Dying two deaths - programmed cell death regulation in development and disease

Sclerotinia sclerotiorum: An Evaluation of Virulence Theories

Oxalic acid production in Sclerotinia sclerotiorum has long been associated with virulence. Research involving UV-induced, genetically undefined mutants that concomitantly lost oxalate accumulation, sclerotial formation, and pathogenicity supported the conclusion that oxalate is an essential pathogenicity determinant of S. sclerotiorum. However, recent investigations showed that genetically defined mutants that lost oxalic acid production but accumulated fumaric acid could cause disease on many plants and substantiated the conclusion that acidic pH, not oxalic acid per se, is the necessary condition for disease development. Critical evaluation of available evidence showed that the UV-induced mutants harbored previously unrecognized confounding genetic defects in saprophytic growth and pH responsiveness, warranting reevaluation of the conclusions about virulence based on the UV-induced mutants. Furthermore, analyses of the evidence suggested a hypothesis for the existence of an unrecognized regulator responsive to acidic pH. Identifying the unknown pH regulator would offer a new avenue for investigating pH sensing/regulation in S. sclerotiorum and novel targets for intervention in disease control strategies.

Evodiamine Induces Apoptosis, G2/M Cell Cycle Arrest, and Inhibition of Cell Migration and Invasion in Human Osteosarcoma Cells via Raf/MEK/ERK Signalling Pathway

Background

Osteosarcoma is a prevalent type of bone tumor mainly reported in children and adolescents. The treatments for osteosarcoma are limited and are associated with serious adverse effects. In this study we evaluated the anticancer activity of Evodiamine, a plant-derived natural product, against a panel of osteosarcoma cells and explored the underlying mechanisms.

Material/Methods

The viability of osteosarcoma cell lines was investigated by MTT assay. Apoptosis was detected by DAPI and annexin V/PI staining and cell cycle analysis was performed by flow cytometry. The expression of the proteins was examined by Western blotting.

Results

The results of the present study indicated that Evodiamine inhibited the proliferation of U2OS osteosarcoma cells with an IC₅₀ of 6 μM. Further investigations indicated the antiproliferative effects of Evodiamine are due to induction of apoptosis and G2/M cell cycle arrest. The results of Western blotting revealed that the expression of several apoptosis (Cytochrome c, Bax, Bid, Caspase 3, 9, 8, and PARP) and cell cycle-related proteins (cyclin B1, Cdc25c, and Cdc2) was significantly altered. Evodiamine also suppressed the migration and invasion of U2OS osteosarcoma cells. Moreover, Evodiamine downregulated the expression of important regulatory proteins such as p-MEK and p-ERK, leading to the inhibition of Raf/MEK/ERK signalling pathways.

Conclusions

We found that Evodiamine exerts anticancer effects on osteosarcoma cells and has potential in the treatment of osteosarcoma.

KIRA1 and ORESARA1 terminate flower receptivity by promoting cell death in the stigma of Arabidopsis

Flowers have a species-specific functional life span that determines the time window in which pollination, fertilization and seed set can occur. The stigma tissue plays a key role in flower receptivity by intercepting pollen and initiating pollen tube growth toward the ovary. In this article, we show that a developmentally controlled cell death programme terminates the functional life span of stigma cells in Arabidopsis. We identified the leaf senescence regulator ORESARA1 (also known as ANAC092) and the previously uncharacterized KIRA1 (also known as ANAC074) as partially redundant transcription factors that modulate stigma longevity by controlling the expression of programmed cell death-associated genes. KIRA1 expression is sufficient to induce cell death and terminate floral receptivity, whereas lack of both KIRA1 and ORESARA1 substantially increases stigma life span. Surprisingly, the extension of stigma longevity is accompanied by only a moderate extension of flower receptivity, suggesting that additional processes participate in the control of the flower's receptive life span.

