Epigenetics: Beyond Chromatin Modifications and Complex Genetic Regulation

From non-coding RNAs to histone modification: The epigenetic mechanisms in tomato fruit ripening and quality regulation

Ripening is one of the most important stages of fruit development and determines the fruit quality. Various factors play a role in this process, with epigenetic mechanisms emerging as important players. Epigenetic regulation encompasses DNA methylation, histone modifications and variants, chromatin remodeling, RNA modifications, and non-coding RNAs. Over the past decade, studies using tomato as a model have made considerable progress in understanding the impact of epigenetic regulation on fleshy fruit ripening and quality. In this paper, we provide an overview of recent advancements in the epigenetic regulation of tomato fruit ripening and quality regulation, focusing on three main mechanisms: DNA/RNA modifications, non-coding RNAs, and histone modifications. Furthermore, we highlight the unresolved issues and challenges within this research field, offering perspectives for future investigations to drive agricultural innovation.

Molecular characterization and induced changes of histone acetyltransferases in the tick Haemaphysalis longicornis in response to cold stress

BACKGROUND

Epigenetic modifications of histones play important roles in the response of eukaryotic organisms to environmental stress. However, many histone acetyltransferases (HATs), which are responsible for histone acetylation, and their roles in mediating the tick response to cold stress have yet to be identified. In the present study, HATs were molecularly characterized and their associations with the cold response of the tick Haemaphysalis longicornis explored.

METHODS

HATs were characterized by using polymerase chain reaction (PCR) based on published genome sequences, followed by multiple bioinformatic analyses. The differential expression of genes in H. longicornis under different cold treatment conditions was evaluated using reverse transcription quantitative PCR (RT-qPCR). RNA interference was used to explore the association of HATs with the cold response of H. longicornis.

RESULTS

Two HAT genes were identified in H. longicornis (Hl), a GCN5-related N-acetyltransferase (henceforth HlGNAT) and a type B histone acetyltransferase (henceforth HlHAT-B), which are respectively 960 base pairs (bp) and 1239 bp in length. Bioinformatics analysis revealed that HlGNAT and HlHAT-B are unstable hydrophilic proteins characterized by the presence of the acetyltransferase 16 domain and Hat1_N domain, respectively. RT-qPCR revealed that the expression of HlGNAT and HlHAT-B decreased after 3 days of cold treatment, but gradually increased with a longer period of cold treatment. The mortality rate following knockdown of HlGNAT or HlHAT-B by RNA interference, which was confirmed by RT-qPCR, significantly increased (P < 0.05) when H. longicornis was treated at the lowest lethal temperature (- 14 °C) for 2 h.

CONCLUSIONS

The findings demonstrate that HATs may play a crucial role in the cold response of H. longicornis. Thus further research is warranted to explore the mechanisms underlying the epigenetic regulation of the cold response in ticks.

Complexity of responses to ionizing radiation in plants, and the impact on interacting biotic factors

In nature, plants are simultaneously exposed to different abiotic (e.g., heat, drought, and salinity) and biotic (e.g., bacteria, fungi, and insects) stresses. Climate change and anthropogenic pressure are expected to intensify the frequency of stress factors. Although plants are well equipped with unique and common defense systems protecting against stressors, they may compromise their growth and development for survival in such challenging environments. Ionizing radiation is a peculiar stress factor capable of causing clustered damage. Radionuclides are both naturally present on the planet and produced by human activities. Natural and artificial radioactivity affects plants on molecular, biochemical, cellular, physiological, populational, and transgenerational levels. Moreover, the fitness of pests, pathogens, and symbionts is concomitantly challenged in radiologically contaminated areas. Plant responses to artificial acute ionizing radiation exposure and laboratory-simulated or field chronic exposure are often discordant. Acute or chronic ionizing radiation exposure may occasionally prime the defense system of plants to better tolerate the biotic stress or could often exhaust their metabolic reserves, making plants more susceptible to pests and pathogens. Currently, these alternatives are only marginally explored. Our review summarizes the available literature on the responses of host plants, biotic factors, and their interaction to ionizing radiation exposure. Such systematic analysis contributes to improved risk assessment in radiologically contaminated areas.

Low H3K27me3 deposition at CYP82E4 determines the nicotinic conversion rate in Nicotiana tabacum

Nicotine conversion is the process by which nornicotine is synthesized from nicotine. The capacity of a plant to carry out this process is represented by the nicotine conversion rate (NCR), which is defined as the percentage of nornicotine content out of the total nicotine + nornicotine content. Nicotine conversion in tobacco is mediated by CYP82E4. Although there are cultivar-specific differences in NCR, these do not correspond to differences in the CYP82E4 promoter or gene body sequences, and little is known about the underlying regulatory mechanism. Here, we found that histone H3 Lysine 27 trimethylation (H3K27me3) was involved in CYP82E4 expression, functioning as a transcriptional repressor. Compared to a high-NCR near-isogenic line, a low-NCR cultivar showed increased levels of the repressive histone modification markers H3K27me3 and H3K9me3 at CYP82E4. Comparison of histone markers between several cultivars with varying NCRs showed that H3K27me3 and H3K9me3 levels were significantly associated with cultivar-specific differences in NCR. Treatment with the H3K27me3 demethylase inhibitor GSK-J4 increased total H3K27me3 levels and enriched H3K27me3 at the CYP82E4 locus; the increased levels of H3K27me3 further inhibited CYP82E4 expression. Knocking out E(z), an indispensable gene for H3K27me3 formation, decreased H3K27me3 levels at CYP82E4, leading to a more than three-fold increase in CYP82E4 expression. Changes in CYP82E4 expression during leaf senescence and chilling stress were also strongly correlated with H3K27me3 levels. These findings reveal a strong correlation between CYP82E4 expression and histone modifications, and demonstrate an instance of histone-mediated alkaloid regulation for the first time.

A review of plant epigenetics through the lens of almond

While genomes were originally seen as static entities that stably held and organized genetic information, recent advances in sequencing have uncovered the dynamic nature of the genome. New conceptualizations of the genome include complex relationships between the environment and gene expression that must be maintained, regulated, and sometimes even transmitted over generations. The discovery of epigenetic mechanisms has allowed researchers to understand how traits like phenology, plasticity, and fitness can be altered without changing the underlying deoxyribonucleic acid sequence. While many discoveries were first made in animal systems, plants provide a particularly complex set of epigenetic mechanisms due to unique aspects of their biology and interactions with human selective breeding and cultivation. In the plant kingdom, annual plants have received the most attention; however, perennial plants endure and respond to their environment and human management in distinct ways. Perennials include crops such as almond, for which epigenetic effects have long been linked to phenomena and even considered relevant for breeding. Recent discoveries have elucidated epigenetic phenomena that influence traits such as dormancy and self-compatibility, as well as disorders like noninfectious bud failure, which are known to be triggered by the environment and influenced by inherent aspects of the plant. Thus, epigenetics represents fertile ground to further understand almond biology and production and optimize its breeding. Here, we provide our current understanding of epigenetic regulation in plants and use almond as an example of how advances in epigenetics research can be used to understand biological fitness and agricultural performance in crop plants.

