In plants, internal signaling is essential for the regulation of growth, reproduction and defense responses. The coordination of these functions is indispensable for normal growth as well as for adaptation to a changing environment. The coordination of these signalling cascades is achieved in part through plant hormones, which are generally derived from lipids or sterols. Recently, however, plant peptides with hormone-like signaling functions were described. Systemin, an 18-aa oligopeptide in tomato (Solanum lycopersicum), attracted interest as a signal that possibly transmits information from an herbivore-attacked site to distal parts of the plant. When applied to tomato plants, systemin induced the accumulation of proteinase inhibitors (PIs) even in plant parts distant from the application site. However, the idea that systemin itself acts as a mobile signal in antiherbivore defense was later discarded. In the current model, systemin is instead supposed to amplify a jasmonate-derived mobile signal at the site of wounding. Tobacco (Nicotiana tabacum) does not possess a systemin homologue, but responds to wounding or herbivory with the elicitation of PIs. In this genus, hydroxyproline-rich glycopeptides (HypSys) are hypothesized to mediate the induction of PIs. Both systemin in tomato and HypSys in cultivated tobacco are, despite their lack of phylogenetic similarity, reported to fulfil similar functions in defense signaling. Similarities in defense signaling activities of systemin and HypSys have led to the inclusion of both in a functionally defined systemin family.