Oxidative stress increased respiration and generation of reactive oxygen species, resulting in ATP depletion, opening of mitochondrial permeability transition, and programmed cell death

In vitro model of normal, immortalized ovarian surface epithelial and ovarian cancer cells for chemoprevention of ovarian cancer

BACKGROUND

Epithelial ovarian cancer has the highest mortality rate among the gynecologic cancers. The synthetic retinoid, N-(4-hydroxyphenyl) retinamide (4-HPR), has been used in the chemoprevention of ovarian cancer. However, the effectiveness of its application for different populations has been questioned because of the genetic differences among normal, high risk, and women with cancer.

OBJECTIVE

To explore the similarities and the differences in 4-HPR effects on different ovarian epithelial cells which mimic different populations of women, normal ovarian surface epithelium to represent the normal population of women, immortalized ovarian surface epithelium to represent premalignant changes, and cells derived from ovarian cancer cells to represent malignant changes were used as in vitro models.

METHODS

Normal ovarian surface epithelial cells, immortalized ovarian surface epithelial cells, and ovarian cancer cells were incubated for different intervals with increasing concentrations of 4-HPR. Growth inhibition, the fraction of apoptotic cells, the expression of apoptosis-related genes, including p53, p16, p21, and caspase-3, and mitochondrial permeability transition were measured before and after 4-HPR treatment.

RESULTS

Treatment with 4-HPR produced growth inhibition and apoptosis in a dose-dependent manner for all 3 cell types. 4-HPR produced the strongest activation of the p53 pathway in normal ovarian epithelial (NOE) cells, while it caused the largest increase in MPT in the cancer cells, suggesting a different mechanism for growth inhibition and/or apoptosis in these cell lines. 4-HPR, at a concentration of 10 muM, had a maximal effect on caspase-3 activity at 72 h in normal cells and at 48 h in immortalized and cancer cells, although the effects were modest.

CONCLUSIONS

Normal ovarian surface epithelial cells, immortalized ovarian surface epithelial cells, and ovarian cancer cells showed a differential response to 4-HPR. Although the same endpoints of growth inhibition and apoptosis induction were present in response to 4-HPR, these endpoints may be regulated through different pathways.

IMPLICATIONS

Clinical trials with higher concentrations of 4-HPR should prove beneficial.

Mitochondrial behaviour in the early stages of ROS stress leading to cell death in Arabidopsis thaliana

BACKGROUND AND AIMS

Reactive oxygen species (ROS) are involved in triggering cell death. To visualize mitochondrial behaviour under ROS stress, transgenic arabidopsis plants possessing mitochondrial-targeted GFP (S65T) were studied.

METHODS

Arabidopsis leaves were treated with ROS and ROS-inducing chemicals such as hydrogen peroxide, paraquat and menadione. Microscopic observations were carried out using a confocal laser scanning microscope system, and electrolyte leakage was also monitored.

KEY RESULTS

After treatment, mitochondria showed morphological changes from a bacillus-like to a round shape. The size of mitochondria treated with H(2)O(2) decreased by half compared with controls. Concurrently, cytoplasmic streaming was blocked and mitochondria eventually swelled. Treatment of leaves with butanedione monoxime, an inhibitor of myosin ATPase, resulted in similar behaviour of mitochondria to that under ROS stress.

CONCLUSIONS

The results indicate that morphological changes of mitochondria and cessation of cytoplasmic streaming may interact, and this phenomenon is one of the features of ROS stress-induced cell death.

An endopolygalacturonase from Sclerotinia sclerotiorum induces calcium-mediated signaling and programmed cell death in soybean cells

A basic endopolygalacturonase (PG) isoform, produced early by Sclerotinia sclerotiorum when infecting soybean seedlings, was used to examine the signaling role of the enzyme in aequorin-expressing soybean cells. A cytosolic Ca2+ elevation was induced, with a rapid increase (phase 1) and a very slow decrease (phase 2) of Ca2+ concentration, indicating the involvement of Ca2+ ions in PG signaling. Within 1 h of PG-cell contact a remarkable level of cell death was recorded, significantly higher than the control cell culture turnover. The observed morphological and biochemical changes were indicative of the activation of programmed cell death; in particular, cytochrome c release in the cytoplasm and activation of both caspase 9-like and caspase 3-like proteases were found. When a polygalacturonase-inhibiting protein (PGIP) and the PG were simultaneously applied to cells, both the Ca2+ increase and cell death were annulled. The possible roles of prolonged sustained cytosolic Ca2+ concentrations in inducing cell death and of the PG-PGIP interaction in preventing PG signaling are discussed.

