The Arabidopsis DNA Methylome Is Stable under Transgenerational Drought Stress

Like mother like son: Transgenerational memory and cross-tolerance from drought to heat stress are identified in chloroplast proteome and seed provisioning in Pinus radiata

How different stressors impact plant health and memory when they are imposed in different generations in wild ecosystems is still scarce. Here, we address how different environments shape heritable memory for the next generation in seeds and seedlings of Pinus radiata, a long-lived species with economic interest. The performance of the seedlings belonging to two wild clonal subpopulations (optimal fertirrigation vs. slightly stressful conditions) was tested under heat stress through physiological profiling and comparative time-series chloroplast proteomics. In addition, we explored the seeds conducting a physiological characterization and targeted transcriptomic profiling in both subpopulations. Our results showed differential responses between them, evidencing a cross-stress transgenerational memory. Seedlings belonging to the stressed subpopulation retained key proteins related to Photosystem II, chloroplast-to-nucleus signalling and osmoprotection which helped to overcome the applied heat stress. The seeds also showed a differential gene expression profile for targeted genes and microRNAs, as well as an increased content of starch and secondary metabolites, molecules which showed potential interest as biomarkers for early selection of primed plants. Thus, these finds not only delve into transgenerational cross-stress memory in trees, but also provide a new biotechnological tool for forest design.

Systematic Analysis of DNA Demethylase Gene Families in Foxtail Millet (Setaria italica L.) and Their Expression Variations after Abiotic Stresses

DNA methylation is a highly conserved epigenetic modification involved in many biological processes, including growth and development, stress response, and secondary metabolism. DNA demethylase (DNA-deMTase) genes have been identified in some plant species; however, there are no reports on the identification and analysis of DNA-deMTase genes in Foxtail millet (Setaria italica L.). In this study, seven DNA-deMTases were identified in S. italica. These DNA-deMTase genes were divided into four subfamilies (DML5, DML4, DML3, and ROS1) by phylogenetic and gene structure analysis. Further analysis shows that the physical and chemical properties of these DNA-deMTases proteins are similar, contain the typical conserved domains of ENCO3c and are located in the nucleus. Furthermore, multiple cis-acting elements were observed in DNA-deMTases, including light responsiveness, phytohormone responsiveness, stress responsiveness, and elements related to plant growth and development. The DNA-deMTase genes are expressed in all tissues detected with certain tissue specificity. Then, we investigated the abundance of DNA-deMTase transcripts under abiotic stresses (cold, drought, salt, ABA, and MeJA). The results showed that different genes of DNA-deMTases were involved in the regulation of different abiotic stresses. In total, our findings will provide a basis for the roles of DNA-deMTase in response to abiotic stress.

Exploring the patterns of evolution: Core thoughts and focus on the saltational model

The Modern Synthesis, a pillar in biological thought, united Darwin's species origin concepts with Mendel's laws of character heredity, providing a comprehensive understanding of evolution within species. Highlighting phenotypic variation and natural selection, it elucidated the environment's role as a selective force, shaping populations over time. This framework integrated additional mechanisms, including genetic drift, random mutations, and gene flow, predicting their cumulative effects on microevolution and the emergence of new species. Beyond the Modern Synthesis, the Extended Evolutionary Synthesis expands perspectives by recognizing the role of developmental plasticity, non-genetic inheritance, and epigenetics. We suggest that these aspects coexist in the plant evolutionary process; in this context, we focus on the saltational model, emphasizing how saltation events, such as dichotomous saltation, chromosomal mutations, epigenetic phenomena, and polyploidy, contribute to rapid evolutionary changes. The saltational model proposes that certain evolutionary changes, such as the rise of new species, may result suddenly from single macromutations rather than from gradual changes in DNA sequences and allele frequencies within a species over time. These events, observed in domesticated and wild higher plants, provide well-defined mechanistic bases, revealing their profound impact on plant diversity and rapid evolutionary events. Notably, next-generation sequencing exposes the likely crucial role of allopolyploidy and autopolyploidy (saltational events) in generating new plant species, each characterized by distinct chromosomal complements. In conclusion, through this review, we offer a thorough exploration of the ongoing dissertation on the saltational model, elucidating its implications for our understanding of plant evolutionary processes and paving the way for continued research in this intriguing field.

DNA methylation: an emerging paradigm of gene regulation under drought stress in plants

Drought is an enormous threat to global crop production. In order to ensure food security for the burgeoning population, we must develop drought tolerant crop varieties. This necessitates the identification of drought-responsive genes and understanding the mechanisms involved in their regulation. DNA methylation is a widely studied mechanism of epigenetic regulation of gene expression, which is known to play vital role in conferring tolerance to various biotic and abiotic stress factors. The recent advances in next-generation sequencing (NGS) technologies, has allowed unprecedented access to genome-wide methylation marks, with single base resolution. The most important roles of DNA methylation have been studied in terms of gene body methylation (gbM), which is associated with regulation of both transcript abundance and its stability. The availability of mutants for the various genes encoding enzymes involved in methylation of DNA has allowed ascertainment of the biological significance of methylation. Even though a vast number of reports have emerged in the recent past, where both genome-wide methylation landscape and locus specific changes in DNA methylation have been studied, a conclusive picture with regards to the biological role of DNA methylation is still lacking. Compounding this, is the lack of sufficient evidence supporting the heritability of these epigenetic changes. Amongst the various epigenetic variations, the DNA methylation changes are observed to be the most stable. This review describes the drought-induced changes in DNA methylation identified across different plant species. We also briefly describe the stress memory contributed by these changes. The identification of heritable, drought-induced methylation marks would broaden the scope of crop improvement in the future.

Heritable responses to stress in plants

Most plants are adapted to their environments through generations of exposure to all elements. The adaptation process involves the best possible response to fluctuations in the environment based on the genetic and epigenetic make-up of the organism. Many plant species have the capacity to acclimate or adapt to certain stresses, allowing them to respond more efficiently, with fewer resources diverted from growth and development. However, plants can also acquire protection against stress across generations. Such a response is known as an intergenerational response to stress; typically, plants lose most of the tolerance in the subsequent generation when propagated without stress. Occasionally, the protection lasts for more than one generation after stress exposure and such a response is called transgenerational. In this review, we will summarize what is known about inter- and transgenerational responses to stress, focus on phenotypic and epigenetic events, their mechanisms and ecological and evolutionary meaning.

