Systemin
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.
The signal transduction processes that occur after the peptides bind are similar to the cytokine-mediated inflammatory immune response in animals.
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.
In 1991 a research group led by Clarence A. Ryan,[8] 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.
[9] Later studies identified homologs of tomato systemin in other members of the Solanaceae including potato, black nightshade and bell pepper.
[3] 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 2006, AtPEP1, a 23 amino acid polypeptide was isolated from Arabidopsis thaliana, which was found to activate components of the innate immune response.
[7] 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 to potato systemin is also localised in a similar manner suggesting it is under the same cell-type-specific regulation in both species.
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.
[16] These mutants are insensitive to brassinolide yet still respond to tomato systemin by producing protease inhibitors and causing an alkalisation response.
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.
[22] Cosilencing of two MAPKs, MPK1 and MPK2, in tomato compromised their defence response against insect larvae compared to wild type plants.
[23] Within minutes of systemin perception, the cytosolic Ca2+ concentration increases, and linolenic acid is released from cell membranes after a phospholipase has been activated.
Methyl jasmonate is volatile and can therefore activate systemic acquired resistance in neighbouring plants, preparing their defences for attack.
[1] This view was challenged by grafting experiments which showed that mutants deficient in jasmonic acid biosynthesis and perception were unable to activate systemic acquired resistance.
[15] The over-expression of the prosystemin resulted in a significant decrease of the larvae damage, indicating that a high level of constitutive protection is superior to an inducible defence mechanism.
[25] However, the continuous activation of prosystemin is costly, affecting the growth, the physiology and the reproductive success of tomato plants.
[26] When systemin was silenced, production of protease inhibitors in tomato was severely impaired and larvae feeding on the plants grew three times as fast.
[28] 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.
[30] 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.
Tomato cell cultures respond similarly, with systemin and UVB acting together to activate MAPKs.
Silencing and over-expression of HypSys does not affect the feeding performance of larvae compared to normal plants.
[36] Berger silenced HypSys and found that it caused changes in flower morphology which reduced the efficiency of self-pollination.
[37] Systemin also increases root growth in Solanum pimpinellifolium suggesting that it may also play some role in plant development.
