Arbuscular mycorrhiza (AM) fungi are key components in sustainable agriculture as they enable efficient nutrient uptake by crops when chemical fertilization is reduced. AM fungal functioning is, however, influenced by other microbes that interact with their hyphae. One of these microorganisms is the recently discovered genus Collimonas. Collimonas bacteria show a great potential application in sustainable agriculture both as a biocontrol agent and as a P-solubilizer, based on its mycophagy (feed on fungi) and mineral weathering abilities. Here, I propose for the first time, the possible application of AM fungi and Collimonas bacteria as a consortium of beneficial microorganisms for sustainable agriculture purposes. The successful application is dependent on our understanding of the interaction between Collimonas and AM fungi. The aim of this proposal is to gain insights on the mechanisms involved in the interaction, its relevance in natural ecosystems and its influence on plant performance. The Collimonas-AM fungal interaction has both positive (increased hyphal branching, higher supply of nutrients) and negative (consumption of AM hyphae) components. My general hypothesis is that the interaction between both microorganisms commonly occurs in nature and results in a positive mutualistic relationship from which plants benefit. The hyphosphere would provide a beneficial environment for Collimonas growth, which in turn would increase nutrient availability for AM fungi and, consequently, for the plant. To test my hypothesis I propose a highly interdisciplinary approach combining advanced molecular, biochemical and microscopic techniques in natural ecosystems, microcosms and in vitro experiments. The results are expected to bring out new leads for novel strategies in sustainable agriculture by improving AM fungi functioning through Collimonas inoculation. The main benefits of the program for my career are enhanced independency and experience in cutting-edge techniques.