguish between "good" and "bad" fungi?

Twenty years of research - mainly in France - has enabled identification of the molecules governing the dialogue between plants and fungi.  The roots of a plant emit small quantities of strigolactones (see Part 5), a hormone that triggers the germination of fungal spores.  In turn, the fungal mycelium emits complex, diffusible sugars (lipochitosaccharides) towards the roots.  In the plant, these factors, called "Myc",  trigger a cascade of molecular mechanisms that lead to formation of the symbiotic exchange structure.

The dialogue between symbiotic fungi and their host plants is highly complex and involves other diffusible molecules acting in concert with the Myc factors.  For example, the colonising mycelium emits plant hormones (auxin, cytokinins; see Part 5) and proteins that control the immune reaction of the plants.

A conditional relationship

This mutually beneficial symbiosis between plants and soil fungi is a general rule and concerns more than 80% of plant species.  Mycorrhizal organisms consume between 5% and 20% of the sugars produced by photosynthesis in the above-ground parts of a plant.  But this uptake of fuel from the plant is largely compensated for by the intake of minerals that stimulates its growth.

It has indeed been demonstrated that if fungi ceased to supply phosphate, the plant would not only cut off its supply of sugars but also digest the exchange structures set up by the fungi in its roots.  A relationship that allows no place for cheating!

Contact(s)
Scientific contact(s):

Associated Division(s):
Forest, Grassland and Freshwater Ecology, Plant Health and Environment, Plant Biology and Breeding
Associated Centre(s):
Grand Est - Nancy, Occitanie-Toulouse

Work by INRA on symbioses

- Characterisation of Myc factors.
- Characterisation of Nod factors, synthesised by Rhizobia, nitrogen-fixing bacteria that live in symbiosis with legumes (French only).
- Truffle: breeding of mycorrhizal oak plants. 90% of current black truffle production in the Périgod region results from this process (French only).
- Participation in sequencing the truffle genome (French only).
- Truffle: discovery of the sex of truffles: truffles result from a cross between a male mycelium and female mycelium.  Development of a sex detection test (French only).
- Sequencing of the genome of Rhizophagus irregularis, the oldest known symbiotic fungus (French only).