Bacteria associated with spores of arbuscular mycorrhizal fungi: a review

Arbuscular mycorrhizal fungi (AMF) are important components of terrestrial ecosystems and sustainable agricultural practices, forming mutually beneficial symbiotic associations with roots of most land plants including crops by providing them with nutrients in exchange for photosynthesis-derived carbon and protection from biotic and abiotic stresses. Microorganisms, including bacteria, are emerging as important new players in the AMF-plant symbiosis by influencing symbiotic outcome. Specifically, bacteria can tightly associate with AMF extraradical mycelium and spores. Spores are produced mainly in the soil and function in reproduction and dissemination. Spores are large and full of lipids and therefore represent an important soil carbon pool as well as an attractive niche for soil bacteria, termed the “sporosphere”. Intriguingly, diverse bacterial communities have been found in different parts of the sporosphere environment, and they may possess roles in complementing and enhancing AMF function. In this review, we describe the AMF sporosphere environment and summarize our current knowledge of spore-associated bacterial communities including their composition, assembly, function and localization. We also identify knowledge gaps and future research directions as well as highlight spores as model systems for studying AMF–microbe interactions.