Proteasome function is required for activation of programmed cell death in heat shocked tobacco Bright-Yellow 2 cells

Transgenic tobacco plants overexpressing a wheat salt stress root protein (TaSSRP) exhibit enhanced tolerance to heat stress

BACKGROUND

Heat stress is a detrimental abiotic stress that limits the development of many plant species and is linked to a variety of cellular and physiological problems. Heat stress affects membrane fluidity, which leads to negative effects on cell permeability and ion transport. Research reveals that heat stress causes severe damage to cells and leads to rapid accumulation of reactive oxygen species (ROS), which could cause programmed cell death.

METHODS AND RESULTS

This current study aimed to validate the role of Triticum aestivum Salt Stress Root Protein (TaSSRP) in plants' tolerance to heat stress by modulating its expression in tobacco plants. The Relative Water Content (RWC), total chlorophyll content, and Membrane Stability Index (MSI) of the seven distinct transgenic lines (T0 - 2, T0 - 3, T0 - 6, T0 - 8, T0 - 9, T0 - 11, and T0 - 13), increased in response to heat stress. Despite the fact that the same tendency was detected in wild-type (WT) plants, changes in physio-biochemical parameters were greater in transgenic lines than in WT plants. The expression analysis revealed that the transgene TaSSRP expressed from 1.00 to 1.809 folds in different lines in the transgenic tobacco plants. The gene TaSSRP offered resistance to heat stress in Nicotiana tabacum, according to the results of the study.

CONCLUSION

These findings could help to improve our knowledge and understanding of the mechanism underlying thermotolerance in wheat, and the novel identified gene TaSSRP could be used in generating wheat varieties with enhanced tolerance to heat stress.

Proteasome activation is critical for cell death induced by inhibitors of polo-like kinase 1 (PLK1) in multiple cancers

Inhibitors of polo-like kinase (PLK) are currently being evaluated as anticancer drugs. However, the molecular mechanism of PLK inhibitor-induced cell death is not fully understood. In this study, we found that GW843682X and BI2536, two inhibitors of PLK1, significantly induced cell death in multiple type cells. The induction of cell death was related to the preferring expression of PLK1. However, in human umbilical vascular endothelial cells (HUVEC) and human colorectal carcinoma cells, which expressed higher levels of both PLK1 and PLK2, PLK1 inhibitors induced very low levels of cell death. Clinical analysis reveals PLK1 presence in 26 of 30 NPC tumor tissues. In in vivo NPC lung metastasis nude mouse models, PLK1 inhibitors decreased NPC progress. Mechanistically, the PLK1 inhibitor did not activate p53, and the cell death was not reversed by p53 inhibition. Moreover, PLK1 inhibitor-induced cell death was PARP- and caspase-independent. Although PLK1 inhibitors induced down-regulation of calpain inhibitor calpastatin and calpain was activated by PLK1 inhibition, calpain blocking did not reverse cell death induced by PLK1 inhibitors, suggesting the non-involvement of calpain. Surprisingly, we found that PLK1 inhibitors induced the activation of proteasome, and the treatment of cells with PLK1 inhibitors reduced the levels of ubiquitinated proteins. And proteasome inhibitors reversed cell death induced by PLK1 inhibitors in various cell types in which PLK1 was preferentially expressed. Moreover, PLK1 inhibition reversed the degradation of proteins including p53, caspase 8, PARP and calpastatin. These results suggest that the activation of proteasome is critical for cell death induced by PLK1 inhibition.

The regulation of Cytochrome f by mannose treatment in broccoli and its relationship with programmed cell death in tobacco BY-2 cells

It is well established that programmed cell death (PCD) occurred in broccoli during postharvest senescence, but no studies have been conducted on the regulation of broccoli cytochrome f by mannose treatment and its relationship with PCD. In this study, we treated broccoli buds with mannose to investigate the changes in color, total chlorophyll content, gene expression related to chlorophyll metabolism, chloroplast structure, and cytochrome f determination during postharvest storage. In addition, to investigate the effect of cytochrome f on PCD, we extracted cytochrome f from broccoli and treated Nicotiana tabacum L. cv Bright Yellow 2 (BY-2) cells with extracted cytochrome f from broccoli at various concentrations. The results showed that cytochrome f can induce PCD in tobacco BY-2 cells, as evidenced by altered cell morphology, nuclear chromatin disintegration, DNA degradation, decreased cell viability, and increased caspase-3-like protease production. Taken together, our study indicated that mannose could effectively delay senescence of postharvest broccoli by inhibiting the expression of gene encoding cytochrome f which could induce PCD.

