Epigenetic regulation in plants

Accessing the Inaccessible: The Organization, Transcription, Replication, and Repair of Heterochromatin in Plants

Eukaryotic genomes often contain large quantities of potentially deleterious sequences, such as transposons. One strategy for mitigating this risk is to package such sequences into so-called constitutive heterochromatin, where the dense chromatin environment is thought to inhibit transcription by excluding transcription factors and RNA polymerase. This type of model makes it tempting to think of heterochromatin as an inert region that is isolated from the rest of the nucleus. Recent work on heterochromatin, however, reveals that it is a dynamic environment. Despite its dense packaging, heterochromatin must remain accessible for a host of processes, including DNA replication and repair, and, paradoxically, transcription. In plants, transcripts produced by specialized RNA polymerases are used to target regions of the genome for silencing via DNA methylation. Thus, the maintenance of heterochromatin requires a careful balancing act of access and exclusion, which is achieved through the action of a host of interrelated pathways.

The molecular hallmarks of epigenetic control

Over the past 20 years, breakthrough discoveries of chromatin-modifying enzymes and associated mechanisms that alter chromatin in response to physiological or pathological signals have transformed our knowledge of epigenetics from a collection of curious biological phenomena to a functionally dissected research field. Here, we provide a personal perspective on the development of epigenetics, from its historical origins to what we define as 'the modern era of epigenetic research'. We primarily highlight key molecular mechanisms of and conceptual advances in epigenetic control that have changed our understanding of normal and perturbed development.

Maintenance of Epigenetic Information

The genome is subject to a diverse array of epigenetic modifications from DNA methylation to histone posttranslational changes. Many of these marks are somatically stable through cell division. This article focuses on our knowledge of the mechanisms governing the inheritance of epigenetic marks, particularly, repressive ones, when the DNA and chromatin template are duplicated in S phase. This involves the action of histone chaperones, nucleosome-remodeling enzymes, histone and DNA methylation binding proteins, and chromatin-modifying enzymes. Last, the timing of DNA replication is discussed, including the question of whether this constitutes an epigenetic mark that facilitates the propagation of epigenetic marks.

Transcriptional profiling of epigenetic regulators in somatic embryos during temperature induced formation of an epigenetic memory in Norway spruce

A significant number of epigenetic regulators were differentially expressed during embryogenesis at different epitype-inducing conditions. Our results support that methylation of DNA and histones, as well as sRNAs, are pivotal for the establishment of the epigenetic memory. As a forest tree species with long generation times, Norway spruce is remarkably well adapted to local environmental conditions despite having recently, from an evolutionary perspective, recolonized large areas following the last glaciation. In this species, there is an enigmatic epigenetic memory of the temperature conditions during embryogenesis that allows rapid adaptation to changing environment. We used a transcriptomic approach to investigate the molecular mechanisms underlying the formation of the epigenetic memory during somatic embryogenesis in Norway spruce. Nine mRNA libraries were prepared from three epitypes of the same genotype resulting from exposure to epitype-inducing temperatures of 18, 23 and 28 °C. RNA-Seq analysis revealed more than 10,000 differentially expressed genes (DEGs). The epitype-inducing conditions during SE were accompanied by marked transcriptomic changes for multiple gene models related to the epigenetic machinery. Out of 735 putative orthologs of epigenetic regulators, 329 were affected by the epitype-inducing temperatures and differentially expressed. The majority of DEGs among the epigenetic regulators was related to DNA and histone methylation, along with sRNA pathways and a range of putative thermosensing and signaling genes. These genes could be the main epigenetic regulators involved in formation of the epigenetic memory. We suggest considerable expansion of gene families of epigenetic regulators in Norway spruce compared to orthologous gene families in Populus and Arabidopsis. Obtained results provide a solid basis for further genome annotation and studies focusing on the importance of these candidate genes for the epigenetic memory formation.

