Sakshi Education
Plant Stress Physiology
“Stress” in plants can be defined as any external factor that decreases plant growth, productivity, reproductive capacity or survival below the potential of the genotype. This includes a wide range of factors which can be broadly divided into two main categories: abiotic (non-living) or environmental stress factors, and biotic (living) or biological stress factors. Though plants have evolved several strategies to cope with and adapt to these stresses imposed by the frequently adverse environment a greater understanding of the molecular and physiological mechanisms underlying plant response to these stresses will greatly enhance the chances of improving the plant performance against different stresses using biotechnological approaches.
Apart from environmental stresses, plants are permanently exposed to potential enemies including microbial pathogens - viruses, bacteria, fungi, nematodes and insects. To protect themselves, plants have in their armory, passive defense mechanisms such as strengthened cell walls and antimicrobial compounds, as well as active healing responses. Only a few microbes can breach these basal defenses, and are then fought by the plant’s innate immune system. Plants have evolved sophisticated mechanisms to perceive such attacks, and translate the perception into an adaptive response.
The innate immune response is well described genetically by what is known as the gene-for-gene model, because it requires a pathogen protein encoded by ‘avirulence’ (Avr) gene to be recognized by a plant protein encoded by a resistance (R) gene. This activates an array of defense mechanisms, including the hypersensitive response, in which a few plant cells at the site of infection die, thereby limiting the spread of disease. Pathogens that are recognized in this way and that therefore fail to cause disease are called avirulent pathogens, the host is called resistant, and the interaction is called incompatible. In the absence of gene-for-gene recognition, due to absence of the avirulence gene in the pathogen and/or of the R gene in the host, the pathogen is virulent, the host is susceptible, and the interaction is compatible. For example the Rpg1-b gene from soybean (Glycine max) confers resistance to Pseudomonas syringae pv glycinea (causing bacterial blight) carrying the avrB gene in a classic gene for gene specific manner. R gene products may not directly bind to avirulence gene products, but rather detect alterations in host proteins that are caused by the pathogen gene products.
This idea is known as the guard model. Initiation of RPS2 mediated disease resistance against Pseudomonas syringae in Arabidopsis through elimination of RIN4 by AvrRpt2 is a typical example of Guard hypothesis. Plant pathogens can be broadly divided into those that kill the host and feed on the contents (necrotrophs) and those that require a living host to complete their life cycle (biotrophs). However, many others behave as both biotrophs and necrotrophs, depending on the conditions in which they find themselves or the stages of their life cycles. Such pathogens are called hemi-biotrophs. Microbial necrotrophy is often accompanied by production of toxins. Viruses are quintessential biotrophs, although infection can lead eventually to host cell death. Bacteria and fungi can adopt either lifestyle. Most microbial pathogens enter the host cell, while others like mildew (e.g., Blumeria) or rust fungi (e.g., Hemelia) remain on the outside and grow external structures for nutrient-absorption inside the host.
Terrestrial plants are food source for an estimated one million or more insect species from diverse taxonomic groups. Insects use various strategies to obtain nutrients from all above and below ground plant parts. Around two-third of all known herbivorous insect species are leaf eating beetle (Coleoptera) or caterpillars (Lepidoptera) that cause damage with mouth parts evolved for chewing and tearing .By contrast, sap-feeding insects and nematodes can adopt more intimate and sophisticated modes of biotrophic parasitism, imposing developmental responses on the plant cells, leading to the appearance of galls, root knots or cysts. Once the infection is established, pathogens can manipulate the host to release water and nutrients necessary for the proliferation of the pathogen, leading to nutrient and water deficiency in the host cells. Infection with pathogens also has a significant impact on the composition of soluble carbohydrates in the infected parts of the plant.
Pathogens, especially biotrophs can manipulate the host metabolism in their favour, causing the host plant to accumulate high concentrations of carbohydrates that interfere with the normal metabolism and can cause intracellular osmotic stress. The activation of pathogen stress responses itself poses a stress to the plant, and both disease and defence mechanism might be the cause for reduced yield. After recognition of the pathogen, plant defence mechanisms are induced. This can include the production of signal compounds such as ROS and SA, which might be harmful in high concentrations to the host itself. For instance, when a threshold for local ROS concentrations is exceeded, the cells will initiate programmed cell death (PCD).
Finally, the secretion of cell death inducing compounds or toxins by necrotrophic and hemibiotrophic pathogens can lead to a high stress level and ultimately to cell death. Defense against microbes can be highly effective on small spatial scales like the hypersensitive response (HR), in which cells immediately surrounding the infection site rapidly die and fill with antimicrobial compounds to prevent the spread of the pathogen. Even HR can be effective against sedentary herbivores like aphids feeding on particular tissue like phloem.
How Plants respond to environmental stresses?
Environmental stresses adversely affect growth, yield and productivity. Resistance or sensitivity of plants to these stresses depends on the species, genotype and the stage of development. Resistance includes adaptation, avoidance and tolerance. Adaptation is permanent resistance to stress in morphology and structure, physiology and biochemistry under long-term stress condition.
Many plants exhibit constitutively expressed traits that are recognized to confer resistance or tolerance to environmental stresses; these are frequently referred to as ‘constitutive adaptations’. Examples of morphological adaptations to heat, light and water deficit stresses might include leaf hairiness and waxiness which influence light absorption and hence heat balance. An example of a combined anatomical and metabolic stress adaptation is C4 photosynthesis and CAM pathway, which reduces photorespiration, leading to a higher temperature optimum for photosynthesis and higher water use efficiency in comparison to C3 photosynthesis.
Avoidance is a manner to avoid facing with stress using neither metabolic process nor energy. Examples include very short lifecycle in desert plants and dormancy during the cool, hot, and drought conditions. Tolerance is a resistant reaction to reduce or repair injury with morphology, structure, physiology, biochemistry or molecular biology, when plant counters with stress. This phenomenon is also known as acclimation or hardening or facultative/inducible metabolic adaptations. Examples include well developed aerenchyma in hydrophytes, induction of glycolytic enzymes and enzymes of lactate and ethanol fermentation which allow continued energy production during anoxia or hypoxia, osmotic adjustment, induction of heat shock proteins, etc.
Signalling components
Plant responses to different stresses are highly complex and involve changes at the transcriptome, cellular, and physiological levels. Recent evidence shows that plants respond to multiple stresses differently from how they do to individual stresses, activating a specific programme of gene expression relating to the exact environmental conditions encountered. The main components of stress-induced signalling pathways (Ca2+, ROS, SA, ABA) and signal cascades seem to have equal importance in responses to environmental changes and pathogen challenge. The cross-talk between the different signalling pathways allows plants to adjust their responses depending on the combination of stimuli.
