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2. Plant responses to oxidative stress
Represented To:
Dr. Ehsan Safdar
Represented By:
Muhammad Shahzad Alam
BAGF14E301
Department of Agronomy
University college of Agriculture, University of
Sargodha, Sargodha

3. Plant responses to
oxidative stress

5. A. Physiological Responses

6. 1. Oxidative damage to Lipid.
2. Oxidative damage to proteins.
3. Oxidative damage to DNA.

7. 1. Oxidative damage to Lipid
 The lipid bilayer membrane is composed of a mixture of
phospholipids and glycolipids that have fatty acid chains.
 Initiation-the production of R‘/PUFA radical/ROO‘ by the
interaction of PUFA with free radicals generated by other means.
 R‘ and ROO‘ are degraded to malon dialdehyde—an indicator of
fatty acid break down by free radicals.

8. Conti...
 Propagation-one free radical generates another free radical in
the neighboring molecules chain reaction destruction of fine
architecture & integrity of the membranes.
 Termination- reactions in membrane lipids are terminated
when the carbon or peroxy radicals cross-link to form
conjugated products that are not radicals.

9. Cont...
 Lipid peroxy radicals react with other lipids, proteins, and
nucleic acids; propagating thereby the transfer of electrons and
bringing about the oxidation of substrates.
 Cell membranes, which are structurally made up of large
amounts of PUFA, are highly susceptible to oxidative attack
and, consequently, changes in membrane fluidity, permeability,
and cellular metabolic functions result.

10. Oxidative damage to Lipid

11. 2. Oxidative damage to proteins
 Oxidative attack on proteins results in site-specific amino acid modifications,
fragmentation of the peptide chain, aggregation of cross-linked reaction
products, altered electrical charge and increased susceptibility to
proteolysis(the breakdown of proteins or peptides into amino acids by the
action of enzymes).
 Sulphur containing amino acids, and thiol groups specifically, are very
susceptible sites.
 The oxidation of iron-sulphur centers by superoxide destroys enzymatic
function.
 Thus it destroys the structure, functions of essential proteins and enzymes and
whole cell metabolism is blocked.

12. Oxidative damage to protein

13. 3. Oxidative damage to DNA
 Activated oxygen and agents that generate oxygen free radicals, such as ionizing
radiation, induce numerous lesions in DNA that cause deletions(the loss or
excision of a section of genetic code, or its product, from a nucleic acid or
protein sequence), mutation (the changing of the structure of a gene) and other
lethal genetic effects.
 Degradation of the base will produce numerous products, including 8-
hydroxyguanine, hydroxymethyl urea, urea, thymine glycol, thymine and
adenine ring-opened and -saturated products.
 Characterizations of this damage to DNA has indicated that both the sugar and
the base moieties(distinct part of a large molecule) are susceptible to oxidation,
causing base degradation, single strand breakage, and cross-linking to protein .

14. Conti...
 Mutation arising from selective modification of G : C sites
specially indicates oxidative attack on DNA by ROS.
 ROS may interfere with normal cell signaling, resulting thereby
in alteration of the gene expression by redox regulation of
transcriptional factors/activator and/or by oxidatively modulating
the protein kinase cascades.
 This results in the generation of more ROS through increased
leakage of electrons, and cell damage.

15. Conti...
 Mixed function oxygenase's that contain a hemi moiety add an
oxygen atom into an organic substrate using NAD(P)H as the
electron donor.
 The generalized reaction catalyzed by cytochrome P450.
 The best characterized cytochrome P450 in plants is cinnamate-4-
hydroxylase which functions in flavonoid and lignin
biosynthesis.

16. Conti...
 The plant NAD(P)H oxidase have an analogous function
to the animal enzyme.
 Heat shock and xenobiotics transiently activate this
superoxide generating reaction, and consequently.
 it has been proposed that these superoxide generating
reactions may serve as a signal in plant cells to elicit
responses to biological, physical or chemical stress.

17. Oxidative damage to DNA

18. B. Biochemical responses

19. DEFENCE MECHANISMS
1. Superoxide Dismutase
2. Catalase
3. Peroxidase
4. Glutathione
5. Antioxidants and oxidative stress

20. 1. Superoxide Dismutase
 SOD is now known to catalyze the dismutation of
superoxide to hydrogen peroxide and oxygen.

21. 2. Catalase
 Catalase is a heme-containing enzyme that catalyzes the
dismutation of hydrogen peroxide into water and oxygen.

22. 3. Peroxidase

23. 4. Glutathione
 Glutathione(GSH) is a tripeptide (Glu-Cys-Gly) whose antioxidant
function is facilitated by the sulphydryl group of cysteine is found
in most tissues, cells and subcellular compartments of higher plants.
 Can react chemically with singlet oxygen, superoxide and hydroxyl
radicals and therefore function directly as a free radical scavenger.
 Stabilize membrane structure by removing acyl peroxides formed
by lipid peroxidation reactions.

24. Conti…
 The reduction of GSSG to GSH is catalyzed by the enzyme
glutathione reductase (GR) in presence NADPH.
 GR is associated mainly with the chloroplast but significant activity is
also found in the cytosol and a lesser amount in the mitochondria.
 Cytosol (The aqueous component of the cytoplasm of a cell, within
which various organelles and particles are suspended.)
 Mitochondria(An organelle found in large numbers in most cells, in
which the biochemical processes of respiration and energy production
occur. It has a double membrane, the inner part being folded inwards
to form layers).

25. Glutathione

26. 5. Antioxidants and oxidative stress
 To counteract oxidative stress, plant produces an armory of
antioxidants to defend itself.
 It's the job of antioxidants to neutralize or 'mop up' free radicals
that can harm plant cells.
 Plant ability to produce antioxidants is controlled by it’s genetic
makeup and influenced by exposure to environmental factors.

27. Antioxidants and oxidative stress

28. 6. Carotenoids
 Carotenoids are C40 isoprenoids and tetraterpenes that are located
in the plastids of both photosynthetic and non-photosynthetic
plant tissues.
 In chloroplasts, the carotenoids function act as accessory
pigments in light harvesting.
 More important role is their ability to detoxify various forms of
activated oxygen and triplet chlorophyll that are produced as a
result of excitation of the photosynthetic complexes by light.

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