Regulation of resource exchange in the arbuscular mycorrhizal symbiosis

Arbuscular mycorrhizal (AM) fungi are one of the most important groups of plant symbionts. These fungi provide mineral nutrients to plants in exchange for carbon. Although substantial amounts of resources are exchanged, the factors that regulate trade in the AM symbiosis are poorly understood. Recent evidence for the reciprocally regulated exchange of resources by AM fungi and plants has led to the suggestion that these symbioses operate according to biological market dynamics, in which interactions are viewed from an economic perspective, and the most beneficial partners are favoured. Here we present five arguments that challenge the importance of reciprocally regulated exchange, and thereby market dynamics, for resource exchange in the AM symbiosis, and suggest that such reciprocity is only found in a subset of symbionts, under specific conditions. We instead propose that resource exchange in the AM symbiosis is determined by competition for surplus resources, functional diversity and sink strength.