Identification and functional characterization of REGULATORY PARTICLE NON-ATPASE 1a-like (AhRPN1a-like) in peanuts during aluminum-induced programmed cell death

The toxicity of aluminum (Al) in acidic soil is a prevalent problem and causes reduced crop yields. In the plant response to Al toxicity, programmed cell death (PCD) appears to be one of the important mechanisms. However, the regulation of Al-induced PCD remains poorly understood. Here, we found that an uncharacterized protein REGULATORY PARTICLE NON-ATPASE 1a-like in peanut (AhRPN1a-like), located in the nucleus and cytoplasm, directly interacted with type I metacaspase in peanut (AhMC1). The overexpression of AhRPN1a-like in Arabidopsis strongly enhanced Al inhibition of root growth with a loss of root tip cell viability. Furthermore, in response to Al treatment, the VIGS knockdown line of AhRPN1a-like in peanut displayed decreased transcription of AhMC1, increased root growth, reduced Al-induced PCD and decreased 26S proteasomal activity. Taken together, these findings demonstrated that AhRPN1a-like interacted directly with AhMC1, and promotes the occurrence of Al-induced PCD via the 26S proteasome pathway, thereby reducing Al-resistance.

The boundary of life and death: changes in mitochondrial and cytosolic proteomes associated with programmed cell death of Arabidopsis thaliana suspension culture cells

Introduction

Despite the critical role of programmed cell death (PCD) in plant development and defense responses, its regulation is not fully understood. It has been proposed that mitochondria may be important in the control of the early stages of plant PCD, but the details of this regulation are currently unknown.

Methods

We used Arabidopsis thaliana cell suspension culture, a model system that enables induction and precise monitoring of PCD rates, as well as chemical manipulation of this process to generate a quantitative profile of the alterations in mitochondrial and cytosolic proteomes associated with early stages of plant PCD induced by heat stress. The cells were subjected to PCD-inducing heat levels (10 min, 54°C), with/without the calcium channel inhibitor and PCD blocker LaCl3. The stress treatment was followed by separation of cytosolic and mitochondrial fractions and mass spectrometry-based proteome analysis.

Results

Heat stress induced rapid and extensive changes in protein abundance in both fractions, with release of mitochondrial proteins into the cytosol upon PCD induction. In our system, LaCl3 appeared to act downstream of cell death initiation signal, as it did not affect the release of mitochondrial proteins, but instead partially inhibited changes occurring in the cytosolic fraction, including upregulation of proteins with hydrolytic activity.

Discussion

We characterized changes in protein abundance and localization associated with the early stages of heat stress-induced PCD. Collectively, the generated data provide new insights into the regulation of cell death and survival decisions in plant cells.

Morphology and permeability transitions in plant mitochondria: Different aspects of the same event?

Plant mitochondria are sensitive organelles affected by changing environmental stressors. Upon heat shock or the presence of reactive oxygen species, plant mitochondria undergo in vivo morphological derangements associated with the extensively characterized opening of the mitochondrial permeability transition pore. Nevertheless, the classic mitochondrial permeability transition is known to be triggered by calcium overload causing mitochondrial swelling and dysfunction. Here we review evidence concerning calcium handling, permeability transition and mitochondrial impairments in plants, supporting the notion that the mitochondrial morphology transition is an in vivo indicator of the permeability transition.

Differential tolerance to heat stress of young leaves compared to mature leaves of whole plants relate to differential transcriptomes involved in metabolic adaptations to stress

Plants respond to heat shock by regulating gene expression. While transcriptomic changes in response to heat stress are well studied, it is not known whether young and old leaves reprogram transcription differently upon stress. When whole plants of Arabidopsis thaliana were subjected to heat shock, young leaves were affected significantly less than older leaves based on measurements of tissue damage. To identify quantitative changes to transcriptomes between young and old leaves upon heat stress, we used RNA sequencing on young and old leaves from plants exposed to control and heat stress at 42 °C for 1 h and 10 h. A total of 6472 differentially expressed genes between young and old leaf were identified under control condition, and 9126 and 6891 under 1 h and 10 h heat stress, respectively. Analyses of differentially expressed transcripts led to the identification of multiple functional clusters of genes that may have potential roles in the increased heat tolerance of young leaves including higher level of expression in young leaves of genes encoding chaperones, heat shock proteins and proteins known in oxidative stress resistance. Differential levels of transcripts for genes implicated in pectin metabolism, cutin and wax biosynthesis, pentose and glucuronate interconversions, cellulose degradation, indole glucosinolate metabolism and RNA splicing between young and old leaves under heat stress suggest that cell wall remodelling, cuticular wax synthesis and carbohydrate modifications impacted by alternative splicing may also have roles in the improved heat stress tolerance of young leaves.

