Hyphosphere microbiome of arbuscular mycorrhizal fungi: a realm of unknowns

The extraradical hyphae-associated microbiome of arbuscular mycorrhizal fungi (AMF), the "hyphosphere microbiome," harbors a diverse reservoir of microbes. The biological interactions in the AMF hyphosphere have major implications for soil carbon and nutrient cycling, soil food web dynamics, and plant nutrition and health. Hyphosphere microbial communities are thought to assist AMF in accessing organic nutrients by degrading complex organic compounds that AMF are unable to do by themselves. The AMF, in return, provide an energy-rich microhabitat supplied with hyphal exudates that facilitates microbial growth and mobility in the hyphosphere. However, our current knowledge of hyphosphere entities, their trophic interactions and functional roles, and the underlying mechanisms facilitating microbial co-occurrence and co-operation is largely incomplete. Here, we review the current state of knowledge on the identity and putative roles of AMF hyphae-associated microbes, with a specific focus on prokaryotes, and potential drivers of such microbial communities in the hyphosphere. Moreover, we discuss the knowledge gaps and open challenges that should be addressed and prioritized in future studies on the AMF microbiomes. We also provide an appraisal of available and emerging tools and technologies and highlight the need for innovative approaches to disentangle AMF hyphosphere processes and answer the many unresolved questions.