Epigenetic regulation in tomato fruit ripening

Fruit ripening is a crucial stage in quality development, influenced by a diverse array of internal and external factors. Among these factors, epigenetic regulation holds significant importance and has garnered substantial research attention in recent years. Here, this review aims to discuss the breakthrough in epigenetic regulation of tomato (Solanum lycopersicum) fruit ripening, including DNA methylation, N6-Methyladenosine mRNA modification, histone demethylation/deacetylation, and non-coding RNA. Through this brief review, we seek to enhance our understanding of the regulatory mechanisms governing tomato fruit ripening, while providing fresh insights for the precise modulation of these mechanisms.

Recent advances in epigenetic triggering of climacteric fruit ripening

During ripening, fleshy fruits undergo irreversible changes in color, texture, sugar content, aroma, and flavor to appeal to seed-dispersal vectors. The onset of climacteric fruit ripening is accompanied by an ethylene burst. Understanding the factors triggering this ethylene burst is important for manipulating climacteric fruit ripening. Here, we review the current understanding and recent insights into the possible factors triggering climacteric fruit ripening: DNA methylation and histone modification, including methylation and acetylation. Understanding the initiation factors of fruit ripening is important for exploring and accurately regulating the mechanisms of fruit ripening. Lastly, we discuss the potential mechanisms responsible for climacteric fruit ripening.

Epigenetic Changes Occurring in Plant Inbreeding

Inbreeding is the crossing of closely related individuals in nature or a plantation or self-pollinating plants, which produces plants with high homozygosity. This process can reduce genetic diversity in the offspring and decrease heterozygosity, whereas inbred depression (ID) can often reduce viability. Inbred depression is common in plants and animals and has played a significant role in evolution. In the review, we aim to show that inbreeding can, through the action of epigenetic mechanisms, affect gene expression, resulting in changes in the metabolism and phenotype of organisms. This is particularly important in plant breeding because epigenetic profiles can be linked to the deterioration or improvement of agriculturally important characteristics.

Comprehensive Survey of ChIP-Seq Datasets to Identify Candidate Iron Homeostasis Genes Regulated by Chromatin Modifications

Vital biochemical reactions including photosynthesis to respiration require iron, which should be tightly regulated. Although increasing evidence reveals the importance of epigenetic regulation in gene expression and signaling, the role of histone modifications and chromatin remodeling in plant iron homeostasis is not well understood. In this study, we surveyed publicly available ChIP-seq datasets of Arabidopsis wild-type and mutants defective in key enzymes of histone modification and chromatin remodeling and compared the deposition of epigenetic marks on loci of genes involved in iron regulation. Based on the analysis, we compiled a comprehensive list of iron homeostasis genes with differential enrichment of various histone modifications. This report will provide a resource for future studies to investigate epigenetic regulatory mechanisms of iron homeostasis in plants.

Minireview: Chromatin-based regulation of iron homeostasis in plants

Plants utilize delicate mechanisms to effectively respond to changes in the availability of nutrients such as iron. The responses to iron status involve controlling gene expression at multiple levels. The regulation of iron deficiency response by a network of transcriptional regulators has been extensively studied and recent research has shed light on post-translational control of iron homeostasis. Although not as considerably investigated, an increasing number of studies suggest that histone modification and DNA methylation play critical roles during iron deficiency and contribute to fine-tuning iron homeostasis in plants. This review will focus on the current understanding of chromatin-based regulation on iron homeostasis in plants highlighting recent studies in Arabidopsis and rice. Understanding iron homeostasis in plants is vital, as it is not only relevant to fundamental biological questions, but also to agriculture, biofortification, and human health. A comprehensive overview of the effect and mechanism of chromatin-based regulation in response to iron status will ultimately provide critical insights in elucidating the complexities of iron homeostasis and contribute to improving iron nutrition in plants.

Nitric oxide, salicylic acid and oxidative stress: Is it a perfect equilateral triangle?

Nitric oxide (NO) is an endogenous free radical involved in the regulation of a wide array of physio-biochemical phenomena in plants. The biological activity of NO directly depend on its cellular concentration which usually changes under stress conditions, it participates in maintaining cellular redox equilibrium and regulating target checkpoints which control switches among development and stress. It is one of the key players in plant signalling and a plethora of evidence supports its crosstalk with other phytohormones. NO and salicylic acid (SA) cooperation is also of great physiological relevance, where NO modulates the immune response by regulating SA linked target proteins i.e., non-expressor of pathogenesis-related genes (NPR-1 and NPR-2) and Group D bZIP (basic leucine zipper domain transcription factor). Many experimental data suggest a functional cooperative role between NO and SA in mitigating the plant oxidative stress which suggests that these relationships could constitute a metabolic "equilateral triangle".

Thermo-Priming Mediated Cellular Networks for Abiotic Stress Management in Plants

Plants can adapt to different environmental conditions and can survive even under very harsh conditions. They have developed elaborate networks of receptors and signaling components, which modulate their biochemistry and physiology by regulating the genetic information. Plants also have the abilities to transmit information between their different parts to ensure a holistic response to any adverse environmental challenge. One such phenomenon that has received greater attention in recent years is called stress priming. Any milder exposure to stress is used by plants to prime themselves by modifying various cellular and molecular parameters. These changes seem to stay as memory and prepare the plants to better tolerate subsequent exposure to severe stress. In this review, we have discussed the various ways in which plants can be primed and illustrate the biochemical and molecular changes, including chromatin modification leading to stress memory, with major focus on thermo-priming. Alteration in various hormones and their subsequent role during and after priming under various stress conditions imposed by changing climate conditions are also discussed.

Multigenerational Exposure to Heat Stress Induces Phenotypic Resilience, and Genetic and Epigenetic Variations in Arabidopsis thaliana Offspring

Plants are sedentary organisms that constantly sense changes in their environment and react to various environmental cues. On a short-time scale, plants respond through alterations in their physiology, and on a long-time scale, plants alter their development and pass on the memory of stress to the progeny. The latter is controlled genetically and epigenetically and allows the progeny to be primed for future stress encounters, thus increasing the likelihood of survival. The current study intended to explore the effects of multigenerational heat stress in Arabidopsis thaliana. Twenty-five generations of Arabidopsis thaliana were propagated in the presence of heat stress. The multigenerational stressed lineage F25H exhibited a higher tolerance to heat stress and elevated frequency of homologous recombination, as compared to the parallel control progeny F25C. A comparison of genomic sequences revealed that the F25H lineage had a three-fold higher number of mutations [single nucleotide polymorphisms (SNPs) and insertions and deletions (INDELs)] as compared control lineages, suggesting that heat stress induced genetic variations in the heat-stressed progeny. The F25H stressed progeny showed a 7-fold higher number of non-synonymous mutations than the F25C line. Methylome analysis revealed that the F25H stressed progeny showed a lower global methylation level in the CHH context than the control progeny. The F25H and F25C lineages were different from the parental control lineage F2C by 66,491 and 80,464 differentially methylated positions (DMPs), respectively. F25H stressed progeny displayed higher frequency of methylation changes in the gene body and lower in the body of transposable elements (TEs). Gene Ontology analysis revealed that CG-DMRs were enriched in processes such as response to abiotic and biotic stimulus, cell organizations and biogenesis, and DNA or RNA metabolism. Hierarchical clustering of these epimutations separated the heat stressed and control parental progenies into distinct groups which revealed the non-random nature of epimutations. We observed an overall higher number of epigenetic variations than genetic variations in all comparison groups, indicating that epigenetic variations are more prevalent than genetic variations. The largest difference in epigenetic and genetic variations was observed between control plants comparison (F25C vs. F2C), which clearly indicated that the spontaneous nature of epigenetic variations and heat-inducible nature of genetic variations. Overall, our study showed that progenies derived from multigenerational heat stress displayed a notable adaption in context of phenotypic, genotypic and epigenotypic resilience.

