The arbuscular mycorrhizal (AM) symbiosis is arguably the worldÂ´s most abundant and important mutualism, and brings together the roots of the majority of land plant species and AM fungi to great mutual advantage. The extraradical mycelium of the fungus acts as an extension of the root system and increases the uptake of nutrients, such as phosphate and nitrogen, but also of trace elements such as copper and zinc. In exchange for the beneficial effects on nutrient uptake, the host plant transfers 4 to 20% of its assimilated carbon to the AM fungus. However, the AM symbiosis follows none of the constraints thought to stabilize cooperation in other well-known mutualisms and despite its significance, the mechanisms that control cooperative behavior between both partners are largely unknown. The goal of this project is to answer the following questions: (i) can fungal strains and plant hosts detect non-cooperative partners; (ii) what are the physiological mechanisms employed by partners to control the interaction and punish non-cooperators; and (iii) how does the diversity of partners influence the strength of punishment? These questions will be studied by (i) tracer experiments to determine cost to benefit ratios of cooperative and non-cooperative partners and to track resource allocation between partners under different conditions, (ii) unique 3-chamber root organ cultures to manipulate levels of defection and cooperation of partners, (iii) stable isotope probing to track carbon allocation to different partners in simulated ecosystems, and (iv) microcosm experiments to determine how plant diversity affects punishment and the fitness of different fungal strains. By studying three levels of interactions (cellular, whole plant, plant community), the different scales that stabilize cooperative behavior in the AM symbiosis will be identified. The AM symbiosis is responsible for massive amounts of global nutrient transfer in ecosystems, and plays a key role in global ecosystem stability and productivity. Investigating these processes is critical to maximizing ecosystem and agricultural benefits of the symbiosis. The project will provide training for undergraduate and graduate students, and the students will become members of an international collaborative effort and will have the opportunity to work in the laboratories of the collaborators in the Netherlands and in Canada.