Two for all: receptor-associated kinases SOBIR1 and BAK1

•SOBIR1 epitomises a long-sought partner specifically required for receptor-like protein function. •BAK1 is required for receptor-like kinase and receptor-like protein function. •SOBIR1 and BAK1 have co existed since land plants emerged and may co-reside in receptor-like protein-containing immune receptor complexes. Leucine-rich repeat-receptor-like proteins (LRR-RLPs) are ubiquitous cell surface receptors lacking a cytoplasmic signalling domain. For most of these LRR-RLPs, it remained enigmatic how they activate cellular responses upon ligand perception. Recently, the LRR-receptor-like kinase (LRR-RLK) SUPPRESSOR OF BIR1-1 (SOBIR1) was shown to be essential for triggering defence responses by certain LRR-RLPs that act as immune receptors. In addition to SOBIR1, the regulatory LRR-RLK BRI1-ASSOCIATED KINASE-1 (BAK1) is also required for LRR-RLP function. Here, we compare the roles of SOBIR1 and BAK1 as regulatory LRR-RLKs in immunity and development. BAK1 has a general regulatory role in plasma membrane-associated receptor complexes comprising LRR-RLPs and/or LRR-RLKs. By contrast, SOBIR1 appears to be specifically required for the function of receptor complexes containing LRR-RLPs. Leucine-rich repeat-receptor-like proteins (LRR-RLPs) are ubiquitous cell surface receptors lacking a cytoplasmic signalling domain. For most of these LRR-RLPs, it remained enigmatic how they activate cellular responses upon ligand perception. Recently, the LRR-receptor-like kinase (LRR-RLK) SUPPRESSOR OF BIR1-1 (SOBIR1) was shown to be essential for triggering defence responses by certain LRR-RLPs that act as immune receptors. In addition to SOBIR1, the regulatory LRR-RLK BRI1-ASSOCIATED KINASE-1 (BAK1) is also required for LRR-RLP function. Here, we compare the roles of SOBIR1 and BAK1 as regulatory LRR-RLKs in immunity and development. BAK1 has a general regulatory role in plasma membrane-associated receptor complexes comprising LRR-RLPs and/or LRR-RLKs. By contrast, SOBIR1 appears to be specifically required for the function of receptor complexes containing LRR-RLPs. an LRR-RLK that interacts with BAK1/SERK3 and negatively regulates defence mediated by LRR-RLKs and cytoplasmic NB-LRR proteins. a RLCK that is part of the receptor complex involving the LRR-RLK BRI1 (involved in development), or LRR-RLK PRRs, such as FLS2 (involved in immunity). Together with its close homologues, AvrPphB-SUSCEPTIBLE-1 (PBS1) and PBS1-like proteins, BIK1 participates in ligand-dependent phosphorylation events and is released from the receptor complexes after ligand perception. BIK1 negatively regulates BRI1 responses, whereas it positively regulates FLS2 responses. a member of the SERK family of regulatory LRR-RLKs. BAK1/SERK3 interacts in a ligand-dependent manner with the LRR-RLK BRI1 (involved in development), as well as with several LRR-RLK-PRRs involved in immunity. BAK1/SERK3 is also required for LRR-RLP function. This LRR-RLK acts as a co-receptor and participates in phosphorylation events and downstream signalling and thereby functions as a regulatory LRR-RLK of both LRR-RLKs and LRR-RLPs involved in plant development and plant innate immunity. a RLCK that interacts with the LRR-RLKs SOBIR1 and HAE/HSL2 and is proposed to promote stabilisation and internalisation of these LRR-RLKs. a class of tomato LRR-RLPs involved in resistance to the biotrophic fungal leaf pathogen Cladosporium fulvum. Cf proteins mediate recognition of small secreted proteins of the fungus that function as effectors. Cf-9 is the founding member and additional Cf proteins that have been identified are Cf-2, Cf-4, Cf-4E, and Cf-5. kinase containing a transmembrane domain and a short extracellular N-terminal sequence. CRN interacts with the LRR-RLP CLV2 and primarily participates in its function, independently of the CLV1 pathway. an LRR-RLK that functions as a PRR for the bacterial MAMP ELONGATION FACTOR-TU (EF-Tu), or the elicitor-active peptide elf18 derived from this MAMP. an LRR-RLK that functions as a PRR for the bacterial MAMP flagellin, or the elicitor-active peptide flg22 derived from this MAMP. syntenic genomic regions inferred from collinear series of homologous genes among two or more genomic regions, which suggests that these genomic regions are derived from a common ancestral genomic region rather than resulting from random gene shuffling. Syntenic regions can be derived from different species, or from the same genome as a result of genome duplication events. an LRR-RLK that is involved in floral organ shedding by perception of the predicted endogenous ligand IDA. structural protein motif primarily involved in protein–protein interactions. a class of cell surface receptors carrying an extracellular LRR domain, a transmembrane domain, and a cytoplasmic kinase domain. The LRRs are involved in ligand binding, whereas the kinase domain can be involved in auto- and/or trans-phosphorylation and is potentially able to initiate a signal transduction cascade. a class of cell surface receptors carrying an extracellular LRR domain, a transmembrane domain, and a short cytoplasmic tail. The cytoplasmic tail lacks any obvious signalling domains. a structurally conserved microbial pattern (MAMP) or damage-derived host pattern (DAMP) triggering a defence response upon recognition by a host PRR. paralogous genes that originated as a result of a polyploidy event, for example a whole-genome duplication (WGD) or triplication (WGT) event. homologous genes that descend from the same ancestral sequence, separated by a speciation event. Orthologues often have retained the same function in different species. homologous genes created by a (block) duplication event within a genome. Paralogues might not have retained the same function and they can show various degrees of sequence divergence. a host-encoded receptor capable of recognising a MAMP and/or a DAMP. a class of proteins carrying a cytoplasmic kinase domain similar to LRR-RLKs, but lacking an extracellular and transmembrane domain. RLCKs can be attached to the membrane via post-translational modifications, such as myristoylation. Some RLCKs interact with LRR-RLKs (e.g., BIK1) and participate in phosphorylation events, whereby these proteins take part in initiating a signal transduction cascade. an LRR-RLK that interacts with a ligand-perceiving receptor, either constitutively or in a ligand-dependent fashion. Regulatory LRR-RLKs may act as co-receptors and participate in downstream signalling by amplifying the initial signal by trans-phosphorylation events. a regulatory LRR-RLK originally identified as a positive regulator of defence responses and an inhibitor of floral organ shedding. SOBIR1/EVR interacts constitutively with LRR-RLPs and is specifically required for LRR-RLP function.