2. INTRODUCTION
• Drought is a period or condition of unusually dry weather within
a geographic area where there is a lack of precipitation.
• Drought is governed by various factors, the most prominent
being extremes in temperature, photon irradiance and paucity of
water.
• The characteristics features of drought stress is low water
potential due to high solute concentration.
• Low water supply causes soil mineral toxicities and can make a
plant more susceptible to damage from high irradiance.
• Affected area(s):Rajasthan, parts of Gujarat, Haryana and
Andhra Pradesh.
3. MECHANISM OF DROUGHT TOLERANCE
• DROUGHT ESCAPE: It is defined as the ability of a plant to
complete its life cycle before supply of water in soil is depleted and
form dormant seeds before the onset of dry season. These plants are
known as drought escapers since they escape drought by rapid
development.
• DROUGHT AVOIDANCE: It is the ability of plants to maintain
relatively high tissue- water potential despite a shortage of soil-
moisture. Drought avoidance is performed by maintenance of turgor
through roots grow deeper in the soil, stomatal control of
transpiration and by reduction of water loss through reduced
epidermal i.e. reduced surface by smaller and thicker leaves.
• DROUGHT TOLERANCE: It is the ability to withstand water-
deficit with low tissue water potential. Drought tolerance is the
maintenance of turgor through osmotic adjustment (a process which
induces solute accumulation in cell), increase in elasticity in the cell
and decrease in cell size.
5. Mechanisms of resistance to drought and the
methods to increase the 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
c. Developed veins and bundle，smaller and more stomata
2. Physiology and biochemistry:
a. Stomatal regulation: ABA accumulation→stomatal closure
b. Increase in capacity of resistance to dehydration of
cytoplasm: Rapid accumulation of Pro, glycinebetaine, Lea
protein, dehydrin, osmotins and ion etc.
6. EFFECT OF DROUGHT STRESS
• Effect on Growth: Reduction in Turgor Pressure, due to cell
sizes will be smaller.
• Effect on Photosynthesis: Photosynthesis decreases due to
disruption of PS II (Photo System II), stomatal closure,
decrease in electron transport.
• Decrease in nuclear acids and proteins: Protease activity↑,
free aa↑, RNAase activity↑，RNA hydrolysis, DNA content
falls down.
• Effect on Nitrogen Metabolism: Nitrate reductase activity↓,
nitrite reductase activity insensitive
• Effect on Carbohydrate metabolism: Loss of starch and
increase in simple sugars, carbohydrate translocation decreases.
7. Synthesis of compatible solutes
• Almost all organisms, ranging from microbes to animals and
plants, synthesize compatible solutes in response to osmotic
stress.
• Compatible solutes are nontoxic molecules such as amino acids,
glycine betaine, sugars, or sugar alcohols which can
accumulate at high concentration without interfering with
normal metabolism.
• They may have a role in osmotic adjustment, stabilizing
proteins and cell structures, scavenging reactive oxygen
species.
8. • Proline is the most widely distributed osmolyte; it occurs in
plant and in many other organisms. Its accumulation correlates
with tolerance to drought and salt stress.
• Roles: Osmotic adjustment, membranes protection, a reservoir
of nitrogen and carbon source for post stress growth, sink for
energy to regulate redox potentials, OH• scavenger.
• Synthesis can occurs via two biosyntetic pathways:
The ornithine dependent, and
The glutamate dependent (predominant under stress
conditions).
Proline
9. Glycine Betaine(GlyBet)
Glycine betaine is a quaternary ammonium compound that
functions as an osmoprotectant. Its functions include:
Protects plant by stabilizing both the highly ordered quaternary
structure of proteins and membranes.
Refolding of enzymes as a molecular chaperone.
Maintenance of the water balance between the plant cell and
the environment and by stabilizing macromolecules.
Glycine betaine is synthesized via a two-step oxidation of
choline: Choline→betaine aldehyde→ glycine betaine. The
first reaction is catalyzed by a ferredoxin-dependent choline
monooxygenase (CMO) and the second step by a NAD+-
dependent betaine aldehyde dehydrogenase (BADH).
11. Late Embryogenesis Abundant (LEA) Protein
• Lea genes encode a diverse group of stress-protection proteins
expressed during embryo maturation in all angiosperms.
• Accumulation of LEA proteins during embryogenesis correlates
with increased levels of ABA and with acquisition of desiccation
tolerance.
• LEA proteins are not normally expressed in vegetative tissues but
they are induced by osmotic stress or exogenous application of
ABA.
• Evidence derived from expression profiles strongly supports a
role for LEA proteins as protective molecules, which enable the
cells to survive protoplasmic water depletion.
12. Engineering drought tolerance using transcription
factors (TFs)
• Conventional and transcriptome-based analyses have revealed
that dozens of transcription factors (TFs) are involved in plant
response to drought stress.
• These TFs are categorizes into large gene families like
AP2/ERF, bZIP, NAC, MYB, MYC, Cys2His2 zinc-finger and
WRKY.
• TFs regulates downstream genes which acting as a cis-elements
more directly act on drought response.
• Regulation pathways:
ABA-dependent: ABF/AREB TFs, acting on genes carrying the
ABRE element.
ABA-independent: CBF/DREB TFs acting on genes carrying
the CRT/DRE –C repeat/dehydration responsive-elements.
13. Transcriptional regulatory networks (cis-acting elements and transcription
factors) involved in osmotic and cold-stress responsiveness in Arabidopsis
