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Stress Tolerance in Plants

INTRODUCTION

India has to support 16% of world food needs in the available less than 2 % land of the country. Hence agriculture has to maximize its efficiency. Which can be achieved only by understanding and engineering the plants to make them survive in the adverse condition.
Plant growth requires not only carbon dioxide and oxygen from the air but also water and mineral nutrients from the soil. Soil has been called the "placenta of life", because it supplies essential nutrients to all land plants and the plants in turn feed all the terrestrial ecosystem. Throughout the history, humanity standard of living has depended on the fertility and productivity of the soil.
Soil erosion and salinization are accelerated by poor agronomic practices. Mismanagement and neglect of soil can ruin the arable land, which is a fragile and precious resource. The harappan civilization in western India, Mesopotamia in Asia minor , and the mayan culture in central America all collapsed partly because of soil degradation. Maintaining productive should be one of society's important goals.
Most crops are salt sensitive or hypersensitive (glycophytes ) in contrast to halophytes , which are native flora of saline environment , halophytes have the capacity to accommodate extreme salinity , because of various special anatomical and morphological and physiological adaptation or avoidance mechanism.
Approximately 330 species of vesicular plants (i.e.<0.15% of total number), have been demonstrated as desiccation tolerant.
The majority of bryophytes which represent 30,000 spp of mosses , liverwort , hornworts are postulated to tolerate at least brief desiccation of low intensity.

HALOPHYTES :

Plants which grow and complete their life cycles in a habit with a high salt concentration are commonly designated as halophytes these are the specialized plants growing under saline environment commonly found near sea shore where the concentration of salts ( NaCl , MgSo4 , MgCl2 etc ) are relatively high. Although such plants grow in water or in area well saturated with water , water absorption is extremely difficult process , thus halophytes are physiologically dry but physically wet habitants. For this reason they have under gone a detail morphological physiological and anatomical adaptation , during their life cycle.

MORPHOLOGICAL ADAPTATIONS :

A ) ROOT:

1.In halophytes in addition to normal roots , many stilt or prop roots develops from the aerial branches of the stem. Example? Rhizophora mucronata.
2.Some times a large number of root buttresses develops from the basal part of the tree trunks.
Example ? Dischidia numularia
3.In order to compensate lack of soil aeration they develop special type of negatively geotropic roots , called pneumatophores , this peg like structures causes numerous lenticells inner surface.

B) STEM:

Stems of several halophytes are succulent. Which is induced only after the accumulation of free ions in this organs. They are either hard or tough or swollen or fleshy and are usually covered with hairs.

C ) LEAVES

1.The leaves of most halophytes are thick , succulent , genrally small sized and glassy in appearance
2.Leaves of aerohalopytes are densely covered with trichomes on their surface ,
3.Leaves of submerged marine halophytes are thin , thorny with thick cutinized cuticle,

D) FRUITS & SEEDS

Fruits, seeds and pollen grains usually light in weight, surface of fruit have waxy covering that prevents damage during their transportation through water medium.
Halophytes specially mangroves growing in the tidal region shows the phenomenon of viviparous germination which can be defined as the process of germination of seeds while the fruit is still attached with the mother plant.

ANATOMICAL ADAPTATION

1.Epidermis is highly cutinized and covered with epidermal outgrowths like hairs which prevents transpiration and salt spray into the plant body. Both dorsiventral and isobilateral leaves shows sunken and reduced stomata
2.Cortex shows mucilage cavities , tannin cells , spicule , lacuna , schlerides , salt glands which are very important characteristic modification of the cortical regions in such plants which are adapted in this saline environment.
3.Vascular bundles are very poorly developed and they are conjoint collateral with exarch xylum strands.
4.stele is well liginified.
5.Most of the cell have elastic cell walls.
6.mesophyll cells are differtiated into palisade and spongy parenchyma.
7.Cholorophyll content is very low within the cells among these halophytes.

diagram [ A ] attached in the blog address given below

PHYSIOLOGICAL ADAPTATION

1.salinity reduces the rate of cell division which promotes the rate of cell elongation ,
2.The cells of free ions which improves its turgidity and increases its adaptability from salinity.
3.The plants show high rates of transpiration which is helpful to tolerate saline condition and to maintain normal rate of metabolism.
4.Halophytes shows exudation of sap that contains dissolved salts.
5.Some halophytes have salt secreting glands and water storage tissues.
6.The viviparous of mangrove plants is one of the most important physiological adaptations responsible for normal growth and development of new seedling.

