Hydrogen peroxide plays multiple roles in plant it is more

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Hydrogen peroxide plays multiple roles in plant it is more

    Hydrogen Peroxide in Plants: A Versatile Molecule of Reactive Oxygen Species Network

    122 Li-Juan Quan, Bo Zhang , Wei-Wei Shi and Hong-Yu Li*

     (12. MOE Key Laboratory of Arid and Grassland Ecology, School of Life sciences, Lanzhou University


*Author for correspondence.

    Tel: +86 (0)13519640428;

    Fax : +86(0)931 891 2561;

    E-mail: lihy

Supported by the National Natural Science Foundation of China (30170238; 30670070)


    Plants often face the challenge of severe environmental conditions, which include various biotic and abiotic stresses, all of which exert adverse effects on plant growth and development. With the evolution of plants, Plants have evolved complex regulatory mechanisms in adapting to various environmental stressors, One of the consequences of much stress is an increase in the cellular concentration of reactive oxygen speciesROS(,

    which is subsequently converted to hydrogen peroxideHO(. Even under normal conditions, higher plants 22

    produce ROS during the metabolic process. Excess concentrations of ROS results in oxidative damage to or

    the apoptotic death of cells, Development of an antioxidant defense system in plants protects them against oxidative stress damage. ROS and, more particularly, HO plays versatile roles in plant normal 22

    physiological processes and resistance to stresses. Recently, HO has been regarded as a signaling molecule 22

    and regulator of the expression of some genes in cells. This review describes various aspects of HOfunction, 22

    generation and scavenging, genes regulation and the crosslink with other physiological functional molecules during plant growth, development and resistance responses.

Key words: antioxidant system; gene regulation; hydrogen peroxide (HO); reactive oxygen species (ROS); 22

    signaling molecule

Abbreviation: ABA, abscisic acid; APX, ascorbate peroxidase; CaM, calmodulin; CAT, catalase; CDPKs,

    cacium-dependent protein kinases; DHA, dehydroascorbate; DHAR, dehydroascorbate reductase; GPX,

    glutathione peroxidase; GR, glutathione reductase; GSH, glutathione; GSSG, glutathione reductase; HO22,

    .hydrogen peroxide; HR, hypersensitive reaction; iNOS, inducible nitric oxide synthase; OH, hydroxyl radical; JA,

    Jasmonic acid; MAPKs, mitogen-activated protein kinases; MAPKKKs, mitogen-activated protein kanase kinase

    -kinases; MDHA, mondehydroascorbate reductase; NO, nitric oxide; NOS, nitric oxide synthase; O, superoxide 2

     radical;ROS, reactive oxygen species; SA, salicylic acid; SAR, systemic acquired resistance; SOD, superoxide

    dismutase; UV, ultra-violet.

As a kind of reactive oxygen species (ROS), hydrogen peroxide (HO) has been given much 22

    attention during the last decades. Ample evidence has proven that HO plays an important role in 22

    plants under severe environmental conditions, which include various biotic and abiotic stresses (Dat et al. 2000). HO participates in many resistance mechanisms, including reinforcement of 22

    the plant cell wall, phytoalexin production, and enhancement of resistance to various stresses (Dempsey and klessig 1995). Recently, HO has also been shown to act as a key regulator in a 22

    broad range of physiological processes such as senescence (Peng et al. 2005), photorespiration

    and photosynthesis (Noctor and Foyer 1998a), stomatal movement (Bright et al. 2006), cell cycle

    (Mittler et al. 2004), and growth and development (Foreman et al. 2003). To some extent, excess

    HO accumulation can lead to oxidative stress in plants, which then triggers cell death. The 22

    evolution of all aerobic organisms is dependent upon the development of efficient

    HO-scavenging mechanisms (Arora et al. 2002), Enzymes, including superoxide dismutase 22

    (SOD), catalase (CAT), peroxidase (POD), ascorbate peroxide (APX) and glutathione reductase (GR) (Zhang et al. 1995; Lee and Lee 2000), and nonenzymatic antioxidants such as tocopherols, ascorbic acid (AsA), and glutathione (GSH) (Wingsle and Hallgren 1993; Kocsy et al. 1996;

    Noctor et al. 1998) work in concert to detoxify HO. Sustaining the HO concentration at an 2222

    appropriate level can promote plant development and reinforce resistance to environment stressors. HO modulates the expression of various genes (Neill et al. 2002). The HOinduced transcripts 2222

    encoded proteins with functions such as metabolism, energy, protein destination and transport, cellular organization and biogenesis, cell rescue of defense, and transcription ( Desikan et al.

