Plant Responses to Abiotic Stresses (Lecture 25)
Plant Responses to Abiotic stress
Types of Stress:
“Stress could also be defined as significant deviation of the optimal condition of life.”
There are many factors that limit plants growth, development and productivity, mainly these factors or stresses are of two distinct types termed as:
- Biotic stresses
- Abiotic stresses
Types of Abiotic Stresses:
There are several A-biotic factors that lead to a series of morphological, physiological, biochemical, and molecular changes that adversely affect plant growth and productivity. Some of these stresses are discussed below.
- Water/ drought stress
- Heat stress
- Cold stress
- Light stress
- Wind stress
- Salinity stress
- Heavy metals
Water Stress/Drought:
There are two types of water stresses:
1.Drought
2.Flooding / Water logging
In case of drought:
- Reduced availability of water for vital cellular function.
- Reduced Turgor pressure.
- Osmotic pressure.
- Stomata closure, reduced carbon dioxide supply and slower the rate of biochemical reactions during prolonged periods of dehydration.
Mechanisms of resistance to drought:
- A) Mechanisms of resistance
(1)Morphology: increase in water absorption and transportation , declination of transpiration.
- a. Developed root system and higher ratio of root to shoot
- b. Thick leaf , smaller leaf area and thick cuticle。
(2)Physiology and biochemistry
- Stomatal regulation:
- ABA accumulation → stomatal closure →
- Increase in capacity of resistance to dehydration of cytoplasm
- Rapid accumulation of Proline, glycine betaine, protein, dehydrin, osmotins and ion etc.
- The mechanism: ABA binds to receptors at the surface of the plasma membrane of the guard cells.
- The receptors activate several interconnecting pathways which converge to produce
–a rise in pH in the cytosol;
–transfer of Ca2+ from the vacuole to the cytosol.
- These changes
–stimulate the loss of negatively-charged ions (anions), especially NO3− and Cl−, from the cell and also
–the loss of K+ from the cell.
- The loss of these solutes in the cytosol reduces the osmotic pressure of the cell and thus turgor.
- The stomata close.
Methods to increase the resistance:
(1)Selection of cultivars with high resistance to drought,high yield and quality.
(2)drought hardening:
- Seed priming special technology to control seed water absorption and re-drying slowly
(3)Suitable fertilizer application:
- Application of more P、K to plants.
(4)Chemical regents application
- Soaking in 0.25% CaCl2 or 0.05%ZnSO4 solution.
- Application of plant substance: ABA
- Cycocel Chlormequat Chloride (CCC), Plant Growth Regulator
Response to water deficit:
- ABA phytohormone production
- Induce expression of drought- inducible genes
- Products of induced genes- 2 groups
A.GROUP 1
1.Protective proteins (osmotins, chaperons, mRNA binding proteins)
2.Water channel proteins, membrane transporters
3.Osmoregulator synthesizing enzymes
4.Detoxifying enzymes (peroxidases, catalases)
- GROUP 2
1.Transcription factors (In molecular biology and genetics, transcription factor is a protein that binds to specific DNA sequences, thereby controlling the rate of transcription of genetic information from DNA to messenger RNA)
2.Protein kinases (A protein kinase is a kinase enzyme that modifies other proteins by chemically adding phosphate groups to them (phosphorylation).
3.Proteinases (phospholipase) (that are capable of hydrolyzing proteins)
Flooding / Water Logging:
- In case of Flooding / Water Logging
- The soil become saturated with water & thus the oxygen content decreased in the soil. This also leads to the deficiency of oxygen also in plant cell & tissue.
- Many physiological functions are disturbed by this oxygen deficiency.
- Cell lysis & rupturing of cells also occur due to excess of water.
- ATP production is lowered
FLOODING/ SURVIVAL STRATEGIES:
production of enzymes for sucrose, starch degradation, glycolysis, ethanol fermentation
-ethylene- long term acclimatization responses-stem elongation
- In brief, plants with the escape strategy:
(i) increase the growth rate of shoot organs, such as petioles and stems, so as to emerge above floodwaters, and
(ii) initiate the development of aerenchyma to facilitate internal gas diffusion.
- Quiescent plants, on the other hand, “wait out the submergence event” and are characterized by:
(i) conservation of energy and carbohydrates via, for example, a reduction of the underwater growth rate, and
(ii) an increase of molecular components that prepare shoot and root organs for future conditions with low O2 and production of protective molecules that counteract harmful cellular changes associated with flooding, such as production of ROS.
Temperature Stress:
1.Heat Stress
2.Cold Stress
- HEAT STRESS:
Effects of Heat Stress:
a. Dehydration.
b. Yellowing of plants and leaves; chlorophyll worsens.
c. Withered leaves
d. Sunburn; reddish-purple tint or freckles on leaves and pseudo bulbs, leaf tip and root may turn brown.
e. Alteration of gene expression is the major cause of heat stress.
f. Leathery leaves; damage on cellular level.
Survival strategy:
Ø In response to high temperature, plants synthesize a set of proteins called as heat shock proteins (HSPs)
Ø The induction of HSPs at permissive temperatures have been associated with the acquisition of thermo-tolerance to withstand the stress.
Ø Decrease in synthesis of normal proteins
Ø Transcription and translation of HSPs
Ø When 5o C rise in optimum temp. Conserved proteins act as chaperons (help in proper folding of other proteins), refolding classes- based on mw Hsp 100, Hsp 90, Hsp 70, Hsp60
2. Cold Stress:
Effects of Cold Stress:
a. Cold stress decreases membrane fluidity.
b. It alters the lipid composition of membrane.
c. Freezing causes ice to form in a plant cell wall and inter-cellular spaces.
d. Metabolism retarded.
e. Delayed energy dissipation leading to radical formation and oxidative stress.
f. Chilling causes protoplast volume shrinkage upon extra-cellular ice formation.
SURVIVAL STRATEGIES:
Low temperature limitations have been overcome by the identification of cold-tolerant genes for applications in genetically transformed crops.
anti freeze proteins (AFP)
-Declines rate of ice crystal growth
-Lowers the efficiency of ice nucleation sites (ice–nucleation proteins are localized at the outer membrane surface and can cause frost damage to many plants)
-Lowers temp. at which ice forms
Osmoprotectants
-osmolytes- quarternary amines, amino acids, sugar alcohols-mannitol, choline, glycine betaine and proline (osmoprotectants)
-Balances the osmotic potential of externally increased osmotic pressure
Salt Stress:
- Salt stress is one of the major Abiotic stresses.
Ø All carbohydrates, fatty acids and protein content are adversely affected due to salinity effect.
Ø Salt stress induces the synthesis of abscisic acid which closes stomata when transported to guard cells, therefore, photosynthesis declines and photo inhibition and oxidative stress occurs.
Ø Some physiological damages also occur due to salt stress that are
Ø Water deficiency
Ø Ion cyto-toxicity
Ø Osmotic stress
Ø The accumulation of Na+ and Cl- ions in the cells is very toxic in terms that these ions can influence the enzymatic action.