GENETIC DIVERSITY FOR SALT TOLERANCE IN PLANTS

The extensive genetic diversity for salt tolerance that exists in plant texa is distributed over numerous genera , researchers of recent decade established that most halophytes and glycophytes tolerate salinity by rather similar strategy often using analogous tactical processes. The cytotoxic ions in saline environments , typically Sodium ion and Chloride ion are compartmentalized into the vacuole and used as osmotic salts , the fact that cellular ion homeostasis is controlled and effected by common molecular entity for the dissection of the plant salt stress response .

GENETICS OF STRESS:

To breed or genetically engineer plant stress tolerance , it is imperative to identify the genes that control these traits and to understand how these genes work and their products are regulated.
The products of some of the stress inducible genes may play role in stress signaling and stress tolerance .
Example : enzymes that function in the biosynthesis compatible solutes ( osmolytes ) or either directly in detoxification of reactive oxidants or in the biosynthesis antioxidant compounds ion transporters , ABA biosynthetic enzymes etc..
The products of some other genes may also have protective roles against stress damage. These are mainly "late embryogenesis abundant" ( LEA) like proteins.

In some cases genes that are physically associated with certain key stress induced genes in a chromatin region may be regulated by stress , although these genes may not be related otherwise.
Example : UFC { upstream of FLC (flowering locus )gene } gene. FLC is a flowering repressor whose transcript level is down regulated by cold treatment (vernalization ). Interestingly, UFC is similarly regulated by vernalization yet it does not relate to FLC either in sequence or function. They are merely neighboring genes on the same chromosome. This suggests that chromosome location has a strong influence on the induction of certain genes.

SIGNAL TRANSDUCTION.
Signal transduction is required for many cellular activites and their coordination. Some signal trasduction process are simple but most others are complex , involving multiple components occurring in time and space dependent manner.
Generally signal transduction starts with the perception of the stimulus by a specific cellular molecule(s). The sensors or receptors may differ in their molecular identities, mode of signal perception and output , as well as subcellular localization.
In plant cells , it is also common for receptor activaton to result in the generation of secondary messenger , so called because they represent intracellular signals being translated form the primary external signal. The intracellular signals are interpreted further by other signals component(s) and result in the activation of down stream pathways that may have multiple outputs.

signal transduction diagram [ B ] given in the blog , link given below

A conceptual signal transduction pathway for drought , cold , and salt stress in plants. Secondary molecules can cause receptor mediated calcium ion release(indicated in feed back arrow ). These partners that modulate the components in the main pathway can be regulated by the main pathway. signalling can also bypass calcium ions or secondary signaling molecules in early signaling step.
GPCR ? G-protein coupled receptor.
RLK ? receptor-like kinase.
InsP ? inositol pol phosphate.

Ca2+ Signaling and the Activation of the Salt Overly Sensitive (SOS) Signal Transduction Pathway

