Genetic and functional diversity of arbuscular mycorrhizal fungal spore associated bacteria occurring in Mediterranean sand dunes

Arbuscular mycorrhizal fungi (AMF) are beneficial soil microorganisms that establish mutualistic symbioses with the roots of most plants. They promote plant growth, nutrition and tolerance to both biotic and abiotic stresses. The key functions of AMF are facilitated and complemented by bacterial communities living associated with their hyphae and spores, and playing important roles as plant growth promoting and mycorrhiza helper bacteria. This study aimed at molecularly and functionally characterizing the bacterial communities strictly associated with AMF spores occurring in the rhizosphere of Ammophila arenaria, a xerophytic plant inhabiting maritime sand dunes, a nutrient-poor and harsh environment. High-throughput Illumina sequencing was used to analyse the bacterial communities associated with the two dominant AMF species, Racocetra fulgida and Racocetra persica. Notably, two endosymbionts, Ca. Moeniiplasma glomeromycotorum and Ca. Glomeribacter gigasporarum, differentially occurred in the two AMF species, together with highly diversified bacterial communities affiliated to 69 and 34 genera in R. persica and R. fulgida, respectively (251 Amplicon Sequence Variants). Most of the identified bacteria belonged to culturable genera and species, reported to possess key beneficial activities. In order to analyse the functional diversity of such bacterial communities we isolated a total of 203 and 81 bacterial strains from R. persica and R. fulgida spores, respectively. Their ability to produce extracellular polymeric substances (EPS), key compounds that enhance water retention and protect roots from desiccation, was assessed. 91 strains identified as high EPS producers were subsequently screened for salinity tolerance. Among them, 13 isolates were able to grow in the presence of up to 10% NaCl, 10 of which also produced 1-aminocyclopropane-1- carboxylate deaminase and indole-3-acetic acid. Such bacterial isolates will be further tested in vivo to identify the best performing ones to be used in the formulation of effective microbial consortia, able to enhance plant productivity and resilience in harsh agroecosystems.