Plants release hydrogen peroxide in response to the presence of a fungal invasion, which attacks by piercing the cell wall of a plant and breaking it down.
This hydrogen peroxide (chemical symbol H202) is a double edged sword in its defence against the antigen.
One Way : Hydrogen peroxide stops the breakdown of the cell wall
Certain pathogens will use pectinase, a digestive enzyme, to break down the cell wall barrier and invade the plant. The pectinase released by the fungus must be stopped. H202 is involved in halting the action of this pectinase in the following example;
- The H202 is created and moves to the cell wall - the site of the invasion.
- It reacts in contact with and enzyme called peroxidase, which promotes the breakdown of pectinase.
- The foreign chemical is rendered useless.
- Threat of the cell wall being compromised is removed.
Another Way : Some of the H202 triggers the creation of phytoalexins
Phytoalexins are similar to the antiviral proteins previously mentioned as a secondary line of defence. Phytoalexins are a family of hormones that inhibit protein synthesis and thus "shut up shop" in the event of a pathogenic attack by halting the protein production process in our cells.
- Chemicals released by the fungus that are being used by it in its attack trigger a chemical response in the plasma membrane that makes the plant aware of the pathogens presence.
- Hydrogen peroxide from the plasma membrane triggers a chemical response to inform the nucleus of the infected cells of the current situation.
- mRNA from the nucleus is transported to ribosomes, as described in the protein synthesis section, where phytoalexins are to be produced (essentially protein synthesis coding for phytoalexins).
- The phytoalexins then take on a role similar to that of antiviral proteins, where the presence of a phytoalexin in a cell inhibits protein synthesis and therefore preventing growth of the foreign agent by removing all possible avenues of invasion for the pathogen, thus eliminating the threat.
Barriers Used by Plants in Defence
Lignin is a strong type of molecule that provides plants with a defensive structure similar to that of fibrous proteins. It acts as a barrier and can be found in wood and is characteristically found in plants that have recently endured pathogen attack.
Callose seals off sieve plates in the plant, effectively shutting off the transport of molecules around the organism. This is done to minimise the chance of the plant transporting infectious material around its own self, and halting the movement of materials that could be used by the pathogen in aid of replicating itself.
Ethylene promotes leaf abscission, and is done to sever the plant of dead or dying plant matter. This is done to prevent the spread of infected material, therefore sacrificing infected sections of plant is more economical than taking the risk of the infection spreading.
Galls and tannins are created by the plant to encapsulate foreign agents found within the plant. A gall is an instance where an infected cell becomes inflamed that contains tannins. These tannins play a protective role by segregating the foreign agent and its chemicals from the rest of the plant
All of the four previous pages have illustrated means of self defence against pathogens (fungi, viruses and bacteria).