Many ways to exit? Cell death categories in plants

Programmed cell death (PCD) is an integral part of plant development and defence. It occurs at all stages of the life cycle, from fertilization of the ovule to death of the whole plant. Without it, tall trees would probably not be possible and plants would more easily succumb to invading microorganisms. Here, we have attempted to categorize plant PCD in relation to three established morphological types of metazoan cell death: apoptosis, autophagy and non-lysosomal PCD. We conclude that (i) no examples of plant PCD conform to the apoptotic type, (ii) many examples of PCD during plant development agree with the autophagic type, and (iii) that other examples are apparently neither apoptotic nor autophagic.     

Cell death in maize

Cell death occurs in plants as a part of normal development and as a response to toxins, pathogens and other environmental stimuli or insults. When cell death occurs as an orderly disassembly of the cell under the control of a genetically determined program, the process is referred to as programmed cell death (PCD). The PCD mechanisms of plants show many striking similarities to, but also intriguing differences from, those of animals. The extensive genetic, developmental and physiological characterizations of maize have made it an excellent system for the study of cell death. We describe the recent advances in the study of cell death in maize in light of what is known in plants and animals.     

植物Metacaspase研究进展

过敏性坏死反应是植物的一种重要的抗病机制, 类似于动物细胞凋亡, 它是一种程序性细胞死亡(programmed cell death, PCD)过程。目前, 已经确定半胱天冬蛋白酶(caspase)在动物PCD过程中起核心作用。在植物中, 尚未发现其直系同源蛋白, 但是有一类与其结构相似的蛋白酶, 称为metacaspase。在植物不同的PCD过程中, 有的依赖于metacaspase, 而有的则不依赖于该类蛋白酶。目前对metacaspase的结构和功能已有了初步的研究, 对其深入的研究则进展缓慢, 其具体的生物学功能和在PCD信号路径中的定位有待进一步探索。     

Caspase inhibitors affect the kinetics and dimensions of tracheary elements in xylogenic Zinnia (<Emphasis Type="Italic">Zinnia elegans</Emphasis>) cell cultures

Background  

The xylem vascular system is composed of fused dead, hollow cells called tracheary elements (TEs) that originate through trans-differentiation of root and shoot cambium cells. TEs undergo autolysis as they differentiate and mature. The final stage of the formation of TEs in plants is the death of the involved cells, a process showing some similarities to programmed cell death (PCD) in animal systems. Plant proteases with functional similarity to proteases involved in mammalian apoptotic cell death (caspases) are suggested as an integral part of the core mechanism of most PCD responses in plants, but participation of plant caspase-like proteases in TE PCD has not yet been documented.     

New types of metacaspases in phytoplankton reveal diverse origins of cell death proteases

Metacaspases are evolutionarily distant homologs of caspases that are found outside the metazoan and are known to have key roles in programmed cell death (PCD). Two types of metacaspases (types I and II) have been defined in plants based on their domain structures; these have similarities to metazoan ‘initiator'' and ‘executioner'' caspases. However, we know little about metacaspases in unicellular organisms and even less about their roles in cell death. We identified a novel group of metacaspases in sequenced phytoplanktonic protists that show domain architectures distinct from either type I or II enzymes; we designate them as type III. Type III metacaspases exhibit a rearrangement of domain structures between N- and C-terminus. In addition, we found a group of metacaspase-like proteases in phytoplankton that show sequence homology with other metacaspases, but defy classification in conventional schemes. These metacaspase-like proteases exist in bacteria alongside a variant of type I metacaspases and we propose these bacterial metacaspases are the origins of eukaryotic metacaspases. Type II and III metacaspases were not detected in bacteria and they might be variants of bacterial type I metacaspases that evolved in plants and phytoplanktonic protists, respectively, during the establishment of plastids through the primary and secondary endosymbiotic events. A complete absence of metacaspases in protists that lost plastids, such as oömycetes and ciliates indicates the gene loss during the plastid-to-nucleus gene transfer. Taken together, our findings suggest endosymbiotic gene transfer (EGT) is a key mechanism resulting in the evolutionary diversity of cell death proteases.     

Nitric oxide: promoter or suppressor of programmed cell death?