Regulation of fleshy fruit ripening: From transcription factors to epigenetic modifications

Fleshy fruits undergo a complex ripening process, developing organoleptic fruit traits that attract herbivores and maximize seed dispersal. Ripening is the terminal stage of fruit development and involves a series of physiological and biochemical changes. In fleshy fruits, ripening always involves a drastic color change triggered by the accumulation of pigments and degradation of chlorophyll, softening caused by cell wall remodeling, and flavor formation as acids and sugars accumulate alongside volatile compounds. The mechanisms underlying fruit ripening rely on the orchestration of ripening-related transcription factors, plant hormones, and epigenetic modifications. In this review, we discuss current knowledge of the transcription factors that regulate ripening in conjunction with ethylene and environmental signals (light and temperature) in the model plant tomato (Solanum lycopersicum) and other fleshy fruits. We emphasize the critical roles of epigenetic regulation, including DNA methylation and histone modification as well as RNA m6A modification, which has been studied intensively. This detailed review was compiled to provide a comprehensive description of the regulatory mechanisms of fruit ripening and guide new strategies for its effective manipulation.

Low-temperature plasma promotes growth of Haematococcus pluvialis and accumulation of astaxanthin by regulating histone H3 lysine 4 tri-methylation

Astaxanthin exhibits strong antioxidant ability, so researchers endeavor to improve astaxanthin production in Haematococcus pluvialis (H. pluvialis). Previous work revealed that low-temperature plasma (LTP) could improve the astaxanthin yield in H. pluvialis, but the mechanism is still elusive. In this work, we therefore explored the mechanism of LTP promoting algal growth astaxanthin yield, especially from the perspective of epigenetics. Through measurements of hormones and transcription genes, it was found that the levels of strigolactone and abscisic acid in H. pluvialis increased significantly after LTP treatment, accompanied by enhanced expression of astaxanthin synthesis genes. Particularly, one of the key genes, namely CRTISO, was specifically up-regulated. Further experiments via immunofluorescence and ChIP-PCR methods confirmed that histone H3 lysine 4 tri-methylation (H3K4me3) in the promoter region of CRTISO was increased. Therefore, this study demonstrates that LTP can regulate CRTISO and promote the algal growth and astaxanthin accumulation by stimulating phytohormones and regulating H3K4me3.

DNA methylation in tomato fruit ripening

Fleshy fruit, the most economical and nutritional value unique to flowering plants, is an important part of our daily diet. Previous studies have shown that fruit ripening is regulated by transcription factors and the plant hormone ethylene, but recent research has also shown that epigenetics also plays an essential role, especially DNA methylation. DNA methylation is the process of transferring -CH3 to the fifth carbon of cytosine residues under the action of methyltransferase to form 5-methylcytosine (5-mC). So far, most works have been focused on tomato. Tomato ripening is dynamically regulated by DNA methylation and demethylation, but the understanding of this mechanism is still in its infancy. The dysfunction of a DNA demethylase, DEMETER-like DNA demethylases 2 (DML2), prevents the ripening of tomato fruits, but immature fruits ripen prematurely under the action of DNA methylation inhibitors. Additionally, studies have shown that the relationship between fruit quality and DNA methylation is not linear, but the specific molecular mechanism is still unclear. Here, we review the recent advances in the role of DNA methylation in tomato fruit ripening, the interaction of ripening transcription factors and DNA methylation, and its effects on quality. Then, a number of questions for future research of DNA methylation regulation in tomato fruit ripening is proposed.

Epigenetic regulation of salinity stress responses in cereals

Cereals are important crops and are exposed to various types of environmental stresses that affect the overall growth and yield. Among the various abiotic stresses, salt stress is a major environmental factor that influences the genetic, physiological, and biochemical responses of cereal crops. Epigenetic regulation which includes DNA methylation, histone modification, and chromatin remodelling plays an important role in salt stress tolerance. Recent studies in rice genomics have highlighted that the epigenetic changes are heritable and therefore can be considered as molecular signatures. An epigenetic mechanism under salinity induces phenotypic responses involving modulations in gene expression. Association between histone modification and altered DNA methylation patterns and differential gene expression has been evidenced for salt sensitivity in rice and other cereal crops. In addition, epigenetics also creates stress memory that helps the plant to better combat future stress exposure. In the present review, we have discussed epigenetic influences in stress tolerance, adaptation, and evolution processes. Understanding the epigenetic regulation of salinity could help for designing salt-tolerant varieties leading to improved crop productivity.

Epigenetics for Crop Improvement in Times of Global Change

Epigenetics has emerged as an important research field for crop improvement under the on-going climatic changes. Heritable epigenetic changes can arise independently of DNA sequence alterations and have been associated with altered gene expression and transmitted phenotypic variation. By modulating plant development and physiological responses to environmental conditions, epigenetic diversity-naturally, genetically, chemically, or environmentally induced-can help optimise crop traits in an era challenged by global climate change. Beyond DNA sequence variation, the epigenetic modifications may contribute to breeding by providing useful markers and allowing the use of epigenome diversity to predict plant performance and increase final crop production. Given the difficulties in transferring the knowledge of the epigenetic mechanisms from model plants to crops, various strategies have emerged. Among those strategies are modelling frameworks dedicated to predicting epigenetically controlled-adaptive traits, the use of epigenetics for in vitro regeneration to accelerate crop breeding, and changes of specific epigenetic marks that modulate gene expression of traits of interest. The key challenge that agriculture faces in the 21st century is to increase crop production by speeding up the breeding of resilient crop species. Therefore, epigenetics provides fundamental molecular information with potential direct applications in crop enhancement, tolerance, and adaptation within the context of climate change.

