Genetic and functional diversity of bacterial communities strictly associated with the spores of arbuscular mycorrhizal fungi isolated from the rhizosphere of Ammophila arenaria

The rhizosphere is a dynamic environment colonized by a wide variety of microorganisms, including arbuscular mycorrhizal fungi (AMF) and bacteria that are strictly associated with their spores and mycelium. They promote plant growth and nutrition, and increase plant tolerance to abiotic stresses by improving plant water and nutrient-use efficiency and plant antioxidant defence systems. The aim of this study was the molecular and functional characterization of bacterial communities strictly associated with AMF spores occurring in the rhizosphere of Ammophila arenaria growing in a maritime sand dune system, a drought-stressed and low-fertility environment. Illumina MiSeq analyses of bacteria strictly associated with two Racocetra species allowed the identification of 14 Phyla, including Bacillota (66.1%) and Actinomycetota (8.2%). 253 Amplicon Sequence Variants out of 281 included culturable bacteria, most of which known for their plant-growth promoting properties. The bacterial community found in Racocetra persica was richer than that of Racocetra fulgida and 16 indicator species were found, including some rhizobia and actinobacteria species. Interestingly, two endosymbiotic bacteria of AMF were found. Culture-dependent analyses allowed the isolation in pure culture of 203 and 81 strains from R. persica and R. fulgida spores, respectively. Interestingly, diverse bacterial communities were associated with the spores of the two AMF, although originated from the same host plants and environmental conditions, showing that each AMF isolate recruits on its spores a different microbiota. Functional analyses showed the ability of many bacterial strains to produce high levels of exopolysaccharides, key compounds for plant drought tolerance, favouring water retention and protecting roots against desiccation. Moreover, different bacterial strains were able to produce a number of plant growth promoting compounds. This work selected the best performing bacterial isolates to be further tested in the formulation of effective microbial consortia and applied as innovative inocula promoting plant growth and resilience under climate change.