Molecular recognition in plant root endosymbiosis

Root endosymbiosis is one of the most intimate types of plant-microbe interactions in which a symbiont is hosted within a living plant cell. In this symbiosis, nitrogen-fixing bacteria and arbuscular mycorrhizal fungi support plant nutrient provisioning in exchange for carbohydrates and lipids. To establish these mutually beneficial interactions, plants have evolved complex surveillance mechanisms. These largely rely on the perception of microbe-derived N-acetylglucosamine derivatives by cell-surface receptors that feature a lysine-motif (LysM) ectodomain. Because these derivatives are not exclusive to symbiotic microbes, understanding how plants specifically recognise microsymbionts from the diverse array of soil rhizosphere microbiota has driven genetic, molecular and structural studies in recent years. In this review, we will summarise the current state of the art in molecular recognition by LysM receptors, provide an updated inventory of LysM receptors in land plants with annotated functionalities and highlight key aspects of signal transduction that lead to symbiont accommodation in living plant cells.