plant stress responses
TRANSCRIPT
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PLANT STRESS RESPONSE
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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
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STRESS
• Conditions that adversely affect growth, development, productivity
1.Abiotic (phy/che environment)2.Biotic (organisms)• Abiotic- water logging, drought, high or low
temperatures, excessive soil salinity, inadequate mineral nutrients, too much or too little light, ozone
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• STRESS FACTOR (stressor) – stress stimulus• STRESS RESPONSE (state of stress)- response
to stimulus + ensuing state of adaptation• Avoidance of stress (protective)• Tolerance • Dynamics of stress- destabilizing, destructive
component (distress) + counter mechanisms to promote restabilization and resistance (eustress)
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RESISTANCE
• Depends on1.Species2.Genotype3.Devt. Age of plant4.Tissue identity5.Duration, severity, rate of stress
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1. Alarm phase – stress reaction2. Restitution- repair3. Hardening4. Adjustment5. Adaptation- normalization6. (Exhaustion)- irreversible damage7. End phase
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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
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• 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
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WATER DEFICIT
• Major abiotic stress• Induced by many environmental conditions:1.No rainfall- drought2.High salt conc.3.Low temp.4.Transient loss of turgor at midday• Rate of onset, duration, acclimatization-
influence the water stress response
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Response to water deficit• ABA phytohormone• Induce expression of drought- inducible genes• Products- 2 groupsA.GROUP 11.Protective proteins (AFP, osmotins, chaperons,
mRNA binding proteins)2.Water channel proteins, membrane transporters3.Osmoregulator synthesizing enzymes4.Detoxifying enzymes (peroxidases, catalases)
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B) GROUP 21.Transcription factors (DREB, MYC)2.Protein kinases (MAP kinases, CDP kinases)3.Proteinases (phospholipase)
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• 4 independent pathways1)2 - ABA-dependent2)2 - ABA- independent• Cis acting elements- in promoter of all stress
inducible genes- ABA-responsive element (ABRE) dehydration responsive element (DREB)
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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
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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
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• Temp below 00C , cellular water freeze.
• Cell shrinkage
• Expansion induced lysis
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• SURVIVAL STRATEGIES: anti freeze proteins (AFP)- Declines rate of ice crystal growth- Lowers the efficiency of ice nucleation sites- Lowers temp. at which ice formsOsmoprotectants- osmolytes- quarternary amines, amino acids,
sugar alcohols- Balances the osmotic potential of externally
increased osmotic pressure
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• 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
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• Proline
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• Sugar alcohols –mannitol
• Trehalose- non reducing disaccharide• Increased water retention and desiccation
tolerance
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SALT STRESS
• Flow of water is reversed- imbalance• Accumulation of excess Na+, Cl- in cytosol• Stress tolerant plants- maintains internal
osmotic pressure
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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
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• Plant maintains high K+, low Na+ in cytosol• 3 tolerance mechanisms-1)Reducing Na+ entry to cells2)Na+ efflux from cell (K+-Na+)3)Active transport to vacuole (vacuolar H+-Na+
ATPase)
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Na+ sequestration
• In vacuoles• By NHX1, NHX2 proteins of tonoplast
membrane• Decreases cytoplasmic Na+
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Salt stress induced proteins
• Transcription of genes oncoding late embryogenesis abundant (LEA) proteins- activated
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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 SCAVENGINGAntioxidant system contains a battery of
enzymes that scavenge ROI- SOD, peroxidases, catalases, glutathione reductases
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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
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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
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BIOTIC STRESSES
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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
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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.
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• Systemic signal transduction, aquired resistance- by salicylic acid, oligogalacturonides from plant cell walls, jasmonic acid, systemin, fatty acids, ethylene
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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
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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.
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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
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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.
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ABSCISSION LAYER
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TYLOSE FORMATION
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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
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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
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HR
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• 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
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ACTIVE O RADICALS, LIPOXYGENASES, CELL MEMBRANE DISRUPTION
• Pathogen attack, exposure to toxins, enzymes-permeability changes of plasma membrane
• Membrane ass. Disease response – 1. release of signal transduction molecules
systematically2.Release, accumulation of O radicals,
lipoxygenases3.Activation of phenol oxidases, oxidation of
phenolics
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• 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
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• Lipoxygenase generated hydroperoxides fom unsaturated fatty acids- lin, len
→converted to bio active molecules- jasmonic acid
role in wound and stress response
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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
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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)
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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)