OsNAC2 positively affects salt-induced cell death and binds to the OsAP37 and OsCOX11 promoters

Plant development and adaptation to environmental stresses are intimately associated with programmed cell death (PCD). Although some of the mechanisms regulating PCD [e.g., accumulation of reactive oxygen species (ROS)] are common among responses to different abiotic stresses, the pathways mediating salt-induced PCD remain largely uncharacterized. Here we report that overexpression of OsNAC2, which encodes a plant-specific transcription factor, promotes salt-induced cell death accompanied by the loss of plasma membrane integrity, nuclear DNA fragmentation, and changes to caspase-like activity. In OsNAC2-knockdown lines, cell death was markedly decreased in response to severe salt stress. Additionally, OsNAC2 expression was enhanced in rice seedlings exposed to a high NaCl concentration. Moreover, the results of quantitative real-time PCR, chromatin immunoprecipitation, dual-luciferase, and yeast one-hybrid assays indicated that OsNAC2 targeted genes that encoded an ROS scavenger (OsCOX11) and a caspase-like protease (OsAP37). Furthermore, K⁺ -efflux channels (OsGORK and OsSKOR) were clearly activated by OsNAC2. Overall, our results suggested that OsNAC2 accelerates NaCl-induced PCD and provide new insights into the mechanisms that affect ROS accumulation, plant caspase-like activity, and K⁺ efflux.

Autophagy contributes to regulate the ROS levels and PCD progress in TMV-infected tomatoes

Programmed cell death (PCD) and autophagy are both important means for plants to resist pathogen. It is also the main biological reaction of plant immunity. In previous studies, we found that TMV local-infection on tomato leaves not only caused the PCD process in the distal root tissues, but also induced autophagy in root-tip cells. However, the reasons for these biological phenomena are unclear. In order to get deeper insight, the role of a putative inducible factor reactive oxidative species (ROS) was investigated. The situ staining and subcellular localization analysis showed that the ROS level in the root tissue of TMV infected plants was significantly promoted. TEM observation showed that the intracellular ROS was excreted into the cell wall and intercellular layer. At the same time, the results of western blot and qRT-PCR showed that the expression of autophagy related protein Atg8 and genes (Atg5, Atg7 and Atg10) were increased. However, in the subsequent DPI inhibition experiments we found that the accumulation of ROS in infected plant root-tip tissues was inhibited and the autophagy in the root-tip cells also decreased. When 3-methyladenine (3-MA) was used to inhibit autophagy, there was no significant change in the ROS level in the apical tissue, while the systemic PCD process of the root-tip cells was elevated. Taken together, these results indicate that local TMV inoculation on the leaves induced the root-tip cells producing and releasing a lot of ROS into the extracellular matrix for defense against pathogen invasion. Meanwhile, ROS acted as a signaling substance and triggered autophagy in root-tip cells, in order to eliminate excessive intracellular ROS oxidative damage and maintain cell survival.

Arabidopsis MLO2 is a negative regulator of sensitivity to extracellular reactive oxygen species

The atmospheric pollutant ozone (O₃ ) is a strong oxidant that causes extracellular reactive oxygen species (ROS) formation, has significant ecological relevance, and is used here as a non-invasive ROS inducer to study plant signalling. Previous genetic screens identified several mutants exhibiting enhanced O₃ sensitivity, but few with enhanced tolerance. We found that loss-of-function mutants in Arabidopsis MLO2, a gene implicated in susceptibility to powdery mildew disease, exhibit enhanced dose-dependent tolerance to O₃ and extracellular ROS, but a normal response to intracellular ROS. This phenotype is increased in a mlo2 mlo6 mlo12 triple mutant, reminiscent of the genetic redundancy of MLO genes in powdery mildew resistance. Stomatal assays revealed that enhanced O₃ tolerance in mlo2 mutants is not caused by altered stomatal conductance. We explored modulation of the mlo2-associated O₃ tolerance, powdery mildew resistance, and early senescence phenotypes by genetic epistasis analysis, involving mutants with known effects on ROS sensitivity or antifungal defence. Mining of publicly accessible microarray data suggests that these MLO proteins regulate accumulation of abiotic stress response transcripts, and transcript accumulation of MLO2 itself is O₃ responsive. In summary, our data reveal MLO2 as a novel negative regulator in plant ROS responses, which links biotic and abiotic stress response pathways.

