Systemin
Encyclopedia
Systemin is a plant peptide hormone involved in the wound response in the Solanaceae
plant family, it was the first plant hormone
that was proven to be a peptide
. It was discovered in tomato leaves in 1991, 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 peptides) were found in Arabidopsis thaliana
in 2006. Their precursors are found both in the cytoplasm
and cell walls of plant cell
s, 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 defensin
s 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.
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 homolog
s 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.
-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.
. 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 ribosome
s 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.
ic 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.
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 brassinolide
s 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.
No receptors for HypSys have so far been reported, but it is thought that they are perceived on the cell membrane by a LRR-RLK.
The receptor for AtPep1 has been identified as a 170KDa LRR-RLK and has been named AtPEPR1. AtPep1 is active at 0.1 nano-molar (nM) concentrations and the receptor saturates at 1nM. An analysis of the structure of the AtPEPR1 receptor has shown that it is a member of the LRR XI subfamily of LRR-RLKs in A. thaliana which includes the receptor for another peptide hormone CLAVATA3. Transforming tobacco cell cultures with AtPEPR1 allowed them to respond to AtPep1 in an alkalisation assay, whereas normal tobacco did not show such a response. 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 assay
s have shown that AtPEPR1 and its closest analog, AtPEPR2, interact with BAK1.
that occurs once the peptide had bound to its receptor. Jasmonic acid
is an essential, albeit late component, in the systemin and wound-signalling pathways. In tomato, the signal is transduced from the receptor by mitogen-activated protein kinase
s (MAPKs). 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. The alkalisation of the apoplast is a downstream effect of signalling processing by MAPKs. Applying fusicoccin
, which activates the H+ ATPase
inhibited by systemin, along with systemin still activates MAPKs, even though the pH of the apoplast does not change.
Within minutes of systemin perception, the cytosol
ic 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. Production of methyl jasmonate is induced by systemins and also upregulates systemin precursor genes creating a feedback loop, amplifying the defensive signal. Methyl jasmonate is volatile and can therefore activate systemic acquired resistance
in neighbouring plants, preparing their defences for attack. These signalling events are analogous to the cytokine
-mediated inflammatory immune response
in animals. When the inflammatory response is activated in animals, MAPKs are activated which in turn activate phospholipases. Lipids in the membrane are converted to arachidonic acid
and then to prostaglandins, which are analogs of jasmonic acid. Both pathways can be inhibited by suramin
.
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. 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. It is now thought that jasmonic acid is the systemic signal and that systemin upregulates the pathways for jasmonic acid synthesis.
Systemin plays a critical role in defence signalling in tomato. In promotes the synthesis of over 20 defence-related proteins, mainly antinutritional proteins, signaling pathway proteins and proteases. 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. When HypSys was over-expressed in tobacco, larvae feeding on transgenic plants weighed half as much after ten days feeding, as those feeding on normal plants. 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 HypSys is silenced the production of protease inhibitors induced by wounding is halved compared to wild type plants indicating that both systemin and HypSys are required for a strong defence response against herbivores in tomato.
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. Defensin expression is also induced by AtPEP1.
Tomato plants over-expressing systemin produced more volatile organic compound
s (VOCs) than normal plants and parasitoid wasps find them more attractive. 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, including oxylipin
pathway that synthesises jasmonic acid aldehyde
s and alcohol
s that function in wound healing.
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.
Silencing systemin did not affect the ability of black nightshade to resist herbivory and, when competing against normal plants, silenced plants produced more above-ground biomass and berries. Upon herbivory, systemin was down-regulated in black nightshade in contrast to the other peptides which are up-regulated after herbivory. By contrast HypSys were up-regulated and activated the synthesis of protease inhibitors. The down-regulation of systemin was associated with increased root mass but did not decrease shoot mass, demonstrating that systemin can cause developmental changes as a result of herbivory, allowing the plant to tolerate, rather than directly resist attack. Tomato roots were also affected by tomato systemin, with root growth increasing at high tomato systemin concentrations. By allocating more resources to the roots, plants under attack are thought to store carbon and then use it to re-grow when the attack ends. Overexpressing AtPEP1 also increased root and shoot biomass in A. thaliana.
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. Similarly, wounded tomato plants were less susceptible to salt stress than unwounded plants. This may be because wounding decreases the growth of the plant and therefore slows the uptake of toxic ions into the roots. An analysis of salt-induced changes in gene expression found that the differences measured between the transgenic and normal plants could not be accounted for by changes in conventional salt stress-induced pathways. Instead Orsini et al. suggested that the activation of the jasmonic acid pathway determines a physiological state that not only directs resources towards the production of compounds active against pests, but also pre-adapts plants to minimize water loss. These effects are achieved by negatively regulating the production of hormones and metabolites that will force plants to invest additional resources to counteract water loss, a secondary effect of herbivores.
