The functionality of arbuscular mycorrhizal networks across scales of experimental complexity and ecological relevance

1. One of the most prevalent symbioses on Earth is that formed between the majority of land plants and arbuscular mycorrhizal (AM) fungi. Through these intimate associations, AM fungi transfer soil nutrients to their plant hosts in exchange for photosynthetically fixed carbon resources. 2. It has been hypothesised that this nutritional mutualism is evolutionarily stable because both partners are in control of the exchange of resources and can discriminate between partners according to whichever offers the highest returns. 3. However, in nature, plant–AM symbioses are exposed to a wealth of additional biotic and abiotic interactions which can affect the regulation of carbon-for-nutrient exchange between symbionts. Moreover, the extraradical hyphae of AM fungi make up underground networks that may be interactive or physically connected, known as common mycorrhizal networks (CMNs). These can link neighbouring plants, potentially further influencing resource distribution across the network. How these layers of complexity interact to influence resource regulation and allocation between plants and AM fungi is not often considered by experimental designs. 4. Here, we review resource allocation in AM symbioses, scaling up from evidence from reductionist experimental systems using axenic root organ cultures to complex systems incorporating multiple neighbouring plants dealing with other, co-occurring symbionts. 5. As experimental designs increase in scale and ecologically relevant complexity, the carbon-for-nutrient exchange between plants and their AM symbionts is increasingly subject to disruption associated with the wider ecological context, such as the intricacies of the plant-fungal interactions in a CMN or the presence of co-occurring organisms.