Post-transcriptional suppression of cytosolic ascorbate peroxidase expression during pathogen-induced programmed cell death in tobacco

Sugarcane ScOPR1 gene enhances plant disease resistance through the modulation of hormonal signaling pathways

KEY MESSAGE

Transgenic plants stably overexpressing ScOPR1 gene enhanced disease resistance by increasing the accumulation of JA, SA, and GST, as well as up-regulating the expression of genes related to signaling pathways. 12-Oxo-phytodienoate reductase (OPR) is an oxidoreductase that depends on flavin mononucleotide (FMN) and catalyzes the conversion of 12-oxophytodienoate (12-OPDA) into jasmonic acid (JA). It plays a key role in plant growth and development, and resistance to adverse stresses. In our previous study, we have obtained an OPR gene (ScOPR1, GenBank Accession Number: MG755745) from sugarcane. This gene showed positive responses to methyl jasmonate (MeJA), salicylic acid (SA), abscisic acid (ABA), and Sporisorium scitamineum, suggesting its potential for pathogen resistance. Here, in our study, we observed that Nicotiana benthamiana leaves transiently overexpressing ScOPR1 exhibited weaker disease symptoms, darker 3,3-diaminobenzidine (DAB) staining, higher accumulation of reactive oxygen species (ROS), and higher expression of hypersensitive response (HR) and SA pathway-related genes after inoculation with Ralstonia solanacearum and Fusarium solanacearum var. coeruleum. Furthermore, the transgenic N. benthamiana plants stably overexpressing the ScOPR1 gene showed enhanced resistance to pathogen infection by increasing the accumulation of JA, SA, and glutathione S-transferase (GST), as well as up-regulating genes related to HR, JA, SA, and ROS signaling pathways. Transcriptome analysis revealed that the specific differentially expressed genes (DEGs) in ScOPR1-OE were significantly enriched in hormone transduction signaling and plant-pathogen interaction pathways. Finally, a functional mechanism model of the ScOPR1 gene in response to pathogen infection was depicted. This study provides insights into the molecular mechanism of ScOPR1 and presents compelling evidence supporting its positive involvement in enhancing plant disease resistance.

Physiological function and regulation of ascorbate peroxidase isoforms

Ascorbate peroxidase (APX) reduces H2O2 to H2O by utilizing ascorbate as a specific electron donor and constitutes the ascorbate-glutathione cycle in organelles of plants including chloroplasts, cytosol, mitochondria, and peroxisomes. It has been almost 40 years since APX was discovered as an important plant-specific H2O2-scavenging enzyme, during which time many research groups have conducted molecular physiological analyses. It is now clear that APX isoforms function not only just as antioxidant enzymes but also as important factors in intracellular redox regulation through the metabolism of reactive oxygen species. The function of APX isoforms is regulated at multiple steps, from the transcriptional level to post-translational modifications of enzymes, thereby allowing them to respond flexibly to ever-changing environmental factors and physiological phenomena such as cell growth and signal transduction. In this review, we summarize the physiological functions and regulation mechanisms of expression of each APX isoform.

Identification of citrus APX gene family and their response to CYVCV infection

Ascorbate peroxidase (APX) is one of the most important antioxidant enzymes in the reactive oxygen metabolic pathway of plants. The role of APX under biotic and abiotic stress conditions has been explored, but the response pattern of APX under biotic stresses is relatively less known. In this study, seven CsAPXs gene family members were identified based on the sweet orange (Citrus sinensis) genome and subjected to evolutionary and structural analysis using bioinformatics software. The APX genes of lemon (ClAPXs) were cloned and showed a high conservation to CsAPXs by sequences alignment. In citrus yellow vein clearing virus (CYVCV)-infected Eureka lemons (C. limon) at 30th day post inoculation, APX activity and accumulation of hydrogen peroxide (H₂O₂) and malondialdehyde were measured to be 3.63, 2.29, and 1.73 times to that of the healthy control. The expression levels of 7 ClAPX genes in different periods of CYVCV-infected Eureka lemon were analyzed. Notably, ClAPX1, ClAPX5, and ClAPX7 showed higher expression levels compared to healthy plants, while ClAPX2, ClAPX3, and ClAPX4 showed lower expression levels. Functional identification of ClAPX1 in Nicotiana benthamiana showed that increasing the expression of ClAPX1 could significantly reduce the accumulation of H₂O₂, and it was verified that ClAPX1 is located in the plasma membrane of the cell. The present study provided information on the evolution and function of citrus APXs and revealed for the first time their response pattern to CYVCV infection.

A Nicotiana benthamiana receptor-like kinase regulates Phytophthora resistance by coupling with BAK1 to enhance elicitin-triggered immunity

Cell-surface-localized leucine-rich-repeat receptor-like kinases (LRR-RLKs) are crucial for plant immunity. Most LRR-RLKs that act as receptors directly recognize ligands via a large extracellular domain (ECD), whereas LRR-RLK that serve as regulators are relatively small and contain fewer LRRs. Here, we identified LRR-RLK regulators using high-throughput tobacco rattle virus (TRV)-based gene silencing in the model plant Nicotiana benthamiana. We used the cell-death phenotype caused by INF1, an oomycete elicitin that induces pattern-triggered immunity, as an indicator. By screening 33 small LRR-RLKs (≤6 LRRs) of unknown function, we identified ELICITIN INSENSITIVE RLK 1 (NbEIR1) as a positive regulator of INF1-induced immunity and oomycete resistance. Nicotiana benthamiana mutants of eir1 generated by CRISPR/Cas9-editing showed significantly compromised immune responses to INF1 and were more vulnerable to the oomycete pathogen Phytophthora capsici. NbEIR1 associates with BRI1-ASSOCIATED RECEPTOR KINASE 1 (NbBAK1) and a downstream component, BRASSINOSTEROID-SIGNALING KINASE 1 (NbBSK1). NbBSK1 also contributes to INF1-induced defense and P. capsici resistance. Upon INF1 treatment, NbEIR1 was released from NbBAK1 and NbBSK1 in vivo. Moreover, the silencing of NbBSK1 compromised the association of NbEIR1 with NbBAK1. We also showed that NbEIR1 regulates flg22-induced immunity and associates with its receptor, FLAGELLIN SENSING 2 (NbFLS2). Collectively, our results suggest that NbEIR1 is a novel regulatory element for BAK1-dependent immunity. NbBSK1-NbEIR1 association is required for maintaining the NbEIR1/NbBAK1 complex in the resting state.