Leaf Mesophyll Mitochondrial Polarization Assessment in Arabidopsis thaliana

Plant leaves present an intricate array of layers providing a robust barrier against pathogens and abiotic stressors. However, these layers may also constitute an obstacle for the assessment of intracellular processes, especially when using fluorescence microscopy approaches. Current methods for leaf mitochondrial membrane potential determinations have been traditionally performed in thin mesophyll sections, in isolated protoplasts or in fluorescent protein-expressing transgenic plants. This may limit the amount of information obtained about overall mitochondrial morphology in intact leaves. Here, we detail a fast and straightforward protocol to assess changes in leaf mitochondrial membrane potential associated with mitochondrial dysfunction in the model plant Arabidopsis thaliana. This protocol also permits mitochondrial shape, dynamics and polarity assessment in leaves subjected to diverse stress conditions.

Inactivation of Carbonyl-Detoxifying Enzymes by H2O2 Is a Trigger to Increase Carbonyl Load for Initiating Programmed Cell Death in Plants

H2O2-induced programmed cell death (PCD) of tobacco Bright Yellow-2 (BY-2) cells is mediated by reactive carbonyl species (RCS), degradation products of lipid peroxides, which activate caspase-3-like protease (C3LP). Here, we investigated the mechanism of RCS accumulation in the H2O2-induced PCD of BY-2 cells. The following biochemical changes were observed in 10-min response to a lethal dose (1.0 mM) of H2O2, but they did not occur in a sublethal dose (0.5 mM) of H2O2. (1) The C3LP activity was increased twofold. (2) The intracellular levels of RCS, i.e., 4-hydroxy-(E)-hexenal and 4-hydroxy-(E)-nonenal (HNE), were increased 1.2-1.5-fold. (3) The activity of a reduced nicotinamide adenine dinucleotide phosphate (NADPH)-dependent carbonyl reductase, scavenging HNE, and n-hexanal was decreased. Specifically, these are the earliest events leading to PCD. The proteasome inhibitor MG132 suppressed the H2O2-induced PCD, indicating that the C3LP activity of the 1 subunit of the 20S proteasome was responsible for PCD. The addition of H2O2 to cell-free protein extract inactivated the carbonyl reductase. Taken together, these results suggest a PCD-triggering mechanism in which H2O2 first inactivates a carbonyl reductase(s), allowing RCS levels to rise, and eventually leads to the activation of the C3LP activity of 20S proteasome. The carbonyl reductase thus acts as an ROS sensor for triggering PCD.

Self-incompatibility in Papaver pollen: programmed cell death in an acidic environment

Self-incompatibility (SI) is a genetically controlled mechanism that prevents self-fertilization and thus encourages outbreeding and genetic diversity. During pollination, most SI systems utilize cell-cell recognition to reject incompatible pollen. Mechanistically, one of the best-studied SI systems is that of Papaver rhoeas (poppy), which involves the interaction between the two S-determinants, a stigma-expressed secreted protein (PrsS) and a pollen-expressed plasma membrane-localized protein (PrpS). This interaction is the critical step in determining acceptance of compatible pollen or rejection of incompatible pollen. Cognate PrpS-PrsS interaction triggers a signalling network causing rapid growth arrest and eventually programmed cell death (PCD) in incompatible pollen. In this review, we provide an overview of recent advances in our understanding of the major components involved in the SI-induced PCD (SI-PCD). In particular, we focus on the importance of SI-induced intracellular acidification and consequences for protein function, and the regulation of soluble inorganic pyrophosphatase (Pr-p26.1) activity by post-translational modification. We also discuss attempts to identify protease(s) involved in the SI-PCD process. Finally, we outline future opportunities made possible by the functional transfer of the P. rhoeas SI system to Arabidopsis.