    2001a).Among these genes, the genes encoding potential transcription factors should be emphasized due to their capacity for activating the expression of downstream target genes ( Desikan et al. 2001a). Using cDNA microarray technology, A large-scale analysis of gene transcription has been undertaken looking in Arabidopsis and tobacco during oxidative stress (Desikan et al. 2001a; Vandenabeele et al.2003Vanderauwera et al. 2005).More studies have

    provided evidence that HO itself is a key signal molecule mediating a series of responses 222+(Desikan et al. 2003) and activating many other important signal molecules (Ca, SA, ABA, JA,

    ethylene, NO) of plants (Gundlach et al. 1992; Dempsey and Klessig 1995; Liu et al. 2004;

    Desikan et al.2004; Wendehenne et al. 2004). These signal molecules function together and play a complex role in signal transduction of resistance responses, and growth and development in plant.

    This article describes various aspects of HOfunction, generation and scavenging, gene 22

    regulation and crosslinks with those physiological functional molecules during plant growth, development and resistance responses.

Origin of HO 22

    Since the oxygen molecule (O) emerged in earth, it is usually said to be the final electron receptor 2

    during the biology respiration. Recently, studies have estimated that 1% of O consumed by plants 2

    is diverted to produce reactive oxygen species (ROS) in various sub-cellular loci (Bhattacharjee 2005).

    Reactive oxygen species (ROS), a collective term for radicals and other non-radical but reactive species derived from the oxygen molecule (O), has been implicated in numerous developmental 2

    and adaptive responses in both animal and plant cells (Dypbukt et al. 1994; De Marco and

    Roubelasis-Angelakis 1996; Lamb and Dixon 1997). The earliest report about ROS production in plants is that challenged potato with incompatible P. infestant lead to reduction of cytochrome C

    that induces a hypersensitive reaction (HR) involving active defense reaction, and the reaction can be inhibited by SOD (Doke 1983a,b). The kinds of ROS have been investigated in plant including

    . 1-hydrogen peroxide (HO), superoxide anion (O), hydroxyl radicals (OH), singlet oxygen (O) 2222..-and nitric oxide (NO) by far, HO, O , OH can transform themselves into each other (figure 1). 222+ H(cell wall)Hydrogen peroxide (HO)22superoxide anionSOD2e-(cytosol (O)2Hydrogen peroxidemitochondriaoxygen moleculechloroplast) (HO)22) (O2PKCHydrogen peroxideO) (H22


    (Feton reaction)Hydroxyl radicals.OH) ( -Figure 1. Transition between the oxygen molecule (O), superoxide anion (O), hydrogen peroxide (HO) and 2222

    .hydroxyl radicals (OH).

    -- During oxidative burst, O is reduced to O, and then the Oundergoes spontaneous dismutation at a higher rate 222

    -and at acidic pH, which is also found in the cell wall (Sutherland 1991). O is also catalyzed by superoxide 2

    dismutase (SOD) enzymes, which occur in the cytosol, chloroplasts, and mitochondria (Scandalios 1993), Ocan 2

    also be reduced to HO by protein kinase C (PKC) (Juan et al. 2004), PKC exists in all organelle of plants (Juan et 22 2004). HO reacts with Fe leading to the HO-dependent formation of OH (Arora et al. 2002). 2222

     It has been estimated that both resistance responses to stresses and normal physiological

    -metabolism can lead to ROS production (Van Breusegem et al. 2001). By comparison, O and 2