Plants grown under UVB light are more resistant to insect herbivory compared with plants grown under filters that exclude the radiation. When tomato plants are exposed to a pulse of UVB radiation and then weakly wounded, PIs accumulate throughout the plant. By themselves, neither the radiation nor weak wounding is sufficient to induce systemic PI accumulation. Tomato cell cultures respond similarly, with systemin and UVB acting together to activate MAPKs. Short pulses of UVB also cause alkalisation of the culturing medium.
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.
Solanaceae
Solanaceae are a family of flowering plants that include a number of important agricultural crops as well as many toxic plants. The name of the family comes from the Latin Solanum "the nightshade plant", but the further etymology of that word is unclear...
plant family, it was the first plant hormone
Plant hormone
Plant hormones are chemicals that regulate plant growth, which, in the UK, are termed 'plant growth substances'. Plant hormones are signal molecules produced within the plant, and occur in extremely low concentrations. Hormones regulate cellular processes in targeted cells locally and, when moved...
that was proven to be a peptide
Peptide
Peptides are short polymers of amino acid monomers linked by peptide bonds. They are distinguished from proteins on the basis of size, typically containing less than 50 monomer units. The shortest peptides are dipeptides, consisting of two amino acids joined by a single peptide bond...
. It was discovered in tomato leaves in 1991, since then other peptides, with similar functions have been identified in tomato and outside of the Solanaceae. Hydroxyproline
Hydroxyproline
-4-Hydroxyproline, or L-hydroxyproline , is a common non-proteinogenic amino acid, abbreviated as HYP, e.g., in Protein Data Bank.-Structure and discovery:...
-rich glycopeptides
Glycoprotein
Glycoproteins are proteins that contain oligosaccharide chains covalently attached to polypeptide side-chains. The carbohydrate is attached to the protein in a cotranslational or posttranslational modification. This process is known as glycosylation. In proteins that have segments extending...
were found in tobacco
Nicotiana
Nicotiana is a genus of herbs and shrubs of the nightshade family indigenous to North and South America, Australia, south west Africa and the South Pacific. Various Nicotiana species, commonly referred to as tobacco plants, are cultivated and grown to produce tobacco. Of all Nicotiana species,...
in 2001 and AtPEPs (Arabidopsis thaliana peptides) were found in Arabidopsis thaliana
Arabidopsis thaliana
Arabidopsis thaliana is a small flowering plant native to Europe, Asia, and northwestern Africa. A spring annual with a relatively short life cycle, arabidopsis is popular as a model organism in plant biology and genetics...
in 2006. Their precursors are found both in the cytoplasm
Cytoplasm
The cytoplasm is a small gel-like substance residing between the cell membrane holding all the cell's internal sub-structures , except for the nucleus. All the contents of the cells of prokaryote organisms are contained within the cytoplasm...
and cell walls of plant cell
Plant cell
Plant cells are eukaryotic cells that differ in several key respects from the cells of other eukaryotic organisms. Their distinctive features include:...
s, upon insect damage, the precursors are processed to produce one or more mature peptides. The receptor
Receptor (biochemistry)
In biochemistry, a receptor is a molecule found on the surface of a cell, which receives specific chemical signals from neighbouring cells or the wider environment within an organism...
for systemin was first thought to be the same as the brassinolide
Brassinolide
Brassinolide is a plant hormone. The first isolated brassinosteroid, it was discovered when it was shown that pollen from rapeseed could promote stem elongation and cell division. The biologically active component was isolated and named brassinolide....
receptor but this is now uncertain. The signal transduction
Signal transduction
Signal transduction occurs when an extracellular signaling molecule activates a cell surface receptor. In turn, this receptor alters intracellular molecules creating a response...
processes that occur after the peptides bind are similar to the cytokine
Cytokine
Cytokines are small cell-signaling protein molecules that are secreted by the glial cells of the nervous system and by numerous cells of the immune system and are a category of signaling molecules used extensively in intercellular communication...
-mediated inflammatory immune response
Inflammation
Inflammation is part of the complex biological response of vascular tissues to harmful stimuli, such as pathogens, damaged cells, or irritants. Inflammation is a protective attempt by the organism to remove the injurious stimuli and to initiate the healing process...
in animals. Early experiments showed that systemin travelled around the plant after insects had damaged the plant, activating systemic acquired resistance
Systemic acquired resistance
The systemic acquired resistance is a "whole-plant" resistance response that occurs following an earlier localized exposure to a pathogen. SAR is analogous to the innate immune system found in animals, and there is evidence that SAR in plants and innate immunity in animals may be evolutionarily...
