Breeding wheat to drought tolerance and its mechanism_KKC


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Breeding wheat to drought tolerance and its mechanism : 

Breeding wheat to drought tolerance and its mechanism Department of Genetics & Plant Breeding Junagadh Agricultural University Junagadh 362 001 Kamlesh Kumar Chandel M.Sc.Agri. (Genetics & Plant Breeding)

Contents : 

Contents Introduction Drought stress What is stress Drought mechanism Different Factors Breeding strategies Approaches Methods of breeding Summary Reference

Introduction : 

Introduction Wheat (Triticum aestivum L.) production is adversely affected by drought in 50% of the area under production in the developing countries. (Trethowan and Pfeiffer 2000) Drought tolerance is a quantitative trait, with complex phenotype and genetic control. (Mc William, 1989) Low heritability, high genotype X environment interaction. Variation and unpredictability of environmental conditions have hindered breeders efforts to select for drought tolerance.

Scenario of Drought in India : 

Scenario of Drought in India Total arable area 143.8 mha Irrigated area 43.8 mha (30.5%) Rainfed area 65.5 mha (45.5%) Dryland area 34.5 mha (23.9%) In North-West Himalayan regions 81% is under rainfed

Stress : 

Stress Stress is an external factor that exerts a disadvantageous influence on the plant and is measured in relation to plant survival, crop yield, growth (biomass accumulation), which are related to overall growth. Taiz and Zeiger (2006)

Proportion of different stresses( world) : 

Proportion of different stresses( world) 26 % Blum (1988)

Stress condition in plant : 

Stress condition in plant

Drought : 

Drought The inadequacy of water availability, including precipitation and soil moisture storage capacity, in quantity and distribution during life cycle of crop to restrict expression of its full genetic potential. - Sinha, 1986 “Drought stress accounts for more production losses than all other factors combined” - John Cushman, University of Nevada, Reno

Major Reasons of drought : 

Major Reasons of drought Atmospheric factors – sudden & beyond control. Soil Factors- slow & steady controlled by agronomic means.

Different types of drought : 

Different types of drought Meteorological drought Agricultural drought Hydrological drought Physiological drought

Categorization of plants on the basis of drought : 

Categorization of plants on the basis of drought Drought-escaping plants Germination and growth in moisture Seeds persist during times of drought Annuals- Arabidopsis thaliana Drought-evading plants  Growth restricted in moisture stress Drought- deciduous shrubs Brittle bush (Encelia farinosa) Drought-enduring plants  Maintain growth in water stress Extensive root systems Morphological and physiological adaptations Evergreen shrubs i.e. Creosote bush Drought-resisting plants  Water stored in swollen leaves and stems succulent perennials Barrel Cactus

How plants fight with drought stress : 

How plants fight with drought stress Different survival mechanisms of plants at dry sites: 1) Drought escape 2) Dehydration avoidance 3) Dehydration tolerance Levitt, 1980

Drought escape : 

Drought escape Ability of plant to complete its life cycle before on set of severe water deficit -Rapid development of plant Early maturing varieties – Terminal drought stress Late maturing varieties – Early season drought stress Early or Late maturation of a crop variety has an adverse effect on its economic yield

Drought (dehydration) avoidance : 

Drought (dehydration) avoidance Maintain realtively high water potential as long as possible under water stress. Two groups of drought avoiders: i) Water savers -Reduce water loss -Leaf characteristics -Stomatal senstivity -Cuticular wax ii) Water spenders -Increase water uptake -Root characteristics Anatomical and morphological traits help the plant to avoid drought.

Drought (dehydration) avoidance : 

Ability to tolerate the water stress by the biochemical and physiological changes Capacity of protoplasm to tolerate severe water loss Physiological processes proceed even at high dehydration levels Tolerance mechanisms take over when tissues are no longer protected by avoidance mechanisms Drought tolerance usually found in xerophytes Tolerance aims at plant survival rather than plant growth Drought (dehydration) avoidance


ADAPTATIONS OF PLANT FOR DROUGHT STRESS Morphological Physiological Biochemical

Morphological Adaptations : 

Morphological Adaptations Earliness:- Early maturing varieties are ready for harvest before the onset of drought Stomatal characters: -Various stomatal characters such as sunken type, small size, less number per unit area and rapid closing nature Leaf characters:- Thick cuticle, waxiness of leaf surface, small and thick leaves with thick layers of palisade tissue, glossiness and hairiness Root characters:- Root length, root density, root dry weight and root to shoot ratio are important trait Growth habit:- Indeterminate genotypes are suitable to drought because determinate gives only one flush of flower and if there is drought period during flowering, it may lead to very heavy loss

Physiological factors : 

