Systemin

Plant peptide hormone

[[File:Systemin structures. png|thumb|238px|Structures and predicted structures of systemins, HypSys and AtPeps 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. 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. The precursor to AtPEP1 is a 92 amino acid polypeptide and also lacks a signal sequence. The structural properties of HypSys, containing hydroxyproline and being glycosylated, indicate that they are synthesised through the secretory system.

The precursors for systemin and AtPEP1 are both processed to yield one active peptide from the C terminus of the precursor. 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. 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. 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. BRI1 associated receptor kinase 1 (BAK1) is an LRR RLK found in A. thaliana, which has been proposed to function as an adaptor protein that is required for the proper functioning of other RLKs. Yeast two hybrid assays have shown that AtPEPR1 and its closest analog, AtPEPR2, interact with BAK1.

[[File:Sytemin signalling. png|thumb|Proposed system of signal transduction for systemin. Cosilencing of two MAPKs, MPK1 and MPK2, in tomato compromised their defence response against insect larvae compared to wild type plants. Cosilencing these genes also decreased production of jasmonic acid and of jasmonic acid dependent defence genes. Applying methyl jasmonate to cosilenced plants rescued them, indicating that jasmonates are the signal responsible for causing changes in gene expression. Within minutes of systemin perception, the cytosolic Ca2+ concentration increases, and linolenic acid is released from cell membranes after a phospholipase has been activated. Linolenic acid is then converted to jasmonic acid via the octadecanoid pathway and jasmonic acid activates defensive genes. Early experiments with radiolabelled systemin in tomato demonstrated that it is transported through the phloem sap in tomato plants and was therefore thought to be the systemic signal that activated systemic acquired resistance. However, the continuous activation of prosystemin is costly, affecting the growth, the physiology and the reproductive success of tomato plants. When systemin was silenced, production of protease inhibitors in tomato was severely impaired and larvae feeding on the plants grew three times as fast. HypSys caused similar changes in gene expression in tobacco, for example polyphenol oxidase activity increased tenfold in tobacco leaves and protease inhibitors caused a 30% decrease in chymotrypsin activity within three days of wounding. The concentration of hydrogen peroxide increased in the vasculature tissues when the production of systemin, HypSys or AtPep1 is induced, this may also be involved in initiating systemic acquired resistance. Tomato plants over expressing systemin also accumulated HypSys but did not if the systemin precursor was silenced, indicating that in tomato, HypSys is controlled by systemin. Each of the three HypSys peptides in tomato is able to activate the synthesis and accumulation of protease inhibitors. When applied through cut petioles in Petunia, HypSys did not induce the production of protease inhibitors, but instead increased expression of defensin, a gene which produces a protein that inserts into microbial membranes, forming a pore. Systemin also upregulates the expression of genes involved in the production of biologically active VOCs. Such a response is crucial if antinutritional defences are to be effective, since without predators, developing insects would consume more plant material while completing their development. It is likely that VOC production is upregulated through different pathways,

Different AtPeps may allow A. thaliana to distinguish between different pathogens. When inoculated with a fungus, oomycete and a bacterium, the increases in AtPep expression varied depending on the pathogen. A. thaliana overexpressing AtProPep1 was more resistant to the oomycete Phythium irregulare. By contrast HypSys were up regulated and activated the synthesis of protease inhibitors. When prosystemin was over expressed in tomato, transgenic plants had lower stomatal conductance than normal plants. When grown in salt solutions, transgenic plants had higher stomatal conductances, lower leaf concentrations of abscisic acid and proline and a higher biomass. These findings suggest that systemin either allowed the plants to adapt to salt stress more efficiently or that they perceived a less stressful environment.

In Nicotiana attenuata HypSys is known to not be involved in defence against insect herbivores. Silencing and over expression of HypSys does not affect the feeding performance of larvae compared to normal plants. Berger silenced HypSys and found that it caused changes in flower morphology which reduced the efficiency of self pollination. The flowers had pistils that protruded beyond their anthers, a similar phenotype to CORONATINE INSENSITIVE1 silenced plants which lack a jasmonate receptor. Measurement of jasmonate levels in the flowers revealed that they were lower than in normal plants. The authors suggested that HypSys peptides in N. attenuata have diversified from their function as defence related peptides to being involved in controlling flower morphology. The signalling processes remain similar however, being mediated through jasmonates. Systemin also increases root growth in Solanum pimpinellifolium suggesting that it may also play some role in plant development.