Bufalin suppresses endometriosis progression by inducing pyroptosis and apoptosis

The steroid receptor coactivator (SRC)-1 isoform/estrogen receptor (ER)-β axis has an essential role in endometriosis progression. In this context, therefore, bufalin was employed as a 'tool compound' to evaluate inhibitors of SRC in alternative endometriosis treatment. Bufalin effectively suppressed the growth of primary human endometrial stroma cells isolated from endometriosis patients compared to women without endometriosis and immortalized human endometrial epithelial and stromal cells expressing the SRC-1 isoform compared to their parental cells in vitroIn vivo, compared to the vehicle, bufalin treatment significantly suppressed the growth of endometriotic lesions in mice with surgically induced endometriosis because bufalin disrupted the functional axis of SRC-1 isoform/ERβ by increasing SRC-1 isoform protein stability, hyperactivating the transcriptional activity of the SRC-1 isoform and degrading the ERβ protein by proteasome 26S subunit, non-ATPase 2 in endometriotic lesions. Bufalin treatment elevated the apoptosis signaling in epithelial cells of endometriotic lesions. In stromal cells of endometriotic lesions, bufalin treatment increased the levels of pyroptosis markers (caspase 1 and the active form of interleukin 1β) and reduced proliferation. In addition, bufalin treatment increased the expression levels of endoplasmic reticulum-stress (ERS) markers (PKR-like ER kinase, protein disulfide isomerase and binding immunoglobulin) in endometriotic lesions. Collectively, the bufalin-induced disruption of the SRC-1 isoform/ERβ axis might induce apoptosis, pyroptosis and ERS signaling in endometriotic lesions, causing the suppression of endometriosis. Therefore, future generations of SRC-modulators could be employed as an alternative medical approach for endometriosis treatment.

Programmed Cell Death in Plants: An Overview

Programmed cell death (PCD) is a controlled mechanism that eliminates specific cells under developmental or environmental stimuli. All organisms-from bacteria to multicellular eukaryotes-have the ability to induce PCD in selected cells. Although this process was first identified in plants, the interest in deciphering the signaling pathways leading to PCD strongly increased when evidence came to light that PCD may be involved in several human diseases. In plants, PCD activation ensures the correct occurrence of growth and developmental processes, among which embryogenesis and differentiation of tracheary elements. PCD is also part of the defense responses activated by plants against environmental stresses, both abiotic and biotic.This chapter gives an overview of the roles of PCD in plants as well as the problems arising in classifying different kinds of PCD according to defined biochemical and cellular markers, and in comparison with the various types of PCD occurring in mammal cells. The importance of understanding PCD signaling pathways, with their elicitors and effectors, in order to improve plant productivity and resistance to environmental stresses is also taken into consideration.

Proteolytic activities in cortex of apical parts of Vicia faba ssp. minor seedling roots during kinetin-induced programmed cell death

Programmed cell death (PCD) is a crucial process in plant development. In this paper, proteolytically related aspects of kinetin-induced PCD in cortex cells of Vicia faba ssp. minor seedlings were examined using morphological, fluorometric, spectrophotometric, and fluorescence microscopic analyses. Cell viability estimation after 46 μM kinetin treatment of seedling roots showed that the number of dying cortex cells increased with treatment duration, reaching maximum after 72 h. Weight of the apical root segments increased with time and was about 2.5-fold greater after 96 h, while the protein content remained unchanged, compared to the control. The total and cysteine-dependent proteolytic activities fluctuated during 1-96-h treatment, which was not accompanied by the changes in the protein amount, indicating that the absolute protein amounts decreased during kinetin-induced PCD. N-ethylmaleimide (NEM), phenylmethylsulfonyl fluoride (PMSF), and Z-Leu-Leu-Nva-H (MG115), the respective cysteine, serine, and proteasome inhibitors, suppressed kinetin-induced PCD. PMSF significantly decreased serine-dependent proteolytic activities without changing the amount of proteins, unlike NEM and MG115. More pronounced effect of PMSF over NEM indicated that in the root apical segments, the most important proteolytic activity during kinetin-induced PCD was that of serine proteases, while that of cysteine proteases may be important for protein degradation in the last phase of the process. Both NEM and PMSF inhibited apoptotic-like structure formation during kinetin-induced PCD. The level of caspase-3-like activity of β1 proteasome subunit increased after kinetin treatment. Addition of proteasome inhibitor MG-115 reduced the number of dying cells, suggesting that proteasomes might play an important role during kinetin-induced PCD.

