From recognition to proteolytic control: NLRs and metacaspases in plant antiviral immunity

Plant immunity requires not only intracellular recognition of pathogen invasion but also downstream execution mechanisms that directly restrict pathogen proliferation and spread. Nucleotide-binding leucine-rich repeat receptors (NLRs) function as core components of innate immunity in both plants and animals. In animals, NLR activation initiates caspase-dependent immune signaling pathways. In contrast, plants lack caspases but instead contain metacaspases (MCAs/MCs). MCs are Ca²⁺-dependent cysteine proteases that mediate selective proteolysis during immune-associated cellular reprogramming. In the context of plant antiviral immunity, recent studies support roles for MCs in antiviral defense and position their activation within NLR-mediated immune networks. Notably, Ca²⁺ influx downstream of NLR activation is proposed to contribute to the coupling of immune recognition and proteolytic execution. In this review, we synthesize current advances in NLR- and MC-mediated plant immunity, with a particular emphasis on antiviral immunity, and propose a conceptual framework in which recognition and proteolytic execution constitute coordinated yet mechanistically distinct layers of plant immune responses, with implications for immune engineering aimed at enhancing resistance against diverse viral diseases.