    HO are weaker oxidizing agents. Under normal condition, the half-life of HO is probably 1ms, 2222-.and other forms of ROS, including superoxide anion (O), hydroxyl radicals (OH) and singlet 21oxygen (O), their half-life are very short, about 2-4 µs (Bhattacharjee 2005). Excess HO leads 222

    to oxidative stress and is capable of injuring cells. During the course of evolution, plants were able to achieve a high degree of control over HO accumulation (Droge 2002). Recent investigations 22

    revealed that ROS, especially HO is a central component of the signal transduction cascade 22

    involved in plant adaptation to the changing environment (Neill et al. 2002). HO participates in 22

    the physiological metabolism of plant and activate defense responses to various stresses. HO is 22 beginning to be accepted as a second messenger for signalsgenerated by means of ROS because

     of its relatively long lifeand high permeability across membranes (Neill et al. 2002; Huang et al.

    2002; Yang and Poovaiah 2002).

Versatile roles of HO 22

    Hydrogen peroxide (HO) plays a dual role in plants: at low concentrations, it acts as a signal 22

    molecule involved in acclimatory signaling triggering tolerance against various abiotic and biotic

    stresses (Laloi et al. 2004; Fukao and Bailey-Serres 2004; Mittler et al. 2004). And, at high

    concentrations, it orchestrates programmed cell death (Dat et al. 2000)

    HO takes part in resistance mechanism, reinforcement of plant cell wall (lignification, 22

    cross-linking of cell wall structural proteins) phytoalexin production and resistance enhancement

    (Dempscy and Klessig 1995). In plant-microbe interaction, HO production in plants can kill 22

    the pathogen directly or induces defense genes to limit infection by the microbe. HO can be used 22

    as a marker in tobacco leaves for testing the occurrence of plant basal defense reactions (Bozso et

    al. 2005). Under other stress conditions, which include UV-radiation, salt stress, drought stresses,

    light stress, metal stress, high or low temperature and so on. HO production in plants induces 22

    resistance to various stresses and protects itself from being hurt. Recently, its been suggested that

    HO is not only a defensive signal molecule but it also functions as a signal molecule during plant 22

    growth and development. Evidence suggested that HOproduction plays a key role in separating 22

    and culturing of protoplast during reproduction of tobacco protoplast (Papadakis and Roubelasis-Angelakis 2002). Using a luminescence probe one can check HO accumulation in 22

    the germinating of radish seeds (Schopfer et al. 2001). HOcan also regulate the plant cell cycle. 22

    Treated tobacco with fungi elicitor produced HO and activated MAPK protein (Suzuki et al. 22

    1999). MAPK as a key signal protein regulates the cell cycle. The links between the HO cell 22

    cycle are orthologous protein of MAPKKK, ANP1 and NPK1 (Suzuki et al. 1999) In addition,

    HO is also a signal molecule related to senescence (Bhattacharjee 2005). It has been proven that 22

    there is more HO accumulation in old leaf than young leaf. Hence, HO also takes part in 2222

    ABA-induced stomatal opening and closing (Pei et al. 2000; Neill et al. 2002).

Distribution of HO 22

    pH-dependent cell wall peroxidase is able to oxidize NADH and in the process catalyze the

    -formation of superoxide anion (O); and cell wall oxidase catalyzes the oxidation of NADH to 2+-NAD, which in turn reduces O to O, consequently is dismutated to produce O and HO22222

    (Bhattacharjee 2005). In addition, germin-like oxalate oxidases and amine oxidases have been

    proposed to generate HO at the apoplast (Bolwell and Wojtaszek 1997; Hu et al. 2003; Walters 22

    2003). Cell membrane NADPH-dependent oxidase (NADPH oxidase) has recently received a lot

    of attention as a source of HO for the oxidative burst; In addition, there are other enzymes at the 22

    surface of plasma membranes capable of generating HO (cell wall polyamine oxidase) (Vianello 22

    and Macri 1991). It has been identified that respiratory burst oxidase homologues (rboh), plant

    phoxhomologues of the catalytic subunit of phagocyte NADPH oxidase (gp91), as a source of ROS

    during the apoplastic oxidative burst (Agrawal et al. 2003). ROPs (Rho-related Gtpases from plant) closely related to the mammalian Rac family, triggering HO production and then the oxidative 22

    burst, most likely by activating the NADPH oxidase (Agrawal et al. 2003).

