What is this surprising form, an erupting sun? In fact, it is the root tip of a young pine seedling, colonized by a symbiotic fungus, Cenococcum geophilum . It is a tiny chimeric organ – less than two millimeters – combining plant cells and mycelial filaments. Called "ectomycorrhiza", or root-fungus, this mutualistic symbiosis has played a fundamental role in the biology of trees and the ecology of forests for more than 180 million years.
Through the eyepiece of the confocal microscope, the root transformed by the presence of the symbiotic fungus is distinguished by three structural components.
On the surface of the root, we first distinguish a dense green mantle (this is the color of the fluorescent probe) consisting of several layers of intertwined fungal mycelium which completely encircle the short root. This mantle plays a major role in the storage of nutrients produced by the metabolic activity of the fungus and its host plant: mineral elements, sugars, amino acids, lipids in particular.
Then, a network of mycelium which colonizes the interior of the root by progressing between the cells of the host, whose walls are here stained by a red fluorescent probe. It is at this plant/fungus interface that the symbiotic exchange of nutrients takes place: glucose and amino acids circulate from the plant to the mycelium, while the mineral elements transit from the mycelium to the plant.
Finally, emerging from the mantle, projections of the mycelium move away from the root and project into the ground. This exterior mycelium absorbs the available mineral elements and transports them to the root, stimulating the nutrition of the tree. Because of this, symbiosis exerts a strong influence on the growth and health of the host plant.
In a few years, it will probably be possible to sow or plant trees “fortified” by selected fungi . These trees, protected by their microbial allies, will be better able to resist the stressful environmental conditions imposed by climate change.
Interact with many species of fungi
A tree can interact simultaneously with several hundred species of symbiotic fungi, for example boletes, milk caps, cortinaria or amanitas. In the forest, any environmental disturbance has a negative impact on this diversity.
Indeed, symbiotic fungi are very sensitive to the various silvicultural management interventions commonly practiced in managed forests. For example, thinnings and “clear cuts” of regeneration cause the massive disappearance of these symbiotic species. Forest tree species substitutions also have a detrimental effect on fungal communities. In many plantations, the original coppice-with-standards is by far the most favorable silvicultural state for fungal diversity and all single-specific plantations , broadleaved or coniferous, lead to a strong reduction in fungal biodiversity. The compaction of soils by the passage of forestry machinery, excessive trampling or the export of dead wood are other factors related to logging that strongly affect fungal communities in forest soils.
These studies show that all disturbances, of natural or anthropogenic origin, affect the symbiotic interactions in more or less unstable equilibrium. Current knowledge of forest-fungi relationships therefore clearly shows that any long-term project for the sustainable management of forest resources must be concerned with the stability of the complex interactions between fungi and plants. Any reduction in the fungal biodiversity of a soil therefore represents a real threat to the stability and sustainability of a forest ecosystem.
At the time of COP15, it is urgent to remember that barely 5% of the five million fungal species on our planet have been identified and listed. It is therefore essential to increase mycological expeditions to “biodiversity hotspots” in order to perfect our knowledge of the astonishing diversity of fungi. Thus, we will be able to protect them and better understand their biological and ecological roles.