Glucosidase II β-subunit, a novel substrate for caspase-3-like activity in rice, plays as a molecular switch between autophagy and programmed cell death

Endoplasmic reticulum (ER) stress activates unfolded protein response (UPR) and autophagy. However, prolonged, severe stresses activate programmed cell death (PCD) in both animal and plant cells. Compared to the well-studied UPR pathway, the molecular mechanisms of ER-stress-induced PCD are less understood. Here, we report the identification of Gas2, the glucosidase II β subunit in the ER, as a potential switch between PCD and autophagy in rice. MS analysis identified Gas2, GRP94, and HSP40 protein in a purified caspase-3-like activity from heat stressed rice cell suspensions. The three corresponding genes were down-regulated under DTT-induced ER stress. Gas2 and GRP94 were localized to the ER, while HSP40 localized to the cytoplasm. Compared to wild-type, a Gas2 RNAi cell line was much sensitive to DTT treatment and had high levels of autophagy. Both caspase-3 and heat-stressed cell suspension lysate could cleave Gas2, producing a 14 kDa N-terminal fragment. Conditional expression of corresponding C-terminal fragment resulted in enhanced caspase-3-like activity in the protoplasts under heat stress. We proposed that mild ER stress causes down-regulation of Gas2 and induces autophagy, while severe stress results in Gas2 cleavage by caspase-3-like activity and the cleavage product amplifies this activity, possibly participating in the initiation of PCD.

Reactive Carbonyl Species Activate Caspase-3-Like Protease to Initiate Programmed Cell Death in Plants

Reactive oxygen species (ROS)-triggered programmed cell death (PCD) is a typical plant response to biotic and abiotic stressors. We have recently shown that lipid peroxide-derived reactive carbonyl species (RCS), downstream products of ROS, mediate oxidative signal to initiate PCD. Here we investigated the mechanism by which RCS initiate PCD. Tobacco Bright Yellow-2 cultured cells were treated with acrolein, one of the most potent RCS. Acrolein at 0.2 mM caused PCD in 5 h (i.e. lethal), but at 0.1 mM it did not (sublethal). Specifically, these two doses caused critically different effects on the cells. Both lethal and sublethal doses of acrolein exhausted the cellular glutathione pool in 30 min, while the lethal dose only caused a significant ascorbate decrease and ROS increase in 1-2 h. Prior to such redox changes, we found that acrolein caused significant increases in the activities of caspase-1-like protease (C1LP) and caspase-3-like protease (C3LP), the proteases which trigger PCD. The lethal dose of acrolein increased the C3LP activity 2-fold more than did the sublethal dose. In contrast, C1LP activity increments caused by the two doses were not different. Acrolein and 4-hydroxy-(E)-2-nonenal, another RCS, activated both proteases in a cell-free extract from untreated cells. H2O2 at 1 mM added to the cells increased C1LP and C3LP activities and caused PCD, and the RCS scavenger carnosine suppressed their activation and PCD. However, H2O2 did not activate the proteases in a cell-free extract. Thus the activation of caspase-like proteases, particularly C3LP, by RCS is an initial biochemical event in oxidative signal-stimulated PCD in plants.

Tomato 26S Proteasome subunit RPT4a regulates ToLCNDV transcription and activates hypersensitive response in tomato

Involvement of 26S proteasomal subunits in plant pathogen-interactions, and the roles of each subunit in independently modulating the activity of many intra- and inter-cellular regulators controlling physiological and defense responses of a plant were well reported. In this regard, we aimed to functionally characterize a Solanum lycopersicum 26S proteasomal subunit RPT4a (SlRPT4) gene, which was differentially expressed after Tomato leaf curl New Delhi virus (ToLCNDV) infection in tolerant cultivar H-88-78-1. Molecular analysis revealed that SlRPT4 protein has an active ATPase activity. SlRPT4 could specifically bind to the stem-loop structure of intergenic region (IR), present in both DNA-A and DNA-B molecule of the bipartite viral genome. Lack of secondary structure in replication-associated gene fragment prevented formation of DNA-protein complex suggesting that binding of SlRPT4 with DNA is secondary structure specific. Interestingly, binding of SlRPT4 to IR inhibited the function of RNA Pol-II and subsequently reduced the bi-directional transcription of ToLCNDV genome. Virus-induced gene silencing of SlRPT4 gene incited conversion of tolerant attributes of cultivar H-88-78-1 into susceptibility. Furthermore, transient overexpression of SlRPT4 resulted in activation of programmed cell death and antioxidant enzymes system. Overall, present study highlights non-proteolytic function of SlRPT4 and their participation in defense pathway against virus infection in tomato.

