Plant Stress Physiology Introductory article

David Rhodes, Purdue University, West Lafayette, Indiana, USA Article Contents Anna Nadolska-Orczyk, Breeding and Acclimatization Institute,Radzikow, Blonie, Poland . Stress Factors, Their Influence on Plant Metabolism, and Tolerance or Resistance to Stress ‘Stress’ in can be defined as any external factor that negatively influences plant . Physiological Correlations between Stress growth, productivity, reproductive capacity or survival. This includes a wide range of Sensitivity/Resistance and Reactions of Different Plants to Stress factors which can be broadly divided into two main categories: abiotic or environmental . Interactions Between Different Stresses in stress factors, and biotic or biological stress factors. Signalling and Metabolism . Integrative Approaches to Understanding Stress Stress Factors, Their Influence on Plant Responses . Ecophysiological Adaptations – Examples Metabolism, and Tolerance or Resistance to Stress Na 1 and Cl 2 inhibition of photosynthesis and protein synthesis, and displacement of Ca2 1 from membranes, Stress factors resulting in altered membrane permeability and K 1 The abiotic stress factors that most commonly influence leakage). Water excesses, or flooding stress, cause anoxia plant performance include deficiencies or excesses of water or hypoxia in the root zone; waterlogging fills soil pores (drought and flooding), extremes of irradiance, excessively and spaces, preventing oxygen diffusion to the root. The low or high temperature, or deficiencies or excesses of failure of waterlogged roots to undergo normal aerobic several nutrients, including macro- and micronutrients, metabolism leads to inhibition of protein synthesis, and 1 2 2 2 inhibition of ion acquisition and transport. This, coupled high salinity (i.e. excesses of Na ,Cl and/or SO4 ), and extremes of soil pH. Abiotic stresses may also include with alterations of water permeability of the roots in mechanical stresses (e.g. wind, hail, mechanical impedance flooded soils, often leads to macronutrient deficiencies in of root growth in compacted soils, and wounding), and the shoot, and ironically, shoot water deficits in response to stresses associated with toxic, manmade chemicals, includ- flooding. ing gaseous pollutants (sulfur dioxide, nitrogen oxides, Because higher plants rely on water loss via transpira- ozone), heavy metals and xenobiotics (e.g. herbicides). tion for leaf cooling, water deficit stress can predispose Biotic stresses include a wide range of plant pathogens plants to greater injury when exposed to high tempera- (bacteria, fungi and viruses) and herbivorous animals. tures. Water deficits prevent evaporative cooling and Because of the enormous diversity of these stresses, this associated heat loss by causing closure of stomates, which article will focus only on the physiology of plant responses dramatically restricts water loss, carbon dioxide fixation, to the abiotic stresses. and hence growth. Chilling stress, and a number of other environmental stresses that disrupt membrane-bound electron transport systems, can predispose plants to high Interaction of stress factors photon fluxes as a result of damage to the photosynthetic machinery. When such stress-injured leaves are exposed to In many cases the abiotic stresses do not occur indepen- high light intensities, this leads to production of toxic dently, and thus the stress environment may involve a active oxygen , including superoxide, hydrogen complex of interacting stress factors. For example, acid peroxide (H2O2), singlet oxygen and hydroxyl radical. stress may frequently be associated with aluminium Superoxide is formed by oxygen photoreduction by toxicity. Aluminium is abundant in many soils, and under photosystem I in the chloroplast. Superoxide is then 3 1 acidic conditions (pH 5 5.5) the phytotoxic species, Al , dismutated to H2O2. The hydroxyl radical can then form is solubilized to levels that inhibit plant growth. Both by the reaction of H2O2 with superoxide. Singlet oxygen salinity and freezing stresses can induce water deficits; it is forms by energy transfer from excited triplet state well established that a high concentration of dissolved chlorophyll to oxygen. The chloroplast is particularly solutes in the root zone lowers the soil water potential, susceptible