Constitutive caspase-like machinery executes programmed cell death in plant cells

Temporal and spatial activation of caspase-like enzymes induced by self-incompatibility in Papaver pollen

Caspase-like proteases are universal mediators of programmed cell death (PCD). Because plants have no caspase homologs, establishing the nature of their caspase-like activities is of considerable importance to our understanding of PCD in plants. Caspase-3, displaying DEVD specificity, is a key executioner caspase in animal cells. Self-incompatibility (SI) is an important mechanism to prevent self-fertilization and inbreeding in higher plants by inhibiting incompatible pollen. In Papaver rhoeas, SI activates a caspase-3-like/DEVDase activity in incompatible pollen that plays a pivotal role in regulating PCD. Here we characterize the SI-induced caspase-like activities in detail; our work provides insights into the temporal and spatial activation of plant caspase-like enzymes. We show that SI also activates a VEIDase and a LEVDase and that the VEIDase plays a role in SI-induced PCD. The DEVDase and VEIDase are activated remarkably rapidly: detectable within 1-2 h after SI induction; the LEVDase activity peaks later. Importantly, we show live-cell imaging of a DEVDase activity in a higher plant cell; the SI-activated DEVDase has a cytosolic and nuclear localization. We also demonstrate that SI induces a rapid and substantial cytosolic acidification that matches the in vitro pH optima for the SI-induced caspase activities. Because both cytosolic acidification and nuclear caspase localization are observed during apoptosis in animal cells, our data provide striking parallels between SI-induced PCD and apoptosis in animal cells.

Nitric Oxide Is Critical for Inducing Phosphatidic Acid Accumulation in Xylanase-elicited Tomato Cells

Nitric Oxide (NO) is a second messenger related to development and (a)biotic stress responses in plants. We have studied the role of NO in signaling during plant defense responses upon xylanase elicitation. Treatment of tomato cell cultures with the fungal elicitor xylanase resulted in a rapid and dose-dependent NO accumulation. We have demonstrated that NO is required for the production of the lipid second messenger phosphatidic acid (PA) via the activation of the phospholipase C (PLC) and diacylglycerol kinase (DGK) pathway. Defense-related responses downstream of PA were studied. PA and, correspondingly, xylanase were shown to induce reactive oxygen species production. Scavenging of NO or inhibition of either the PLC or the DGK enzyme diminished xylanase-induced reactive oxygen species production. Xylanase-induced PLDbeta1 and PR1 mRNA levels decreased when NO or PA production were compromised. Finally, we have shown that NO and PA are involved in the induction of cell death by xylanase. Treatment with NO scavenger cPTIO, PLC inhibitor U73122, or DGK inhibitor R59022 diminished xylanase-induced cell death. On the basis of biochemical and pharmacological experimental results, we have shown that PLC/DGK-derived PA represents a novel downstream component of NO signaling cascade during plant defense.

Dinitro-o-cresol induces apoptosis-like cell death but not alternative oxidase expression in soybean cells

In plants, programmed cell death is thought to be activated during differentiation and in response to biotic and abiotic stresses. Although its mechanisms are far less clear, several morphological and biochemical features have been described in different experimental systems, including DNA laddering and cytosolic protease activation. Moreover, plant mitochondria have an alternative terminal oxidase (AOX), which is thought to be involved in protection against increased reactive oxygen species production, perhaps representing a mechanism to prevent programmed cell death. In this study, we analysed cell death induced by the herbicide dinitro-o-cresol (DNOC) in soybean (Glycine max) suspension cell cultures and evaluated biochemical and molecular events associated with programmed cell death. AOX capacity and expression were also determined. DNOC-treated cells showed fragmented nuclear DNA as assessed by an in situ assay that detects 3'-OH ends. In addition, specific colorimetric assays and immunoblot analysis revealed activation of caspase-3-like proteins and release of cytochrome c from mitochondria, respectively, confirming the apoptotic-like phenotype. Surprisingly, AOX capacity and protein levels decreased in DNOC-treated cells, suggesting no association between cell death and AOX under these experimental conditions. In conclusion, the results show that DNOC induces programmed cell death in soybean cells, suggesting that plants and animals might share similar pathways. Further, the role of AOX in cell death has not been confirmed, and may depend on the nature and intensity of stress conditions.