    Plant mitochondria as an“energy factory” is believed to be a major site of HO production 22

    related to continuous physiological processes under aerobic conditions (Rasmusson et al. 1998).

The mitochondria electron transport chain (ETC) is comprised of four complex NADH

    dehydrogenase(C?), succinate dehydrogenase(C?), ubiquinol-cytochrome bc1(C?), and

    cytochrome c oxidase (C ?) (Rasmusson et al. 1998). There are also five enzymes existing only

    in plants: they are one alternative oxidase (AOX), four NAD(P)H dehydrogenase assembled to flavoproteins, so they are a potential source of ROS production (Mller 2001). During respiration, Omay undergo an univalent reduction at the sites of HOgeneration in complexes ?and ? of 2 22

    the respiratory chain (Figure 2). The ubiquinone site in complex ? appears as the major site of

    mitochondrial HO production (Braidot et al. 1999), this site catalyzes the conversion of Ointo 222 -the O by a single electron. Of some substrates along respiratory chain. Flavoproteins, Quinols, 2

    especially semiquinols, its energy barrier of redox is very low, the electron before transporting to

    --final oxidase reacts with O to form O (Elstner 1991). In aqueous solution, O is moderately 222

    reactive but can generate HO by dismutation (Rasmusson et al. 1998). About 1-5% of 22

    mitochondria O consumption leads to HO production (Mller 2001). The activity of C? can be 222

    inhibited by rotenone and diphenyleneiodo (DPI) (Meloamp et al. 1996); and the activity of C?

    can be inhibited by KCN, KCN interdicts the Q cycle, so inhibits the semi-quinone production (Rasmusson et al. 1998).




    MnSODsuperoxide anionhydrogen peroxide- ) (O (HO)222

    Figure 2. Sites of hydrogen peroxide formation in mitochondria electron transfer system. HO production is at the two main sites, Complex I and III. The ubiquinone site (UQ) in complex ? catalyzes 22

    -the conversion of O to O by a single electron transfer (Rasmusson et al. 1998). Since UQ is bound to two sites in 22

    complex III, one close to the inner surface of the inner mitochondria membrane, the other close to the out surface,

    -ROS might be found on either side of the membranes (Rasmusson et al. 1998). O is converted into HOby 222

    Mn-SOD (Mller 2001). CI NADH dehydrogenase; CII succinate dehydrogenase; CIII ubiquinol-cytochrome bc1; c? cytochrome C oxidase

    Chloroplasts are also a major source for HO production. Chloroplasts consist of pigment and 22

    protein, two photo reaction systems: photo-system ?(PS?) and photo-system ?(PS ?)

    (Asada and Takahashi 1987). There is a photosynthesis electron transport, calling Z-scheme.

    Recently the electron transport chains (ETC) in photo-system ?(PS?) have been considered to

    -be the source of O in chloroplasts(figure 3). Normally, the electron flow from the excited PS 2+centers is directed to NADP, which is reduced to NADPH. It then enters the Calvin cycle and

    reduces the final electron acceptor, CO. In situations of overloading of the ETC, a part of the 2

    electron flow is diverted from ferredoxin to O, reducing it to superoxide anion via a Mehler 2

    reaction (Wise and Naylor 1987; Elstner 1991). Later studies have revealed that the acceptor side

    -of ETC in PS ?also provides sides (Q, Q) with electron leakage to O producing OAB22-Takahashi and Asada 1988(,Figure 3(.On the external, stromal membrane surface O is 2

    enzymatically by CuZn-SOD or spontaneously dismutated to HOTakahashi and Asada 1988( 22

    P680P680* PS?

    P700* PS?