Nitric oxide (NO) is a short-lived gaseous free radical that predominantly functions as a messenger and effector molecule. It affects a variety of physiological processes, including programmed cell death (PCD) through cyclic guanosine monophosphate (cGMP)-dependent and-independent pathways. In this field, dominant discoveries are the diverse apoptosis networks in mammalian cells, which involve signals primarily via death receptors (extrinsic pathway) or the mitochondria (intrinsic pathway) that recruit caspases as effector molecules. In plants, PCD shares some similarities with animal cells, but NO is involved in PCD induction via interacting with pathways of phytohormones. NO has both promoting and suppressing effects on cell death, depending on a variety of factors, such as cell type, cellular redox status, and the flux and dose of local NO. In this article, we focus on how NO regulates the apoptotic signal cascade through protein S-nitrosylation and review the recent progress on mechanisms of PCD in both mammalian and plant cells.     

Multiple mediators of plant programmed cell death: interplay of conserved cell death mechanisms and plant-specific regulators

Programmed cell death (PCD) is a process aimed at the removal of redundant, misplaced, or damaged cells and it is essential to the development and maintenance of multicellular organisms. In contrast to the relatively well-described cell death pathway in animals, often referred to as apoptosis, mechanisms and regulation of plant PCD are still ill-defined. Several morphological and biochemical similarities between apoptosis and plant PCD have been described, including DNA laddering, caspase-like proteolytic activity, and cytochrome c release from mitochondria. Reactive oxygen species (ROS) have emerged as important signals in the activation of plant PCD. In addition, several plant hormones may exert their respective effects on plant PCD through the regulation of ROS accumulation. The possible plant PCD regulators discussed in this review are integrated in a model that combines plant-specific regulators with mechanisms functionally conserved between animals and plants.     

Programmed cell death in plants: distinguishing between different modes

Programmed cell death (PCD) in plants is a crucial componentof development and defence mechanisms. In animals, differenttypes of cell death (apoptosis, autophagy, and necrosis) havebeen distinguished morphologically and discussed in these morphologicalterms. PCD is largely used to describe the processes of apoptosisand autophagy (although some use PCD and apoptosis interchangeably)while necrosis is generally described as a chaotic and uncontrolledmode of death. In plants, the term PCD is widely used to describemost instances of death observed. At present, there is a vastarray of plant cell culture models and developmental systemsbeing studied by different research groups and it is clear fromwhat is described in this mass of literature that, as with animals,there does not appear to be just one type of PCD in plants.It is fundamentally important to be able to distinguish betweendifferent types of cell death for several reasons. For example,it is clear that, in cell culture systems, the window of timein which ‘PCD’ is studied by different groups varieshugely and this can have profound effects on the interpretationof data and complicates attempts to compare different researcher'sdata. In addition, different types of PCD will probably havedifferent regulators and modes of death. For this reason, inplant cell cultures an apoptotic-like PCD (AL-PCD) has beenidentified that is fairly rapid and results in a distinct corpsemorphology which is visible 4–6 h after release of cytochromec and other apoptogenic proteins. This type of morphology, distinctfrom autophagy and from necrosis, has also been observed inexamples of plant development. In this review, our model systemand how it is used to distinguish specifically between AL-PCDand necrosis will be discussed. The different types of PCD observedin plants will also be discussed and the importance of distinguishingbetween different forms of cell death will be highlighted. Key words: Apoptosis, apoptosis-like programmed cell death (AL-PCD), Arabidopsis, autophagy, mitochondria, necrosis, programmed cell death (PCD) Received 5 June 2007; Revised 13 September 2007 Accepted 20 September 2007     

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

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

ABCA1-dependent but apoA-I-independent cholesterol efflux mediated by fatty acid-bile acid conjugates (FABACs)

