Recruitment of SERK co-receptors determines signaling specificity within the systemin peptide family

The phytocytokine systemin (SYS1) regulates defense responses of tomato plants against herbivores and necrotrophic pathogens. For more than three decades, systemin signaling was thought to rely solely on SYS1, although the tomato genome encodes the precursors of at least three additional systemin-like peptides (SYS2, SYS3 and SYS4). We show that these recently discovered peptides are processed in a wound-inducible manner and act as genuine wound signals that activate early defense signaling via the receptor SYR1. Although all systemins depend on SYR1 for perception, the new peptides elicit downstream responses distinct from those triggered by SYS1, including sustained ethylene production and extensive transcriptional reprogramming. Signaling specificity is governed by a single C-terminal residue that determines the ability of SYS peptides to recruit SERK co-receptors into the receptor complex. The resulting increase in complex stability underlies the amplified and prolonged signaling responses observed for the new SYS peptides compared to SYS1. Our findings reveal how subtle sequence changes within the systemin peptide family can remodel receptor-co-receptor interactions to generate ligand-specific signaling outputs. This mechanism allows a single receptor to decode related ligands into distinct physiological responses and may represent a broader principle allowing for the diversification of peptide-mediated signaling in plants.