A Structural Perspective on Readout of Epigenetic Histone and DNA Methylation Marks

This article outlines the protein modules that target methylated lysine histone marks and 5mC DNA marks, and the molecular principles underlying recognition. The article focuses on the structural basis underlying readout of isolated marks by single reader molecules, as well as multivalent readout of multiple marks by linked reader cassettes at the histone tail and nucleosome level. Additional topics addressed include the role of histone mimics, cross talk between histone marks, technological developments at the genome-wide level, advances using chemical biology approaches, the linkage between histone and DNA methylation, the role for regulatory lncRNAs, and the promise of chromatin-based therapeutic modalities.

50 years of Arabidopsis research: highlights and future directions

922 I. 922 II. 922 III. 925 IV. 925 V. 926 VI. 927 VII. 928 VIII. 929 IX. 930 X. 931 XI. 932 XII. 933 XIII. Natural variation and genome-wide association studies 934 XIV. 934 XV. 935 XVI. 936 XVII. 937 937 References 937 SUMMARY: The year 2014 marked the 25(th) International Conference on Arabidopsis Research. In the 50 yr since the first International Conference on Arabidopsis Research, held in 1965 in Göttingen, Germany, > 54 000 papers that mention Arabidopsis thaliana in the title, abstract or keywords have been published. We present herein a citational network analysis of these papers, and touch on some of the important discoveries in plant biology that have been made in this powerful model system, and highlight how these discoveries have then had an impact in crop species. We also look to the future, highlighting some outstanding questions that can be readily addressed in Arabidopsis. Topics that are discussed include Arabidopsis reverse genetic resources, stock centers, databases and online tools, cell biology, development, hormones, plant immunity, signaling in response to abiotic stress, transporters, biosynthesis of cells walls and macromolecules such as starch and lipids, epigenetics and epigenomics, genome-wide association studies and natural variation, gene regulatory networks, modeling and systems biology, and synthetic biology.

Creating Order from Chaos: Epigenome Dynamics in Plants with Complex Genomes

Flowering plants have strikingly distinct genomes, although they contain a similar suite of expressed genes. The diversity of genome structures and organization is largely due to variation in transposable elements (TEs) and whole-genome duplication (WGD) events. We review evidence that chromatin modifications and epigenetic regulation are intimately associated with TEs and likely play a role in mediating the effects of WGDs. We hypothesize that the current structure of a genome is the result of various TE bursts and WGDs and it is likely that the silencing mechanisms and the chromatin structure of a genome have been shaped by these events. This suggests that the specific mechanisms targeting chromatin modifications and epigenomic patterns may vary among different species. Many crop species have likely evolved chromatin-based mechanisms to tolerate silenced TEs near actively expressed genes. These interactions of heterochromatin and euchromatin are likely to have important roles in modulating gene expression and variability within species.

Histone deacetylation-mediated cellular dedifferentiation in Arabidopsis

Chromatin structure determines the accessibility of transcriptional regulators to target DNA and contributes to regulation of gene expression. Posttranslational modifications of core histone proteins underlie the reversible changes in chromatin structure. Epigenetic regulation is closely associated with cellular differentiation. Consistently, we found that histone deacetylation is required for callus formation from leaf explants in Arabidopsis . Treatment with trichostatin A (TSA) led to defective callus formation on callus-inducing medium (CIM). Gene expression profiling revealed that a subset of HDAC genes, including HISTONE DEACETYLASE 9 (HDA9), HD-TUINS PROTEIN 1 (HDT1), HDT2, HDT4, and SIRTUIN 1 (SRT1), was significantly up-regulated in calli. In support of this, genetic mutations of HDA9 or HDT1 showed reduced capability of callus formation, probably owing to their roles in regulating auxin and embryonic and meristematic developmental signaling. Taken together, our findings suggest that histone deacetylation is intimately associated with the leaf-to-callus transition, and multiple signaling pathways are controlled by means of histone modification during cellular dedifferentiation.

