A hierarchical immune receptor network in lettuce reveals contrasting patterns of evolution in sensor and helper NLRs

Nucleotide-binding domain and leucine-rich repeat immune receptors (NLRs) are known for their rapid evolution, yet evolutionary rates differ significantly across NLR classes. In Asterids, within the NRC (NLR Required for Cell Death) network, NLRs function in sensor–helper configurations to confer immunity against diverse pathogens. While helper NLRs are typically conserved, sensor NLRs tend to evolve rapidly. However, the functional connections between these slow- and fast-evolving NLRs remain poorly understood, especially in crops. Here, we conducted a comparative analysis of NLRs across 40 Solanales and 29 Asterales genomes to investigate NRC network expansion and diversification within the lesser-studied Asterales. Our findings show that the NRC network has expanded less in Asterales relative to Solanales. We functionally validated a minimal Asterales NRC network with two helpers and nine sensors in common lettuce (Lactuca sativa). Through selection and entropy analyses coupled with structural modeling of NRC helper and sensor subclades in the Lactuca genus, we observed sensor diversification rates correlated with helper dependency, with sensors relying on a phylogenetically conserved helper experiencing limited diversification. Our results highlight the lineage- and function-specific evolution of the NRC network and offer insights into the evolutionary pressures shaping plant immune receptor networks.