, now it is thought that it increases the production of jasmonic acid
Jasmonic acid
Jasmonic acid is derived from the fatty acid linolenic acid. It is a member of the jasmonate class of plant hormones. It is biosynthesized from linolenic acid by the octadecanoid pathway....
causing the same result. The main function of systemins is to coordinate defensive responses against insect
Insect
Insects are a class of living creatures within the arthropods that have a chitinous exoskeleton, a three-part body , three pairs of jointed legs, compound eyes, and two antennae...
herbivores but they also affect plant development. Systemin induces the production of protease inhibitors
Protease inhibitor (biology)
In biology and biochemistry, protease inhibitors are molecules that inhibit the function of proteases. Many naturally occurring protease inhibitors are proteins....
which protect against insect herbivores, other peptides activate defensin
Defensin
Defensins are small cysteine-rich cationic proteins found in both vertebrates and invertebrates. They have also been reported in plants. They are, and function as, host defense peptides. They are active against bacteria, fungi and many enveloped and nonenveloped viruses. They consist of 18-45 amino...
s 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
Nicotiana attenuata
Nicotiana attenuata is a species of wild tobacco known by the common name coyote tobacco. It is native to western North America from British Columbia to Texas and northern Mexico, where it grows in many types of habitat. It is a glandular and sparsely hairy annual herb exceeding a meter in maximum...
, some of the peptides have stopped being involved in defensive roles and instead affect flower morphology.
Discovery and structure
In 1991, an 18 amino acid polypeptide was isolated from tomato leaves, that induced the production of protease inhibitorProtease inhibitor
Protease inhibitor can refer to:* Protease inhibitor : a class of medication that inhibits viral protease* Protease inhibitor : molecules that inhibit proteases...
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 homolog
Homology (biology)
Homology forms the basis of organization for comparative biology. In 1843, Richard Owen defined homology as "the same organ in different animals under every variety of form and function". Organs as different as a bat's wing, a seal's flipper, a cat's paw and a human hand have a common underlying...
s of tomato systemin in other members of the Solanaceae
Solanaceae
Solanaceae are a family of flowering plants that include a number of important agricultural crops as well as many toxic plants. The name of the family comes from the Latin Solanum "the nightshade plant", but the further etymology of that word is unclear...
including potato
Potato
The potato is a starchy, tuberous crop from the perennial Solanum tuberosum of the Solanaceae family . The word potato may refer to the plant itself as well as the edible tuber. In the region of the Andes, there are some other closely related cultivated potato species...
, black nightshade
Black nightshade
Black nightshade may refer to:* Solanum americanum of much of North America* Solanum nigrum of Europe* Solanum ptychanthum of the Caribbean region...
and bell pepper
Bell pepper
Bell pepper, also known as sweet pepper or a pepper and capsicum , is a cultivar group of the species Capsicum annuum . Cultivars of the plant produce fruits in different colors, including red, yellow, orange and green. Bell peppers are sometimes grouped with less pungent pepper varieties as...
. Systemins have only been identified in the Solaneae subtribe of the Solanaceae, but other members of the family, such as tobacco
Nicotiana
Nicotiana is a genus of herbs and shrubs of the nightshade family indigenous to North and South America, Australia, south west Africa and the South Pacific. Various Nicotiana species, commonly referred to as tobacco plants, are cultivated and grown to produce tobacco. Of all Nicotiana species,...
, also respond to wounding by systemically producing protease inhibitors.
Peptides with similar functions
In 2001, biologically active hydroxyprolineHydroxyproline
-4-Hydroxyproline, or L-hydroxyproline , is a common non-proteinogenic amino acid, abbreviated as HYP, e.g., in Protein Data Bank.-Structure and discovery:...
-rich glycopeptides
Glycoprotein
Glycoproteins are proteins that contain oligosaccharide chains covalently attached to polypeptide side-chains. The carbohydrate is attached to the protein in a cotranslational or posttranslational modification. This process is known as glycosylation. In proteins that have segments extending...
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
Petunia
Petunia is a widely cultivated genus of flowering plants of South American origin, closely related with tobacco, cape gooseberries, tomatoes, deadly nightshades, potatoes and chili peppers; in the family Solanaceae. The popular flower derived its name from French, which took the word petun, meaning...
and black nightshade
Black nightshade
Black nightshade may refer to:* Solanum americanum of much of North America* Solanum nigrum of Europe* Solanum ptychanthum of the Caribbean region...
. In 2007, HypSys were found outside the Solanaceae, in sweet potato (Ipomoea batatas) and sequence analysis identified HypSys analogs in poplar (Populus trichocarpa
Populus trichocarpa
Populus trichocarpa is a deciduous broadleaf tree species native to western North America. It is used for timber, and is notable as a model organism in plant biology. Its full genome sequence was published in 2006...
) and coffee (Coffea canephora
Coffea canephora
Robusta coffee is a variety of coffee which has its origins in central and western sub-Saharan Africa. It is a species of flowering plant in the Rubiaceae family. Though widely known as Coffea robusta, the plant is scientifically identified as Coffea canephora, which has two main varieties -...
