The document discusses the impact of drought stress on plant growth and yield, emphasizing its importance in arid regions and the physiological mechanisms behind drought tolerance. It details various plant responses, including biochemical adaptations, gene expression, and the role of phytohormones like abscisic acid, as well as strategies to enhance drought tolerance through genetic modification, plant breeding, and microbial interactions. The conclusion highlights the need for developing high-yielding, drought-tolerant crops to address future food and shelter challenges due to global warming.

1. Drought Tolerance in Plants
By Syed Inam Ul Haq
MSc. Molecular Biology & Plant Biotechnology.
DPB, FOH, SKUAST-K.

2. Drought
• Water scarcity is a universal problem
• Anthropogenic activities affect the balance between incoming
solar radiation and outgoing radiation.
• Global warming increases water evaporation and consequently
leads to drought stress.
• At the end of the twenty-first century, the waves of heat will
be frequent and more intense
• High summer temperatures and drought
• High temperature and water scarcity are two important
interconnected stresses
• Determine the morphophysiological mechanisms and
molecular signalling pathways responsible for increased
drought tolerance.

3. Effects of Drought Stress on Plant Growth and Yield
• Most important stresses in arid and semi-arid areas
• Effects seed germination, plant growth, and yield, specifically
transpiration rate.
• Photosynthetic rate, stomatal conductance, leaf relative water
content and water potential.
• 67% of plant yield reduced in the US last 50 years
• Uncontrolled stress which damages almost all stages of plants
directly or indirectly.
• Abnormal physiological processes like loss of turgidity, rate of
• Carbon assimilation, gaseous exchange, oxidative damage, and
affects translocation of nutrients.
• Affects the composition of minerals, antioxidants, and proteins.
• Decline in leaf development, suffer various enzymatic activities,
• And absorption of ions which result in loss of crop yield.

4. Sensing Drying Environments
Plant leaves
• Leaf-air vapor pressure difference
• Expression of the gene encoding abscisic aldehyde oxidase has been
revealed in the guard cells of dehydrated arabidopsis
Roots
• Evaporation lowers the water potential and increases the salt
concentration of soil.
• Deficits in the water content of the soil environment > increase in the
salt concentration around root surfaces and/or an increase in the osmotic
pressure of root cells
• No water sensor or water potential sensor has so far been identified in
plants.
• Aba is synthesized in parenchyma cells of vascular bundles by drought
and salt stresses.
• Conjugate with glucose, and from here is transported to the leaves.

5. Plant Response to Drought Stress
Leaf Structure and Shape
• Dropping and reduction in leaf size
• Development of waxy and thick leaf cuticle layer
• Develop xeromorphic characters e.g. Smaller and less number
of stomata.
• Thick palisade tissues, large number of trichomes.
• Thicker and tiny leaves and developed vascular tissues.

6. Responses of Leaf Photosynthetic Systems to Drying Environments
• Molecular and biochemical characteristics of photosynthetic organs have
evolved to maximize photon capture.
• Stomatal closure under drought stress deprives plants of their largest
consumer of solar energy i.e Photosynthetic system
• Stomatal closure leads to the unavailability of fixable CO2.
• Utilization of NADPH slows down >ATP/ADP ratio increases >acidified
lumen of Thylakoids.
• Synthesis of zeaxanthin from violaxanthin (low luminal pH)
• Zeaxanthin blocks energy transfer from LHC to chlorophyll P680.
• Transfer of electrons from water in PSII to oxygen in PSI is also
• Superoxide formed in PSI is reduced by thylakoid-bound Cu, Zn-superoxide
dismutase to H2O2, and then to water by thylakoid-bound ascorbate
peroxidase.
• However, no up-regulation of enzymes involved in decomposing active
oxygen in the chloroplasts.
• The First Sacrifice in Drought is impaired ATP synthesis.

7. Biochemical and Molecular Adaptations
Plants show multiple response mechanisms to drought stress
such as
• Production of specific proteins
• High level of metabolites and gene expression
• Accumulation of soluble solutes
• Phytohormones

8. Abscisic Acid
• Principal phytohormone that is involved in response to abiotic stresses
• crucial plant stress regulator
• ABA synthesis increases under drought stress.
• Activates drought response signalling pathways.
Triggers various drought related genes lead to
• Closing of stomata
• Improve root architecture
• Increase synthesis of drought tolerance substances
Jasmonates
• Signal development and various gene expression, responsible for stresses
• Synthesised in plastids, cytosol, and peroxisomes.
• Closing of stomata, root development and scavenging of ros.
• Hydraulic uptake of water from soil in slight humid condition.

9. Auxin
• Synthesized in leaf primordial, juvenile leaves and developing
seed
• Development of plant roots while roots have crucial role in
• Improving drought tolerance
Ethylene
• Seed germination, plant growth, flowering, fruit ripening and
response to different stresses
• Produced from methionine
Cytokinin
• Promotes cell division, root nodule development, delay leaf
senescence,
• Regulate nutrient allocation and plant response to pathogen
interactions enhance drought tolerance
• Protection of photosynthetic apparatus, increase of antioxidant
substances, regulate water balance, control plant growth, and
regulate stress-related hormones.