Landscape and evolutionary dynamics of terminal repeat retrotransposons in miniature in plant genomes

Background

Terminal repeat retrotransposons in miniature (TRIMs) are a unique group of small long terminal repeat retrotransposons that are difficult to identify. Thus far, only a few TRIMs have been characterized in the euphyllophytes, and their evolutionary and biological significance as well as their transposition mechanisms are poorly understood.

Results

Using a combination of de novo and homology-based methods, we annotate TRIMs in 48 plant genome sequences, spanning land plants to algae. The TRIMs are grouped into 156 families including 145 that were previously undefined. Notably, we identify the first TRIMs in a lycophyte and non-vascular plants. The majority of the TRIM families are highly conserved and shared within and between plant families. Unlike other long terminal repeat retrotransposons, TRIMs are enriched in or near genes; they are also targeted by sRNAs between 21 and 24 nucleotides in length, and are frequently found in CG body-methylated genes. Importantly, we also identify putative autonomous retrotransposons and very recent transpositions of a TRIM element in Oryza sativa.

Conclusions

We perform the most comprehensive analysis of TRIM transposons thus far and report that TRIMs are ubiquitous across plant genomes. Our results show that TRIMs are more frequently associated with large and CG body-methylated genes that have undergone strong purifying selection. Our findings also indicate that TRIMs are likely derived from internal deletions of large long terminal repeat retrotransposons. Finally, our data and methodology are important resources for the characterization and evolutionary and genomic studies of long terminal repeat retrotransposons in other genomes.

Electronic supplementary material

The online version of this article (doi:10.1186/s13059-015-0867-y) contains supplementary material, which is available to authorized users.

DNA Methylation and Chromatin Regulation during Fleshy Fruit Development and Ripening

Fruit ripening is a developmental process that results in the leaf-like carpel organ of the flower becoming a mature ovary primed for dispersal of the seeds. Ripening in fleshy fruits involves a profound metabolic phase change that is under strict hormonal and genetic control. This work reviews recent developments in our understanding of the epigenetic regulation of fruit ripening. We start by describing the current state of the art about processes involved in histone post-translational modifications and the remodeling of chromatin structure and their impact on fruit development and ripening. However, the focus of the review is the consequences of changes in DNA methylation levels on the expression of ripening-related genes. This includes those changes that result in heritable phenotypic variation in the absence of DNA sequence alterations, and the mechanisms for their initiation and maintenance. The majority of the studies described in the literature involve work on tomato, but evidence is emerging that ripening in other fruit species may also be under epigenetic control. We discuss how epigenetic differences may provide new targets for breeding and crop improvement.

Crop epigenetics and the molecular hardware of genotype × environment interactions

Crop plants encounter thermal environments which fluctuate on a diurnal and seasonal basis. Future climate resilient cultivars will need to respond to thermal profiles reflecting more variable conditions, and harness plasticity that involves regulation of epigenetic processes and complex genomic regulatory networks. Compartmentalization within plant cells insulates the genomic central processing unit within the interphase nucleus. This review addresses the properties of the chromatin hardware in which the genome is embedded, focusing on the biophysical and thermodynamic properties of DNA, histones and nucleosomes. It explores the consequences of thermal and ionic variation on the biophysical behavior of epigenetic marks such as DNA cytosine methylation (5mC), and histone variants such as H2A.Z, and how these contribute to maintenance of chromatin integrity in the nucleus, while enabling specific subsets of genes to be regulated. Information is drawn from theoretical molecular in vitro studies as well as model and crop plants and incorporates recent insights into the role epigenetic processes play in mediating between environmental signals and genomic regulation. A preliminary speculative framework is outlined, based on the evidence of what appears to be a cohesive set of interactions at molecular, biophysical and electrostatic level between the various components contributing to chromatin conformation and dynamics. It proposes that within plant nuclei, general and localized ionic homeostasis plays an important role in maintaining chromatin conformation, whilst maintaining complex genomic regulation that involves specific patterns of epigenetic marks. More generally, reversible changes in DNA methylation appear to be consistent with the ability of nuclear chromatin to manage variation in external ionic and temperature environment. Whilst tentative, this framework provides scope to develop experimental approaches to understand in greater detail the internal environment of plant nuclei. It is hoped that this will generate a deeper understanding of the molecular mechanisms underlying genotype × environment interactions that may be beneficial for long-term improvement of crop performance in less predictable climates.