ROS generated from biotic stress: Effects on plants and alleviation by endophytic microbes

Aerobic living is thought to generate reactive oxygen species (ROS), which are an inevitable chemical component. They are produced exclusively in cellular compartments in aerobic metabolism involving significant energy transfer and are regarded as by-products. ROS have a significant role in plant response to pathogenic stress, but the pattern varies between necrotrophs and biotrophs. A fine-tuned systemic induction system is involved in ROS-mediated disease development in plants. In regulated concentrations, ROS act as a signaling molecule and activate different pathways to suppress the pathogens. However, an excess of these ROS is deleterious to the plant system. Along with altering cell structure, ROS cause a variety of physiological reactions in plants that lower plant yield. ROS also degrade proteins, enzymes, nucleic acids, and other substances. Plants have their own mechanisms to overcome excess ROS and maintain homeostasis. Microbes, especially endophytes, have been reported to maintain ROS homeostasis in both biotic and abiotic stresses by multiple mechanisms. Endophytes themselves produce antioxidant compounds and also induce host plant machinery to supplement ROS scavenging. The structured reviews on how endophytes play a role in ROS homeostasis under biotic stress were very meager, so an attempt was made to compile the recent developments in ROS homeostasis using endophytes. This review deals with ROS production, mechanisms involved in ROS signaling, host plant mechanisms in alleviating oxidative stress, and the roles of endophytes in maintaining ROS homeostasis under biotic stress.

Ferroptosis in plants: regulation of lipid peroxidation and redox status

Regulated cell death (RCD) is an essential process that plays key roles along the plant life cycle. Unlike accidental cell death, which is an uncontrolled biological process, RCD involves integrated signaling cascades and precise molecular-mediated mechanisms that are triggered in response to specific exogenous or endogenous stimuli. Ferroptosis is a cell death pathway characterized by the iron-dependent accumulation of lipid reactive oxygen species. Although first described in animals, ferroptosis in plants shares all the main core mechanisms observed for ferroptosis in other systems. In plants as in animals, oxidant and antioxidant systems outline the process of lipid peroxidation during ferroptosis. In plants, cellular compartments such as mitochondria, chloroplasts and cytosol act cooperatively and coordinately to respond to changing redox environments. This particular context makes plants a unique model to study redox status regulation and cell death. In this review, we focus on our most recent understanding of the regulation of redox state and lipid peroxidation in plants and their role during ferroptosis.

Oryza-Specific Orphan Protein Triggers Enhanced Resistance to Xanthomonas oryzae pv. oryzae in Rice

All genomes carry lineage-specific orphan genes lacking homology in their closely related species. Identification and functional study of the orphan genes is fundamentally important for understanding lineage-specific adaptations including acquirement of resistance to pathogens. However, most orphan genes are of unknown function due to the difficulties in studying them using helpful comparative genomics. Here, we present a defense-related Oryza-specific orphan gene, Xio1, specifically induced by the bacterial pathogen Xanthomonas oryzae pv. oryzae (Xoo) in an immune receptor XA21-dependent manner. Salicylic acid (SA) and ethephon (ET) also induced its expression, but methyl jasmonic acid (MeJA) reduced its basal expression. C-terminal green fluorescent protein (GFP) tagged Xio1 (Xio1-GFP) was visualized in the nucleus and the cytosol after polyethylene glycol (PEG)-mediated transformation in rice protoplasts and Agrobacterium-mediated infiltration in tobacco leaves. Transgenic rice plants overexpressing Xio1-GFP showed significantly enhanced resistance to Xoo with reduced lesion lengths and bacterial growth, in company with constitutive expression of defense-related genes. However, all of the transgenic plants displayed severe growth retardation and premature death. Reactive oxygen species (ROS) was significantly produced in rice protoplasts constitutively expressing Xio1-GFP. Overexpression of Xio1-GFP in non-Oryza plant species, Arabidopsis thaliana, failed to induce growth retardation and enhanced resistance to Pseudomonas syringae pv. tomato (Pst) DC3000. Our results suggest that the defense-related orphan gene Xio1 plays an important role in distinctive mechanisms evolved within the Oryza and provides a new source of Oryza-specific genes for crop-breeding programs.

The Phytophthora effector Avh241 interacts with host NDR1-like proteins to manipulate plant immunity

Plant pathogens rely on effector proteins to suppress host innate immune responses and facilitate colonization. Although the Phytophthora sojae RxLR effector Avh241 promotes Phytophthora infection, the molecular basis of Avh241 virulence remains poorly understood. Here we identified non-race specific disease resistance 1 (NDR1)-like proteins, the critical components in plant effector-triggered immunity (ETI) responses, as host targets of Avh241. Avh241 interacts with NDR1 in the plasma membrane and suppresses NDR1-participated ETI responses. Silencing of GmNDR1s increases the susceptibility of soybean to P. sojae infection, and overexpression of GmNDR1s reduces infection, which supports its positive role in plant immunity against P. sojae. Furthermore, we demonstrate that GmNDR1 interacts with itself, and Avh241 probably disrupts the self-association of GmNDR1. These data highlight an effective counter-defense mechanism by which a Phytophthora effector suppresses plant immune responses, likely by disturbing the function of NDR1 during infection.

Differential Alternative Splicing Genes and Isoform Regulation Networks of Rapeseed (Brassica napus L.) Infected with Sclerotinia sclerotiorum

Alternative splicing (AS) is a post-transcriptional level of gene expression regulation that increases transcriptome and proteome diversity. How the AS landscape of rapeseed (Brassica napus L.) changes in response to the fungal pathogen Sclerotinia sclerotiorum is unknown. Here, we analyzed 18 RNA-seq libraries of mock-inoculated and S. sclerotiorum-inoculated susceptible and tolerant B. napus plants. We found that infection increased AS, with intron retention being the main AS event. To determine the key genes functioning in the AS response, we performed a differential AS (DAS) analysis. We identified 79 DAS genes, including those encoding splicing factors, defense response proteins, crucial transcription factors and enzymes. We generated coexpression networks based on the splicing isoforms, rather than the genes, to explore the genes’ diverse functions. Using this weighted gene coexpression network analysis alongside a gene ontology enrichment analysis, we identified 11 modules putatively involved in the pathogen defense response. Within these regulatory modules, six DAS genes (ascorbate peroxidase 1, ser/arg-rich protein 34a, unknown function 1138, nitrilase 2, v-atpase f, and amino acid transporter 1) were considered to encode key isoforms involved in the defense response. This study provides insight into the post-transcriptional response of B. napus to S. sclerotiorum infection.