Up-regulation of the mitochondrial alternative oxidase pathway enhances photosynthetic electron transport under drought conditions

The aim of this study was to explore the role of the mitochondrial alternative oxidase (AOX) in the protection of photosynthesis during drought in wheat leaves. The relative water contents of water-replete and drought-exposed wheat plants were 97.2+/-0.3 and 75+/-2, respectively. Drought increased the amount of leaf AOX protein and also enhanced the rate of AOX-dependent O(2) uptake by the respiratory electron transport chain. The amount of the reduced, active form of the AOX protein was specifically increased by drought. The AOX inhibitor salicylhydroxamic acid (1 mM; SHAM) inhibited 70% of AOX activity in vivo in both water-replete and drought-exposed plants. Plants treated with SHAM were then exposed to low (100), high (350), or excess light (800 mumol photons m(-2) s(-1)) for 90 min. SHAM did not modify chlorophyll a fluorescence quenching parameters in water-replete controls after any of these treatments. However, while the maximal quantum yield of photosystem II (PSII) electron transport (F(v)/F(m)) was not affected by SHAM, the immediate quantum yield of PSII electron transport (Phi(PSII)) and photochemical quenching (qP) were gradually reduced by increasing irradiance in SHAM-treated drought-exposed plants, the decrease being most pronounced at the highest irradiance. Non-photochemical quenching (NPQ) reached near maximum levels in plants subjected to drought at high irradiance. However, a combination of drought and low light caused an intermediate increase in NPQ, which attained higher values when AOX was inhibited. Taken together, these results show that up-regulation of the respiratory AOX pathway protects the photosynthetic electron transport chain from the harmful effects of excess light.

Legume nodule senescence: roles for redox and hormone signalling in the orchestration of the natural aging process

Research on legume nodule development has contributed greatly to our current understanding of plant-microbe interactions. However, the factors that orchestrate root nodule senescence have received relatively little attention. Accumulating evidence suggests that redox signals contribute to the establishment of symbiosis and senescence. Although degenerative in nature, nodule senescence is an active process programmed in development in which reactive oxygen species (ROS), antioxidants, hormones and proteinases have key roles. Nodules have high levels of the redox buffers, ascorbate and glutathione, which are important in the nodulation process and in senescence. These metabolites decline with N-fixation as the nodule ages but the resultant decrease in redox buffering capacity does not necessarily lead to enhanced ROS or oxidative stress. We propose models by which ROS and antioxidants interact with hormones such as abscisic acid in the orchestration of nodule senescence.

Redox regulation: a broadening horizon

Initially discovered in the context of photosynthesis, regulation by change in the redox state of thiol groups (S-S <--> 2SH) is now known to occur throughout biology. Several systems, each linking a hydrogen donor to an intermediary disulfide protein, act to effect changes that alter the activity of target proteins: the ferredoxin/thioredoxin system, comprised of reduced ferredoxin, a thioredoxin, and the enzyme, ferredoxin-thioredoxin reductase; the NADP/thioredoxin system, including NADPH, a thioredoxin, and NADP-thioredoxin reductase; and the glutathione/glutaredoxin system, composed of reduced glutathione and a glutaredoxin. A related disulfide protein, protein disulfide isomerase (PDI) acts in protein assembly. Regulation linked to plastoquinone and signaling induced by reactive oxygen species (ROS) and other agents are also being actively investigated. Progress made on these systems has linked redox to the regulation of an increasing number of processes not only in plants, but in other types of organisms as well. Research in areas currently under exploration promises to provide a fuller understanding of the role redox plays in cellular processes, and to further the application of this knowledge to technology and medicine.

The role of [Delta]1-pyrroline-5-carboxylate dehydrogenase in proline degradation

In response to stress, plants accumulate Pro, requiring degradation after release from adverse conditions. Delta1-Pyrroline-5-carboxylate dehydrogenase (P5CDH), the second enzyme for Pro degradation, is encoded by a single gene expressed ubiquitously. To study the physiological function of P5CDH, T-DNA insertion mutants in AtP5CDH were isolated and characterized. Although Pro degradation was undetectable in p5cdh mutants, neither increased Pro levels nor an altered growth phenotype were observed under normal conditions. Thus AtP5CDH is essential for Pro degradation but not required for vegetative plant growth. External Pro application caused programmed cell death, with callose deposition, reactive oxygen species production, and DNA laddering, involving a salicylic acid signal transduction pathway. p5cdh mutants were hypersensitive toward Pro and other molecules producing P5C, such as Arg and Orn. Pro levels were the same in the wild type and mutants, but P5C was detectable only in p5cdh mutants, indicating that P5C accumulation may be the cause for Pro hypersensitivity. Accordingly, overexpression of AtP5CDH resulted in decreased sensitivity to externally supplied Pro. Thus, Pro and P5C/Glu semialdehyde may serve as a link between stress responses and cell death.