Differentially methylated obligatory epialleles modulate context-dependent LAM gene expression in the honeybee Apis mellifera

Differential intragenic methylation in social insects has been hailed as a prime mover of environmentally driven organismal plasticity and even as evidence for genomic imprinting. However, very little experimental work has been done to test these ideas and to prove the validity of such claims. Here we analyze in detail differentially methylated obligatory epialleles of a conserved gene encoding lysosomal α-mannosidase (AmLAM) in the honeybee. We combined genotyping of progenies derived from colonies founded by single drone inseminated queens, ultra-deep allele-specific bisulfite DNA sequencing, and gene expression to reveal how sequence variants, DNA methylation, and transcription interrelate. We show that both methylated and non-methylated states of AmLAM follow Mendelian inheritance patterns and are strongly influenced by polymorphic changes in DNA. Increased methylation of a given allele correlates with higher levels of context-dependent AmLAM expression and appears to affect the transcription of an antisense long noncoding RNA. No evidence of allelic imbalance or imprinting involved in this process has been found. Our data suggest that by generating alternate methylation states that affect gene expression, sequence variants provide organisms with a high level of epigenetic flexibility that can be used to select appropriate responses in various contexts. This study represents the first effort to integrate DNA sequence variants, gene expression, and methylation in a social insect to advance our understanding of their relationships in the context of causality.

RNAi and heterochromatin assembly

SUMMARY

The involvement of RNA interference (RNAi) in heterochromatin formation has become clear largely through studies in the fission yeast Schizosaccharomyces pombe and plants like Arabidopsis thaliana. This article discusses how heterochromatic small interfering RNAs are produced and how the RNAi machinery participates in the formation and function of heterochromatin.

Epigenetic regulatory functions of DNA modifications: 5-methylcytosine and beyond

The chemical modification of DNA bases plays a key role in epigenetic gene regulation. While much attention has been focused on the classical epigenetic mark, 5-methylcytosine, the field garnered increased interest through the recent discovery of additional modifications. In this review, we focus on the epigenetic regulatory roles of DNA modifications in animals. We present the symmetric modification of 5-methylcytosine on CpG dinucleotide as a key feature, because it permits the inheritance of methylation patterns through DNA replication. However, the distribution patterns of cytosine methylation are not conserved in animals and independent molecular functions will likely be identified. Furthermore, the discovery of enzymes that catalyse the hydroxylation of 5-methylcytosine to 5-hydroxymethylcytosine not only identified an active demethylation pathway, but also a candidate for a new epigenetic mark associated with activated transcription. Most recently, N6-methyladenine was described as an additional eukaryotic DNA modification with epigenetic regulatory potential. Interestingly, this modification is also present in genomes that lack canonical cytosine methylation patterns, suggesting independent functions. This newfound diversity of DNA modifications and their potential for combinatorial interactions indicates that the epigenetic DNA code is substantially more complex than previously thought.

Roles and activities of chromatin remodeling ATPases in plants

Chromatin remodeling ATPases and their associated complexes can alter the accessibility of the genome in the context of chromatin by using energy derived from the hydrolysis of ATP to change the positioning, occupancy and composition of nucleosomes. In animals and plants, these remodelers have been implicated in diverse processes ranging from stem cell maintenance and differentiation to developmental phase transitions and stress responses. Detailed investigation of their roles in individual processes has suggested a higher level of selectivity of chromatin remodeling ATPase activity than previously anticipated, and diverse mechanisms have been uncovered that can contribute to the selectivity. This review summarizes recent advances in understanding the roles and activities of chromatin remodeling ATPases in plants.

