Community assembly of root-colonizing arbuscular mycorrhizal fungi : beyond carbon and into defence?

The arbuscular mycorrhizal (AM) fungi form symbiotic associations with the majority of terrestrial plants in a relationship estimated to be at least 470 million years old. This symbiosis supported the terrestrialization of plants by facilitating their access to belowground nutrients, such as phosphorus. Today, AM fungi associate with most land plants where, as obligate symbionts, they rely entirely on their hosts for access to carbon as carbohydrates and lipids. Yet the AM symbiosis does not exist in isolation. Simultaneous to AM fungal colonization, almost all plant hosts are subject to foliar damage from herbivores and pathogens. These antagonistic relationships are as ubiquitous as the AM symbiosis itself and have had significant impacts on the evolution and diversification of vegetation. Thus, this complex interplay among AM fungi, plants, and their herbivorous and pathogenic antagonists serves as a key driver in the ecological and evolutionary dynamics not only of the individual partners but also of global ecosystems. A fundamental goal in ecology is to understand the under- lying mechanisms of the assembly of communities. One of the most studied drivers of community assembly in ecology, particularly for plants and animals, is variation in resource availability. In this context, community assembly is driven by distinct niches reflecting differences among species in either their preferred type of resources, their resource requirements, or how they acquire resources in time and space. The principles that govern resource allocation and utilization in plant communities are, logically, considered applicable to AM fungi as well, underlining the universal relevance of resource availability in shaping ecological communities. Thus, it is not surprising that resource availability is one of the predominant mechanisms invoked to understand the assembly of AM fungi in plant roots from a local species pool. Dispersal factors, along with AM fungal preferences for certain soil characteristics and certain plant hosts, are all important in shaping the local species pool of AM fungi and ultimately the assembly of AM fungi in plant roots. What is surprising, however, is the limited attention given to understanding how antagonistic interactions, such as herbivory or pathogen infection, determine the outcome of resource availability on AM fungal community assembly.