It was identified that three genetically linked Arabidopsis loci (SOS1, SOS2 and SOS3), which are components of a stress-signaling pathway that controls ion homeostasis and salt tolerance . Genetic analysis of Na+/Li+ sensitivity established that sos1 is epistatic to sos2 and sos3 . These sos mutants also exhibit a K+ deficient phenotype in medium supplemented with ?M [K+]ext and [Ca2+]ext. Na+ and K+ deficiency of sos2 and sos3 is suppressed with mM [Ca2+]ext . sos1 exhibits hyperosmotic sensitivity unlike sos3 and sos2. Together, these results indicate that the SOS signaling pathway regulates Na+ and K+ homeostasis and is Ca2+ activated. SOS3 encodes a Ca2+ binding protein with sequence similarity to the regulatory B subunit of calcineurin (protein phosphatase 2B) and neuronal Ca2+ sensors Interaction of SOS3 with the SOS2 kinase and SOS2 activation is Ca2+ dependent The in planta function of SOS3 as a salt tolerance determinant is dependent on Ca2+ binding and Nmyristoylation . The SOS2 serine/threonine kinase (446 amino acids) has a 267 amino acid N-terminal catalytic domain that is similar in sequence to yeast SNF1 (sucrose nonfermenting) kinase and the mammalian AMPK (AMP-activated protein kinase). The kinase activity of SOS2 is essential for its salt tolerance determinant function . The SOS2 C-terminal regulatory domain interacts with the kinase domain to cause autoinhibition. A 21 amino acid motif in the regulatory domain of SOS2 is the site where SOS3 interacts with the kinase and is the autoinhibitory domain of the kinase . Binding of SOS3 to this motif blocks autoinhibition of SOS2 kinase activity. Deletion of the autoinhibitory domain results in constitutive SOS2 activation, independent of SOS3. Also, a Thr168 to Asp mutation in the activation loop of the kinase domain constitutively activates SOS2.Genetic and biochemical evidence indicates that components of the SOS signal pathway function in the hierarchical sequence . Ca2+ binds to SOS3, which leads to interaction with SOS2 and activation of the kinase. Among the SOS signal pathway outputs are transport systems that facilitate ion homeostasis. The plasma membrane sited Na+/H+ antiporter SOS1 is controlled by the SOS pathway at the transcriptional and post-transcriptional level Recently, functional disruption of AtHKT1 was shown to suppress the salt sensitive phenotype of sos3-1, indicating that the SOS pathway negatively controls this Na+ influx system. Also, the SOS pathway negatively controls expression of AtNHX family members that are implicated as determinants in the salt stress response.[Ca2+]ext enhances salt tolerance and salinity stress elicits a transient [Ca2+]cyt increase, from either an internal or external source, that has been implicated in adaptation . Yeast has provided insight into Ca2+ activation of salt stress signaling that controls ion homeostasis and tolerance.The hyperosmotic component of high salinity induces a short duration (1 min) rise in [Ca2+]cyt that is due substantially to influx across the plasma membrane through the Cch1p and Mid1p Ca2+ transport system. The transient increase in [Ca2+]cyt activates the PP2B phosphatase calcineurin (a key intermediate in salt stress signaling controlling ion homeostasis) leading to the transcription of ENA1, which encodes the P-type ATPase that is primarily responsible for Na+ efflux across the plasma membrane .The model proposes that the hyperosmotically-induced localized [Ca2+]cyt transient activates calmodulin that is tethered to Cch1p-Midp. Calmodulin in turn activates signaling through the calcineurin pathway, which mediates ion homeostasis and salt tolerance. From these results, a paradigm for salt-induced Ca2+ signaling and the activation of the SOS pathway can be suggested. Components of the SOS pathway, either SOS3 or upstream elements, might be associated with an osmotically responsive channel through which Ca2+ influx could initiate signaling through the pathway. These are constituent of signal pathways that respond to different inducers but are still components of the plant response to salt stress. SOS signaling transduction by physical interaction with the positive effectors or competition for substrate required for signaling. Such positive and negative regulation of signal modulation constitute a fine tuning necessary to achieve the appropriate plant response for stress adaptation and ill stability.

CELLULAR MECHANISMS OF SALT STRESS SURVIVAL RECOVERY AND GROWTH

Plant are either dormant during the salt episode or they must they be cellular adjust to tolerate saline environment. The chemical potential of the saline solution initially establishes a water potential imbalance between the apoplast and the symplast that leads to turgor decrease , which is severe enough to cause growth reduction . cellular dehydration begins when the water potential difference is greater than can be compensated for by the tugor loss. The cellular response to turgor response is osmotic adjustment which is achieved in this compartments by accumulation of compatible osmolytes . however Na + and Cl - are energetically efficient osmolytes for osmotic adjustment and are compartmentalized into the vacuole to minimize cytotoxicity.Compartmentalization of Na+ and Cl - facilitates osmotic adjustment that is very essential for cellular development. Movement of ion into the vacuole might occur directly from the apoplast into the vacuole through membrane vesicles or a cytological processes through the plasma membrane to the tonoplast. The bulk of Na+ and Cl- from the apoplast to the vacuole is mediated through ion transport system located in the plasma membrane and tonoplast. The SOS signallig pathway is the key transport system required for ion homeostasis.