). Systemins are highly conserved between species, whereas HypSys are more divergent but all contain a conserved proline
Proline
Proline is an α-amino acid, one of the twenty DNA-encoded amino acids. Its codons are CCU, CCC, CCA, and CCG. It is not an essential amino acid, which means that the human body can synthesize it. It is unique among the 20 protein-forming amino acids in that the α-amino group is secondary...
or hydroxyproline
Hydroxyproline
-4-Hydroxyproline, or L-hydroxyproline , is a common non-proteinogenic amino acid, abbreviated as HYP, e.g., in Protein Data Bank.-Structure and discovery:...
-rich central domain.
In 2006, AtPEP1, a 23 amino acid polypeptide was isolated from Arabidopsis thaliana
Arabidopsis thaliana
Arabidopsis thaliana is a small flowering plant native to Europe, Asia, and northwestern Africa. A spring annual with a relatively short life cycle, arabidopsis is popular as a model organism in plant biology and genetics...
, which was found to activate components of the innate immune response
Innate immune system
The innate immune system, also known as non-specific immune system and secondary line of defence, comprises the cells and mechanisms that defend the host from infection by other organisms in a non-specific manner...
. Unlike HypSys, AtPEP1 is not post-translationally modified
Posttranslational modification
Posttranslational modification is the chemical modification of a protein after its translation. It is one of the later steps in protein biosynthesis, and thus gene expression, for many proteins....
by hydroxylation
Hydroxylation
Hydroxylation is a chemical process that introduces a hydroxyl group into an organic compound. In biochemistry, hydroxylation reactions are often facilitated by enzymes called hydroxylases. Hydroxylation is the first step in the oxidative degradation of organic compounds in air...
or glycosylation
Glycosylation
Glycosylation is the reaction in which a carbohydrate, i.e. a glycosyl donor, is attached to a hydroxyl or other functional group of another molecule . In biology glycosylation refers to the enzymatic process that attaches glycans to proteins, lipids, or other organic molecules...
. Six paralogs of the precursor have been identified in A. thaliana as well as orthologs in grape
Grape
A grape is a non-climacteric fruit, specifically a berry, that grows on the perennial and deciduous woody vines of the genus Vitis. Grapes can be eaten raw or they can be used for making jam, juice, jelly, vinegar, wine, grape seed extracts, raisins, molasses and grape seed oil. Grapes are also...
, rice
Rice
Rice is the seed of the monocot plants Oryza sativa or Oryza glaberrima . As a cereal grain, it is the most important staple food for a large part of the world's human population, especially in East Asia, Southeast Asia, South Asia, the Middle East, and the West Indies...
, maize
Maize
Maize known in many English-speaking countries as corn or mielie/mealie, is a grain domesticated by indigenous peoples in Mesoamerica in prehistoric times. The leafy stalk produces ears which contain seeds called kernels. Though technically a grain, maize kernels are used in cooking as a vegetable...
, wheat
Wheat
Wheat is a cereal grain, originally from the Levant region of the Near East, but now cultivated worldwide. In 2007 world production of wheat was 607 million tons, making it the third most-produced cereal after maize and rice...
, barley
Barley
Barley is a major cereal grain, a member of the grass family. It serves as a major animal fodder, as a base malt for beer and certain distilled beverages, and as a component of various health foods...
, canola
Canola
Canola refers to a cultivar of either Rapeseed or Field Mustard . Its seeds are used to produce edible oil suitable for consumption by humans and livestock. The oil is also suitable for use as biodiesel.Originally, Canola was bred naturally from rapeseed in Canada by Keith Downey and Baldur R...
, soybean
Soybean
The soybean or soya bean is a species of legume native to East Asia, widely grown for its edible bean which has numerous uses...
, medicago
Medicago
Medicago is a genus of flowering plants, commonly known as medick or burclover. The name is based on Latin medica 'alfalfa, lucerne,' from 'Median .'...
and poplar
Poplar
Populus is a genus of 25–35 species of deciduous flowering plants in the family Salicaceae, native to most of the Northern Hemisphere. English names variously applied to different species include poplar , aspen, and cottonwood....
, 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
Sequence motif
In genetics, a sequence motif is a nucleotide or amino-acid sequence pattern that is widespread and has, or is conjectured to have, a biological significance...
. 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 cytosolCytosol
The cytosol or intracellular fluid is the liquid found inside cells, that is separated into compartments by membranes. For example, the mitochondrial matrix separates the mitochondrion into compartments....
