Receptors localizing to the cell surface play an essential role in immunity of plants to microbial pathogens. Receptor-like proteins (RLPs) are ubiquitous and activate defence upon recognition of an appropriate pathogen-associated ligand. RLPs frequently contain extracellular leucine-rich repeats (LRRs) involved in pathogen perception. However, in contrast to receptor-like kinases (RLKs), they lack a cytoplasmic kinase domain for cellular signalling. The Cf proteins and Ve1 of tomato (Solanum lycopersicum; Sl) are examples of LRR-RLPs mediating resistance to the fungal pathogens Cladosporium fulvum and Verticillium dahliae, respectively. The mechanism by which these proteins trigger immunity has remained enigmatic for a long time. Recently, we discovered that the LRR-RLK SOBIR1 interacts with Cfs and Ve1 in planta and that this LRR-RLK is essential for their function (Liebrand, et al. (2013) PNAS 110:10010-10015). SOBIR1, which is highly conserved in plants, interacts with additional LRR-RLPs involved in immunity or development. We propose that SOBIR1 plays a key role by acting as a regulatory RLK for LRR-RLPs. Here we will study the role of SOBIR1 in LRR-RLP-mediated plant immunity, by (I) identifying its downstream interactors, (II) determining the phosphorylation status of its kinase domain in the presence or absence of the ligand matching the interacting LRR-RLP and (III) determining where SOBIR1 and LRR-RLPs interact at the subcellular level. Understanding SOBIR1 function in relation to LRR-RLP-mediated immunity provides insight into this novel and yet unexplored segment of the defence arsenal of plants and allows exploiting this regulator for generating durable and broad-spectrum resistance of plants to pathogens.