Physiological factors Rate of photosynthesis Rate of transpiration Leaf turgidity Osmoregulation

Biochemical factors : 

Biochemical factors ABA Abscisic Acid content ABAR genes PROLINE CONTENT gene P5CS GLYCINE BETAINE CONTENT Gene bet A- choline dehydrogenase Gene bet B- betaine aldehyde dehydrogenase

Drought stress affects plant physiology : 

Drought stress affects plant physiology A plant responds to a lack of water by halting growth and reducing photosynthesis and other plant processes in order to reduce water use. As water loss progresses, leaves of some species may appear to change color usually to blue-green. Foliage begins to wilt and, if the plant is not irrigated, leaves will fall off and the plant will eventually die. Drought symptoms resemble salt stress because high concentrations of salts in the root zone cause water loss from roots. Close examination of environmental and cultural conditions should help identify the specific problem. The time required for drought injury to occur depends on the water-holding capacity of the soil, environmental conditions, stage of plant growth, and plant species.

Drought stress affects plant physiology : 

Plants growing in sandy soils with low water-holding capacity are more susceptible to drought stress than plants growing in clay soils. A limited root system will accelerate the rate at which drought stress develops. A root system may be limited by the presence of competing root systems, by site conditions such as compacted soils or high water tables, or by container size (if growing in a container). A plant with a large mass of leaves in relation to the root system is prone to drought stress because the leaves may lose water faster than the roots can supply it. Newly installed plants and poorly established plants may be especially susceptible to drought stress because of the limited root system or the large mass of stems and leaves in comparison to roots. Drought stress affects plant physiology

ABA (Abscisic acid) : 

ABA (Abscisic acid) ABA, a plant stress hormone Role Signal transduction of stress Induces the closure of leaf stomata thereby reducing water loss through transpiration and decreasing the rate of photosynthesis Promotion of root growth and increase root hydraulic conductivity These responses improve the water use efficiency of the plant on the short term

Proline : 

Proline Proline is the most widely distributed osmolyte; it occurs in plant and in many other organisms. Its accumulation correlates with tolerance to drought stress. Role Osmotic adjustment Membranes protection Reservoir of nitrogen and carbon source for post stress growth Regulate redox potentials

Glycine Betaine : 

Glycine Betaine Glycine betaine is a quaternary ammonium compound that functions as an osmo-protectant. Role Stabilizing complex protein structure Protecting transcriptional machinery Maintain integrity of membrane Maintenance of the water balance between the plant cell and the environment.

Drought tolerance traits : 

Drought tolerance traits Tolerance to drought is a quantitative trait, with a complex phenotype, often confounded by plant phenology. Breeding for drought tolerance is further complicated since several types of abiotic stress, such as high temperatures, high irradiance, and nutrient toxicities or deficiencies can challenge crop plants simultaneously.

Osmotic adjustment : 

Osmotic adjustment As a plant detects a water-deficit stress, it may accumulate a variety of osmotically active compounds such as amino acids, sugars and ions inside its cells, resulting in a lowering of the cell osmotic potential. Water present in inter-cellular spaces then flows towards the inside of those cells. Thisprocess, called “osmotic adjustment” (OA), was proposed as a potential factor that could enable plants to maintain turgor and survive better at low water status. It has, however, been argued that osmotic adjustment probably does not allow the plant to draw much extra water from the soil and that this could come at a cost in yield potential.

Cell membrane stability : 

Cell membrane stability The ability to survive dehydration is influenced by a cell’s ability to survive at reduced water content. This can be considered complementary to OA because both traits will help maintain leaf growth (or prevent leaf death) during drought. Crop varieties differ in dehydration tolerance and an important factor for such differences is the capacity of the cell membrane to prevent electrolyte leakage at decreasing water content, or “cell membrane stability (CMS)”. The maintenance of membrane function is assumed to mean that cell activity is also maintained. Measurements of CMS have been used in different crops and are known to be correlated with yields under high temperature and possibly under drought stress.

Fundamentals of Breeding : 

Fundamentals of Breeding Genetic variation Source of the desired trait Transfer of trait Methods and approaches of breeding

Genetic sources : 

Genetic sources Cultivated varieties Adapted variety No compromise on yield Breeding material Transfer of trait is easy and with minimum linkage drag Landraces problem of undesirable linkages Subjected to artificial and natural selection Wild relatives Aim is to survive not the yield Transfer of trait is major problem Transgenes Cloning of target gene Transfer requires technical expertise Aegilops Squarrosa Imperata cylindrica

Breeding approach : 

