Molecular mechanisms of plant NLR activation and signalling

Plants rely on NLRs (nucleotide-binding leucine-rich repeat receptors) to recognise effector proteins secreted by pathogens into plant cells and to deliver disease resistance. Plant NLRs are broadly characterised by their N-terminal domains, which include the TIR (Toll/interleukin-1 receptor) and the CC (coiled-coil) domains. Effector recognition triggers NLR oligomerisation into complexes termed resistosomes, which initiate immune signalling. Some NLRs function as singletons that detect pathogens and activate immune responses, while there are NLRs that only recognise effectors and thereby require helper NLRs or genetically linked 'paired' NLRs to execute immune signalling. Recent studies have enhanced our understanding of the molecular mechanisms of different classes of NLRs, as well as how downstream proteins are recruited to signal upon effector recognition. In this review, we discuss the current knowledge of the NLR activation mechanisms, based on findings from recent structural and functional studies. We also highlight the remaining unknowns in the field and discuss current and potential future applications for enhancing plant immunity by engineering plant NLRs.