Can arbuscular mycorrhizal fungi and associated bacteria affect the diversity of the endophytic microbiota recruited by plant roots?

Arbuscular mycorrhizal fungi (AMF) are beneficial soil microorganisms, which establish mutualistic symbioses with the roots of about 80% of land plants, comprising most crops, such as cereals, legumes, vegetables, fruit trees, cotton, sunflower, tobacco. They play a key role in plant growth and nutrition, increase plant tolerance to biotic and abiotic stresses, and provide multiple ecosystem services as biofertilizers and biostimulants. AMF live closely associated with bacteria colonizing spores and extraradical hyphae, originating the composite and metabolically active environment called mycorrhizosphere. Many members of such bacterial communities have been identified as plant growth–promoting (PGP) bacteria, given their diverse functional activities in the production of plant hormones, antibiotic, and siderophore, in N2 fixation, P solubilization, and in the facilitation of mycorrhizal establishment. The fate of mycorrhizospheric bacterial communities has not been adequately investigated, in order to reveal whether they can colonise the roots and become endophytes, like the rhizospheric bacteria which are able to establish such an intimate relationship with plant roots. In this perspective, AMF could act either as sources and/or drivers of the beneficial microbiota associated with their spores and mycelium, many of which showed specific PGP properties. The only findings available so far detected major shifts in the composition of root bacterial endophytes of durum wheat inoculated with AMF, which increased the abundance of Actinobacteria and Bacteroidetes, and Pantoea spp. (Proteobacteria). However, the putative role of AMF in shaping the structure of root bacterial endophytic communities, as well as the mechanisms underlying their recruitment are far from being understood and should be thoroughly investigated in the years to come, also considering the crucial importance of mycorrhizal symbioses in sustainable and organic agriculture.