Cytosolic Ascorbate Peroxidases Plays a Critical Role in Photosynthesis by Modulating Reactive Oxygen Species Level in Stomatal Guard Cell

Photosynthetic rate is one of the key factors limiting yield of cotton. Reactive oxygen species (ROS) generated by abiotic stress imposes numerous detrimental effects and causes tremendous loss of yield. It is worth to study whether ROS scavenging enzymes could affect yield through regulating photosynthetic rate in cotton. In this study, we created transgenic cotton with changes of endogenous ROS by overexpressing or suppressing the expression of cytosolic ascorbate peroxidases (APXs), which are hydrogen peroxide (H₂O₂) scavenging enzymes in plants. The suppression of cytosolic APXs by RNAi brings about a great influence on plant growth and development. Plant height and leaf size declined, and yield-related traits including single boll weight, seed weight, seed size, and lint weight dropped significantly, in IAO lines (cytosolic APX-suppressed lines). The stunted plant growth was due to the decrease of plant photosynthetic rate. The evidences showed that increased ROS level in guard cells inhibited stomatal opening and suppressed the absorption of CO₂ and H₂O in IAO line. The decrease of water content and the increase of water loss rate in leaf exacerbated the decline of photosynthetic rate in cytosolic APX-suppressed lines. Based on these results, it implies that cytosolic APXs as a whole play an important role in maintaining REDOX balance to regulate photosynthetic rate and yield in cotton.

Using Foldscope to Monitor Superoxide Production and Cell Death During Pathogen Infection in Arabidopsis Under Different Nitrogen Regimes

Nitrogen nutrition plays a role in plant growth development and resistance against biotic and abiotic stress. During pathogen infection various signal molecules such as reactive oxygen species, calcium, reactive nitrogen species, salicylic acid, and ethylene plays an important role. The form of nitrogen nutrition such as nitrate or ammonium plays a role in production of these molecules. Under nitrate nutrition NO is predominant. The produced NO plays a role in reacting with superoxide to generate peroxynitrite to induce cell death during hypersensitive response elicited by avirulent pathogens. Excess of ROS is also detrimental to plants and NO plays a role in regulating ROS. Hence it is important to observe superoxide production during infection. By using an avirulent Pseudomonas syringae and Arabidopsis differential N nutrition we show superoxide production in leaves using a paper microscope called Foldscope, which can be applied as a simple microscope to observe objects. The data also compared with root system infected with pathogenic Fusarium oxysporum. Taken together here we show that Foldscope is a cost-effective and powerful technique to visualize superoxide and cell death in plants during infection.

The chaperone-like protein CDC48 regulates ascorbate peroxidase in tobacco

There is increasing evidence that the chaperone-like protein CDC48 (cell division cycle 48) plays a role in plant immunity. Cytosolic ascorbate peroxidase (cAPX), which is a major regulator of the redox status of plant cells, has previously been shown to interact with CDC48. In this study, we examined the regulation of cAPX by the ATPase NtCDC48 during the cryptogein-induced immune response in tobacco cells. Our results not only confirmed the interaction between the proteins but also showed that it occurs in the cytosol. cAPX accumulation was modified in cells overexpressing NtCDC48, a process that was shown to involve post-translational modification of cAPX. In addition, cryptogein-induced increases in cAPX activity were suppressed in cells overexpressing NtCDC48 and the abundance of the cAPX dimer was below the level of detection. Furthermore, the levels of both reduced (GSH) and oxidized glutathione (GSSG) and the GSH/GSSG ratio decreased more rapidly in response to the elicitor in these cells than in controls. A decrease in cAPX activity was also observed in response to heat shock in the cells overexpressing NtCDC48, indicating that the regulation of cAPX by NtCDC48 is not specific to the immune response.

Regulation of programmed cell death by phytoglobins

Programmed cell death (PCD) is a fundamental plant process in growth and development and in response to both biotic and abiotic stresses. Nitric oxide (NO) is a central component in determining whether a cell undergoes PCD, either as a direct elicitor of the response or as a factor in signal transduction from various hormones. Both NO and hormones that use NO as a signal transducer are mobile in the plant. Why do one set of cells die while adjacent cells remain alive, if this is the case? There is evidence to suggest that phytoglobins (Pgbs; previously termed non-symbiotic hemoglobins) may act as binary switches to determine plant cellular responses to perturbations. There are anywhere from one to five Pgb genes in plants that are expressed in response to growth and development and to stress. One of their main functions is to scavenge NO. This review will discuss how Pgb modulates cellular responses to auxin, cytokinin, and jasmonic acid during growth and development and in response to stress. The moderation in the production of reactive oxygen species (ROS) by Pgbs and the effects on PCD will also be discussed. An overall mechanism for Pgb involvement will be presented.

Synergistic interaction among begomoviruses leads to the suppression of host defense-related gene expression and breakdown of resistance in chilli

Chilli (Capsicum sp.) is one of the economically important spice and vegetable crops grown in India and suffers great losses due to the infection of begomoviruses. Conventional breeding approaches have resulted in development of a few cultivars of chilli resistant to begomoviruses. A severe leaf curl disease was observed on one such resistant chilli cultivar (Capsicum annuum cv. Kalyanpur Chanchal) grown in the experimental field of the Jawaharlal Nehru University, New Delhi. Four different viral genomic components namely, Chilli leaf curl virus (DNA A), Tomato leaf curl Bangladesh betasatellite (DNA β), Tomato leaf curl New Delhi virus (DNA A), and Tomato leaf curl Gujarat virus (DNA B) were associated with the severe leaf curl disease. Further, frequent association of these four genomic components was also observed in symptomatic plants of other chilli cultivars (Capsicum annuum cv. Kashi Anmol and Capsicum chinense cv. Bhut Jolokia) grown in the experimental field. Interaction studies among the isolated viral components revealed that Nicotiana benthamiana and chilli plants inoculated with four genomic components of begomoviruses exhibited severe leaf curl disease symptoms. In addition, this synergistic interaction resulted in increased viral DNA accumulation in infected plants. Resistant chilli plants co-inoculated with four genomic components of begomoviruses showed drastic reduction of host basal (ascorbate peroxidase, thionin, polyphenol oxidase) and specific defense-related gene (NBS-LRR) expression. Our results suggested that synergistic interaction among begomoviruses created permissive cellular environment in the resistant chilli plants which leads to breakdown of natural resistance, a phenomenon observed for the first time in chilli.

Proteome and transcript analysis of Vitis vinifera cell cultures subjected to Botrytis cinerea infection

Gray mold caused by Botrytis cinerea is one of the most important diseases of grapevine resulting in significant reductions in yield and fruit quality. In order to examine the molecular mechanisms that characterize the interaction between B. cinerea and the host plant, the grapevine cytoplasmic proteome was analyzed by two-dimensional polyacrylamide gel electrophoresis. The interaction between Vitis vinifera cv. Gamay cells and B. cinerea was characterized by the increase in spot abundance of 30 proteins, of which 21 were successfully identified. The majority of these proteins were related to defence and stress responses and to cell wall modifications. Some of the modulated proteins have been previously found to be affected by other pathogens when they infect V. vinifera but interestingly, the proteins related to cell wall modification that were influenced by B. cinerea have not been shown to be modulated by any other pathogen studied to date. Transcript analysis using the quantitative real time polymerase chain reaction additionally revealed the up-regulation of several acidic, probably extracellular, chitinases. The results indicate that cell wall strengthening, accumulation of PR proteins and excretion of lytic enzymes are likely to be important mechanisms in the defence of grapevine against B. cinerea.

