A physical model links structure and function in the plant immune system.

Effector-Triggered Immunity (ETI) is an important part of the plant immune system, allowing plants to sense and respond to harmful pathogen proteins known as "effectors." Effectors can be sensed directly or indirectly by NLR (Nucleotide-binding Leucine-rich Repeat) proteins, many of which "guard" the plant proteins targeted by effectors. Although a few effector-target-NLR interactions have been characterized, a general understanding of how these molecular interactions give rise to a functioning immune system is lacking. Here, we present a physics-based model of ETI based on protein-protein interactions. We show that the simplest physical model consistent with the biology gives rise to a robust immune sensor and explains the empirical phenomenon of effector interference as a generic consequence of molecules competing for binding partners. Using the evolutionarily conserved ZAR1 defense gene as a model, we explain how more complex interaction networks integrate multiple pathogen signals into a single response. We then examine alternatives to a guarding architecture, including direct sensing, decoys, and blended "integrated decoy" strategies, and reveal that these sensing architectures obey functional trade-offs between their sensitivity, target protection, and proteomic cost. This allows a quantitative analysis of the trade-offs between different forms of ETI. We discuss these findings in the context of the evolutionary forces shaping the plant immune system.