Abiotic stresses in plant

1. ABIOTIC STRESS IN PLANT
Avjinder Singh Kaler

2. Defining Plant Stress
 Ideal conditions: allows the plant to achieve its maximum growth and
reproductive potential as measured by plant weight, height, and seed
number, which together comprise the total biomass of the plant.
 Stress: any environmental condition that prevents the plant from achieving
its full genetic potential.
 For example, decrease in water availability would have deleterious effect
on growth because of reduce in water potential is by closing their stomata,
which reduce water loss by transpiration. It also reduces the CO2 uptake
which decrease the photosynthesis.
 Physiological adjustment to abiotic stress involves trades offs between
vegetative and reproductive development.

3. Acclimation and Adaption
 Acclimation: nonpermanent change in physiology or morphology of the
individual to improve response with exposure to environmental stress.
 Epigenetic mechanism that alter expression of genes without changing the
genetic code
 Adaptation: Fixed genetic change over many generations by selective
environmental pressure

4. Environmental Factors and their
biological impacts on plants
Environmental
Factor
Primary Effect Secondary effect
Water Deficit Water potential
reduction,
Cell dehydration,
Hydraulic
resistance
Reduced cell/leaf expansion, cellular/metabolic
activities, stomatal closure, photosynthetic inhibition,
leaf abscission, Altered Carbon partitioning,
Cytorrhysis, cavitation, membrane and protein
destabilization, ROS production, Ion cytotoxicity, cell
death
Salinity Water potential
reduction,
Cell dehydration,
Ion Cytotoxicity
Same as for Water deficit
Light Stress Photo inhibition
ROS production
Inhibition of PSII repair
Reduced CO2 fixation

5. Environmental Factors and their
biological impacts on plants
Environmental
Factor
Primary Effect Secondary effect
High Temperature Membrane and
protein
destabilization
Photosynthetic and respiratory inhibition, ROS
production, Cell death
Chilling Membrane
Destabilization
Membrane dysfunction
Flooding and soil
Compaction
Hypoxia,
Anoxia
Reduced respiration, Fermentative metabolism,
inadequate ATP production, production of toxins by
anaerobic microbes, ROS production, stomatal
closure

6. Environmental Factors and their
biological impacts on plants
Environmental
Factor
Primary Effect Secondary effect
Freezing Water Potential
reduction,
Cell hydration,
Symplastic ice
crystal formation
Same as for water deficit
Physical destruction
Trace element
toxicity
Disturbed cofactor
binding to proteins
and DNA,
ROS production
Disruption of metabolism
Mimic other essential metals
Mineral Nutrient
deficiencies
Reduced growth
and unavailable
for uptake
Ceases energy production

7. Environmental Factors and their
biological impacts on plants
 Ozone and ultraviolet light generate ROS that cause lesions and induce
PCD
 Combination of abiotic stresses induce unique signaling and metabolic
pathways
 Combination of abiotic stresses have both positive as well as negative
impacts
 Sequential exposure to different abiotic stresses sometimes confers cross
protection, for examples molecular chaperones and osmoprotectants for
ROC scavenging

8. Stress Sensing Mechanisms in Plants
 Physical Sensing: mechanical effect of stress on plant such contraction of
plasm membrane
 Biophysical Sensing: Change in protein or enzyme structure
 Metabolic Sensing: accumulation of ROS
 Biochemical Sensing: specialized protein to sense a particular stress, Ca
channel
 Epigenetic Sensing: modification of DNA or RNA such change in chromatin

9. Signaling Pathways activated in
Response to abiotic Stress
 Different pathways such as calcium, protein kinases, protein phosphatases,
ROS signaling, activation of transcriptional regulators, accumulations of
plant hormones
 Stress specific signals that emerge from these pathways, in turn, activate or
suppress various network that may allow growth under stress conditions until
favorable conditions returns
 Increase in the concentration of Ca and ROS are early signaling events
 Ca regulates the transcription factors by binding directly or to form Ca
complexes.
 Ca activates various protein kinases and phosphatases that regulate gene
expression either by phosphorylating (activating) or dephosphorylating
(inhibiting) transcriptional factors

10. Signaling Pathways activated in
Response to abiotic Stress
 Steady state level of ROS is governed by the balance of ROS generating
and ROS scavenging reactions
 ROS generation: Activities of Specialized oxidases
 ROS Scavenging: Antioxidant molecules such as APX, CAT, SOD
 ROS can trigger the opening of calcium channels which activate Ca
dependent protein kinases
 For example, mitogen activated protein kinases regulates stresses (MAPK)

11. Acclimation to stress involves transcriptional
regulatory network called regulons
 Transcriptional regulators or factors binds to specific DNA sequences and
activate or suppress the expression of genes.
 Chloroplast genes respond to high intensity light by sending stress signals to
the nucleus
 Epigenetic mechanisms and small RNAs provide additional protection
against stress
 Hormonal interactions regulate normal development and abiotic stress
resonses

12. Developmental and Physiological
Mechanisms that protect plants
 Plants adjust osmotically to drying soil by accumulating solutes
 Submerged organs develop aerenchym tissue in response to hypoxia
 Antioxidant and ROS scavenging pathways protect cells from oxidative stress
 Molecular chaperones and molecular shields protect proteins and membranes
during abiotic stress (Heat shock Protein)
 Plant can alter their membrane lipids in response to temperature and other
biotic stresses
 Exclusion (block entry) and internal tolerance mechanisms allow plants to cope
with toxic ions: glycophytes and halophytes
 Phytochelatins and other chelators contribute to internal tolerance of toxic
metal ions: chelating molecule have ligation sites
 Plants use cryoprotectant molecules and antifreeze proteins to prevent ice
crystal formations

13. ABA signaling during water stress
 During water stress, ABA increases in leaves, which leads to stomatal closure
 Stomata closure is due to reduction in turgor pressure that follows the massive efflux of K
and anions from guard cells
 Activation of specialized ion efflux channels on the plasma membrane is required for such
a large scale loss of K and anions
 Plasma membrane K efflux channels are voltage gated, they open only if plasma
membrane become depolarized.
 ABA causes membrane depolarization by elevating the cytosolic Ca in two ways: transient
influx of Ca ions and release of Ca from internal stores
 Increase in Ca open the Ca activated anion channel on the plasma membrane
 Opening of anion channels allow Cl and malate ions to escape, moving down their
electrochemical gradients

14. ABA signaling during water stress
 Outflow of negatively charged Cl and malate trigger the opening of voltage gated K efflux
channels
 Elevated level of Ca cause K influx channels to close
 ABA cause alkalization that further stimulate the opening of K efflux channels
 ABA also inhibit the activity of the plasma membrane H-ATPase
 During stomatal closure, surface area of guard cell contract 50 %. Extra membrane taken
up as small vesicles by endocytosis
 Signal Transduction involves protein kinases and phosphatases

15. Plant can alter their morphology in
response to abiotic stress
 Phenotype plasticity: plant activate developmental program that alter the
phenotype
 Leaf area, leaf orientations, trichrome, cuticle, root: shoot ratio
 Metabolic shifts enable plants to cope with variety of abiotic stresses
 The process of recovery from the stress can be dangerous to the plant and
require a coordinated adjustment of plant metabolism and physiology
 High level of ROS could form and damage cells.
 Plants needs to remove recycle all the unneeded mRNAs and protein

16. Abiotic stress tolerance crop
 Developing crops with enhanced tolerance to both biotic and abiotic
stress conditions is a major goal
 Such crops would decrease the yield penalty and prevent annual losses of
billions of dollars