OSMOLYTES AND OSMOPROTECTANTS

Some compatible osmolytes are essential elemental ions such as K+ but the majority are organic solutes. The major cateogory of organic osmotic solutes consists of simple sugars like fructose and glucose : sugar alcohols like glycerol , inositols: complex sugars like raffinose. Other include quaternary amino acids like proline , glycine, beta alanine : tertiary amines and sulfonium compounds like dimethyl sulfonium , propyronate.An adaptable biochemical function of osmoprotectants is scavenging of reactive oxygen species that are by product of hyper osmotic and ionic stresses which causes cell death.Compatible solutes have the capacity to preserve the activity of enzymes in saline conditions. The synthesis of compatible osmolytes is often achieved by diversion of basic intermediately metabolites into unique biochemical reactions often stress triggers this metabolic diversions.

ION HOMEOSTASIS - TRANSPORT DETERMINANTS AND THEIR REGULATIONS.

Intracellular Na+ homeostasis and salt tolerance are modulated by ca++ and high Na+ concentration which effects K+ acquisition. Na + competes with K+ for uptake through common transport system , and does this effectively since the Na+ oncentration in saline environment is usually greater than extracellular K+ concentration , Ca++ enhances K+/Na+ selective intracellular accumulation.
The molecular entitites that mediate Na+ and K+ homeostasis is one of the function of Ca++ in the regulation of these transport systems. The SOS stress signaling pathway is identified to be a important regulator of plant ion homeostasis and salt tolerance.

ION TRANSPORT SYSTEM : Na + HOMEOSTASIS

( a ) H+ pumps ( proton pumps)

H+ pumps in the plasma memebrane and tonoplast fecilitate solute transport necessary to compartmentalize cytotoxic ions away from the cytoplasm and the function of ions as signal determinants.
These pumps provide the driving force ( H+ electro chemical potential ) for secondary active transport and function to establish membrane potential variants that facilitate electrophoretic ion flux. The plasma membrane loclised H+ pump is a p-type ATPase and is primarily responsible for the large membrane potential gradient across the gradient. A vacuolar type H+ ATPase generate the membrane potential across the tonoplast. The activity of the H+ pumps is increased by salt treatment and induced gene expression.
The plasma membrane H+ ATPase is confirmed as a salt tolerant determinant based on analysis of phenotypes caused by the semidominant "aha4-1" mutation. The mutation to aha4 which is expressed predominantly in the root causes a reduction in root and shoot and root growth. The decreased root length of salt treated "aha4-1" plants is due to reduced cell length. It is postulated that leaves of "aha4-1" plant accumulate more Na + and less K+ than those of wild type. So it can be said that "aha4-1" functions in the control of Na+ flux across the endodermis.

( b ) Na+ influx & Efflux across the plasma membrane

Transport system with greater selectivity for K+ are presumed to facilitate Na+ leakage in cells. Na is a competitor for uptake through plasma membrane K+ inward rectify channels. K+ outward rectifying channels also facilitate Na+ influx. Na+ when expressed in heterologous systems providing evidence of the function as a Na+ , H+ dependent K+ transporter.
Energy dependent Na+ transport across the plasma membrane is also mediated by the secondary active Na+/ H+ antiport.

( c ) Na+ vacuolar compartmentalization

Na+/ H+ antiport across the tonoplast facilitate vacuolar compartmentalization of the cation. The SOS pathway negatively regulates transcriptional expression of these Na+/H+ antiporter genes.

