Increased Maternal Genome Dosage Bypasses the Requirement of the FIS Polycomb Repressive Complex 2 in Arabidopsis Seed Development

Seed development in flowering plants is initiated after a double fertilization event with two sperm cells fertilizing two female gametes, the egg cell and the central cell, leading to the formation of embryo and endosperm, respectively. In most species the endosperm is a polyploid tissue inheriting two maternal genomes and one paternal genome. As a consequence of this particular genomic configuration the endosperm is a dosage sensitive tissue, and changes in the ratio of maternal to paternal contributions strongly impact on endosperm development. The FERTILIZATION INDEPENDENT SEED (FIS) Polycomb Repressive Complex 2 (PRC2) is essential for endosperm development; however, the underlying forces that led to the evolution of the FIS-PRC2 remained unknown. Here, we show that the functional requirement of the FIS-PRC2 can be bypassed by increasing the ratio of maternal to paternal genomes in the endosperm, suggesting that the main functional requirement of the FIS-PRC2 is to balance parental genome contributions and to reduce genetic conflict. We furthermore reveal that the AGAMOUS LIKE (AGL) gene AGL62 acts as a dosage-sensitive seed size regulator and that reduced expression of AGL62 might be responsible for reduced size of seeds with increased maternal genome dosage.     

Arabidopsis ACCELERATED CELL DEATH2 modulates programmed cell death

The Arabidopsis thaliana chloroplast protein ACCELERATED CELL DEATH2 (ACD2) modulates the amount of programmed cell death (PCD) triggered by Pseudomonas syringae and protoporphyrin IX (PPIX) treatment. In vitro, ACD2 can reduce red chlorophyll catabolite, a chlorophyll derivative. We find that ACD2 shields root protoplasts that lack chlorophyll from light- and PPIX-induced PCD. Thus, chlorophyll catabolism is not obligatory for ACD2 anti-PCD function. Upon P. syringae infection, ACD2 levels and localization change in cells undergoing PCD and in their close neighbors. Thus, ACD2 shifts from being largely in chloroplasts to partitioning to chloroplasts, mitochondria, and, to a small extent, cytosol. ACD2 protects cells from PCD that requires the early mitochondrial oxidative burst. Later, the chloroplasts of dying cells generate NO, which only slightly affects cell viability. Finally, the mitochondria in dying cells have dramatically altered movements and cellular distribution. Overproduction of both ACD2 (localized to mitochondria and chloroplasts) and ascorbate peroxidase (localized to chloroplasts) greatly reduces P. syringae-induced PCD, suggesting a pro-PCD role for mitochondrial and chloroplast events. During infection, ACD2 may bind to and/or reduce PCD-inducing porphyrin-related molecules in mitochondria and possibly chloroplasts that generate reactive oxygen species, cause altered organelle behavior, and activate a cascade of PCD-inducing events.     

Characterization of Aquilegia Polycomb Repressive Complex 2 homologs reveals absence of imprinting

Epigenetic regulation is important for maintaining gene expression patterns in multicellular organisms. The Polycomb Group (PcG) proteins form several complexes with important and deeply conserved epigenetic functions in both the plant and animal kingdoms. The plant Polycomb Repressive Complex 2 (PRC2) contains four core proteins, Enhancer of Zeste (E(z)), Suppressor of Zeste 12 (Su(z)12), Extra Sex Combs (ESC), and Multicopy Suppressor of IRA 1 (MSI1), and functions in many developmental transitions. In some plant species, including rice and Arabidopsis, duplications in the core PRC2 proteins allow the formation of PRC2s with distinct developmental functions. In addition, members of the plant specific VEL PHD family have been shown to associate with the PRC2 complex in Arabidopsis and may play a role in targeting the PRC2 to specific loci. Here we examine the evolution and expression of the PRC2 and VEL PHD families in Aquilegia, a member of the lower eudicot order Ranunculales and an emerging model for the investigation of plant ecology, evolution and developmental genetics. We find that Aquilegia has a relatively simple PRC2 with only one homolog of Su(z)12, ESC and MSI1 and two ancient copies of E(z), AqSWN and AqCLF. Aquilegia has four members of the VEL PHD family, three of which appear to be closely related to Arabidopsis proteins known to associate with the PRC2. The PRC2 and VEL PHD family proteins are expressed at a relatively constant level throughout Aquilegia vulgaris development, with the VEL PHD family and MSI1 expressed at higher levels during and after vernalization and in the inflorescence. Both AqSWN and AqCLF are expressed in Aquilegia endosperm but neither copy is imprinted.     