to these active oxygen species, whose synthesis creating a situation similar to soil water deficits, and that is enhanced on exposure to excess excitation energy, slow freezing of plant tissue resulting in extracellular ice particularly when carbon fixation is impaired. These active formation will result in cellular dehydration. Consequently oxygen species denature proteins, damage nucleic acids there are some common osmotic stress features of water and cause lipid peroxidation. Injury from these active deficit, salinity and freezing stresses. However, salinity oxygen species is common with damage from pollutants stress involves both osmotic and specific ion effects (e.g. such as ozone, nitrogen oxides and sulfur dioxide, because

ENCYCLOPEDIA OF LIFE SCIENCES © 2002, John Wiley & Sons, Ltd. www.els.net 1 Plant Stress Physiology they generate active oxygen and other free radicals, with Facultative/inducible adaptations ultraviolet B (UV-B) radiation stress, and with post-anoxic injury, which occurs when roots are exposed to oxygen Many plant species, when exposed to a sublethal stress after a period of oxygen deficiency (waterlogging). (such as a heat shock, or a period of chilling, or exposure to water deficits), acquire tolerance and/or resistance to a subsequent stress that would be lethal or severely dama- Stress resistance versus tolerance ging to nonpretreated plants. This phenomenon is known It is evident from the brief discussion above that stresses as ‘acclimation’ or ‘hardening’. Acclimation responses are result in cellular injury or damage which frequently result generally distinguished by their stress inducibility from the from impaired or perturbed metabolism. The production inherent, constitutively expressed tolerance or resistance of damaging active oxygen species as a consequence of mechanisms or ‘adaptations’ discussed above. However, impairments in the photosynthetic apparatus is an the terminology is often used loosely in the literature. In excellent example of this. Stress damage or injury is our opinion, metabolic acclimation responses can best be typically referred to as a ‘strain’. By analogy with regarded as ‘facultative metabolic adaptations’. Several engineering terminology, stress resistance 5 stress/strain. examples of facultative metabolic adaptations of plants to The smaller the strain incurred by a specific stress, the different stresses which are proposed to confer increased greater the stress ‘resistance’. The term stress ‘tolerance’ stress resistance/tolerance are summarized in Table 1. implies that the development of stress-induced strains can Asterisks denote that confirmatory evidence for the be tolerated and/or actively repaired during the stress or proposed function(s) have been obtained with mutants following stress relief. and/or transgenic plants. An example of a morphological acclimation to anaero- bic stress exhibited by certain plant species is the production of intracellular gas channels (aerenchyma) which improve aeration of the root. Aerenchyma forma- Physiological Correlations between tion accelerates the transfer of oxygen from the aerial Stress Sensitivity/Resistance and tissues to the oxygen-deficient tissues of the stem base and root; however, in some flooding-tolerant species aerench- Reactions of Different Plants to Stress yma formation is constitutive, occurring in the absence of oxygen deficiency. An example of a facultative metabolic Constitutive adaptations adaptation to anaerobic stress is the coordinated induction Many plants exhibit constitutively expressed traits that are of glycolytic enzymes and enzymes of lactate and ethanol recognized to confer resistance or tolerance to environ- fermentation which allow continued energy production mental stresses; these are frequently referred to as during anoxia or hypoxia (Table 1). Phosphate starvation ‘adaptations’. Examples of morphological adaptations to may lead to a host of facultative metabolic adaptations heat, light and water deficit stresses might include leaf including induction/derepression of phosphate transpor- hairiness and waxiness which influence light absorption ters and alterations of pathways of glycolytic carbon flow and hence heat balance. An example of a combined and mitochondrial respiration to circumvent the adeny- late- and inorganic phosphate (Pi) dependent