By Catherine Couturier
Since time immemorial, plants and arbuscular mycorrhizal fungi have coexisted in a mutually beneficial relationship. The fungi colonize plant roots and help them absorb nutrients. In return, plants provide the fungi with the carbon they need.
"These fungi are what we call biotrophs; they can't get the nutrients they need on their own, so they form a symbiotic relationship, or mycorrhiza, with plant roots," explained Mohamed Hijri, a professor in the Department of Biological Sciences at Université de Montréal.
The fungi settle in the plant root, between its wall and membrane, and form highly branched structures called arbuscules.
Today, the agricultural sector is turning its attention to this beneficial interaction, hoping to replicate what nature does so well and to use these biotrophs to ensure more productive and healthier crops.
Give and take
Although most plants can survive without mycorrhizal fungi, the vast network of white filaments beneath our feet, known as the extraradical mycelium, helps plants absorb water and nutrients from the soil.
"These fungi are like an extension of the roots, allowing the plant to explore the soil more widely for nutrients," said Zakaria Lahrach, a Ph.D. student supervised by Hijri.
Beyond this plant-fungus co-evolution lies another phenomenon that scientists are just beginning to understand: how these fungi interact with bacterial communities in the soil.
It turns out that fungi release some of the carbon they take up back into the soil to recruit bacteria, which then break down the organic matter and release nutrients into the soil, which in turn are absorbed by the plant.
This means the fungi sequester some of the carbon that would otherwise have been released into the air, an effect not lost on scientists working on ways to mitigate climate change.
For his Ph.D. thesis, Lahrach wanted to explore the mechanisms by which arbuscular mycorrhizal fungi recruit soil bacteria and make phosphate available to plants. Certain bacteria in the soil promote solubilization of phosphate, which is key to plant growth and development.
After two years of experimental work, Lahrach's results were published last October in Environmental Microbiome.
Three fungal species tested
Lahrach focused on bacterial recruitment by three fungal species, Rhizophagus irregularis, Rhizophagus clarus and Rhizophagus cerebriforme. He also examined the effects of adding a bacterial community, or microbial bioaugmentation, to certain samples.
"We wanted to see if adding bacteria that were not naturally present in the soil would affect the fungi's bacterial recruitment," explained Lahrach.
Before carrying out his experiments, he spent five months in the greenhouse and more than a month delicately harvesting extraradical mycelium and preparing samples.
"What makes our study original is that it isolates the separate effects of the plant and the fungi on bacterial recruitment," he said.
Unique fungus-bacteria relationships
Lahrach and his colleagues found that all three species of arbuscular mycorrhizal fungi increased the growth of the experimental plants (the leek Allium porrum), as measured by shoot and root biomass and chlorophyll concentration.
Although the three fungal species were genetically close, each recruited a different bacterial community. The fungi adapted to the experimental conditions in which they were placed and played a central role in these newly created ecosystems.
Furthermore, microbial bioaugmentation only worked in the presence of fungi, highlighting the key role of fungi in enhancing plant growth.
Basically, the fungi act as conductors in the bacterial recruitment process via their mycelium, modulating the interaction between soil bacteria and the plant and enabling the plant to access nutrients such as phosphorus and nitrogen.
"The mycorrhizal fungi are like a transmission belt between the soil microbiota and the roots," said Hijri.
Benefits for agriculture
At a time when agriculture worldwide is under stress, the use of mycorrhizal fungi could help keep soil healthy by reducing the use of chemical inputs such as fertilizers and pesticides and their harmful effects.
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