Like animals, plants are also exposed to a wide variety of enemy organisms, which can damage the plants. These organisms include insect pests, nematodes, pathogenic fungi, bacterium, viruses and many other organisms. Plants are also exposed to many types of environmental stress called abiotic stresses. The stress of living organisms is known as biotic stress. Biotic stress can cause a severe reduction in the quantity as well as quality of the crops. In spite of the attack by pathogenic organisms and other animals, they remain healthy. This is because plants also have a defense mechanism to fight against the invading organisms. Studies about plant-defense mechanisms are very important because the identification and isolation of any genes related to the defense response can be used for genetically engineering other crop plants if needed. The defense system can be classified into two categories based on the defense response: passive or constitutive if it is a preexisting method of response and active or inducible if the method of response is a new type developed after the infection or attack by the pathogen.
Passive Defense:
This type of defense response is due to the presence of some structural components or some type of metabolites present in the body of the plant. The outer covering of the plant surface may be a special type such as cuticle or wax, which cannot be attacked or digested by the infecting fungus or bacteria. The presence of strong material such as lignin, tough bark, cuticle, etc. can effectively prevent the organisms from penetrating the plant surface. There are a large number of secondary metabolites such as alkaloids, tannins, phenols, resins, etc., which are toxic to pests and pathogens. Some of these compounds may have antimicrobial, antibacterial, or insecticidal properties. In addition to the secondary metabolites, there are certain proteins or peptides that have antimicrobial properties. For example, the antifungal pep tides present in the seeds, which help in preventing the seeds from fungal infection; hydrolytic enzymes, which can lysing the bacteria and fungus; and proteins that inactivates the viral particle by digesting its coat protein and nucleic acids.
Active Defense :
The defense response, which is produced newly and is not present previously in the cell or body, is called the active defense. The plant-cell wall is one of the sites where the change due to the defense response can be observed. All changes that happen in the cell wall due to an infection are collectively known as wall apposition. When a microorganism such as a fungus or bacteria starts infecting the plant body through the surface, immediately cell-wall thickness at that part is increased to make the penetration impossible. The change in thickness is due to the addition of new wall materials to the cell wall, specifically to the area of infection. Another interesting mechanism or response is called hypersensitive response (HR). In this response, the cells around the site of infection become necrotic. The metabolic activities of these cells also change. Their respiration becomes very slow or completely stopped. They begin to accumulate toxic compounds. Thus, an inhibitory effect or an unfavorable condition is created for the further growth and spread of the pathogen around the site of infection. The plant system or those cells (cells around the site of infection) also produce certain new chemicals in response to the infection known as phytoalexins. Phytoalexins are small molecular weight compounds produced when there is microbial attack or under conditions of stress, which are completely absent in healthy tissues.
It has been experimentally observed that if the phytoalexins production by an infected tissue is blocked or inhibited using some selective inhibitors, the resistance of the plant against the infection has reduced substantially. Similarly, it has been demonstrated that those pathogens, which can produce the enzyme for degrading the phytoalexins, had a pathogenisity that was very high compared to those that cannot produce such enzymes.
Tags: Bio Technology, Bio Genetics, Immune System
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