Plant Stress Biology || Cold, Salinity, and Drought Stress

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<ul><li><p>7Cold, Salinity, and Drought Stress</p><p>Narendra Tuteja</p><p>Abstract</p><p>Genetically modied crops are emerging as a key weapon to ght the</p><p>negative impact of abiotic stresses on agricultural production. Among</p><p>abiotic stresses, cold mainly causes mechanical constraint to the membrane,</p><p>whereas salinity and drought exert their negative impact essentially by dis-</p><p>rupting the ionic and osmotic equilibrium of the cell. Cytosolic free Ca2</p><p>concentration ([Ca2 ]cyt) has been found to increased in response to theabiotic stress. The stress signal is rst perceived at the membrane level by</p><p>the receptors and then transduced in the cell to switch on various stress-</p><p>responsive genes for mediating tolerance. The products of stress-inducible</p><p>genes function both in the initial stress response and in establishing plant</p><p>stress tolerance. Some genes have been reported to be upregulated in</p><p>response to more than one stress, indicating the presence of cross-talk</p><p>between the different stress signaling pathways. The generation of reactive</p><p>oxygen species represents a universal mechanism in cellular responses to</p><p>environmental stresses. Plants also accumulate a variety of osmoprotectants</p><p>that improve their ability to combat abiotic stresses. Understanding the</p><p>mechanism of abiotic stress tolerance is important for crop improvement. In</p><p>this chapter various aspects of cold, salinity, and drought stresses along with</p><p>the role of calcium are discussed.</p><p>7.1</p><p>Introduction</p><p>The world population is increasing at an alarming rate and is expected to reach</p><p>more than 9 billion by the end of 2050 (http://www.unfpa.org/swp/2007/presskit).</p><p>However, food productivity is decreasing due to the negative effect of various stress</p><p>factors. Minimizing these losses is a major area of concern for all nations. Among</p><p>these, abiotic stress is the principal cause of decreasing the average yield of major</p><p>Plant Stress Biology. Edited by H. HirtCopyright r 2009 WILEY-VCH Verlag GmbH &amp; Co. KGaA, WeinheimISBN: 978-3-527-32290-9</p><p>| 137</p></li><li><p>crops by more than 50%, which causes losses worth hundreds of millions of</p><p>dollars each year. In 2000, the United Nations Secretary-General, Ko Annan,</p><p>called for a Blue Revolution and said that there was an urgent need for more</p><p>crops out of dry land. In 2007, the United Nations Food and Agriculture Orga-</p><p>nization warned of food shortages in new climates (food security is in danger).</p><p>Recently, in 2008, Neena Fedoroff, Science and Technology adviser to the United</p><p>States Secretary of State, emphasized the acute need for a Second Green Revo-</p><p>lution. Climate change and the decreased availability of fertile land will create a</p><p>problem for future crop production. In fact, these stresses threaten the sustain-</p><p>ability of agricultural industry. The challenge now is to produce additional food</p><p>under stress conditions and in less soil. Therefore, it is now necessary to obtain</p><p>stress-tolerant varieties to cope with this upcoming problem of food security.</p><p>It is important, rst of all, to understand the notion of stress. Stress in physical</p><p>terms is dened as a mechanical force per unit area applied to an object. In bio-</p><p>logical systems stress can be dened as an adverse force, effect, or inuence that</p><p>tends to inhibit normal systems from functioning [1, 2]. Various stress signals,</p><p>both abiotic as well as biotic, serve as elicitors for the plant cell. Abiotic stresses</p><p>include heat, cold, drought, salinity, wounding, heavy metals toxicity, excess light,</p><p>excess water (ooding), high speed wind, nutrient loss, anaerobic conditions, and</p><p>radiation. Biotic stresses include pathogens (bacteria, fungus, virus), herbivores,</p><p>weeds, insects, nematodes, and mycoplasma. Plants respond to stress as individual</p><p>cells and synergistically as a whole organism. In general, the stress signal is rst</p><p>perceived by receptors of the plant cells. Following this the signal information is</p><p>transduced, resulting in the activation of various stress-responsive genes. The</p><p>products of these stress genes ultimately lead to a stress tolerance response or</p><p>plant adaptation, and help the plant to survive and surpass unfavorable conditions</p><p>[1, 2]. The response could also result in growth inhibition or cell death, which will</p><p>depend upon how many and which kinds of genes are up- or downregulated in</p><p>response to the stress. The various stress-responsive genes can be broadly cate-</p><p>gorized as early- and late-induced genes. Early genes are induced within minutes</p><p>of stress signal perception, which include various transcription factors. Late genes</p><p>include the major stress-responsive genes such as RD (RESPONSIVE TODEHYDRATION)/KIN (COLD INDUCED)/COR (COLD RESPONSIVE), whichencode and modulate the proteins needed for synthesis, for example, late</p><p>embryogenesis abundant (LEA)-like proteins, antioxidants, membrane-stabilizing</p><p>proteins, and osmolytes [2]. Overall, the stress response is a coordinated action of</p><p>many genes encoding signaling proteins/factors, including protein modiers</p><p>(methylation, ubiquitination, glycosylation, etc.), adaptors, and scaffolds [2, 3].