Abiotic stress resistance for plants

1. BREEDING FOR ABIOTIC STRESS
RESISTANCE IN FRUIT CROPS

2. OUTLINE
 Introduction
 Drought tolerance
 Salt tolerance
 Cold tolerance
 Oxidative stress tolerance

3. What’s in name?
Stress: Factors of environment interfering the complete
expression of genotypic potential.
Abiotic stress: The negative impact of non-living
factors on the living organisms in a specific
environment.
Abiotic stress factors or stressors are naturally
occurring, often intangible factors
The four major abiotic stresses: drought , salinity,
temperature and heavy metals, cause drastic yield
reduction in most crops.

4. Environmental conditions that can cause stress
Water-logging & drought
Excessive soil salinity
High or low temperatures
Ozone
Low oxygen
Phytotoxic compounds
Inadequate mineral in the soil
Too much or too little light
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5. Characteristics of abiotic stresses
 Unpredictable occurrence.
 Some stresses are impossible to manage.
 One stress may increase or decrease the level of
another stress.
 Differential response of plant sp. to a given stress
 Effects generated by one abiotic stress may
overlap with some effects of another stress.

6. PLANT RESPONSE TO STRESS
Stresses trigger a wide range of plant responses:
 Altered gene expression
 Cellular metabolism
 Changes in growth rates and crop yields
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7. Ozone
Extreme
temperature
Flooding
Drought
Salt
Physiological &
developmental event
Altered cellular
metabolism
PLANT RESPONSE TO STRESS
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8. Resistance or sensitivity of plants to stress depends on
Environmental
stress
Stress
characteristics
Plant
characteristics
Response Result
Severity
Duration
No. of
exposures
Continuation
of stress
Organ or
tissue in
question
Stage of
develop-ment
Genotype
Resistance
Susceptibility
Survival &
growth
Death
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9. Stress resistance mechanisms
 Avoidance
- prevents exposure to stress
 Tolerance
- permit the plant to withstand stress
 Acclimation
- alter their physiology in response to stress
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10. Stress resistance mechanisms
Saguaro Honey mesquite
Spinach
Mohave desert star Black spruce
Abiotic
Stress
Acclimation
Resistance
- Stress avoidance
- Stress tolerance
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11.  Drought:
I. DROUGHT STRESS
Moisture scarcity which restricts the full expression
of genetic yield potential of a plant.
 Mechanisms of drought resistance:
a) Drought escape: mature early
b) Drought avoidance: Maintain water balance
c) Drought tolerance: higher yield even under low
water potential
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12. Effects of drought at cellular level
 Structures of membrane and organelles
 Structures of macromolecules like proteins
and nucleic acids
 Amounts of specific mRNA
 Rate of translation
 Pressure differential across the membrane-cell
wall complex; in turn affects cell
expansion.
 A combination of the above.
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13. Morphological features
providing drought resistance
• Earliness
• Reduced tillering
• Leaf rolling, folding, shedding, leaf reflectance
• Reduced leaf area; narrow leaf, change in leaf angle
• Hairiness
• Color of leaves
• Wax coating
• Root systems
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14. Physiological response to drought
 Photosynthetic efficiency is reduced due to
chloroplast damage
 Reduced transpiration and reduced respiration losses
 Stomatal behavior
Biochemical response to drought
 Accumulation of compatible solutes
 Increase in ABA & Ethylene
Protein synthesis
Nitrate reductase activity
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15. Sources of drought resistance
 Cultivated varieties
 Land races
 Wild relatives:
e.g. Wheat: A.variabilis, A,speltoides, A.squarrosa
Sugar cane: S. spontaneum
 Transgenes :
Rab (Responsive to abscisic acid) in rice
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16. Breeding methods and approaches
Approaches:
 Varieties adapted to a specific environment
 Adaptation to variable environment
 Combining drought resistance traits with high
yield potential
Breeding methods:
Introduction
 Selection
 Hybridization
 Mutation
 Genetic engineering
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17. Osmotic adjustment
 In response to dehydration or osmotic stress a series of
compatible solutes/ osmolytes are accumulated for osmotic
adjustment, water retention and free radical scavenging.
 The cell actively accumulates solutes and as a result the
solute potential (s) drops, promoting the flow of water into
the cell.
