SOBIR1-mediated immunity requires conserved tyrosine residues in its kinase domain

Receptor-like proteins (RLPs) that sense non-self molecules, induce immune signalling to promote plant resistance against pathogens. Leucine-rich repeat (LRR)-RLPs are transmembrane (TM) proteins without an intracellular kinase domain, which constitutively interact with the LRR receptor-like kinase (RLK) SOBIR1/EVR (SUPPRESSOR OF BIR1-1/EVERSHED) to form signalling-competent receptor complexes. Perception of Avr4 from Cladosporium fulvum by the tomato RLP Cf-4 recruits the co-receptor BRI1-ASSOCIATED KINASE 1 (BAK1) to the SOBIR1-Cf-4 complex, thereby activating Cf-4-mediated immunity. Here we identify that SOBIR1 induces auto-immunity in N. tabacum and N. benthamiana in a BAK1-dependent manner. The SOBIR1 ectodomain, the GxxxGxxxG protein-protein interaction motif located in its TM domain, and a functional kinase domain are all essential for SOBIR1 auto-immune activity. The SOBIR1 kinase domain contains highly conserved tyrosine (Tyr) residues, one of which is located between the N- and C-lobe, and a second Tyr residue which is located just after the activation segment. We determine that the Tyr residue located between the N- and C-lobe is required for both SOBIR1 auto-immunity and the Cf-4-mediated hypersensitive response (HR), whereas the second Tyr residue located close to the C-terminal end of the activation segment is essential for SOBIR1 auto-immunity and plays a minor role in the HR.