. The precursor
Protein precursor
A protein precursor, also called a pro-protein or pro-peptide, is an inactive protein that can be turned into an active form by posttranslational modification. The name of the precursor for a protein is often prefixed by pro...
to tomato systemin is transcribed
Transcription (genetics)
Transcription is the process of creating a complementary RNA copy of a sequence of DNA. Both RNA and DNA are nucleic acids, which use base pairs of nucleotides as a complementary language that can be converted back and forth from DNA to RNA by the action of the correct enzymes...
as a 200 amino acid polypeptide. It does not contain a putative signal sequence
Protein targeting
Protein targeting or protein sorting is the mechanism by which a cell transports proteins to the appropriate positions in the cell or outside of it. Sorting targets can be the inner space of an organelle, any of several interior membranes, the cell's outer membrane, or its exterior via secretion...
suggesting that it is synthesised on free ribosome
Ribosome
A ribosome is a component of cells that assembles the twenty specific amino acid molecules to form the particular protein molecule determined by the nucleotide sequence of an RNA molecule....
s 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
Phloem
In vascular plants, phloem is the living tissue that carries organic nutrients , in particular, glucose, a sugar, to all parts of the plant where needed. In trees, the phloem is the innermost layer of the bark, hence the name, derived from the Greek word "bark"...
in vascular bundles of mid veins. The precursor accumulates exclusively in the phloem parenchyma
Parenchyma
Parenchyma is a term used to describe a bulk of a substance. It is used in different ways in animals and in plants.The term is New Latin, f. Greek παρέγχυμα - parenkhuma, "visceral flesh", f. παρεγχεῖν - parenkhein, "to pour in" f. para-, "beside" + en-, "in" + khein, "to pour"...
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
Secretory pathway
The secretory pathway is a series of steps a cell uses to move proteins out of the cell; a process known as secretion. The path of a protein destined for secretion has its origins in the rough endoplasmic reticulum, a membrane-bound compartment in the cell...
. The precursor to HypSys in tomato is a 146 amino acid polypeptide, exclusively synthesised within the vascular bundles of leaves and petioles
Petiole (botany)
In botany, the petiole is the stalk attaching the leaf blade to the stem. The petiole usually has the same internal structure as the stem. Outgrowths appearing on each side of the petiole are called stipules. Leaves lacking a petiole are called sessile, or clasping when they partly surround the...
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.
Processing of precursors
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 apoplastApoplast
Within a plant, the apoplast is the free diffusional space outside the plasma membrane. It is interrupted by the Casparian strip in roots, air spaces between plant cells and the cuticula of the plant....
ic aspartic protease
Aspartate protease
Aspartic proteases are a family of protease enzymes that use an aspartate residue for catalysis of their peptide substrates. In general, they have two highly-conserved aspartates in the active site and are optimally active at acidic pH...
. 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.
Receptors
Exceedingly small amounts of tomato systemin are active, femto-Femto-
Femto- is a prefix in the metric system denoting a factor of 10−15 or . Adopted by the 11th Conférence Générale des Poids et Mesures, it was added in 1964 to the SI...
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
Receptor (biochemistry)
In biochemistry, a receptor is a molecule found on the surface of a cell, which receives specific chemical signals from neighbouring cells or the wider environment within an organism...
for tomato systemin was identified as a 160KDa
Dalton
Dalton may refer to:-In Canada:* Dalton, Algoma District, Ontario* Dalton Armoury, a Canadian Forces facility primarily used by the Queen's Own Rifles of Canada- In the United Kingdom :* Dalton, Cumbria, England* Dalton, Dumfries and Galloway, Scotland...
leucine-rich repeat
Leucine-rich repeat
A leucine-rich repeat is a protein structural motif that forms an α/β horseshoe fold. It is composed of repeating 20–30 amino acid stretches that are unusually rich in the hydrophobic amino acid leucine...
receptor like kinase
Kinase
In chemistry and biochemistry, a kinase is a type of enzyme that transfers phosphate groups from high-energy donor molecules, such as ATP, to specific substrates, a process referred to as phosphorylation. Kinases are part of the larger family of phosphotransferases...
(LRR-RLK), SR160. After being isolated it was found that was very similar in structure to BRI1 from A. thaliana, the receptor that brassinolide
Brassinolide
Brassinolide is a plant hormone. The first isolated brassinosteroid, it was discovered when it was shown that pollen from rapeseed could promote stem elongation and cell division. The biologically active component was isolated and named brassinolide....
s bind to on the cell membrane. This was the first receptor which was found to be able to bind both a steroid
Steroid
A steroid is a type of organic compound that contains a characteristic arrangement of four cycloalkane rings that are joined to each other. Examples of steroids include the dietary fat cholesterol, the sex hormones estradiol and testosterone, and the anti-inflammatory drug dexamethasone.The core...
and a peptide ligand
Ligand
In coordination chemistry, a ligand is an ion or molecule that binds to a central metal atom to form a coordination complex. The bonding between metal and ligand generally involves formal donation of one or more of the ligand's electron pairs. The nature of metal-ligand bonding can range from...
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
Null allele
A null allele is a mutant copy of a gene that completely lacks that gene's normal function. This can be the result of the complete absence of the gene product at the molecular level, or the expression of a non-functional gene product...
with a stop codon
Stop codon
In the genetic code, a stop codon is a nucleotide triplet within messenger RNA that signals a termination of translation. Proteins are based on polypeptides, which are unique sequences of amino acids. Most codons in messenger RNA correspond to the addition of an amino acid to a growing polypeptide...
