DROUGHT TOLERANCE

SUBMITTED BY

SUDHANSHU SHEKHAR

M.TECH (BT) IInd SEM

A7110709009

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.

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.

Figure: Drought Avoidance : Reduced surface by smaller and thicker leaves

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.

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.

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.

• 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

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).

Osmolytes, polypeptides and other compounds induced during abiotic drought stresses

 • Polyamines : Spermine, Spermidine, Putrescine,

Cadaverine• Sugars : Sucrose, Trehalose, Fructans• Polyols : Mannitol, Sorbitol, myo-inositol, pinitol

(Sugar alcohols)• Amino Acids : Proline, Ectoine• Quarternary amines : Glycine betaine (GlyBet), Proline betaine,

Alanine betaine, 4-hydroxy proline betaine

• Ions : Potassium• Proteins : LEA/dehydrins, SOD/catalase, PR proteins• Pigments : Anthocyanins, Betalines, Carotenoids

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.

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.

Transcriptional regulatory networks (cis-acting elements and transcriptionfactors) involved in osmotic and cold-stress responsiveness in Arabidopsis

Strategies for the genetic engineering of drought tolerance

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