An evolutionary epigenetic clock in plants

Molecular clocks are the basis for dating the divergence between lineages over macroevolutionary timescales (~105 to 108 years). However, classical DNA-based clocks tick too slowly to inform us about the recent past. Here, we demonstrate that stochastic DNA methylation changes at a subset of cytosines in plant genomes display a clocklike behavior. This "epimutation clock" is orders of magnitude faster than DNA-based clocks and enables phylogenetic explorations on a scale of years to centuries. We show experimentally that epimutation clocks recapitulate known topologies and branching times of intraspecies phylogenetic trees in the self-fertilizing plant Arabidopsis thaliana and the clonal seagrass Zostera marina, which represent two major modes of plant reproduction. This discovery will open new possibilities for high-resolution temporal studies of plant biodiversity.

Methylome and transcriptome analysis of flowering branches building of Citrus plants induced by drought stress

Citrus plants exhibit positive floral response under water stress conditions, however, the mechanistic understanding of floral induction remains largely unexplored in water deficit. In this study, DNA methylomic and transcriptomic analyses were integrated to explore the flowering bud formation as well as branches building after light drought stress. While comparing with the conventional watering group (CK), the light drought group treated with five months (LD) showed a significant increase in the flowering branches, whereas an apparent decrease in vegetative branches. Global DNA methylation analysis showed that the LD Group acquired DNA methylation in more than 70090 genomic regions and lost DNA methylation in about 18421 genomic regions compared with normal watering group, this indicates that water deficiency leads to a global increase in the expression of DNA methylation in citrus. In the same time, we verified that the increase of DNA methylation level in LD group was correlated with the decrease of DNA demethylase related gene expression. Interestingly, in transcription analysis, it was found that the promoting flower genes of the LD group did not increase but decreased similarly with repressing genes, which is contrary to the intended result. Thus, we thought the lower expression of suppressors FLC and BFT were the key influencing factor to stimulate the flowering branches formation after LD treatment. Moreover, there was a strong negative correlation between the genes expression level and methylation level of the flowering induction/flower development genes. In general, we thought high global DNA methylation level induced by water deficit regulate the flowering branches building by reducing FLC and BFT genes expression.

Transgenerational effects of intertidal environment on physiological phenotypes and DNA methylation in Pacific oysters

Climate change and intensifying human activity are posing serious threats to marine organisms. The fluctuating intertidal zone forms a miniature ecosystem of a rapidly changing environment for studying biological adaptation. Transgenerational plasticity (TGP), an evolutionary phenomenon in which parental experience influences offspring phenotypes, provides an avenue for adaptation, but the molecular mechanism was poorly understood in marine molluscs. In this study, wild Pacific oysters (Crassostrea gigas), which were collected from intertidal zones, were used to conduct two-generation breeding in a subtidal area combined with a heat shock experiment in the laboratory to investigate the intertidal environment-induced TGP under temperate subtidal condition and thermally exposed condition, respectively. We showed that TGP could influence the physiological phenotypes related to the status of oxidation and energy in non-stress-exposed subtidal offspring for at least two generations. Genomic DNA methylation exhibited heritable divergence between intertidal and subtidal oysters, and 1655 (or 42.83 %) differentially methylated genes (DMGs) in F₀ were continuously reserved to F₂, which may mediate physiological TGP by participating in biological processes including macromolecule metabolism, cellular responses to stress, and the positive regulation of molecular function, especially fatty acid metabolism. The intertidal experience also influenced the thermal plasticity of physiological phenotypes within and across generations. Totally, 320 (or 14.74 %) specific thermal response DMGs in the intertidal F₀ generation were identified in F₁ and F₂, participating in pathways including carbohydrate, lipid, and energy metabolism, signal transduction, and the organismal immune system, which suggested transgenerational intertidal effect mediated by these genes could positively contribute to stress adaptation and had potential applications for aquaculture. This study demonstrates an epigenetic mechanism for TGP in stress adaptation in marine molluscs, and provides new avenues to improve the stress adaptation for marine resource conservation and aquaculture.

An evolutionary epigenetic clock in plants

Molecular clocks are the basis for dating the divergence between lineages over macro-evolutionary timescales (~10 5 -10 8 years). However, classical DNA-based clocks tick too slowly to inform us about the recent past. Here, we demonstrate that stochastic DNA methylation changes at a subset of cytosines in plant genomes possess a clock-like behavior. This 'epimutation-clock' is orders of magnitude faster than DNA-based clocks and enables phylogenetic explorations on a scale of years to centuries. We show experimentally that epimutation-clocks recapitulate known topologies and branching times of intra-species phylogenetic trees in the selfing plant A. thaliana and the clonal seagrass Z. marina , which represent two major modes of plant reproduction. This discovery will open new possibilities for high-resolution temporal studies of plant biodiversity.

Environmental and genealogical effects on DNA methylation in a widespread apomictic dandelion lineage

DNA methylation in plant genomes occurs in different sequences and genomic contexts that have very different properties. DNA methylation that occurs in CG (mCG) sequence context shows transgenerational stability and high epimutation rate, and can thus provide genealogical information at short time scales. However, due to meta-stability and because mCG variants may arise due to other factors than epimutation, such as environmental stress exposure, it is not clear how well mCG captures genealogical information at micro-evolutionary time scales. Here, we analysed DNA methylation variation between accessions from a geographically widespread, apomictic common dandelion (Taraxacum officinale) lineage when grown experimentally under different light conditions. Using a reduced-representation bisulphite sequencing approach, we show that the light treatment induced differentially methylated cytosines (DMCs) in all sequence contexts, with a bias towards transposable elements. Accession differences were associated mainly with DMCs in CG context. Hierarchical clustering of samples based on total mCG profiles revealed a perfect clustering of samples by accession identity, irrespective of light conditions. Using microsatellite information as a benchmark of genetic divergence within the clonal lineage, we show that genetic divergence between accessions correlates strongly with overall mCG profiles. However, our results suggest that environmental effects that do occur in CG context may produce a heritable signal that partly dilutes the genealogical signal. Our study shows that methylation information in plants can be used to reconstruct micro-evolutionary genealogy, providing a useful tool in systems that lack genetic variation such as clonal and vegetatively propagated plants.