Perturbation of NgTRF1 expression induces apoptosis-like cell death in tobacco BY-2 cells and implicates NgTRF1 in the control of telomere length and stability

Telomeres are specialized nucleoprotein complexes that are essential for preserving chromosome integrity in eukaryotic cells. Several potential telomere binding proteins have recently been identified in higher plants, but nothing is known about their in vivo functions. We previously identified NgTRF1 as a double-stranded telomeric repeat binding factor in tobacco (Nicotiana tabacum) and here show that the binding of NgTRF1 to telomeric repeats inhibits telomerase-mediated telomere extension. To determine whether NgTRF1 is involved in telomere length regulation, we established transgenic tobacco BY-2 cell lines that overexpress or suppress NgTRF1. Pulsed-field gel electrophoresis showed that 35S::NgTRF1 cells exhibited significantly shortened telomeres (45 to 10 kb), whereas 35S::antisense-NgTRF1 cells contained longer telomeres (80 to 25 kb) compared with wild-type and 35S::GUS control cells (65 to 15 kb), indicating that telomere length inversely correlates with the amount of functional NgTRF1 in BY-2 cells. 35S::NgTRF1 cells with shorter telomeres displayed a progressive reduction in cell viability and stopped dividing after 25 to 40 successive rounds of 12-d batch subculture, in sharp contrast with control cells, which have an unlimited capacity for division. Internucleosomal DNA fragmentation, mitochondrial release of cytochrome c, and terminal deoxynucleotidyltransferase-mediated dUTP nick end labeling positive nuclei were detected in 35S::NgTRF1 cells during prolonged subculture, indicating that enhanced cell death was attributable to an apoptosis-like mechanism. 35S::antisense-NgTRF1 cells containing low levels of NgTRF1 also exhibited a progressive decrease in cell viability and apoptotic cell death, but less so than did 35S::NgTRF1 cells, suggesting that the level of NgTRF1 is critically associated with cell viability. Taken together, these data indicate that perturbation of NgTRF1 expression results in changes in telomere length and stability, which in turn causes apoptotic cell death in transgenic BY-2 cells. These results are discussed in light of the suggestion that NgTRF1 is involved in the mechanism by which telomere length and stability are maintained. We further suggest that the structural stability of telomeres, in addition to length maintenance, is essential for their function and for the immortality of BY-2 cells.

The mitochondrion--an organelle commonly involved in programmed cell death in Arabidopsis thaliana

Plant cells undergoing programmed cell death (PCD) at late stages typically show chromatin condensation and endonucleolytic cleavage prior to obvious membrane or organelle ultrastructural changes. To investigate possible early PCD-associated events, we used microscopic observations and flow cytometry to quantitate mitochondrial membrane potential (DeltaPsim) changes during PCD at the single cell and population levels using Arabidopsis protoplasts. A DeltaPsim loss was commonly induced early during plant PCD and was important for PCD execution, as evidenced by the concomitant reduction of the change in DeltaPsim and PCD by cyclosporin A, which inhibits mitochondrial permeability transition pores in animal cells. DeltaPsim loss occurred prior to nuclear morphological changes and was only associated with mitochondrial cytochrome c release (an apoptotic trigger in animals) in response to one of three PCD elicitors. Three different stimuli in wild type implicated DeltaPsim changes in PCD: ceramide, protoporphyrin IX, and the hypersensitive response elicitor AvrRpt2. Additionally, the behavior of the conditional ectopic cell death mutant accelerated cell death2 and ACD2-overproducing plants also implicated DeltaPsim alteration as key for PCD execution. Because ACD2 is largely a chloroplast component in mature plants, the observation that the cell death in acd2 mutants requires changes in mitochondrial functions implicates communication between chloroplasts and mitochondria in mediating PCD activation. We suggest that DeltaPsim loss is a common early marker in plant PCD, similar to what has been documented in animals. However, unlike in animal cells, in plant cells, mitochondrial cytochrome c release is not an obligatory step in PCD control.