reactions of anatomical and metabolic stress adaptation is C4 photo- synthesis, which reduces photorespiration, leading to a respiration, facilitating continued respiration in Pi de- higher temperature optimum for photosynthesis and prived plants (Table 1). Osmotic adjustment, the active accumulation of solutes in response to drought or salinity higher water use efficiency in comparison to C3 photo- stress, lowering solute potential, and facilitating main- synthesis. C4 photosynthesis requires spatial separation of distinct metabolic functions between discrete mesophyll tenance of turgor, is an example of a facultative metabolic and bundle-sheath cells of the leaf (see ‘Ecophysiological adaptation to drought and salinity stress (Table 1). The Adaptations – Examples’, below). Constitutive - induction of heat-shock proteins, which may play roles as cean acid metabolism (CAM) allows for noctural carbon molecular chaperones, stabilizing proteins that are in a dioxide fixation into malate, and photoprotection as a heat-induced unstable state, is an example of a facultative result of high intracellular carbon dioxide concentrations metabolic adaptation to high temperature stress that generated by decarboxylation of malate during the day confers a higher level of thermotolerance, i.e. protection to a subsequent heat treatment that would be lethal to when stomates are closed. Like C4 metabolism, CAM suppresses photorespiration, increasing the efficiency of nonacclimated plants. There is growing evidence for cross- protection between heat stress, dehydration/drought, cold/ photosynthesis at high temperatures in comparison to C3 plants. The CAM pathway requires strict temporal control chilling/freezing, heavy metal stress and oxidative stress in over malate synthesis and decarboxylation coordinated plants. with control over stomatal aperture (see ‘Ecophysiological Certain facultative metabolic adaptations may in part Adaptations – Examples’). serve to afford protection against injury when constitutive

2 Plant Stress Physiology

Table 1 Examples of facultative metabolic adaptations to environmental stresses in plants, and their proposed functions Stress Facultative metabolic adaptation Proposed function(s) Aluminium toxicity Excretion of organic acids (malate, citrate, Organic acids chelate toxic Al3 1 , oxalate) preventing its absorption* Anaerobic (flooding) Induction of glycolytic enzymes and Provides energy under O2 deficiency; enzymes of alcohol and lactate ADH and LDH recycle NAD 1 from fermentation (e.g. alcohol and lactate NADH required to sustain glycolysis; dehydrogenases (ADH and LDH, ADH may divert flux away from lactate, respectively)) delaying cytoplasmic acidification* Anaerobic (flooding) Low pH and/or Ca2 1 /calmodulin GABA synthesis is H 1 -consuming, and activation of glutamate decarboxylase, may function in delaying cytoplasmic leading to g-aminobutyrate (GABA) acidosis under anaerobic stress accumulation Chilling Induction of cold-responsive genes and Cold-responsive proteins may protect genes encoding antifreeze proteins membranes from freezing/dehydration induced injury*; antifreeze proteins may inhibit intracellular ice formation Chilling Induction of fatty acid desaturases Increased unsaturated fatty acid content of membranes may restore membrane fluidity and function at low temperatures* Heat Induction of heat-shock proteins (HSPs) HSPs may serve as molecular chaperones, interacting with proteins rendered in an unstable state by heat stress, conferring thermotolerance* Heat Increased synthesis of isoprene Isoprene may stabilize photosynthetic membranes at high temperatures Heavy metals Cadmium activation of phytochelatin Phytochelatins (PCs) may chelate and synthase; copper induction of detoxify cadmium*, metallothioneins metallothioneins may chelate and detoxify copper High light Activation of a de-epoxidase that leads to Zeaxanthin may serve to quench synthesis of zeaxanthin from chlorophyll fluorescence under intense violaxanthin illumination, preventing overexcitation and photodamage Phosphate (Pi) deficiency Induction/derepression of Pi transporters; May facilitate increased Pi uptake; may activation of alternative respiratory permit respiration via adenylate- and Pi- pathways independent pathways K 1 deficiency, osmotic and Induction of polyamine synthesis, Polyamines