Stress memory and its regulation in plants experiencing recurrent drought conditions

Developing stress-tolerant plants continues to be the goal of breeders due to their realized yields and stability. Plant responses to drought have been studied in many different plant species, but the occurrence of stress memory as well as the potential mechanisms for memory regulation is not yet well described. It has been observed that plants hold on to past events in a way that adjusts their response to new challenges without altering their genetic constitution. This ability could enable training of plants to face future challenges that increase in frequency and intensity. A better understanding of stress memory-associated mechanisms leading to alteration in gene expression and how they link to physiological, biochemical, metabolomic and morphological changes would initiate diverse opportunities to breed stress-tolerant genotypes through molecular breeding or biotechnological approaches. In this perspective, this review discusses different stress memory types and gives an overall view using general examples. Further, focusing on drought stress, we demonstrate coordinated changes in epigenetic and molecular gene expression control mechanisms, the associated transcription memory responses at the genome level and integrated biochemical and physiological responses at cellular level following recurrent drought stress exposures. Indeed, coordinated epigenetic and molecular alterations of expression of specific gene networks link to biochemical and physiological responses that facilitate acclimation and survival of an individual plant during repeated stress.

Unravelling Differential DNA Methylation Patterns in Genotype Dependent Manner under Salinity Stress Response in Chickpea

DNA methylation is one of the epigenetic mechanisms that govern gene regulation in response to abiotic stress in plants. Here, we analyzed the role of epigenetic variations by exploring global DNA methylation and integrating it with differential gene expression in response to salinity stress in tolerant and sensitive chickpea genotypes. Genome-wide DNA methylation profiles showed higher CG methylation in the gene body regions and higher CHH methylation in the TE body regions. The analysis of differentially methylated regions (DMRs) suggested more hyper-methylation in response to stress in the tolerant genotype compared to the sensitive genotype. We observed higher enrichment of CG DMRs in genes and CHH DMRs in transposable elements (TEs). A positive correlation of gene expression with CG gene body methylation was observed. The enrichment analysis of DMR-associated differentially expressed genes revealed they are involved in biological processes, such as lateral root development, transmembrane transporter activity, GTPase activity, and regulation of gene expression. Further, a high correlation of CG methylation with CHG and CHH methylation under salinity stress was revealed, suggesting crosstalk among the methylation contexts. Further, we observed small RNA-mediated CHH hypermethylation in TEs. Overall, the interplay between DNA methylation, small RNAs, and gene expression provides new insights into the regulatory mechanism underlying salinity stress response in chickpeas.

Active DNA demethylation in plants: 20 years of discovery and beyond

Maintaining proper DNA methylation levels in the genome requires active demethylation of DNA. However, removing the methyl group from a modified cytosine is chemically difficult and therefore, the underlying mechanism of demethylation had remained unclear for many years. The discovery of the first eukaryotic DNA demethylase, Arabidopsis thaliana REPRESSOR OF SILENCING 1 (ROS1), led to elucidation of the 5-methylcytosine base excision repair mechanism of active DNA demethylation. In the 20 years since ROS1 was discovered, our understanding of this active DNA demethylation pathway, as well as its regulation and biological functions in plants, has greatly expanded. These exciting developments have laid the groundwork for further dissecting the regulatory mechanisms of active DNA demethylation, with potential applications in epigenome editing to facilitate crop breeding and gene therapy.

Environmental response in gene expression and DNA methylation reveals factors influencing the adaptive potential of Arabidopsis lyrata

Understanding what factors influence plastic and genetic variation is valuable for predicting how organisms respond to changes in the selective environment. Here, using gene expression and DNA methylation as molecular phenotypes, we study environmentally induced variation among Arabidopsis lyrata plants grown at lowland and alpine field sites. Our results show that gene expression is highly plastic, as many more genes are differentially expressed between the field sites than between populations. These environmentally responsive genes evolve under strong selective constraint – the strength of purifying selection on the coding sequence is high, while the rate of adaptive evolution is low. We find, however, that positive selection on cis-regulatory variants has likely contributed to the maintenance of genetically variable environmental responses, but such variants segregate only between distantly related populations. In contrast to gene expression, DNA methylation at genic regions is largely insensitive to the environment, and plastic methylation changes are not associated with differential gene expression. Besides genes, we detect environmental effects at transposable elements (TEs): TEs at the high-altitude field site have higher expression and methylation levels, suggestive of a broad-scale TE activation. Compared to the lowland population, plants native to the alpine environment harbor an excess of recent TE insertions, and we observe that specific TE families are enriched within environmentally responsive genes. Our findings provide insight into selective forces shaping plastic and genetic variation. We also highlight how plastic responses at TEs can rapidly create novel heritable variation in stressful conditions.

Transcriptional Stress Memory and Transgenerational Inheritance of Drought Tolerance in Plants

Plants respond to drought stress by producing abscisic acid, a chemical messenger that regulates gene expression and thereby expedites various physiological and cellular processes including the stomatal operation to mitigate stress and promote tolerance. To trigger or suppress gene transcription under drought stress conditions, the surrounding chromatin architecture must be converted between a repressive and active state by epigenetic remodeling, which is achieved by the dynamic interplay among DNA methylation, histone modifications, loop formation, and non-coding RNA generation. Plants can memorize chromatin status under drought conditions to enable them to deal with recurrent stress. Furthermore, drought tolerance acquired during plant growth can be transmitted to the next generation. The epigenetically modified chromatin architectures of memory genes under stressful conditions can be transmitted to newly developed cells by mitotic cell division, and to germline cells of offspring by overcoming the restraints on meiosis. In mammalian cells, the acquired memory state is completely erased and reset during meiosis. The mechanism by which plant cells overcome this resetting during meiosis to transmit memory is unclear. In this article, we review recent findings on the mechanism underlying transcriptional stress memory and the transgenerational inheritance of drought tolerance in plants.