Reactive Oxygen Species
ROS production is induced by a number of stresses, which include hyperosmotic stress, drought, cold, ozone and pathogen attack. ROS are partially reduced forms of atmospheric oxygen and include H2O2, OH and O2-. The nature, timing and amount of ROS is important for the effect on the plant: low levels of ROS are involved in stress signalling inducing protective genes, while high concentrations usually lead to the induction of PCD.
ROS levels are regulated by a fine tuned interplay of ROS producing and scavenging enzymes. The main enzymes of plants include superoxide dismutase (SOD), ascorbate peroxidase (APX), non-specific peroxidase (POX), glutathione reductase (GR) and catalase (CAT).SOD reacts with O2- to produce H2O2, which is scavenged by POXs and CATs. Among the different peroxidases APX, which uses ascorbates an electron donor, has an important role in H2O2 detoxification and is well described in plants. GR is involved in the reduction of oxidized glutathione to continue H2O2 scavenging.
CAT has been mainly found in leaves (peroxisomes) to remove H2O2 (formed by photorespiration and ß-oxidation of fatty acids). The activity of these enzymes is highly differentiated in separate plant organs and under different stresses. This complex system of enzymes and antioxidants needs an even more complex network of genes to regulate the antioxidative activity in the plant. One explanation for this array of genes involved in ROS management could be that the source of ROS production and the localisation of the different ROS species are important for the effect: in contrast to OH and O2-, H2O2 can permeate cellular membranes and is therefore not restricted to the original location of production. In pathogen defence mechanisms, the oxidative burst is one of the earliest responses to pathogen recognition.
Plants generate ROS to mediate cell death as part of the host resistance mechanisms. Recognition of an invading pathogen results in the activation of plasma membrane-associated NAD(P)H oxidases and peroxidases, which leads to enhanced production of O2-.Dismutation of O2- by SOD generates H2O2, which can diffuse into cells and activate defence mechanisms, ultimately leading to PCD. These responses in pathogen challenge are similar to those under abiotic stress. ROS and NO act as secondary messengers in regulating stomatal movements, providing a link to regulatory mechanisms responding to drought tolerance. Cross-talk of ROS with other signalling networks involves SA, Ca2+ and ABA signalling. While ROS is needed for SA signalling increase in Ca2+ concentration seem to be essential for ROS-mediated cellular defence.
Calcium Signalling
Calcium (Ca2+) is a major convergence point of signalling cross-talk, which is elicited by different stress conditions. Alterations in the concentration mark early cellular responses for a number of processes, for instance stomatal closure or stress adaptation. Differences in the intracellular Ca2+ level as a stress response can be caused by Ca2+ influx or release from internal storages and have been observed for a range of stresses. In addition, several stresses induce the biosynthesis of second messengers that require Ca2+ for signal transduction, e.g., ROS, ABA and IP3. Elevation of Ca2+ in the cytosol marks early responses of the plant to abiotic stresses such as cold, mechanical, ozone and salt stress but also to pathogens or pathogen elicitors. Rapid increases in cytosolic Ca2+ are observed in response to touch or cold shock, involving Ca2+ fluxes from internal vacuolar stores and from the plasma membrane.
Transient increases in Ca2+ of varying duration are initiated by oxidative stress, pathogen elicitors and hyperosmotic treatments, often in conjunction with an oxidative burst. During drought and other water limiting stresses, oscillations of Ca2+ are important for the regulation of guard cell-mediated stomatal pore closure. Other than Ca2+ extrusion, which is an active ATP dependent process, the entry into the cytosol is mainly a passive process mediated by ion channels. Such channels are activated by membrane depolarization, reported in response to different stresses such as high light intensity and fungal elicitors. Because high levels of Ca2+ are toxic for the cell, the calcium influx into the cell has to be controlled.
Many different proteins have a role in keeping Ca2+ concentrations in the acceptable nano-molar range, usually by calcium binding or Ca2+ extrusion. Hence, Ca2+ binding proteins (like calmodulin or calcineurin) help to regulate the spatial and temporal release of Ca2+ and thereby have multiple roles in stress response. At the cellular level, calmodulins reduce the effective diffusion in the cytosol and restrict calcium to specific locations thereby regulating the activity .Consequently, calmodulin gene expression is induced by a variety of stresses like salinity, osmotic stress and wounding in different plants. Various findings have revealed that Ca2+ signals have specific characteristics: Ca2+ signals can be slow or fast, short- or long-lived and oscillations of the flux are of major importance for the specificity of the signal.
The “signature” changes in response to the stress and previous exposure to the stress. Hence, the magnitude, frequency, localization and duration of Ca2+ transients all specify the pathway in which Ca2+ signals participate. Another specific characteristic influencing the signal is the source of Ca2+: influx from extracellular space or internal stores like the vacuole can release Ca2+. For the induction of plant defence against pathogens, extracellular Ca2+ influx and the involvement of the specific Ca2+ channels on the Ca2+ signal appears to be crucial. Finally, the Ca2+ signal is influenced by channel guarding proteins and the localization of the channels. Taken together, alterations of Ca2+ transients and Ca2+ oscillation are mediated by many stresses and underline the importance of Ca2+ as a multi recognizer of stresses. Thus, it is quite clear that Ca2+ has a major role in cross-tolerance and deciphering the Ca2+ signatures could be a promising step towards the manipulation of cellular Ca2+ signalling in favour of more resistant crops.
Abscisic Acid
ABA is involved in many cellular processes like germination, gravitropism and guard cell mediated stomatal opening. Particularly well established is the regulatory function of ABA in water balance and osmotic stress tolerance. Under non-stress conditions, plants retain a low level of ABA, which increases in response to low temperature, drought and osmotic stress. Drought and salinity activate de novo ABA synthesis to prevent further water loss by evaporation through stomata, mediated by changes in the guard cell turgor pressure.
The mechanism involves fast changes in intracellular Ca2+concentration and stimulates further signalling in the cell. Under osmotic stress, ABA induces the accumulation of proteins involved in the biosynthesis of osmolytes (e.g., proline, trehalose), which increases the stress tolerance of plants. ABA or abiotic stresses, which lead to the accumulation of ABA can significantly impair biotic defence mechanisms.
However the positive effects of ABA application during biotic stress have also been reported. ABA is required for the multiple effects of ß-aminobutyric acid (BABA), a non-protein amino acid, which enhances resistance through potentiated defence responses, leading to a restriction of pathogen growth. ABA shows an overlap with other signalling networks, and the role of ABA might be based on the repression of SA responses. ABA antagonizes SA-mediated responses in a number of species, but might, on the other hand, positively regulate structural defence mechanisms, e.g., callose deposition and papillae formation.
Salicylic acid, Jasmonic Acid and Ethylene.