PCD (programmed cell death) in plants presents important morphological and biochemical differences compared with apoptosis in animal cells. This raises the question of whether PCD arose independently or from a common ancestor in plants and animals. In the present study we describe a cell-free system, using wheat grain nucellar cells undergoing PCD, to analyse nucleus dismantling, the final stage of PCD. We have identified a Ca2+/Mg2+ nuclease and a serine protease localized to the nucleus of dying nucellar cells. Nuclear extracts from nucellar cells undergoing PCD triggered DNA fragmentation and other apoptotic morphology in nuclei from different plant tissues. Inhibition of the serine protease did not affect DNA laddering. Furthermore, we show that the nuclear extracts from plant cells triggered DNA fragmentation and apoptotic morphology in nuclei from human cells. The inhibition of the nucleolytic activity with Zn2+ or EDTA blocked the morphological changes of the nucleus. Moreover, nuclear extracts from apoptotic human cells triggered DNA fragmentation and apoptotic morphology in nuclei from plant cells. These results show that degradation of the nucleus is morphologically and biochemically similar in plant and animal cells. The implication of this finding on the origin of PCD in plants and animals is discussed.     

Detection of apoptosis induced by topoisomerase inhibitors and serum deprivation in syrian hamster embryo cells

The sensitivity of normal diploid Syrian hamster embryo (SHE) cells to apoptosis was tested after treatment with the topoisomerase inhibitors camptothecin and etoposide and after serum withdrawal. Programmed cell death (PCD) was identified through morphological, biochemical, and molecular changes and compared with that of HL60 cell line. The results showed that topoisomerase inhibitors, which were shown to be potent PCD inducers in the HL60 cell line, induced a weaker apoptotic response in SHE cells than after growth factor deprivation. In addition, serum-free medium, which rapidly induced apoptosis in SHE cells, did not affect the HL60 cell line. In both cell types, PCD was expressed by condensed chromatin, fragmented nuclei, and DNA laddering on electrophoretic gels, an indisputable sign of apoptosis. In apoptotic HL60 cells, the cleavage of 113-kDa poly(ADP-ribose)polymerase (PARP) resulted in the so-called apoptotic 89-kDa fragment and was associated with increased caspase-3 activity. In apoptotic SHE cells, PARP degraded early but the degradation profile was not characterized by the appearance of an 89-kDa fragment. Moreover, no activation of caspase-3 was noted. ZnCl(2), which is known to prevent protease activity responsible for apoptosis features, inhibited PARP cleavage and nuclear modifications induced by apoptotic stimuli in both cell types, but with a higher sensitivity in SHE cells. Apoptosis induced by serum deprivation was linked with c-myc negative regulation in SHE cells, but not with p53 protein accumulation, while topoisomerase inhibitors led to p53 stabilization without any change in c-myc expression. Serum-free medium and topoisomerase inhibitors did not modify c-myc expression in the HL60 cell line. The overall results demonstrated that apoptosis, which is a carefully regulated process of cell death, may proceed through mechanisms varying according to cell type or apoptosis inducer. In addition, markers which are generally considered hallmarks of apoptosis may fail to appear in some cell types.     

Implication of reactive oxygen species and mitochondrial dysfunction in the early stages of plant programmed cell death induced by ultraviolet-C overexposure

Recent studies have suggested that ultraviolet-C (UV-C) overexposure induces programmed cell death (PCD) in Arabidopsis thaliana (L.) Heynh, and this process includes participation of caspase-like proteases, DNA laddering as well as fragmentation of the nucleus. To investigate possible early signal events, we used microscopic observations to monitor in vivo the behaviour of mitochondria, as well as the production and localization of reactive oxygen species (ROS) during protoplast PCD induced by UV-C. A quick burst of ROS was detected when the protoplasts were kept in continuous light after UV-C exposure, which was restricted in chloroplasts and the adjacent mitochondria. Pre-incubation with ascorbic acid (AsA, antioxidant molecule) or 3-(3, 4-dichlorophenyl)-1, 1-dimethylurea (DCMU, an inhibitor of photosynthetic electron transport) decreased the ROS production and partially protected protoplasts from PCD. A mitochondrial transmembrane potential (MTP) loss occurred prior to cell death; thereafter, the mitochondria irregularly clumped around chloroplasts or aggregated in other places within the cytoplasm, and the movement of mitochondria was concomitantly blocked. Pre-treatment with an inhibitor of mitochondrial permeability transition pores (MPTP), cyclosporine (CsA), effectively retarded the decrease of MTP and reduced the percentage of protoplasts undergoing PCD after UV-C overexposure. Our results suggest that the MTP loss and the changes in distribution and mobility of mitochondria, as well as the production of ROS play important roles during UV-induced plant PCD, which is in good accordance with what has been reported in many types of apoptotic cell death, both in animals and plants. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