Epigenetic variation: A major player in facilitating plant fitness under changing environmental conditions

Recent research in plant epigenetics has increased our understanding of how epigenetic variability can contribute to adaptive phenotypic plasticity in natural populations. Studies show that environmental changes induce epigenetic switches either independently or in complementation with the genetic variation. Although most of the induced epigenetic variability gets reset between generations and is short-lived, some variation becomes transgenerational and results in heritable phenotypic traits. The short-term epigenetic responses provide the first tier of transient plasticity required for local adaptations while transgenerational epigenetic changes contribute to stress memory and help the plants respond better to recurring or long-term stresses. These transgenerational epigenetic variations translate into an additional tier of diversity which results in stable epialleles. In recent years, studies have been conducted on epigenetic variation in natural populations related to various biological processes, ecological factors, communities, and habitats. With the advent of advanced NGS-based technologies, epigenetic studies targeting plants in diverse environments have increased manifold to enhance our understanding of epigenetic responses to environmental stimuli in facilitating plant fitness. Taking all points together in a frame, the present review is a compilation of present-day knowledge and understanding of the role of epigenetics and its fitness benefits in diverse ecological systems in natural populations.

Effect of DNA methylation, modified by 5-azaC, on ecophysiological responses of a clonal plant to changing climate

Epigenetic regulation of gene expression is expected to be an important mechanism behind phenotypic plasticity. Whether epigenetic regulation affects species ecophysiological adaptations to changing climate remains largely unexplored. We compared ecophysiological traits between individuals treated with 5-azaC, assumed to lead to DNA demethylation, with control individuals of a clonal grass originating from and grown under different climates, simulating different directions and magnitudes of climate change. We linked the ecophysiological data to proxies of fitness. Main effects of plant origin and cultivating conditions predicted variation in plant traits, but 5-azaC did not. Effects of 5-azaC interacted with conditions of cultivation and plant origin. The direction of the 5-azaC effects suggests that DNA methylation does not reflect species long-term adaptations to climate of origin and species likely epigenetically adjusted to the conditions experienced during experiment set-up. Ecophysiology translated to proxies of fitness, but the intensity and direction of the relationships were context dependent and affected by 5-azaC. The study suggests that effects of DNA methylation depend on conditions of plant origin and current climate. Direction of 5-azaC effects suggests limited role of epigenetic modifications in long-term adaptation of plants. It rather facilitates fast adaptations to temporal fluctuations of the environment.

Comprehensive Analysis of Differentially Expressed Genes and Epigenetic Modification-Related Expression Variation Induced by Saline Stress at Seedling Stage in Fiber and Oil Flax, Linum usitatissimum L

The ability of different germplasm to adapt to a saline-alkali environment is critical to learning about the tolerance mechanism of saline-alkali stress in plants. Flax is an important oil and fiber crop in many countries. However, its molecular tolerance mechanism under saline stress is still not clear. In this study, we studied morphological, physiological characteristics, and gene expression variation in the root and leaf in oil and fiber flax types under saline stress, respectively. Abundant differentially expressed genes (DEGs) induced by saline stress, tissue/organ specificity, and different genotypes involved in plant hormones synthesis and metabolism and transcription factors and epigenetic modifications were detected. The present report provides useful information about the mechanism of flax response to saline stress and could lead to the future elucidation of the specific functions of these genes and help to breed suitable flax varieties for saline/alkaline soil conditions.

Activating stress memory: eustressors as potential tools for plant breeding

Plants are continuously exposed to stress conditions, such that they have developed sophisticated and elegant survival strategies, which are reflected in their phenotypic plasticity, priming capacity, and memory acquisition. Epigenetic mechanisms play a critical role in modulating gene expression and stress responses, allowing malleability, reversibility, stability, and heritability of favourable phenotypes to enhance plant performance. Considering the urgency to improve our agricultural system because of going impacting climate change, potential and sustainable strategies rely on the controlled use of eustressors, enhancing desired characteristics and yield and shaping stress tolerance in crops. However, for plant breeding purposes is necessary to focus on the use of eustressors capable of establishing stable epigenetic marks to generate a transgenerational memory to stimulate a priming state in plants to face the changing environment.

How do plants remember drought?

Plants develop both short-term and transgenerational memory of drought stress through epigenetic regulation of transcription for a better response to subsequent exposure. Recurrent spells of droughts are more common than a single drought, with intermittent moist recovery intervals. While the detrimental effects of the first drought on plant structure and physiology are unavoidable, if survived, plants can memorize the first drought to present a more robust response to the following droughts. This includes a partial stomatal opening in the watered recovery interval, higher levels of osmoprotectants and ABA, and attenuation of photosynthesis in the subsequent exposure. Short-term drought memory is regulated by ABA and other phytohormone signaling with transcriptional memory behavior in various genes. High levels of methylated histones are deposited at the drought-tolerance genes. During the recovery interval, the RNA polymerase is stalled to be activated by a pause-breaking factor in the subsequent drought. Drought leads to DNA demethylation near drought-response genes, with genetic control of the process. Progenies of the drought-exposed plants can better adapt to drought owing to the inheritance of particular methylation patterns. However, a prolonged watered recovery interval leads to loss of drought memory, mediated by certain demethylases and chromatin accessibility factors. Small RNAs act as critical regulators of drought memory by altering transcript levels of drought-responsive target genes. Further studies in the future will throw more light on the genetic control of drought memory and the interplay of genetic and epigenetic factors in its inheritance. Plants from extreme environments can give queues to understanding robust memory responses at the ecosystem level.

Chromatin-Based Transcriptional Reprogramming in Plants under Abiotic Stresses

Plants' stress response machinery is characterized by an intricate network of signaling cascades that receive and transmit environmental cues and ultimately trigger transcriptional reprogramming. The family of epigenetic regulators that are the key players in the stress-induced signaling cascade comprise of chromatin remodelers, histone modifiers, DNA modifiers and regulatory non-coding RNAs. Changes in the histone modification and DNA methylation lead to major alterations in the expression level and pattern of stress-responsive genes to adjust with abiotic stress conditions namely heat, cold, drought and salinity. The spotlight of this review falls primarily on the chromatin restructuring under severe abiotic stresses, crosstalk between epigenetic regulators along with a brief discussion on stress priming in plants.

