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Plants defense mechanism
- Plants diseases or phytopathogens affects plants right from the planting stage up to the harvesting and storage of the produce.
- Consequently, the plants have evolved several different mechanisms by which they defend themselves. These defense mechanisms in plants are termed as “BASAL RESISTANCE”.
- This term referring to two distinct aspects of plant-pathogen interaction.
- Constitutive defense.
- Induced defense.
- Conditions under which pathogens cause disease in the plant.
- For the development of the disease in plants, there must be a successful interaction between three components.
Besides these the potential for any pathogen to cause disease also depends upon.
- Host-microbe compatibility/incompatibility.
- Host receptors and sites for toxins
- Essential nutrients and growth factors.
- Recognition factors.
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Pre-existing structural defense
These structures not only protect the plant from invasion but also give the plant strength and rigidity and also exists as an integral structural component throughout the life span of plants.
- waxes are made of long-chain of aliphatic compounds, which prevents water retention on the plant surfaces.
- The epidermal cells of the aerial part of the plants are often covered with a waxy layer made of fatty acids, thus plants get a negative charge that helps to repel airborne spores of microbes.
- Its hydrophobic nature of it also protects the plants from fungal pathogens.
- These are modified branches that protect the plants from grazing vertebrates.
- In some plants e.g. rose plants the “thorns” on the stem are not true thorns.
- These are actually an outgrowth of the epidermis which serves the same function.
- it is the first line of defense against invading pathogens.
- Its toughness is because of the polymers of cellulose, hemicellulose, and lignin, etc.
- It serves as an excellent barrier that prevents many pathogens and insects from reaching the living cells underneath.
- The epidermis constitutes the outermost protective tissue system of leaves, floral parts, fruits, stem and roots of the plants.
- Structurally it incorporates a wide variety of chemical defenses that can be rapidly activated when the cell detects the pathogen.
- The presence of complex polysaccharides in the cell wall also makes it difficult for a pathogen to get an entry into the cell.
- The actin cytoskeleton is the fundamental structural component of eukaryotic cells.
- It is involved in the vesicular transportation of the cell wall components, callose, and antimicrobial compounds to the site of infection.
- Also referred to as “crazy cells” are the important immunological cells of the plant that helps them protecting against herbivory because they contain toxic chemicals or sharp crystals of calcium oxalate which tear the mouthparts of insects and mammals.
Pre-existing biochemical defense
Plants liberate different chemicals, which interferes with the activities of the pathogen and pathogenesis, thereby preventing or reduce infection.
1. Anti-Microbial Compounds
The plant produces different anti-microbial compounds during their growth which directly affects the environment. Here we discuss some of these compounds
- these are highly volatile compounds that contribute to the fragrance of plants that produce them.
- Terpenoid are the primary component of the essential oils.
- Essential oils often function as insect toxins and many protect from fungal and bacterial infections.
- Examples: – mint, basil, oregano, black paper, cinnamon etc.
- These are monoterpenoid esters.
- Produced by chrysanthemum plants that act as insect neurotoxins.
- Many commercially available insecticides are actually synthetic analogs of pyrethrins.
- gossypol produced by cotton has strong antifungal and antibacterial properties.
- When produced by plant-like spinach they disturb the larval development and increase insect mortality.
- These are the glycolate triterpenoids that are present in the cell membrane of many plants.
- They have detergent properties and act by disrupting the cell membranes of invading fungal pathogens.
2. Toxic inhibitors
- In most of the host-parasite interactions, pre-existing toxic substances form the basis of resistance.
- Several phenolic compounds, tannins and some fatty acids like dienes preexisting in high concentration in cells for resistance against parasitic fungi.
- Many of the plants produced toxic substances have been studied well and used in medicines.
- Example: Atropine is a neurotoxin and cardiac stimulant produced by the nightshade plants.
- These are the compounds produced during normal plant growth, thus excreted in the rhizosphere, or may be present in the vacuoles in an inactive form that inhibits the development of pathogens.
- Example: saponins and, avenin of oats provide resistance to the host cells.
- Dead cells of brown onion skins contain the quinones catechol and protocatechuic acid, which inhibit the germination of smudge pathogen within its rhizosphere.
- Production of phytohormones auxins and cytokinins are important virulence factors for many of the bacterial disease.
- They also play an important role in systemic signaling that facilitates resistance against pathogens.
- Example: this signaling is observed in tomatoes.
Induced histological defense
After the establishment of infection in plant cells, the host defense tries to create barriers for further colonization of tissues.
This may be at various levels
- The lignified cell wall provides an effective barrier to hyphal penetration.
- The lignin layer in the cell wall thickens when pathogen gets an entry into the tissues.
- Act as an impermeable barrier for free movement of nutrients causing starvation of the pathogen.
- In several plants, the infected cells are surrounded by suberized cells. Thus isolating them from healthy cells.
- Corky layer formation is a part of the natural healing system of plants.
- Example: – Scab of potato and rot of sweet potato are good examples.
3. Abscission layers:
- It is a gap between the host cell layers and devices for dropping-off old leaves and mature fruits.
- The plant may use this for defense mechanism also i.e. to drop-off infected or invaded plant tissue or part.
- Example: – a shot hole in the leaves of fruit trees is a common example.
4. Gum deposition:
- The gums and vascular gels quickly accumulate and fill the intercellular spaces or within the cell surroundings the infection thread.
- They are formed by the protrusion of the xylem parenchymatous cells through pits into xylem vessels.
- The tyloses are inductively formed much ahead of infection thus blocking the spread of the pathogen.
Induced chemical defense
It is the last line of host defense in plants.
This may condition a plant or plant tissue from susceptible to resistance as per their genetic potential.
1. Hypersensitivity response (hr)
- The HR is characterized by the rapid death of cells in the local region surrounding an infection. It restricts the spread of the pathogen to other parts.
- It occurs in two phases
- Phase one: – there will be activation of R genes triggers an ion flux, involving an efflux of hydroxide and potassium outside the cells, and an influx of hydrogen and calcium ions into the cells.
- Phase two: – the cells involved in HR generate an oxidative burst by producing reactive oxygen species.
- These compounds affect the cell membrane function, and also by lipid peroxidation leading to lipid damage.
- The alteration of ionic components in the presence of ROS results in the death of affected cells and the formation of local lesions.
2. Production of antimicrobial compounds
- These destroy many species of fungi and bacteria nonspecifically.
- It forms as a result of injury in living plant tissues maybe by the adjacent cells to the infected site.
- Examples: medicarpin, rishitin, and camalexin.
- It is generally antibodies.
- It is encoded by animal genes, nut produced in and by the plants that’s why called plantibodies.
- Transgenic plants have been produced which are genetically engineered, and to express foreign genes.
- It shown in transgenic plants.
- Example: Artichoke mottled crinkle virus.
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