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.

Fusaric acid induces apoptosis in saffron root-tip cells: roles of caspase-like activity, cytochrome c, and H2O2

Programmed cell death (PCD), now known as apoptosis, is accompanied by specific morphological features. In this study, fusaric acid, a fusarium mycotoxin, was used to examine cell death in saffron (Crocus sativus Linnaeus) roots, using several apoptosis assays. Our results show that moderate FA doses (50-100 microM) induce apoptotic features while high FA doses (> 200 microM) stimulate necrosis. The apoptotic-like features induced by moderate doses of FA include chromatin condensation, formation of condensed chromatin spheres which bud from the nucleus, fragmentation of nucleosomal DNA into approximately 180 bp fragments, exposure of phosphatidyl serine to the external membrane leaflet, delivery of cytochrome c to cytosol, and generation of H(2)O(2). These apoptotic alterations in root cells are not observed in the presence of serine protease, caspase-1 or caspase-3 inhibitors. It is proposed that production of H(2)O(2) and release of cytochrome c into the cytosol may activate caspase-like proteases and thus establish the apoptotic pathway. As nuclei budding spheres formed in plant root cells after exposure to 50-100 microM FA doses seem to be digested inside the cytosol, we suggest labeling them as internal apoptotic bodies (IAB) that may be more informative than previously used term, apoptotic-like bodies.

Glutathione half-cell reduction potential: a universal stress marker and modulator of programmed cell death?

The elucidation of factors that contribute to cell viability loss is presently compromised by the lack of a universal measure that quantifies "stress." We have investigated mechanisms of viability loss in plant seeds to find a reliable marker of stress response. Oxidative damage has previously been correlated with degenerative processes and death, but how exactly this contributes to viability loss is unknown. We show in four species subjected to ageing or desiccation that seed viability decreased by 50% when the half-cell reduction potential of glutathione (E(GSSG/2GSH)), a major cellular antioxidant and redox buffer, increased to -180 to -160 mV. We then conducted a metaanalysis of data representative of 13 plant and fungal orders to show that plant stress generally becomes lethal when E(GSSG/2GSH) exceeds -160 mV. We put forward that this change in E(GSSG/2GSH) is part of the signaling cascade that initiates programmed cell death (PCD), finally causing internucleosomal DNA fragmentation in the final, or execution phase, of PCD. E(GSSG/2GSH) is therefore a universal marker of plant cell viability and allows us to predict whether a seed will live, germinate, and produce a new plant, or if it will die.

Toxin release in response to oxidative stress and programmed cell death in the cyanobacterium Microcystis aeruginosa

An unprecedented bloom of the cyanobacterium Microcystis aeruginosa Kütz. occurred in the St. Lucie Estuary, FL in the summer of 2005. Samples were analyzed for toxicity by ELISA and by use of the polymerase chain reaction (PCR) with specific oligonucleotide primers for the mcyB gene that has previously been correlated with the biosynthesis of toxic microcystins. Despite the fact that secreted toxin levels were relatively low in dense natural assemblages (3.5 microg l(-1)), detectable toxin levels increased by 90% when M. aeruginosa was stressed by an increase in salinity, physical injury, application of the chemical herbicide paraquat, or UV irradiation. The application of the same stressors caused a three-fold increase in the production of H(2)O(2) when compared to non-stressed cells. The application of micromolar concentrations of H(2)O(2) induced programmed cell death (PCD) as measured by a caspase protease assay. Catalase was capable of inhibiting PCD, implicating H(2)O(2) as the inducing oxidative species. Our results indicate that physical stressors induce oxidative stress, which results in PCD and a concomitant release of toxin into the surrounding media. Remediation strategies that induce cellular stress should be approached with caution since these protocols are capable of releasing elevated levels of microcystins into the environment.

