Throughout the Earth’s history, the mutualism between plants and their fungal partners has mediated nutrient cycles and energy flow in ecosystems. Underground, mycorrhizal fungi and plant roots form vast networks of connected individuals, in which sugars from roots are exchanged for nutrients from fungi.How is cooperation maintained in plant-fungal networks? Selfish individuals can potentially exploit the collaboration, reaping nutrient benefits while paying no costs. So, why cooperate at all?I recently demonstrated that plant and fungal partners are able to detect variation in nutrient provisioning by the other, and adjust their own strategy accordingly (Kiers et al. Science 2011). We argued that the partnership functions like an economic market: partners compete by trading resources, and those offering the best rate of exchange are rewarded.While this work suggests that plants and fungi can successfully negotiate conditions of trade, we have yet to conclusively demonstrate what drives ‘fair’ trade dynamics. In particular, we do not know how partner performance is evaluated, nor how trade strategies respond to changes in resource levels.I present an interdisciplinary program of research to address this problem by investigating four aspects critical to market regulation in nature: (1) Responses to external resources, (2) Partner decisions, (3) Network formation, (4) Conflict resolution within networks.Using a combination of gene-level characterization, microscale manipulation of nutrient landscapes, experimental evolution, and game theory, I will test: (1) how plant and fungal trading strategies respond to changing resource levels; (2) how hosts control fungal ‘behavior’, stimulating them to collect specific nutrients; (3) the role of fungal fusion in network formation; (4) how genetic conflicts within a fungal network are resolved.This work opens up a new field of research into how markets evolve and are stabilized in non-animal systems.