BIOLOGICAL SIGNIFICANCE

Although gray mold caused by Botrytis cinerea is one of the most important diseases of grapevine, little information is available about proteomic changes in this pathosystem. These results suggest that cell wall strengthening, accumulation of PR proteins and excretion of lytic enzymes are important molecular mechanisms in the defence of grapevine against B. cinerea. Surprisingly, the proteins related to cell wall modification that were modulated by B. cinerea have not been shown to be affected by any other pathogen studied to date.

Extracellular matrix-associated proteome changes during non-host resistance in citrus-Xanthomonas interactions

Non-host resistance (NHR) is a most durable broad-spectrum resistance employed by the plants to restrict majority of pathogens. Plant extracellular matrix (ECM) is a critical defense barrier. Understanding ECM responses during interaction with non-host pathogen will provide insights into molecular events of NHR. In this study, the ECM-associated proteome was compared during interaction of citrus with pathogen Xanthomonas axonopodis pv. citri (Xac) and non-host pathogen Xanthomonas oryzae pv. oryzae (Xoo) at 8, 16, 24 and 48 h post inoculation. Comprehensive analysis of ECM-associated proteins was performed by extracting wall-bound and soluble ECM components using both destructive and non-destructive procedures. A total of 53 proteins was differentially expressed in citrus-Xanthomonas host and non-host interaction, out of which 44 were identified by mass spectrometry. The differentially expressed proteins were related to (1) defense-response (5 pathogenesis-related proteins, 3 miraculin-like proteins (MIR, MIR1 and MIR2) and 2 proteases); (2) enzymes of reactive oxygen species (ROS) metabolism [Cu/Zn superoxide dismutase (SOD), Fe-SOD, ascorbate peroxidase and 2-cysteine-peroxiredoxin]; (3) signaling (lectin, curculin-like lectin and concanavalin A-like lectin kinase); and (4) cell-wall modification (α-xylosidase, glucan 1, 3 β-glucosidase, xyloglucan endotransglucosylase/hydrolase). The decrease in ascorbate peroxidase and cysteine-peroxiredoxin could be involved in maintenance of ROS levels. Increase in defense, cell-wall remodeling and signaling proteins in citrus-Xoo interaction suggests an active involvement of ECM in execution of NHR. Partially compromised NHR in citrus against Xoo, upon Brefeldin A pre-treatment supported the role of non-classical secretory proteins in this phenomenon.

Selective recruitment of mRNAs and miRNAs to polyribosomes in response to rhizobia infection in Medicago truncatula

Translation of mRNAs is a key regulatory step that contributes to the coordination and modulation of eukaryotic gene expression during development or adaptation to the environment. mRNA stability or translatability can be regulated by the action of small regulatory RNAs (sRNAs), which control diverse biological processes. Under low nitrogen conditions, leguminous plants associate with soil bacteria and develop a new organ specialized in nitrogen fixation: the nodule. To gain insight into the translational regulation of mRNAs during nodule formation, the association of mRNAs and sRNAs to polysomes was characterized in roots of the model legume Medicago truncatula during the symbiotic interaction with Sinorhizobium meliloti. Quantitative comparison of steady-state and polysomal mRNAs for 15 genes involved in nodulation identified a group of transcripts with slight or no change in total cellular abundance that were significantly upregulated at the level of association with polysomes in response to rhizobia. This group included mRNAs encoding receptors like kinases required either for nodule organogenesis, bacterial infection or both, and transcripts encoding GRAS and NF-Y transcription factors (TFs). Quantitative analysis of sRNAs in total and polysomal RNA samples revealed that mature microRNAs (miRNAs) were associated with the translational machinery, notably, miR169 and miR172, which target the NF-YA/HAP2 and AP2 TFs, respectively. Upon inoculation, levels of miR169 pronouncedly decreased in polysomal complexes, concomitant with the increased accumulation of the NF-YA/HAP2 protein. These results indicate that both mRNAs and miRNAs are subject to differential recruitment to polysomes, and expose the importance of selective mRNA translation during root nodule symbiosis.

Transgenic expression of Tobacco mosaic virus capsid and movement proteins modulate plant basal defense and biotic stress responses in Nicotiana tabacum

Plant viruses cause metabolic and physiological changes associated with symptomatic disease phenotypes. Symptoms involve direct and indirect effects, which result in disruption of host physiology. We used transgenic tobacco expressing a variant of Tobacco mosaic virus (TMV) coat protein (CP(T42W)) or movement protein (MP), and a hybrid line (MP×CP(T42W)) that coexpresses both proteins, to study the plant response to individual viral proteins. Findings employing microarray analysis of MP×CP(T42W) plants and silenced mp×cp(T42W)* controls revealed that altered transcripts were mostly downregulated, suggesting a persistent shut-off due to MP×CP(T42W) expression. Next, we showed that MP triggered reactive oxygen species (ROS) accumulation, reduction of total ascorbate, and expression of ROS scavenging genes. These effects were enhanced when both proteins were coexpressed. MP and MP×CP(T42W) plants showed increased levels of salicylic acid (SA) and SA-responsive gene expression. Furthermore, these effects were partially reproduced in Nicotiana benthamiana when GMP1 transcript was silenced. CP(T42W) seems to be playing a negative role in the defense response by reducing the expression of PR-1 and RDR-1. MP and MP×CP(T42W) transgenic expression promoted a recovery-like phenotype in TMV RNA infections and enhanced susceptibility to Pseudomonas syringae and Sclerotinia sclerotiorum. The individual effects of viral proteins may reflect the ability of a virus to balance its own virulence.

Antioxidant and prooxidant effects of lanthanum ions on Vicia faba L. seedlings under cadmium stress, suggesting ecological risk

The present study combined chemical analyses and biological measurements to investigate biphasic effects of La on Cd stress in leaves of Vicia faba seedlings, which were hydroponically cultivated for 15 d in the combination of 6 µM CdCl(2) and 2 to 480 µM La(NO(3))(3), respectively. The results showed that contents of Cd first elevated above and then declined below the 6 µM single Cd treatment when 2 to 30 µM extraneous La were combined. Contents of mineral nutrients altered differentially and became imbalanced. No distinct band was observed in catalase (CAT), guaiacol peroxidase (GPX), or ascorbate peroxidase (APX) patterns, but in superoxide dismutase (SOD) isozymes by the supplementation with 8 to 480 µM of extraneous La. Superoxide dismutase and APX activities changed as a U-shaped curve; however, CAT and GPX changed as an inverted U-shaped curve along with increasing La. Moreover, heat shock protein 70 (HSP 70) production was reduced below the single treatment of Cd at 2 to 8 µM of extraneous La and enhanced thereafter. Thus, La at lower concentrations promoted antioxidation against Cd stress; La at higher concentrations turned to prooxidant effects, implicating potential ecological risk. Heat shock protein 70, combined with the antioxidant enzymes, constitutes an integrative defense system, which can be used to estimate the degree of antioxidation or prooxidation of extraneous La to Cd-induced oxidative stress in the seedlings.

