Systemin, hydroxyproline-rich systemin and the induction of protease inhibitors

Systemin, an 18-amino acid signaling peptide isolated from tomato leaves, has been found to be an integral component of the jasmine acid signaling pathway, leading to the synthesis of protease inhibitors (PIs). The discovery of systemin has led to a search for other peptide signals involved in defense in the Solanaceae and in other plant families. A new class of peptides having similar signaling properties but little sequence homology to systemin have been found and termed hydroxyproline-rich glycopeptide systemins (HypSys). These small (18-20 amino acids) glycopeptides, like systemin, are derived from larger precursor proteins (proHypSys) and until recently were thought to function only in protection from herbivore attack. However, HypSys peptides isolated from petunia induced the defensin gene, known for its involvement in pathogen defense. More recently, a HypSys glycopeptide was isolated from sweet potato, a member of the Convolvulaceae family and found to induce the sporamin gene which codes for the major storage protein in tubers with trypsin inhibitor activity. These recent discoveries expand the function and range of the HypSys family of glycopeptides and establish these unique inducible signaling molecules as potential components of defense pathways throughout the Eudicots. Herein we review the signaling and structural properties of systemin and the HypSys glycopeptides and their roles in the induction of PIs.