SA is a phenolic compound, which was identified as a major signal for accumulation of PR proteins in virus infected tobacco. Pathogen recognition also triggers various inducible systemic defenses in addition to the locally effective hypersensitive response. . In plant parts distant from the site of primary infection, systemic responses establish an enhanced defensive capacity against subsequent infection. This biologically induced resistance in systemic tissue is known as systemic acquired resistance (SAR) and has been shown to be effective in many plant species.
The attained state of resistance is long-lasting and effective against a broad spectrum of pathogens, including pathogenic bacteria, fungi and viruses. SAR requires both local and systemic salicylic acid (SA) accumulation and the induction of a subset of the pathogenesis-related (PR) genes, but SA itself is not the mobile signal. SA thus plays a major role in defence signalling and is essential for different defence mechanisms, such as SAR and hypersensitive responses (HRs). Stress acclimation has been observed for SA mediated stress responses. In plant defense research, PR genes serve as powerful molecular markers for the onset of SAR.
Although SA can be synthesized from phenylalanine, the predominant pathway for de novo SA biosynthesis during pathogen infection is through chorismate via isochorismate synthase (ICS).SA, synthesized from chorismate by means of ICS, is required for local and systemic acquired resistance responses. Non-expressor of pathogenesis related 1 (NPR1, also known as non-immunity 1 [NIM1]) is a central positive regulator of SAR signalling. NPR1 protein contains an ankyrin repeat and a BTB/ POZ domain and functions downstream of SA. NPR1 regulate PR gene expression through interaction with TGA transcription factors. PR proteins that are mainly associated with pathogen defence, especially PR-1, -2 and -5, are well characterised in pathogen response and confer resistance when expressed transient or constitutively. Yet, PR genes are also activated in abiotic stress: PR-3 protein, which functions as a chitinase, is also induced during cold stress and may function as an antifreeze protein.
A PR-1 gene of hot pepper (CABPR1) is induced by environmental stresses such as high salinity and low temperatures and by pathogen infections. In potato, PR-5 protein is induced during abiotic stress and responds to SA, ABA, salinity, wounding and pathogen stress. PR-10 proteins have been identified in various species and accumulate in response to pathogens (fungi) in pea, rice and potato, but also in pea under salinity and in white pine following wounding, fungal infection and cold-hardening.
Colonization of plant roots with selected strains of non-pathogenic, growth promoting rhizosphere bacteria can also provoke broad-spectrum disease resistance in plants. This type of induced resistance is called induced systemic resistance (ISR). ISR is less ‘broad spectrum’ in nature. In contrast to SAR, the ISR response does not require SA and generally requires jasmonic acid (JA) and ethylene (ET).
Jasmonate synthesis occurs through the octadecanoid pathway and begins with the release of linolenic acid from the chloroplast membrane. Attack by necrotrophic pathogens, as well as herbivorous insects, elicits the production of a large chemically diverse set of oxygenated fatty acids (oxylipins) that can be potent regulators of defense signalling. JA and precursors of JA that are synthesized during JA biosynthesis have an important function as signalling molecules in various processes, including plant defense. JA-dependent signalling proceeds through increased JA synthesis in response to pathogen attack and consequent increase in expression of defense effector genes such as PDF1.2.
The gaseous plant hormone, ethylene, is synthesized from the amino acid 1-aminocyclopropane-1-carboxylic acid (ACC), derived from methionine via S-adenosylmethionine (Adomet). ACCformation from Adomet is catalysed by ACC synthase. ACC oxidation to ethylene is catalysed by an oxygen-dependentACC oxidase. A large number of stresses (including wounding, flooding, chilling, high temperature and osmotic stresses) induce ethylene biosynthesis primarily bymodulating the expression of genes encoding ACC synthase. Some JA regulated genes are also regulated by ET. In the case of PDF1.2, induced expression requires both JA and ET.
In contrast, ET is not required for expression of the JA inducible gene VSP1. Defense responses dependent on SA are often effective against biotrophic pathogens, whereas defences dependent on JA are mostly effective against necrotrophic pathogens and insects. To achieve an effective state of resistance after recognition of the invader, plants are thought to fine-tune different defense signalling pathways by means of synergistic and antagonistic interactions. A synergistic effect was reported between SA-dependent SAR and rhizobacteria-mediated induced systemic resistance (ISR).
ISR is activated by root-colonizing, non-pathogenic fluorescent Pseudomonas spp. and provides broad spectrum resistance to pathogen attack. ISR signal transduction is dependent on an intact JA/ethylene response and, interestingly, requires the function of the regulatory protein NPR1. Thus, SAR and ISR signalling pathways are distinct in their requirement for SA and JA/ethylene, yet unite in their downstream requirement of NPR1. Besides signal synergy between SA- and JA-dependent defense responses, cases of antagonism between these two signalling molecules have also been reported. The JA-resisted tobacco hornworm Manduca sexta inflicted more damage on SAR induced tobacco plants compared to control plants. Moreover, tobacco plants silenced for the expression of the phenylpropanoid biosynthesis gene PAL (phenyl ammonia-lyase) exhibit reduced SAR against TMV, but exhibited enhanced resistance to insect infestation. Conversely, plants overexpressing PAL were more resistant to TMV, whereas resistance to insect attack was lost.
Plant responses to herbivores are broadly categorized as direct and indirect defences and tolerance. Compounds that exert repellent, antinutritive, or toxic effects on herbivores are commonly referred to as direct defences. Proteinase inhibitors (PI) (antidigestive proteins) are inducible by wounding and herbivory and influence herbivore performance by inhibiting insect digestive enzymes. Physical barriers such as leaf toughness and trichomes that increase plant fitness in the presence of herbivores come under direct defences. Toxic compounds (e.g., alkaloids, terpenoids, phenolics) that poison generalist herbivores also constitute a part of direct defense. Indirect defences are plant traits that attract predators and parasitoids of herbivores. Volatile organic compounds (VOCs) released by herbivore-attacked plants not only mediate interactions with herbivores and their predators and the damage induced volatiles can provide a signal that allows neighbouring plants to prepare for imminent herbivory. This process is called priming.
Protein/MAP kinases
A highly conserved signal mechanism between all eukaryotes is the phosphorylation of proteins by protein kinases. Ca2+ dependent protein kinases (CDPK), a class of serine/threonine kinases, and mitogen activated kinases (MAPK) are key signal transmitters in various stress responses. Specific MAPK cascades are activated by a range of stimuli including cold, drought, salinity, ROS, heat, shaking, wounding, pathogens and pathogen elicitors, ethylene, ABA and SA. MAPKs are central for the transduction of cellular signals by activation and repression of downstream target protein activity.
The mechanism of MAPK signalling cascades is usually three fold. Environmental factors stimulate a MAP kinase kinase kinase (MAPKKK), which phosphorylates kinase kinases (MAPKK). MPKK activates MAPK, which in turn activates downstream cellular proteins by phosphorylation. MAP kinases can be highly specific and distinct MAPKs are only activated by distinct stimuli. The MAPK activation by fungal and bacterial elicitors and SA is well-documented and MAPKs are involved in regulating a range of defence pathways.