Cytochrome c is released in a reactive oxygen species-dependent manner and is degraded via caspase-like proteases in tobacco Bright-Yellow 2 cells en route to heat shock-induced cell death

To gain some insight into the mechanism of plant programmed cell death, certain features of cytochrome c (cyt c) release were investigated in heat-shocked tobacco (Nicotiana tabacum) Bright-Yellow 2 cells in the 2- to 6-h time range. We found that 2 h after heat shock, cyt c is released from intact mitochondria into the cytoplasm as a functionally active protein. Such a release did not occur in the presence of superoxide anion dismutase and catalase, thus showing that it depends on reactive oxygen species (ROS). Interestingly, ROS production due to xanthine plus xanthine oxidase results in cyt c release in sister control cultures. Maximal cyt c release was found 2 h after heat shock; later, activation of caspase-3-like protease was found to increase with time. Activation of this protease did not occur in the presence of ROS scavenger enzymes. The released cyt c was found to be progressively degraded in a manner prevented by either the broad-range caspase inhibitor (zVAD-fmk) or the specific inhibitor of caspase-3 (AC-DEVD-CHO), which have no effect on cyt c release. In the presence of these inhibitors, a significant increase in survival of the cells undergoing programmed cell death was found. We conclude that ROS can trigger release of cyt c, but do not cause cell death, which requires caspase-like activation.

PH4 of Petunia is an R2R3 MYB protein that activates vacuolar acidification through interactions with basic-helix-loop-helix transcription factors of the anthocyanin pathway

The Petunia hybrida genes ANTHOCYANIN1 (AN1) and AN2 encode transcription factors with a basic-helix-loop-helix (BHLH) and a MYB domain, respectively, that are required for anthocyanin synthesis and acidification of the vacuole in petal cells. Mutation of PH4 results in a bluer flower color, increased pH of petal extracts, and, in certain genetic backgrounds, the disappearance of anthocyanins and fading of the flower color. PH4 encodes a MYB domain protein that is expressed in the petal epidermis and that can interact, like AN2, with AN1 and the related BHLH protein JAF13 in yeast two-hybrid assays. Mutation of PH4 has little or no effect on the expression of structural anthocyanin genes but strongly downregulates the expression of CAC16.5, encoding a protease-like protein of unknown biological function. Constitutive expression of PH4 and AN1 in transgenic plants is sufficient to activate CAC16.5 ectopically. Together with the previous finding that AN1 domains required for anthocyanin synthesis and vacuolar acidification can be partially separated, this suggests that AN1 activates different pathways through interactions with distinct MYB proteins.

AtBAG6, a novel calmodulin-binding protein, induces programmed cell death in yeast and plants

Calmodulin (CaM) influences many cellular processes by interacting with various proteins. Here, we isolated AtBAG6, an Arabidopsis CaM-binding protein that contains a central BCL-2-associated athanogene (BAG) domain. In yeast and plants, overexpression of AtBAG6 induced cell death phenotypes consistent with programmed cell death (PCD). Recombinant AtBAG6 had higher affinity for CaM in the absence of free Ca2 + than in its presence. An IQ motif (IQXXXRGXXXR, where X denotes any amino-acid) was required for Ca2 +-independent CaM complex formation and single amino-acid changes within this motif abrogated both AtBAG6-activated CaM-binding and cell death in yeast and plants. A 134-amino-acid stretch, encompassing both the IQ motif and BAG domain, was sufficient to induce cell death. Agents generating oxygen radicals, which are known to be involved in plant PCD, specifically induced the AtBAG6 transcript. Collectively, these results suggest that AtBAG6 is a stress-upregulated CaM-binding protein involved in plant PCD.