植物细胞程序死亡的机理及其与发育的关系

细胞程序死亡（PCD）是在植物体发育过程中普遍存在的,在发育的特定阶段发生的自然的细胞死亡过程,这一死亡过程是由某些特定基因编码的“死亡程序”控制的。PCD是细胞分化的最后阶段。细胞分化的临界期就处于死亡程序执行中的某个阶段。PCD包含启动期、效应期和清除期三个阶段,其间caspase家族起着重要作用。PCD在细胞和组织的平衡、特化,以及组织分化、器官建成和对病原体的反应等植物发育过程中起着重要作用。PCD中的形态学变化和生物化学变化都有着严格的时序性。植物的PCD和动物的PCD有许多共性,包括细胞形态和DNA降解等变化。也有一些不同,植物PCD的产物既可被其它细胞吸收利用;也可用于构建自身的次生细胞壁。     

植物细胞程序死亡的机理及其与发育的关系

细胞程序死亡（ＰＣＤ）是在植物体发育过程中普遍存在的,在发育的特定阶段发生的自然的细胞死亡过程,这一死亡过程是由某些特定基因编码的“死亡程序”控制的。ＰＣＤ的细胞分化的最后阶段。细胞分化的临界期就牌死亡程序执行中的某个阶段。ＰＣＤ包含启动期和清除期三个阶段,其间ＣＡＳＰＡＳＥ家族起着重要作用。ＰＣＤ在细胞和组织的平衡、特化,以及组织分化、器官建成和对病原体的反应等植物发育过程中起着重要作用。ＰＣＤ     

The gene expression profiling in murine cortical cells undergoing programmed cell death (PCD) induced by serum deprivation

PCD (programmed cell death) is important mechanism for development, homeostasis and disease. To analyze the gene expression pattern in brain cells undergoing PCD in response to serum deprivation, we analyzed the cDNA microarray consisting of 2,300 genes and 7 housekeeping genes of cortical cells derived from mouse embryonic brain. Cortical cells were induced apoptosis by serum deprivation for 8 hours. We identified 69 up-regulated genes and 21 down-regulated genes in apoptotic cells. Based on the cDNA microarray data four genes were selected and analyzed by RT-PCR and northern blotting. To characterize the role of UNC-51-like kinase (ULK2) gene in PCD, we investigated cell death effect by ULK2. And we examined expression of several genes that related with PCD. Especially GAPDH was increased by ULK2. Theses findings indicated that ULK2 is involved in apoptosis through p53 pathway.     

Cell death in the unicellular chlorophyte Dunaliella tertiolecta. A hypothesis on the evolution of apoptosis in higher plants and metazoans

Apoptosis is essential for normal growth and development of multicellular organisms, including metazoans and higher plants. Although cell death processes have been reported in unicellular organisms, key elements of apoptotic pathways have not been identified. Here, we show that when placed in darkness, the unicellular chlorophyte alga Dunaliella tertiolecta undergoes a form of cell death reminiscent of apoptosis in metazoans. Many morphological criteria of apoptotic cell death were met, including an increase in chromatin margination, degradation of the nucleus, and DNA fragmentation. Biochemical assays of the activities of cell death-associated proteases, caspases, measured using highly specific fluorogenic substrates, increased with time in darkness and paralleled the morphological changes. The caspase-like activities were inhibited by caspase-specific inhibitors. Antibodies raised against mammalian caspases cross-reacted with specific proteins in the alga. The pattern of expression of these immunologically reactive proteins was correlated with the onset of cell death. The occurrence of key components of apoptosis, and particularly a caspase-mediated cell death cascade in a relatively ancient linage of eukaryotic photoautotrophs, argues against current theories that cell death evolved in multicellular organisms. We hypothesize that key elements of cell death pathways were transferred to the nuclear genome of early eukaryotes through ancient viral infections in the Precambrian Ocean before the evolution of multicellular organisms and were subsequently appropriated in both metazoan and higher plant lineages.     

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.