The Pik NLR pair accumulates at the plasma membrane as a hetero-oligomeric sensor-helper immune protein complex prior to activation
Following the perception of pathogen virulence proteins in plants, nucleotide-binding and leucine-rich repeat immune receptors (NLRs) are activated via a wide range of mechanisms. Singleton NLRs can both perceive effectors and trigger an immune response, whereas other NLRs specialise in either pathogen recognition (sensor NLRs) or activation of the immune response (helper NLRs). Sensor and helper NLRs can function as genetically linked pairs or in unlinked receptor networks. Although growing evidence suggests that NLRs conditionally oligomerise upon activation, our understanding of the resting state of NLRs prior to effector perception remains limited. Here, we investigated the oligomeric state of the genetically linked rice (Oryza sativa) sensor Pik-1 and helper Pik-2 NLR pair prior to effector activation when transiently expressed in Nicotiana benthamiana leaves. We show that both wild-type Pikm-1 and engineered Pikm-1Enhancer sensors associate with Pikm-2 and form ~1 MDa hetero-complexes in the resting state that accumulate at the plasma membrane. Our findings contribute to the growing evidence that pre-activation mechanisms vary widely across NLRs. This knowledge could be leveraged for disease resistance engineering strategies complementary to approaches focussing solely on effector binding.
