ABIOTIC STRESS

Adverse environmental conditions such as droughts, extreme temperatures and salinity impose stress on plants and effect plant growth and yield. These abiotic stress cause cell death and senescence.
The soil water potential is a combination of the effects of surface area of soil particles and small soil pores that absorb water (matrix potential) the effects of dissolved substances (solute/osmotic potential) and the atmospheric pressure effects(pressure potential).
Water potential = matrix potential + solute potential + pressure potential
ΨW = ΨM + ΨS + ΨP)
In non salty well drained soil the matrix potential is almost equal to the water potential
When solubility is high the contribution of solute potential (Ψ)S to water potential(ΨW) is high.
Soil capacity of a soil is the soil moisture % held in the soil at a water potential(ΨW) of -1/3 barr, which is the condition of newly wetted soil after gravitational water have drained from macro-pores.
The permanent wilting potential is the soil moisture at -15 barr
Plant available water (water available to plants) is the water in soils held between -1/3 barr and -15 barr.
Some plant species can extract water held at water potential of -60 barrs or lower but water held at potential less than -15 barrs is absorbed only slowly and in small quantities.
Crop plants wilt if only -15 barrs water is present because water loss through transpiration is faster and greater than that amount absorbed by roots at these low moisture potentials.
Most soluble salts in soils are composed of cations of sodium, calcium and magnesium and anions of chlorine, sulphate and carbonate ions.
Although toxicity due to high concentration of sodium and chlorine and other ions can occur, salts usually effect plants growth because of the osmotic effect.
High salt concentration lowers the solute potential there by lowering the water potential it more difficult for plant roots to extract water from soil
During a drying period salts in soil solutions may be so concentrated so as to cause water to move from root to soil (exosmosis) there by killing the plant.
Barley & cotton have considerable salt tolerance
Several abiotic stresses such as drought, salinity and extreme temperature have a common consequence of causing cellular water deficit.
The response of plants to water deficit is the synthesis and accumulation of low molecular weight compounds called as osmoprotectants.
These osmoprotectants within the cells lower the osmotic potential and help maintain turgor
These solutes include inorganic ions, organic ions, soluble carbohydrates including polyols (sugar, alchohols) , aminoacid (proline) and ammonium compounds such as glycine betaine.
Several plant genes that encode key enzymes of osmolyte biosyntheic pathways including sugars, alchols, glycine betaine and proline have been cloned.
Strong correlations have been reported between proline accumulation and tolerance to draught and salinity stresses.
Gamma pyrroline – 5 – carboxylate synthetase (P5 CS) is the limiting enzyme in the synthesis of proline.
Transgenic tobacco plants were generated with P5 CS enzyme & they showed enhanced plant growth under salt & draught stresses.
The ability to synthesize & accumulate glycine betaine is synthesized by the two step oxidation of choline via the intermediate betainealdehyde the reaction beung catalysed by choline monoxygenase (CMO) and betaine aldehyde dehydrogenase (BADH)
Transgenic plants of tobacco, arabidopsis and rice with BADH have shown increased salt resistance.
Polyols like mannitol contribute to drought tolerance in plants.
Polyols are derivatives of sugars.
An E.coli gene MtPD (mannitol – 1 posphate dehydrogenase) evolved in the biosynthesis of mannitol was introduced into tobacco through disarmed Ti plasmid.
The transgenic tobacco plants producing mannitol were tolerant to draught & salt stresses.
Fructose and poly fructose molecules that are produced by many plants and bacteria and play a role in adaptation to osmotic stress due to their high soluble nature.
Fructosyl transferase gene of Bacillus subtilis was transferred to potato and the transgenic potato showed tolerance to drought stresses.
Trehalose a non – reducing disaccharide of glucose is known as a trehalose has been shown to stabilize protein and membrane lipids
Transgenic tobacco plants with the trehalose – 6 – phosphate synthetase gene (TPS 1) from E.coli accumulated trehalose and exhibited improved drought tolerance.
Heat shock protein (stress response proteins) synthesized in increased amounts in response to elevated temperature or other stressful treatments and which usually helps the cells to survive the stress.
Osmotic stress induces the accumulation of a set of low molecular weight proteins such as late embryogenesis abundant protein (LEA).
A barley LEA gene HVA, when transferred to rice resulted in rice tolerance to drought and salinity.
High temperature stress – tolerant transgenic Arabidopsis plant were produced by increased production of heat shock proteins transcription factor (Ab HSP) by a CaMV 35S promoter.
In conditions of drought and salt stress there is over production of over active oxygen species in plant cells.
Hence a plants ability to be scavenge these toxic oxygen species is considered to be critical for abiotic stress tolerance.
Among the enzymatic antioxidants, super oxide dismutase (SOD) catalyses the dismutation of superoxide to catalase(CAT) and peroxidase breakdown hydrogen peroxide to water
An effective detoxification mechanism against hydrogen peroxide is the ascorbate glutathione cycle, where ascorbate peroxidase (AP) , monodehydro ascorbate reductase and glutathione reductase (GR) work together
Genes pr cDNA’s encoding SOD, CAT, AP & GR have been isolated from several crops and transferred to other crops to confer draught & salt resistance. SOD from rice and AP from pea were transferred to tobacco to confer tolerance to salt draught and chilling stresses.
Oxygen deprivation is detrimental to most species of higher plants.
This can occur during periods of flooding and in drained soils.
Several toxic oxidant products are frequently formed.
Genes encoding proteins that increase intracellular oxygen concentration and/or its transport hence been isolated.
Expression of nonsymbiotic haemoglobin from barley (Hb) and bacteria vitriscella iteroacia (VHB) in maize and tobacco resulted in faster seed germination & higher plant growth rates.
Arabidopsis is a small green plant. It contains acyl glycerol phosphate (AGP) which makes the plant tolerant to prolonged chilling.
AGP is made from glycerol diphosphate by the enzyme glycerolposphate acyl transferase (GPAT).
The GPAT gene of arabidopsis along with CaMv 35S promoter and transferred to tobacco through disarmed Ti plasmid
The transgenic tobacco accumulates AGP in the cells and tolerates chilling to allow for its cultivation in cold area.
Nishizaura et al in 1996 introduced a desaturase gene from cyanocobacterium into tobacco. This resulted in increased chilling tolerance in tobacco ssedling.
Expression of fish (winter flounder fish) antifreeze protien (AfP) in transgenic tobacco plants inhibit the information of ice crystals in cells and there by confer tolerance to freezing stress.