    Fdacceptor side



    Calvin cycle

    Figure 3. Production of hydrogen peroxide in chloroplast at the site of PSI and PSII.. 680*, P700*:photo reaction center ? and ?,electron flows from PS? to PS?. Q: quinone A. Q: quinone B. PQ: proton quinone. FeS: AB

    --ironsulfur protein. Fd:ferredoxin. At these sites of electron leakage provides electrons for O producing O, O is 222

    dismutated to HO by CuZn-SODTakahashi and Asada 1988(. 22

    Peroxisomes are subcellular organelles with an essentially oxidative type of metabolism. It is

    -also called glyoxysome. peroxisomes produce superoxide radicals (O) as a consequence of their 2-normal metabolism. At least, two sites of O generation are demonstrated (Figure 4) (refer to Del 2

    Río et al. 2002). One is in the organelle matrix, in which the generating system is identified as

    Xanthine oxidase (XOD), Xanthine oxidase (XOD) catalyzes the oxidation of Xanthine and

    -hypoxanthine to uric acid and is a well-known producer of O (Corpas et al. 2001). Another site is 2

    in the peroxisome membranes dependent on NAD (P) H. Peroxisome membrane, a small electron

    -transport chain, is composed of a flavoprotein NADH and cytochrome b, and O is produced by 2

    the peroxisome electron-transport chain. Monodehydroascorbate reductase (MDHAR)

    --participating in O production by peroxisome membranes (Del Río et al. 1989). O radicals are 22

    rapidly converted into HO and O by CuZn-SOD(Del Río et al. 2002). 222



    -O2Uric acid








    HO22 Figure 4. Production of hydrogen peroxide in peroxisomes.

    The model is based on results recently described (Jimenez et al. 1997) monodehydroascorbate reductase(MDHAR)

    -is an NADH-dependent enzyme. Matrix and membrane are two sites of O generation. XOD oxidizes Xanthine to 2

    ---Uric acid, providing electrons for O to product O, cyt b also provides electrons for O to produce O; O then 22222is converted into HO by SOD, XOD (Xanthine oxidase) and cyt b (cytochrome b). 22

Localization of HO scavenging enzymes 22

    The accumulation of HO increases the probability of hydroxyl radical formation via Teton-type 22

    reaction. This leads to the phenomenon known as oxidative stress (Bartosz 1997; Foyer and

    Noctor 2000). In plant cells, enzymes and redox metabolites act in synergy to carry out HO 22scavenging (Table 1)

    TABLE 1. HO scavenging enzymes 22

    Enzyme EC number Reaction catalyzed

    +--Superoxide dismutase O+ O +2 H <=> 2 HO + O 2222

    Catalase 2 HO <=> 2 HO +O 2222

    Glutathione peroxidase 2GSH+PUFA-OOH<=>GSSG+PUFA+2 HO 2

    Glutathine reductase NADPH+GSSG <=> NADP +2GSH

    Ascorbate peroxidase AA+ HO <=> DHA+2 HO 222

    Guaiacol type peroxidase Donor + HO<=>Oxidized donor +2 HO 222

    Major ROS-scavenging enzymes of plants include superoxide dismutase (SOD), ascorbate peroxidase (APX), catalase (CAT), and glutathione peroxidase (GPX) (Table1). These enzymes

    -provide cells with highly efficient machinery for detoxifying O and HO. The balance between 222

    SOD and the different HO-scavenging enzymes in cells is considered to be crucial in 22-determining the steady state level of O and HO (Asada and Takahashi. 1987Bowler et al. 222


    In plants, the main enzymatic HO scavenger of photosynthetic cells is CAT, which convert 22

    HO into HO and O(Scandalios 1987). CAT scavenges HO generated during mitochondrial 2222 22

    electron transport, β-oxidation of the fatty acids, and most importantly in photorespiratory oxidation (Scandalios et al. 1997). In perxoxisomes/ glyoxysomes, CAT predominates. CAT

    isoforms are distinguished on the basis of organ specificity and responses to environmental stress (Willekens et al 1994a). A CAT isoform has been reported to be present in maize mitochondria (Scandalios et al 1980), but no mitochondrial form has been reported in C species (Foyer and 3

