Unearthing the mechanisms of the mycorrhizal-bacterial symbiosis in plant roots using a metatranscriptomic approach

Arbuscular mycorrhizal fungi participate in a widely conserved symbiosis with a majority of land plants which provides plant hosts with increased capability for soil nutrient uptake. These endosymbiotic fungi are themselves colonized by a diverse group of bacteria, including both parasitic and symbiotic species. Recently several obligate endosymbionts of the arbuscular mycorrhizal fungi have been identified, and these bacteria have been shown to modulate both the metabolism and morphology of the fungal symbionts. However, molecular and functional characterization of these bacterial endosymbionts has been limited by an inability to isolate and culture such obligate symbionts, which have significant metabolic dependencies on the host fungi. In this work, a metatranscriptomic approach is applied in order to determine the transcriptional mechanisms underlying this multilayered symbiosis. Different mycorrhizal fungal species were found to be colonized by distinct communities of bacteria, and the study identified bacterial genes with significant differential abundance in mycorrhiza- inoculated plant roots as well as bacterial genes with varying abundance across the life cycle of the symbiosis. Overall, arbuscular mycorrhizal fungi harbor a diverse and metabolically active community of bacteria, and metatranscriptomics provides a capable tool to uncover the functional basis of such complex, obligate symbioses