Suppression of PCD-related genes affects salt tolerance in Arabidopsis

This work aims at examining a natural exciting phenomenon suggesting that suppression of genes inducing programmed cell death (PCD) might confer tolerance against abiotic stresses in plants. PCD-related genes were induced in tobacco under oxalic acid (OA) treatment (20 mM), and plant cells were characterized to confirm the incidence of PCD. The results indicated that PCD was triggered 24 h after the exposure to OA. Then, RNAs were extracted from tobacco cells 0, 2, 6, 12 and 24 h after treatment for deep sequencing. RNA-Seq analyses were done with a special emphasis to clusters whose PCD-related genes were upregulated after 2 h of OA exposure. Accordingly, 23 tobacco PCD-related genes were knocked down via virus-induced gene silencing (VIGS), whereas our results indicated the influence of five of them on inducing or suppressing PCD. Knockout T-DNA insertion mutants of these five genes in Arabidopsis were tested under salt stress (0, 100, 150, and 200 mM NaCl), and the results indicated that a mutant of an antiapoptotic gene, namely Bax Inhibitor-1 (BI-1), whose VIGS induced PCD in tobacco, was salt sensitive, while a mutant of an apoptotic gene, namely mildew resistance locus O (Mlo), whose VIGS suppressed PCD, was salt tolerant as compared to the WT (Col) control. These data support our hypothesis that retarding PCD-inducing genes can result in higher levels of salt tolerance, while retarding PCD-suppressing genes can result in lower levels of salt tolerance in plants.

The impact of environmental stress on male reproductive development in plants: biological processes and molecular mechanisms

In plants, male reproductive development is extremely sensitive to adverse climatic environments and (a)biotic stress. Upon exposure to stress, male gametophytic organs often show morphological, structural and metabolic alterations that typically lead to meiotic defects or premature spore abortion and male reproductive sterility. Depending on the type of stress involved (e.g. heat, cold, drought) and the duration of stress exposure, the underlying cellular defect is highly variable and either involves cytoskeletal alterations, tapetal irregularities, altered sugar utilization, aberrations in auxin metabolism, accumulation of reactive oxygen species (ROS; oxidative stress) or the ectopic induction of programmed cell death (PCD). In this review, we present the critically stress-sensitive stages of male sporogenesis (meiosis) and male gametogenesis (microspore development), and discuss the corresponding biological processes involved and the resulting alterations in male reproduction. In addition, this review also provides insights into the molecular and/or hormonal regulation of the environmental stress sensitivity of male reproduction and outlines putative interaction(s) between the different processes involved.

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 fatal effect of tungsten on Pisum sativum L. root cells: indications for endoplasmic reticulum stress-induced programmed cell death

Programmed cell death (PCD) is a widespread response of plants against abiotic stress, such as heavy metal toxicity. Tungsten (W) is increasingly considered toxic for plants since it irreversibly affects their growth. Therefore, we investigated whether W could induce some kind of PCD in plants, like other heavy metals do. The morphology of cell and nucleus, the integrity of the cytoskeleton, Evans Blue absorbance and the expression of PCD-related genes were used as indicators of PCD in W-treated roots of Pisum sativum (pea). TEM and fluorescence microscopy revealed mitotic cycle arrest, protoplast shrinkage, disruption of the cytoskeleton and chromatin condensation and peripheral distribution in the nucleus of W-affected cells. Moreover, Evans Blue absorbance in roots increased in relation to the duration of W treatment. These effects were suppressed by inhibitors of the 26S proteasome, caspases and endoplasmic reticulum stress. In addition, silencing of DAD-1 and induction of HSR203J, BiP-D, bZIP28 and bZIP60 genes were also recorded in W-treated pea roots by semi-quantitative RT-PCR. The above observations show that W induces a kind of PCD in pea roots, further substantiating its toxicity for plants. Data imply that endoplasmic reticulum stress-unfolded protein response may be involved in W-induced PCD.

Programmed cell death of the megagametophyte during post-germinative growth of white spruce (Picea glauca) seeds is regulated by reactive oxygen species and the ubiquitin-mediated proteolytic system

The megagametophyte of white spruce (Picea glauca) seeds undergoes programmed cell death following seed germination. This process is characterized by distinct morphological and biochemical features, such as DNA fragmentation and the induction of proteases. Biphasic production of hydrogen peroxide was detected in the megagametophyte following seed germination. ROS scavengers or inhibitors of ROS production decreased caspase-like protease activity and slowed the progression of cell death. One catalase (CAT) of white spruce reacted with antibodies directed against cotton-seed CAT. The corresponding CAT gene was cloned and compared with the catalase genes of other plant species. The activity of the white spruce CAT enzyme was stimulated by tyrosine phosphorylation. The phosphorylated CAT was subjected to ubiquitination and degraded by the proteasome. Furthermore, the proteasome inhibitor MG132 inhibited the degradation of CAT and delayed cell death. These results suggest that the interplay of CAT and the ubiquitin-mediated proteolytic system is critical in the control of ROS production and subsequent cell death.

