Now published in Current Opinion in Plant Biology, 50:121-131. NLRs are modular plant and animal proteins that are intracellular sensors of pathogen-associated molecules that trigger a potent broad-spectrum immune reaction known as the hypersensitive response. An emerging paradigm is that plant NLRs form immune signalling networks with varying degrees of complexity. NLRs may have evolved from multifunctional singleton receptors, which combine pathogen detection (sensor activity) and immune signalling (helper or executor activity) into a single...