    Noctor 2000). Peroxisomes contain a large amount of CAT, but its properties suggest that the

    enzyme is inefficient in removing low concentrations of HO (Willekens et al. 1994a) 22

     Peroxidase (POD) is a heme-containing glycoprotein encoded by a large mutigene family in plants and involved in various physiological processes. Studies have suggested that POD plays a role in lignification, cross-linking of cell wall structure proteins and defense against pathogen (Kawano 2003). POD exists as isoenzymes in individual plant species (Hiraga et al. 2001).

    Ascorbate peroxidaseAPX( is the main enzyme responsible for HO removal in the chloroplast, 22

    peroxisomes and mitochondria. APX utilizes ascorbate as its specific electron donor to reduce HO to water (Asada 1992). Glutathione peroxidase (GPX) is a family of isoenzymes that uses 22

    glutathione to reduce HO and organic and lipid hydro-peroxides, thereby protecting cells against 22

    oxidative damage. GPX is an important HO scavenging enzyme in mammals. In plants, GPX 22

    exists in the cytosol to reduce HO to water. But the ability of plant GPX to scavenge HO 2222

    decreases largely due to its Cys residue without selenium. Hence, the major functions of GPX in plants are lignin biosynthesis, degradation of indole-3-acetic acid and resistance to pathogens (Asada 1992). However, except for the donor specific peroxidase mentioned above, there is a group of non-donor specific peroxidase in plant cells, for which guaiacol is a common donor, named guaiacol peroxidase (Mika and Luthje 2003). Recently, two distinct guaiacol peroxidases (pm POD1 and pm POD2) have been separated from the plasma membrane. However more functions of guaiacol peroxidase are still unclear (Mika and Luthje 2003).

Balance between HOand cell redox 22

    An appropriate intracellular balance between HO generation and scavenging exists in all cells 22

    (figure 5) (refer to Mittler et al 2004). This redox homeostasis requires the efficient coordination

    of reactions in different cell compartments and is governed by a complex network of prooxidant and antioxidant systems. The latter include nonenzymatic scavengers such as ascorbate,

    glutathione, hydrophobic molecules, tocopherols and detoxifying enzymes (Noctor and Foyer 1998a).



    MDAMDA--eeCuZnSODCuZnSOD ComplexesComplexesOO--Ascorbate22MDAHHOOOOHHOO2222222222 ubiquinone ubiquinoneAPXCuZnSODCuZnSODee






    Figure 5. Localization of hydrogen peroxide (HO) scavenging pathways in plant cells(chloroplast, peroxisome, 22

    cytosol and mitochondria).

    -The enzymatic pathways responsible for HO detoxification are shown. The water-water cycle detoxifies O and 222

    HO. HOdistributes in peroxisomes, mitochondria, chloroplast and cytosol. Catalase (CAT), ascorbate 2222

    peroxidase(APX). SOD and other components of the Ascorbate-glutathione cycle are also present in mitochondria and peroxisomal. Glutathione peroxidase(GPX) is involved in HO removal in the cytosol. HO can easily 2222

    diffuse through membranes and antioxidants such as glutathione and ascorbic acid (reduced or oxidized) can be transported between the different compartments. Abbreviations: DHA, dehydroascrobate; DHAR,DHA reductase; FD, ferredoxin; GLR, glutaredoxin; GR, glutathione reductase; GSH, reduced glutathione; GSSG, Oxidized glutathione; IM, inner membrane; MDA, monodehydroascorbate; MDAR, MDA reductase; PS?,photosystem?;

    PS?, photosystem?.

