Plant beneficial activities of the microbiota strictly associated with 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. Here we investigated the plant beneficial activities of the microbiota 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. In particular, key bacterial traits fundamental for the improvement of plant resilience toward abiotic stresses, such as drought and salinity were investigated on the strains isolated in pure culture from R. persica and R. fulgida spores, respectively. Functional analyses were performed to reveal the ability of the bacterial isolates to produce exopolysaccharides (EPS), key compounds favouring water retention and protecting roots against desiccation. The 90 strains producing the highest EPS levels were further analysed for selecting those able to tolerate high salinity concentrations. The 13 selected ones were successively tested for their ability to produce the enzyme 1-aminocyclopropane-1- carboxylate (ACC) deaminase, able to lower the level of ethylene in stressed plants, alleviating its adverse effects, and promoting plant growth. The best performing bacterial isolates will be used for the formulation of effective synthetic microbial consortia as innovative inocula promoting plant growth and resilience under climate change.