A cellular suicide strategy of plants: vacuole-mediated cell death

Programmed cell death (PCD) occurs in animals and plants under various stresses and during development. Recently, vacuolar processing enzyme (VPE) was identified as an executioner of plant PCD. VPE is a cysteine protease that cleaves a peptide bond at the C-terminal side of asparagine and aspartic acid. VPE exhibited enzymatic properties similar to that of a caspase, which is a cysteine protease that mediates the PCD pathway in animals, although there is limited sequence identity between the two enzymes. VPE and caspase-1 share several structural properties: the catalytic dyads and three amino acids forming the substrate pockets (Asp pocket) are conserved between VPE and caspase-1. In contrast to such similarities, subcellular localizations of these proteases are completely different from each other. VPE is localized in the vacuoles, while caspases are localized in the cytosol. VPE functions as a key molecule of plant PCD through disrupting the vacuole in pathogenesis and development. Cell death triggered by vacuolar collapse is unique to plants and has not been seen in animals. Plants might have evolved a VPE-mediated vacuolar system as a cellular suicide strategy.

Signal role for activation of caspase-3-like protease and burst of superoxide anions during Ce4+-induced apoptosis of cultured Taxus cuspidata cells

The signal events of 1 mM Ce4+ (Ce(NH4)2(NO3)6)-induced apoptosis of cultured Taxus cuspidata cells were investigated. The percentage of apoptotic cells increased from 0.82% to 51.32% within 6 days. Caspase-3-like protease activity became notable during the second day of Ce4+-treatment, and the maximum activity was 5-fold higher than that of control cells at the fourth day. When the experiment system was pretreated with acetyl-Asp-Glu-Val-Asp-aldehyde (Ac-DEVD-CHO) at 100 microM, caspase-3-like activity resulted in distinct inhibition by 70% and 77.3% after 3 and 4 days of induction. Furthermore, 100 microM Ac-DEVD-CHO partially reduced the apoptotic cells by 58.6% and 60.8% at day 4 and 5 respectively. Ce4+ induced superoxide anions (O2*-) transient burst, and the first peak appeared at around 3.7-4 h, the second appeared at about 7 h. Both O2*- burst and cell apoptosis were effectively suppressed by application of diphenyl iodonium (NADPH oxidase inhibitor). Inhibition of O2*- production attenuated caspase-3-like activation by 49% and 53.6% during day 3 and 4 respectively. In addition, a total of 15 protein spots changed in response to caspase-3-like protease activation were identified by two-dimensional gel electrophoresis. These results suggest that Ce4+ of 1 mM induces apoptosis in suspension cultures of T. cuspidata through O2*- burst as well as caspase-3-like protease activation. The burst of O2*- exerts its activity as an upstream of caspase-3-like activation. Our results also implicate that other signal pathways independent of an O2*- burst possibly participate in mediating caspase-3-like protease activation.

Identification of a novel gene family, paralogs of inhibitor of apoptosis proteins present in plants, fungi, and animals

Only few orthologs of animal apoptosis regulators have been found in plants. Recently, the ectopic expression of mammalian inhibitor of apoptosis proteins (IAPs) has been shown to affect plant programmed cell death. Here, we identified two novel proteins homologous to Arabidopsis thaliana IAP-like protein (AtILP) 1 and 2 by applying an improved motif searching method. Furthermore, homologs of AtILP1 were found to occur as a novel gene family in other organisms such as fungi and animals including Homo sapiens (HsILP1). Like baculovirus IAP repeats (BIRs) in IAPs, ILPs contain two highly conserved BIR-like domains (BLDs) with a putative C2HC-type zinc finger. Phylogenetic analyses indicated that ILPs are putative paralogs of IAPs. Homology modeling revealed that the three-dimensional structure of BLD in HsILP1 is similar to that of BIR. Transient expression of HsILP1 resulted in inhibition of etoposide-induced apoptosis in HEK293 and HeLaS3 cells. These findings suggest that ILPs are conserved in a wide range of eukaryotes including plants, and that their functions are closely related to those of IAPs.