may function in charge acid stresses primarily via induction/activation of balance in K 1 -deficient plants; arginine decarboxylase, leading to synthesis of putrescine may play a role in increased putrescine synthesis intracellular pH regulation Numerous abiotic stresses Induction of superoxide dismutase, May function to scavenge reactive oxygen ascorbate peroxidase, catalase, and species* monodehydroascorbate, dehydroascorbate and glutathione reductases Salinity and water deficits Osmotic adjustment (OA); active OA may restrict dehydration, and accumulation of solutes, including facilitate turgor maintenance*; organic compatible organic solutes (sugars, solutes may function as osmo- and polyols, amino acids and onium cryoprotectants*; certain organic compounds) solutes may also serve as antioxidants*

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Table 1 – continued Water deficits Induction of dehydrins and late- May stabilize membranes and/or proteins embryogenesis abundant (LEA) during desiccation* and freezing proteins Xenobiotics Induction of glutathione S-transferases GSTs may conjugate the xenobiotics, (GSTs) rendering them nontoxic*

* An asterisk denotes that the proposed function has been corroborated with mutants and/or transgenic plants. defence mechanisms are overwhelmed. For example, Plants, like other organisms, must perceive environ carotenoid pigments and the action of the photorespira- mental signals via specific receptors; these receptors then tory carbon oxidation (PCO) cycle in C3 plants, afford a trigger a cascade of events leading to modification of cellular basal level of protection against excess light energy by or metabolic activity, including regulation of the expression rapidly quenching the excited state of chlorophyll, and of specific genes. Ion channels, intracellular signalling dissipating excess adenosine triphosphate (ATP) and proteins and second messengers play a key role in these reducing power from the light reactions of photosynthesis, signal transduction cascades. Although specific environmen- respectively. These constitutive defence mechanisms are tal stress receptors have not yet been identified in plants, it is supplemented by facultative/inducible mechanisms; a light reasonable to postulate that they may be membrane bound, activation of a de-epoxidase that converts violaxanthin to as in other eukaryotes. Osmotic stresses may compress or zeaxanthin via antheraxanthin, which may serve to quench stretch membranes to modulate directly or indirectly the chlorophyll fluorescence, and the stress induction of activity of proton pumps and ion channels. Chilling, freezing enzymes (e.g. superoxide dismutase, ascorbate peroxidase, and heat stresses directly influence membrane fluidity. monodehydroascorbate reductase, dehydroascorbate re- Putative temperature stress sensors may detect such mem- ductase and glutathione reductase) that function to brane fluidity or conformational changes. These sensors are scavenge reactive oxygen species (Table 1). Ascorbate postulated to be either histidine kinases or Ca2 1 channels. provides the electrons for violaxanthin de-epoxidase, and Early events in the response of plant cells to many so is particularly important in these facultative defensive environmental stimuli are known to involve membrane tiers in the chloroplast. Only when all these defence systems depolarization and elevations of cytosolic Ca2 1 .Calcium are overwhelmed, may injuries result. The D1 protein of may in turn regulate a host of enzyme activities via calcium- photosystem II (PS-II) is known to be particularly dependent protein kinases and calmodulin. Calcineurin (a susceptible to reactive photoproducts, and its photo- Ca2 1 - and calmodulin-dependent protein phosphatase) has damage may be the primary cause of photoinhibition in recently been shown to play an important role in salinity stressed leaves. Damaged D1 proteins must be excised stress signalling and salt tolerance in plants. from the PS-II reaction centres, and replaced with newly synthesized D1 proteins. This repair will be inhibited if protein synthesis is impaired in stressed leaves. Abscisic acid and stress signalling One of the best studied examples of the cascade of signalling events in stress responses in plants is the process of stomatal closure, mediated by elevated