Thus, defects can have a significant impact on plant defences and mutations in MAPKs can lead to de-repression of defence mechanisms such as enhanced SA accumulation, constitutive PR gene expression and growth inhibition. MAPK signalling is of equal importance in biotic and abiotic signalling, hence defects in MAPK mediated signalling affect not only pathogen related responses but also abiotic stress responses. Thus MAP kinase cascades constitute a major mechanism for activation of biotic defence responses and in mediating other signals, such as hormones in response to abiotic stresses like cold and drought, and osmotic stress.
Signalling networks are interconnected
Stress sensing is traditionally categorized in separate linear signal chains, but does in fact consist of a network of parallel and interacting signalling chains, which often share common components or have signalling convergence points. As a consequence, signalling compounds, which are classical abiotic or biotic stress signals, often induce non-related stress responses. ROS, Ca2+, ABA, SA, and Kinases serve as key components in mediating these signalling networks.
One situation where plants achieve cross tolerance is inherent tolerance of a plant species to a stress condition enables this plant to withstand other stresses, which share an overlapping stress potential. For example, effective antioxidative machinery, which allows the plant to cope efficiently with oxidative stress, would lead to enhanced tolerance to other stresses that involve oxidative stress. The other situation is stress acclimation: usually the exposure of plants to sub-lethal doses of a stress is referred to as acclimation. A typical example is acquired cold/freezing tolerance of plants following low temperature exposure. The understanding of signalling interactions involved in different stresses is complicated by the fact that signalling compounds can act as stress signals and as stress themselves.
The effects of these compounds is modulated by their absolute levels and a range of mechanisms is involved in keeping the intracellular concentrations in balance, as small changes result in drastically different responses. High levels of a single secondary messenger might be beneficial in one plant species, but can be harmful to another. The responses of plants are highly tuned to the absolute levels of these key compounds, distinguishing whether they induce defence gene activity or exhibit secondary stress (i.e. oxidation) leading to plant cell death. The intracellular concentration and localization of these signalling compounds can influence the effect of the stress and activate sensors responding to different stress intensities.
Conclusion
These signalling compounds and components exhibit vivid signal cross-talk and show an increasingly complex network of interacting signal pathways. If this limited number of signalling molecules or components can be used to combine different defence or stress tolerance responses, the cell can respond specifically to numerous signals. Given the amount of information that plants receive from different overlapping stress signals, it is intriguing to see how plants read between the lines of all this crosstalk and initiate appropriate cellular responses. Untangling this network of interconnected signal pathways and understanding the underlying mechanisms remains a challenge for the future that could help to develop more durable and resistant plants.
-The End-
“Stress” in plants can be defined as any external factor that decreases plant growth, productivity, reproductive capacity or survival below the potential of the genotype. This includes a wide range of factors which can be broadly divided into two main categories: abiotic (non-living) or environmental stress factors, and biotic (living) or biological stress factors. Though plants have evolved several strategies to cope with and adapt to these stresses imposed by the frequently adverse environment a greater understanding of the molecular and physiological mechanisms underlying plant response to these stresses will greatly enhance the chances of improving the plant performance against different stresses using biotechnological approaches.
- Abiotic Stress Abiotic stresses include drought, excess water, heat, cold, salinity, deficiencies or excesses of several nutrients, extremes of soil pH, exposure to xenobiotics (now including chemical pollution) and UV radiation.
Drought Stress
Insufficient availability of water, i.e., drought, is presumably the most common stress experienced by terrestrial plants. On the cellular level drought stress will affect vital metabolic functions and maintenance of turgor pressure. Cell expansion and cell wall formation are therefore especially sensitive to water limitation. In order to minimise water loss, plants respond to lower water availability with the closure of stomata. However, this protective measure decreases photosynthetic fixation due to decrease in CO2 supply and increases ROS generation.
ROS including singlet oxygen (1O2-), superoxide anions (O2-), hydrogen peroxide (H2O2) and hydroxyl radicals (OH) are highly reactive and damage cells by ROS mediated oxidative processes such as membrane lipid peroxidation, protein oxidation, enzyme inhibition and damage of nucleic acids. Consequently, protective mechanisms against this oxidative stress have evolved in all aerobic organisms and acclimation to this stress generally correlates with strengthening of the anti-oxidative system. Another important messenger during drought stress adaptation is the phytohormone abscisic acid (ABA). ABA biosynthesis is initiated by decreasing water potential, and seems to be essential for the activation of many protective measures towards abiotic stresses mediated through ABA responsive cis- and trans-acting factors and protein kinases or phosphatases interacting with Ca2+. The investigation of drought-induced genes has also revealed ABA-independent signal transduction pathways containing the so called dehydration responsive element.
Additionally, the volatile phyto-hormone ethylene accumulates under drought stress and possibly initiates specific gene expression. Cytokinine, on the other hand, has antagonistic effects and its levels usually decrease under water stress. Drought-induced genes encode proteins involved in metabolic, osmotic or structural adjustment as well as proteins with damage control and repair functions. Biosynthesis of proline for instance increases the concentration of compatible osmoprotectants in the cells, while aquaporins can facilitate water permeability of cellular membranes and maximise water uptake potential of the plant, and ROS scavenging proteins can limit damage by secondary oxidative stress. The specific function of the majority of drought induced genes is, however, still unknown and revelation of their function should give new insights into the plant protective mechanisms against drought.
1.2 Water Logging
Water excesses, or flooding stress, cause anoxia or hypoxia in the root zone. Water logging fills soil pores and spaces, preventing oxygen diffusion to the root. The failure of waterlogged roots to undergo normal aerobic metabolism leads to inhibition of protein synthesis, and inhibition of ion acquisition and transport. This, coupled with alterations of water permeability of the roots in flooded soils, often leads to macronutrient deficiencies in the shoot, and ironically, shoots water deficits in response to flooding.
1.3 Salt Stress
High concentrations of salt (i.e., ions, mostly Na+) in the soil solution can impair water and nutrient uptake, reduce growth and photosynthetic activity. Furthermore high salinity can lead to an unfavourable Ca2+ or K+ to Na+ ratio, toxic intracellular Na+ concentrations and per-oxidation of membrane lipids. The reduced water intake induces effects comparable to drought stress such as ABA biosynthesis and accumulation, which in turn trigger closure of stomata and increased production of compatible osmoprotectants and antioxidants. Limitations in photosynthesis and changes in redox status of the mitochondrion promote increased production of ROS and downstream signalling pathways.