</p><p>In this chapter I have emphasized the general response to abiotic stress followed</p><p>by cold, salt, and drought stresses, and the reason for these stresses being injur-</p><p>ious for plants. Various genes involved in cold acclimation and their role towards</p><p>membrane stabilization are discussed. The role of calcium in relation to stress is</p><p>covered. Furthermore, the role of the salt overly sensitive (SOS) pathway in salt</p><p>tolerance and the role of glycine betaine (GB, N,N,N-trimethylglycine-betaine) as amajor osmolyte in response to salt stress are also described.</p><p>138 | 7 Cold, Salinity, and Drought Stress</p></li><li><p>7.2</p><p>Abiotic Stress Response and Stress-Induced Genes</p><p>A generic pathway in response to salinity, drought, and cold stresses is depicted in</p><p>Figure 7.1.</p><p>To sense these environmental signals, higher plants have evolved a complex</p><p>signaling network, which may also cross-talk. Stress signal transduction pathways</p><p>start with signal perception by receptors (phytochromes, histidine kinases,</p><p>receptor-like kinases, G-protein-coupled receptors (GPCR), hormone receptors,</p><p>etc.). Heterotrimeric G-proteins mediate the coupling of signal transduction from</p><p>activated GPCRs to generate secondary signaling molecules (inositol phosphatase,</p><p>reactive oxygen species (ROS), abscisic acid (ABA), etc.). These secondary mole-</p><p>cules can modulate the intracellular Ca2 levels by receptor-mediated Ca2</p><p>Figure 7.1 Generic pathway under salinity, drought, and cold</p><p>stresses. Salinity and drought stresses mainly disrupt the</p><p>ionic and osmotic equilibrium of the cell. These stresses can</p><p>also cause injury to the cellular physiology, which leads to</p><p>metabolic dysfunctions followed by growth inhibition. Cold</p><p>stress mainly exerts its negative effect by disruption of</p><p>membrane integrity and solute leakage. Finally, in response to</p><p>all these stresses several stress-responsive genes are</p><p>upregulated whose products can directly or indirectly help the</p><p>plant in stress tolerance.</p><p>7.2 Abiotic Stress Response and Stress-Induced Genes | 139</p></li><li><p>release or can bypass Ca2 in early signaling steps and initiate protein phos-phorylation cascades (protein phosphatase, mitogen-activated protein kinase</p><p>(MAPK), calcium-dependent protein kinase (CDPK), SOS3/protein kinase S, etc.),</p><p>which activate specic stress-responsive genes for cellular protection through</p><p>transcription control (MYC/MTB, C-repeat binding (CBF)/dehydration-responsive</p><p>element binding (DREB) factors) [2, 3, 5, 6]. Salinity and drought exert their</p><p>inuence on a cell mainly by disrupting the ionic and osmotic equilibrium [2].</p><p>Thus, excess of Na ions and osmotic changes in the form of turgor pressure arethe initial triggers, leading to a cascade of events, which can be grouped under</p><p>ionic and osmotic signaling pathways, the outcome of which is ionic and osmotic</p><p>homeostasis, leading to stress tolerance. These stresses are marked by symptoms</p><p>of stress injury, including chlorosis and necrosis, and may also exert negative</p><p>inuences on cell division resulting in growth retardation of plants [2]. Reduction</p><p>in shoot growth, especially leaves, is benecial for plants as it reduces the surface</p><p>area exposed for transpiration, hence minimizing water loss. Plants may also</p><p>sacrice or shed older leaves, which is another adaptation in response to drought.</p><p>Stress injury may occur through denaturation of cellular proteins/enzymes or</p><p>through the production of ROS, Na toxicity, and disruption of membraneintegrity. In response to injury stress plants trigger a detoxication process, which</p><p>may include change in the expression of LEA/dehydrin-type gene synthesis of</p><p>molecular chaperones, proteinases, enzymes for scavenging ROS, and other</p><p>detoxication proteins. This process functions in the control and repair of stress-</p><p>induced damage, and results in stress tolerance. Cold stress mainly results in</p><p>disruption of membrane integrity, leading to severe cellular dehydration and</p><p>osmotic imbalances. Cold acclimation results in the triggering of various genes,</p><p>which result in a restructuring of the cellular membranes by changes in the lipid</p><p>composition and the generation of osmolytes, which prevent cellular dehydration</p><p>and enhances stress tolerance (Figure 7.1).</p><p>Plants suffer from dehydration or osmotic stress under drought, salinity, and</p><p>also under low-temperature conditions that cause reduced availability of water for</p><p>cellular function and maintenance of cellular turgor pressure. Prolonged periods</p><p>of dehydration lead to high production of ROS in the chloroplasts, causing irre-</p><p>versible cellular damage and photoinhibition. Overall, in response to all these</p><p>stresses several stress-responsive genes are upregulated whose products can</p><p>directly or indirectly help the plant through stress tolerance. Understanding the</p><p>molecular mechanism for abiotic stress tolerance is still a major challenge in</p><p>biology. Many chemicals are also critical for plant growth and development, and</p><p>play an important role in integrating various stress signals and controlling</p><p>downstream stress responses by modulating gene expression machinery and</p><p>regulating various transporters/pumps and biochemical reactions. Some of the</p><p>chemicals include calcium (Ca2 ), cyclic nucleotides, polyphosphoinositides,nitric oxide, sugars, ABA, jasmonates, salicylic acid, and polyamines [7].