Transcriptome analysis of embryonic domains in Norway spruce reveals potential regulators of suspensor cell death

The terminal differentiation and elimination of the embryo-suspensor is the earliest manifestation of programmed cell death (PCD) during plant ontogenesis. Molecular regulation of suspensor PCD remains poorly understood. Norway spruce (Picea abies) embryos provide a powerful model for studying embryo development because of their large size, sequenced genome, and the possibility to obtain a large number of embryos at a specific developmental stage through somatic embryogenesis. Here, we have carried out global gene expression analysis of the Norway spruce embryo-suspensor versus embryonal mass (a gymnosperm analogue of embryo proper) using RNA sequencing. We have identified that suspensors have enhanced expression of the NAC domain-containing transcription factors, XND1 and ANAC075, previously shown to be involved in the initiation of developmental PCD in Arabidiopsis. The analysis has also revealed enhanced expression of Norway spruce homologues of the known executioners of both developmental and stress-induced cell deaths, such as metacaspase 9 (MC9), cysteine endopeptidase-1 (CEP1) and ribonuclease 3 (RNS3). Interestingly, a spruce homologue of bax inhibitor-1 (PaBI-1, for Picea abies BI-1), an evolutionarily conserved cell death suppressor, was likewise up-regulated in the embryo-suspensor. Since Arabidopsis BI-1 so far has been implicated only in the endoplasmic reticulum (ER)-stress induced cell death, we investigated its role in embryogenesis and suspensor PCD using RNA interference (RNAi). We have found that PaBI-1-deficient lines formed a large number of abnormal embryos with suppressed suspensor elongation and disturbed polarity. Cytochemical staining of suspensor cells has revealed that PaBI-1 deficiency suppresses vacuolar cell death and induces necrotic type of cell death previously shown to compromise embryo development. This study demonstrates that a large number of cell-death components are conserved between angiosperms and gymnosperms and establishes a new role for BI-1 in the progression of vacuolar cell death.

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.

Macroautophagy occurs in distal TMV-uninfected root tip tissue of tomato taking place systemic PCD

Autophagy is an important mechanism for recycling cell materials upon encountering stress conditions. Our previous studies had shown that TMV infection could lead to systemic PCD in the distal uninfected tissues, including root tip and shoot tip tissues. But it is not clear whether there is autophagy in the distal apical meristem of TMV-induced plants. To better understand the autophagy process during systemic PCD, here we investigated the formation and type of autophagy in the root meristem cells occurring PCD. Transmission electron microscopy assay revealed that the autophagic structures formed by the fusion of vesicles, containing the sequestered cytoplasm, multilamellar bodies, and degraded mitochondria. In the PCD progress, many mitochondria appeared degradation with blurred inner membrane structure. And the endoplasmic reticulum was broke into small fragments. Finally, the damaged mitochodria were engulfed and degraded by the autophagosomes. These results indicated that during the systemic PCD process of root tip cells, the classical macroautophagy occurred, and the cell contents and damaged organelles (mitochondria) would be self-digested by autophagy.

Autophagy as a mediator of life and death in plants

Autophagy is a major pathway for degradation and recycling of cytoplasmic material, including individual proteins, aggregates, and entire organelles. Autophagic processes serve mainly survival functions in cellular homeostasis, stress adaptation and immune responses but can also have death-promoting activities in different eukaryotic organisms. In plants, the role of autophagy in the regulation of programmed cell death (PCD) remained elusive and a subject of debate. More recent evidence, however, has resulted in the consensus that autophagy can either promote or restrict different forms of PCD. Here, we present latest advances in understanding the molecular mechanisms and functions of plant autophagy and discuss their implications for life and death decisions in the context of developmental and pathogen-induced PCD.