Understanding plant stress memory response for abiotic stress resilience: Molecular insights and prospects

As sessile species and without the possibility of escape, plants constantly face numerous environmental stresses. To adapt in the external environmental cues, plants adjust themselves against such stresses by regulating their physiological, metabolic and developmental responses to external environmental cues. Certain environmental stresses rarely occur during plant life, while others, such as heat, drought, salinity, and cold are repetitive. Abiotic stresses are among the foremost environmental variables that have hindered agricultural production globally. Through distinct mechanisms, these stresses induce various morphological, biochemical, physiological, and metabolic changes in plants, directly impacting their growth, development, and productivity. Subsequently, plant's physiological, metabolic, and genetic adjustments to the stress occurrence provide necessary competencies to adapt, survive and nurture a condition known as "memory." This review emphasizes the advancements in various epigenetic-related chromatin modifications, DNA methylation, histone modifications, chromatin remodeling, phytohormones, and microRNAs associated with abiotic stress memory. Plants have the ability to respond quickly to stressful situations and can also improve their defense systems by retaining and sustaining stressful memories, allowing for stronger or faster responses to repeated stressful situations. Although there are relatively few examples of such memories, and no clear understanding of their duration, taking into consideration plenty of stresses in nature. Understanding these mechanisms in depth could aid in the development of genetic tools to improve breeding techniques, resulting in higher agricultural yield and quality under changing environmental conditions.

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.

Decoding the sorghum methylome: understanding epigenetic contributions to agronomic traits

DNA methylation is a chromatin modification that plays an essential role in regulating gene expression and genome stability and it is typically associated with gene silencing and heterochromatin. Owing to its heritability, alterations in the patterns of DNA methylation have the potential to provide for epigenetic inheritance of traits. Contemporary epigenomic technologies provide information beyond sequence variation and could supply alternative sources of trait variation for improvement in crops such as sorghum. Yet, compared with other species such as maize and rice, the sorghum DNA methylome is far less well understood. The distribution of CG, CHG, and CHH methylation in the genome is different compared with other species. CG and CHG methylation levels peak around centromeric segments in the sorghum genome and are far more depleted in the gene dense chromosome arms. The genes regulating DNA methylation in sorghum are also yet to be functionally characterised; better understanding of their identity and functional analysis of DNA methylation machinery mutants in diverse genotypes will be important to better characterise the sorghum methylome. Here, we catalogue homologous genes encoding methylation regulatory enzymes in sorghum based on genes in Arabidopsis, maize, and rice. Discovering variation in the methylome may uncover epialleles that provide extra information to explain trait variation and has the potential to be applied in epigenome-wide association studies or genomic prediction. DNA methylation can also improve genome annotations and discover regulatory elements underlying traits. Thus, improving our knowledge of the sorghum methylome can enhance our understanding of the molecular basis of traits and may be useful to improve sorghum performance.

Exploration of the Potential Transcriptional Regulatory Mechanisms of DNA Methyltransferases and MBD Genes in Petunia Anther Development and Multi-Stress Responses

Cytosine-5 DNA methyltransferases (C5-MTases) and methyl-CpG-binding-domain (MBD) genes can be co-expressed. They directly control target gene expression by enhancing their DNA methylation levels in humans; however, the presence of this kind of cooperative relationship in plants has not been determined. A popular garden plant worldwide, petunia (Petunia hybrida) is also a model plant in molecular biology. In this study, 9 PhC5-MTase and 11 PhMBD proteins were identified in petunia, and they were categorized into four and six subgroups, respectively, on the basis of phylogenetic analyses. An expression correlation analysis was performed to explore the co-expression relationships between PhC5-MTases and PhMBDs using RNA-seq data, and 11 PhC5-MTase/PhMBD pairs preferentially expressed in anthers were identified as having the most significant correlations (Pearson’s correlation coefficients > 0.9). Remarkably, the stability levels of the PhC5-MTase and PhMBD pairs significantly decreased in different tissues and organs compared with that in anthers, and most of the selected PhC5-MTases and PhMBDs responded to the abiotic and hormonal stresses. However, highly correlated expression relationships between most pairs were not observed under different stress conditions, indicating that anther developmental processes are preferentially influenced by the co-expression of PhC5-MTases and PhMBDs. Interestingly, the nuclear localization genes PhDRM2 and PhMBD2 still had higher correlations under GA treatment conditions, implying that they play important roles in the GA-mediated development of petunia. Collectively, our study suggests a regulatory role for DNA methylation by C5-MTase and MBD genes in petunia anther maturation processes and multi-stress responses, and it provides a framework for the functional characterization of C5-MTases and MBDs in the future.

Molecular properties of epimutation hotspots

Mistakes in the maintenance of CG methylation are a source of heritable epimutations in plants. Multigenerational surveys indicate that the rate of these stochastic events varies substantially across the genome, with some regions harbouring localized 'epimutation hotspots'. Using Arabidopsis as a model, we show that epimutation hotspots are indexed by a specific set of chromatin states that map to subregions of gene body methylation genes. Although these regions comprise only ~12% of all CGs in the genome, they account for ~63% of all epimutation events per unit time. Molecular profiling revealed that these regions contain unique sequence features, harbour steady-state intermediate methylation levels and act as putative targets of antagonistic DNA methylation pathways. We further demonstrate that experimentally induced shifts in steady-state methylation in these hotspot regions are sufficient to significantly alter local epimutation intensities. Our work lays the foundation for dissecting the molecular mechanisms and evolutionary consequences of epimutation hotspots in plants.

A complex network of interactions governs DNA methylation at telomeric regions

DNA methylation modulates telomere function. In Arabidopsis thaliana, telomeric regions have a bimodal chromatin organization with unmethylated telomeres and methylated subtelomeres. To gain insight into this organization we have generated TAIR10-Tel, a modified version of the Arabidopsis reference genome with additional sequences at most chromosome ends. TAIR10-Tel has allowed us to analyse DNA methylation at nucleotide resolution level in telomeric regions. We have analysed the wild-type strain and mutants that encode inactive versions of all currently known relevant methyltransferases involved in cytosine methylation. These analyses have revealed that subtelomeric DNA methylation extends 1 to 2 kbp from Interstitial Telomeric Sequences (ITSs) that abut or are very near to telomeres. However, DNA methylation drops at the telomeric side of the telomere-subtelomere boundaries and disappears at the inner part of telomeres. We present a comprehensive and integrative model for subtelomeric DNA methylation that should help to decipher the mechanisms that govern the epigenetic regulation of telomeres. This model involves a complex network of interactions between methyltransferases and subtelomeric DNA sequences.

