HEAT STRESS

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HEAT STRESS

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Heat stress: Plant Responses and Adaptation

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Contents Introduction Molecular responses Oxidative stress and antioxidants Stress proteins Heat-stress threshold Plant responses to heat stress Morpho-anatomical and phenological responses Morphological symptoms Anatomical changes Phenological changes Physiological responses Waters relations Accumulation of compatible osmolytes Photosynthesis Assimilate partitioning Cell membrane thermostability Hormonal changes Secondary metabolites Mechanism of heat tolerance Conclusion and future prospects

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INTRODUCTION Heat stress is often defined as the rise in temperature beyond a threshold level for a period of time sufficient to cause irreversible damage to plant growth and development. Heat stress affects plant growth throughout its ontogeny , though heat-threshold level varies considerably at different developmental stages.

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Heat stress due to high ambient temperature is a serious threat to crop production worldwide (Hall, 2001). Different global circulation models predict that green house gases will gradually increase world’s average ambient temperature. At very high temperatures, severe cellular injury and even cell death may occur within minutes ( Schoffl et al., 1999). At moderately high temperatures, direct injuries include protein degradation, aggregation and increased fluidity of membrane lipids. Indirect injuries include inactivation of enzymes, inhibition of protein synthesis, protein degradation and loss of membrane integrity ( Howrath , 2005).

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Due to high temperatures changes occur at the molecular level altering the expression of genes and accumulation of transcripts, thereby leading to the synthesis of stress related proteins as a stress tolerance strategy (Iba, 2002). There exists tremendous variation within and between species, providing opportunities to improve crop heat stress tolerance through genetic means (Camejo et al., 2005). Recently, however, advanced techniques of molecular breeding and genetic engineering have provided additional tools, which could be employed to develop crops with improved heat tolerance hence, to achieve it concerted efforts of plant physiologist, molecular biologists and crop breeders are imperative.

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Heat-stress threshold It is a value of daily mean temperature at which a detectable reduction in growth begins / the temperature at which growth and development of plant cease. Upper threshold: is the temperature above which growth and development cease. Lower threshold (base temperature ): is the temperature below which plant growth and development stop. Cool season and temperate crops often have lower threshold temperature values compared to tropical crops.

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Crop plants Threshold temperature ( 0 C) Growth stage References Wheat 26 Post -anthesis Stone and Nicolas (1994) Corn 38 Grain filling Thompson (1986) Cotton 45 Reproductive Rehman et al., (2004) Pearl millet 35 Seedling Ashraf and Hafeez (2004) Tomato 30 Emergence Camejo et al., (2005) Brassica 29 Flowering Morrison Stewart (2002) Cool season pluses 25 Flowering Siddique et al., (1999) Ground nut 34 Pollen production Vara Prasad et al., (2000) Cow Pea 41 Flowering Patel and Hall (1990) Rice 34 Grain yield Morita et al., (2004) Threshold high temperatures for some crops plants

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PLANT RESPONSES TO HEAT STRESS Morpho-anatomical and phenological responses Morphological symptoms : scorching of leaves and twigs sunburns on leaves branches and stems Leaf senescence and abscission shoot and root growth inhibition fruit discoloration and damage and reduced yield High temperatures caused significant declines in shoot dry mass, relative growth rate and net assimilation rate in maize, pearl millet and sugarcane (Wahid, 2007). Major impact of high temperatures on shoot growth is a severe reduction in the first internode length resulting in premature death of plants (Hall, 1992). sugarcane plants grown under high temperatures exhibited smaller internodes, increased tillering, early senescence, and reduced total biomass ( Ebrahim et al., 1998).

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Reproductive phases most sensitive to high temperature are gametogenesis (8–9 days before anthesis) and fertilization (1–3 days after anthesis) in various crop plants (Foolad, 2005). In wheat, both grain weight and grain number appeared to be sensitive to heat stress, as the number of grains per ear at maturity declined with increasing temperature (Ferris et al., 1998). In tomato, reproductive processes were adversely affected by high temperature, which included meiosis in both male and female organs, pollen germination and pollen tube growth, ovule viability, stigmatic and style positions, number of pollen grains retained by the stigma, fertilization and post-fertilization processes (Foolad, 2005). The most noticeable effect of high temperatures on reproductive processes in tomato is the production of an exerted style (i.e., stigma is elongated beyond the anther cone), which may prevent self-pollination. Poor fruit set at high temperature has also been associated with low levels of carbohydrates and growth regulators released in plant sink tissues (Kinet and Peet, 1997).

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Anatomical changes : At the whole plant level Reduced cell size Closure of stomata and curtailed water loss Increased stomatal and trichomatous density Greater xylem vessels of both root and shoot Damaged the mesophyll cells and increased permeability of plasma membrane High temperatures reduced photosynthesis by changing the structural organization of thylakoids ( Karim et al., 1997). Loss of grana stacking or its swelling The cumulative effects of all these changes under high temperature stress may result in poor plant growth and productivity.