Redox regulation in plant programmed cell death

Programmed cell death (PCD) is a genetically controlled process described both in eukaryotic and prokaryotic organisms. Even if it is clear that PCD occurs in plants, in response to various developmental and environmental stimuli, the signalling pathways involved in the triggering of this cell suicide remain to be characterized. In this review, the main similarities and differences in the players involved in plant and animal PCD are outlined. Particular attention is paid to the role of reactive oxygen species (ROS) as key inducers of PCD in plants. The involvement of different kinds of ROS, different sites of ROS production, as well as their interaction with other molecules, is crucial in activating PCD in response to specific stimuli. Moreover, the importance is stressed on the balance between ROS production and scavenging, in various cell compartments, for the activation of specific steps in the signalling pathways triggering this cell suicide process. The review focuses on the complexity of the interplay between ROS and antioxidant molecules and enzymes in determining the most suitable redox environment required for the occurrence of different forms of PCD.

The antagonistic effect of hydroxyl radical on the development of a hypersensitive response in tobacco

Reactive oxygen species (ROS) are important signalling molecules in living cells. It is believed that ROS molecules are the main triggers of the hypersensitive response (HR) in plants. In the present study of the effect of riboflavin, which is excited to generate ROS in light, on the development of the HR induced by the elicitin protein ParA1 in tobacco (Nicotiana tabacum), we found that the extent of the ParA1-induced HR was diminished by hydroxyl radical (OH•), a type of ROS. As compared with the zones treated with ParA1 only, the HR symptom in the zones that were infiltrated with ParA1 plus riboflavin was significantly diminished when the treated plants were placed in the light. However, this did not occur when the plants were maintained in the dark. Trypan blue staining and the ion leakage measurements confirmed HR suppression in the light. Further experiments proved that HR suppression is attributed to the involvement of the photoexcited riboflavin, and that the suppression can be eliminated with the addition of hydrogen peroxide scavengers or OH• scavengers. Fenton reagent treatment and EPR measurements demonstrated that it is OH• rather than hydrogen peroxide that contributes to HR suppression. Accompanying the endogenous OH• formation, suppression of the ParA1-induced HR occurred in the tobacco leaves that had been treated with high-level abscisic acid, and that suppression was also removed by OH• scavengers. These results offer evidence that OH•, an understudied and little appreciated ROS, participates in and modulates biologically relevant signalling in plant cells.

Analysis of ascorbate peroxidase genes expressed in resistant and susceptible wheat lines infected by the cereal cyst nematode, Heterodera avenae

Changes in ascorbate peroxidase (APX) enzyme activity in response to nematode (Heterodera avenae) attack were studied in roots of three hexaploid wheat lines carrying Cre2, Cre5, or Cre7 nematode resistance genes and the susceptible Triticum aestivum cv. Anza. A spectrophotometric analysis was carried out with root extracts of infected plants 4, 7, 11, and 14 days after nematode inoculation using uninfected plant as control. APX induction in infected resistant genotypes was similar and higher than in the susceptible control. The introgression wheat/Aegilops ventricosa H-93-8 line, carrying the Cre2 gene, and its parental line H-10-15 as susceptible control were used to analyze whether this increase of activity was correlated with the induction of APX gene expression. Genes encoding cytosolic forms of APX were induced in roots of both lines in response to nematode infection. This induction took place both earlier and with greater intensity in the resistant line than in the susceptible one, and it was also higher in the root area at the site of nematode attachment.

Nitric oxide and hydrogen peroxide involvement during programmed cell death of Sechium edule nucellus

The nucellus is a maternal tissue that feeds the developing embryo and the secondary endosperm. During seed development the cells of the nucellus suffer a degenerative process early after fertilization as the cellular endosperm expands and accumulates reserves. Nucellar cell degeneration has been characterized as a form of developmentally programmed cell death (PCD). In this work we show that nucellus PCD is accompanied by a considerable production of both nitric oxide and hydrogen peroxide (NO and H(2)O(2)). Interestingly, each of the two molecules is able to induce the production of the other and to cause cell death when applied to a living nucellus. We show that the induced cell death has features of a PCD, accompanied by profound changes in the morphology of the nuclei and by a massive degradation of nuclear DNA. Moreover, we report that NO and H(2)O(2) cause an induction of caspase-like proteases previously characterized in physiological nucellar PCD.

The expression, function and regulation of mitochondrial alternative oxidase under biotic stresses

To survive, plants possess elaborate defence mechanisms to protect themselves against virus or pathogen invasion. Recent studies have suggested that plant mitochondria may play an important role in host defence responses to biotic stresses. In contrast with animal mitochondria, plant mitochondria possess a unique respiratory pathway, the cyanide-insensitive alternative pathway, which is catalysed by the alternative oxidase (AOX). Much work has revealed that the genes encoding AOX, AOX protein and the alternative respiratory pathway are frequently induced during plant-pathogen (or virus) interaction. This raises the possibility that AOX is involved in host defence responses to biotic stresses. Thus, a key to the understanding of the role of mitochondrial respiration under biotic stresses is to learn the function and regulation of AOX. In this article, we focus on the theoretical and experimental progress made in the current understanding of the function and regulation of AOX under biotic stresses. We also address some speculative aspects to aid further research in this area.

Chloroplast-generated reactive oxygen species play a major role in localized cell death during the non-host interaction between tobacco and Xanthomonas campestris pv. vesicatoria

Attempted infection of plants by pathogens elicits a complex defensive response. In many non-host and incompatible host interactions it includes the induction of defence-associated genes and a form of localized cell death (LCD), purportedly designed to restrict pathogen advance, collectively known as the hypersensitive response (HR). It is preceded by an oxidative burst, generating reactive oxygen species (ROS) that are proposed to cue subsequent deployment of the HR, although neither the origin nor the precise role played by ROS in the execution of this response are completely understood. We used tobacco plants expressing cyanobacterial flavodoxin to address these questions. Flavodoxin is an electron shuttle present in prokaryotes and algae that, when expressed in chloroplasts, specifically prevents ROS formation in plastids during abiotic stress episodes. Infiltration of tobacco wild-type leaves with high titres of Xanthomonas campestris pv. vesicatoria (Xcv), a non-host pathogen, resulted in ROS accumulation in chloroplasts, followed by the appearance of localized lesions typical of the HR. In contrast, chloroplast ROS build-up and LCD were significantly reduced in Xcv-inoculated plants expressing plastid-targeted flavodoxin. Metabolic routes normally inhibited by pathogens were protected in the transformants, whereas other aspects of the HR, including the induction of defence-associated genes and synthesis of salicylic and jasmonic acid, proceeded as in inoculated wild-type plants. Therefore, ROS generated in chloroplasts during this non-host interaction are essential for the progress of LCD, but do not contribute to the induction of pathogenesis-related genes or other signalling components of the response.