Salt stress induces programmed cell death in Thellungiella halophila suspension-cultured cells

Thellungiella halophila (T. halophila) suspension-cultured cells were used to gain knowledge of the pathway of programmed cell death (PCD) in halophytes under salt stress. Several apoptotic-like features occurred in T. halophila cells after exposure to 300 mM NaCl, including the retraction of the plasma membrane from the cell wall, nuclear condensation, DNA laddering and the release of cytochrome c accompanying the increase of caspase 3-like protease activity. This process resulted in ultrastructural changes of mitochondria and Golgi bodies, and autophagy was also induced by high salinity stress. DNA laddering and caspase 3-like activity were inhibited prior to the inhibition of cell death by a specific caspase 3 inhibitor, Ac-DEVD-CHO. The results indicate that 300 mM NaCl stress-induced PCD in T. halophila is similar to animal apoptosis, and this process occurs partly through a caspase 3-like dependent pathway.

The ubiquitin/26S proteasome system in plant-pathogen interactions: a never-ending hide-and-seek game

The ubiquitin/26S proteasome system (UPS) plays a central role in plant protein degradation. Over the past few years, the importance of this pathway in plant-pathogen interactions has been increasingly highlighted. UPS is involved in almost every step of the defence mechanisms in plants, regardless of the type of pathogen. In addition to its proteolytic activities, UPS, through its 20S RNase activity, may be part of a still unknown antiviral defence pathway. Strikingly, UPS is not only a weapon used by plants to defend themselves, but also a target for some pathogens that have evolved mechanisms to inhibit and/or use this system for their own purposes. This article attempts to summarize the current knowledge on UPS involvement in plant-microbe interactions, a complex scheme that illustrates the never-ending arms race between hosts and microbes.

Redox homeostasis in plants. The challenge of living with endogenous oxygen production

Plants are not only obligate aerobic organisms requiring oxygen for mitochondrial energy production, but also produce oxygen during photosynthesis. Therefore, plant cells have to cope with a hyperoxic cellular environment that determines a production of reactive oxygen species (ROS) higher than the one occurring in animal cells. In order to maintain redox homeostasis under control, plants evolved a particularly complex and redundant ROS-scavenging system, in which enzymes and metabolites are linked in a network of reactions. This review gives an overview of the mechanisms active in plant cells for controlling redox homeostasis during optimal growth conditions, when ROS are produced in a steady-state low amount, and during stress conditions, when ROS production is increased. Particular attention is paid to the aspects of oxygen/ROS management for which plant and animal cells differ.

Heat stress activates phospholipase D and triggers PIP accumulation at the plasma membrane and nucleus

Heat stress induces an array of physiological adjustments that facilitate continued homeostasis and survival during periods of elevated temperatures. Here, we report that within minutes of a sudden temperature increase, plants deploy specific phospholipids to specific intracellular locations: phospholipase D (PLD) and a phosphatidylinositolphosphate kinase (PIPK) are activated, and phosphatidic acid (PA) and phosphatidylinositol 4,5-bisphosphate (PIP(2)) rapidly accumulate, with the heat-induced PIP(2) localized to the plasma membrane, nuclear envelope, nucleolus and punctate cytoplasmic structures. Increases in the steady-state levels of PA and PIP(2) occur within several minutes of temperature increases from ambient levels of 20-25 degrees C to 35 degrees C and above. Similar patterns were observed in heat-stressed Arabidopsis seedlings and rice leaves. The PA that accumulates in response to temperature increases results in large part from the activation of PLD rather than the sequential action of phospholipase C and diacylglycerol kinase, the alternative pathway used to produce this lipid. Pulse-labelling analysis revealed that the PIP(2) response is due to the activation of a PIPK rather than inhibition of a lipase or a PIP(2) phosphatase. Inhibitor experiments suggest that the PIP(2) response requires signalling through a G-protein, as aluminium fluoride blocks heat-induced PIP(2) increases. These results are discussed in the context of the diverse cellular roles played by PIP(2) and PA, including regulation of ion channels and the cytoskeleton.