concentrations Interactions Between Different Stresses of the plant stress hormone, abscisic acid (ABA), in guard in Signalling and Metabolism cells. ABA is synthesized from carotenoid precursors in both leaves and roots; its synthesis is greatly stimulated by Abiotic stress perception water deficits and other stresses. ABA synthesis increases markedly in the roots that are in contact with drying soil, As alluded to above, different stresses can interact in and is transported from the root to the shoot via the xylem. complex ways to lead to a wide variety of metabolic This transport can occur before the low water potential of responses, including both facultative metabolic adapta- the soil causes any measurable decrease in water potential tions that may afford stress protection, and metabolic of the leaf. Under normal conditions the xylem sap is impairments or injuries; e.g. inhibition of photosynthesis, slightly acidic, favouring the uptake of the unassociated inhibition of protein synthesis, and protein and membrane form of ABA by the mesophyll cells. During the early damage. Many of the facultative metabolic adaptations stages of water stress, however, the pH of the xylem involve coordinated changes in gene expression in response increases. This alkalinization favours formation of the to environmental stimuli (Table 1). How do plants perceive dissociated form of ABA, which is less readily taken up by these environmental signals and transduce them into the mesophyll cells; thus, more ABA reaches the guard changes of gene expression and/or metabolic rate? cells. Activation of ABA receptors in guard cells leads to

4 Plant Stress Physiology

Ca2 1 -dependent and Ca2 1 -independent signalling events Ethylene and stress signalling that regulate ion channel activity in complex ways. The rapid activation of nonselective ion channels by ABA The gaseous plant hormone, ethylene, is synthesized from triggers membrane depolarization and allows Ca2 1 influx the amino acid 1-aminocyclopropane-1-carboxylic acid from the extracellular space. The increase of Ca2 1 and the (ACC), derived from methionine via S-adenosylmethio- production of other signalling intermediates, such as nine (Adomet). ACC formation from Adomet is catalysed inositol 1,4,5-trisphosphate, then triggers Ca2 1 release by ACC synthase. ACC oxidation to ethylene is catalysed from internal stores. Membrane depolarization, elevated by an oxygen-dependent ACC oxidase. A large number of cytosolic Ca2 1 and other signalling events activate anion stresses (including wounding, flooding, chilling, high channels resulting in anion release and long-term mem- temperature and osmotic stresses) induce ethylene bio- brane depolarization. K 1 efflux through voltage-depen- synthesis primarily by modulating the expression of genes dent outward K 1 channels is driven by membrane encoding ACC synthase. ACC synthase has a short half- depolarization and enhanced by ABA-induced cytosolic life, and is the rate-limiting enzyme in ethylene biosynth- 1 2 esis. Ethylene production in response to hypoxia is alkalinization. Loss of K and a parallel loss of Cl and 2 1 malate and/or conversion of malate to starch, leads to loss implicated in the Ca -dependent signal transduction of guard cell turgor, and stomatal closure. Activation of pathway leading to cell death and lysis in the root cortex anion channels may involve Ca2 1 -independent signalling resulting in aerenchyma formation. Ethylene regulates pathways. Phosphorylation is implicated in the activation numerous genes possessing ethylene response elements. of anion channels. The ABA-insensitive loci – abi1 and This cascade likely involves an ethylene receptor, ETR1, a abi2 –inArabidopsis encode type 2Cprotein phosphatases, transmembrane protein that is similar to prokaryotic two- which may function as negative regulators in ABA component histidine kinases. Binding of ethylene to this signalling. Protein farnesylation, a posttranslational mod- receptor is proposed to result in inactivation of a negative ification process, which mediates the COOH-terminal regulator, CTR1, a member of the Raf family of protein lipidation of specific cellular signalling proteins, has kinases. The inactivation of CTR1 then allows a channel- recently been implicated in the ABA signal transduction like transmembrane protein, EIN2, to