A novel plant cysteine protease has a dual function as a regulator of 1-aminocyclopropane-1-carboxylic Acid synthase gene expression

The hormone ethylene influences plant growth, development, and some defense responses. The fungal elicitor Ethylene-Inducing Xylanase (EIX) elicits ethylene biosynthesis in tomato (Lycopersicon esculentum) and tobacco (Nicotiana tabacum) leaves by induction of 1-aminocyclopropane-1-caboxylic acid synthase (Acs) gene expression. A minimal promoter element in the LeAcs2 gene required for EIX responsiveness was defined by deletion analysis in transgenic tomato plants. The sequence between -715 and -675 of the tomato Acs2 gene was found to be essential for induction by EIX. A Cys protease (LeCp) was isolated that specifically binds to this cis element in vitro. Ectopic expression of LeCp in tomato leaves induced the expression of Acs2. Moreover, chromatin immunoprecipitation showed that LeCp binds in vivo to the Acs promoter. We propose a mechanism for the dual function of the LeCp protein. The protease acts enzymatically in the cytoplasm. Then, upon signaling, a small ubiquitin-related modifier protein binds to it, enabling entrance into the nucleus, where it acts as a transcription factor. Thus, LeCp can be considered a dual-function protein, having enzymatic activity and, upon elicitor signaling, exhibiting transcriptional factor activity that induces LeAcs2 expression.

Imaging plant cell death: GFP-Nit1 aggregation marks an early step of wound and herbicide induced cell death

BACKGROUND

A great deal is known about the morphological endpoints of plant cell death, but relatively little is known about its sequence of events and/or its execution at the biochemical level. Live cell imaging using GFP-tagged markers is a powerful way to provide dynamic portraits of a cellular process that can in turn provide a descriptive foundation valuable for future biochemical and genetic investigations.

RESULTS

While characterizing a collection of random GFP-protein fusion markers we discovered that mechanical wounding induces rapid aggregation of a GFP-Nitrilase 1 fusion protein in Arabidopsis cells directly abutting wound sites. Time-lapse imaging of this response shows that the aggregation occurs in cells that subsequently die 30-60 minutes post-wounding, indicating that GFP-Nit1 aggregation is an early marker of cell death at wound sites. Time-lapse confocal imaging was used to characterize wound-induced cell death using GFP-Nit1 and markers of the nucleus and endoplasmic reticulum. These analyses provide dynamic portraits of well-known death-associated responses such as nuclear contraction and cellular collapse and reveal novel features such as nuclear envelope separation, ER vesiculation and loss of nuclear-lumen contents. As a parallel system for imaging cell death, we developed a chemical method for rapidly triggering cell death using the herbicides bromoxynil or chloroxynil which cause rapid GFP-Nit1 aggregation, loss of nuclear contents and cellular collapse, but not nuclear contraction, separating this response from others during plant cell death.

CONCLUSION

Our observations place aggregation of Nitrilase 1 as one of the earliest events associated with wound and herbicide-induced cell death and highlight several novel cellular events that occur as plant cells die. Our data create a detailed descriptive framework for future investigations of plant cell death and provide new tools for both its cellular and biochemical analysis.

Bax-induced cell death of Arabidopsis is meditated through reactive oxygen-dependent and -independent processes