Epigenetic inheritance and reproductive mode in plants and animals

Epigenetic inheritance is another piece of the puzzle of nongenetic inheritance, although the prevalence, sources, persistence, and phenotypic consequences of heritable epigenetic marks across taxa remain unclear. We systematically reviewed over 500 studies from the past 5 years to identify trends in the frequency of epigenetic inheritance due to differences in reproductive mode and germline development. Genetic, intrinsic (e.g., disease), and extrinsic (e.g., environmental) factors were identified as sources of epigenetic inheritance, with impacts on phenotype and adaptation depending on environmental predictability. Our review shows that multigenerational persistence of epigenomic patterns is common in both plants and animals, but also highlights many knowledge gaps that remain to be filled. We provide a framework to guide future studies towards understanding the generational persistence and eco-evolutionary significance of epigenomic patterns.

Epigenome and Epitranscriptome: Potential Resources for Crop Improvement

Crop breeding faces the challenge of increasing food demand, especially under climatic changes. Conventional breeding has relied on genetic diversity by combining alleles to obtain desired traits. In recent years, research on epigenetics and epitranscriptomics has shown that epigenetic and epitranscriptomic diversity provides additional sources for crop breeding and harnessing epigenetic and epitranscriptomic regulation through biotechnologies has great potential for crop improvement. Here, we review epigenome and epitranscriptome variations during plant development and in response to environmental stress as well as the available sources for epiallele formation. We also discuss the possible strategies for applying epialleles and epitranscriptome engineering in crop breeding.

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.

Plant epigenomics for extenuation of abiotic stresses: challenges and future perspectives

Climate change has escalated abiotic stresses, leading to adverse effects on plant growth and development, eventually having deleterious consequences on crop productivity. Environmental stresses induce epigenetic changes, namely cytosine DNA methylation and histone post-translational modifications, thus altering chromatin structure and gene expression. Stable epigenetic changes are inheritable across generations and this enables plants to adapt to environmental changes (epipriming). Hence, epigenomes serve as a good source of additional tier of variability for development of climate-smart crops. Epigenetic resources such as epialleles, epigenetic recombinant inbred lines (epiRILs), epigenetic quantitative trait loci (epiQTLs), and epigenetic hybrids (epihybrids) can be utilized in epibreeding for improving stress tolerance of crops. Epigenome engineering is also gaining momentum for developing sustainable epimarks associated with important agronomic traits. Different epigenome editing tools are available for creating, erasing, and reading such epigenetic codes in plant genomes. However, epigenome editing is still understudied in plants due to its complex nature. Epigenetic interventions such as epi-fingerprinting can be exploited in the near future for health and quality assessment of crops under stress conditions. Keeping in view the challenges and opportunities associated with this important technology, the present review intends to enhance understanding of stress-induced epigenetic changes in plants and its prospects for development of climate-ready crops.

Harnessing the power of multi-omics data for predicting climate change response

Predicting how species will respond to future climate change is of central importance in the midst of the global biodiversity crisis, and recent work has demonstrated the utility of population genomics for improving these predictions. Here, we suggest a broadening of the approach to include other types of genomic variants that play an important role in adaptation, like structural (e.g. copy number variants) and epigenetic variants (e.g. DNA methylation). These data could provide additional power for forecasting response, especially in weakly structured or panmictic species. Incorporating structural and epigenetic variation into estimates of climate change vulnerability, or maladaptation, may not only improve prediction power but also provide insight into the molecular mechanisms underpinning species' response to climate change.

Epigenome plasticity in plants

Plant intra-individual and inter-individual variation can be determined by the epigenome, a set of covalent modifications of DNA and chromatin that can alter genome structure and activity without changes to the genome sequence. The epigenome of plant cells is plastic, that is, it can change in response to internal or external cues, such as during development or due to environmental changes, to create a memory of such events. Ongoing advances in technologies to read and write epigenomic patterns with increasing resolution, scale and precision are enabling the extent of plant epigenome variation to be more extensively characterized and functionally interrogated. In this Review, we discuss epigenome dynamics and variation within plants during development and in response to environmental changes, including stress, as well as between plants. We review known or potential functions of such plasticity and emphasize the importance of investigating the causality of epigenomic changes. Finally, we discuss emerging technologies that may underpin future research into plant epigenome plasticity.

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.

Region-level epimutation rates in Arabidopsis thaliana

Failure to maintain DNA methylation patterns during plant development can occasionally give rise to so-called "spontaneous epimutations". These stochastic methylation changes are sometimes heritable across generations and thus accumulate in plant genomes over time. Recent evidence indicates that spontaneous epimutations have a major role in shaping patterns of methylation diversity in plant populations. Using single CG dinucleotides as units of analysis, previous work has shown that the epimutation rate is several orders of magnitude higher than the genetic mutation rate. While these large rate differences have obvious implications for understanding genome-methylome co-evolution, the functional relevance of single CG methylation changes remains questionable. In contrast to single CG, solid experimental evidence has linked methylation gains and losses in larger genomic regions with transcriptional variation and heritable phenotypic effects. Here we show that such region-level changes arise stochastically at about the same rate as those at individual CG sites, are only marginal dependent on region size and cytosine density, but strongly dependent on chromosomal location. We also find consistent evidence that region-level epimutations are not restricted to CG contexts but also frequently occur in non-CG regions at the genome-wide scale. Taken together, our results support the view that many differentially methylated regions (DMRs) in natural populations originate from epimutation events and may not be effectively tagged by proximal SNPs. This possibility reinforces the need for epigenome-wide association studies (EWAS) in plants as a way to identify the epigenetic basis of complex traits.

