1. Drought Tolerance in Plants
By Syed Inam Ul Haq
MSc. Molecular Biology & Plant Biotechnology.
DPB, FOH, SKUAST-K.
• 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
• Determine the morphophysiological mechanisms and
molecular signalling pathways responsible for increased
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
• 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
• Leaf-air vapor pressure difference
• Expression of the gene encoding abscisic aldehyde oxidase has been
revealed in the guard cells of dehydrated arabidopsis
• 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
• 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
• 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
• 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
• Production of specific proteins
• High level of metabolites and gene expression
• Accumulation of soluble solutes
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
• 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.
• Synthesized in leaf primordial, juvenile leaves and developing
• Development of plant roots while roots have crucial role in
• Improving drought tolerance
• Seed germination, plant growth, flowering, fruit ripening and
response to different stresses
• Produced from methionine
• Promotes cell division, root nodule development, delay leaf
• 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
• 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
11. Defence mechanisms against ROS
• 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
• 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).
• α-tocopherol, carotenoids, osmolyte proline and
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
• Sugar alcohols (mannitol and pinitol)
• Di- and oligo-saccharides (sucrose, trehalose, and
• Glycine betaine is synthesized in xerophytes and
• 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
• 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
MITOGEN ACTIVATED PROTEIN KINASE
• Play diverse roles in intra- and extra-cellular
• Comprise a family of ubiquitous proline-directed,
• Acute responses to hormones.
• Transfer information from sensors to activate cellular
• 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
• Expression of specific sets of genes in response to
15. Calcium Signalling
• Important ubiquitous intracellular second messenger
• 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 proteins activate many calcium-dependent
• 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
• 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
• 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.
• transcription activator-like effectors nucleases (TALENs),
• zinc fingers nucleases (ZFNs)
• homing meganucleases
• clustered regularly interspaced short palindromic repeats
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