Propagation of patterns of gene expression through the cell cycle requires prompt restoration of epigenetic marks after the twofold dilution caused by DNA replication. Here we show that the ...transcriptional repressive mark H3K27me3 (histone H3 lysine 27 trimethylation) is restored in replicating plant cells through DNA replication–coupled modification of histone variant H3.1. Plants evolved a mechanism for efficient K27 trimethylation on H3.1, which is essential for inheritance of the silencing memory from mother to daughter cells. We illustrate how this mechanism establishes H3K27me3-mediated silencing during the developmental transition to flowering. Our study reveals a mechanism responsible for transmission of H3K27me3 in plant cells through cell divisions, enabling H3K27me3 to function as an epigenetic mark.
Gregor Mendel's work on segregation of traits in plants established the basic methodology and rules of genetics. The interruption of Mendel's research activities in 1870 impeded the immediate ...recognition of the value of his work until the dawn of the 20th century. Only then were his founding laws of genetics validated, propelling the development of biological research toward the birth of molecular biology in the second half of the 20th century. While molecular plant genetics can be viewed as the spiritual heir of Mendel's research, one might wonder whether in the 21st century Gregor Mendel would prefer to practice scientific approaches other than molecular genetics such as population genetics, comparative genomics, or the emerging field of evo-chromo. In this perspective, I review aspects of these fields that might have attracted or perplexed a 21st century Mendel.
Epigenetic reprogramming consists of global changes in DNA methylation and histone modifications. In mammals, epigenetic reprogramming is primarily associated with sexual reproduction and occurs ...during both gametogenesis and early embryonic development. Such reprogramming is crucial not only to maintain genomic integrity through silencing transposable elements but also to reset the silenced status of imprinted genes. In plants, observations of stable transgenerational inheritance of epialleles have argued against reprogramming. However, emerging evidence supports that epigenetic reprogramming indeed occurs during sexual reproduction in plants and that it has a major role in maintaining genome integrity and a potential contribution to epiallelic variation.
The flowering plant germline is produced during the haploid gametophytic stage. Defining the germline is complicated by the extreme reduction of the male and female gametophytes, also referred to as ...pollen and embryo sac, respectively. Both male and female gamete progenitors are segregated by an asymmetric cell division, as is the case for the germline in animals. Genetic studies and access to the transcriptome of isolated gametes have provided a regulatory framework for the mechanisms that define the male germline. What specifies female germline identity remains unknown. Recent evidence indicates that an auxin gradient provides positional information and plays a role in defining the identity of the female gamete lineage. The animal germline is also marked by production of small RNAs, and recent evidence indicates that this trait might be shared with the plant gamete lineage.
It is generally considered that Polycomb Repressive Complex (PRC)2 deposits the histone mark H3K27me3 on silent protein-coding genes, while transposable elements are repressed by DNA and/or H3K9 ...methylation. Yet, there is increasing evidence that PRC2 also targets and even silences transposable elements in representatives of several distantly related eukaryotic lineages. In plants and animals, H3K27me3 is present on transposable elements in mutants and specific cell types devoid of DNA methylation. In this Opinion, we summarize the experimental evidence for this phenomenon across the eukaryotic kingdom, and discuss its functional and evolutionary significance. We hypothesize that an ancestral role of Polycomb group (PcG) proteins was to silence transposable elements.
It has been generally accepted that H3K9me2-3/DNA methylation and H3K27me3 silent heterochromatin marks are found in mutually exclusive genomic regions and display distinct properties.This view is challenged by recent observations of transposable elements marked by H3K27me3.We hypothesize an ancestral association of H3K27me3 with silencing of transposable elements. As multicellular eukaryotes evolved, H3K27me3 became associated with gene silencing, while H3K9/DNA methylation became dedicated to silencing transposable elements.
Nucleosome dynamics and properties are central to all forms of genomic activities. Among the core histones, H3 variants play a pivotal role in modulating nucleosome structure and function. Here, we ...focus on the impact of H3 variants on various facets of development. The deposition of the replicative H3 variant following DNA replication is essential for the transmission of the epigenomic information encoded in posttranscriptional modifications. Through this process, replicative H3 maintains cell fate while, in contrast, the replacement H3.3 variant opposes cell differentiation during early embryogenesis. In later steps of development, H3.3 and specialized H3 variants are emerging as new, important regulators of terminal cell differentiation, including neurons and gametes. The specific pathways that regulate the dynamics of the deposition of H3.3 are paramount during reprogramming events that drive zygotic activation and the initiation of a new cycle of development.
The endosperm is an essential constituent of seeds in flowering plants. It originates from a fertilization event parallel to the fertilization that gives rise to the embryo. The endosperm nurtures ...embryo development and, in some species including cereals, stores the seed reserves and represents a major source of food for humankind. Endosperm biology is characterized by specific features, including idiosyncratic cellular controls of cell division and epigenetic controls associated with parental genomic imprinting. This review attempts a comprehensive summary of our current knowledge of endosperm development and highlights recent advances in this field.
New work by Hisanaga et al () uncovers the central module that regulates sexual dimorphism in haploid early land plants, opening doors toward understanding evolution of sex chromosomes in ancestors ...of diploid plants.
Discovery of the central module regulating sexual dimorphism in haploid early land plants open doors towards understanding evolution of sex chromosomes in ancestors of diploid plants.
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