Defence mechanism in plants

Defence mechanism in plants, defence against pathogens, defence against herbivores, structural defence mechanism, biochemical defence mechanism

1. DEFENCE MECHANISM IN PLANTS Submitted by Nipun Govind T 1 st Semester MSc Botany NGM College Pollachi

2. Contents Plant Defence against herbivores Host, Pathogen, Resistance Structural defence mechanism Biochemical defence mechanism

3. Plant Defenses Against Herbivores Many plants have impenetrable barriers, such as bark and waxy cuticles, or adaptations, such as thorns and spines, to protect them from herbivores.
If herbivores breach a plant’s barriers, the plant can respond with secondary metabolites, which are often toxic compounds, such as glycol cyanide, that may harm the herbivore.
When attacked by a predator, damaged plant tissue releases jasmonate hormones that promote the release of volatile compounds, attracting parasitoids, which use, and eventually kill, the predators as host insects.

4. HOST :- A living organism from which other organism derive its food. PATHOGEN :- Any agent (living organism or virus) which can cause disease. RESISTANCE :-The ability of an organism to exclude or overcome completely or in some degree, the effect of a pathogens, or other damaging factor.

5. STRUCTURAL DEFENCE MECHANISM The surface of the Plants or Hosts is first line of Defence against the pathogen. Structural Defence Mechanism are mainly of 2 types:- Pre-existing structural defence mechanism. Post-infectional or Induced structural defence mechanism.

6. Pre-existing structural defence mechanism The first line of defence in plants present in its surface. If the pathogen is succeeds in penetration, it encounters pre existing internal structural barriers. The external and internal structural barrier existing before pathogen attack are called pre existing defence structure or passive, static or anti-infection structures.

7. Pre-existing structural defence includes:- Wax Thick cuticle and epidermal cells Stomata Lenticels Schlerenchyma cells

8. Wax It is the mixture of long chain of apolar lipid. It forming a protective coating on plant leaves and fruit. Synthesized by epidermis. Extremely hydrophobic.

9. STOMATA Most of pathogen enter plants through natural opening. Some pathogen like stem rust of wheat ( Puccinia graminis f.sp. Tritici) can enters its host only when the stomata are open. Structure of stomata provides resistance to penetration by certain plant pathogenic bacteria.

10. Cuticle & Epidermal cell Ex: Disease resistance in Barberry species infected with Puccinia graminis tritici has been attributed to the tough outer epidermal cells with a thick cuticle. In linseed, cuticle acts as a barrier against Melampsora lini. Silicification and lignifications of epidermal cells offers protection against Pyricularia oryzae and Streptomyces scabies in paddy and potato, respectively.

11. Lenticels Lenticels are openings on fruit, stem and tubers that are filled with loosely connected cells that allow the passage of air. Shape and internal structure of lenticels can increase or decrease the incidence of fruit diseases. Ex. Small and suberised lenticels will offer resistance to potato scab pathogen, Streptomyces scabies.

12. Sclerenchyma cells: It composed of thickened walls of lignin. Sclerenchyma cells is present in stem and leaf veins. Brittle cells help in mechanical support of the plant. Effectively block the spread of some fungal and bacterial pathogens that cause angular leaf spot.

13. Post-Infectional / Induced structural defence mechanism Even after the establishment of infection in plant cells, the host defence system tries to create barriers for further colonization of tissues. Most pathogen manage to penetrate their host through wounds and natural opening to produce various degree of infection.

14. These may be regarded as:- Histological defence barrier Cellular defence structures

15. Histological defence structures:- Even after the establishment of infection in plant cells,the host defence system tries to create barriers for further colonization of tissues.These are:- Cork layer
Abscission layer
Tyloses
Gum deposition

16. Cork layer Infection by fungi, bacteria, some viruses and nematodes induce plants to form several layers of cork cells beyond the point of infection. These cork cells inhibits the further invasion by the pathogen beyond the initial lesion and also blocks the spread of toxin substances secreted by the pathogen. It also stop the flow of nutrients and water from the healthy to the infected area and deprive the pathogen of nourishment.

17. ABSCISSION LAYERS An abscission layer consists of a gap formed between infected and healthy cells of leaf surrounding the locus of infection. Due to the disintegration of middle lamella of parenchymatous tissue. Gradually, infected area shrivels, dies, and sloughs off, carrying with it the pathogen.

18. Ex: Xanthomonas pruni , and Closterosporium carpophylum on peach leaves

19. GUM DEPOSITION Various types of gums are produced by many plants around lesions after infection by pathogen or injury Generallyy these gums are exudated by plant under stressed condition. Gummosis is the process in which gum produced by the plants and trees.

20. Tyloses Tyloses are the overgrowths of the protoplast of adjacent living parenchymatous cells, which protrude into xylem vessels through pits. Tyloses have cellulosic walls. It formed quickly ahead of the pathogen and may clog the xylem vessels completely blocking the further advance of the pathogen in resistant varieties.

21. Ex: Tyloses form in xylem vessels of most plants under invasion by most of the vascular wilt pathogens.

22. Cellular defence structures The cellular defence structures i.e. Changes in cell walls,have only limited role in defence. Example :- Hyphal sheathing

23. Hyphal sheathing is observed in flax infected with Fusarium oxysporum f.sp. Lini.

24. Bio-chemical Defence Mechanism Although structural defence mechanisms do prevent the attack of pathogen , the defence mechanism also include the chemical substances produced in the plant cells before or after the infection Types of Biochemical defence mechanism are:- • Pre-existing chemical defence • Induced chemical defence

25. 1.Inhibitors:- • Released by plants in its environment • Present in plant cells before infection 2.Phenolics:- • Tannins • Glucanases • Dienes • Chitinase Pre-existing chemical defence

26. Induced Chemical Defence The induced biochemical changes in most plants are the last line of host defence.This may condition a plant or plant tissue from susceptible to resistant to immune status as per their genetic potential. These are :- Hypersensitivity response (HR) Production of antimicrobial substances Phytoalexins
Plantibodies

27. Hypersensitive response (HR) The term hypersensitivity was first used by Stakman (1915) in wheat infected by rust fungus, Puccinia graminis. The HR is a localized induced cell death in the host plant at the site of infection by a pathogen, thus limiting the growth of pathogen. HR occurs only in incompatible host-pathogen combinations. HR is initiated by the recognition of specific pathogen-produced signal molecules, known as elicitors .

28. antimicrobial substances Phytoalexins Phytoalexins are toxic antimicrobial substances. Muller and Borger (1940) first used the term phytoalexins for fungistati compoundss produced by plants in response to injury (mechanical or chemical) or infection. It produced in appreciable amounts in plants only after stimulation by phytopathogenic micro-organisms or by chemical or mechanical injury. Phytoalexins are not produced during compatable reaction.

29. antimicrobial substances Plantibodies It is generally antibodies. It is encoded by animal genes, but produced in and by the plants. So that it is called as plantibodies. Transgenic plants have been produced which are genetically engineered to incorporate into their genome, and to express foreign genes.

30. Mouse genes that produce antibodies against certain plant pathogens.
It shown in transgenic plant. Ex:-Artichoke mottled crinkle virus

31. Thanks...