Biochemical basis of activation and inhibition of an NLR/PRR immune receptor network

The plant immune system utilizes intracellular nucleotide-binding domain and leucine-rich repeat (NLR) receptors to recognize and respond to pathogen-secreted proteins, termed effectors. NLRs can function individually or in higher-order configurations, such as pairs or networks. In solanaceous plants, the NLR- required for cell death (NRC) network is a complex NLR network that mediates resistance against diverse pathogens by connecting sensor NLRs and cell surface receptors to helper NLRs of the NRC clade. While the genetics underlying the NRC network are well characterized, until recently, our understanding of the biochemical mechanisms of activation in this network were limited. We established native PAGE electrophoresis using the NRC-dependent sensor NLR Rx and its helper NRCs as an experimental system to study the activation mechanisms of the NRC network. We report that upon pathogen perception, Rx mediates the formation of NRC homo-oligomers. The activated NRC resistosomes then accumulate at the plant plasma membrane without the sensor NLR Rx. Additionally, we discovered that the previously characterized nematode effector SPRYSEC15, which suppresses NRC function, can inhibit NRC2 oligomerization and plasma membrane association to prevent immune signaling. To overcome this suppression, we bioengineered an NRC2 variant that is resilient to this inhibition and capable of restoring immune signalling. Our work establishes an activation-and-release model for sensor-helper activation of NLRs in the NRC network and leverages this understanding to bioengineer disease resistance.