Most land plants rely on the symbiosis with arbuscular mycorrhizal (AM) fungi to acquire sufficient mineral nutrients from soils.Plants host AM fungi inside root cortex cells,where the fungus forms highly branched structures called arbuscules.Arbuscules are surrounded by a specialized host membrane across which nutrients are exchanged in a cooperative manner.In retu for mineral nutrients,the fungus obtains sugars and fatty acids from the plant (Luginbuehl and Oldroyd,2017).Intriguingly,it has been shown that both plant and fungus can adjust the amounts of nutrients exchanged depending on the benefits that they obtain from their partner (Kiers et al.,2011).This is thought to constitute a biological market that stabilizes the symbiosis.The plant monitors its needs and the nutrients that it obtains and controls arbuscule development accordingly.Therefore,understanding the molecular basis of arbuscule development and reciprocal nutrient exchange is of major importance for optimizing AM-plant benefits.