The GYF domain protein PSIG1 dampens the induction of cell death during plant-pathogen interactions

The induction of rapid cell death is an effective strategy for plants to restrict biotrophic and hemi-biotrophic pathogens at the infection site. However, activation of cell death comes at a high cost, as dead cells will no longer be available for defense responses nor general metabolic processes. In addition, necrotrophic pathogens that thrive on dead tissue, take advantage of cell death-triggering mechanisms. Mechanisms by which plants solve this conundrum remain described. Here, we identify PLANT SMY2-TYPE ILE-GYF DOMAIN-CONTAINING PROTEIN 1 (PSIG1) and show that PSIG1 helps to restrict cell death induction during pathogen infection. Inactivation of PSIG1 does not result in spontaneous lesions, and enhanced cell death in psig1 mutants is independent of salicylic acid (SA) biosynthesis or reactive oxygen species (ROS) production. Moreover, PSIG1 interacts with SMG7, which plays a role in nonsense-mediated RNA decay (NMD), and the smg7-4 mutant allele mimics the cell death phenotype of the psig1 mutants. Intriguingly, the psig1 mutants display enhanced susceptibility to the hemi-biotrophic bacterial pathogen. These findings point to the existence and importance of the SA- and ROS-independent cell death constraining mechanism as a part of the plant immune system.

Programmed cell death (PCD) has crucial roles in development and immunity in multicellular organisms. In plants, rapid PCD induction, so-called hypersensitive response (HR) cell death, can be triggered as a part of immune system, and plays an important role in restricting pathogen growth. Despite its importance, cell death induction can backfire on plants because of the diversified infection strategies of plant pathogens. It is therefore assumed that plants have mechanisms by which they are able to minimize PCD induction during plant-pathogen interactions. However, their existence and biological significance are not clear yet. Here, we demonstrate that PSIG1, which has the GYF domain that is highly conserved among diverse eukaryotic species, restricts cell death induction during pathogen invasions. Importantly, psig1 mutants do not display autoimmune phenotypes, and are more susceptible to the virulent bacterial pathogen. Our findings suggest that the restriction of cell death can have benefits for plants to defend themselves against hemi-biotrophic bacterial pathogen infections. We further provide evidence suggesting a mechanism by which PSIG1 may contain cell death by regulating the RNA metabolism machinery.

Plant life needs cell death, but does plant cell death need Cys proteases?

Caspases are key regulators of apoptosis in animals. This correlation has driven plant researchers for decades to look for caspases regulating programmed cell death (PCD) in plants. These studies revealed caspase-like activities, caspase-related proteases, and cysteine (Cys) proteases regulating PCD in plants, but identified no caspases and no conserved, apoptosis-like death pathway. Here, we critically review the evidence for Cys proteases implicated in PCD in plants. We discuss the role of papain-like Cys proteases, vacuolar processing enzymes, and metacaspases in PCD during the development of tracheary elements, seed coat, suspensor, and tapetum, and during the hypersensitive response. There are several convincing cases where these Cys proteases are required for PCD, but this requirement is often not conserved across different plant species. There are also cases where Cys proteases contribute to the speed, but not the timing of PCD, while other Cys proteases are nonessential for PCD, but have other roles, e.g., in the clearance of cell remains after PCD. These data illustrate the need for caution when generalizing the role of Cys proteases in regulating PCD in plants, and call for studies that further investigate plant Cys proteases and other PCD regulators.

Back to the roots of regulated necrosis

Todorov and Linkermann highlight the recent discovery by Distéfano et al. that heat triggers ferroptosis-like death in plants.

In recent years, our knowledge of how cells die by regulated pathways of necrosis has increased tremendously. In this issue, Distéfano et al. (2017. J. Cell Biol.https://doi.org/10.1083/jcb.201605110) provide yet another milestone in our understanding of regulated necrosis as they identify a ferroptosis-like cell death in Arabidopsis thaliana.