Plant immune co-receptors: bridging gaps toward next-generation crop defense

Plant innate immunity is a crucial, multi-layered defense system that protects crops from a wide range of bacterial, fungal, and viral pathogens, all of which pose a significant threat to global food security. Co-receptors function as central molecular modulators that amplify and fine-tune immune signaling. This signaling is initiated by cell-surface pattern-recognition receptors (PRRs) and intracellular nucleotide-binding leucine-rich repeat (NLR) proteins. Critical co-receptors for strong immune signal propagation include the SOMATIC EMBRYOGENESIS RECEPTOR KINASE (SERK) protein family, represented by the versatile BAK1, and the SUPPRESSOR OF BIR1-1 (SOBIR1) adaptor. These components work cooperatively to convert pathogen-derived molecular signals into effective cellular defense mechanisms, including the production of reactive oxygen species (ROS), calcium ion (Ca²⁺) fluxes, and the activation of mitogen-activated protein kinase (MAPK) signaling. However, several major knowledge gaps persist. It remains largely unclear how co-receptors physically and functionally orchestrate cross-talk between the two main tiers of defense, pattern-triggered immunity (PTI) and effector-triggered immunity (ETI). A persistent challenge is understanding how shared co-receptors, such as BAK1, avoid disrupting essential developmental pathways, including brassinosteroid (BL) signaling. Furthermore, the spatial and temporal coordination of downstream events within specific plasma membrane microdomains remains poorly understood. In this review, we integrate recent functional and genetic evidence to explain the multifaceted roles of these co-receptors in immune amplification and signaling integration. We discuss the challenges posed by functional redundancy and signaling specificity, and highlight their transformative potential for engineering broad-spectrum disease resistance in crops through precision breeding and targeted modulation strategies.