Catalase deficiency drastically affects gene expression induced by high light in Arabidopsis thaliana

In plants, hydrogen peroxide (H(2)O(2)) plays a major signaling role in triggering both a defense response and cell death. Increased cellular H(2)O(2) levels and subsequent redox imbalances are managed at the production and scavenging levels. Because catalases are the major H(2)O(2) scavengers that remove the bulk of cellular H(2)O(2), altering their levels allows in planta modulation of H(2)O(2) concentrations. Reduced peroxisomal catalase activity increased sensitivity toward both ozone and photorespiratory H(2)O(2)-induced cell death in transgenic catalase-deficient Arabidopsis thaliana. These plants were used as a model system to build a comprehensive inventory of transcriptomic variations, which were triggered by photorespiratory H(2)O(2) induced by high-light (HL) irradiance. In addition to an H(2)O(2)-dependent and -independent type of transcriptional response during light stress, microarray analysis on both control and transgenic catalase-deficient plants, exposed to 0, 3, 8, and 23 h of HL, revealed several specific regulatory patterns of gene expression. Thus, photorespiratory H(2)O(2) has a direct impact on transcriptional programs in plants.

Reactive oxygen signalling: the latest news

During the past two years, a wide range of plant responses have been found to be triggered by hydrogen peroxide that is generated in a genetically controlled manner by NADPH oxidases. Several studies have revealed examples of how changes in the concentrations of reactive oxygen species (ROS) are perceived and transferred into signals that change the transcription of genes. Moreover, both the chemical identity of a given ROS and the intracellular site of its production seem to affect the specificity of its biological activity, further increasing the complexity of ROS signalling within plants.

A proteomic analysis of plant programmed cell death

Programmed cell death (PCD) is an active cellular suicide that occurs in animals and plants throughout development and in response to both abiotic and biotic stresses. In contrast to animals, little is known about the molecular machinery that regulates plant PCD. We have previously identified transcriptomic changes associated with heat- and senescence-induced PCD in an Arabidopsis cell suspension culture [Plant J. 30 (2002) 431]. However, since plant PCD is also likely to involve elements that are regulated post-transcriptionally, we have undertaken a proteomic analysis in the Arabidopsis system. We identified 11 proteins that increased in abundance relative to total protein in both treatments despite extensive degradation of other proteins. We argue that some of these proteins are maintained during PCD and may therefore have specific functions in the PCD pathway. The increased abundance of several antioxidant proteins as well as a measured increase in free Fe2+ content of the cells indicates an oxidative stress in this system. Several mitochondrial proteins were identified, confirming the importance of this organelle during PCD. We also identified an extracellular glycoprotein that may function in the transmission of a 'death signal' from cell to cell. Putative roles for the identified proteins are presented.

Rapid transmission of oxidative and nitrosative stress signals from roots to shoots in Arabidopsis

Protein kinases play a central role in signal transduction pathways in eukaryotes. A highly conserved group of kinases, termed mitogen-activated-protein kinases (MAPKs) was shown to mediate many diverse stress responses. In plants, MAPKs were shown to function in resistance responses to many biotic and abiotic stresses. Here, we show that exposure of Arabidopsis roots to hydrogen peroxide or to nitric oxide resulted in rapid activation of protein kinases in the shoots that exhibited MAPK properties. The same pattern of kinases was induced by direct injection of these compounds into leaves, indicating accurate long-distance transmission of H2O2 and NO signals. These results are important for the understanding of redox signal transmission from the rhizosphere throughout the plant.

Ectopic expression of maize polyamine oxidase and pea copper amine oxidase in the cell wall of tobacco plants

To test the feasibility of altering polyamine levels by influencing their catabolic pathway, we obtained transgenic tobacco (Nicotiana tabacum) plants constitutively expressing either maize (Zea mays) polyamine oxidase (MPAO) or pea (Pisum sativum) copper amine oxidase (PCuAO), two extracellular and H(2)O(2)-producing enzymes. Despite the high expression levels of the transgenes in the extracellular space, the amount of free polyamines in the homozygous transgenic plants was similar to that in the wild-type ones, suggesting either a tight regulation of polyamine levels or a different compartmentalization of the two recombinant proteins and the bulk amount of endogenous polyamines. Furthermore, no change in lignification levels and plant morphology was observed in the transgenic plants compared to untransformed plants, while a small but significant change in reactive oxygen species-scavenging capacity was verified. Both the MPAO and the PCuAO tobacco transgenic plants produced high amounts of H(2)O(2) only in the presence of exogenously added enzyme substrates. These observations provided evidence for the limiting amount of freely available polyamines in the extracellular space in tobacco plants under physiological conditions, which was further confirmed for untransformed maize and pea plants. The amount of H(2)O(2) produced by exogenously added polyamines in cell suspensions from the MPAO transgenic plants was sufficient to induce programmed cell death, which was sensitive to catalase treatment and required gene expression and caspase-like activity. The MPAO and PCuAO transgenic plants represent excellent tools to study polyamine secretion and conjugation in the extracellular space, as well as to determine when and how polyamine catabolism actually intervenes both in cell wall development and in response to stress.