    Ascorbate is present in chloroplasts, cytol, and vacuole and apoplastic spaces of leaf cells in high concentration (Foyer et al. 1991). It is perhaps the most important antioxidant in plants, with a fundamental role in the removal of HO (Polle et al. 1990). The ascorbate/glutathione cycle is 22

    the most important HO detoxifying system in the chloroplasts. But it also has been an 22

    identifying system in the cytosol (Nakano and Asada 1981), peroxisomes, and mitochondria (Jimenez et al. 1997). Two enzymes are involved in the regeneration of reduced ascorbate, namely mono-dehydro-ascorbate reductase (MDHR) which uses NAD (P) H directly to recycle ascorbate and dehydro-ascorbate reductase (DHR). Mono-dehydro-ascorbate is reduced directly to ascorbate by using electrons derived from the photosynthetic electrons transport chain as follows (Arora et

    al. 2002):

    4 Mono-dehydro- ascorbate(MDHR) + 2 HO? 4 Ascorbate + O 22




Figure 6. Ascorbate-glutathione cycle (Halliwell-Asada pathway) of HO scavenging. AsA, ascorbate; APX, 22

    ascorbate-peroxidase; MDHAR, mondehydroascorbate reductase; DHA, dehydroascorbate; DHAR, dehydroascorbate reductase; GR, glutathione reductase; GSH, Glutathione.

Regulation of genes expression related to HO22

    Hydrogen peroxide (HO) has been regarded as the second messenger for gene activation in 22

    mammalian systems as well as in plant. In plants, increased HO level induces the expressing not 22

    only of defense genes, but also other resistance genes (Mitter et al. 2004).

    Temperature-independent induction of smHSPs has been observed in response to high light (HL) (Pnueli et al. 2003; Yamamoto et al. 2004) and to various other abiotic stress conditions

    (Zimmermann et al. 2004). A subset of genes within the heat shock response might be triggered by increased levels of HO(Larkindale and Knight 2002). HO is clearly able to induce the smHSPs 22 22

    17.6 class. In the different assessed abiotic stresses, these smHSPs are coexpressed with AtHsf 2A; recently, this class of cytoplasmic smHSPs has been shown not to be under transcriptional control of HsfA1a/HsfA1b during heat shock (Busch et al. 2005). And recently a role of AtHsfA4a in the

    early sensing of HOstress has been demonstrated in Arabidopsis (Davletova et al. 2005) 22

    Recent studies of knockout and antisense lines for Cat2, Apx1, chlAOX, mitAOX, CSD2, 2-cysteine PrxR and various NADPH oxidases have revealed a strong link between HO and 22

    processes such as growth, development, stomatal responses and biotic and abiotic stress responses (Mittler 2004). Based on the analysis of the different mutants. Cat2, Apx1, ChlAOX, CSD2 and 2-cysteine PrxR are essential for the protection of chloroplasts against oxidative damage. Suppression of CSD2, for example, results in the induction of a High-light (HL) stress response in Arabidopsis plants grown under a low light intensity (Rizhksy et al. 2003). Catalase deficiency

    triggers growth retardation and high sensitivity to ozone and high light stress (Vandenabeele et al.

    2004).the absence of Apx1 results in reduced photosynthetic activity, augmented induction of heat shock proteins during light stress and altered stomatal responses (Pnueli et al. 2003). Catalase

    deficiency triggers growth retardation and high sensitivity to Ozone and high light stress (Vandenabeele et al. 2004). By contrast, the absence of the NADPH oxidase genes AtrbohD and AtrbohF suppresses HOproduction and the defense responses of Arabidopsis against pathogen 22

    attack (Torres et al. 2002). And knockout of atrbohC has an altered root phenotype (Foreman et al.

    2003). AtrbohD and AtrbohF are also essential for abscisic acid signaling in guard cells (Kwak et

    al. 2003).

    Figure 7 (refer to Vanderauwera et al. 2005) presents the overlap of the HO-induced genes 22

    with each of the environmental stresses. Twenty genes were induced in response to HO and 22

    under at least two stress conditions. Within these 20 commonly induced genes, two transcription factors, DREB2A and ZAT12, could be identified, which have already been linked to H2O2 responses (Rizhksy et al. 2004). DREB2A is known to be a key regulator of drought response (Shinozaki and Yamaguchi-Shinozaki 2000), whereas ZAT12 participates in regulation of

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