Peroxisome biogenesis and function

Peroxisomes are small and single membrane-delimited organelles that execute numerous metabolic reactions and have pivotal roles in plant growth and development. In recent years, forward and reverse genetic studies along with biochemical and cell biological analyses in Arabidopsis have enabled researchers to identify many peroxisome proteins and elucidate their functions. This review focuses on the advances in our understanding of peroxisome biogenesis and metabolism, and further explores the contribution of large-scale analysis, such as in sillco predictions and proteomics, in augmenting our knowledge of peroxisome function In Arabidopsis.

Redox regulation in photosynthetic organisms: signaling, acclimation, and practical implications

Reactive oxygen species (ROS) have multifaceted roles in the orchestration of plant gene expression and gene-product regulation. Cellular redox homeostasis is considered to be an "integrator" of information from metabolism and the environment controlling plant growth and acclimation responses, as well as cell suicide events. The different ROS forms influence gene expression in specific and sometimes antagonistic ways. Low molecular antioxidants (e.g., ascorbate, glutathione) serve not only to limit the lifetime of the ROS signals but also to participate in an extensive range of other redox signaling and regulatory functions. In contrast to the low molecular weight antioxidants, the "redox" states of components involved in photosynthesis such as plastoquinone show rapid and often transient shifts in response to changes in light and other environmental signals. Whereas both types of "redox regulation" are intimately linked through the thioredoxin, peroxiredoxin, and pyridine nucleotide pools, they also act independently of each other to achieve overall energy balance between energy-producing and energy-utilizing pathways. This review focuses on current knowledge of the pathways of redox regulation, with discussion of the somewhat juxtaposed hypotheses of "oxidative damage" versus "oxidative signaling," within the wider context of physiological function, from plant cell biology to potential applications.

Proteomic analysis of tobacco mosaic virus-infected tomato (Lycopersicon esculentum M.) fruits and detection of viral coat protein

Tobacco mosaic virus (TMV) is a widespread plant virus from the genus Tobamovirus that affects tobacco and tomato plants causing a pathology characterised by cell breakage and disorganisation in plant leaves and fruits. In this study we undertook a proteomic approach to investigate the molecular and biochemical mechanisms potentially involved in tomato fruit defence against the viral infection. The comparison of 2-D gels from control and TMV-infected but asymptomatic tomato fruits revealed changes in several proteins. The differential expression of peptidases, endoglucanase, chitinase and proteins participating in the ascorbate-glutathione cycle in infected fruits suggests that pathogenesis-related proteins and antioxidant enzymes may play a role in the protection against TMV infection. TMV coat protein appeared as a prominent spot in 2-D gels from TMV-infected asymptomatic fruits. A Triton X-114 phase-partitioning step of tomato protein extracts favoured the solubilisation of TMV coat protein and the enrichment of two aminopeptidases not present in control fruits. PMF and MS/MS data of the 2-D gel-isolated TMV coat protein is proposed as a powerful analysis method for the simultaneous tobamovirus detection, species determination and strain differentiation in virus-infected fruit commodities.

Suppression of tobacco mosaic virus-induced hypersensitive-type necrotization in tobacco at high temperature is associated with downregulation of NADPH oxidase and superoxide and stimulation of dehydroascorbate reductase

Tissue necroses and resistance during the hypersensitive response (HR) of tobacco to tobacco mosaic virus (TMV) are overcome at temperatures above 28 degrees C and the virus multiplies to high levels in the originally resistant N-gene expressing plants. We have demonstrated that chemical compounds that generate reactive oxygen species (ROS) or directly applied hydrogen peroxide (H(2)O(2)) are able to induce HR-type necroses in TMV-inoculated Xanthi-nc tobacco even at high temperatures (e.g. 30 degrees C). The amount of superoxide (O(2)(*-)) decreased, while H(2)O(2) slightly increased in TMV- and mock-inoculated leaves at 30 degrees C, as compared with 20 degrees C. Activity of NADPH oxidase and mRNA levels of genes that encode NADPH oxidase and an alternative oxidase, respectively, were significantly lower, while activity of dehydroascorbate reductase was significantly higher at 30 degrees C, as compared with 20 degrees C. It was possible to reverse or suppress the chemically induced HR-type necrotization at 30 degrees C by the application of antioxidants, such as superoxide dismutase and catalase, demonstrating that the development of HR-type necroses indeed depends on a certain level of superoxide and other ROS. Importantly, high TMV levels at 30 degrees C were similar in infected plants, whether the HR-type necrotization developed or not. Suppression of virus multiplication in resistant, HR-producing tobacco at lower temperatures seems to be independent of the appearance of necroses but is associated with temperatures below 28 degrees C.

The developmental transition to flowering in Arabidopsis is associated with an increase in leaf chloroplastic lipoxygenase activity

The developmental transition from vegetative growth to flowering in Arabidopsis is associated with a precipitous decline in the activity of leaf ascorbate peroxidase (APx), an enzymatic scavenger of hydrogen peroxide, and an increase in specific lipid peroxidation leading to the accumulation of 13-hydroperoxy-9,11,15 (Z,E,Z) octadecatrienoic acid (13 HOO-FA). The appearance of this specific isomer suggests that it is of enzymatic origin and may represent the activation of an oxylipin signaling pathway. We thus hypothesized that leaf 13-lipoxygenase (LOX) activity increases at the floral transition and leads to the observed elevation of 13-HOO-FA levels. Leaf protein extracts were prepared from seven distinct life stages of Arabidopsis plants and used to assay for LOX activity. We report that leaf 13-LOX enzymatic activity increases two- to three-fold from the vegetative stage to the immediate post-floral transition stage. We found two forms of LOX activity in cell extracts and show that the higher pH optimum form is the isoenzyme activated. This increase is correlated with a small increase in H(2)O(2), perhaps resulting from the previously reported decline in leaf APx activity. Very low levels of exogenous H(2)O(2) activate the induced form in vegetative leaf extracts in vitro, suggesting that the floral transition-dependent APx decline and subsequent H(2)O(2) elevation are involved in activating plastid 13-LOX and thus a second messenger oxylipin pathway.