become active. pathway in plants. Arabidopsis mutants lacking the Activation of EIN2 may ultimately lead to the activation farnesyltransferase (FTase) b-subunit exhibit ABA hyper- of a transcription factor, EIN3, that alters gene expres- sensitivity of guard cell anion channel activation and of sion. It seems likely that other putative ethylene receptors stomatal closure. The FTase may function downstream to that have been identified (ERS, ETR2 and EIN4) in the ABI protein phosphatases. plants, may have similar functions to ETR1; all three ABA regulates the expression of a large number of genes share sequence similarity to ETR1 and have a putative that possess ABA response elements. Desiccation/dehy- histidine protein kinase transmitter module. The different dration may lead to induction of these genes as a result of receptors may have tissue- or stage-specific functions. water stress-induced ABA accumulation. Many of these desiccation/ABA responsive genes are also responsive to Oxidative stress signals heat shock, chilling temperatures and salinity stress. Interacting ABA-dependent and -independent signal There is increasing evidence that the reactive oxygen transduction pathways have been identified in both cold species, hydrogen peroxide (H2O2), is perceived by plants and osmotic signal transduction in plants. ABA appears to as a signal of environmental change, acting as a diffusible play a role in inducing freezing tolerance; thus, the ABA- signal-transduction molecule to alert metabolism to both insensitive mutant (abi1)ofArabidopsis is unable to abiotic and biotic stresses. As discussed above, increased undergo low-temperature acclimation to freezing. Cold production of active oxygen species is a common response acclimation in part involves induction of cold-responsive of plants to a number of environmental stresses that result genes that encode hydrophilic polypeptides that may help in impaired carbon dioxide assimilation at high light to stabilize membranes against freeze-induced injury. intensities. Pathogen attack also elicits an oxidative burst These cold-responsive genes are regulated by the transcrip- of superoxide production via activation of a neutrophil tional activator, CBF1, which binds to a DNA regulatory NADPH oxidase (plasmalemma-bound NADPH-depen- element that stimulates transcription in response to both dent superoxide synthase). Apoplastic superoxide dis- low temperature and water deficits. Constitutive over- mutase rapidly converts this superoxide to H2O2.Cell expression of CBF1 induces expression of the cold- death results if H2O2 production overwhelms the anti- responsive/dehydration-responsive genes and results in oxidative defence mechanisms (catalase, ascorbate per- enhanced freezing tolerance. The cold-induced proteins are oxidase, monodehydroascorbate reductase, dehydroas- homologous to the ABA and desiccation inducible late- corbate reductase, glutathione peroxidase, glutathione embryogenesis abundant (LEA) proteins that are thought reductase and glutathione S-transferase). Cell death to play a role in dehydration tolerance of seeds, but which triggered by the oxidative burst plays a key role in can also potentially contribute to freezing tolerance. pathogen defence. There is growing evidence that osmotic

5 Plant Stress Physiology and UV-B stresses may also activate neutrophil NADPH Ecophysiological Adaptations – oxidase, suggesting that certain abiotic and biotic stresses may share a common signalling pathway. Examples The best studied examples of ecophysiological adaptations are those of carbon assimilation in plants. We will briefly Integrative Approaches to compare and contrast C3,C4 and crassulacean acid metabolism (CAM) modes of photosynthesis to point out Understanding Stress Responses key features of these carbon assimilation pathways that determine relative fitness of C ,C and CAM plants for Rapid advances in plant molecular genetics now permits 3 4 growth in cool/temperate versus hot/arid environments. detailed dissection of the signal transduction pathways involved in stress perception (e.g. the salt, osmotic, cold, ABA and ethylene signal transduction pathways de- C3 photosynthesis scribed above). These advances also permit the modula- tion of the activity or abundance of specific transcription C3 plants assimilate carbon dioxide (CO2) by the Calvin factors, regulatory proteins, transport proteins, proteins cycle; the first reaction of this cycle is the condensation of that are implicated in membrane or protein stabilization, CO2 with ribulose-1,5-bisphosphate to generate two or key enzymes of metabolism, in transgenic plants, in molecules of 3-phosphoglycerate, catalysed by ribulose- order to determine their effects on plant growth during or 1,5-bisphosphate carboxylase/oxygenase (Rubisco). As after recovery from a specific stress treatment. This the name implies, Rubisco also has an oxygenase function, progress is clearly shifting the balance of evidence away and will utilize oxygen (O2) in place of CO2 as substrate, from correlations or associations between physiological generating 2-phosphoglycolate. This must be recycled to 3- responses to specific stresses, to firmer cause–effect phosphoglycerate via a complex set of reactions known as relationships between response and resistance/suscept- the photorespiratory carbon oxidation (PCO) cycle. The ibility. The enhanced expression of cold-responsive PCO cycle results in CO2 loss and O2 consumption, and genes in response to cold temperature stress, discussed expends energy. In normal air, at 258C, the ratio of above, is an excellent example of a physiological carboxylation to oxygenation catalysed by Rubisco is response that has long been recognized as correlated between 2.5 and 3.0. However, as the temperature with the acquisition of cold (freezing) tolerance, and/or increases, the concentration of CO2 in solution in dehydration tolerance. Modulation of the expression of equilibrium with air decreases more than does the the transcriptional activator, CBF1, now provides direct concentration of O2; consequently the ratio of CO2:O2 evidence that these changes in gene expression do indeed decreases as the temperature rises, driving greater flux via contribute to increased freezing tolerance. Several other the PCO cycle. Photorespiration therefore results in examples of these advances are noted with asterisks in inefficiency of CO2 fixation, particularly at high tempera- Table 1. tures. However, as noted earlier, it may play an important Although genetics and molecular biology are becoming role in protecting C3 plants against high light intensities at increasingly important tools in our understanding of low CO2 concentrations (e.g. during water stress when plant stress responses, the tools of electrophysiology, stomates are closed), functioning to dissipate excess ATP enzymology, analytical biochemistry and classical plant and reducing power from the light reactions of photo- physiology remain key to interpreting the consequences synthesis, preventing damage to the photosynthetic of specific genetic interventions on metabolism, ion fluxes, apparatus (photooxidation and photoinhibition). compartmentation, water relations, growth and overall plant fitness. More than ever before, integrative, inter- C photosynthesis disciplinary approaches are needed to probe the under- 4 lying mechanisms, and the complex interactions in stress C4 plants suppress photorespiration by concentrating CO2 signalling and metabolism. Although model organisms, in bundle-sheath cells where Rubisco and other enzymes of such as Arabidopsis, have been central to the rapid the Calvin cycle are exclusively localized. Fixation of CO2 advances described above, it is clear that the plant occurs via the carboxylation of phosphoenolpyruvate kingdom exhibits an enormous diversity of stress adapta- (PEP) in the mesophyll cells, to form the C4 acid, tions. Continued work on plant species uniquely adapted oxaloacetate, which is then converted to either malate or to environmental extremes is essential if we are to aspartate, transported to the bundle-sheath cells and appreciate the full spectrum of adaptive strategies decarboxylated to generate the CO2 for assimilation via employed by plants. the Calvin cycle. A C3 acid (pyruvate) formed from the aforementioned decarboxylation is then recycled to the mesophyll cells to regenerate the PEP which serves as CO2 acceptor in the cycle. Because the CO2 concentrating

6 Plant Stress Physiology