The Underlying Nature of Epigenetic Variation: Origin, Establishment, and Regulatory Function of Plant Epialleles

In plants, the gene expression and associated phenotypes can be modulated by dynamic changes in DNA methylation, occasionally being fixed in certain genomic loci and inherited stably as epialleles. Epiallelic variations in a population can occur as methylation changes at an individual cytosine position, methylation changes within a stretch of genomic regions, and chromatin changes in certain loci. Here, we focus on methylated regions, since it is unclear whether variations at individual methylated cytosines can serve any regulatory function, and the evidence for heritable chromatin changes independent of genetic changes is limited. While DNA methylation is known to affect and regulate wide arrays of plant phenotypes, most epialleles in the form of methylated regions have not been assigned any biological function. Here, we review how epialleles can be established in plants, serve a regulatory function, and are involved in adaptive processes. Recent studies suggest that most epialleles occur as byproducts of genetic variations, mainly from structural variants and Transposable Element (TE) activation. Nevertheless, epialleles that occur spontaneously independent of any genetic variations have also been described across different plant species. Here, we discuss how epialleles that are dependent and independent of genetic architecture are stabilized in the plant genome and how methylation can regulate a transcription relative to its genomic location.

Plant Responses to Abiotic Stresses and Rhizobacterial Biostimulants: Metabolomics and Epigenetics Perspectives

In response to abiotic stresses, plants mount comprehensive stress-specific responses which mediate signal transduction cascades, transcription of relevant responsive genes and the accumulation of numerous different stress-specific transcripts and metabolites, as well as coordinated stress-specific biochemical and physiological readjustments. These natural mechanisms employed by plants are however not always sufficient to ensure plant survival under abiotic stress conditions. Biostimulants such as plant growth-promoting rhizobacteria (PGPR) formulation are emerging as novel strategies for improving crop quality, yield and resilience against adverse environmental conditions. However, to successfully formulate these microbial-based biostimulants and design efficient application programs, the understanding of molecular and physiological mechanisms that govern biostimulant-plant interactions is imperatively required. Systems biology approaches, such as metabolomics, can unravel insights on the complex network of plant-PGPR interactions allowing for the identification of molecular targets responsible for improved growth and crop quality. Thus, this review highlights the current models on plant defence responses to abiotic stresses, from perception to the activation of cellular and molecular events. It further highlights the current knowledge on the application of microbial biostimulants and the use of epigenetics and metabolomics approaches to elucidate mechanisms of action of microbial biostimulants.

Polycomb-dependent differential chromatin compartmentalization determines gene coregulation in Arabidopsis

In animals, distant H3K27me3-marked Polycomb targets can establish physical interactions forming repressive chromatin hubs. In plants, growing evidence suggests that H3K27me3 acts directly or indirectly to regulate chromatin interactions, although how this histone modification modulates 3D chromatin architecture remains elusive. To decipher the impact of the dynamic deposition of H3K27me3 on the Arabidopsis thaliana nuclear interactome, we combined genetics, transcriptomics, and several 3D epigenomic approaches. By analyzing mutants defective for histone H3K27 methylation or demethylation, we uncovered the crucial role of this chromatin mark in short- and previously unnoticed long-range chromatin loop formation. We found that a reduction in H3K27me3 levels led to a decrease in the interactions within Polycomb-associated repressive domains. Regions with lower H3K27me3 levels in the H3K27 methyltransferase clf mutant established new interactions with regions marked with H3K9ac, a histone modification associated with active transcription, indicating that a reduction in H3K27me3 levels induces a global reconfiguration of chromatin architecture. Altogether, our results reveal that the 3D genome organization is tightly linked to reversible histone modifications that govern chromatin interactions. Consequently, nuclear organization dynamics shapes the transcriptional reprogramming during plant development and places H3K27me3 as a key feature in the coregulation of distant genes.

Deciphering the Epigenetic Alphabet Involved in Transgenerational Stress Memory in Crops

Although epigenetic modifications have been intensely investigated over the last decade due to their role in crop adaptation to rapid climate change, it is unclear which epigenetic changes are heritable and therefore transmitted to their progeny. The identification of epigenetic marks that are transmitted to the next generations is of primary importance for their use in breeding and for the development of new cultivars with a broad-spectrum of tolerance/resistance to abiotic and biotic stresses. In this review, we discuss general aspects of plant responses to environmental stresses and provide an overview of recent findings on the role of transgenerational epigenetic modifications in crops. In addition, we take the opportunity to describe the aims of EPI-CATCH, an international COST action consortium composed by researchers from 28 countries. The aim of this COST action launched in 2020 is: (1) to define standardized pipelines and methods used in the study of epigenetic mechanisms in plants, (2) update, share, and exchange findings in epigenetic responses to environmental stresses in plants, (3) develop new concepts and frontiers in plant epigenetics and epigenomics, (4) enhance dissemination, communication, and transfer of knowledge in plant epigenetics and epigenomics.

Fruit development and epigenetic modifications

Fruit development is a complex process that is regulated not only by plant hormones and transcription factors, but also requires epigenetic modifications. Epigenetic modifications include DNA methylation, histone post-translational modifications, chromatin remodeling and noncoding RNAs. Together, these epigenetic modifications, which are controlled during development and in response to the environment, determine the chromatin state of genes and contribute to the transcriptomes of an organism. Recent studies have demonstrated that epigenetic regulation plays an important role in fleshy fruit ripening. Dysfunction of a DNA demethylase delayed ripening in tomato, and the application of a DNA methylation inhibitor altered ripening process in the fruits of several species. These studies indicated that manipulating the epigenome of fruit crops could open new ways for breeding in the future. In this review, we highlight recent progress and address remaining questions and challenges concerning the epigenetic regulation of fruit development and ripening.

The methylome is altered for plants in a high CO2 world: Insights into the response of a wild plant population to multigenerational exposure to elevated atmospheric [CO2 ]

Unravelling plant responses to rising atmospheric CO₂ concentration ([CO₂ ]) has largely focussed on plastic functional attributes to single generation [CO₂ ] exposure. Quantifying the consequences of long-term, decadal multigenerational exposure to elevated [CO₂ ] and the genetic changes that may underpin evolutionary mechanisms with [CO₂ ] as a driver remain largely unexplored. Here, we investigated both plastic and evolutionary plant responses to elevated [CO₂ ] by applying multi-omic technologies using populations of Plantago lanceolata L., grown in naturally high [CO₂ ] for many generations in a CO₂ spring. Seed from populations at the CO₂ spring and an adjacent control site (ambient [CO₂ ]) were grown in a common environment for one generation, and then offspring were grown in ambient or elevated [CO₂ ] growth chambers. Low overall genetic differentiation between the CO₂ spring and control site populations was found, with evidence of weak selection in exons. We identified evolutionary divergence in the DNA methylation profiles of populations derived from the spring relative to the control population, providing the first evidence that plant methylomes may respond to elevated [CO₂ ] over multiple generations. In contrast, growth at elevated [CO₂ ] for a single generation induced limited methylome remodelling (an order of magnitude fewer differential methylation events than observed between populations), although some of this appeared to be stably transgenerationally inherited. In all, 59 regions of the genome were identified where transcripts exhibiting differential expression (associated with single generation or long-term natural exposure to elevated [CO₂ ]) co-located with sites of differential methylation or with single nucleotide polymorphisms exhibiting significant inter-population divergence. This included genes in pathways known to respond to elevated [CO₂ ], such as nitrogen use efficiency and stomatal patterning. This study provides the first indication that DNA methylation may contribute to plant adaptation to future atmospheric [CO₂ ] and identifies several areas of the genome that are targets for future study.

