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The innate immune system detects pathogen-derived molecules via specialized immune receptors to prevent infections^{, –}. Plant immune receptors include cell surface-resident pattern recognition receptors (PRRs, including receptor-like kinases (RLKs)), and intracellular nucleotide-binding domain leucine-rich repeat proteins (NLRs). It remains enigmatic how RLK- and NLR-mediated signalling are connected. Disruption of an immune-activated MEKK1–MKK1/2–MPK4 MAPK cascade activates the NLR SUMM2 via the MAPK kinase kinase MEKK2, leading to autoimmunity^{, , , , –}. To gain insights into the mechanisms underlying SUMM2 activation, we used an RNA interference-based genetic screen for mekk1 autoimmune suppressors and identified an uncharacterized malectin-like RLK, named LETUM1 (LET1), as a specific regulator of mekk1–mkk1/2–mpk4 autoimmunity via complexing with both SUMM2 and …