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Phenological changes During vegetative stage, for example, high day temperature can damage leaf gas exchange properties. During reproduction, a short period of heat stress can cause significant increases in floral buds and opened flowers abortion. Impairment of pollen and anther development by elevated temperatures is another important factor contributing to decreased fruit set in many crops at moderate to high temperatures (Peet et al., 1998; Sato et al., 2006). Heat stress is a major factor affecting the rate of plant development, which may increase to a certain limit and decrease afterwards (Hall, 1992; Marcum, 1998; Howarth, 2005).

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. Under high temperature conditions, earlier heading is advantageous in the retention of more green leaves at anthesis, leading to a smaller reduction in yield (Tewolde et al., 2006). High temperature hastens the phenological development in wheat i.e. decrease in days to ear emergence, anthesis and maturity has been reported. Consequently, grain filing duration is also decreased .

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Physiological responses Waters relations Heat stress perturbed the leaf water relations and root hydraulic conductivity (Morales et al., 2003). Enhanced transpiration induces water deficiency in plants, causing a decrease in water potential and leading to perturbation of many physiological processes (Tsukaguchi et al., 2003).

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Accumulation of compatible osmolytes Under stress, different plant species may accumulate a variety of osmolytes such as sugars and sugar alcohols (polyols), proline, tertiary and quaternary ammonium compounds, and tertiary sulphonium compounds (Sairam and Tyagi, 2004) Glycinebetaine (GB), an amphoteric quaternary amine, plays an important role as a compatible solute in plants under various stresses, such as salinity or high temperature (Sakamoto and Murata, 2002) High level of GB accumulation was reported in maize (Quan et al., 2004) and sugarcane (Wahid and Close, 2007) In contrast, plant species such as rice ( Oryza sativa), mustard ( Brassica spp.), Arabidopsis (Arabidopsis thaliana) and tobacco ( Nicotiana tabacum) naturally do not produce GB under stress conditions.

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Proline is also known to occur widely in higher plants and normally accumulates in large quantities in response to environmental stresses (Kavi Kishore et al., 2005). Under high temperatures, fruit set in tomato plants failed due to the disruption of sugar metabolism and proline transport during the narrow window of male reproductive development (Sato et al., 2006). Other osmolytes, -4-aminobutyric acid (GABA), a non-protein amino acid, is widely distributed throughout the biological world to act as a compatible solute.

Proline : A Multifunctional Amino Acid: 

Proline : A Multifunctional Amino Acid

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Adaptation to Heat Stress Is Mediated by Cytosolic Calcium

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PHOTOSYNTHESIS

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Photosynthesis PSII is highly thermolabile

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Photochemical reactions in thylakoid lamellae and carbon metabolism in the stroma of chloroplast have been suggested as the primary sites of injury at high temperatures (Wise et al., 2004). In tomato genotypes differing in their capacity for thermotolerance as well as in sugarcane, an increased chlorophyll a:b ratio and a decreased chlorophyll:carotenoids ratio were observed in the tolerant genotypes under high temperatures, High temperature influences the photosynthetic capacity of C 3 plants more strongly than in C 4 plants. ( RuBP regeneration by the disruption of electron transport and inactivation of the oxygen evolving enzymes of PSII) (Todorov et al., 2003)

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At High Temperatures, Photosynthesis Is Inhibited before Respiration

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Assimilate partitioning Under low to moderate heat stress, a reduction in source and sink activities may occur leading to severe reductions in growth, economic yield and harvest index mobilization efficiency of reserves To elucidate causal agents of reduced grain filling in wheat under high temperatures, Wardlaw (1974) examined three main components of the plant system including source (flag leaf blade), sink (ear), and transport pathway (peduncle). It was determined that photosynthesis had a broad temperature optimum from 20 to 30 ◦C, however it declined rapidly at temperatures above 30 ◦C.

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Cell membrane thermostability The integrity and functions of biological membranes are sensitive to high temperature Heat stress Alters the tertiary and quaternary structures of membrane proteins Enhance the permeability of membranes increased loss of electrolytes The increased solute leakage, as an indication of decreased cell membrane thermostability (CMT), has long been used as an indirect measure of heat-stress tolerance in diverse plant species Influence of saturation levels of membrane lipids on the temperature sensitivity of plants

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Heat stress kinetic energy thereby loosening chemical bonds within molecules of biological membranes. This makes the lipid bilayer of biological membranes more fluid by either denaturation of proteins or an increase in unsaturated fatty acids. (Savchenko et al., 2002) PM Movement molecules

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Hormonal changes Abscisic acid (ABA) and ethylene (C 2 H 4 ), as stress hormones, They involved in the regulation of many physiological processes Acting as signal molecules High temperature- levels of ABA Action of ABA involves modification of gene expression modulating the up- or down-regulation of numerous genes (Xiong et al., 2002). Ethylene (C 2 H 4 ) : High temperature - Level ACC induced abscission of reproductive organs levels and transport capacity of auxins reproductive organs ACC

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Another class of hormones, Brassinosteroids have recently been shown to confer thermotolerance to tomato and oilseed rape ( Brassica napus) Other hormones SA Brassinosteroids Salicylic acid (SA) has been suggested to be involved in heat-stress responses elicited by plants.