Comparative methylome reveals regulatory roles of DNA methylation in melon resistance to Podosphaera xanthii

Powdery mildew caused by Podosphaera xanthii (P. xanthii) severely endangers melon (Cucumis melo L.) production, while the mechanistic understanding about its resistance to powdery mildew remains largely limited. In this study, we integrated transcriptomic and methylomic analyses to explore whether DNA methylation was involved in modulating transcriptional acclimation of melon to P. xanthii infection. Net photosynthetic rate (Pn), stomatal conductance (Gs), actual photochemical efficiency (ФPSII) and maximum PSII quantum yield (Fv/Fm) were significantly decreased in P. xanthii-infected plants relative to uninfected ones (Control), revealing apparent physiological disorders. Totally 4808 differentially expressed genes (DEGs) were identified by global analysis of gene expression in Control and P. xanthii-infected plants. Comparative methylome uncovered that 932 DEGs were associated with hypermethylation, while 603 DEGs were associated with hypomethylation in melon upon P. xanthii infection. Among these differential methylation-involved DEGs, a set of resistance-related genes including R genes and candidate genes in metabolic and defense pathways were further identified, demonstrating that DNA methylation might function as a new regulatory layer for melon resistance to P. xanthii infection. Altogether our study sheds new insights into the molecular mechanisms of melon against powdery mildew and provides some potential targets for improving melon disease resistance in future.

Beyond the gene: epigenetic and cis-regulatory targets offer new breeding potential for the future

For millennia, natural and artificial selection has combined favourable alleles for desirable traits in crop species. While modern plant breeding has achieved steady increases in crop yields over the last century, on the current trajectory we will simply not meet demand by 2045. Novel breeding strategies and sources of genetic variation will be required to sustainably fill predicted yield gaps and meet new consumer preferences. Here, we highlight that stepping up to meet this grand challenge will increasingly require thinking 'beyond the gene'. Significant progress has been made in understanding the contributions of both epigenetic variation and cis-regulatory variation to plant traits. This non-genic variation has great potential in future breeding, synthetic biology and biotechnology applications.

Epimutations Define a Fast-Ticking Molecular Clock in Plants

Stochastic gains and losses of DNA methylation at CG dinucleotides are a frequent occurrence in plants. These spontaneous 'epimutations' occur at a rate that is 100 000 times higher than the genetic mutation rate, are effectively neutral at the genome-wide scale, and are stably inherited across mitotic and meiotic cell divisions. Mathematical models have been extraordinarily successful at describing how epimutations accumulate in plant genomes over time, making this process one of the most predictable epigenetic phenomena to date. Here, we propose that their high rate and effective neutrality make epimutations a powerful new molecular clock for timing evolutionary events of the recent past and for age dating of long-lived perennials such as trees.

Potential evidence for transgenerational epigenetic memory in Arabidopsis thaliana following spaceflight

Plants grown in spaceflight exhibited differential methylation responses and this is important because plants are sessile, they are constantly exposed to a variety of environmental pressures and respond to them in many ways. We previously showed that the Arabidopsis genome exhibited lower methylation level after spaceflight for 60 h in orbit. Here, using the offspring of the seedlings grown in microgravity environment in the SJ-10 satellite for 11 days and returned to Earth, we systematically studied the potential effects of spaceflight on DNA methylation, transcriptome, and phenotype in the offspring. Whole-genome methylation analysis in the first generation of offspring (F₁) showed that, although there was no significant difference in methylation level as had previously been observed in the parent plants, some residual imprints of DNA methylation differences were detected. Combined DNA methylation and RNA-sequencing analysis indicated that expression of many pathways, such as the abscisic acid-activated pathway, protein phosphorylation, and nitrate signaling pathway, etc. were enriched in the F₁ population. As some phenotypic differences still existed in the F₂ generation, it was suggested that these epigenetic DNA methylation modifications were partially retained, resulting in phenotypic differences in the offspring. Furthermore, some of the spaceflight-induced heritable differentially methylated regions (DMRs) were retained. Changes in epigenetic modifications caused by spaceflight affected the growth of two future seed generations. Altogether, our research is helpful in better understanding the adaptation mechanism of plants to the spaceflight environment.

In order to investigate whether the effects of spaceflight on plants persist in future generations, Xu et al studied the offspring of Arabidopsis thaliana seedlings that had been grown in a microgravity environment for 11 days. They found that epigenetic modifications caused by spaceflight potentially affected the growth of two future seed generations, shedding light on the longevity of the effects of spaceflight on plants.

Exploration of Epigenetics for Improvement of Drought and Other Stress Resistance in Crops: A Review

Crop plants often have challenges of biotic and abiotic stresses, and they adapt sophisticated ways to acclimate and cope with these through the expression of specific genes. Changes in chromatin, histone, and DNA mostly serve the purpose of combating challenges and ensuring the survival of plants in stressful environments. Epigenetic changes, due to environmental stress, enable plants to remember a past stress event in order to deal with such challenges in the future. This heritable memory, called "plant stress memory", enables plants to respond against stresses in a better and efficient way, not only for the current plant in prevailing situations but also for future generations. Development of stress resistance in plants for increasing the yield potential and stability has always been a traditional objective of breeders for crop improvement through integrated breeding approaches. The application of epigenetics for improvements in complex traits in tetraploid and some other field crops has been unclear. An improved understanding of epigenetics and stress memory applications will contribute to the development of strategies to incorporate them into breeding for complex agronomic traits. The insight in the application of novel plant breeding techniques (NPBTs) has opened a new plethora of options among plant scientists to develop germplasms for stress tolerance. This review summarizes and discusses plant stress memory at the intergenerational and transgenerational levels, mechanisms involved in stress memory, exploitation of induced and natural epigenetic changes, and genome editing technologies with their future possible applications, in the breeding of crops for abiotic stress tolerance to increase the yield for zero hunger goals achievement on a sustainable basis in the changing climatic era.

Distinct cellular strategies determine sensitivity to mild drought of Arabidopsis natural accessions

The worldwide distribution of Arabidopsis (Arabidopsis thaliana) accessions imposes different types of evolutionary pressures, which contributes to various responses of these accessions to environmental stresses. Responses to drought stress have mostly been studied in the Columbia accession, which is predominantly used in plant research. However, the reactions to drought stress are complex and our understanding of the responses that contribute to maintaining plant growth during mild drought (MD) is very limited. Here, we studied the mechanisms with which natural accessions react to MD at a physiological and molecular level during early leaf development. We documented variations in MD responses among natural accessions and used transcriptome sequencing of a drought-sensitive accession, ICE163, and a drought-insensitive accession, Yeg-1, to gain insights into the mechanisms underlying this discrepancy. This revealed that ICE163 preferentially induces jasmonate- and anthocyanin-related pathways, which are beneficial in biotic stress defense, whereas Yeg-1 has a more pronounced activation of abscisic acid signaling, the classical abiotic stress response. Related physiological traits, including the content of proline, anthocyanins, and reactive oxygen species, stomatal closure, and cellular leaf parameters, were investigated and linked to the transcriptional responses. We can conclude that most of these processes constitute general drought response mechanisms that are regulated similarly in drought-insensitive and -sensitive accessions. However, the capacity to close stomata and maintain cell expansion under MD appeared to be major factors that allow to better sustain leaf growth under MD.