Epigenetic Regulation of Chromatin States in Schizosaccharomyces pombe

This article discusses the advances made in epigenetic research using the model organism fission yeast Schizosaccharomyces pombe. S. pombe has been used for epigenetic research since the discovery of position effect variegation (PEV). This is a phenomenon in which a transgene inserted within heterochromatin is variably expressed, but can be stably inherited in subsequent cell generations. PEV occurs at centromeres, telomeres, ribosomal DNA (rDNA) loci, and mating-type regions of S. pombe chromosomes. Heterochromatin assembly in these regions requires enzymes that modify histones and the RNA interference (RNAi) machinery. One of the key histone-modifying enzymes is the lysine methyltransferase Clr4, which methylates histone H3 on lysine 9 (H3K9), a classic hallmark of heterochromatin. The kinetochore is assembled on specialized chromatin in which histone H3 is replaced by the variant CENP-A. Studies in fission yeast have contributed to our understanding of the establishment and maintenance of CENP-A chromatin and the epigenetic activation and inactivation of centromeres.

RNA based viral silencing suppression in plant pararetroviruses

The 35S promoter of cauliflower mosaic virus and that of other plant pararetroviruses gives rise to an RNA, which is both a pre-genome and a polycistronic mRNA. The 600 nucleotide long very structured leader of this RNA is also transcribed separately. The resulting 8S RNA is then converted to a double strand giving rise to a huge set of siRNAs, which suppress silencing. In this Mini-Review I discuss how this versatile stretch of 600 nts constitutes a masterpiece of evolution.

Epigenetic regulation in plant responses to the environment

In this article, we review environmentally mediated epigenetic regulation in plants using two case histories. One of these, vernalization, mediates adaptation of plants to different environments and it exemplifies processes that are reset in each generation. The other, virus-induced silencing, involves transgenerationally inherited epigenetic modifications. Heritable epigenetic marks may result in heritable phenotypic variation, influencing fitness, and so be subject to natural selection. However, unlike genetic inheritance, the epigenetic modifications show instability and are influenced by the environment. These two case histories are then compared with other phenomena in plant biology that are likely to represent epigenetic regulation in response to the environment.

Somatic embryogenesis - Stress-induced remodeling of plant cell fate

Plants as sessile organisms have remarkable developmental plasticity ensuring heir continuous adaptation to the environment. An extreme example is somatic embryogenesis, the initiation of autonomous embryo development in somatic cells in response to exogenous and/or endogenous signals. In this review I briefly overview the various pathways that can lead to embryo development in plants in addition to the fertilization of the egg cell and highlight the importance of the interaction of stress- and hormone-regulated pathways during the induction of somatic embryogenesis. Somatic embryogenesis can be initiated in planta or in vitro, directly or indirectly, and the requirement for dedifferentiation as well as the way to achieve developmental totipotency in the various systems is discussed in light of our present knowledge. The initiation of all forms of the stress/hormone-induced in vitro as well as the genetically provoked in planta somatic embryogenesis requires extensive and coordinated genetic reprogramming that has to take place at the chromatin level, as the embryogenic program is under strong epigenetic repression in vegetative plant cells. Our present knowledge on chromatin-based mechanisms potentially involved in the somatic-to-embryogenic developmental transition is summarized emphasizing the potential role of the chromatin to integrate stress, hormonal, and developmental pathways leading to the activation of the embryogenic program. The role of stress-related chromatin reorganization in the genetic instability of in vitro cultures is also discussed. This article is part of a Special Issue entitled: Stress as a fundamental theme in cell plasticity.

DNA methylation in mammals

DNA methylation is one of the best characterized epigenetic modifications. In mammals it is involved in various biological processes including the silencing of transposable elements, regulation of gene expression, genomic imprinting, and X-chromosome inactivation. This article describes how DNA methylation serves as a cellular memory system and how it is dynamically regulated through the action of the DNA methyltransferase (DNMT) and ten eleven translocation (TET) enzymes. Its role in the regulation of gene expression, through its interplay with histone modifications, is also described, and its implication in human diseases discussed. The exciting areas of investigation that will likely become the focus of research in the coming years are outlined in the summary.