Polycomb Repressive Complex 2 Regulates Lineage Fidelity during Embryonic Stem Cell Differentiation

Polycomb Repressive Complex 2 (PRC2) catalyzes histone H3 lysine 27 tri-methylation (H3K27me3), an epigenetic modification associated with gene repression. H3K27me3 is enriched at the promoters of a large cohort of developmental genes in embryonic stem cells (ESCs). Loss of H3K27me3 leads to a failure of ESCs to properly differentiate, making it difficult to determine the precise roles of PRC2 during lineage commitment. Moreover, while studies suggest that PRC2 prevents DNA methylation, how these two epigenetic regulators coordinate to regulate lineage programs is poorly understood. Using several PRC2 mutant ESC lines that maintain varying levels of H3K27me3, we found that partial maintenance of H3K27me3 allowed for proper temporal activation of lineage genes during directed differentiation of ESCs to spinal motor neurons (SMNs). In contrast, genes that function to specify other lineages failed to be repressed in these cells, suggesting that PRC2 is also necessary for lineage fidelity. We also found that loss of H3K27me3 leads to a modest gain in DNA methylation at PRC2 target regions in both ESCs and in SMNs. Our study demonstrates a critical role for PRC2 in safeguarding lineage decisions and in protecting genes against inappropriate DNA methylation.     

Caspase-like proteases regulate aluminum-induced programmed cell death in peanut

Recent evidence has proved that caspase protease activities are detected in both mammals and plants during programmed cell death (PCD). The characteristics and functions of caspase-like proteases play important roles in understanding the mechanisms of PCD in plants. In this work, we report firstly the involvement of caspase-like protease activities and effects in aluminum (Al) stress in two contrasting peanut genotypes. Caspase-like activities in the root tip cells of ‘Zhonghua 2’ (Al-sensitive) and ‘99-1507’ (Al-tolerant) were detected using synthetic caspase substrates during Al-triggered PCD. Caspase-1-, -2-, -3-, -4-, -5-, -6-, -8- and -9-like proteases were found in peanut root tip cells. VDQQDase (caspase-2-like) and WEHD (caspase-5-like) were the first detected in the plants, and almost all of the caspase-like proteases were activated during Al-induced PCD, especially caspase-3-like and caspase-1-like, which was higher in ‘Zhonghua 2’ than in ‘99-1507’. The highest activity levels of caspase-3- and caspase-1-like proteases occurred 8 and 4 h after 100 µM Al treatment, respectively. Compared with 100 µM AlCl₃ treatment alone, specific caspase-3 protease inhibitor Ac-DEVD-CHO inhibited the increase of caspase-3-like protease activity, Al content, Hsr203j expression, cell death and DNA fragmentation, and the decrease in root growth induced by 100 µM AlCl₃ treatment, but it was more obvious in ‘Zhonghua 2’ than in ‘99-1507’. In conclusion, there were different caspase-like proteases in root tips of peanut, and caspase-3-like protease was a crucial executioner in Al-induced PCD. Its effects in the ‘Zhonghua 2’ genotype were higher than in ‘99-1507’. An improved model of the mechanism of Al-induced PCD and Al tolerance differences in different genotypes is proposed.     

