1. PLANT STRESS RESPONSE

2. STRESS
Manifold unfavourable, but not necessarily immediately lethal conditions, occurring either permanently or sporadically in a locality
Significant deviation from optimal conditions for life
Elicit responses and changes at all functional levels of the organism
May be first reversible, but may become permanent

3. STRESS
Conditions that adversely affect growth, development, productivity
Abiotic (phy/che environment)
Biotic (organisms)
Abiotic- water logging, drought, high or low temperatures, excessive soil salinity, inadequate mineral nutrients, too much or too little light, ozone

4. RESISTANCE Depends on Species Genotype Age of plant Tissue identity
Duration, severity, rate of stress

5. Alarm phase – stress reaction
Restitution- repair
Hardening
Adjustment
Adaptation- normalization
(Exhaustion)- irreversible damage
End phase

6. STRESS RESPONSE
Race b/w effort to adapt and potentially lethal processes in protoplasm
Triggered by stress or stress- induced injury (membrane integrity loss)
Some- enable plant to acclimatize to stress

7. Altered gene expression- changes in development, metabolism
Initiated when plant recognizes stress at cellular level- proteins that sense abiotic stress
Transmit information within individual cells and through out the plant
increase in specific mRNA, enhanced translation, stabilization of proteins, alteration of protein activity

8. WATER DEFICIT Major a biotic stress
Induced by many environmental conditions:
No rainfall- drought
High salt conc.
Low temp.
Transient loss of turgor at midday
Rate of onset, duration, acclimatization- influence the water stress response

9. Response to water deficit
ABA phytohormone
Induce expression of drought- inducible genes
Products- 2 groups
GROUP 1
Protective proteins
Water channel proteins, membrane transporters
Osmoregulator synthesizing enzymes
Detoxifying enzymes (peroxidases, catalases)

11. B) GROUP 2
Transcription factors (DREB, MYC)
Protein kinases (MAP kinases, CDP kinases)
Proteinases (phospholipase)

12. 4 independent pathways
2 - ABA-dependent
2 - ABA- independent
Cis acting elements- in promoter of all stress inducible genes-
ABA-responsive element (ABRE)
dehydration responsive element (DREB)

13. COLD- STRESS
Plants produce a no. of proteins in response to cold and freezing temp.
54 cold inducible genes
10% of drought induced genes- also induced by cold
Genes- contain a cis element repeat (CRT)- 5 bp seq.
Transcription factor- C repeat binding factor (CBF)- Main controlling switch in monocots, dicots

14. Cold stress reactions Injury to cell membrane – chilling, freezing
Ratio of saturated to unsaturated fatty acids- degree of tolerance, particularly in plastids
Non- acclimatized plants- killed or injured at -100 C or below.
Freeze acclimatized trees- survive between -40 to -500C
Injury- by severe dehydration during freeze-thaw cycles

15. Temp below 00C , cellular water freeze.
Cell shrinkage
Expansion induced lysis

16. SURVIVAL STRATEGIES:
anti freeze proteins (AFP)
Declines rate of ice crystal growth
Lowers the efficiency of ice nucleation sites
Lowers temp. at which ice forms
Osmoprotectants
osmolytes- quarternary amines, amino acids, sugar alcohols
Balances the osmotic potential of externally increased osmotic pressure

17. Glycine betaine
Quaternary amine, soluble
CH3 gps- interact with hydrophobic and hydrophilic molecules
Oxidation-
choline (choline monooxygenase)> betaine aldehyde
Betaine aldehyde (betaine aldehyde dehydrogenase)> glycine betaine

18. Proline

19. Sugar alcohols –mannitol
Trehalose- non reducing disaccharide
Increased water retention and desiccation tolerance

20. SALT STRESS Flow of water is reversed- imbalance
Accumulation of excess Na+, Cl- in cytosol
Stress tolerant plants- maintains internal osmotic pressure

21. Sensing salt stress Ion specific signals of salt stress
High Na+- increases Ca2+ conc. In cytoplasm- Key component of Na+ signalling
SOS3 – Ca2+ binding protein
Activates protein kinase (SOS2)
Phosphorylates (activates) plasma membrane H+-Na+ antiporter (SOS1)
SOS1 mRNA- stabilized, accumulates -

22. Plant maintains high K+, low Na+ in cytosol
3 tolerance mechanisms-
Reducing Na+ entry to cells
Na+ efflux from cell (K+-Na+)
Active transport to vacuole (vacuolar H+-Na+ ATPase)

24. Na+ sequestration In vacuoles
By NHX1, NHX2 proteins of tonoplast membrane
Decreases cytoplasmic Na+

25. Salt stress induced proteins
Transcription of genes oncoding late embryogenesis abundant (LEA) proteins- activated

26. Antioxidant production
Abiotic stress – drought, salt, chill- increases reactive O intermediates (ROI) in plants
ROI- stress signal- due to altered metabolic functions of chloroplast, mitochondria
ROI SCAVENGING
Antioxidant system contains a battery of enzymes that scavenge ROI- SOD, peroxidases, catalases, glutathione reductases

27. HEAT STRESS Decrease in synthesize of normal proteins
Transcription and translation of HSPs
When 5o C rise in optimum temp.
Conserved proteins
Act as chaperons, refolding
classes- based on mw
Hsp 100, Hsp 90, Hsp 70, Hsp60

29. FLOODING
-Decreases O2 availability of plant roots -ATP production is lowered -SURVIVAL STRATEGIES: production of enzymes for sucrose, starch degradation, glycolysis, ethanol fermentation -ethylene- long term acclimatization responses-stem elongation

31. BIOTIC STRESSES

33. INDUCED STRUCTURAL AND BIOCHEMICAL DEFENCES
Plants receive signal molecules as soon as pathogen contact
Elicitors of recognition
Host receptors – on plasma membrane or cytoplasm
Bichemical reactions, structural changes – to fend off pathogen, toxins

35. Signal transduction
Transmission of alarm signal to host defense providers
To host proteins, nucleur genes- activated- products that inhibit pathogen
Signals to adjacent cells, usually systematically
Intracellular signal transducers- protein kinases, Ca2+ ,phosphorylases, phospholipases, ATPases, H2O2,ethylene.

