Lec 24 Plant Defense mechanism for safety.pptx

Zplant defense

1. Plant Defense

2. PLANT DEFENCE AGAINST PATHOGENS Host-parasite relationships the substrate requirements of the pathogen and the response of the plant to the pathogen Necrotrophs and Biotrophs Five possible relationships between plants and potential pathogens

3. Plant Defense Plants have some built- in mechanism of defense that enables them to survive in presence of so many diseases around them Classification of Mechanism of defense 1. Pre- infectional or pre existing defense mechanism is determined by the genetic constitution of the plant and pathogen In resistant varieties attack of the pathogen is controlled by physical as well as biochemical substances . Structural and biochemical defense are always present in these plants regardless of the plant is affected or not. a. Physical defense /structural barriers: b. Biochemical defense: . 2. Post- infectional defense mechanism a . Physical defense /structural barriers. b . Biochemical defense .

4. Some defense mechanisms in plants.   Falcarinol falcarindiol  

5. Pre-existing structural defense. The first line of structural defense is present on the plant surface . The structure of the epidermis along with the overlying cuticle can obstruct penetration of the pathogen. Cutin and waxes which constitute the cuticle, is a thick layer which cannot be degraded by any parasitic enzymes Due to presence of waxes in cuticle, plant surface is hydrophobic and prevents water from accumulating as a flow or drops. Most fungi need water on host surface for spore germination and subsequent growth. The wall layers of the epidermis are important in pre- existing structural defense. The degree of defense of these wall layers depends on the thickness and toughness of wall materials. The periderm formed in many perennial plant is found to be wax resistant to pathogen.

6. Pre- existing chemical defense Physiological or biochemical defense is more important for resisting invasions by plant pathogens. Eg : Red scales of onion contains catechol and related compound which impart resistance to the attack of Colletotrichum in cells of young fruits and leaves afford resistance to pathogens. The other antimicrobial substances in plant cells include unsaturated lactones, saponins , glycosides etc . Phenolic compounds are widely distributed in plants; coumarins , flavanoids , tannins , saponins etc . The defense mechanism which develops in the host tissue as a result of host parasite interaction is called host infection defense or autonomous and parasitic defense reaction . These prevent further spread of the pathogen with the host after infection

7. (a) Post Infectional Structural or Physical defence Cork Layer : Some bacteria, fungi and viruses stimulate host to form multilayered cork cells beyond the point of infection by substances secreted by pathogen Abscision Layer : Develops on an active young leaves infected by fungi , bacteria etc. Such layer develops at the point of infection. Ultimately helps discarding the infected area along with a few healthy cells from the host. Tyloses :develop under pathogenic stress in response to invasion to vascular pathogens. Gum deposition : Plants produce variety of gummy substances around the lesions. Gum deposition is common in store fruits

8. Induced mechanical barriers to infection. ( A) Lignituber formation in a root of a wheat seedling in response to infection by Baeumannomaces graminis . ( B) Diagrammatic representation of tyloses in r esponse to invasion of the xylem by a vascular wilt fungus. (C) Scanning electron micrograph of tyloses formed in the xylem of maize in response to infection by Verticilltum alboatrum .

9. Post- infectional biochemical defence Toxic substances produced in response to infection. Synthesis of inhibitory substances in response to injury caused by the pathogen When plant tissues are injured by a pathogen or by other means, a series of reactions start to isolate the irritant and heal the wound . Mostly these reactions form fungicidal substances around the site of infection.

10. Phytoalexins In literal sense ‘ alexin ’ means to ward-off. Common chemical substances produced in host plant interaction- phenolic compounds which are produced in antiparasitic response . Phytoalexin synthesis parallels the mechanism of defense through antibodies in animal system. Their production is induced by any physical or chemical injury and by fungi, bacteria etc . P roduced by healthy cells adjacent to damaged or necrotic cells. Produced in response to materials diffusing from damaged cells . Stimulated in host by the presence of certain pathogenic substances called elicitors . Elicitors are generally high molecular weight substances of fungal cell wall like glucan , glycoprotein etc. Phytoalexin synthesis has been found in families like Leguminaceae , Solanaceae , Malvaceae , Apiaceae , Asteraceae etc. Example: Phaseolin from Phaseolis Vulgaris due to pathogen Sclerotina ; Isocumarin in Daucus carota due to pathogen Ceratocystis ; Medicarpin by Medicage sativa due to Helminthosporum ; Cicerin by Cicer arietinum due to Ascochyte

11. Induced susceptibility to stripe rust of wheat induced by simultaneous inoculation of plants with compatible and incompatible isolates of Puccinta striformis .

12. Wheat leaves showing the hypersensitive reaction in response to infection by (A) Blumer tagramtnis and (B) Puccinia graminis

13. Sequence of events leading to the hypersensitive reaction in plants infected by incompatible pathogens.

14. Diagrammatic representation of phytoalexin production by bean pods in response to inoculation with Monilinia fructicola .

15. Examples of phytoalexins produced by higher plants

16. Accumulation of phaseolin in beans inoculated with compatible and incompatible strains of Colletotrichum lindemutttianum Compatible I ncompatible

17. roles of chitinase and -B-1 , 3-glucanase in a bean plant's defense against pathogen attacks

18. Systemic A cquired R esistance The induction of SAR usually requires the development of a slowly expanding necrotic lesion. Induction of SAR may be associated with other localised responses such as hypersensitive cell death, phytoalexin accumulation, papilla deposition and lignification . Two or three days after the inducing lesion first appears, a signal is released that is systemically translocated in the phloem. This signal is graft transmissible and is not cultivar, species or genus specific, but is not active once plants have begun flowering. All of the signal originates from the induction site. The systemic signal primes the rest of the plant against further pathogen challenge . Defence responses such as the rapid release of reactive oxygen species , hypersensitive cell death, phytoalexin accumulation, and enhanced levels of pathogenesis-related proteins are expressed more rapidly and intensely than in uninduced plants .

19. Events involved in the coordination of defence response in plants to challenge by pathooens . Time Event Minutes Membrane depolarisation and electrolyte leakage Reactive oxygen generation Expression of genes involved in phytoalexin biosynthesis Hours Oxidative burst Membrane lipid peroxidation Rise in salicylic acid levels Cytoplasmic aggregation, cell collapse and hypersensitive cell death Phytoalexin accumulation Cell wall reinforcements Days Accumulation of pathogenesis-related proteins Svstemic acouired resistance

20. Root exudates:the hidden part of plant defense Concerted action of root border cells and their exudates in root tip resistance against pathogens. ( A) Pathogen attraction and penetration of physiologically independent border cells to prevent deleterious root tip infection. ( B) The mucilage layer comprising mainly polysaccharides, proteins, and extracellular DNA is secreted by border cells and represents a defensive matrix that binds, immobilizes, and aggregates pathogens. ( C) Depicted is the inhibition or killing of microbes through high-and low-molecular-weight compounds that are released by root border cells and exhibit direct antimicrobial and/or antifungal properties. A B C