First known polypeptide signal in plants. Systemically induces plant defenses in response to wounding of leaves by insects or pathogens. Proline-rich palindromic structure. Identification of activity: C. A. Ryan, Plant Physiol. 54, 328 (1974). Isolation from tomato leaves and characterization: G. Pearce et al., Science 253, 895 (1991). Purification by capillary electrophoresis: P. Mucha et al., J. Chromatogr. A 734, 410 (1996). Proton NMR assignments: D. J. Russell et al., J. Protein Chem. 11, 265 (1992). Structure-activity: T. Meindl et al., Plant Cell 10, 1561 (1998). Tertiary structure and mechanism of action: T. Specht et al., Plants Proteins Eur. Crops 1998, 41. Review: C. A. Ryan, G. Pearce, Annu. Rev. Cell Dev. Biol. 14, 1-17 (1998).