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Below are some of the proposed role of reactive oxygen species (ROS) in plant defense.

Question : Below are some of the proposed role of reactive oxygen species (ROS) in plant defense.

A. H2O2 may be directly toxic to pathogens.

B. In presence of iron, H2O2 gives rise to an extremely reactive hydroxyl radical.
 
C.  H2O2 leads to induced biosynthesis of salicylic acid (SA).
 
D. H2O2  production is always delayed during incompatible interactions.

which one of the following  options represents the combination of all correct statement?
 
  1. A, B and C 
  2. B, C and D 
  3. A, C and D
  4. A, B and D 
 Answer:  1.  A, B and C

Explanation:  
  

Reactive oxygen species(ROS)
  •  Free radical derived from molecular oxygen.




  1.  The hypersensitive response is often preceded by the production of reactive oxygen species. Cells in the vicinity of the infection synthesize a burst of toxic compounds formed by the reduction of molecular oxygen, including the superoxide anion (O2 •–), hydrogen peroxide (H2O2) and the hydroxyl radical (•OH). An NADPH-dependent oxidase located on the plasma membrane is thought to produce O2 •–, which in turn is converted to •OH and H2O2. The hydroxyl radical is the strongest oxidant of these active oxygen species and can initiate radical chain reactions with a range of organic molecules, leading to lipid peroxidation, enzyme inactivation, and nucleic acid degradation (Lamb and Dixon 1997). Active oxygen species may contribute to cell death as part of the hypersensitive response or act to kill the pathogen directly.


 2. When plants encounter pathogens, they frequently produce reactive oxygen species (ROS), such as hydrogen peroxide (H2O2). This molecule serves as a signaling agent to activate defense responses and can also directly damage pathogens. Its high reactivity allows it to interact with and disrupt vital components of pathogen cells, including proteins, nucleic acids, and membranes, leading to their inactivation and eventual cell death. In the presence of iron (Fe), hydrogen peroxide can participate in the Fenton reaction, resulting in the formation of highly reactive hydroxyl radicals (·OH). Moreover, hydrogen peroxide can enhance the activity of enzymes like phenylalanine ammonia-lyase (PAL) and isochorismate synthase, which are involved in the synthesis of salicylic acid from precursor molecules. The production of hydrogen peroxide (H2O2) in response to incompatible interactions between plants and pathogens generally occurs as an early defense mechanism rather than a delayed one. The precise chemical composition of signal molecules exchanged between hosts and pathogens remains uncertain. Various molecules have been suggested to play roles in intracellular signal transduction, with the most common being different protein kinases, calcium ions, phosphorylases, phospholipases, ATPases, hydrogen peroxide (H2O2), ethylene, among others. Systemic signal transduction, which leads to systemic acquired resistance, is believed to involve salicylic acid, oligogalacturonides from plant cell walls, jasmonic acid, systemin, fatty acids, ethylene, and more. Certain natural or synthetic compounds, such as salicylic acid and the synthetic dichloroisonicotinic acid, are known to activate the signaling pathway that induces systemic acquired resistance against a variety of plant pathogenic viruses, bacteria, and fungi.



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