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Most of the secondary metabolites are synthesized from the intermediates of primary carbon metabolism via phenyl propanoid, shikimate, mevalonate or methyl erythritol phosphate (MEP) pathways (Wahid and Ghazanfar, 2006). Secondary metabolites

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Shikimic acid pathway Thermal stress induces the biosynthesis of phenolics and suppresses their oxidation, which is considered to trigger the acclimation to heat stress. Increased activity of PAL is considered as the main acclamatory response of cells to heat stress. PAL PAL is considered to be the principal enzyme of the phenylpropanoid pathway.

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Carotenoids of the xanthophyll family and some other terpenoids, such as isoprene or -tocopherol, stabilize and photoprotect the lipid phase of the thylakoid membranes High temperature decreases synthesis of anthocyanins in reproductive parts of red apples (Tomana and Yamada, 1988), chrysanthemums (Shibata et al., 1988) and asters (Sachray et al., 2002). One of the causes of low anthocyanin concentration in plants at high temperatures is a decreased rate of its synthesis and stability (Sachray et al., 2002).

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Plants capable of emitting greater amounts of isoprene generally display better photosynthesis under heat stress, thus there is a relationship between isoprene emission and heat-stress tolerance (Velikova and Loreto, 2005). Sharkey (2005) opined that isoprene production protects the PSII from the damage caused by ROS, including H 2 O 2 , produced during heat-induced oxygenase action of rubisco.

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Heat stress may induce oxidative stress Generation and reactions of activated oxygen species (AOS) singlet oxygen ( 1 O 2 ), super oxide radical (O 2 .- ), hydrogen peroxide (H 2 O 2 ) hydroxyl radical (OH - ) AOS cause -- autocatalytic peroxidation of membrane lipids and pigments leading to the loss of membrane semi- permeability and modifying its function The scavenging of O 2 .- by superoxide dismutase (SOD) results in the production of H 2 O 2 , which is removed by APX or CAT. Protection against oxidative stress is an important component in determining the survival of a plant under heat stress. MOLECULAR RESPONSES

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Halliwell Asada Pathway

STRESS PROTIENS: 

STRESS PROTIENS Expression of stress proteins is an important adaption to cope with environmental stresses. Most of the stress proteins are soluble in water and therefore contribute to stress tolerance presumably via hydration of cellular structures. In higher plants, HSPs is usually induced under heat shock at any stage of development. Heat shock proteins

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LIST OF HEAT SHOCK PROTEINS

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The heat shock factor (HSF) cycle activates the synthesis of heat shock protein mRNAs.

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Planta (2003) 218: 1–14

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Heat-stress tolerance mechanisms in plants

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Proposed heat-stress tolerance mechanisms in plants. MAPK, mitogen activated protein kinases; ROS, reactive oxygen species; HAMK, heat shock activated MAPK; HSE, heat shock element; HSPs, heat shock proteins; CDPK, calcium dependent protein kinase; HSK, histidine kinase. Partly adopted from Sung et al. (2003).

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Plants regularly face elevated temperature throughout their multi-seasonal life cycle. Basal thermotolerance: A plant’s ability to tolerate elevated temperatures, without prior conditioning. Acquired thermotolerance: A plant’s adaptive capacity to survive lethal high temperatures after pre exposure to sub-lethal temperature. Adaptation to thermotolerance

Conclusion: 

Conclusion Plants exhibit a variety of responses to high temperatures High temperatures affect plant growth at all developmental stages Stress proteins are helping in folding and unfolding of essential proteins under stress, and ensuring three-dimensional structure of membrane proteins for sustained cellular functions and survival under heat stress The induction of signaling cascades leading to profound changes in specific gene expression is considered an important heat-stress adaptation

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Attempts have been made to induce heat tolerance in a range of plant species using different approaches. -preconditioning of plants to heat stress -exogenous applications of osmo protectants or plant growth- regulating compounds on seeds or whole plants. Physiological mechanisms of heat tolerance are relatively well understood, further studies are essential to determine -physiological basis of assimilate partitioning from source to sink, plant phenotypic flexibility which leads to heat tolerance, -factors that modulate plant heat-stress response. -an understanding of root responses to heat stress, -most likely involving root–shoot signaling, is crucial and warrants further exploration. future prospects

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Molecular knowledge of response and tolerance mechanisms will pave the way for engineering plants that can tolerate heat stress and could be the basis for production of crops which can produce economic yield under heat-stress conditions.

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THANKS