</p><p>Microarrays employing cDNAs or oligonucleotides are a powerful tool for ana-</p><p>lyzing the gene expression proles of plants exposed to abiotic stresses. A 7000</p><p>full-length cDNA microarray was utilized to identify 299 drought-inducible genes,</p><p>140 | 7 Cold, Salinity, and Drought Stress</p></li><li><p>54 cold-inducible genes, 213 high salinity-inducible genes, and 245 ABA-inducible</p><p>genes in Arabidopsis [8, 9]. More than half of these drought-inducible genes werealso induced by high salinity and/or ABA treatments, implicating signicant</p><p>cross-talk between the drought, high salinity, and ABA response pathways.</p><p>Recently, Shinozaki and Yamaguchi-Shinozaki [10] summarized the gene net-</p><p>works involved in drought stress response and tolerance. By using transgenic</p><p>technology, Bhatnagar-Mathur et al. [11] have also described the recent progress inthe improvement of abiotic stress tolerance in plants, which includes a discussion</p><p>on the evaluation of abiotic stress responses and protocols for testing transgenic</p><p>plants for their tolerance under close-to-eld conditions. Emerging evidence</p><p>indicates CDPKs sense the Ca2 concentration changes in plant cells, and playimportant roles in signaling pathways for disease resistance and various stress</p><p>responses. Among the 20 wheat CDPK genes studied, 10 were found to respond to</p><p>drought, salinity, and ABA treatment [12].</p><p>7.3</p><p>Cold Stress</p><p>Each plant has its own set of temperature requirements, which are optimum for its</p><p>proper growth and development. Deviation from optimum temperature may lead</p><p>to plant growth inhibition and yield loss. The cold stress experienced by plants can</p><p>be classied into two types: those occurring at (i) temperatures below freezing and</p><p>(ii) low temperatures above freezing (nonfreezing temperatures).This section</p><p>covers various aspects of cold stress.</p><p>7.3.1</p><p>Effect of Low-Temperature Stress on Plant Physiology</p><p>Many plants such as maize, soybean, cotton, tomato, and banana are sensitive to</p><p>nonfreezing temperatures (1015 1C) and exhibit signs of injury [1315]. Variousphenotypic symptoms in response to chilling stress include reduced leaf expan-</p><p>sion, wilting, and chlorosis, which may lead to necrosis. Low temperature can also</p><p>severely hamper the reproductive development of plants, as reported in rice [16].</p><p>Freezing temperatures can induce severe membrane damage, which is largely due</p><p>to the acute dehydration associated with freezing [14, 17]. The temperature at</p><p>which a membrane changes from a semiuid state to a semicrystalline state is</p><p>known as the transition temperature. Chilling-sensitive plants usually have a</p><p>higher transition temperature as compared to the chilling-resistant plants, which</p><p>have a lower transition temperature. An understanding of how freezing induces</p><p>plant injuries is essential for the development of frost-tolerant crops. The real</p><p>cause of freeze-induced injury to plants is ice formation rather than the low</p><p>temperatures. Ice formation in plants begins in the apoplastic space having rela-</p><p>tively low solute concentrations. This causes a mechanical strain on the cell wall</p><p>and plasma membrane leading to cell rupture [18]. Freezing temperatures exert</p><p>7.3 Cold Stress | 141</p></li><li><p>their effects largely by membrane damage due to severe cellular dehydration, but</p><p>certain additional factors including ROS also contribute to damage induced by</p><p>freezing. Overall, chilling ultimately results in loss of membrane integrity, which</p><p>leads to solute leakage. The integrity of intracellular organelles is also disrupted,</p><p>leading to the loss of compartmentalization and impairment of photosynthesis,</p><p>protein assembly, and general metabolic processes. The primary environmental</p><p>factor responsible for triggering increased tolerance against freezing is the phe-</p><p>nomenon known as cold acclimation. It is the process where certain plants</p><p>increase their freezing tolerance upon prior exposure to low nonfreezing tem-</p><p>peratures [2].</p><p>7.3.2</p><p>Cold Acclimation</p><p>Cold temperatures induce a number of alterations in cellular components,</p><p>including the extent of unsaturated fatty acids, the composition of glycerolipids,</p><p>changes in protein and carbohydrate composition, and the activation of ion channels</p><p>[2, 19]. For cold acclimation, membranes have to be stabilized against freeze injury,</p><p>which can be achieved through changes in the lipid composition and induction of</p><p>other nonenzymatic proteins that alter the freezing point of water. Accumulation</p><p>of sucrose and other simple sugars also contributes to the stabilization of mem-</p><p>branes as these molecules can protect membranes against freeze-damage. Low</p><p>temperatures activate a number of cold-inducible genes, such as those encoding</p><p>dehydrins, lipid transfer proteins, translation...</p></li></ul>

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