The clathrin adaptor ECA4 negatively regulates plant immunity through modulating BDA1 levels

Plants employ plasma membrane-localized pattern recognition receptors (PRRs) to sense pathogens and activate downstream defense responses. One class of plant PRRs is receptor-like proteins (RLPs). In Arabidopsis, activation of RLP SUPPRESSOR OF NPR1-1, CONSTITUTIVE 2 (SNC2) signals through parallel transcription factors SYSTEMIC ACQUIRED RESISTANCE DEFICIENT 1 (SARD1) and CALMODULIN-BINDING PROTEIN 60 g (CBP60g). The autoimmunity of snc2-1D was partially suppressed in sard1-1 snc2-1D and cbp60g-1 snc2-1D. Here, we report the identification of a gain-of-function allele of EPSIN-LIKE CLATHRIN ADAPTOR 4 (ECA4), which suppresses the autoimmunity of sard1-1 snc2-1D. ECA4 encodes a clathrin adaptor involved in clathrin-mediated endocytosis (CME). The eca4-4D single mutant as well as ECA4 overexpression lines exhibit compromised resistance against pathogens, supporting its negative role in plant immunity. Further analysis showed that ECA4 negatively regulates the protein levels of BDA1, a transmembrane protein required for SNC2-mediated immunity. Consistently, eca4-4D partially suppresses the autoimmunity of bda1-17D, a gain-of-function allele of bda1. Taken together, unlike the known roles of clathrin and its homologous clathrin adaptor EPSIN1 in PRR endocytosis, ECA4 appears to suppress immunity through modulating the protein levels of BDA1.