Few osmolytes:
 Proline
 Glycine betaine
 Mannitol
 Osmotin
 D-Pinitol
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18. Stepwise gene expression in molecular responses to
dehydration and low temperature
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19. Limitations
 Generally resistant varieties have low yield; Do not
have much adaptability.
 Drought resistant genes may have linkage with
undesirable genes.
 Drought resistant traits may reduce yield.
 Transfer of resistant genes from wild types may pose a
problem
 Creation of controlled moisture stress environment is a
problem, selection require considerable resources
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20. Achievements
 Rice: Akashi, Bala, IRS
 Barley: Karan 280
 Durum wheat: Gulab, Motia, Jay, Vijay
 Ground nut: Jyothi, DH3-10
 Sugarcane: Co 1148, Co 11158, Co 997
 Maize: Early triumph, silver king
 Potato: Rila, Viking
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21. II. SALT TOLERANCE
Salt tolerance: Ability of plants to prevent ,reduce or overcome
injurious effects of soluble salts present in their root zone
Salinity can be overcome by
1) Soil reclamation: costly ,time consuming & short lived
2) Resistant varieties: less costly, more effective, long
lasting but require longer period to develop.
Characteristics of plants to salt :
 Land races more tolerant than high yielding varieties
 Salt tolerance capacity differs from species to species
 Different plants show differential response to salinity
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22. Classification of plants based on salt tolerance
1. Highly tolerant crops:
Sugar beet, barley, cotton, date palm, asparagus.
2. Moderately tolerant:
Barley, rye, sorghum, wheat, safflower, Soya been
3. Moderately sensitive:
Rice, corn, foxtail millet, cow pea, peanut, sugar cane,
tomato, potato, radish, cabbage
4. Extremely sensitive:
Citrus, strawberry, melon, peas, carrot, okra, onion.
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23. Symptoms of plant to salt stress
Retardation of growth
Necrosis
Leaf abscission
Loss of turgor
Ultimate death of plant
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24. Mechanism of salt tolerance
1. Salt tolerance:
By accumulating salt, generally in their cells or glands
& roots.
Halophytes show tolerance by ion accumulation
mechanism
2. Salt avoidance:
By maintaining their cell salt concentration unchanged
either by water absorption (e.g. Rice,
chenopodiaceae) or by salt exclusion (e.g. tomato,
Soya bean, citrus, wheat grass)
Glycophytes (nonhalophytes) owe their resistance
primarily to avoidance e.g. barley
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26. Over-expression of AtNHX1 increases salt tolerance
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27. Breeding strategies
 Breeding for yield potential should have greater emphasis
than breeding for salt resistance.
 Selection should be done in stresses target environments
Screening techniques
1. Sand culture by using nutrient solution in sand & irrigation
with saline water.
2. Solution culture by using solution culture tanks
3. Micro plot techniques by using small micro plots.
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28. Achievements
Rice : Mohan, pokkali, SR23b, SR26b, CSR-2, CSR-3,
CSR-6.
Onion: Hissar-2, pb selection, Karchia, Karna-92.
Okra: Pusa Sawani
Barley: Ratna, RS-16, Karan-18, 19, 92
Sugar cane: Co7717, Co1148, Bo91
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29. III. COLD TOLERANCE
Chilling: When temp remain above freezing i.e. >0°C
to < 10-15°C.
Freezing: When temp remains below freezing i.e.
<0°C.
a) Chilling resistance
Chilling sensitive plants are typically tropical plants.
Temperate plants generally tolerate chilling injury.

30. Effects of chilling on plants
 Reduced germination
 Poor seedling establishment
 Stunted growth
 Wilting, Chlorosis, necrosis
 Pollen sterility
 Poor seed set/ seed formation.
 Locked open stomata.
 ABA accumulation
At subcellular level:
 Reduces membrane stability
 Poor chlorophyll synthesis
 Reduced photosynthesis
 Toxicity due to H2O2 formation

31. Chilling tolerance mechanisms involve
 Membrane lipid un-saturation
 Reduced sensitivity of photosynthesis
 Increased chlorophyll accumulation
 Improved germination
 Improved fruit/seed set
 Pollen fertility
Sources of chilling tolerance:
 Late adopted breeding populations e.g. maize
 Germ plasm
 Induced mutants for cold tolerance
 Cold tolerant somaclonal variants
 Related wild species eg.tomato

32. b) Freezing resistance
Dormant state is conducive to freezing resistance,
while resistance is rare in actively growing tissue.
As water in plants cool below 0°C, it may either
1) freeze i.e. form ice.
2) super cool with out forming ice.