More than half of the known drought-responsive genes show also induced gene expression by salt stress. Excessive amounts of salt can enter the plant and accumulate at high concentrations disturbing the nutritional homeostasis in the plant. In the cells, high ion concentration can seriously affect the activity of enzymes and increasing Na+ concentration can further inhibit K+ uptake, causing K+ deficiency as a secondary stress. K+ has a key role in several physiological processes, such as osmotic regulation, protein synthesis, and enzyme activation and substitution of K+ by Na+ may lead to an ionic imbalance. Plants have therefore developed protective measures against the accumulation of toxic salt concentration in the cytoplasm, e.g., the active compartmentation of Na+ into the vacuole, selective import of K+ or salt excretion by specific glands.
1.4 Temperature Stress
Every plant species performs optimally only in a characteristic temperature range, which depending on the thermo-sensitivity of the species might be very narrow. Extremely high or low temperatures affect vital cell functions such as enzyme activity, cell division and membrane integrity. Nevertheless, heat and cold acclimation is possible, as mild stress pre-treatment can significantly enhance the thermo-tolerance of plants.
1.4.1 Heat Stress
Effects of high temperature stress can range from moderate effects such as oxidative stress and enhanced transpiration to fatal consequences for the plant, leading to tissue collapse and plant death. To cope with the high temperature stress, plants usually react with enhanced transpiration rates to achieve evaporative cooling. Since high temperatures are often accompanied with limited water availability, the water potential in the plant cell can decrease, leading to drought conditions and initiate responses similar to drought stress.
Under these conditions, different events take place: small heat shock proteins (HSPs), acting as molecular chaperones, are synthesized at high abundance and are thought to contribute the stabilisation of cellular structures and prevent the thermal aggregation of proteins, Ca2+ influx is enhanced and ROS accumulate rapidly in different parts of the cell. Based on the results obtained in different plant species, ABA, SA, ethylene and Ca2+ are involved in heat stress signalling and can also confer protection against heat stress damage.
1.4.2 Cold Stress
Chilling stress is a sub-optimal temperature, where the plant faces reduced enzyme activity and maybe water availability, yet the temperature is above the freezing point of water. Cold stress affects mainly metabolic processes, impairing enzyme reactions, substrate diffusion rates and membrane transport properties. Thereby some reactions are more affected by the cold then others. In particular, the dark reaction of photosynthesis and oxidative phosphorylation seem to be sensitive to chilling. The discrepancy between the speeds of biochemical reactions can cause ROS accumulation in the chloroplast and mitochondrion. One early response to cold and osmotic stress is the rapid Ca2+ influx into the cell.
Physical alterations in the cellular structure may cause this Ca2+ influx by activation of Ca2+ channels and initiate downstream Ca2+ dependent signalling pathways. When temperature drops below zero degrees, plants can experience freezing stress. Intracellular freezing of water can damage the protoplast membrane structure, mechanically injure and finally kill the cells by the expanding ice crystals. During extracellular freezing, the protoplasm of the plant becomes severely dehydrated when water is transferred to the ice crystals in the intercellular spaces. Freezing acclimation can be achieved by the expression of a class of cold-induced “cold-regulated genes” (COR), which encode hydrophilic polypeptides and stabilize membranes against freeze-induced injury.
1.5 Pollutants
Many natural occurring substances, termed xenobiotics, are toxic for plants at high concentrations. Human activity contributed substantially to the accumulation of heavy metals in soils and ground water. Most plants do not possess specific mechanisms to prevent the excessive uptake of heavy metals from the soil. Accumulation of heavy metal ions in the plant can impair membrane integrity, affect enzyme activity and hinder nutrient uptake. Metal ions can generate ROS by auto-oxidation, Haber-Weiss cycle or Fenton reaction and disturb the redox status of cells.
Metals without redox capacity such as cadmium, mercury and lead can disturb the antioxidative glutathione pool, activate Ca2+ dependent systems and iron mediated processes. Apart from soil pollution human activity also increased concentration of air pollutants such as O3, SO2, NO, NO2, NH3, HNO3 and HF. Through stomata they can enter the leaves and affect the plant metabolism. Entry of O3 into the cell forces the creation of ROS and induces oxidative stress.
1.6 UV-B radiation
Energy-rich UV-B radiation represents a small, but important part of terrestrial solar irradiance. The potentially harmful effects of UV-B radiation on plant life include damage to DNA, the photosynthetic apparatus and membrane lipids. However, during evolution, plants have developed powerful protection and repair mechanisms. Increased UV-B radiation initiates morphological adaptation to high-light environments such as stunted growth, leaf thickening, trichome development, flavanoid biosynthesis and decreased stomatal densities. UV-B exposure triggers ROS generation from multiple sources (e.g. NADPH oxidase, peroxidase) and the source of ROS origin is apparently important for the specific changes in gene regulation. In addition to O2- and H2O2, nitric oxide (NO) plays a role in UV-B stress response.
- Biotic Stress
Apart from environmental stresses, plants are permanently exposed to potential enemies including microbial pathogens - viruses, bacteria, fungi, nematodes and insects. To protect themselves, plants have in their armory, passive defense mechanisms such as strengthened cell walls and antimicrobial compounds, as well as active healing responses. Only a few microbes can breach these basal defenses, and are then fought by the plant’s innate immune system. Plants have evolved sophisticated mechanisms to perceive such attacks, and translate the perception into an adaptive response.
The innate immune response is well described genetically by what is known as the gene-for-gene model, because it requires a pathogen protein encoded by ‘avirulence’ (Avr) gene to be recognized by a plant protein encoded by a resistance (R) gene. This activates an array of defense mechanisms, including the hypersensitive response, in which a few plant cells at the site of infection die, thereby limiting the spread of disease. Pathogens that are recognized in this way and that therefore fail to cause disease are called avirulent pathogens, the host is called resistant, and the interaction is called incompatible. In the absence of gene-for-gene recognition, due to absence of the avirulence gene in the pathogen and/or of the R gene in the host, the pathogen is virulent, the host is susceptible, and the interaction is compatible. For example the Rpg1-b gene from soybean (Glycine max) confers resistance to Pseudomonas syringae pv glycinea (causing bacterial blight) carrying the avrB gene in a classic gene for gene specific manner. R gene products may not directly bind to avirulence gene products, but rather detect alterations in host proteins that are caused by the pathogen gene products.
This idea is known as the guard model. Initiation of RPS2 mediated disease resistance against Pseudomonas syringae in Arabidopsis through elimination of RIN4 by AvrRpt2 is a typical example of Guard hypothesis. Plant pathogens can be broadly divided into those that kill the host and feed on the contents (necrotrophs) and those that require a living host to complete their life cycle (biotrophs). However, many others behave as both biotrophs and necrotrophs, depending on the conditions in which they find themselves or the stages of their life cycles. Such pathogens are called hemi-biotrophs. Microbial necrotrophy is often accompanied by production of toxins. Viruses are quintessential biotrophs, although infection can lead eventually to host cell death. Bacteria and fungi can adopt either lifestyle. Most microbial pathogens enter the host cell, while others like mildew (e.g., Blumeria) or rust fungi (e.g., Hemelia) remain on the outside and grow external structures for nutrient-absorption inside the host.