Epigenetic Mechanisms of Plant Adaptation to Biotic and Abiotic Stresses

Unlike animals, plants are immobile and could not actively escape the effects of aggressive environmental factors, such as pathogenic microorganisms, insect pests, parasitic plants, extreme temperatures, drought, and many others. To counteract these unfavorable encounters, plants have evolved very high phenotypic plasticity. In a rapidly changing environment, adaptive phenotypic changes often occur in time frames that are too short for the natural selection of adaptive mutations. Probably, some kind of epigenetic variability underlines environmental adaptation in these cases. Indeed, isogenic plants often have quite variable phenotypes in different habitats. There are examples of successful “invasions” of relatively small and genetically homogenous plant populations into entirely new habitats. The unique capability of quick environmental adaptation appears to be due to a high tendency to transmit epigenetic changes between plant generations. Multiple studies show that epigenetic memory serves as a mechanism of plant adaptation to a rapidly changing environment and, in particular, to aggressive biotic and abiotic stresses. In wild nature, this mechanism underlies, to a very significant extent, plant capability to live in different habitats and endure drastic environmental changes. In agriculture, a deep understanding of this mechanism could serve to elaborate more effective and safe approaches to plant protection.

Epigenetics: possible applications in climate-smart crop breeding

To better adapt transiently or lastingly to stimuli from the surrounding environment, the chromatin states in plant cells vary to allow the cells to fine-tune their transcriptional profiles. Modifications of chromatin states involve a wide range of post-transcriptional histone modifications, histone variants, DNA methylation, and activity of non-coding RNAs, which can epigenetically determine specific transcriptional outputs. Recent advances in the area of '-omics' of major crops have facilitated identification of epigenetic marks and their effect on plant response to environmental stresses. As most epigenetic mechanisms are known from studies in model plants, we summarize in this review recent epigenetic studies that may be important for improvement of crop adaptation and resilience to environmental changes, ultimately leading to the generation of stable climate-smart crops. This has paved the way for exploitation of epigenetic variation in crop breeding.

Systemic lupus erythematosus: an expert insight into emerging therapy agents in preclinical and early clinical development

INTRODUCTION

Systemic lupus erythematosus (SLE) is a chronic disease that is potentially fatal. There is no cure for SLE and the medications used are associated with toxic side effects. In the era of revolutionary emerging novel biologic agents, the design and investigation of targeted therapy for these patients is necessary. Novel therapies under investigation in phase II-III clinical trials showed promising results. Therapies can target various pathways involved in SLE including cytokines, signal transduction inhibitors, B-cell depletion and interference with co-stimulation. Of interest is the proof of concept of sequential therapy.

AREAS COVERED

We performed an extensive literature search via PubMed, Medline, Elsevier Science and Springer Link databases between the years 2014-2020 using the following terms: SLE, novel treatments. We have reviewed 232 articles and selected those articles that (i) focus on phase II-III emerging therapies and (ii) offer new findings from existing therapies, which reveal breakthrough concepts in SLE treatment.

EXPERT OPINION

It is still difficult to crack the puzzle of a successful SLE treatment approach. New strategies with potential may encompass the targeting of more than one protein. Another way forward is to identify each SLE patient and personalize therapy by clinical manifestations, disease activity, serology and activated protein.

Exploration of space to achieve scientific breakthroughs

Living organisms adapt to changing environments using their amazing flexibility to remodel themselves by a process called evolution. Environmental stress causes selective pressure and is associated with genetic and phenotypic shifts for better modifications, maintenance, and functioning of organismal systems. The natural evolution process can be used in complement to rational strain engineering for the development of desired traits or phenotypes as well as for the production of novel biomaterials through the imposition of one or more selective pressures. Space provides a unique environment of stressors (e.g., weightlessness and high radiation) that organisms have never experienced on Earth. Cells in the outer space reorganize and develop or activate a range of molecular responses that lead to changes in cellular properties. Exposure of cells to the outer space will lead to the development of novel variants more efficiently than on Earth. For instance, natural crop varieties can be generated with higher nutrition value, yield, and improved features, such as resistance against high and low temperatures, salt stress, and microbial and pest attacks. The review summarizes the literature on the parameters of outer space that affect the growth and behavior of cells and organisms as well as complex colloidal systems. We illustrate an understanding of gravity-related basic biological mechanisms and enlighten the possibility to explore the outer space environment for application-oriented aspects. This will stimulate biological research in the pursuit of innovative approaches for the future of agriculture and health on Earth.

Mechanisms of Plant Responses and Adaptation to Soil Salinity

Soil salinity is a major environmental stress that restricts the growth and yield of crops. Understanding the physiological, metabolic, and biochemical responses of plants to salt stress and mining the salt tolerance-associated genetic resource in nature will be extremely important for us to cultivate salt-tolerant crops. In this review, we provide a comprehensive summary of the mechanisms of salt stress responses in plants, including salt stress-triggered physiological responses, oxidative stress, salt stress sensing and signaling pathways, organellar stress, ion homeostasis, hormonal and gene expression regulation, metabolic changes, as well as salt tolerance mechanisms in halophytes. Important questions regarding salt tolerance that need to be addressed in the future are discussed.

Extending the Genotype in Brachypodium by Including DNA Methylation Reveals a Joint Contribution with Genetics on Adaptive Traits

Epigenomic changes have been considered a potential missing link underlying phenotypic variation in quantitative traits but is potentially confounded with the underlying DNA sequence variation. Although the concept of epigenetic inheritance has been discussed in depth, there have been few studies attempting to directly dissect the amount of epigenomic variation within inbred natural populations while also accounting for genetic diversity. By using known genetic relationships between Brachypodium lines, multiple sets of nearly identical accession families were selected for phenotypic studies and DNA methylome profiling to investigate the dual role of (epi)genetics under simulated natural seasonal climate conditions. Despite reduced genetic diversity, appreciable phenotypic variation was still observable in the measured traits (height, leaf width and length, tiller count, flowering time, ear count) between as well as within the inbred accessions. However, with reduced genetic diversity there was diminished variation in DNA methylation within families. Mixed-effects linear modeling revealed large genetic differences between families and a minor contribution of DNA methylation variation on phenotypic variation in select traits. Taken together, this analysis suggests a limited but significant contribution of DNA methylation toward heritable phenotypic variation relative to genetic differences.