DROUGHT RESISTANT PLANTS ( XEROPHYTES )

Plants which grow in dry habitats or xeric conditions can with stand low humidity ,high temperature are called as xerophytes. Xerophytic plants are characteristics of desert and semi desert regions.
These plants develops certain structural , anatomical physiological adaptations to absorb as much as water possible they can get from the surrounding and to retain water in their organs for long time by reducing the transpiration rate.

EFFECT ON PLANTS:

o Decrease in growth ( Ex: limitation in leaf expansion).
o Decrease in leaf area decreases the photosynthetic activities.
o Decrease in water content increases the solute concentration.
o The first effect on root system is the death of root hairs, which decreases the capacity of the roots to absorb water.
o Production of phytohormones like cytokinins and gibberlic acid decreases.
o It decreases the production of secondary metabolites, which leads to decrease in the defense mechanism against certain insects and diseases.

MORPHOLOGICAL ADAPTATIONS

A ) ROOT

Xerophytes have well developed root system which may be profusely branched and more elobarate than shoot system. The roots of perennial xerophytes grow very deep in soil and reach the layer where water is available in plenty.

B) STEM

1. Hard and woody stem are covered with thick coating of wax and silica or may be cover with hairs ( Calotropis sp ).
2. In some xerophytes stem may be modified with thorns. Example? Ulex sp
3. Stem of some extereme are modified to leaf like , flattened and fleshy structures , which are called phylloclades. Example? Muehlenbeckia sp
4. In some plants a number of axullary branches become modified into small needle like green structure which looks like leaves and are called cladodes. Example ? Asparagus sp

C) LEAVES.

1.In some xerophytes the leaves fall early in the season , but in majority of plants leaves are generally reduced to scales. Example? Casuarina equisitifolia,
2.Some ever green have needle shaped leaves. Example? Pinus roxburghii
3.In some species the leaves become succulent and swell remarkably and becomes very fleshy for the storage of excess amount of and latex in them. Example? Aloe spinossina
4.Leaves may be reduced to spines and are provided with thick coating of wax or silica. Example? Opumtia polardii.
5.Leaves blades have thick network of veins, In some cases the green petiole swells and becomes flattened to form phyllode. Example? Acacia auriculiformis.
6.Many xerophytic plants shows trichophylly for protecting the stomatal guard cells against stong winds. Example? Zizyphus numularis.
7.Leaves in some extreme xerophytic grasses have the capacity for rolling and folding.

D) FRUITS & SEEDS.

Flowers usually develop in favourable conditions and they complete their reproduction in very short period of time. Fruits and seeds are protected by very hard coverings and they can remain dormant for a long period of time.

ANATOMICAL ADAPTATIONS

1.Epidermal cells are small compact with thick cuticle and it is single layered.
2.Wax , tannin , rasin , cellulose etc. are deposited on the surface of epidermis this forms a protective measure against high intensity of light.
3.Some of the epidermal cells found in the depression become more enlarged are called motor cells or hinge cells which felicitates the rolling of leaves by becoming flaccid during dry period. Example? Amnophilla.
4.The hypodermal cells are thick walled and compactly grouped and may be filled with tannin and mucilage.
5.Stomatal number per units area is reduced and they are sunken type. Walls of the guard cells and subsidiary cells are heavily cutinized and lignified. Such specialized stomata reduces the rate of transpiration.
6.In case of reduced leaves the photosynthetic activity is taken up by outer Chlorenchymatous cortex. Example? Capparis. Decidua
7.In succulent stem the ground tissue is filled with thin walled parenchymatous cells which store excess quanitity of water , mucilage , latex. Example? Agave americana.
8.The mesophyll cells are very compact , intracellular spaces are reduced. Palisade tissue develops in several layers and in some cases mesophyll is surrounded by a sheath of sclerenchyma.
9.In Pinus sp spongy cells in the mesophyll cells are star shaped.
10.Both the conducting tissues xylum & phloem are very well developed in the xerophytes.

Diagram [ C ] given in the blog , link given below.