Terrestrial plants are food source for an estimated one million or more insect species from diverse taxonomic groups. Insects use various strategies to obtain nutrients from all above and below ground plant parts. Around two-third of all known herbivorous insect species are leaf eating beetle (Coleoptera) or caterpillars (Lepidoptera) that cause damage with mouth parts evolved for chewing and tearing .By contrast, sap-feeding insects and nematodes can adopt more intimate and sophisticated modes of biotrophic parasitism, imposing developmental responses on the plant cells, leading to the appearance of galls, root knots or cysts. Once the infection is established, pathogens can manipulate the host to release water and nutrients necessary for the proliferation of the pathogen, leading to nutrient and water deficiency in the host cells. Infection with pathogens also has a significant impact on the composition of soluble carbohydrates in the infected parts of the plant.
Pathogens, especially biotrophs can manipulate the host metabolism in their favour, causing the host plant to accumulate high concentrations of carbohydrates that interfere with the normal metabolism and can cause intracellular osmotic stress. The activation of pathogen stress responses itself poses a stress to the plant, and both disease and defence mechanism might be the cause for reduced yield. After recognition of the pathogen, plant defence mechanisms are induced. This can include the production of signal compounds such as ROS and SA, which might be harmful in high concentrations to the host itself. For instance, when a threshold for local ROS concentrations is exceeded, the cells will initiate programmed cell death (PCD).
Finally, the secretion of cell death inducing compounds or toxins by necrotrophic and hemibiotrophic pathogens can lead to a high stress level and ultimately to cell death. Defense against microbes can be highly effective on small spatial scales like the hypersensitive response (HR), in which cells immediately surrounding the infection site rapidly die and fill with antimicrobial compounds to prevent the spread of the pathogen. Even HR can be effective against sedentary herbivores like aphids feeding on particular tissue like phloem.
How Plants respond to environmental stresses?
Environmental stresses adversely affect growth, yield and productivity. Resistance or sensitivity of plants to these stresses depends on the species, genotype and the stage of development. Resistance includes adaptation, avoidance and tolerance. Adaptation is permanent resistance to stress in morphology and structure, physiology and biochemistry under long-term stress condition.
Many plants exhibit constitutively expressed traits that are recognized to confer resistance or tolerance to environmental stresses; these are frequently referred to as ‘constitutive adaptations’. Examples of morphological adaptations to heat, light and water deficit stresses might include leaf hairiness and waxiness which influence light absorption and hence heat balance. An example of a combined anatomical and metabolic stress adaptation is C4 photosynthesis and CAM pathway, which reduces photorespiration, leading to a higher temperature optimum for photosynthesis and higher water use efficiency in comparison to C3 photosynthesis.
Avoidance is a manner to avoid facing with stress using neither metabolic process nor energy. Examples include very short lifecycle in desert plants and dormancy during the cool, hot, and drought conditions. Tolerance is a resistant reaction to reduce or repair injury with morphology, structure, physiology, biochemistry or molecular biology, when plant counters with stress. This phenomenon is also known as acclimation or hardening or facultative/inducible metabolic adaptations. Examples include well developed aerenchyma in hydrophytes, induction of glycolytic enzymes and enzymes of lactate and ethanol fermentation which allow continued energy production during anoxia or hypoxia, osmotic adjustment, induction of heat shock proteins, etc.
Signalling components
Plant responses to different stresses are highly complex and involve changes at the transcriptome, cellular, and physiological levels. Recent evidence shows that plants respond to multiple stresses differently from how they do to individual stresses, activating a specific programme of gene expression relating to the exact environmental conditions encountered. The main components of stress-induced signalling pathways (Ca2+, ROS, SA, ABA) and signal cascades seem to have equal importance in responses to environmental changes and pathogen challenge. The cross-talk between the different signalling pathways allows plants to adjust their responses depending on the combination of stimuli.
Reactive Oxygen Species
ROS production is induced by a number of stresses, which include hyperosmotic stress, drought, cold, ozone and pathogen attack. ROS are partially reduced forms of atmospheric oxygen and include H2O2, OH and O2-. The nature, timing and amount of ROS is important for the effect on the plant: low levels of ROS are involved in stress signalling inducing protective genes, while high concentrations usually lead to the induction of PCD.
ROS levels are regulated by a fine tuned interplay of ROS producing and scavenging enzymes. The main enzymes of plants include superoxide dismutase (SOD), ascorbate peroxidase (APX), non-specific peroxidase (POX), glutathione reductase (GR) and catalase (CAT).SOD reacts with O2- to produce H2O2, which is scavenged by POXs and CATs. Among the different peroxidases APX, which uses ascorbates an electron donor, has an important role in H2O2 detoxification and is well described in plants. GR is involved in the reduction of oxidized glutathione to continue H2O2 scavenging.
CAT has been mainly found in leaves (peroxisomes) to remove H2O2 (formed by photorespiration and ß-oxidation of fatty acids). The activity of these enzymes is highly differentiated in separate plant organs and under different stresses. This complex system of enzymes and antioxidants needs an even more complex network of genes to regulate the antioxidative activity in the plant. One explanation for this array of genes involved in ROS management could be that the source of ROS production and the localisation of the different ROS species are important for the effect: in contrast to OH and O2-, H2O2 can permeate cellular membranes and is therefore not restricted to the original location of production. In pathogen defence mechanisms, the oxidative burst is one of the earliest responses to pathogen recognition.
Plants generate ROS to mediate cell death as part of the host resistance mechanisms. Recognition of an invading pathogen results in the activation of plasma membrane-associated NAD(P)H oxidases and peroxidases, which leads to enhanced production of O2-.Dismutation of O2- by SOD generates H2O2, which can diffuse into cells and activate defence mechanisms, ultimately leading to PCD. These responses in pathogen challenge are similar to those under abiotic stress. ROS and NO act as secondary messengers in regulating stomatal movements, providing a link to regulatory mechanisms responding to drought tolerance. Cross-talk of ROS with other signalling networks involves SA, Ca2+ and ABA signalling. While ROS is needed for SA signalling increase in Ca2+ concentration seem to be essential for ROS-mediated cellular defence.
Calcium Signalling
Calcium (Ca2+) is a major convergence point of signalling cross-talk, which is elicited by different stress conditions. Alterations in the concentration mark early cellular responses for a number of processes, for instance stomatal closure or stress adaptation. Differences in the intracellular Ca2+ level as a stress response can be caused by Ca2+ influx or release from internal storages and have been observed for a range of stresses. In addition, several stresses induce the biosynthesis of second messengers that require Ca2+ for signal transduction, e.g., ROS, ABA and IP3. Elevation of Ca2+ in the cytosol marks early responses of the plant to abiotic stresses such as cold, mechanical, ozone and salt stress but also to pathogens or pathogen elicitors. Rapid increases in cytosolic Ca2+ are observed in response to touch or cold shock, involving Ca2+ fluxes from internal vacuolar stores and from the plasma membrane.