33. Effects of freezing stress
1. Ice formation :
 Intercellular ice formation:
 Intracellular ice formation: it is most lethal may
be due to physical disruption of sub cellular
structure by ice crystals.
2. Membrane disruption:
• Freezing causes disruption and alter the semi
permeable properties of plasma membrane
• Loss of solutes from the cells occur
• Cells remain plasmolyzed even after thawing

34. 3. Super cooling:
 In plants cooling of water below 0°C with out ice
crystal formation is called super cooling
 In plants water may cool down to -1 to -15°C
 It is possible because internal ice-nucleators are
absent.
 This is regarded as imp mechanism of freezing
avoidance.

35. Mechanism of freezing resistance
1. Freezing avoidance :
The ability of plant tissues / or genes to avoid ice
formation at sub zero temperature
Super cooling is a mechanism of freezing avoidance
which is controlled by
 Lack of ice nucleators
 Small cell size
 Little or no intercellular space
 Low moisture contents
 Barriers against external nucleators

36. 2. Freezing tolerance:
Ability of plants to survive the stress generated
by extra cellular ice formation and to recover
and re grow after thawing
Components of freezing tolerance
 Osmotic adjustment
 Amount of bound water
 Plasma membrane stability
 Cell wall components properties
 Cold responsive proteins, e.g. ABA

37. Genetic resources for freezing tolerance
 Cultivated varieties
 Germplasm lines
 Induced mutations
 Related wild species
e.g. Wheat: Agropyron sp., Rye
Oats: Avena sterilis
 Transgenes:
e.g. chemical synthesized anti freeze
protein gene, ala3 in tobacco

38. Selection criteria:
• Freezing test in laboratory
• Cryo freezing
• Osmoregulation
• Field survival
Problems in breeding for freezing tolerance:
o Breeding work under field conditions is highly
influenced by other environmental factors and biotic
stresses
o Due to large G x E interaction field survival shows
poor heritability
o Lab tests yet to be developed to screen large
breeding populations

39. IV. OXIDATIVE STRESS
 Results from conditions promoting the formation of
active oxygen species that damage or kill cells
Environmental factors that cause oxidative stress:
 Air pollution (increased amounts of ozone or sulfur
dioxide)
 oxidant forming herbicides e.g. Paraquat dichloride
 heavy metals
 drought
 heat and cold stress
 wounding
 UV light
 Intense light that stimulate photoinhibition
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40. Reactive oxygen species (ROS)
 Formed during certain redox reactions and during
incomplete reduction of oxygen or oxidation of water by
the mitochondrial or chloroplast electron transfer chain.
 e.g. Singlet oxygen, hydrogen peroxide, superoxide
anion, hydroxyl and perhydroxyl radicals
The negative effects of ozone on plants
 Decreased rates of photosynthesis
 Leaf injury
 Reduced growth of shoots and roots
 Accelerated senescence
 Reduced crop yield
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41. Ozone Damage
 Alters ion transport
 Increases membrane permeability
 Inhibits H+-pump activity
 Collapses membrane potential
 Increases Ca2+ uptake from the apoplasm
 Oxidative damage to biomolecules
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42. Resistance to ozone
 Utilizes either avoidance or tolerance
 Avoidance involves physically excluding the pollutant
by closing the stomata, the principal site at which
ozone enters the plant
 Tolerance - biochemical responses that induce or
activate the antioxidant defence system and possibly
also various repair mechanisms
Anti-oxidants:
Over expression of certain enzymes such as superoxide
dismutase, ascorbate peroxidase and glutathione
reductase has been implicated in free radical
detoxification and scavenging of free radicals under
oxidative stress.
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43. Salicylic acid and ethylene
• Ozone exposure results in increased amounts of H2O2,
which stimulate the production of SA
• Results in a transient increase in the number of transcripts
that encode defence-related secondary metabolites
e.g. phytoalexins, cellular barrier molecules e.g. lignins,
callose, and extensins, PR proteins e.g. (13) -glucanase,
chitinase, gluthatione S-transferase and phenylalanine
ammonia lyase
• Increases ethylene production by inducing increases in ACC
synthase and ACC oxidase gene transcription
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45. QUERIES….
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