Systemins have only been identified in the Solaneae subtribe of the Solanaceae, but other members of the family, such as tobacco, also respond to wounding by systemically producing protease inhibitors. Peptides with similar functions In 2001, biologically active hydroxyproline-rich glycopeptides were isolated from tobacco which activated the production of protease inhibitors in a similar way to systemin in tomatoes. Although they are structurally unrelated to systemins, their similar function resulted in them being named hydroxyproline-rich systemins (HypSys). Following the initial discovery other HypSys peptides were found in tomato, Petunia and black nightshade. In 2007, HypSys were found outside the Solanaceae, in sweet potato (Ipomoea batatas) and sequence analysis identified HypSys analogs in poplar (Populus trichocarpa) and coffee (Coffea canephora). Systemins are highly conserved between species, whereas HypSys are more divergent but all contain a conserved proline or hydroxyproline-rich central domain. In 2006, AtPEP1, a 23 amino acid polypeptide was isolated from Arabidopsis thaliana, which was found to activate components of the innate immune response. Unlike HypSys, AtPEP1 is not post-translationally modified by hydroxylation or glycosylation. Six paralogs of the precursor have been identified in A. thaliana as well as orthologs in grape, rice, maize, wheat, barley, canola, soybean, medicago and poplar, although the activity of these orthologs has not been tested in assays. The predicted structures of the paralogs of AtPEP1 are varied within A. thaliana but all contain a SSGR/KxGxxN sequence motif. The orthologs identified in other species are more varied but still contain components of the sequence motif. Localisation and precursors Systemin and AtPEP1 are found in the cell cytosol. The precursor to tomato systemin is transcribed as a 200 amino acid polypeptide. It does not contain a putative signal sequence suggesting that it is synthesised on free ribosomes in the cytosol. The precursor to AtPEP1 is a 92 amino acid polypeptide and also lacks a signal sequence. In tomato, mRNA encoding the precursor for systemin is present at very low levels in unwounded leaves but accumulates upon wounding, particularly in the cells surrounding the sieve elements of the phloem in vascular bundles of mid veins. The precursor accumulates exclusively in the phloem parenchyma cells of leaves in tomato after wounding. The precursor to potato systemin is also localised in a similar manner suggesting it is under the same cell-type-specific regulation in both species. HypSys are localised in the cell wall. The precursor for tobacco HypSys is transcribed