Knowing your friends and foes – plant receptor‐like kinases as initiators of symbiosis or defence

<jats:title>Summary</jats:title><jats:p>The decision between defence and symbiosis signalling in plants involves alternative and modular plasma membrane‐localized receptor complexes. A critical step in their activation is ligand‐induced homo‐ or hetero‐oligomerization of leucine‐rich repeat (<jats:styled-content style="fixed-case">LRR</jats:styled-content>)‐ and/or lysin motif (LysM) receptor‐like kinases (<jats:styled-content style="fixed-case">RLK</jats:styled-content>s). In defence signalling, receptor complexes form upon binding of pathogen‐associated molecular patterns (<jats:styled-content style="fixed-case">PAMP</jats:styled-content>s), including the bacterial flagellin‐derived peptide flg22, or chitin. Similar mechanisms are likely to operate during the perception of microbial symbiont‐derived (lipo)‐chitooligosaccharides. The structurally related chitin‐oligomer ligands chitooctaose and chitotetraose trigger defence and symbiosis signalling, respectively, and their discrimination involves closely related, if not identical, LysM‐<jats:styled-content style="fixed-case">RLK</jats:styled-content>s. This illustrates the demand for and the challenges imposed on decision mechanisms that ensure appropriate signal initiation. Appropriate signalling critically depends on abundance and localization of <jats:styled-content style="fixed-case">RLK</jats:styled-content>s at the cell surface. This is regulated by internalization, which also provides a mechanism for the removal of activated signalling <jats:styled-content style="fixed-case">RLK</jats:styled-content>s. Abundance of the malectin‐like domain (<jats:styled-content style="fixed-case">MLD</jats:styled-content>)‐<jats:styled-content style="fixed-case">LRR</jats:styled-content>‐<jats:styled-content style="fixed-case">RLK</jats:styled-content> Symbiosis Receptor‐like Kinase (<jats:styled-content style="fixed-case">SYMRK</jats:styled-content>) is additionally controlled by cleavage of its modular ectodomain, which generates a truncated and rapidly degraded <jats:styled-content style="fixed-case">RLK</jats:styled-content> fragment. This review explores <jats:styled-content style="fixed-case">LRR</jats:styled-content>‐ and LysM‐mediated signalling, the involvement of <jats:styled-content style="fixed-case">MLD</jats:styled-content>‐<jats:styled-content style="fixed-case">LRR</jats:styled-content>‐<jats:styled-content style="fixed-case">RLK</jats:styled-content>s in symbiosis and defence, and the role of endocytosis in <jats:styled-content style="fixed-case">RLK</jats:styled-content> function.</jats:p>