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
Gene silencing
Gene silencing is a general term describing epigenetic processes of gene regulation. The term gene silencing is generally used to describe the "switching off" of a gene by a mechanism other than genetic modification...
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
Dibasic
Dibasic is an adjective meaning:# containing two hydrogen atoms that can be replaced by metal ions, or# "of or relating to salts or acids forming salts with two atoms of a univalent metal."...
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.
No receptors for HypSys have so far been reported, but it is thought that they are perceived on the cell membrane by a LRR-RLK.
The receptor for AtPep1 has been identified as a 170KDa LRR-RLK and has been named AtPEPR1. AtPep1 is active at 0.1 nano-molar (nM) concentrations and the receptor saturates at 1nM. An analysis of the structure of the AtPEPR1 receptor has shown that it is a member of the LRR XI subfamily of LRR-RLKs in A. thaliana which includes the receptor for another peptide hormone CLAVATA3. Transforming tobacco cell cultures with AtPEPR1 allowed them to respond to AtPep1 in an alkalisation assay, whereas normal tobacco did not show such a response. BRI1-associated receptor kinase 1 (BAK1) is an LRR-RLK found in A. thaliana, which has been proposed to function as an adaptor protein
Adaptor protein
Signal transducing adaptor proteins are proteins which are accessory to main proteins in a signal transduction pathway. These proteins tend to lack any intrinsic enzymatic activity themselves but instead mediate specific protein–protein interactions that drive the formation of protein complexes...
that is required for the proper functioning of other RLKs. Yeast two-hybrid assay
Two-hybrid screening
Two-hybrid screening is a molecular biology technique used to discover protein–protein interactions and protein–DNA interactions by testing for physical interactions between two proteins or a single protein and a DNA molecule, respectively.The premise behind the test is the activation of...
s have shown that AtPEPR1 and its closest analog, AtPEPR2, interact with BAK1.
Signal transduction
Although the receptors for systemins and HypSys remain poorly understood, we have a better understanding of the signal transductionSignal transduction
Signal transduction occurs when an extracellular signaling molecule activates a cell surface receptor. In turn, this receptor alters intracellular molecules creating a response...
that occurs once the peptide had bound to its receptor. Jasmonic acid
Jasmonic acid
Jasmonic acid is derived from the fatty acid linolenic acid. It is a member of the jasmonate class of plant hormones. It is biosynthesized from linolenic acid by the octadecanoid pathway....
is an essential, albeit late component, in the systemin and wound-signalling pathways. In tomato, the signal is transduced from the receptor by mitogen-activated protein kinase
Mitogen-activated protein kinase
Mitogen-activated protein kinases are serine/threonine-specific protein kinases that respond to extracellular stimuli and regulate various cellular activities, such as gene expression, mitosis, differentiation, proliferation, and cell survival/apoptosis.-Activation:MAP kinases are activated...
s (MAPKs). Cosilencing of two MAPKs, MPK1 and MPK2, in tomato compromised their defence response against insect larvae compared to wild type
Wild type
Wild type refers to the phenotype of the typical form of a species as it occurs in nature. Originally, the wild type was conceptualized as a product of the standard, "normal" allele at a locus, in contrast to that produced by a non-standard, "mutant" allele...
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. The alkalisation of the apoplast is a downstream effect of signalling processing by MAPKs. Applying fusicoccin
Fusicoccin
Fusicoccin is an organic compound produced by a fungus. It has detrimental effect on plants and causes their death.Fusicoccin is synthesized by the fungus Fusicoccum amygdali, which is a parasite of mainly almond and peach trees...
, which activates the H+ ATPase
V-ATPase
Vacuolar-type H+-ATPase is a highly conserved evolutionarily ancient enzyme with remarkably diverse functions in eukaryotic organisms. V-ATPases acidify a wide array of intracellular organelles and pump protons across the plasma membranes of numerous cell types...
inhibited by systemin, along with systemin still activates MAPKs, even though the pH of the apoplast does not change.
Within minutes of systemin perception, the cytosol
Cytosol
The cytosol or intracellular fluid is the liquid found inside cells, that is separated into compartments by membranes. For example, the mitochondrial matrix separates the mitochondrion into compartments....
ic Ca2+
Calcium in biology
Calcium plays a pivotal role in the physiology and biochemistry of organisms and the cell. It plays an important role in signal transduction pathways, where it acts as a second messenger, in neurotransmitter release from neurons, contraction of all muscle cell types, and fertilization...
concentration increases, and linolenic acid is released from cell membranes after a phospholipase
Phospholipase
A phospholipase is an enzyme that hydrolyzes phospholipids into fatty acids and other lipophilic substances. There are four major classes, termed A, B, C and D, distinguished by the type of reaction which they catalyze:*Phospholipase A...