Breeding approach FOUR APPROACHES Breeding for high yield under optimum condition No intentional selection for drought tolerance Breeding for other characters indirectly effect drought Screening for drought is done Lines perform well in optimal condition show decline in yield under drought Breeding for High yield under Stress condition Choice of Parents Selection under the stress environment Drought varies from year and location Breeding for High yield under both stress and non-stress environment Simultaneous selection Use of conventional method Multi disciplinary approach Integrate drought tolerant mechanisms Use of genomic tools

Breeding methods for drought tolerance : 

Breeding methods for drought tolerance Conventional methods Non-conventional methods

Conventional methods : 

Conventional methods INTRODUCTION Primary Secondary SELECTION Desirable Adaptation HYBRIDIZATION LINE A High yielding LINE B Drought resistant F1 Conventional breeding focus on drought avoidance than drought tolerance (Blum, 2005)

Conventional methods : 

Conventional methods

Non - Conventional methods : 

BIOTECHNOLOGY GENETIC ENGINEERING Agrobacterium mediated gene transfer Particle Bombardment (Gene Gun) Electroporation of protoplast MAS QTL MOLECULAR CYTOGENETICS Non - Conventional methods

Two type Genetic engineering methods : 

Two type Genetic engineering methods

Drought Forward Genetics : 

Drought Forward Genetics Tolerance to drought is a quantitative trait, with a complex phenotype, often confounded by plant phenology. For increasing drought tolerance of wheat tackle the problem in a multi-disciplinary approach, considering interaction between multiple stresses and plant phenology, and integrating the physiological dissection of drought tolerance traits and the genetic and genomics tools. Physiological analysis, population development and phenotyping and the various ‘omics technologies are integrated to support a gene discovery path.

Drought Forward Genetics : 

Drought Forward Genetics

Drought Forward Genetics : 

Drought Forward Genetics

Roadmap for identification of drought Tolerance gene : 

Roadmap for identification of drought Tolerance gene

Marker-assisted Breeding (MAS) : 

Marker-assisted Breeding (MAS) F2 P2 F1 P1 x large populations consisting of thousands of plants Drought Resistant High yield but Drought Susceptible Selected plants are resistance to Drought tolerance

Crop ideotype for drougth tolerance : 

Crop ideotype for drougth tolerance Usually ideotypes are developed to create a ideal plant variety. The following traits constitutes ideotype of wheat by CIMMYT.

Crop ideotype for drougth tolerance : 

1) Large seed size. Helps emergence, early ground cover, and initial biomass. 2) Long coleoptiles. For emergence from deep sowing 3) Early ground cover. Thinner, wider leaves (i.e., with a relatively low specific leaf weight) and a more prostrate growth habit help to increase ground cover, thus conserving soil moisture and potentially increasing radiation use efficiency. 4) High pre-anthesis biomass. 5) Good capacity for stem reserves and remobilization 6) High spike photosynthetic capacity 7) High RLWC/Gs/CTD during grain filling to indicate ability to extract water Crop ideotype for drougth tolerance

Crop ideotype for drougth tolerance : 

8) Osmotic adjustment (9) Accumulation of ABA. The benefit of ABA accumulation under drought has been demonstrated (Innes et al. 1984). It appears to pre-adapt plants to stress by reducing stomatal conductance, rates of cell division, organ size,and increasing development rate. However, high ABA can also result in sterility problems since high ABA levels may abort developing florets 10) Heat Tolerance. The contribution of heat tolerance to performance under moisture stress needs to be quantified, but it is relatively easy to screen for (Reynolds et al. 1998). 11) Leaf anatomy: waxiness, pubescence, rolling, thickness, posture. These traits decrease radiation load to the leaf surface. Benefits include a lower evapotranspiration rate and reduced risk of irreversible photo-inhibition. However, they may also be associated with reduce radiation use efficiency, which would reduce yield under more favorable conditions. Crop ideotype for drougth tolerance

Summary : 

Summary Wheat is the second most important cereal crop of the India Drought stress is polygenic in nature for which breeding require much effort Only conventional breeding is not sufficient in development of drought tolerant wheat variety Non conventional approaches i.e. Molecular breeding can play a significant role in the sphere of abiotic stress mainly drought tolerance Genomic regions carrying drought tolerant gene can contribute for abiotic stress tolerance

The way to go … : 

The way to go … Identification of major QTLs linked to different abiotic stresses and their pyramiding with minor effects QTLs elite lines. Molecular genetic dissection of the genomic regions to understand the physiological mechanism. Marker assisted selection and Molecular cytogenetic techniques for efficient screening. Transgenic with stress protein/TF gene over- expression

Slide 47: 

Thank you Cereal production in the rain-fed areas still remains relatively unaffected by the impact of the green revolution, but significant change and progress are now becoming evident in several countries. - Norman Borlaug

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