Epigenetic Changes in Host Ribosomal DNA Promoter Induced by an Asymptomatic Plant Virus Infection

DNA cytosine methylation is one of the main epigenetic mechanisms in higher eukaryotes and is considered to play a key role in transcriptional gene silencing. In plants, cytosine methylation can occur in all sequence contexts (CG, CHG, and CHH), and its levels are controlled by multiple pathways, including de novo methylation, maintenance methylation, and demethylation. Modulation of DNA methylation represents a potentially robust mechanism to adjust gene expression following exposure to different stresses. However, the potential involvement of epigenetics in plant-virus interactions has been scarcely explored, especially with regard to RNA viruses. Here, we studied the impact of a symptomless viral infection on the epigenetic status of the host genome. We focused our attention on the interaction between Nicotiana benthamiana and Pelargonium line pattern virus (PLPV, family Tombusviridae), and analyzed cytosine methylation in the repetitive genomic element corresponding to ribosomal DNA (rDNA). Through a combination of bisulfite sequencing and RT-qPCR, we obtained data showing that PLPV infection gives rise to a reduction in methylation at CG sites of the rDNA promoter. Such a reduction correlated with an increase and decrease, respectively, in the expression levels of some key demethylases and of MET1, the DNA methyltransferase responsible for the maintenance of CG methylation. Hypomethylation of rDNA promoter was associated with a five-fold augmentation of rRNA precursor levels. The PLPV protein p37, reported as a suppressor of post-transcriptional gene silencing, did not lead to the same effects when expressed alone and, thus, it is unlikely to act as suppressor of transcriptional gene silencing. Collectively, the results suggest that PLPV infection as a whole is able to modulate host transcriptional activity through changes in the cytosine methylation pattern arising from misregulation of methyltransferases/demethylases balance.

Differential Gene Expression between Fungal Mating Types Is Associated with Sequence Degeneration

Degenerative mutations in non-recombining regions, such as in sex chromosomes, may lead to differential expression between alleles if mutations occur stochastically in one or the other allele. Reduced allelic expression due to degeneration has indeed been suggested to occur in various sex-chromosome systems. However, whether an association occurs between specific signatures of degeneration and differential expression between alleles has not been extensively tested, and sexual antagonism can also cause differential expression on sex chromosomes. The anther-smut fungus Microbotryum lychnidis-dioicae is ideal for testing associations between specific degenerative signatures and differential expression because 1) there are multiple evolutionary strata on the mating-type chromosomes, reflecting successive recombination suppression linked to mating-type loci; 2) separate haploid cultures of opposite mating types help identify differential expression between alleles; and 3) there is no sexual antagonism as a confounding factor accounting for differential expression. We found that differentially expressed genes were enriched in the four oldest evolutionary strata compared with other genomic compartments, and that, within compartments, several signatures of sequence degeneration were greater for differentially expressed than non-differentially expressed genes. Two particular degenerative signatures were significantly associated with lower expression levels within differentially expressed allele pairs: upstream insertion of transposable elements and mutations truncating the protein length. Other degenerative mutations associated with differential expression included nonsynonymous substitutions and altered intron or GC content. The association between differential expression and allele degeneration is relevant for a broad range of taxa where mating compatibility or sex is determined by genes located in large regions where recombination is suppressed.

Salicylic acid and nitric oxide signaling in plant heat stress

In agriculture, heat stress (HS) has become one of the eminent abiotic threats to crop growth, productivity and nutritional security because of the continuous increase in global mean temperature. Studies have annotated that the heat stress response (HSR) in plants is highly conserved, involving complex regulatory networks of various signaling and sensor molecules. In this context, the ubiquitous-signaling molecules salicylic acid (SA) and nitric oxide (NO) have diverted the attention of the plant science community because of their putative roles in plant abiotic and biotic stress tolerance. However, their involvement in the transcriptional regulatory networks in plant HS tolerance is still poorly understood. In this review, we have conceptualized current knowledge concerning how SA and NO sense HS in plants and how they trigger the HSR leading to the activation of transcriptional-signaling cascades. Fundamentals of functional components and signaling networks associated with molecular mechanisms involved in SA/NO-mediated HSR in plants have also been discussed. Increasing evidences have suggested the involvement of epigenetic modifications in the development of a 'stress memory', thereby provoking the role of epigenetic mechanisms in the regulation of plant's innate immunity under HS. Thus, we have also explored the recent advancements regarding the biological mechanisms and the underlying significance of epigenetic regulations involved in the activation of HS responsive genes and transcription factors by providing conceptual frameworks for understanding molecular mechanisms behind the 'transcriptional stress memory' as potential memory tools in the regulation of plant HSR.

Distinct Metabolic Signals Underlie Clone by Environment Interplay in "Nebbiolo" Grapes Over Ripening

Several research studies were focused to understand how grapevine cultivars respond to environment; nevertheless, the biological mechanisms tuning this phenomenon need to be further deepened. Particularly, the molecular processes underlying the interplay between clones of the same cultivar and environment were poorly investigated. To address this issue, we analyzed the transcriptome of berries from three "Nebbiolo" clones grown in different vineyards, during two ripening seasons. RNA-sequencing data were implemented with analyses of candidate genes, secondary metabolites, and agronomical parameters. This multidisciplinary approach helped to dissect the complexity of clone × environment interactions, by identifying the molecular responses controlled by genotype, vineyard, phenological phase, or a combination of these factors. Transcripts associated to sugar signalling, anthocyanin biosynthesis, and transport were differently modulated among clones, according to changes in berry agronomical features. Conversely, genes involved in defense response, such as stilbene synthase genes, were significantly affected by vineyard, consistently with stilbenoid accumulation. Thus, besides at the cultivar level, clone-specific molecular responses also contribute to shape the agronomic features of grapes in different environments. This reveals a further level of complexity in the regulation of genotype × environment interactions that has to be considered for orienting viticultural practices aimed at enhancing the quality of grape productions.

Epigenetic Mechanisms and Posttranslational Modifications in Systemic Lupus Erythematosus

The complex physiology of eukaryotic cells is regulated through numerous mechanisms, including epigenetic changes and posttranslational modifications. The wide-ranging diversity of these mechanisms constitutes a way of dynamic regulation of the functionality of proteins, their activity, and their subcellular localization as well as modulation of the differential expression of genes in response to external and internal stimuli that allow an organism to respond or adapt to accordingly. However, alterations in these mechanisms have been evidenced in several autoimmune diseases, including systemic lupus erythematosus (SLE). The present review aims to provide an approach to the current knowledge of the implications of these mechanisms in SLE pathophysiology.

Basic-leucine zipper 17 and Hmg-CoA reductase degradation 3A are involved in salt acclimation memory in Arabidopsis

Salt acclimation, which is induced by previous salt exposure, increases the resistance of plants to future exposure to salt stress. However, little is known about the underlying mechanism, particularly how plants store the "memory" of salt exposure. In this study, we established a system to study salt acclimation in Arabidopsis thaliana. Following treatment with a low concentration of salt, seedlings were allowed to recover to allow transitory salt responses to subside while maintaining the sustainable effects of salt acclimation. We performed transcriptome profiling analysis of these seedlings to identify genes related to salt acclimation memory. Notably, the expression of Basic-leucine zipper 17 (bZIP17) and Hmg-CoA reductase degradation 3A (HRD3A), which are important in the unfolded protein response (UPR) and endoplasmic reticulum-associated degradation (ERAD), respectively, increased following treatment with a low concentration of salt and remained at stably high levels after the stimulus was removed, a treatment which improved plant tolerance to future high-salinity challenge. Our findings suggest that the upregulated expression of important genes involved in the UPR and ERAD represents a "memory" of the history of salt exposure and enables more potent responses to future exposure to salt stress, providing new insights into the mechanisms underlying salt acclimation in plants.