Functional redundancy in the Arabidopsis Cathepsin B gene family contributes to basal defence, the hypersensitive response and senescence

Cysteine proteases are required for programmed cell death (PCD) in animals. Recent work in Nicotiana benthamiana has implicated cathepsin B-like cysteine proteases in the hypersensitive response (HR) in plants, a form of PCD involved in disease resistance. Here, we investigate the function and regulation of Cathepsin B (CathB) genes in plant defence, and in both pathogen-inducible and developmental forms of PCD. Single, double and triple knockout mutants were isolated for the three Arabidopsis thaliana AtCathB genes. AtCathB genes were redundantly required for full basal resistance against the virulent bacterial pathogen Pseudomonas syringae pv. tomato (Pst) DC3000. By contrast, AtCathB genes were not required for R gene-mediated resistance to Pst DC3000 expressing AvrB or AvrRps4. Neither did they contribute to PCD triggered by AvrRps4, although they were crucial for the full development of PCD during HR triggered by AvrB. Cathepsin B has also been proposed to play a positive regulatory role in senescence. Atcathb triple mutants showed a delay in senescence and a seven-fold decrease in accumulation of senescence marker gene SAG12. Our results demonstrate a redundant function for AtCathB genes in basal defence as well as a potential regulatory role in distinct forms of plant PCD.

Mitochondrial proteome during salt stress-induced programmed cell death in rice

It has been shown that mitochondria play a pivotal role in plant programmed cell death (PCD). Previous study established a salt stress-induced PCD model in rice (Oryza sativa L. cv. WYJ 8th) root tip cells, demonstrated by DNA laddering, cytochrome c release, and TUNEL positive reaction. In this study, the role of mitochondria during the early phase of PCD (2h-PCD) was analyzed in rice roots. After 2h-PCD induction, the integrity of mitochondria decreased slightly, consistent with a small release of cytochrome c. 2h-PCD partially inhibited electron transport, resulting in oxidative burst in mitochondria. However, ATP production maintained constant. Mitochondria proteome were analyzed by two-dimensional IEF/SDS-PAGE before and after 2h-PCD induction, and eight PCD-related proteins were identified. Among them, four proteins were up-regulated after PCD induction, which included glycoside hydrolase, mitochondrial heat shock protein 70, 20S proteasome subunit, and Cu/Zn-superoxide dismutase, and four were down-regulated, namely ATP synthase beta subunit, cytochrome c oxidase subunit 6b, S-adenosylmethionine synthetase 2, and transcription initiation factor eIF-3 epsilon. These results suggested that ATP synthase may not be the major producer of ATP in mitochondria during the early stage of PCD in rice. Glycoside hydrolase may be involved in ETC impairment and ROS burst, and mitochondrial HSP70 is a potential candidate for PCD regulation. The possible roles of other proteins on PCD initiation were also discussed.

Structure, function and regulation of plant proteasomes

Proteasomes are large multisubunit, multicatalytic proteases responsible for most of the cytosolic and nuclear protein degradation, and their structure and functions are conserved in eukaryotes. Proteasomes were originally identified as the proteolytic module of the ubiquitin-dependent proteolysis pathway. Today we know that proteasomes also mediate ubiquitin-independent proteolysis, that they have RNAse activity, and play a non-proteolytic role in transcriptional regulation. Here we present an overview of the current knowledge of proteasome function and regulation in plants and highlight the role of proteasome-dependent protein degradation in the control of plant development and responses to the environment.

A transient proteasome activation is needed for acetic acid-induced programmed cell death to occur in Saccharomyces cerevisiae

To gain further insight into the mechanism by which yeast programmed cell death (PCD) occurs, we investigated whether and how proteasome activity changes in Saccharomyces cerevisiae cells undergoing PCD as a result of treatment with acetic acid (AA-PCD). We show that proteasome activation starts 60 min after AA-PCD induction, with a maximum at 90 min, and decreases at 150 min. Moreover, cell survival measurements carried out in the absence or presence of MG132, which inhibits proteasome function, show that the inhibition of proteasome activity partially prevents AA-PCD, thus indicating that a transient proteasome activation is needed for AA-PCD to occur.

Programmed cell death in plants: new insights into redox regulation and the role of hydrogen peroxide

Programmed cell death (PCD), the highly regulated dismantling of cells, is essential for plant growth and survival. PCD plays key roles in embryo development, formation and maturation of many cell types and tissues, and plant reaction/adaptation to environmental conditions. Reactive oxygen species (ROS) are not only toxic by products of aerobic metabolism with strictly controlled cellular levels, but they also function as signaling agents regulating many biological processes and producing pleiotropic effects. Over the last decade, ROS have become recognized as important modulators of plant PCD. Molecular genetic approaches using plant mutants and transcriptome studies related to ROS-mediated PCD have revealed a wide array of plant-specific cell death regulators and have contributed to unraveling the elaborate redox signaling network. This review summarizes the biological processes, in which plant PCD participates and discusses the signaling functions of ROS with emphasis on hydrogen peroxide.