Abiotic stress responses in Plants

with special reference to drought

Presented byShobha Devidas SurbhaiyyaM.Sc. Agril. Biotech, Dr. PDKV, Akola

CONTENTSIntroductionAbiotic stress and its mechanismDrought stress and its types and synthesis of compatible solutes The genetic bases of drought tolerance The abscisic acid transcriptional regulation pathway Plant response to abiotic stress Plant responses to abiotic stressStress tolerance mechanism

Detoxification ChaperoningLate embryogenesis abundant protein functionOsmoprotectionWater and ion movement

Case study

Abiotic stresses are serious threats to agriculture and the environment

Abiotic stress leads to a series of morphological, physiological, biochemical and molecular changes in plants that adversely affect growth and productivity.

Abiotic stresses cause losses worth hundreds of million dollars each year due to reduction in crop productivity and crop failure.

INTRODUCTION

What is stress:-

External conditions that adversely affect growth, development or productivity.

Types of stresses:- Biotic:- Imposed by organism(i.e. living things) Abiotic:- Arising from an excess or deficit in the physical

The term abiotic stress refers to factors such as sub and supra optimal temperature, excess salt level, reduced water availability leading to dehydration stress, excess water resulting in flooding stress and oxidative stress.

1. Avoidance - 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 tissues.

2 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.

3 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 .

STRESS RESISTANCE MECHANISM

An extended period of deficient rainfall < 75% as compared to normal rainfall of the region is called drought.

The limitations of water over a prolonged period of time or In plant denotes the loss of water from tissues and cells

Water stress may arise as a result of two conditions, either due to excess of water or water deficit

Drought is the most significant environment stress on agricultural production and enormous effort is being made by plant scientist to improve crop yields in the face of decreasing water availability.

DROUGHT

TYPES OF DROUGHT

1. Meteorological Drought- rainfall < 25 % of the average of the region.( <50 %- severe drought)

2. Agricultural Drought- lack of rainfall result in insufficient moisture in the root zone.

3. Hydrological Drought- extended dry period leading to marked depletion of surface water leading to drying up of reservoir, lacks ,streams, rivers etc.

EFFECT OF DROUGHT STRESS1. Effect on Growth: Reduction in Turgor Pressure, due to cell

sizes will be smaller.2. Effect on Photosynthesis: Photosynthesis decreases,

stomatal closure, decrease in electron transport.3. Decrease in nuclear acids and proteins: Protease activity↑,

free a.a.↑, RNAase activity↑ ， RNA hydrolysis, DNA content falls down.

4. Effect on Nitrogen Metabolism: Nitrate reductase activity↓

5. 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 Proline is the most widely distributed osmolyte; in plant- it is

produced from ornithine under normal condition but under stress it is made directly from glutamate.

Roles: Osmotic adjustment, membranes protection, etc Synthesis can occurs via two biosyntetic pathways: 1. The ornithine dependent, and 2. The glutamate dependent (under stress conditions). Glutamate-Glutamate phosphate-Glutamate

semialdehydes- ProlineP5CS – pyrroline-5-carboxylate synthataseP5CR- pyrroline-5-carboxylate reductase

GLYCINE BETAINE(GlyBet) Glycine betaine is a quaternary ammonium compound that

functions as an osmoprotectant. 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. 1. The first reaction is catalyzed by a ferredoxin-dependent

choline monooxygenase (CMO) 2. The second step by a NAD+-dependent betaine aldehyde

dehydrogenase (BADH).

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

(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 Pigments : Anthocyanins, Betalines, Carotenoids

OSMOLYTES, POLYPEPTIDES AND OTHER COMPOUNDS INDUCED DURING ABIOTIC DROUGHT STRESSES

THE GENETIC BASIS OF DROUGHT TOLERANCE

Expression studies have shown that drought specific genes can be grouped into three major categories

1. Genes involved in signal transduction pathways and transcriptional control

2. Genes with membrane and protein protection functions3. Genes assisting with water and ion uptake and transport

ABSCISIC ACID AND TRANSCRIPTIONAL REGULATION

PLANT RESPONSE TO ABIOTIC STRESSES

The outcome of stress signal perception, transduction and transcriptional up or down regulation of gene is the production of molecules with various plant protection, repair and stabilization functions.This molecule can be broadly grouped into five functional groups1. Detoxification 2. Chaperoning3. Late embryogenesis abundant protein function4. Osmoprotection5. Water and ion movement

STRESS TOLERANCE MECHANISM

To prevent stress injury, cellular ROS needs to remain at nontoxic level under drought stress

Antioxidant involve in plant strategies to degrade ROS include,1. Enzymes such as catalase, superoxide dismutase, ascorbate

peroxidase 2. Non-enzyme such as ascorbate, caratenoide, glutatione etc. Some proteins, osmolytes have antioxidative functionality

DETOXIFICATION

LEA protein are produced in response to dehydration stress.

They play role in water status stabilization, production of cytosolic structure , transport of nuclear targeted protein, prevention of membrane leakage.

LEA and LEA type genes are found in plant, they accumulate in seeds during the late stage of embryogenesis.

E.g. – The barley group 3 LEA gene, HVA1 was constitutively over expressed in rice plant to increase drought tolerance.

LATE EMBRYOGENESIS ABUNDANT

PROTEIFUNCTION

Chaperoning functions involve specific stress associated proteins

It is responsible for protein synthesis, targeting, maturation and degradation and also membrane stabilization

HSPs, which are induced by heat, have been implicated in plant cell protection mechanisms under drought stress

One such protein, Nt HSP 70-1, was constitutively overexpressed in tobacco at as certain it’s role in plant drought response and tolerance(cho and hong 2006)

CHAPERORING

OSMOPROTECTION

Osmoprotection involves the upregulation of compatible solutes (osmolyte) that function primary to maintain cell turgor, but are also involved in antioxidation and chaperoning through direct stabilization of membrane .

Compatible solute are low molecular weight, high soluble compound that are usually nontoxic at high cellular concentration.

The three major groups of compatible solutes are1. Amino acids (eg. proline).2. Amines (glycine betaine, polyamines).3. Sugars(mannitol, trehalose).

Water and ions move through plants via transcelluar and interacellular pathway.

Aquaporins (major intrinsic protein, MIPs) which are either tonoplast or plasma membrane localized, facilitate water, glycerol, small molecules and gas transfer through membrane and therefore have a role in water homeostasis.

WATER AND ION MOVEMENT

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Dhariwal et.al, Genetic engineering for abiotic stress tolerance in plant ,

plant biotechnology, 15(1) 1-10.1998.

Yang et.al, Narrowing down the target, towards successful genetic

engineering of drought tolerant crop, molecular plant, vol-3:469-490,

2010.

Ajay K. Garg, Trehalose accumulation of rice plant confer high tolerance

level to different abiotic stresses, vol-99: 25, 2002.

REFERENCES

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