The victorin-induced mitochondrial permeability transition precedes cell shrinkage and biochemical markers of cell death, and shrinkage occurs without loss of membrane integrity

Summary In this study, we determined the timing of events associated with cell death induced by the host-selective toxin, victorin. We show that the victorin-induced collapse in mitochondrial transmembrane potential (Deltapsi(m)), indicative of a mitochondrial permeability transition (MPT), on a per cell basis, did not occur simultaneously in the entire mitochondrial population. The loss of Deltapsi(m) in a predominant population of mitochondria preceded cell shrinkage by 20-35 min. Rubisco cleavage, DNA laddering, and victorin binding to the P protein occurred concomitantly with cell shrinkage. During and following cell shrinkage, tonoplast rupture did not occur, and membranes, including the plasma membrane and tonoplast, retained integrity. Ethylene signaling was implicated upstream of a victorin-induced loss in mitochondrial motility and the collapse in Deltapsi(m). Results suggest that the victorin-induced collapse in Deltapsi(m) is a consequence of an MPT and that the timing of the victorin-induced MPT is poised to influence the cell death response. The retention of plasma membrane and tonoplast integrity during cell shrinkage supports the interpretation that victorin induces an apoptotic-like cell death response.

Production of reactive oxygen species, alteration of cytosolic ascorbate peroxidase, and impairment of mitochondrial metabolism are early events in heat shock-induced programmed cell death in tobacco Bright-Yellow 2 cells

To gain some insight into the mechanisms by which plant cells die as a result of abiotic stress, we exposed tobacco (Nicotiana tabacum) Bright-Yellow 2 cells to heat shock and investigated cell survival as a function of time after heat shock induction. Heat treatment at 55 degrees C triggered processes leading to programmed cell death (PCD) that was complete after 72 h. In the early phase, cells undergoing PCD showed an immediate burst in hydrogen peroxide (H2O2) and superoxide (O2*-) anion production. Consistently, death was prevented by the antioxidants ascorbate (ASC) and superoxide dismutase (SOD). Actinomycin D and cycloheximide, inhibitors of transcription and translation, respectively, also prevented cell death, but with a lower efficiency. Induction of PCD resulted in gradual oxidation of endogenous ASC; this was accompanied by a decrease in both the amount and the specific activity of the cytosolic ASC peroxidase (cAPX). A reduction in cAPX gene expression was also found in the late PCD phase. Moreover, changes of cAPX kinetic properties were found in PCD cells. Production of ROS in PCD cells was accompanied by early inhibition of glucose (Glc) oxidation, with a strong impairment of mitochondrial function as shown by an increase in cellular NAD(P)H fluorescence, and by failure of mitochondria isolated from cells undergoing PCD to generate membrane potential and to oxidize succinate in a manner controlled by ADP. Thus, we propose that in the early phase of tobacco Bright-Yellow 2 cell PCD, ROS production occurs, perhaps because of damage of the cell antioxidant system, with impairment of the mitochondrial oxidative phosphorylation.

Structural and kinetic properties of a novel purple acid phosphatase from phosphate-starved tomato (Lycopersicon esculentum) cell cultures

An intracellular acid phosphatase (IAP) from P(i)-starved (-P(i)) tomato ( Lycopersicon esculentum ) suspension cells has been purified to homogeneity. IAP is a purple acid phosphatase (PAP), as the purified protein was violet in colour (lambda(max)=546 nm) and was insensitive to L-tartrate. PAGE, periodic acid-Schiff staining and peptide mapping demonstrated that the enzyme exists as a 142 kDa heterodimer composed of an equivalent ratio of glycosylated and structurally dissimilar 63 (alpha-subunit) and 57 kDa (beta-subunit) polypeptides. However, the nine N-terminal amino acids of the alpha- and beta-subunits were identical, exhibiting similarity to the deduced N-terminal portions of several putative plant PAPs. Quantification of immunoblots probed with rabbit anti-(tomato acid phosphatase) immune serum revealed that the 4-fold increase in IAP activity due to P(i)-deprivation was correlated with similar increases in the amount of antigenic IAP alpha- and beta-subunits. IAP displayed optimal activity at pH 5.1, was activated 150% by 10 mM Mg(2+), but was potently inhibited by Zn(2+), Cu(2+), Fe(3+), molybdate, vanadate, fluoride and P(i). Although IAP demonstrated broad substrate selectivity, its specificity constant ( V (max)/ K (m)) with phosphoenolpyruvate was >250% greater than that obtained with any other substrate. IAP exhibited significant peroxidase activity, which was optimal at pH 9.0 and insensitive to Mg(2+) or molybdate. This IAP is proposed to scavenge P(i) from intracellular phosphate esters in -P(i) tomato. A possible secondary IAP role in the metabolism of reactive oxygen species is discussed. IAP properties are compared with those of two extracellular PAP isoenzymes that are secreted into the medium of -P(i) tomato cells [Bozzo, Raghothama and Plaxton (2002) Eur. J. Biochem. 269, 6278-6286].

