A conserved extracellular LRR-only protein disrupts BAK1-mediated immune receptor formation to negatively regulate...

Pattern-triggered immunity (PTI) forms the first barrier of plant defense against pathogens, but prolonged immune activation is costly and hampers growth. Therefore, plants employ diverse negative regulators to maintain immune homeostasis. Although intracellular regulators have been well characterized, the roles of extracellular components remain elusive. Notably, plants encode numerous extracellular leucine-rich repeat-only (eLRR) proteins lacking signaling domains, but their functions in immune regulation are largely unexplored. We performed genome-wide screening in Nicotiana benthamiana and identified NbLRRP1, an eLRR protein with high structural similarity to the extracellular domain of central hub NbBAK1 in PTI signaling, suggesting a potential functional association. NbLRRP1 is secreted into the apoplast, where it broadly suppresses PTI induced by multiple pathogen-associated molecular patterns (PAMPs). NbLRRP1-knockout plants display no visible growth defects but markedly enhance resistance to Sclerotinia sclerotiorum. Mechanistically, NbLRRP1 directly interacts with NbBAK1 and disrupts the formation of immune receptor complexes, such as NbBAK1-RE02 and NbBAK1-NbFLS2, thereby attenuating prolonged PTI activation. NbLRRP1 orthologs are broadly distributed among diverse food crops, cash crops, and forest trees, and seem functionally conserved. Collectively, these findings reveal a conserved eLRR-mediated mechanism and highlight this eLRR protein as a promising molecular target for crop disease resistance breeding.