An Arabidopsis protoplast system was developed for dissecting plant cell death in individual cells. Bax, a mammalian pro-apoptotic member of the Bcl-2 family, induces apoptotic-like cell death in Arabidopsis. Bax accumulation in Arabidopsis mesophyll protoplasts expressing murine Bax cDNA from a glucocorticoid-inducible promoter results in cytological characteristics of apoptosis, namely DNA fragmentation, increased vacuolation, and loss of plasma membrane integrity. In vivo targeting analysis monitored using jellyfish green fluorescent protein (GFP) reporter indicated full-length Bax was localized to the mitochondria, as it does in animal cells. Deletion of the carboxyl-terminal transmembrane domain of Bax completely abolished targeting to mitochondria. Bax expression was followed by reactive oxygen species (ROS) accumulation. Treatment of protoplasts with the antioxidant N -acetyl- -cysteine (NAC) during induction of Bax expression strongly suppressed Bax-mediated ROS production and the cell death phenotype. However, some population of the ROS depleted cells still induced cell death, indicating that there is a process that Bax-mediated plant cell death is independent of ROS accumulation. Accordingly, suppression of Bax-mediated plant cell death also takes place in two different processes. Over-expression of a key redox-regulator, Arabidopsis nucleoside diphosphate kinase 2 (AtNDPK2) down-regulated ROS accumulation and suppressed Bax-mediated cell death and transient expression of Arabidopsis Bax inhibitor-1 (AtBI-1) substantially suppressed Bax-induced cell death without altering cellular ROS level. Taken together, our results collectively suggest that the Bax-mediated cell death and its suppression in plants is mediated by ROS-dependent and -independent processes.

Caspases: an ancient cellular sword of Damocles

Caspases are a family of cysteine proteases homologous to the Caenorhabditis elegans programmed cell death gene product CED-3. Caspases and their distant relatives, meta- and paracaspases, have been found in phylogenetically distant nonmetazoan groups, including plants, fungi and prokaryotes. This review summarizes the current information on the mechanisms and functions of non-mammalian caspases and their relatives in apoptotic and nonapoptotic processes, and explores the possible evolutionary origin of the caspase family.

The plant proteolytic machinery and its role in defence

The diverse roles of plant proteases in defence responses that are triggered by pathogens or pests are becoming clearer. Some proteases, such as papain in latex, execute the attack on the invading organism. Other proteases seem to be part of a signalling cascade, as indicated by protease inhibitor studies. Such a role has also been suggested for the recently discovered metacaspases and CDR1. Some proteases, such as RCR3, even act in perceiving the invader. These exciting recent reports are probably just the first examples of what lies beneath. More roles for plant proteases in defence, as well as the regulation and substrates of these enzymes, are waiting to be discovered.

Regulation of senescence by eukaryotic translation initiation factor 5A: implications for plant growth and development

Regulation of protein synthesis is increasingly being recognized as an important determinant of cell proliferation and senescence. In particular, recent evidence indicates that eukaryotic translation initiation factor 5A (eIF-A) plays a pivotal role in this determination. Separate isoforms of eIF-5A appear to facilitate the translation of mRNAs required for cell division and cell death. This raises the possibility that eIF-5A isoforms are elements of a biological switch that is in one position in dividing cells and in another position in dying cells. Changes in the position of this putative switch in response to physiological and environmental cues are likely to have a significant impact on plant growth and development.

The role and regulation of programmed cell death in plant-pathogen interactions

It is commonly known that animal pathogens often target and suppress programmed cell death (pcd) pathway components to manipulate their hosts. In contrast, plant pathogens often trigger pcd. In cases in which plant pcd accompanies disease resistance, an event called the hypersensitive response, the plant surveillance system has learned to detect pathogen-secreted molecules in order to mount a defence response. In plants without genetic disease resistance, these secreted molecules serve as virulence factors that act through largely unknown mechanisms. Recent studies suggest that plant bacterial pathogens also secrete antiapoptotic proteins to promote their virulence. In contrast, a number of fungal pathogens secrete pcd-promoting molecules that are critical virulence factors. Here, we review recent progress in determining the role and regulation of plant pcd responses that accompany both resistance and susceptible interactions. We also review progress in discerning the mechanisms by which plant pcd occurs during these different interactions.

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.