Transient increases in Ca2+ of varying duration are initiated by oxidative stress, pathogen elicitors and hyperosmotic treatments, often in conjunction with an oxidative burst. During drought and other water limiting stresses, oscillations of Ca2+ are important for the regulation of guard cell-mediated stomatal pore closure. Other than Ca2+ extrusion, which is an active ATP dependent process, the entry into the cytosol is mainly a passive process mediated by ion channels. Such channels are activated by membrane depolarization, reported in response to different stresses such as high light intensity and fungal elicitors. Because high levels of Ca2+ are toxic for the cell, the calcium influx into the cell has to be controlled.
Many different proteins have a role in keeping Ca2+ concentrations in the acceptable nano-molar range, usually by calcium binding or Ca2+ extrusion. Hence, Ca2+ binding proteins (like calmodulin or calcineurin) help to regulate the spatial and temporal release of Ca2+ and thereby have multiple roles in stress response. At the cellular level, calmodulins reduce the effective diffusion in the cytosol and restrict calcium to specific locations thereby regulating the activity .Consequently, calmodulin gene expression is induced by a variety of stresses like salinity, osmotic stress and wounding in different plants. Various findings have revealed that Ca2+ signals have specific characteristics: Ca2+ signals can be slow or fast, short- or long-lived and oscillations of the flux are of major importance for the specificity of the signal.
The “signature” changes in response to the stress and previous exposure to the stress. Hence, the magnitude, frequency, localization and duration of Ca2+ transients all specify the pathway in which Ca2+ signals participate. Another specific characteristic influencing the signal is the source of Ca2+: influx from extracellular space or internal stores like the vacuole can release Ca2+. For the induction of plant defence against pathogens, extracellular Ca2+ influx and the involvement of the specific Ca2+ channels on the Ca2+ signal appears to be crucial. Finally, the Ca2+ signal is influenced by channel guarding proteins and the localization of the channels. Taken together, alterations of Ca2+ transients and Ca2+ oscillation are mediated by many stresses and underline the importance of Ca2+ as a multi recognizer of stresses. Thus, it is quite clear that Ca2+ has a major role in cross-tolerance and deciphering the Ca2+ signatures could be a promising step towards the manipulation of cellular Ca2+ signalling in favour of more resistant crops.
Abscisic Acid
ABA is involved in many cellular processes like germination, gravitropism and guard cell mediated stomatal opening. Particularly well established is the regulatory function of ABA in water balance and osmotic stress tolerance. Under non-stress conditions, plants retain a low level of ABA, which increases in response to low temperature, drought and osmotic stress. Drought and salinity activate de novo ABA synthesis to prevent further water loss by evaporation through stomata, mediated by changes in the guard cell turgor pressure.
The mechanism involves fast changes in intracellular Ca2+concentration and stimulates further signalling in the cell. Under osmotic stress, ABA induces the accumulation of proteins involved in the biosynthesis of osmolytes (e.g., proline, trehalose), which increases the stress tolerance of plants. ABA or abiotic stresses, which lead to the accumulation of ABA can significantly impair biotic defence mechanisms.
However the positive effects of ABA application during biotic stress have also been reported. ABA is required for the multiple effects of ß-aminobutyric acid (BABA), a non-protein amino acid, which enhances resistance through potentiated defence responses, leading to a restriction of pathogen growth. ABA shows an overlap with other signalling networks, and the role of ABA might be based on the repression of SA responses. ABA antagonizes SA-mediated responses in a number of species, but might, on the other hand, positively regulate structural defence mechanisms, e.g., callose deposition and papillae formation.
Salicylic acid, Jasmonic Acid and Ethylene.
SA is a phenolic compound, which was identified as a major signal for accumulation of PR proteins in virus infected tobacco. Pathogen recognition also triggers various inducible systemic defenses in addition to the locally effective hypersensitive response. . In plant parts distant from the site of primary infection, systemic responses establish an enhanced defensive capacity against subsequent infection. This biologically induced resistance in systemic tissue is known as systemic acquired resistance (SAR) and has been shown to be effective in many plant species.
The attained state of resistance is long-lasting and effective against a broad spectrum of pathogens, including pathogenic bacteria, fungi and viruses. SAR requires both local and systemic salicylic acid (SA) accumulation and the induction of a subset of the pathogenesis-related (PR) genes, but SA itself is not the mobile signal. SA thus plays a major role in defence signalling and is essential for different defence mechanisms, such as SAR and hypersensitive responses (HRs). Stress acclimation has been observed for SA mediated stress responses. In plant defense research, PR genes serve as powerful molecular markers for the onset of SAR.
Although SA can be synthesized from phenylalanine, the predominant pathway for de novo SA biosynthesis during pathogen infection is through chorismate via isochorismate synthase (ICS).SA, synthesized from chorismate by means of ICS, is required for local and systemic acquired resistance responses. Non-expressor of pathogenesis related 1 (NPR1, also known as non-immunity 1 [NIM1]) is a central positive regulator of SAR signalling. NPR1 protein contains an ankyrin repeat and a BTB/ POZ domain and functions downstream of SA. NPR1 regulate PR gene expression through interaction with TGA transcription factors. PR proteins that are mainly associated with pathogen defence, especially PR-1, -2 and -5, are well characterised in pathogen response and confer resistance when expressed transient or constitutively. Yet, PR genes are also activated in abiotic stress: PR-3 protein, which functions as a chitinase, is also induced during cold stress and may function as an antifreeze protein.
A PR-1 gene of hot pepper (CABPR1) is induced by environmental stresses such as high salinity and low temperatures and by pathogen infections. In potato, PR-5 protein is induced during abiotic stress and responds to SA, ABA, salinity, wounding and pathogen stress. PR-10 proteins have been identified in various species and accumulate in response to pathogens (fungi) in pea, rice and potato, but also in pea under salinity and in white pine following wounding, fungal infection and cold-hardening.
Colonization of plant roots with selected strains of non-pathogenic, growth promoting rhizosphere bacteria can also provoke broad-spectrum disease resistance in plants. This type of induced resistance is called induced systemic resistance (ISR). ISR is less ‘broad spectrum’ in nature. In contrast to SAR, the ISR response does not require SA and generally requires jasmonic acid (JA) and ethylene (ET).
Jasmonate synthesis occurs through the octadecanoid pathway and begins with the release of linolenic acid from the chloroplast membrane. Attack by necrotrophic pathogens, as well as herbivorous insects, elicits the production of a large chemically diverse set of oxygenated fatty acids (oxylipins) that can be potent regulators of defense signalling. JA and precursors of JA that are synthesized during JA biosynthesis have an important function as signalling molecules in various processes, including plant defense. JA-dependent signalling proceeds through increased JA synthesis in response to pathogen attack and consequent increase in expression of defense effector genes such as PDF1.2.