10. Reactive Oxygen Species
• Come from the incomplete reduction of atmospheric oxygen
• AKA reactive oxygen intermediate (ROI) or active oxygen species (AOS).
• Four forms,
Hydroxyl radicle (HO•) Hydrogen peroxide (H2O2)
Singlet oxygen (1O2) Superoxide anion radical (O2−)
• Prolonged drought stress increases the pro- duction of ROS in the cell
components
• Increased production of ROS is harmful to nucleic acid, lipids and proteins
within the cell.
• Two important sources of reactive oxygen species
i.e., Metabolic ROS and signaling ROS.
• If the oxidation of these components is not controlled, it may lead to cell
death

11. Defence mechanisms against ROS
• Enzymatic
• Important role in detoxification and scavenging of the
ROS and increase drought tolerance.
• Various enzymes present in different parts of the plant
cells are involved in scavenging of ROS
• SOD( convert the o2*- into hydrogen peroxide).
POD(disintegrate the hydrogen peroxide into oxygen and
water)
• Glutathione peroxidase (GPX) & ascorbate peroxidase
(APX) detoxify the H2O2.
• Monodehydroascorbate reductase (MDHAR)
• Glutathione reductase (GR), & CAT (remove the H2O2
via Halliwell- Asada pathway).
• Reduced glutathione (GSH).
• Non-enzymatic
• α-tocopherol, carotenoids, osmolyte proline and
flavonoids.

12. COMPATIBLE SOLUTES
• Small molecular compounds synthesized by organisms
as a way of tolerating stresses such as drought, and
high salt concentrations. For example
• Amino acids (proline and citrulline)
• Onium compounds (glycine betaine, 3- dimethyl
sulfoniopropionate)
• Monosaccharide(fructose)
• Sugar alcohols (mannitol and pinitol)
• Di- and oligo-saccharides (sucrose, trehalose, and
fructan)
• Glycine betaine is synthesized in xerophytes and
halophytes
• Citrulline accumulates in leaves of wild watermelon
plants under drought.

13. Functions of compatible solutes
• Acts as osmoregulator, since their high solubility in water.
• Acts as a substitute for water molecules released from
leaves.
• Acts as active oxygen scavengers or thermostabilizers.
• Increase cellular osmotic pressure.
• High hydrophilicity helps maintain the turgor pressure.
• Replace water molecules around nucleic acids, proteins, and
membranes during water shortages.
• Prevent interaction between these ions and cellular
components by replacing the water molecules around these
components, thereby protecting against destabilization.
• Stabilize enzymes
• For example stabilization of RuBisCO, PSII super complex
• Stabilize membrane during freeze-drying
• Scavengers of hydroxyl radicals

14. SIGNAL TRANSDUCTION UNDER DROUGHT
STRESS
MITOGEN ACTIVATED PROTEIN KINASE
• Play diverse roles in intra- and extra-cellular
signaling.
• Comprise a family of ubiquitous proline-directed,
protein-serine/threonine kinases.
• Acute responses to hormones.
• Transfer information from sensors to activate cellular
responses.
• Contain sub-families, i.e., MAP4K, MAP3K,
MAP2K, MAPK. that are sequentially activated.
• Results in the activation of transcription factors,
phospholipases or cytoskeletal proteins, and
microtubule-associated proteins.
• Expression of specific sets of genes in response to
environmental stimuli.

15. Calcium Signalling
• Important ubiquitous intracellular second messenger
molecules.
• Cytosolic free Ca2+ concentration ([Ca2+]cyt) increases in
response to drought stress
• Increased level of Ca2+ is recognised by some Ca2+-sensors or
calcium-binding protein.
• Calcium-binding proteins activate many calcium-dependent
protein kinases.
• CDPKs phosphorylate various transcription factors.
• CDPKs regulate the function of stress-responsive genes,
resulting in the phenotypic response of stress tolerance.

16. SCIENTIFIC STRATEGIES TO IMPROVE DROUGHT
TOLERANCE
• Exogenous Application of Substances
• (Example Nitric oxide, 24-epibrassinoide,Glycine betaine, Proline)
• Plant Microbe Interactions
• Plant growth promoting bacteria (PGPB) and mycorrhizal fungi assist
plant growth and development under biotic and abiotic stresses.
• PGPB secrete osmolytes, antioxidants, phytohormones, etc. that
enhance the root osmotic potential under drought stress.
• Arbuscular mycorrhizal fungi (AMF) promote water uptake and
nutrients to control abiotic stresses

17. Transgenic Approach
• DNA is modified using genetic engineering techniques to make
plants resistant to biotic and abiotic stresses.
• Expression of stress response genes makes the plant cope with
abiotic stresses.
• Identification of genes involved in drought stress tolerance.
• AtNAC2 gene (related with phytohormones signalling).
• AtNHX1 gene induces salinity and drought tolerance in plants.
• ScALDH21gene transformation of Syntrichia carninervis
enhanced drought tolerance in cotton cultivar .
• Use of genome editing technologies to produce targeted genetic
modification in organisms of choice.
• Example
• transcription activator-like effectors nucleases (TALENs),
• zinc fingers nucleases (ZFNs)
• homing meganucleases
• clustered regularly interspaced short palindromic repeats
(CRISPR)

18. Plant Breeding
• Development of stress-tolerant plants that have stable
production under stress conditions.
• Few of the modern varieties are tolerant to stresses
• Find stress tolerance alleles in conventional landraces and
wild relatives of the important crops
• Provide a solid platform to promote new gene discoveries
and mechanisms of physiological adaptations

19. Conclusions and Future Recommendations
• Increasing world population and global warming challenges our
capability to feed the world
• International interest in increasing yield and plant drought tolerance due
to the severe losses in crop production.
• Aim of the current study was to aggregate various drought tolerance
mechanisms and to further improve these processes.
• In order to be prepared for the upcoming food and shelter crisis, high
yielding drought tolerant crops should be developed via integrating
above discussed approaches.
• In this regard, various new technologies
• Containing gene selection (GS), microarrays,
• Transcriptomics, next-generation sequencing,
• RT-PCR, , ELISA, AFLP, TALEN, CRISPR
• Immunofluorescence, and Western blotting,
will enable the scientists to understand and improve drought tolerance in
major crops.

20. Thank you for
your attention!
Questions?