Two residues reprogram immunity receptor kinases to signal in nitrogen-fixing symbiosis

Receptor signalling determines cellular responses and is crucial for defining specific biological outcomes. In legume root cells, highly similar and structurally conserved chitin and Nod factor receptor kinases activate immune or symbiotic pathways, respectively, upon perception of chitinous ligands. Here, we show that specific amino acid residues in the intracellular part of the Nod factor receptor NFR1 determine signalling specificity and enable the distinction between immune and symbiotic responses. Functional investigation of CERK6, NFR1 and receptor variants hereof revealed a conserved motif that we term Symbiosis Determinant 1 in the juxtamembrane region of the kinase domain that is key for symbiotic signalling. We demonstrate that two residues in Symbiosis Determinant 1 are indispensable hallmarks for NFR1-type receptors and are sufficient to convert Lotus CERK6 and barley RLK4 kinase outputs to enable symbiotic signalling in Lotus japonicus.