Monitoring the interplay between transposable element families and DNA methylation in maize

DNA methylation and epigenetic silencing play important roles in the regulation of transposable elements (TEs) in many eukaryotic genomes. A majority of the maize genome is derived from TEs that can be classified into different orders and families based on their mechanism of transposition and sequence similarity, respectively. TEs themselves are highly methylated and it can be tempting to view them as a single uniform group. However, the analysis of DNA methylation profiles in flanking regions provides evidence for distinct groups of chromatin properties at different TE families. These differences among TE families are reproducible in different tissues and different inbred lines. TE families with varying levels of DNA methylation in flanking regions also show distinct patterns of chromatin accessibility and modifications within the TEs. The differences in the patterns of DNA methylation flanking TE families arise from a combination of non-random insertion preferences of TE families, changes in DNA methylation triggered by the insertion of the TE and subsequent selection pressure. A set of nearly 70,000 TE polymorphisms among four assembled maize genomes were used to monitor the level of DNA methylation at haplotypes with and without the TE insertions. In many cases, TE families with high levels of DNA methylation in flanking sequence are enriched for insertions into highly methylated regions. The majority of the >2,500 TE insertions into unmethylated regions result in changes in DNA methylation in haplotypes with the TE, suggesting the widespread potential for TE insertions to condition altered methylation in conserved regions of the genome. This study highlights the interplay between TEs and the methylome of a major crop species.

Epigenetics in the plant-virus interaction

Plants have developed diverse molecular mechanisms to resist viruses. RNA silencing plays a dominant role in antiviral defense. Recent studies have correlated plant antiviral silencing to epigenetic modification in genomic DNA and protein by remodeling the expression levels of coding genes. The plant host methylation level is reprogrammed in response to viral challenge. Genomes of some viruses have been implicated in the epigenetic modification via small RNA-mediated transcriptional gene silencing and post-transcriptional gene silencing. These mechanisms can be primed prior to a virus attack through methylation changes for antiviral defense. This review highlights the findings concerning the methylation changes in plant-virus interactions and demonstrates a possible direction to improve the understanding of plant host methylation regulation in response to viral infection.

Identification of histone methylation modifiers and their expression patterns during somatic embryogenesis in Hevea brasiliensis

Histone methylation plays a crucial role in various biological processes, from heterochromatin formation to transcriptional regulation. Currently, no information is available regarding histone methylation modifiers in the important rubber-producing plant Hevea brasiliensis. Here, we identified 47 histone methyltransferase (HMT) genes and 25 histone demethylase (HDM) genes as possible members of the histone methylation modifiers in the rubber tree genome. According to the structural features of HMT and HDM, the HbHMTs were classified into two groups (HbPRMs and HbSDGs), the HbHDMs have two groups (HbLSDs and HbJMJs). Expression patterns were analyzed in five different tissues and at different phases of somatic embryogenesis. HbSDG10, 21, 25, 33, HbJMJ2, 18, 20 were with high expression at different phases of somatic embryogenesis. HbSDG10,14, 20, 21, 33 and HbPRMT4 were expressed highly in anther, HbSDG14, 20, 21, 22, 23, 33, 35 and HbPRMT1 HbJMJ7 and HbLSD1, 2, 3, 4 showed high expression levels in callus. HbSDG1, 7, 10, 13, 14, 18, 19, 21, 22, 23, 35, HbPRMT1, 8, HbJMJ5, 7, 11, 16, 20 and HbLSD2, 3, 4 were expressed highly in somatic embryo. HbSDG10, 21, 25, 33, HbLSD2, 3 were expressed highly in bud of regenerated plant. The analyses reveal that HbHMTs and HbHDMs exhibit different expression patterns at different phases during somatic embryogenesis, implying that some HbHMTs and HbHDMs play important roles during somatic embryogenesis. This study provide fundamental information for further studies on histone methylation in Hevea brasiliensis.

Decrease in DNA methylation 1 (DDM1) is required for the formation of m CHH islands in maize

DNA methylation plays a crucial role in suppressing mobilization of transposable elements and regulation of gene expression. A number of studies have indicated that DNA methylation pathways and patterns exhibit distinct properties in different species, including Arabidopsis, rice, and maize. Here, we characterized the function of DDM1 in regulating genome-wide DNA methylation in maize. Two homologs of ZmDDM1 are abundantly expressed in the embryo and their simultaneous disruption caused embryo lethality with abnormalities in cell proliferation from the early stage of kernel development. We establish that ZmDDM1 is critical for DNA methylation, at CHG sites, and to a lesser extent at CG sites, in heterochromatic regions, and unexpectedly, it is required for the formation of ^m CHH islands. In addition, ZmDDM1 is indispensable for the presence of 24-nt siRNA, suggesting its involvement in the RdDM pathway. Our results provide novel insight into the role of ZmDDM1 in regulating the formation of ^m CHH islands, via the RdDM pathway maize, suggesting that, in comparison to Arabidopsis, maize may have adopted distinct mechanisms for regulating ^m CHH.

Rapid changes in seed dispersal traits may modify plant responses to global change

When climatic or environmental conditions change, plant populations must either adapt to these new conditions, or track their niche via seed dispersal. Adaptation of plants to different abiotic environments has mostly been discussed with respect to physiological and demographic parameters that allow local persistence. However, rapid modifications in response to changing environmental conditions can also affect seed dispersal, both via plant traits and via their dispersal agents. Studying such changes empirically is challenging, due to the high variability in dispersal success, resulting from environmental heterogeneity, and substantial phenotypic variability of dispersal-related traits of seeds and their dispersers. The exact mechanisms that drive rapid changes are often not well understood, but the ecological implications of these processes are essential determinants of dispersal success, and deserve more attention from ecologists, especially in the context of adaptation to global change. We outline the evidence for rapid changes in seed dispersal traits by discussing variability due to plasticity or genetics broadly, and describe the specific traits and biological systems in which variability in dispersal is being studied, before discussing some of the potential underlying mechanisms. We then address future research needs and propose a simulation model that incorporates phenotypic plasticity in seed dispersal. We close with a call to action and encourage ecologists and biologist to embrace the challenge of better understanding rapid changes in seed dispersal and their consequences for the reaction of plant populations to global change.

Microspore embryogenesis: targeting the determinant factors of stress-induced cell reprogramming for crop improvement

Under stress, isolated microspores are reprogrammed in vitro towards embryogenesis, producing doubled haploid plants that are useful biotechnological tools in plant breeding as a source of new genetic variability, fixed in homozygous plants in only one generation. Stress-induced cell death and low rates of cell reprogramming are major factors that reduce yield. Knowledge gained in recent years has revealed that initiation and progression of microspore embryogenesis involve a complex network of factors, whose roles are not yet well understood. Here, I review recent findings on the determinant factors underlying stress-induced microspore embryogenesis, focusing on the role of autophagy, cell death, auxin, chromatin modifications, and the cell wall. Autophagy and cell death proteases are crucial players in the response to stress, while cell reprogramming and acquisition of totipotency are regulated by hormonal and epigenetic mechanisms. Auxin biosynthesis, transport, and action are required for microspore embryogenesis. Initial stages involve DNA hypomethylation, H3K9 demethylation, and H3/H4 acetylation. Cell wall remodelling, with pectin de-methylesterification and arabinogalactan protein expression, is necessary for embryo development. Recent reports show that treatments with small modulators of autophagy, proteases, and epigenetic marks reduce cell death and enhance embryogenesis initiation in several crops, opening up new possibilities for improving in vitro embryo production in breeding programmes.