Anthocyanin, antioxidant activity and stress-induced gene expression in high CO2-treated table grapes stored at low temperature

A pretreatment with 20kPa CO2+20 kPa O2+60 kPa N2 for 3 days proved effective in maintaining the fruit quality and controlling decay in table grapes (Vitis vinifera cv. Cardinal) stored at 0 degrees C. In the present work, we analyzed whether total anthocyanin content, the molecular mechanism implicated in their biosynthesis and antioxidant activity is related to the beneficial effect of this gaseous treatment. We isolated partial cDNAs that codified for enzymes implicated in the anthocyanin biosynthesis such as l-phenylalanine ammonia-lyase (PAL) and chalcone synthase (CHS), and an antioxidant enzyme such as ascorbate peroxidase (APX). Low temperatures induced an accumulation of total anthocyanin content in the skin of both treated and non-treated grapes, although levels were lower in CO2-treated fruit. By contrast, antioxidant activity decreased during storage at 0 degrees C in non-treated grapes but did not change in CO2-treated grapes. The up-regulation of anthocyanin biosynthesis gene expression and VcAPX mRNA observed in non-treated grape is not enhanced in CO2-treated grapes, which presented low total decay. These results point out the ability of CO2-treated grapes to prevent the generation of reactive oxygen species rather than their inactivation by means of induction of studied defense systems.

Metabolism of reactive oxygen species is attenuated in leghemoglobin-deficient nodules of Lotus japonicus

Leghemoglobins together with high rates of respiration are believed to be major sources of reactive oxygen species (ROS) in root nodules of leguminous plants. High capacities of antioxidative systems apparently protect this organ from oxidative damage. Using leghemoglobin-RNA interference (LbRNAi) lines of Lotus japonicus, we found that loss of leghemoglobin results in significantly lower H(2)O(2) levels in nodules. Transcript levels and catalytic activities of ascorbate-glutathione cycle enzymes involved in H(2)O(2) detoxification as well as concentrations of reduced ascorbate were also altered in LbRNAi nodules. Thus, symbiotic leghemoglobins contribute significantly to ROS generation in functional nodules.

Protein changes in the albedo of citrus fruits on postharvesting storage

In this work, major protein changes in the albedo of the fruit peel of Murcott tangor (tangerine x sweet orange) during postharvest ageing were studied through 2D PAGE. Protein content in matured on-tree fruits and in fruits stored in nonstressing [99% relative humidity (RH) and 25 degrees C], cold (99% RH and 4 degrees C), and drought (60% RH and 25 degrees C) conditions was initially determined. Protein identification through MS/MS determinations revealed in all samples analyzed the occurrence of manganese superoxide dismutase (Mn SOD), actin, ATP synthase beta subunit (ATPase), citrus salt-stress associated protein (CitSap), ascorbate peroxidase (APX), translationally controlled tumor protein (TCTP), and a cysteine proteinase (CP) of the papain family. The latter protein was identified in two different gel spots, with different molecular mass, suggesting the simultaneous presence of the proteinase precursor and its active form. While Mn SOD, actin, ATPase, and CitSap were unchanged in the assayed conditions, TCTP and APX were downregulated during the postharvest ageing process. Ageing-induced APX repression was also reversed by drought. CP contents in albedo, which were similar in on- and off-tree fruits, were strongly dependent upon cold storage. The active/total CP protein ratio significantly increased after cold exposure. This proteomic survey indicates that major changes in protein content in the albedo of the peel of postharvest stored citrus fruits are apparently related to the activation of programmed cell death (PCD).

Changes in the antioxidant status in leaves of Solanum species in response to elicitor from Phytophthora infestans

Three Solanum genotypes with various polygenic resistance levels to the oomycete pathogen Phytophthora infestans (Mont.) De Bary were studied for their antioxidant response to the pathogen culture filtrate (CF). Detached plant leaves were treated with CF for 6, 18 and 30 h, and assayed for changes in hydrogen peroxide content, total ascorbate and glutathione pools and redox ratios (reduced form to total pool), as well as for changes in activities of ascorbate peroxidase, glutathione reductase and glutathione-S-transferase. In CF treated leaves of non-host resistant S. nigrum var. gigantea and field resistant S. tuberosum cv Bzura, the H(2)O(2) content did not change in comparison to water treated control leaves, whereas in the susceptible S. tuberosum clone H-8105 it decreased below the control level. In CF treated leaves of all genotypes, the total ascorbate pools were relatively unaltered and their redox ratio changed only transiently. In Bzura leaves the total glutathione content increased earlier than in the two other genotypes. The glutathione redox ratio remained rather stable, except for the susceptible clone H-8105, where it decreased transiently by about 42%. The relative increases in activity of all the studied enzymes were the highest in the susceptible clone H-8105. The results are discussed in the light of oxidative processes occurring in CF treated leaves. We conclude that stringent control of pro- and anti-oxidant reactions bringing the H(2)O(2) and/or cellular redox state to the threshold level is decisive for deployment of an effective defense strategy.

Protein tyrosine dephosphorylation during copper-induced cell death in rice roots

Early signalling events that control the process of heavy metal-induced cell death are largely unknown in plants. In mammals protein tyrosine phosphorylation plays an important role in the activation of programmed cell death. We thus examined the involvement of tyrosine phosphorylation in Cu-induced rice cell death. This investigation demonstrates that Cu induces cell death and DNA fragmentation in rice root cells. In the presence of Cu, the level of phosphotyrosine accumulation declined in the band of 45 kDa, p45. To analyze the role of tyrosine dephosphorylation for the regulation of Cu-induced cell death more precisely, we increased levels of tyrosine phosphorylation using the protein tyrosine phosphatase inhibitor, sodium orthovanadate (Na(3)VO(4)). Treatment of rice roots with Na(3)VO(4) blocked Cu-induced cell death and protein tyrosine dephosphorylation. In addition, the antioxidant GSH and the calcium chelator EGTA significantly abolished Cu-induced cell death and protein tyrosine dephosphorylation. These results provide evidence that dephosphorylation of a tyrosine-phosphorylated protein, p45, is an important step in the Cu-triggered signalling transduction pathway.

Enhanced sensitivity to oxidative stress in an Arabidopsis nitric oxide synthase mutant

The possible involvement of nitric oxide (NO) in oxidative stress tolerance was studied using Arabidopsis thaliana wild type (WT) and Atnos1 mutant plants, in which endogenous NO production is greatly diminished because 80% of nitric oxide synthase (NOS) activity is eliminated due to T-DNA insertion in the first exon of the NOS1 gene. Compared with WT, Atnos1 mutant plants showed increased hypersensitivity to salt stress and methyl viologen (MV) treatment. The maximal photochemical efficiency of photosystem II (F(v)/F(m)) and membrane integrity decreased in WT and Atnos1 mutant plants under stresses, but the extent was higher in the mutant. Treatment with sodium nitroprusside (SNP) (a NO donor) to Atnos1 mutant plants alleviated the damage. Instead, inhibition of nitric oxide accumulation in the WT plants produced opposite effects. Hydrogen peroxide and lipid peroxidation increased and the extent was higher in Atnos1 mutant plants than that in WT plants under MV stress. These results indicated that nitric oxide could protect the damage against NaCl and MV treatments.