Dissection of Arabidopsis Bax inhibitor-1 suppressing Bax-, hydrogen peroxide-, and salicylic acid-induced cell death

Overexpression of plant Bax Inhibitor-1 (BI-1) was able to suppress Bax-mediated cell death in yeast and Arabidopsis. Here, we demonstrate that reactive oxygen species production induced by the ectopic expression of Bax was insensitive to the coexpression of AtBI-1. Similarly, H2O2- or salicylic acid-mediated cell death also was suppressed in tobacco BY-2 cells overexpressing AtBI-1. To define the functional domain of AtBI-1 as a cell death suppressor, a truncated series of the AtBI-1 protein was analyzed in yeast possessing a galactose-inducible mammalian Bax. The results showed that DeltaC-AtBI-1 (with the C-terminal 14 amino acids deleted) lost the ability to sustain cell growth. Furthermore, a mutant protein in which the C-terminal seven amino acid residues of AtBI-1 were replaced with others lacking a coiled-coil structure failed to inhibit cell death, suggesting that the C-terminal region is essential for the inhibition of cell death. We also noted that the C-terminal hydrophilic region was interchangeable between animal and plant Bax inhibitors.

Induction of salt and osmotic stress tolerance by overexpression of an intracellular vesicle trafficking protein AtRab7 (AtRabG3e)

Adaptation to stress requires removal of existing molecules from various cellular compartments and replacing them with new ones. The transport of materials to and from the specific compartments involved in the recycling and deposition of macromolecules is carried out by an intracellular vesicle trafficking system. Here, we report the isolation of a vesicle trafficking-regulating gene, AtRabG3e (formerly AtRab7), from Arabidopsis. The gene was induced during programmed cell death after treatment of intact leaves with superoxide and salicylic acid or infection with necrogenic pathogens. Transgenic plants that expressed the AtRabG3e gene under the constitutive 35S promoter from cauliflower mosaic virus exhibited accelerated endocytosis in roots, leaves, and protoplasts. The transgenic plants accumulated sodium in the vacuoles and had higher amounts of sodium in the shoots. The transgenic plants also showed increased tolerance to salt and osmotic stresses and reduced accumulation of reactive oxygen species during salt stress. These results imply that vesicle trafficking plays an important role in plant adaptation to stress, beyond the housekeeping function in intracellular vesicle trafficking.

Reactive oxygen species: metabolism, oxidative stress, and signal transduction

Several reactive oxygen species (ROS) are continuously produced in plants as byproducts of aerobic metabolism. Depending on the nature of the ROS species, some are highly toxic and rapidly detoxified by various cellular enzymatic and nonenzymatic mechanisms. Whereas plants are surfeited with mechanisms to combat increased ROS levels during abiotic stress conditions, in other circumstances plants appear to purposefully generate ROS as signaling molecules to control various processes including pathogen defense, programmed cell death, and stomatal behavior. This review describes the mechanisms of ROS generation and removal in plants during development and under biotic and abiotic stress conditions. New insights into the complexity and roles that ROS play in plants have come from genetic analyses of ROS detoxifying and signaling mutants. Considering recent ROS-induced genome-wide expression analyses, the possible functions and mechanisms for ROS sensing and signaling in plants are compared with those in animals and yeast.

VDAC is a conserved element of death pathways in plant and animal systems

Programmed cell death (PCD) is very much a part of plant life, although the underlying mechanisms are not so well understood as in animals. In animal cells, the voltage-dependent anion channel (VDAC), a major mitochondrial outer membrane transporter, plays an important role in apoptosis by participating in the release of intermembrane space proteins. To characterize plant PCD pathways by investigating the function of putative components in a mammalian apoptotic context, we have overexpressed a rice VDAC (osVDAC4) in the Jurkat T-cell line. Overexpression of osVDAC4 induces apoptosis, which can be blocked by Bcl-2 and the VDAC inhibitor DIDS. Modifying endogenous VDAC function by DIDS and hexokinase II (HxKII) in Jurkat cells inhibits mitochondria-mediated apoptotic pathways. Finally, we show that DIDS also abrogates heat-induced PCD in cucumber cotyledons. Our data suggest that VDAC is a conserved mitochondrial element of the death machinery in both plant and animal cells.

