Extraradical hyphae (ERH) of arbuscular mycorrhizal fungi (AMF) extend from plant roots into the soil environment and interact with soil microbial communities. Evidence of positive and negative interactions between AMF and soil bacteria point to functionally important ERH-associated communities. To characterize communities associated with ERH and test controls on their establishment and composition, we utilized an in-growth core system containing a live soil–sand mixture that allowed manual extraction of ERH for 16S rRNA gene amplicon profiling. Across experiments and soils, consistent enrichment of members of the Betaproteobacteriales, Myxococcales, Fibrobacterales, Cytophagales, Chloroflexales, and Cellvibrionales was observed on ERH samples, while variation among samples from different soils was observed primarily at lower taxonomic ranks. The ERH-associated community was conserved between two fungal species assayed, Glomus versiforme and Rhizophagus irregularis, though R. irregularis exerted a stronger selection and showed greater enrichment for taxa in the Alphaproteobacteria and Gammaproteobacteria. A distinct community established within 14 days of hyphal access to the soil, while temporal patterns of establishment and turnover varied between taxonomic groups. Identification of a conserved ERH-associated community is consistent with the concept of an AMF microbiome and can aid the characterization of facilitative and antagonistic interactions influencing the plant-fungal symbiosis.

Sorghum is an important crop grown worldwide for feed and fibre. Like most plants, it has the capacity to benefit from symbioses with arbuscular mycorrhizal (AM) fungi, and its diverse genotypes likely vary in their responses. Currently, the genetic basis of mycorrhiza-responsiveness is largely unknown. Here, we investigated transcriptional and physiological responses of sorghum accessions, founders of a bioenergy nested association mapping panel, for their responses to four species of AM fungi.Transcriptome comparisons across four accessions identified mycorrhiza-inducible genes; stringent filtering criteria revealed 278 genes that show mycorrhiza-inducible expression independent of genotype and 55 genes whose expression varies with genotype. The latter suggests variation in phosphate transport and defence across these accessions. The mycorrhiza growth and nutrient responses of 18 sorghum accessions varied tremendously, ranging from mycorrhiza-dependent to negatively mycorrhiza-responsive. Additionally, accessions varied in the number of AM fungi to which they showed positive responses, from one to several fungal species. Mycorrhiza growth and phosphorus responses were positively correlated, whereas expression of two mycorrhiza-inducible phosphate transporters, SbPT8 and SbPT9, correlated negatively with mycorrhizal growth responses. AM fungi improve growth and mineral nutrition of sorghum, and the substantial variation between lines provides the potential to map loci influencing mycorrhiza responses.