Defense Mechanism in Plants

When plants are exposed to insects, nematodes, pests, microorganisms like bacteria, virus, fungi and pathogenic organism may endanger the growth of plants. Plants undergo stresses like

• Abiotic stress: Exposure to environmental factors
• Biotic stress: Exposure to living organisms

It reduces the quality and quantity of plant production. Some plants remain strong and healthy even after exposure due to active defense mechanism in plants. Detailed study of defense mechanism helps in proper isolation of defense genes and genetic construction of crop plants.

Types of defense mechanism

Plant defense mechanisms are classified in to two types and are as follows.

• Passive or constitutive defense mechanism
• Active or inducible defense mechanism

 Passive defense mechanism

The defensive response is by plant metabolites and certain structural compounds that are present inside the plant. This is referred as passive defense mechanism or constitutive defense mechanism. Plant outer wall is made of either cuticle or wax that is easily damaged by fungi and bacteria. It is due to the fact that outer cell wall is made of lignin, cuticles and tough bark as structural compounds. The plant metabolites such as phenols, resins, tannins and alkaloids are found pathogenic to some pathogens. Antimetabolic compounds may be proteins, peptides, and hydrolytic enzymes, insecticidal, antimicrobial, antifungal or antibacterial compounds.

Active defense mechanism

Defensive mechanisms which are not present in the body but produced newly are referred as active defense. Active defense mechanism is also referred as inducible defense mechanism. Changes that are prominent in cell wall are referred as wall opposition. Plant cell becomes so thick in such a way that microorganisms cannot invade through surface of plant body. The other way of developing active defense is through hypersensitive responses. As a result of hypersensitiveness cells that are near infection site becomes necrotic thereby it changes it metabolic response. Cells will not respire anymore thereby, it starts accumulating toxic compounds. Ultimately, plants start producing chemicals called phytoalexins as an immune response. Phytoalexins are produced when plants are under stress and are of small molecular weight.

Plants are subjected to environmental factors such as salinity, Droughtness, toxicity, deficiency of minerals and exposure to high and low temperatures. The genetically engineered proteins, Late Embryogeensis Abundant Protein (LEA proteins) are injected into plants to overcome abiotic stresses. Similarly to withstand high temperature exposure Heat Shock Proteins (HSP) protect them.

Questions:

• What are the types of defense mechanism?
• How genetically engineered plants are produced?
• In what ways genetically engineered plants are effective in defense?
• State the types of stress and its components.