Plant carbon nourishment of arbuscular mycorrhizal fungi

•Carbohydrates are required to sustain intraradical AM fungal growth.•Carbohydrate metabolism and signalling plays a role in fine-tuning AM colonization.•AM fungi are fatty acid (FA) heterotrophs.•Arbuscules, FA uptake and completion of the AM fungal life cycle are functionally linked. Reciprocal nutrient exchange between the majority of land plants and arbucular mycorrhizal (AM) fungi is the cornerstone of a stable symbiosis. To date, a dogma in the comprehension of AM fungal nourishment has been delivery of host organic carbon in the form of sugars. More recently a role for lipids as alternative carbon source or as a signalling molecule during AM symbiosis was proposed. Here we review the symbiotic requirement for carbohydrates and lipids across developmental stages of the AM symbiosis. We present a role for carbohydrate metabolism and signalling to maintain intraradical fungal growth, as opposed to lipid uptake at the arbuscule as an indispensible requirement for completion of the AM fungal life cycle.