Damage to the leaves of many plant species by insects or other mechanical wounds activates the defense gene throughout the plants within hours. An 18-amino acid polypeptide, called systemin, has been isolated from tomato leaves and is a potent inducer of more than 15 defense genes when supplemented with tomato plants at fmol/plant levels. Systemin is easily transported from wound sites and is the primary systemic signal.

The polypeptide is processed from a precursor of 200 amino acids called prosystemin, similar to polypeptide hormones in animals. However, the plant prohormone does not possess typical diploid cleavage sites nor contain signaling sequences or typical membranous regions.

The signal transduction pathway mediates the signaling system. It involves the release of linolenic acid from membranes and subsequent conversion to jasmonic acid, a potent activator of defense gene transcription. The pathway is similar to animal arachidonic acid/prostaglandin signaling, which triggers inflammatory and acute phase responses.

Plants have various defense mechanisms to protect themselves from insect damage. The hormonal system plays a vital role in plants of the nightshade family (peppers, potatoes, tomatoes, and eggplant).

Systemin is a short peptide cut from a more significant protein called Prosystemin. Protamine is a "guardian" because it is constantly in plant cells. When insects attack plants, the active system breaks down and accumulates at the site of the attack to protect the plant from the insects.

 Systemin acts as a signal to damaged plant cells. It belongs to DAMPs (damage-associated molecular patterns), which refer to plant-specific substances produced only when plant cells are damaged.

The cell produces a system when insects chew the plant or damage it. This peptide is, in turn, the starting signal for the production of the hormone jasmonic acid, which initiates the synthesis of protease inhibitors and quinolines.

Eggplant spoils the appetites of insects with both systems.

Both protease inhibitors and quinolines directly interfere with insects' metabolism by altering their saliva and disrupting the animal's digestion process.

So, insects can get fewer nutrients from eaten leaves, and their attackers let them go. The researchers showed that African cotton worm larvae (Spodoptera littoralis) developed worse on organ-producing tomatoes than caterpillars fed on tomatoes without this defense system.

There are two types of receivers responsible for recognizing the system.

However, precisely how the system is perceived at the plant and how the alarm signal is sent has yet to be discovered. However, the researchers note that cultivated tomatoes (Solanum lycopersicum) respond to the system by producing ethylene and reactive oxygen species, while wild tomatoes (Solanum pennellii) show no response. 

One of the most important defenses against plant attackers is the formation of ethylene and reactive oxygen species, which is why they are frequently formed in addition to other actions. If ethylene and reactive oxygen species can be detected, this is a strong indication that the plant recognizes the attacker and begins to defend itself.

 The working group examined different junctions of Solanum lycopersicum and Solanum pennellii to identify receptors critical for system recognition and their location in the genome. 

Each plant contained a short portion of the chromosome of one wild tomato at different locations, and the rest of the genome came from cultivated tomatoes. The plants were tested to see if they could still form ethylene like the transplanted tomatoes after the diet was given.

If not, the genome portion with the system receptor in this plant line has been replaced by a chromosome portion of wild tomato. This enabled the scientists to localize the system receptor gene. The receptor, which the team called Systemin Receptor 1 (SYR1), belongs to a class of "receptor-like kinases" and is highly sensitive to systemin.

 Indeed, two systemic receptors, SYR1 and SYR2, have been identified. The receptors are structurally identical but have different sensitivity to the two systems. Scientists think this could be a fine-tuning mechanism. The plant will have the opportunity to react differently depending on whether one or both receptors are active. Whether this is really the case needs further investigation.

 SYR is only found in eggplant.

Similar forms of this receptor are also found in other nightshade plants, such as potatoes (Solanum tuberosum), peppers (Capsicum annuals), and eggplant (Solanum melongena). Since systemin and its receptors are not present in other plant families, the systemin recognition system is specific to nightshade shades.

 It has also been shown that the receptor can be transferred to other plants and can respond to systemic therapy, for example, by producing reactive oxygen species. Australian tobacco (Nicotiana benthamiana) has been successfully equipped with the Systemin defense system.

The systemin gene must also be transmitted for the defensive system to work. Since plants from other families lack both the receptor and the systemin gene, two genes are needed to make a functional unit when combined. 

This "disadvantage" might be deliberately used in plant breeding to create a system that can be triggered from the outside. To do this, just the SYR1 gene would be added to the unprotected plants. This would allow plants to detect systemin but prevent them from producing any. Spraying the plants with systemin might trigger the plants' defenses and avoid significant harm if an insect invasion arises in a field.