HR and SAR Mechanism 1.pptx
- 1. HYPERSENSITIVE RESPONSE (HR) AND SECONDARY ACQUIRED RESISTANCE (SAR)
- 2. C O N T E N T S 1.Introduction 2.Biotic stress 3.Hypersensitive response 4.Secondary Acquired Resistance 5.Reference 6.Conclusion
- 3. INTRODUCTION Biotic stress is the stress that occurs as a result of damage done to an organism by an other living organism.Such as Bacteria,Viruses,Fungi, Parasites, Beneficial and harmful insects, Weeds and Cultivated or native plants.
- 4. It is different from abiotic stress, which is the negative impact of non-living factors on the organism such as Temperature,Sunlight, Wind, Salinity,Flooding and Drought. The damage caused by these living and non-living agents can appear very similar.Even with close observation accurate diagnosis can be difficult. For example: Browning of leaves on an oak tree caused by drought stress may appear similar to leaf browning caused by oak wilt, a serious vascular disease caused by a fungus.
- 5. The herbivore insects and disease caused by fungi, bacteria and nematodes that are ever present in the environment represent potential biotic stresses in plants. Although, plants do not have an immune system as compared to animals, but the plants have developed intricate defense strategies including production of secondary metabolites to ward off insects
- 6. The parts of diverse mechanisms adopted by plants to resist infection are; 1. Hypersensitive response (HR) 2. Secondary acquired resistance (SAR)
- 7. 1.HYPERSENSITIVE RESPONSE The Hypersensitive response is a mechanism used by plants, to prevent the spread of infection by microbial pathogens. The Hypersensitive response serves to restrict the growth and spread of pathogen to other parts of the Plant.
- 8. On being attacked by insects or a pathogenic microorganisms, typically a plant responds with, 1. Formation of pathogenesis-related proteins 2. Biosynthesis of phytoalexins 3. Changes in composition and physical properties of cell walls 4. Process of programmed cell death All these responses are collectively known as Hypersensitive response or reaction (HR).
- 9. 1. FORMATION OF PATHOGENESIS-RELATED PROTEINS These are the products of defense-related genes that are activated by microbial infection and include hydrolytic enzymes such as, A. Proteinase inhibitors which inhibit activities of proteolytic enzymes secreted by the pathogen. B. Lytic enzymes such as glucanases, chitinases and other hydrolases that attack and degrade the cell walls of the
- 10. 2. BIOSYNTHESIS OF PHYTOALEXINS The microbial infection also activates genes that encode enzymes for the synthesis of Phytoalexins. Phytoalexins are a Chemical diverse group of secondary metabolites (chiefly isoflavonoids and sesquiterpenes ) with strong antimicrobial activity. The isoflavonoids medicarpin from Alfalfa and Glyceolin from Soyabean, and Sesquiterpenes such as rishitin from tomato and potato, and capsidol from tobacco and pepper are well known examples of
- 11. 3. CHANGES IN COMPOSITION AND PHYSICAL PROPERTIES OF HOST CELL WALLS In response to pathogen invasion, lignin, callose, suberin and some hydroxy-proline rich glycoproteins are synthesized and accumulated in host cell walls to strengthen the latter and physically blocking the spread of the invading pathogen.
- 12. 4. PROGRAMMED CELL DEATH The Hypersensitive response culminates in rapid death of cells around the infection site depriving pathogen of the nutrient supply and limiting it’s spread in host plant and leaving necrotic lesions at the site of invasion. The rest of the plant however, remains unaffected.
- 13. MECHANISM OF RECOGNITION OF THE POTENTIAL PATHOGEN TO INITIATE DEFENSE RESPONSE IN PLANTS Not all plants are resistant to disease caused by pathogens. Researchers have shown that resistance of microbial pathogens has an underlying genetic basis. The pathogens carry avr genes (avirulence genes) while the host plants carry corresponding resistance genes called R genes. Diseases occurs when the pathogen lacks avr genes or the host plant does not carry dominant R genes.
- 14. 2. SECONDARY ACQUIRED RESISTANCE (SAR) The Hypersensitive response is limited to near vicinity of the initial site of infection by pathogen. But, often the entire host plants develops increased resistance against pathogens over a period of time ranging from few hours to several days following initial infection at one site of the plant. This phenomenon wherein “a single encounter with the pathogen increases resistance to entire plant to future attacks by pathogens” is called as secondary
- 15. SAR appears to result from increased levels of some secondary metabolites and other defense compounds such as chitinases and other hydrolytic enzymes. However,the mechanism of SAR is not clearly understood. One component of the signaling pathway is likely to be Salicylic acid (SA) that is benzoic acid derivative and a secondary metabolite. It has been shown in variety of plants that the infection by pathogen results in increased levels of SA in the zone of infection that establishes SAR in
- 16. POSSIBLE ROLES OF SALICYLIC ACID AND METHYL SALICYLATE IN SAR
- 17. According to Van Bel and Gaupels (2004), the transmission of SAR signal from infection site to other parts of the plant is very rapid and possibly occurs through vascular tissue. Phloem is now considered to be the pathway of SAR signal. Salicylic acid is not the mobile SAR signal. Maldonado et.al., in 2002 have shown that in Arabidopsis, mutations in DIR1 gene (Defective in Induced Resistance 1 gene) inhibit SAR response.
- 18. been suggested that the long distance SAR signal might be a substance derived from a Apart from Phloem-mobile signals, the plant may develop SAR through air-borne signals.
- 19. Salicylic acid may be converted into it’s methyl ester, methyl Salicylate that is a moderately volatile substance. Methyl Salicylate may function as volatile air- borne SAR-inducing signal that is transmitted distant non-infected parts of the plant and even to non-infected neighbouring plants making them resistant to pathogen attack.
- 20. CONCLUSION Recent researchers have shown that the Hypersensitive response is preceded by accumulation of nitric oxide and active Oxygen species. The production of active oxygen species appears to be prerequisite for activation of Hypersensitive response. Induction of PCD is prevented in absence of any of these two signals.
- 21. REFERENCE: Dr.V.K.Jain, Fundamentals of Plant Physiology, Biotic stresses in plants, First edition,Nirja Publishers,1974,584-586.