Hyperphosphorylation of a mitochondrial protein, prohibitin, is induced by calyculin A in a rice lesion-mimic mutant cdr1

The rice (Oryza sativa) lesion-mimic mutants, cell death and resistance (cdr), show spontaneous cell death on the entire leaf and exhibited significant resistance to the rice blast fungus. Our previous studies showed that CDR1 and CDR2 genes negatively regulated the phosphorylation steps leading to the activation of NADPH oxidase, which is associated with oxidative burst. To identify novel factors involved in the phosphorylation steps, the phosphorylation level of total proteins was compared between cdr mutants and wild type using two-dimensional gel electrophoresis. Here, we show that the phosphorylation level of four proteins in cdr1 was increased as compared with the wild type after calyculin A treatment. Partial amino acid sequences revealed that one of the four proteins is homologous to prohibitin (PHB), which has been shown to be associated with senescence and cell death and to function as a chaperone in the assembly of mitochondrial respiratory chain complex in yeast and mammals. Analysis of green fluorescent protein fusions indicated that rice PHB (OsPHB1) was targeted to mitochondria as found in yeast and mammals, suggesting a possibility that PHB is involved in defense response and/or programmed cell death through the mitochondrial function.

Environmental stresses inhibit and stimulate different protein import pathways in plant mitochondria

The impact of various environmental stresses (drought, chilling or herbicide treatment) on the capacity of plant mitochondria to import precursor proteins was investigated. Drought treatment stimulated import and processing of various precursor proteins via the general import pathway. The stimulatory effect of drought on the general import pathway was due to an increased rate of import, was accompanied by an increased rate of processing, and could be attributed to the presequence of the precursor protein. Interestingly, drought decreased the import of the F(A)d subunit of ATP synthase suggesting a bypass of the point of stimulation during import of this precursor. Both chilling and herbicide treatment of plants, on the other hand, caused inhibition of import with all precursors tested. No decrease in processing of imported proteins was observed by these stress treatments. Western analysis of several mitochondrial proteins indicated that the steady-state level of several mitochondrial components, including the TOM20 receptor and the core subunits of the cytochrome bc(1) complex responsible for processing, remained largely unchanged. Thus environmental stresses differentially affect import of precursor proteins in a complicated manner dependent on the import pathway utilised.

Reactive oxygen species and hormonal control of cell death

The accumulation of reactive oxygen species (ROS) is involved in regulating cell death. Pathogen- and ozone-induced processes have become important models for the study of cell death regulation by ROS. Hydrogen peroxide and superoxide have emerged as the two key ROS and recent studies have addressed their sources and control of their production. ROS signals interact directly or indirectly with several other signaling pathways, such as nitric oxide, and the stress hormones salicylic acid, jasmonic acid and ethylene. The interaction and balance of these pathways determines whether the cell lives or dies.

AtCYS1, a cystatin from Arabidopsis thaliana, suppresses hypersensitive cell death

In plants, cysteine protease inhibitors are involved in the regulation of protein turnover and play an important role in resistance against insects and pathogens. AtCYS1 from Arabidopsis thaliana encodes a protein of 102 amino acids that contains the conserved motif of cysteine protease inhibitors belonging to the cystatin superfamily (Gln-Val-Val-Ala-Gly). Recombinant A. thaliana cystatin-1 (AtCYS1) was expressed in Escherichia coli and purified. AtCYS1 inhibits the catalytic activity of papain (Kd = 4.0 x 10-2 micro m, at pH 7.0 and 25 degrees C), generally taken as a molecular model of cysteine proteases. The molecular bases for papain inhibition by AtCYS1 have been analysed taking into account the three-dimensional structure of the papain-stefin B complex. AtCYS1 is constitutively expressed in roots and in developing siliques of A. thaliana. In leaves, AtCYS1 is strongly induced by wounding, by challenge with avirulent pathogens and by nitric oxide (NO). The overexpression of AtCYS1 blocks cell death activated by either avirulent pathogens or by oxidative and nitrosative stress in both A. thaliana suspension cultured cells and in transgenic tobacco plants. The suppression of the NO-mediated cell death in plants overexpressing AtCYS1 provides the evidence that NO is not cytotoxic for the plant, indicating that NO functions as cell death trigger through the stimulation of an active process, in which cysteine proteases and theirs proteinaceous inhibitors appear to play a crucial role.