mechanism suppresses photorespiration, C4 plants are form of the enzyme is dephosphorylated, and is sensitive to more efficient than C3 plants in CO2 fixation at high malate. Certain plant species (e.g. Mesembryanthemum temperatures. Moreover, PEP carboxylase has a higher crystallinum) exhibit facultative CAM, i.e. a switch from 2 affinity for HCO3 than Rubisco has for CO2, enabling C4 C3 to CAM in response to salinity or drought stress; this plants to reduce stomatal aperture and conserve water requires coordinated expression of CAM genes, including while maintaining high CO2 assimilation rates. C4 plants the gene encoding PEP carboxylase. are therefore better adapted to drier, hotter climates than C3 plants, and show greater water-use efficiency. However, C4 metabolism has a higher energy requirement than C3 Further Reading photosynthesis, consuming two extra molecules of ATP Drew MC(1997) Oxygen deficiency and root metabolism; injury and per CO2 fixed. This renders C4 plants less efficient in CO2 acclimation under hypoxia and anoxia. Annual Review of Plant fixation, particularly at low temperatures where the PCO Physiology and Plant Molecular Biology 48: 233–250. cycle is suppressed in C3 plants. Consequently C3 plants Kieber JJ (1997) The ethylene response pathway in Arabidopsis. Annual have a competitive advantage over C4 plants in cool, Review of Plant Physiology and Plant Molecular Biology 48: 277–296. temperate climates. Lamb Cand Dixon RA (1997) The oxidative burst in plant disease resistance. Annual Review of Plant Physiology and Plant Molecular Biology 48: 251–275. Crassulacean acid metabolism Levitt J (1980) Responses of Plants to Environmental Stress, vols 1 and 2, 2nd edn. New York: Academic Press. Crassulacean acid metabolism (CAM) is another mechan- Leung J and Giraudat J (1998) Abscisic acid signal transduction. Annual ism of concentrating CO2 at the site of action of Rubisco Review of Plant Physiology and Plant Molecular Biology 49: 199–222. that is found in numerous angiosperm species adapted to Murata N and Los DA (1997) Membrane fluidity and temperature perception. Plant Physiology 115: 875–879. hot, arid environments. In CAM plants, CO2 is captured by PEP carboxylase at night when stomates are open, Noctor G and Foyer CH (1998) Ascorbate and glutathione: keeping active oxygen under control. Annual Review of Plant Physiology and generating oxaloacetate from which malate is then derived Plant Molecular Biology 49: 249–279. and stored in the vacuole. During the day when stomates Nuccio ML, Rhodes D, McNeil SD and Hanson AD (1999) Metabolic are closed, malate is transported to the chloroplast, engineering of plants for osmotic stress resistance. Current Opinions in releasing CO2 for assimilation via the Calvin cycle. The Plant Biology 2: 128–134. elevated CO2 concentrations achieved by this mechanism Roberts DM and Harmon A (1993) Calcium-modulated proteins: targets of intracellular calcium signals in higher plants. Annual Review effectively suppress photorespiration, as in C4 plants. Closure of stomates during the day greatly restricts both of Plant Physiology and Plant Molecular Biology 43: 375–414. Scho¨fflF,Pra¨ndl R and Reindl A (1998) Regulation of the heat shock water and CO2 losses. CAM plants have specialized response. Plant Physiology 117: 1135–1141. mechanisms to cope with the high leaf temperatures that Smirnoff N (ed.) (1995) Environment and Plant Metabolism: Flexibility can occur at peak irradiances in the absence of transpira- and Acclimation. Oxford: Bios Scientific. tional cooling. They rely on re-emission of infrared Taiz L and Zeiger E (1998) Plant Physiology, 2nd edn. Sunderland, MA: radiation and heat loss by convection and conduction for Sinauer Associates. leaf cooling. A key regulatory mechanism in CAM Thomashow MF (1998) Role of cold-responsive genes in freezing metabolism is the diurnal phosphorylation and depho- tolerance. Plant Physiology 118: 1–7. Ward JM, Pei Z-M and Schroeder JI (1995) Roles of ion channels in sphorylation of PEP carboxylase. The active night form of initiation of signal transduction in higher plants. Plant Cell 7: 833–844. PEP carboxylase is phosphorylated; this phosphorylated Zielinksi RE (1998) Calmodulin and calmodulin-binding proteins in form of the enzyme is insensitive to malate, permitting the plants. Annual Review of Plant Physiology and Plant Molecular Biology large accumulation of malate at night. The inactive day 49: 697–725.

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