Mechanisms of cooperation in the plants-arbuscular mycorrhizal fungi-bacteria continuum

In nature, cooperation is an essential way for species, whether they belong to the same kingdom or to different kingdoms, to overcome the scarcity of resources and improve their fitness. Arbuscular mycorrhizal fungi are symbiotic microorganisms whose origin date back 400 million years. They form symbiotic associations with the vast majority of terrestrial plants, helping them to obtain nutrients from the soil in exchange for carbon. At the more complex level, soil bacteria participate in the symbiosis between arbuscular mycorrhizal fungi and plants: they obtain carbon from the exudation of hyphae connected to the roots and compensate for the limited saprophytic capacity of arbuscular mycorrhizal fungi by mineralizing organic compounds. Therefore, plants, arbuscular mycorrhizal fungi and soil bacteria constitute a continuum that may be accompanied by multiple forms of cooperation. In this review, we first analyzed the functional complementarities and differences between plants and arbuscular mycorrhizal fungi in arbuscular mycorrhizal symbiosis. Secondly, we discussed the resource exchange relationship between plants and arbuscular mycorrhizal fungi from the perspective of biological market theory and “surplus carbon” hypothesis. Finally, on the basis of mechanisms for maintaining cooperation, direct and indirect reciprocity in the hyphosphere, induced by the availability of external resource and species fitness, were examined. Exploring these reciprocal cooperations will provide a better understanding of the intricate ecological relationships between plants, arbuscular mycorrhizal fungi and soil bacteria as well as their evolutionary implications.