Systemin

Systemin is a plant peptide hormone involved in the wound response in the Solanaceae family. It was the first plant hormone that was proven to be a peptide having been isolated from tomato leaves in 1991 by a group led by Clarence A. Ryan. Since then, other peptides with similar functions have been identified in tomato and outside of the Solanaceae. Hydroxyproline-rich glycopeptides were found in tobacco in 2001 and AtPEPs (Arabidopsis thaliana Plant Elicitor Peptides) were found in Arabidopsis thaliana in 2006. Their precursors are found both in the cytoplasm and cell walls of plant cells, upon insect damage, the precursors are processed to produce one or more mature peptides. The receptor for systemin was first thought to be the same as the brassinolide receptor but this is now uncertain. The signal transduction processes that occur after the peptides bind are similar to the cytokine-mediated inflammatory immune response in animals. Early experiments showed that systemin travelled around the plant after insects had damaged the plant, activating systemic acquired resistance, now it is thought that it increases the production of jasmonic acid causing the same result. The main function of systemins is to coordinate defensive responses against insect herbivores but they also affect plant development. Systemin induces the production of protease inhibitors which protect against insect herbivores, other peptides activate defensins and modify root growth. They have also been shown to affect plants' responses to salt stress and UV radiation. AtPEPs have been shown to affect resistance against oomycetes and may allow A. thaliana to distinguish between different pathogens. In Nicotiana attenuata, some of the peptides have stopped being involved in defensive roles and instead affect flower morphology. Systemin is a plant peptide hormone involved in the wound response in the Solanaceae family. It was the first plant hormone that was proven to be a peptide having been isolated from tomato leaves in 1991 by a group led by Clarence A. Ryan. Since then, other peptides with similar functions have been identified in tomato and outside of the Solanaceae. Hydroxyproline-rich glycopeptides were found in tobacco in 2001 and AtPEPs (Arabidopsis thaliana Plant Elicitor Peptides) were found in Arabidopsis thaliana in 2006. Their precursors are found both in the cytoplasm and cell walls of plant cells, upon insect damage, the precursors are processed to produce one or more mature peptides. The receptor for systemin was first thought to be the same as the brassinolide receptor but this is now uncertain. The signal transduction processes that occur after the peptides bind are similar to the cytokine-mediated inflammatory immune response in animals. Early experiments showed that systemin travelled around the plant after insects had damaged the plant, activating systemic acquired resistance, now it is thought that it increases the production of jasmonic acid causing the same result. The main function of systemins is to coordinate defensive responses against insect herbivores but they also affect plant development. Systemin induces the production of protease inhibitors which protect against insect herbivores, other peptides activate defensins and modify root growth. They have also been shown to affect plants' responses to salt stress and UV radiation. AtPEPs have been shown to affect resistance against oomycetes and may allow A. thaliana to distinguish between different pathogens. In Nicotiana attenuata, some of the peptides have stopped being involved in defensive roles and instead affect flower morphology. In 1991 a research group led by Clarence A. Ryan, isolated an 18 amino acid polypeptide from tomato leaves that induced the production of protease inhibitor proteins (PIs) in response to wounding. Experiments using synthetic radio-labelled forms of the polypeptide demonstrated that it was able to travel systemically through the plant and induce PI production in unwounded leaves. Because of the systemic nature of the wounding signal, it was named systemin, it was the first polypeptide found to function as a hormone in plants. mRNA encoding for systemin is found in all tissues of the plant except the roots. Later studies identified homologs of tomato systemin in other members of the Solanaceae including potato, black nightshade and bell pepper. 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. 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. 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 as a 165 amino acid polypeptide which has no structural homology to the precursor for systemin in tomato. The structural properties of HypSys, containing hydroxyproline and being glycosylated, indicate that they are synthesised through the secretory system. The precursor to HypSys in tomato is a 146 amino acid polypeptide, exclusively synthesised within the vascular bundles of leaves and petioles associated with parenchyma cells of phloem bundles. Unlike systemin, it is primarily associated with the cell wall. The precursors to HypSys appear to represent a distinct subfamily of hydroxyproline-rich proteins found in cell walls. Upon wounding it is thought that a protease from the cytosol, the cell wall matrix, or the pathogen, processes the precursor producing active HypSys peptides. The precursors for systemin and AtPEP1 are both processed to yield one active peptide from the C-terminus of the precursor. It has been speculated that ProAtPEP1 is processed by CONSTITUTIVE DISEASE RESISTANCE 1, an apoplastic aspartic protease. The precursors to HypSys are processed into more than one active peptide. In tobacco, it is processed into two peptides, in petunia into three, and in sweet potato, possibly into six. At 291 amino acids long, the precursor to HypSys in sweet potato is the longest precursor described. The production of multiple signalling peptides from one precursor is a common feature found in animals. Exceedingly small amounts of tomato systemin are active, femto-molar concentrations of the peptide are sufficient to elicit a response at the whole plant level, making it one of the most potent gene activators identified.A receptor for tomato systemin was identified as a 160KDa leucine-rich repeat receptor like kinase (LRR-RLK), SR160. After being isolated it was found that was very similar in structure to BRI1 from A. thaliana, the receptor that brassinolides bind to on the cell membrane. This was the first receptor which was found to be able to bind both a steroid and a peptide ligand and also to be involved in both defensive and developmental responses. Recent studies have found that the initial conclusion that BRI1 is the receptor for tomato systemin may be incorrect. In cu3 mutants of tomato, a null allele with a stop codon present in the extracellular LRR domain of BRI1 prevents the receptor from being localised correctly and it also lacks the kinase domain, required for signalling. These mutants are insensitive to brassinolide yet still respond to tomato systemin by producing protease inhibitors and causing an alkalisation response. This led Holton et al. to suggest that there is another mechanism by which systemin is perceived. Further investigation showed that binding of systemin to BRI1 does not cause the receptor to become phosphorylated, as when brassinolides bind, suggesting that it does not transduce a signal. When BRI1 is silenced in tomato, the plants have a similar phenotype to cu3 mutants yet are still able to respond normally to systemin, strengthening the view that BRI1 is not the systemin receptor. In 1994, tomato systemin was found to bind to a 50KDa protein in the cell membrane of tomato. The protein has a structure similar to proteases of the Kex2p-like prohormone convertases. This led Schaller and Ryan to suggest that it is not a receptor, but instead is involved in the processing of ProSys into the active form, or the degradation of Sys. Synthetic forms of tomato systemin, with substituted amino acids at the predicted dibasic cleavage site, remained stable in cell cultures for longer than the native form. Later studies have noted that the enzymes responsible for processing ProSys remain unidentified. No further research has been reported on the 50KDa protein to date, and the gene has not been identified.