Chitosan induces Ca2+ -mediated programmed cell death in soybean cells

•  Chitosan, a component of the cell wall of many fungi, has been widely used to mimic pathogen attack and has been shown to induce several defence responses. •  Here we show that low concentrations (50 µg ml^-1 ) of chitosan are able to induce an increase in cytosolic Ca²⁺ concentration ([Ca²⁺ ]_cyt ), accumulation of H₂ O₂ in the culture medium, induction of the defence gene chalcone synthase (chs), and cell death in soybean cells (Glycine max). •  Chitosan-induced cell death occurred through cytoplasmic shrinkage, chromatin condensation and activation of caspase 3-like protease, suggesting the activation of a programmed cell death (PCD) pathway. Buffering extracellular Ca²⁺ with the Ca²⁺ chelator EGTA prevents [Ca²⁺ ]_cyt elevation, H₂ O₂ production and all downstream PCD features, but not cell death. •  Higher doses (200 µg ml^-1 ) of chitosan evoked neither Ca²⁺ transient and H₂ O₂ production nor caspase 3-like activation, but caused cell death, possibly as a result of plasma membrane disturbance.

VEIDase is a principal caspase-like activity involved in plant programmed cell death and essential for embryonic pattern formation

Plant embryogenesis is intimately associated with programmed cell death. The mechanisms of initiation and control of programmed cell death during plant embryo development are not known. Proteolytic activity associated with caspase-like proteins is paramount for control of programmed cell death in animals and yeasts. Caspase family of proteases has unique strong preference for cleavage of the target proteins next to asparagine residue. In this work, we have used synthetic peptide substrates containing caspase recognition sites and corresponding specific inhibitors to analyse the role of caspase-like activity in the regulation of programmed cell death during plant embryogenesis. We demonstrate that VEIDase is a principal caspase-like activity implicated in plant embryogenesis. This activity increases at the early stages of embryo development that coincide with massive cell death during shape remodeling. The VEIDase activity exhibits high sensitivity to pH, ionic strength and Zn(2+) concentration. Altogether, biochemical assays show that VEIDase plant caspase-like activity resembles that of both mammalian caspase-6 and yeast metacaspase, YCA1. In vivo, VEIDase activity is localised specifically in the embryonic cells during both the commitment and in the beginning of the execution phase of programmed cell death. Inhibition of VEIDase prevents normal embryo development via blocking the embryo-suspensor differentiation. Our data indicate that the VEIDase activity is an integral part in the control of plant developmental cell death programme, and that this activity is essential for the embryo pattern formation.

Expression of the baculovirus p35 protein in tobacco affects cell death progression and compromises N gene-mediated disease resistance response to Tobacco mosaic virus

The p35 protein from baculovirus is a broad-range caspase inhibitor and suppresses programmed cell death in animals. We report here the effects of transgenic expression in tobacco of the p35 protein during the hypersensitive response (HR). Expression of p35 causes partial inhibition of nonhost HR triggered by bacteria and gene-for-gene HR triggered by virus. Infection of p35-expressing tobacco plants with Tobacco mosaic virus (TMV) disrupts N-mediated disease resistance, causing systemic spreading of the virus within a resistant background. Mutant variants altered in aspartate residues within the loop region of p35 are inefficient substrates for caspases in vitro, and they do not suppress caspase proteolytic activity in animal systems. Tobacco plants expressing these mutant variants of the p35 protein do not show inhibition of HR cell death or enhanced virus systemic movement. Thus, HR inhibition and TMV systemic spreading phenotype in p35-expressing plants correlate with the ability of the p35 protein to suppress caspase activity in animal systems. In addition, a C-terminal truncated variant of p35 is unable to suppress cell death in animals as well as HR cell death in transgenic tobacco. Our results provide evidence for the participation of caspase-like proteases during the HR. In addition, they suggest that timely activation of cell death is necessary for effective TMV containment within the primary infection site.

How plants make tubes

The plant body requires the transport of various materials over large distances. Two cell types that bear a striking resemblance morphologically are the cells specialized for water transport and those responsible for the transport of oxygen: xylem and lysigenous aerenchyma, respectively. Each of these cell types undergoes programmed cell death and cellular autolysis, resulting in the production of a functional space within the plant body. The major morphological difference observed is the presence of the lignified secondary wall in water-conducting tissues. The prevalence of tubular structures in other plant tissues suggests that the ability to form spaces through cellular autolysis is a fundamental paradigm in plant development and evolution.