From pathogen effectors to plant enhancers - Harpin proteins as novel biostimulants: A review

Harpin proteins were originally identified as hypersensitive response-eliciting factors produced by bacterial plant pathogens. Over the past three decades, however, accumulating evidence has revealed a paradoxical and agriculturally valuable role for these proteins: when applied exogenously, harpins function as potent plant biostimulants that enhance immunity, stress tolerance, and productivity. Despite this growing body of work, a unified synthesis linking their structural properties, molecular mechanisms, and agronomic outcomes remains lacking. This review is an integration of structural, biophysical, molecular, and applied studies to clarify how harpins transition from pathogen-associated effectors to plant enhancers. Harpins activate multiple defense signaling pathways, prominently salicylic acid-mediated systemic acquired resistance (SAR), while also modulating ethylene, jasmonic acid, and abscisic acid networks in a context-dependent manner. Interaction with host components such as HIPM proteins and aquaporins emerges as a key mechanistic theme linking perception to downstream transcriptional reprogramming. Beyond immunity, harpins promotes growth, yield, and crop quality across diverse species, with some inconsistencies observed under field conditions. A phylogenetic analysis clarifies the evolutionary relationships within the harpin superfamily, distinguishing ancestral core lineages from more recently diversified clades. Collectively, this review reframes harpins not merely as bacterial virulence-associated proteins, but as multifunctional signaling molecules with translational potential in sustainable agriculture. We identify current mechanistic gaps and propose future research directions aimed at improving formulation stability, receptor characterization, and field reliability.