Complex networks of nutritional interactions between plant beneficial symbionts and associated bacteria

Arbuscular mycorrhizal fungi (AMF) form mutualistic symbiosis with the roots of most food crops and play a key role in plant nutrient uptake, while protecting host plants from biotic and abiotic stresses. The establishment and efficiency of mycorrhizal symbioses are affected by bacteria living in association with AMF spores, hyphae and mycorrhizal roots (mycorrhizosphere), which show different functional abilities, including indole acetic acid (IAA) production, nitrogen fixation, solubilization of phosphate and phytates. By using a culture-independent approach, PCR-DGGE and sequence analysis, we previously identified bacterial species belonging to Arthrobacter, Bacillus, Herbaspirillum, Massilia, Pseudomonas, Rhizobium, Streptomyces associated with Rhizoglomus intraradices IMA6 spores. Here, a culture-dependent approach was utilized to isolate such bacteria and detect their functional traits. From a pool of 374 strains, 122 were selected and characterized for P-solubilizing, chitinase, nitrogen fixing activity and siderophores and IAA production. Phosphatase and phytase activities were detected in 73% and 100% of Actinobacteria, in 74% and 83% of chitinolytic bacteria and in 44% and 52% of nitrogen-fixers. The emerging picture of mycorrizospheric interactions is one of a previously unimagined complexity, where different partners of a tripartite association - host plants, AMF and bacteria - may positively interact and provide new multifunctional benefits. Indeed, AMF associated bacteria may be transported along hyphae to the relevant soil volume explored, where they may enhance nutrient availability, control plant pathogens and promote plant growth. Further studies should investigate whether different compositions of AMF-associated bacterial communities may determine differential performances of AMF isolates, in order to select the best AMF/bacteria combinations to be utilised as biofertilisers and bioenhancer