Signatures of the coevolutionary arms race in response to effector-triggered immunity in the Pseudomonas syringae HopZ type III effector family

Host-pathogen interactions are coevolutionary battles often described as arms races, requiring evolutionary change in both partners, functional consequences, and selection driven by the interaction. Plant-pathogen systems provide strong models, especially through effectors and host immune receptors. We investigated the HopZ family of Pseudomonas syringae type III effectors and their recognition by Arabidopsis thaliana receptors ZAR1 and RPM1. Using ancestral reconstruction, we synthesized ancestral HopZ alleles, mapped evolutionary changes, and functionally tested extant and ancestral variants for effector-triggered immunity (ETI) elicitation. We identified four independent transitions from ETI-eliciting to non-eliciting alleles via sequence diversification, pseudogenization, and expression modulation, the latter revealing that reduced effector expression mitigates ETI costs to bacterial fitness. We also detected a host-driven transition from non-eliciting to eliciting alleles through recruitment of a new immune receptor. These findings highlight the diverse strategies by which pathogens evade recognition and hosts expand immune surveillance.