The gaseous plant hormone, ethylene, is synthesized from the amino acid 1-aminocyclopropane-1-carboxylic acid (ACC), derived from methionine via S-adenosylmethionine (Adomet). ACCformation from Adomet is catalysed by ACC synthase. ACC oxidation to ethylene is catalysed by an oxygen-dependentACC oxidase. A large number of stresses (including wounding, flooding, chilling, high temperature and osmotic stresses) induce ethylene biosynthesis primarily bymodulating the expression of genes encoding ACC synthase. Some JA regulated genes are also regulated by ET. In the case of PDF1.2, induced expression requires both JA and ET.
In contrast, ET is not required for expression of the JA inducible gene VSP1. Defense responses dependent on SA are often effective against biotrophic pathogens, whereas defences dependent on JA are mostly effective against necrotrophic pathogens and insects. To achieve an effective state of resistance after recognition of the invader, plants are thought to fine-tune different defense signalling pathways by means of synergistic and antagonistic interactions. A synergistic effect was reported between SA-dependent SAR and rhizobacteria-mediated induced systemic resistance (ISR).
ISR is activated by root-colonizing, non-pathogenic fluorescent Pseudomonas spp. and provides broad spectrum resistance to pathogen attack. ISR signal transduction is dependent on an intact JA/ethylene response and, interestingly, requires the function of the regulatory protein NPR1. Thus, SAR and ISR signalling pathways are distinct in their requirement for SA and JA/ethylene, yet unite in their downstream requirement of NPR1. Besides signal synergy between SA- and JA-dependent defense responses, cases of antagonism between these two signalling molecules have also been reported. The JA-resisted tobacco hornworm Manduca sexta inflicted more damage on SAR induced tobacco plants compared to control plants. Moreover, tobacco plants silenced for the expression of the phenylpropanoid biosynthesis gene PAL (phenyl ammonia-lyase) exhibit reduced SAR against TMV, but exhibited enhanced resistance to insect infestation. Conversely, plants overexpressing PAL were more resistant to TMV, whereas resistance to insect attack was lost.
Plant responses to herbivores are broadly categorized as direct and indirect defences and tolerance. Compounds that exert repellent, antinutritive, or toxic effects on herbivores are commonly referred to as direct defences. Proteinase inhibitors (PI) (antidigestive proteins) are inducible by wounding and herbivory and influence herbivore performance by inhibiting insect digestive enzymes. Physical barriers such as leaf toughness and trichomes that increase plant fitness in the presence of herbivores come under direct defences. Toxic compounds (e.g., alkaloids, terpenoids, phenolics) that poison generalist herbivores also constitute a part of direct defense. Indirect defences are plant traits that attract predators and parasitoids of herbivores. Volatile organic compounds (VOCs) released by herbivore-attacked plants not only mediate interactions with herbivores and their predators and the damage induced volatiles can provide a signal that allows neighbouring plants to prepare for imminent herbivory. This process is called priming.
Protein/MAP kinases
A highly conserved signal mechanism between all eukaryotes is the phosphorylation of proteins by protein kinases. Ca2+ dependent protein kinases (CDPK), a class of serine/threonine kinases, and mitogen activated kinases (MAPK) are key signal transmitters in various stress responses. Specific MAPK cascades are activated by a range of stimuli including cold, drought, salinity, ROS, heat, shaking, wounding, pathogens and pathogen elicitors, ethylene, ABA and SA. MAPKs are central for the transduction of cellular signals by activation and repression of downstream target protein activity.
The mechanism of MAPK signalling cascades is usually three fold. Environmental factors stimulate a MAP kinase kinase kinase (MAPKKK), which phosphorylates kinase kinases (MAPKK). MPKK activates MAPK, which in turn activates downstream cellular proteins by phosphorylation. MAP kinases can be highly specific and distinct MAPKs are only activated by distinct stimuli. The MAPK activation by fungal and bacterial elicitors and SA is well-documented and MAPKs are involved in regulating a range of defence pathways.
Thus, defects can have a significant impact on plant defences and mutations in MAPKs can lead to de-repression of defence mechanisms such as enhanced SA accumulation, constitutive PR gene expression and growth inhibition. MAPK signalling is of equal importance in biotic and abiotic signalling, hence defects in MAPK mediated signalling affect not only pathogen related responses but also abiotic stress responses. Thus MAP kinase cascades constitute a major mechanism for activation of biotic defence responses and in mediating other signals, such as hormones in response to abiotic stresses like cold and drought, and osmotic stress.
Signalling networks are interconnected
Stress sensing is traditionally categorized in separate linear signal chains, but does in fact consist of a network of parallel and interacting signalling chains, which often share common components or have signalling convergence points. As a consequence, signalling compounds, which are classical abiotic or biotic stress signals, often induce non-related stress responses. ROS, Ca2+, ABA, SA, and Kinases serve as key components in mediating these signalling networks.
One situation where plants achieve cross tolerance is inherent tolerance of a plant species to a stress condition enables this plant to withstand other stresses, which share an overlapping stress potential. For example, effective antioxidative machinery, which allows the plant to cope efficiently with oxidative stress, would lead to enhanced tolerance to other stresses that involve oxidative stress. The other situation is stress acclimation: usually the exposure of plants to sub-lethal doses of a stress is referred to as acclimation. A typical example is acquired cold/freezing tolerance of plants following low temperature exposure. The understanding of signalling interactions involved in different stresses is complicated by the fact that signalling compounds can act as stress signals and as stress themselves.
The effects of these compounds is modulated by their absolute levels and a range of mechanisms is involved in keeping the intracellular concentrations in balance, as small changes result in drastically different responses. High levels of a single secondary messenger might be beneficial in one plant species, but can be harmful to another. The responses of plants are highly tuned to the absolute levels of these key compounds, distinguishing whether they induce defence gene activity or exhibit secondary stress (i.e. oxidation) leading to plant cell death. The intracellular concentration and localization of these signalling compounds can influence the effect of the stress and activate sensors responding to different stress intensities.
Conclusion
These signalling compounds and components exhibit vivid signal cross-talk and show an increasingly complex network of interacting signal pathways. If this limited number of signalling molecules or components can be used to combine different defence or stress tolerance responses, the cell can respond specifically to numerous signals. Given the amount of information that plants receive from different overlapping stress signals, it is intriguing to see how plants read between the lines of all this crosstalk and initiate appropriate cellular responses. Untangling this network of interconnected signal pathways and understanding the underlying mechanisms remains a challenge for the future that could help to develop more durable and resistant plants.
-The End-
Published date : 31 May 2014 05:34PM
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