The receptor-like protein (RLP) Cf-4, present in a constitutive complex with SOBIR1/EVR, recruits BAK1 to mount plant immunity

P382 - Plants perceive microbial patterns by cell surface receptors that are either receptor-like kinases (RLKs) or receptor-like proteins (RLPs), usually containing extracellular leucine-rich repeats (LRRs). However, RLPs lack an intracellular kinase domain for activation of downstream signaling upon ligand perception. Recently, we showed that tomato (Solanum lycopersicum, Sl) Cf-4, an LRR-RLP mediating resistance to Avr4-expressing strains of the fungal pathogen Cladosporium fulvum, constitutively interacts with the RLK SOBIR1. SOBIR 1 is unrelated to the RLK BRI1-Associated Kinase (BAK1), which is an essential regulatory LRR-RLK for pattern recognition receptors (PRRs), such as the LRR-RLK FLS2. Interestingly,Arabidopsis thaliana (At) SOBIR1 complements a knock-down in Nicotiana benthamiana (Nb) of NbSOBIR1 by restoring the Cf-4/Avr4-triggered HR, whereas a kinase-dead version of At SOBIR1 does not. Furthermore, overexpression of SOBIR1 in N. benthamiana results in an HR, which does not take place with a kinase-dead version of this RLK. The Cf-4/SOBIR1 complex does not dissociate in the presence of Avr4, suggesting that the complex acts as a two-component RLK. We found that the Cf-4/SOBIR1 complex localizes at the ARA7/ARA6-positive late endosomes upon activation with Avr4. Similar trafficking takes place for FLS2 upon its activation, a process which is BAK1-d ependent. Strikingly, we discovered that the Cf-4/SOBIR1 complex also recruits BAK1, an interaction which is required for both Avr4-triggered HR and Cf-4/SOBIR1 endocytosis. Thus, Cf-4 immune signalling is initiated by the formation of at least a tripartite receptor complex, showing that RLPs, ina complex with SOBIR1, function like bona fide PRRs. Future studies will focus on (1) the requirements of tripartite complex formation, (2) possible differential phosphorylation of SOBIR1 before and after Avr4/Cf-4-triggered BAK1 recruitment and (3) identification of downstream events that take place upon Avr4-mediated activation of the Cf-4/SOBIR1 complex.