PHYSIOLOGICAL ADAPTATION

1. Xerophytes have high osmotic pressure which increases the turgidity of the cell sap exerts tension force on the cell wall. In this way wilting of cell is prevented.
2. Presence of cuticle, sunken stomata protected with stomatal hair regulates the transpiration.
3. The capacity of xerophytes to survive during dry period lies not only on the structural features but also in the resistance of hardened protoplasm to heat and dessication.
4. Some enzymes such as catalases, peroxidases are more active in xerophytes. Low concentration of hydrolytic enzymes prevents higher rate of water consumption.
5. In xerophytes conversion of chemical compounds of cell sap such as polysaccharides into anhydrous forms like cellulose suberin etc are noted.
6. In some xerophytes stomata opens during night hours and remain closed during the day. These unusual features are associated with the metabolic activity of thee plants.
7. In these plants some polysaccharides are converted into pentosens which have water building capacity.
8. In xerophytes respiratory carbon dioxide release during night leads to the biosynthesis of large amount of organic acids which are helpful for the plants to survive in extreme draught condition.

HEAT SHOCK PROTEINS

Heat shock proteins (HSP) are a group of proteins whose expression is increased when the cells are exposed to elevated temperatures or other stress. This increase in expression is transcriptionally regulated. This dramatic upregulation of the heat shock proteins induced mostly by heat shock factor (HSF) is a key part of the heat shock response.
The HSPs are named according to their molecular weights. For example, Hsp60, Hsp70 and Hsp90 (the most widely-studied HSPs) refer to families of heat shock proteins on the order of 60, 70 and 90 kilodaltons in size, respectively. The small 8 kilodalton protein ubiquitin, which marks proteins for degradation, also has features of a heat shock protein.
Molecular chaperones, including the heat-shock proteins (Hsps), are a ubiquitous feature of cells in which these proteins cope with stress-induced denaturation of other proteins. Hsps have received the most attention in model organisms undergoing experimental stress in the laboratory, and the function of Hsps at the molecular and cellular level is becoming well understood in this context. A complementary focus is now emerging on the Hsps of both model and non model organisms undergoing stress in nature, on the roles of Hsps in the stress physiology of whole multicellular eukaryotes and the tissues and organs they comprise, and on the ecological and evolutionary correlates of variation in Hsps and the genes that encode them. This focus discloses that (a) expression of Hsps can occur in nature, (b) all species have hsp genes but they vary in the patterns of their expression, (c) Hsp expression can be correlated with resistance to stress, and (d) species' thresholds for Hsp expression are correlated with levels of stress that they naturally undergo. These conclusions are now well established and may require little additional confirmation; many significant questions remain unanswered concerning both the mechanisms of Hsp-mediated stress tolerance at the organismal level and the evolutionary mechanisms that have diversified the hsp genes.
Upregulation through stress
Production of high levels of heat shock proteins can also be triggered by exposure to different kinds of environmental stress conditions, such as infection, inflammation, exercise, exposure of the cell to toxins (ethanol, arsenic, trace metals and ultraviolet light, among many others), starvation, hypoxia (oxygen deprivation), nitrogen deficiency (in plants), or water deprivation. Consequently, the heat shock proteins are also referred to as stress proteins and their upregulation is sometimes described more generally as part of the stress response.

EFFECT OF ABA IN STRESS :

STRESS-RESPONSIVE GENES ARE REGULATED BY ABA-DEPENDENT AND ABA-INDEPENDENT PROCESS.

Gene transcription is controlled through the interaction of regulatory proteins with specific regulatory sequences in the promoters of the genes they regulate. Different genes that are induced by the same signal are controlled by a signaling pathway
leading to the activation of these specific transcription factors. Studies of the promoters of several stress-induced genes have led to the identification of specific regulatory sequences for genes involved in different stresses. For example, the RD29 gene contains DNA sequences that can be activated by osmotic stress, by cold, and ABA.

EFFECT OF ABA IN STOMATAL CLOSING IN DROUGHT CONDITIONS

Diagram [ D ]given in the blog , link given below.

The Acidity , alkalinity and salinity of soils are important determinants of productivity.