Comparative sequence and methylation analysis of chloroplast and amyloplast genomes from rice

Grain amyloplast and leaf chloroplast DNA sequences are identical in rice plants but are differentially methylated. The leaf chloroplast DNA becomes more methylated as the rice plant ages. Rice is an important crop worldwide. Chloroplasts and amyloplasts are critical organelles but the amyloplast genome is poorly studied. We have characterised the sequence and methylation of grain amyloplast DNA and leaf chloroplast DNA in rice. We have also analysed the changes in methylation patterns in the chloroplast DNA as the rice plant ages. Total genomic DNA from grain, old leaf and young leaf tissues were extracted from the Oryza sativa ssp. indica cv. MR219 and sequenced using Illumina Miseq. Sequence variant analysis revealed that the amyloplast and chloroplast DNA of MR219 were identical to each other. However, comparison of CpG and CHG methylation between the identical amyloplast and chloroplast DNA sequences indicated that the chloroplast DNA from rice leaves collected at early ripening stage was more methylated than the amyloplast DNA from the grains of the same plant. The chloroplast DNA became more methylated as the plant ages so that chloroplast DNA from young leaves was less methylated overall than amyloplast DNA. These differential methylation patterns were primarily observed in organelle-encoded genes related to photosynthesis followed by those involved in transcription and translation.

Assessing Global DNA Methylation Changes Associated with Plasticity in Seven Highly Inbred Lines of Snapdragon Plants (Antirrhinum majus)

Genetic and epigenetic variations are commonly known to underlie phenotypic plastic responses to environmental cues. However, the role of epigenetic variation in plastic responses harboring ecological significance in nature remains to be assessed. The shade avoidance response (SAR) of plants is one of the most prevalent examples of phenotypic plasticity. It is a phenotypic syndrome including stem elongation and multiple other traits. Its ecological significance is widely acknowledged, and it can be adaptive in the presence of competition for light. Underlying genes and pathways were identified, but evidence for its epigenetic basis remains scarce. We used a proven and accessible approach at the population level and compared global DNA methylation between plants exposed to regular light and three different magnitudes of shade in seven highly inbred lines of snapdragon plants (Antirrhinum majus) grown in a greenhouse. Our results brought evidence of a strong SAR syndrome for which magnitude did not vary between lines. They also brought evidence that its magnitude was not associated with the global DNA methylation percentage for five of the six traits under study. The magnitude of stem elongation was significantly associated with global DNA demethylation. We discuss the limits of this approach and why caution must be taken with such results. In-depth approaches at the DNA sequence level will be necessary to better understand the molecular basis of the SAR syndrome.

Defence-related priming and responses to recurring drought: Two manifestations of plant transcriptional memory mediated by the ABA and JA signalling pathways

Collective evidence from agricultural practices and from scientific research has demonstrated that plants can alter their phenotypic responses to repeated biotic and abiotic stresses or their elicitors. A coordinated reaction at the organismal, cellular, and genome levels has suggested that plants can "remember" an earlier stress and modify their future responses, accordingly. Stress memory may increase a plant's survival chances by improving its tolerance/avoidance abilities and may provide a mechanism for acclimation and adaptation. Understanding the mechanisms that regulate plant stress memory is not only an intellectually challenging topic but has important implications for agricultural practices as well. Here, I focus exclusively on specific aspects of the transcription memory in response to recurring dehydration stresses and the memory-type responses to insect damage in a process known as "priming." The questions discussed are (a) whether/how the two memory phenomena are connected at the level of transcriptional regulation; (b) how differential transcription is achieved mechanistically under a repeated stress; and (c) whether similar molecular and/or epigenetic mechanisms are involved. Possible biological relevance of transcriptional stress memory and its preservation in plant evolution are also discussed.

Evolutionary and ecological functional genomics, from lab to the wild

Plant phenotypes are the result of both genetic and environmental forces that act to modulate trait expression. Over the last few years, numerous approaches in functional genomics and systems biology have led to a greater understanding of plant phenotypic variation and plant responses to the environment. These approaches, and the questions that they can address, have been loosely termed evolutionary and ecological functional genomics (EEFG), and have been providing key insights on how plants adapt and evolve. In particular, by bringing these studies from the laboratory to the field, EEFG studies allow us to gain greater knowledge of how plants function in their natural contexts.

Parental experience modifies the Mimulus methylome

BACKGROUND

Transgenerational plasticity occurs when the environmental experience of an organism modifies the growth and development of its progeny. Leaf damage in Mimulus guttatus exhibits transgenerational plasticity mediated through differential expression of hundreds of genes. The epigenetic mechanisms that facilitate this response have yet to be described.

RESULTS

We performed whole genome bisulfite sequencing in the progeny of genetically identical damaged and control plants and developed a pipeline to compare differences in the mean and variance of methylation between treatment groups. We find that parental damage increases the variability of CG and CHG methylation among progeny, but does not alter the overall mean methylation. Instead it has positive effects in some regions and negative in others. We find 3,396 CHH, 203 CG, and 54 CHG Differentially Methylated Regions (DMRs) ranging from tens to thousands of base pairs scattered across the genome. CHG and CHH DMRs tended to overlap with transposable elements. CG DMRs tended to overlap with gene coding regions, many of which were previously found to be differentially expressed.

CONCLUSIONS

Genome-wide increases in methylome variation suggest that parental conditions can increase epigenetic diversity in response to stress. Additionally, the potential association between CG DMRs and differentially expressed genes supports the hypothesis that differential methylation is a mechanistic component of transgenerational plasticity in M. guttatus.

DNA methylation footprints during soybean domestication and improvement

BACKGROUND

In addition to genetic variation, epigenetic variation plays an important role in determining various biological processes. The importance of natural genetic variation to crop domestication and improvement has been widely investigated. However, the contribution of epigenetic variation in crop domestication at population level has rarely been explored.

RESULTS

To understand the impact of epigenetics on crop domestication, we investigate the variation of DNA methylation during soybean domestication and improvement by whole-genome bisulfite sequencing of 45 soybean accessions, including wild soybeans, landraces, and cultivars. Through methylomic analysis, we identify 5412 differentially methylated regions (DMRs). These DMRs exhibit characters distinct from those of genetically selected regions. In particular, they have significantly higher genetic diversity. Association analyses suggest only 22.54% of DMRs can be explained by local genetic variations. Intriguingly, genes in the DMRs that are not associated with any genetic variation are enriched in carbohydrate metabolism pathways.

CONCLUSIONS

This study provides a valuable map of DNA methylation across diverse accessions and dissects the relationship between DNA methylation variation and genetic variation during soybean domestication, thus expanding our understanding of soybean domestication and improvement.