has been activated. Linolenic acid is then converted to jasmonic acid via the octadecanoid pathway
Octadecanoid pathway
The octadecanoid pathway is a reasonably well-characterized biosynthetic pathway for the production of the phytohormone jasmonic acid , an important hormone for induction of defense genes. JA is synthesized from alpha-linolenic acid, which can be released from the plasma membrane by certain lipase...
and jasmonic acid activates defensive genes. Production of methyl jasmonate is induced by systemins and also upregulates systemin precursor genes creating a feedback loop, amplifying the defensive signal. Methyl jasmonate is volatile and can therefore activate systemic acquired resistance
Systemic acquired resistance
The systemic acquired resistance is a "whole-plant" resistance response that occurs following an earlier localized exposure to a pathogen. SAR is analogous to the innate immune system found in animals, and there is evidence that SAR in plants and innate immunity in animals may be evolutionarily...
in neighbouring plants, preparing their defences for attack. These signalling events are analogous to the cytokine
Cytokine
Cytokines are small cell-signaling protein molecules that are secreted by the glial cells of the nervous system and by numerous cells of the immune system and are a category of signaling molecules used extensively in intercellular communication...
-mediated inflammatory immune response
Inflammation
Inflammation is part of the complex biological response of vascular tissues to harmful stimuli, such as pathogens, damaged cells, or irritants. Inflammation is a protective attempt by the organism to remove the injurious stimuli and to initiate the healing process...
in animals. When the inflammatory response is activated in animals, MAPKs are activated which in turn activate phospholipases. Lipids in the membrane are converted to arachidonic acid
Arachidonic acid
Arachidonic acid is a polyunsaturated omega-6 fatty acid 20:4.It is the counterpart to the saturated arachidic acid found in peanut oil, Arachidonic acid (AA, sometimes ARA) is a polyunsaturated omega-6 fatty acid 20:4(ω-6).It is the counterpart to the saturated arachidic acid found in peanut oil,...
and then to prostaglandins, which are analogs of jasmonic acid. Both pathways can be inhibited by suramin
Suramin
Suramin is a drug developed by Oskar Dressel and Richard Kothe of Bayer, Germany in 1916, and is still sold by Bayer under the brand name Germanin.According to the National Cancer Institute there are no active clinical trials...
.
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. 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. It is now thought that jasmonic acid is the systemic signal and that systemin upregulates the pathways for jasmonic acid synthesis.
Defence
Gene silencing
Gene silencing is a general term describing epigenetic processes of gene regulation. The term gene silencing is generally used to describe the "switching off" of a gene by a mechanism other than genetic modification...
, 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
Polyphenol oxidase
Polyphenol oxidase is a tetramer which contains four atoms of copper per molecule, and binding sites for two aromatic compounds and oxygen...
activity increased tenfold in tobacco leaves and protease inhibitors caused a 30% decrease in chymotrypsin
Chymotrypsin
Chymotrypsin is a digestive enzyme that can perform proteolysis. Chymotrypsin preferentially cleaves peptide amide bonds where the carboxyl side of the amide bond is a tyrosine, tryptophan, or phenylalanine. These amino acids contain an aromatic ring in their sidechain that fits into a...
activity within three days of wounding. When HypSys was over-expressed in tobacco, larvae feeding on transgenic plants weighed half as much after ten days feeding, as those feeding on normal plants. The concentration of hydrogen peroxide
Hydrogen peroxide
Hydrogen peroxide is the simplest peroxide and an oxidizer. Hydrogen peroxide is a clear liquid, slightly more viscous than water. In dilute solution, it appears colorless. With its oxidizing properties, hydrogen peroxide is often used as a bleach or cleaning agent...
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 HypSys is silenced the production of protease inhibitors induced by wounding is halved compared to wild type plants indicating that both systemin and HypSys are required for a strong defence response against herbivores in tomato.
When applied through cut petioles
Petiole (botany)
In botany, the petiole is the stalk attaching the leaf blade to the stem. The petiole usually has the same internal structure as the stem. Outgrowths appearing on each side of the petiole are called stipules. Leaves lacking a petiole are called sessile, or clasping when they partly surround the...
in Petunia
Petunia
Petunia is a widely cultivated genus of flowering plants of South American origin, closely related with tobacco, cape gooseberries, tomatoes, deadly nightshades, potatoes and chili peppers; in the family Solanaceae. The popular flower derived its name from French, which took the word petun, meaning...
, HypSys did not induce the production of protease inhibitors, but instead increased expression of defensin
Defensin
Defensins are small cysteine-rich cationic proteins found in both vertebrates and invertebrates. They have also been reported in plants. They are, and function as, host defense peptides. They are active against bacteria, fungi and many enveloped and nonenveloped viruses. They consist of 18-45 amino...
, a gene which produces a protein that inserts into microbial membranes, forming a pore. Defensin expression is also induced by AtPEP1.