Epigenetic regulation of rice flowering and reproduction

Current understanding of the epigenetic regulator roles in plant growth and development has largely derived from studies in the dicotyledonous model plant Arabidopsis thaliana. Rice (Oryza sativa) is one of the most important food crops in the world and has more recently becoming a monocotyledonous model plant in functional genomics research. During the past few years, an increasing number of studies have reported the impact of DNA methylation, non-coding RNAs and histone modifications on transcription regulation, flowering time control, and reproduction in rice. Here, we review these studies to provide an updated complete view about chromatin modifiers characterized in rice and in particular on their roles in epigenetic regulation of flowering time, reproduction, and seed development.

Epigenetics of ciliates

Research using ciliates revealed early examples of epigenetic phenomena and continues to provide novel findings. These protozoans maintain separate germline and somatic nuclei that carry transcriptionally silent and active genomes, respectively. Examining the differences in chromatin within distinct nuclei of Tetrahymena identified histone variants and established that transcriptional regulators act by modifying histones. Formation of somatic nuclei requires both transcriptional activation of silent chromatin and large-scale DNA elimination. This somatic genome remodeling is directed by homologous RNAs, acting with an RNA interference (RNAi)-related machinery. Furthermore, the content of the parental somatic genome provides a homologous template to guide this genome restructuring. The mechanisms regulating ciliate DNA rearrangements reveal the surprising power of homologous RNAs to remodel the genome and transmit information transgenerationally.

The use of mouse models to study epigenetics

Much of what we know about the role of epigenetics in the determination of phenotype has come from studies of inbred mice. Some unusual expression patterns arising from endogenous and transgenic murine alleles, such as the Agouti coat color alleles, have allowed the study of variegation, variable expressivity, transgenerational epigenetic inheritance, parent-of-origin effects, and position effects. These phenomena have taught us much about gene silencing and the probabilistic nature of epigenetic processes. Based on some of these alleles, large-scale mutagenesis screens have broadened our knowledge of epigenetic control by identifying and characterizing novel genes involved in these processes.

Neurospora crassa, a model system for epigenetics research

The filamentous fungus Neurospora crassa has provided a rich source of knowledge on epigenetic phenomena that would have been difficult or impossible to gain from other systems. Neurospora sports features found in higher eukaryotes but absent in both budding and fission yeast, including DNA methylation and H3K27 methylation, and also has distinct RNA interference (RNAi)-based silencing mechanisms operating in mitotic and meiotic cells. This has provided an unexpected wealth of information on gene silencing systems. One silencing mechanism, named repeat-induced point mutation (RIP), has both epigenetic and genetic aspects and provided the first example of a homology-based genome defense system. A second silencing mechanism, named quelling, is an RNAi-based mechanism that results in silencing of transgenes and their native homologs. A third, named meiotic silencing, is also RNAi-based but is distinct from quelling in its time of action, targets, and apparent purpose.

Position-effect variegation, heterochromatin formation, and gene silencing in Drosophila

Position-effect variegation (PEV) results when a gene normally in euchromatin is juxtaposed with heterochromatin by rearrangement or transposition. When heterochromatin packaging spreads across the heterochromatin/euchromatin border, it causes transcriptional silencing in a stochastic pattern. PEV is intensely studied in Drosophila using the white gene. Screens for dominant mutations that suppress or enhance white variegation have identified many conserved epigenetic factors, including the histone H3 lysine 9 methyltransferase SU(VAR)3-9. Heterochromatin protein HP1a binds H3K9me2/3 and interacts with SU(VAR)3-9, creating a core memory system. Genetic, molecular, and biochemical analysis of PEV in Drosophila has contributed many key findings concerning establishment and maintenance of heterochromatin with concomitant gene silencing.