37. Systemic signal transduction, aquired resistance- by salicylic acid, oligogalacturonides from plant cell walls, jasmonic acid, systemin, fatty acids, ethylene

38. Induced structural defenses
After pathogen has penetrated preformed defense structures- plant respond by one or more structures to prevent further pathogen invasion
Defense structures:
1) cytoplasmic defense reaction
2) cell wall defense structures
3) histological dfense structures
4) necrotic/ hypersensitive defense reaction

39. Cytoplasmic defense reaction
In response to weakly pathogenic and mycorrhizal fungi
Induce chronic diseases / nearly symbiotic conditions
Cytoplasm surrounds hyphal clump
Cytoplasm and nucleus enlarge
Dense granular cytoplasm
Mycelium disintegrates, invasion stops.

40. Cell wall defense structures
Morphological changes of cell wall
Limited effectiveness
a) parenchymatous cells’ walls swell, produces amorphous, fibrillar material that surrounds, traps bacteria
b) cell wall thickens by a cellulosic material infused with phenolics
c) callose papillae laid of inner surface of cell wall (2-3 mins ) (fungi)
formation of lignituber around fungal hyphae

41. Histological defense structures
Formation of cork layers- fungi, bacteria, virus, nematodes
inhibits invasion beyond initial lesion
prevents flow of nutrients
Abscission layers- fungi, bacteria, virus
gap b/w 2 circular cell layers surrounding infection site
Tyloses- over growth of protoplasts of adjacent parenchymatous cells- protrude into xylem vessels through piths
Gums- around lesions
intracellular spaces, within surr. cells.

42. ABSCISSION LAYER

43. TYLOSE FORMATION

44. Necrotic defense reaction
Hypersensitive response
Brown resin-like granules in cytoplasm
Browning continues, cell dies
Invading hypha- degenerates
Bacterial infections- destruction of cell membranes, desiccation, necrosis of tissue
Obligate parasites- fungi, bacteria, nematode, viruses

45. INDUCED BIOCHEMICAL DEFENSES
HYPERSENSITIVE RESPONSE(HR)
Initiated by elicitor recognition
Rapid burst of oxidative reactions
↑sed ion movement (H+, K+)
Loss of cellular compartmentalization
Crosslinking of phenolics with cell wall
Production of antimicrobials

46. HR

48. due to plant R (resistance) gene
Pathogen produced elicitor- from its Avirulence gene
Eg:- arv D gene of P. syringae- enzyme involved in synthesis of syringolides (hypersensitive response in soya bean)
Eg:- protein of tobacco R gene- protect against leaf spotting bacterium – in cytoplasm
Eg:- protein of Cf9 R gene of tomato- against race 9 of leaf mould fungus- outside plasma membrane

49. ACTIVE O RADICALS, LIPOXYGENASES, CELL MEMBRANE DISRUPTION
Pathogen attack, exposure to toxins, enzymes-permeability changes of plasma membrane
Membrane ass. Disease response –
release of signal transduction molecules systematically
Release, accumulation of O radicals, lipoxygenases
Activation of phenol oxidases, oxidation of phenolics

51. O2-, H2O2, .OH released by multi subunit NADPH oxidase enzyme complex of plasma membrane
Sec or mins
Hydroperoxidation of membrane phospholipids, forming lipid hydroperoxides (toxic)
Involved in HR induced response
Oxidises phenols to more toxcs quinones
Lipoxygenases oxidizes membranes as well

52. Lipoxygenase generated hydroperoxides fom unsaturated fatty acids- lin, len
→converted to bio active molecules- jasmonic acid
role in wound and stress response

55. Antimicrobials
Pathogenesis related proteins (PR)- toxic to invading fungi
Trace amounts normally, but high after pathogen attack (stress induced trancscription)
Extremely acidic or basic – hence soluble, reactive
PR1, chitinases, β 1,3-glucanases,proteinases, peroxidases, cystein rich proteins

56. Phytoalexins
Antimicrobials produced by phytopathogens/ chemical/ mechanical injury
inhibit fungi, also toxic to bacteria, nematodes
Chemical structure- quite similar
Eg;- isoflavonoids in legumes
Accumulates around healthy cells around wounded cells
Phytoalexin elicitors- glucans, chitosan, glycoproteins (constituents of fungal cell wall)

58. OTHER MECHANISMS SIMPLE PHENOLICS- chlorogenic acid, caffeic acid
TOXIC PHENOLICS FROM NON-TOXIC PHENOL GLYCOSIDES (sugar+ phenolic)- microbial glycosidases
DETOXIFICATION OF PATHOGEN TOXINS-
Eg:-fungal HC toxin (Cochliobolus carbonum),
Pyricularin (Magnoporthe grisea)