Acetylsalicylic acid induces programmed cell death in Arabidopsis cell cultures

Acetylsalicylic acid (ASA), a derivative from the plant hormone salicylic acid (SA), is a commonly used drug that has a dual role in animal organisms as an anti-inflammatory and anticancer agent. It acts as an inhibitor of cyclooxygenases (COXs), which catalyze prostaglandins production. It is known that ASA serves as an apoptotic agent on cancer cells through the inhibition of the COX-2 enzyme. Here, we provide evidences that ASA also behaves as an agent inducing programmed cell death (PCD) in cell cultures of the model plant Arabidopsis thaliana, in a similar way than the well-established PCD-inducing agent H(2)O(2), although the induction of PCD by ASA requires much lower inducer concentrations. Moreover, ASA is herein shown to be a more efficient PCD-inducing agent than salicylic acid. ASA treatment of Arabidopsis cells induces typical PCD-linked morphological and biochemical changes, namely cell shrinkage, nuclear DNA degradation, loss of mitochondrial membrane potential, cytochrome c release from mitochondria and induction of caspase-like activity. However, the ASA effect can be partially reverted by jasmonic acid. Taking together, these results reveal the existence of common features in ASA-induced animal apoptosis and plant PCD, and also suggest that there are similarities between the pathways of synthesis and function of prostanoid-like lipid mediators in animal and plant organisms.     

Polycomb Repressive Complex 2 Regulates Normal Hematopoietic Stem Cell Function in a Developmental-Stage-Specific Manner

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Steroid hormones regulate programmed cell death: a review

Programmed cell death (PCD) is a physiologically active process which is essential for the proper functioning of any living tissue. The steroid hormones modulate the programme in the immunological and reproductive organs and tissues, such as the thymus gland, circulating thymocytes, uterus, vagina, testis, ovary and prostrate gland. The influence of steroid hormones on cell death is tissue specific; the same hormone can inhibit PCD in one tissue, and may promote PCD in another tissue. The roles of apoptosis and terminal differentiation have been examined, and the regulation of PCD by steroid hormones, assessed.     

Photoperiodic Control of the Floral Transition through a Distinct Polycomb Repressive Complex

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Characterization of non-canonical Polycomb Repressive Complex 1 subunits during early mouse embryogenesis

An intense period of chromatin remodeling takes place after fertilization in mammals, which is thought necessary for epigenetic reprogramming to start a new developmental program. While much attention has been given to the role of Polycomb Repressive Complex 2 (PRC2) and to canonical PRC1 complexes during this process, little is known as to whether there is any contribution of non-canonical PRC1 in shaping the chromatin landscape after fertilization. Here, we first describe in detail the temporal dynamics and abundance of H2A ubiquitylation (H2AK119ub), a histone modification catalyzed by PRC1, during pre-implantation mouse development. In addition, we have analyzed the presence of the 2 characteristic subunits of non-canonical PRC1 complexes, RYBP and its homolog YAF-2. Our results indicate that H2AK119ub is inherited from the sperm, rapidly removed from the paternal chromatin after fertilization, but detected again prior to the first mitosis, suggesting that PRC1 activity occurs as early as the zygotic stage. RYBP and YAF-2, together with the non-canonical subunit L3MBTL2, are all present during pre-implantation development but show different temporal dynamics. While RYBP is absent in the zygote, it is strongly induced from the 4-cell stage onwards. YAF-2 is inherited maternally and localizes to the pericentromeric regions in the zygote, is strongly induced between the 2- and 4-cell stages but then remains weak to undetectable subsequently. All together, our data suggest that non-canonical PRC1 is active during pre-implantation development and should be regarded as an additional component during epigenetic reprogramming and in the establishment of cellular plasticity of the early embryo.     

Bcl-2 proteins and autophagy regulate mitochondrial dynamics during programmed cell death in the Drosophila ovary

The Bcl-2 family has been shown to regulate mitochondrial dynamics during cell death in mammals and C. elegans, but evidence for this in Drosophila has been elusive. Here, we investigate the regulation of mitochondrial dynamics during germline cell death in the Drosophila melanogaster ovary. We find that mitochondria undergo a series of events during the progression of cell death, with remodeling, cluster formation and uptake of clusters by somatic follicle cells. These mitochondrial dynamics are dependent on caspases, the Bcl-2 family, the mitochondrial fission and fusion machinery, and the autophagy machinery. Furthermore, Bcl-2 family mutants show a striking defect in cell death in the ovary. These data indicate that a mitochondrial pathway is a major mechanism for activation of cell death in Drosophila oogenesis.