Tomato plants over-expressing systemin produced more volatile organic compound
Volatile organic compound
Volatile organic compounds are organic chemicals that have a high vapor pressure at ordinary, room-temperature conditions. Their high vapor pressure results from a low boiling point, which causes large numbers of molecules to evaporate or sublimate from the liquid or solid form of the compound and...
s (VOCs) than normal plants and parasitoid wasps find them more attractive. 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, including oxylipin
Oxylipin
Oxylipins constitute a family of oxygenated natural products which are formed from fatty acids by pathways involving at least one step of dioxygen-dependent oxidation. Many of oxylipins have physiological significance....
pathway that synthesises jasmonic acid aldehyde
Aldehyde
An aldehyde is an organic compound containing a formyl group. This functional group, with the structure R-CHO, consists of a carbonyl center bonded to hydrogen and an R group....
s and alcohol
Alcohol
In chemistry, an alcohol is an organic compound in which the hydroxy functional group is bound to a carbon atom. In particular, this carbon center should be saturated, having single bonds to three other atoms....
s that function in wound healing.
Different AtPeps may allow A. thaliana to distinguish between different pathogens. When inoculated with a fungus
Fungus
A fungus is a member of a large group of eukaryotic organisms that includes microorganisms such as yeasts and molds , as well as the more familiar mushrooms. These organisms are classified as a kingdom, Fungi, which is separate from plants, animals, and bacteria...
, 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.
Silencing systemin did not affect the ability of black nightshade to resist herbivory and, when competing against normal plants, silenced plants produced more above-ground biomass and berries. Upon herbivory, systemin was down-regulated in black nightshade in contrast to the other peptides which are up-regulated after herbivory. By contrast HypSys were up-regulated and activated the synthesis of protease inhibitors. The down-regulation of systemin was associated with increased root mass but did not decrease shoot mass, demonstrating that systemin can cause developmental changes as a result of herbivory, allowing the plant to tolerate, rather than directly resist attack. Tomato roots were also affected by tomato systemin, with root growth increasing at high tomato systemin concentrations. By allocating more resources to the roots, plants under attack are thought to store carbon and then use it to re-grow when the attack ends. Overexpressing AtPEP1 also increased root and shoot biomass in A. thaliana.
Abiotic stress resistance
Overexpression of systemin and HypSys has been found to improve plants' tolerance to abiotic stress, including salt stress and UV radiation. 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 acidAbscisic acid
Abscisic acid , also known as abscisin II and dormin, is a plant hormone. ABA functions in many plant developmental processes, including bud dormancy. It is degraded by the enzyme -abscisic acid 8'-hydroxylase.-Function:...
and proline
Proline
Proline is an α-amino acid, one of the twenty DNA-encoded amino acids. Its codons are CCU, CCC, CCA, and CCG. It is not an essential amino acid, which means that the human body can synthesize it. It is unique among the 20 protein-forming amino acids in that the α-amino group is secondary...
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. Similarly, wounded tomato plants were less susceptible to salt stress than unwounded plants. This may be because wounding decreases the growth of the plant and therefore slows the uptake of toxic ions into the roots. An analysis of salt-induced changes in gene expression found that the differences measured between the transgenic and normal plants could not be accounted for by changes in conventional salt stress-induced pathways. Instead Orsini et al. suggested that the activation of the jasmonic acid pathway determines a physiological state that not only directs resources towards the production of compounds active against pests, but also pre-adapts plants to minimize water loss. These effects are achieved by negatively regulating the production of hormones and metabolites that will force plants to invest additional resources to counteract water loss, a secondary effect of herbivores.
Plants grown under UVB light are more resistant to insect herbivory compared with plants grown under filters that exclude the radiation. When tomato plants are exposed to a pulse of UVB radiation and then weakly wounded, PIs accumulate throughout the plant. By themselves, neither the radiation nor weak wounding is sufficient to induce systemic PI accumulation. Tomato cell cultures respond similarly, with systemin and UVB acting together to activate MAPKs. Short pulses of UVB also cause alkalisation of the culturing medium.
Development
In Nicotiana attenuataNicotiana attenuata
Nicotiana attenuata is a species of wild tobacco known by the common name coyote tobacco. It is native to western North America from British Columbia to Texas and northern Mexico, where it grows in many types of habitat. It is a glandular and sparsely hairy annual herb exceeding a meter in maximum...
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
Self-pollination
Self-pollination is a form of pollination that can occur when a flower has both stamen and a carpel in which the cultivar or species is self fertile and the stamens and the sticky stigma of the carpel contact each other in order to accomplish 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
Solanum pimpinellifolium
Solanum pimpinellifolium, commonly known as the Currant Tomato, is a species of nightshade native to Ecuador and Peru but naturalized elsewhere, such as the Galápagos Islands. Its small fruits are edible.-Breeding purposes:...
suggesting that it may also play some role in plant development.