Arbuscular mycorrhizal fungal highways – What, how and why?

Arbuscular mycorrhizal (AM) fungal structures, such as spores, vesicles, extra- and intra-radical hyphae, are home to diverse communities of prokaryotic (bacterial and archaeal) taxa and can have significant impact on the movement and behavior of those microbes within the soil. The ability to disperse via fungal hyphae, known as the fungal highway effect, was first observed in ectomycorrhiza and later in other fungal groups. This effect may benefit soil prokaryotes allowing them to explore new microhabitats in soil, offering advantages such as improved nutrient access, enhanced dispersal and colonization. Although the term “fungal highways” is well established, there still are only few studies that address the partner interactions and movement of microorganisms through the hyphal networks when referring to AM fungi. Bacteria can colonize the surfaces of hyphae and form biofilms that provide protection for both the bacteria and the fungus, influence the nutrient cycles, giving the bacteria access to resources transported by the fungus. Bacteria movement on AM fungal hyphae is facilitated by several mechanisms, including physical transport along the hyphal networks, swimming within the continuous water films that develop along the hyphal surfaces, and chemotaxis, where the bacteria move towards or away from specific chemical signals. Overall, the interactions between bacteria and AM fungi appears as a dynamic and complex process. Yet, we still do not know much about the influence of soil properties, plant age and species, seasonality, soil management and different climate with respect to AM fungal highways and microbiomes. Here, we review the current knowledge on prokaryotic movement through AM fungal hyphosphere and the possible factors that could affect it. Future research needs to elucidate mechanisms involved in the recruitment and/or migration of microbes in the AM fungal hyphosphere. Understanding these interactions may eventually help developing more sustainable agricultural practices and/or support environmental conservation.