Because soil acidity influences the physical properties , the availability of certain plant nutrients , and the biological activity of the soil, it greatly affects the plant growth , the soils degree of acidity depends on the concentration of H+ ion dissolved in the soil water. In a neutral soil the H+ ion concentration is about one part per billion parts of water and the acid soil may have a concentration of H+ that is 100 to 1000 times higher , where as a alkaline H+ ion concentration.
Neither extreme acidity nor extreme alkalinity is suitable for plant growth or for most other soil organisms. such conditions also upset soil weathering and the availability of the nutrients , although some plants can grow in strongly acidic or alkaline soil , most crop plants grow best in neutral or slightly acidic soils . just over a quarter ( 26 % )
Of the worlds arable land is classified as acidic. In the tropics the % is even greater ( 43%). Acidic soils account for 68% of tropical America , 38% of tropical asia and
27 % of tropical Africa.

Diagram [E ] given in the blog , link given below.

IMPROVEMENT

A)CROP RESISTANCE TO WATER DEFICIT CAN BE IMPROVED:
Improving drought resistance is an important aim of plant breeders.
Four basic approaches to the drought resistance are being used :
1.breed for high yields under optimal condition ; i.e breed for yield potential - assuming this will provide yield advantage under suboptimal condition.
2.breed for maximum yield in the target environment.
3.select and incorporate morphological and physiological mechanisms of drought resistance into traditional breeding programmes.
4.do not use multiple physiological selection criteria , but established without doubt that a single drought-resistance character will benefit yield under water limited conditions , and then incorporate the character into an existing yield breeding programme.

Using molecular techniques several classes of genes have been identified that confer resistance to water deficit . Some of the genes could be used to engineer plant for drought resistance and better crop yield under drought condition. First the enzymes that synthesize osmoprotectants , small molecules that accumulate in the cytoplasm of drought stress plants , have been identified.
Plants genetically engineered with the genes encoding these enzymes are more drought tolerant. Second the genes that encode transcription factors that regulate entire metabolic pathways leading to drought adaptation were identified. By incorporating such genes , one can hope to ensure that plants respond rapidly and efficiently to any water deficit and continue all their developmental processes.

B) Better performance on saline soil.
Salt tolerance is a complex , quantitative , genetic trait controlled by many genes. Recently a few genes have been identified that provide information useful in screening and selection programmes for salt tolerance.
Four major stratergies that to develop salt tolerant crops are :
1.gradually improve the salt tolerance to conventional breeding and selection.
Example : development of salt tolerance in rice ( Pokkali Rice) of kerala, India has been used extensively to develop salt tolerance in other , more desirable rice genotypes.
2.Introduce traits for salt tolerance from wild relatives into the crops by the process of back crossing.
Example : tomato (Lycopersicon esculentum),
Barley (Hordeum vulgare ) and Wheat ( triticum aestivum).
3.Domesticate wild species that currently inhabit saline environment ( halophytes ) by breeding and selecting for improved agronomic characteristic.
4.use molecular techniques to identify genes associated with salt tolerance , and enhance their expression in the crop species or transfer the genes from the non crop to a crop species. Example : On the molecular front , the genes involved in sensing salt in the environment ( signal- transduction ) , transcription factor genes that turn on batteries of other genes that prepare cell to withstand a higher rate of salt influx , and genes that are a part of plant's adaptation to the presence of salt are being identified. An example of the later category is the gene that encodes that vacuolar sodium pump. Plants that can turn this gene on rapidly when the cells are exposed to salt , will be able to transport the salt from the cytoplasm into the vacuole , there by detoxifying the cytoplasm. Example : Lycopersicon esculentum ( tomato)

CONCULSION :

Conventional and GM breeding are complementary approaches and can be expected to enhance the draught resistance and yield of crops. People have entered in new era in which enhance knowledge of both the physiology of yield accumulation and the physiological basis of genetic variation in both salt and draught resistance traits offer the potential for improving breeding efficiency for major food crops in different target environments. Using physiological knowledge and powerful tools

blog address:http://stresstolerance.blogspot.com/

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