Seleno-independent glutathione peroxidases

Glutathione peroxidases (GPXs, EC 1.11.1.9) were first discovered in mammals as key enzymes involved in scavenging of activated oxygen species (AOS). Their efficient antioxidant activity depends on the presence of the rare amino-acid residue selenocysteine (SeCys) at the catalytic site. Nonselenium GPX-like proteins (NS-GPXs) with a Cys residue instead of SeCys have also been found in most organisms. As SeCys is important for GPX activity, the function of the NS-GPX can be questioned. Here, we highlight the evolutionary link between NS-GPX and seleno-GPX, particularly the evolution of the SeCys incorporation system. We then discuss what is known about the enzymatic activity and physiological functions of NS-GPX. Biochemical studies have shown that NS-GPXs are not true GPXs; notably they reduce AOS using reducing substrates other than glutathione, such as thioredoxin. We provide evidence that, in addition to their inefficient scavenging action, NS-GPXs act as AOS sensors in various signal-transduction pathways.

Temporal gene expression profiling of the wheat leaf rust pathosystem using cDNA microarray reveals differences in compatible and incompatible defence pathways

In this study, we detail the construction of a custom cDNA spotted microarray containing 7728 wheat ESTs and the use of the array to identify host genes that are differentially expressed upon challenges with leaf rust fungal pathogens. Wheat cultivar RL6003 (Thatcher Lr1) was inoculated with Puccinia triticina virulence phenotypes BBB (incompatible) or TJB (7-2) (compatible) and sampled at four different time points (3, 6, 12, and 24 hours) after inoculation. Transcript expression levels relative to a mock treatment were measured. One hundred ninety two genes were found to have significantly altered expression between the compatible and incompatible reactions. Among those were genes involved in photosynthesis, the production of reactive oxygen species, ubiquitination, signal transduction, as well as in the shikimate/phenylpropanoid pathway. These data indicate that various metabolic pathways are affected, some of which might be used by RL6003 to mount a coordinated defense against an incompatible fungal pathogen.

Reactive oxygen species, nitric oxide, and their interactions play different roles in Cupressus lusitanica cell death and phytoalexin biosynthesis

Beta-thujaplicin Is a natural troponoid with strong antifungal, antiviral, and anticancer activities. Beta-thujaplicin production in yeast elicitor-treated Cupressus lusitanica cell culture and its relationships with reactive oxygen species (ROS) and nitric oxide (NO) production and hypersensitive cell death were investigated. Superoxide anion radical (O2*-) induced cell death and inhibited beta-thujaplicin accumulation, whereas hydrogen peroxide (H2O2) induced beta-thujaplicin accumulation but did not significantly affect cell death. Both elicitor and O2*- induced programmed cell death, which can be blocked by protease inhibitors, protein kinase inhibitors, and Ca2+ chelators. Elicitor-induced NO generation was nitric oxide synthase (NOS)-dependent. Inhibition of NO generation by NOS inhibitors and NO scavenger partly blocked the elicitor-induced beta-thujaplicin accumulation and cell death, and NO donors strongly induced cell death. Interaction among NO, H2O2, and O2*- shows that NO production and H2O2 production are interdependent, but NO and O2*- accumulation were negatively related because of coconsumption of NO and O2*-. NO- and O2*- -induced cell death required each other, and both were required for elicitor-induced cell death. A direct interaction between NO and O2*- was implicated in the production of a potent oxidant peroxynitrite, which might mediate the elicitor-induced cell death.

Cloning and expression of peroxisomal ascorbate peroxidase gene from wheat

A full-length cDNA encoding wheat peroxisomal ascorbate peroxidase (pAPX) was cloned by Suppression Subtractive Hybridization (SSH) and in silico approach. The cDNA was 1027 bp in length and contained a complete ORF of 876 bp, which encodes a protein of 292 amino acid residues. Its deduced amino acids sequence had 84% identity with that of pAPX from barley. The gene was designated as Ta-pAPX. The Ta-pAPX homologous genes were mapped on wheat chromosome 7A and 7D using Chinese Spring nulli-tetrasomic lines analysis. Northern analysis indicated that, after inoculation by Erysiphe graminis Dc.f.sp. tritici, the expression of Ta-pAPX gene in Yangmai5 was enhanced, but its expression in wheat-Haynaldia villosa 6VS/6AL translocation lines changed a little. The results implied that Ta-pAPX may be related to susceptibility of wheat to powdery mildew. The complete coding sequence of Ta-pAPX was cloned into an expression vector pET32 (a+) and a protein with the same deduced molecular weight (MW) was expressed in E. coli BL21 (DE3), which showed ascorbate peroxidase activity.

Hydrogen peroxide, nitric oxide and cytosolic ascorbate peroxidase at the crossroad between defence and cell death

An increase in the production of reactive oxygen species (ROS) is a typical event occurring during different stress conditions and activating conflicting responses in plants. In order to investigate the relevance of different timing and amounts of ROS production, tobacco (Nicotiana tabacum) Bright Yellow-2 (TBY-2) cells were incubated with different amounts of glucose plus glucose oxidase, for generating H(2)O(2) during time, or directly with known amounts of H(2)O(2). Data presented here indicate that, in TBY-2 cells, a difference in H(2)O(2) level is a critical point for shifting metabolic responses towards strengthening of antioxidant defences, or their depletion with consequent cell death. Timing of ROS production is also critical because it can determine programmed cell death (PCD) or necrosis. Depending on the different kinds of activated cell death, ascorbate (ASC) and glutathione (GSH) pools are altered differently. Moreover, an H(2)O(2)-dependent activation of nitric oxide synthesis is triggered only in the conditions inducing PCD. Ascorbate peroxidase (APX) has been analysed under different conditions of H(2)O(2) generation. Under a threshold value of H(2)O(2) overproduction, a transient increase in APX occurs, whereas under conditions inducing cell necrosis, the activity of APX decreases in proportion to cell death without any evident alteration in APX gene expression. Under conditions triggering PCD, the suppression of APX involves both gene expression and alteration of the kinetic characteristics of the enzyme. The changes in ASC, GSH and APX are involved in the signalling pathway leading to PCD, probably contributing to guaranteeing the cellular redox conditions required for successful PCD.