Symbiotic root infections in Medicago truncatula require remorin-mediated receptor stabilization in membrane nanodomains
Plant cell infection is tightly controlled by cell surface receptor-like kinases (RLKs) Alike other RLKs the Medicago truncatula entry receptor LYK3 laterally segregates into membrane nanodomains in a stimulus-dependent manner. Although nanodomain localization arises as a generic feature of plant membrane proteins, molecular mechanisms underlying such dynamic transitions and their functional relevance remained poorly understood. Here, we demonstrate that actin and the flotillin protein FLOT4 form the primary and indispensable core of a specific nanodomain. Infection-dependent induction of the remorin protein and secondary molecular scaffold SYMREM1 results in subsequent recruitment of ligand-activated LYK3 and its stabilization within these membrane subcompartments. Reciprocally, the majority of this LYK3 receptor pool is destabilized at the plasma membrane and undergoes rapid endocytosis in symrem1 mutants upon rhizobial inoculation resulting in premature abortion of host cell infections. These data reveal that receptor recruitment into nanodomains is indispensable for their function during host cell infection.\n\nSIGNIFICANCE STATEMENTPattern recognition receptors control the cellular entry of pathogenic as well as symbiotic microbes. While ligand-induced changes in receptor mobility at the plasma membrane and their localization in membrane nanodomains appears as a general feature, the molecular mechanism and the biological relevance of this phenomenon remained unknown. Here, we show that immobilization of the symbiotic cell entry receptor LYK3 in nanodomains requires the presence of actin and the two molecular scaffold proteins FLOT4 and SYMREM1. While FLOT4 forms the initial core structure, infection-induced expression and subsequent physical interaction of SYMREM1 with LYK3 stabilizes the activated receptors in membrane nanodomains. This recruitment prevents its stimulus-dependent endocytosis and ensures progression of the primary infection thread into root cortical cells.