Changes in hydrogen peroxide homeostasis trigger an active cell death process in tobacco

In transgenic tobacco plants with reduced catalase activity, high levels of hydrogen peroxide (H2O2) can accumulate under photorespiratory conditions. Such a perturbation in H2O2 homeostasis induced cell death in clusters of palisade parenchyma cells, primarily along the veins. Ultrastructural alterations, such as chromatin condensation and disruption of mitochondrial integrity, took place before cell death. Furthermore, enhanced transcript levels of mitochondrial defense genes accompanied these mitochondrial changes. Pharmacological data indicated that the initiation and execution of cell death require de novo protein synthesis and that the signal transduction pathway leading to cell death involved changes in ion homeostasis, (de)phosphorylation events and an oxidative burst, as observed during hypersensitive responses. This oxidase-dependent oxidative burst is essential for cell death, but it is not required for the accumulation of defense proteins, suggesting a more prominent role for the oxidative burst in abiotic stress-induced cell death.

The impact of oxidative stress on Arabidopsis mitochondria

Treatment of Arabidopsis cell culture for 16 h with H2O2, menadione or antimycin A induced an oxidative stress decreasing growth rate and increasing DCF fluorescence and lipid peroxidation products. Treated cells remained viable and maintained significant respiratory rates. Mitochondrial integrity was maintained, but accumulation of alternative oxidase and decreased abundance of lipoic acid-containing components during several of the treatments indicated oxidative stress. Analysis of the treatments was undertaken by IEF/SDS-PAGE, comparison of protein spot abundances and tandem mass spectrometry. A set of 25 protein spots increased >3-fold in H2O2/menadione treatments, a subset of these increased in antimycin A-treated samples. A set of 10 protein spots decreased significantly during stress treatments. A specific set of mitochondrial proteins were degraded by stress treatments. These damaged components included subunits of ATP synthase, complex I, succinyl CoA ligase, aconitase, and pyruvate and 2-oxoglutarate dehydrogenase complexes. Nine increased proteins represented products of different genes not found in control mitochondria. One is directly involved in antioxidant defense, a mitochondrial thioredoxin-dependent peroxidase, while another, a thioredoxin reductase-dependent protein disulphide isomerase, is required for protein disulfide redox homeostasis. Several others are generally considered to be extramitochondrial but are clearly present in a highly purified mitochondrial fraction used in this study and are known to play roles in stress response. Using H2O2 as a model stress, further work revealed that this treatment induced a protease activity in isolated mitochondria, putatively responsible for the degradation of oxidatively damaged mitochondrial proteins and that O2 consumption by mitochondria was significantly decreased by H2O2 treatment.

Ca(2+)-induced high amplitude swelling and cytochrome c release from wheat (Triticum aestivum L.) mitochondria under anoxic stress

Under stress conditions, mitochondria sense metabolic changes, e.g. in pH, cytoplasmic Ca(2+), energy status, and reactive oxygen species (ROS), and respond by induction of the permeability transition pore (PTP) and by releasing cytochrome c, thus initiating the programmed cell death (PCD) cascade in animal cells. In plant cells, the presence of all the components of the cascade has not yet been shown. In wheat (Triticum aestivum L.) root mitochondria, the onset of anoxia caused rapid dissipation of the inner membrane potential, initial shrinkage of the mitochondrial matrix and the release of previously accumulated Ca(2+). Ca(2+) uptake by mitochondria was dependent on the presence of inorganic phosphate. Treatment of mitochondria with high micromolar and millimolar Ca(2+) (but not Mg(2+)) concentrations induced high amplitude swelling, indicative of PTP opening. Alterations in mitochondrial volume were confirmed by transmission electron microscopy. Mitochondrial swelling was not sensitive to cyclosporin A (CsA)-an inhibitor of mammalian PTP. The release of cytochrome c was monitored under lack of oxygen. Anoxia alone failed to induce cytochrome c release from mitochondria. Oxygen deprivation and Ca(2+) ions together caused cytochrome c release in a CsA-insensitive manner. This process correlated positively with Ca(2+) concentration and required Ca(2+) localization in the mitochondrial matrix. Functional characteristics of wheat root mitochondria, such as membrane potential, Ca(2+) transport, swelling, and cytochrome c release under lack of oxygen are discussed in relation to PCD.