Stories that can't be told by SNPs; DNA methylation variation in plant populations

Epigenetic variation has been observed in many plant populations. This variation can influence qualitative and quantitative traits. A key question is whether there is novel information in the epigenome that is not captured by SNP-based genetic markers. The answer likely varies depending on the sources and stability of epigenetic variation as well as the type of population being studied. We consider the epigenetic variation in several plant systems and how this relates to potential for hidden information that could increase our understanding of phenotypic variation.

Prospects for plant productivity: from the canopy to the nucleus

Population growth has been closely associated with agricultural production, since the first famine predicted by Malthus (1798) up to the Green Revolution of the past century. Today, we continue to face increasing demand for food and crop production (Tilman et al., 2011). Considering the combined caloric or protein content of the 275 major crops used directly as human foods or as livestock and fish feeds, Tilman et al. (2011) forecast a 100% increase in global demand for crops from 2005 to 2050. Meeting this demand with the lowest impact on the environment could be achieved by sustainable intensification of existing cropland with reduced land clearing (Tilman et al., 2011; Fischer and Connor, 2018).

The epiallelic potential of transposable elements and its evolutionary significance in plants

DNA provides the fundamental framework for heritability, yet heritable trait variation need not be completely ‘hard-wired’ into the DNA sequence. In plants, the epigenetic machinery that controls transposable element (TE) activity, and which includes DNA methylation, underpins most known cases of inherited trait variants that are independent of DNA sequence changes. Here, we review our current knowledge of the extent, mechanisms and potential adaptive contribution of epiallelic variation at TE-containing alleles in this group of species. For the purpose of this review, we focus mainly on DNA methylation, as it provides an easily quantifiable readout of such variation. The picture that emerges is complex. On the one hand, pronounced differences in DNA methylation at TE sequences can either occur spontaneously or be induced experimentally en masse across the genome through genetic means. Many of these epivariants are stably inherited over multiple sexual generations, thus leading to transgenerational epigenetic inheritance. Functional consequences can be significant, yet they are typically of limited magnitude and although the same epivariants can be found in nature, the factors involved in their generation in this setting remain to be determined. On the other hand, moderate DNA methylation variation at TE-containing alleles can be reproducibly induced by the environment, again usually with mild effects, and most of this variation tends to be lost across generations. Based on these considerations, we argue that TE-containing alleles, rather than their inherited epiallelic variants, are the main targets of natural selection. Thus, we propose that the adaptive contribution of TE-associated epivariation, whether stable or not, lies predominantly in its capacity to modulate TE mobilization in response to the environment, hence providing hard-wired opportunities for the flexible exploration of the phenotypic space.

This article is part of the theme issue ‘How does epigenetics influence the course of evolution?’

Integrative analysis of chloroplast DNA methylation in a marine alga-Saccharina japonica

KEY MESSAGE

We applied an integrative approach using multiple methods to verify cytosine methylation in the chloroplast DNA of the multicellular brown alga Saccharina japonica. Cytosine DNA methylation is a heritable process which plays important roles in regulating development throughout the life cycle of an organism. Although methylation of nuclear DNA has been studied extensively, little is known about the state and role of DNA methylation in chloroplast genomes, especially in marine algae. Here, we have applied an integrated approach encompassing whole-genome bisulfite sequencing, methylated DNA immunoprecipitation, gene co-expression networks and photophysiological analyses to provide evidence for the role of chloroplast DNA methylation in a marine alga, the multicellular brown alga Saccharina japonica. Although the overall methylation level was relatively low in the chloroplast genome of S. japonica, gametophytes exhibited higher methylation levels than sporophytes. Gene-specific bisulfite-cloning sequencing provided additional evidence for the methylation of key photosynthetic genes. Many of them were highly expressed in sporophytes whereas genes involved in transcription, translation and biosynthesis were strongly expressed in gametophytes. Nucleus-encoded photosynthesis genes were co-expressed with their chloroplast-encoded counterparts potentially contributing to the higher photosynthetic performance in sporophytes compared to gametophytes where these co-expression networks were less pronounced. A nucleus-encoded DNA methyltransferase of the DNMT2 family is assumed to be responsible for the methylation of the chloroplast genome because it is predicted to possess a plastid transit peptide.

Multifaceted Chromatin Structure and Transcription Changes in Plant Stress Response

Sessile plants are exposed throughout their existence to environmental abiotic and biotic stress factors, such as cold, heat, salinity, drought, dehydration, submergence, waterlogging, and pathogen infection. Chromatin organization affects genome stability, and its dynamics are crucial in plant stress responses. Chromatin dynamics are epigenetically regulated and are required for stress-induced transcriptional regulation or reprogramming. Epigenetic regulators facilitate the phenotypic plasticity of development and the survival and reproduction of plants in unfavorable environments, and they are highly diversified, including histone and DNA modifiers, histone variants, chromatin remodelers, and regulatory non-coding RNAs. They contribute to chromatin modifications, remodeling and dynamics, and constitute a multilayered and multifaceted circuitry for sophisticated and robust epigenetic regulation of plant stress responses. However, this complicated epigenetic regulatory circuitry creates challenges for elucidating the common or differential roles of chromatin modifications for transcriptional regulation or reprogramming in different plant stress responses. Particularly, interacting chromatin modifications and heritable stress memories are difficult to identify in the aspect of chromatin-based epigenetic regulation of transcriptional reprogramming and memory. Therefore, this review discusses the recent updates from the three perspectives—stress specificity or dependence of transcriptional reprogramming, the interplay of chromatin modifications, and transcriptional stress memory in plants. This helps solidify our knowledge on chromatin-based transcriptional reprogramming for plant stress response and memory.

Small RNAs and their targets are associated with the transgenerational effects of water-deficit stress in durum wheat

Water-deficit stress negatively affects wheat yield and quality. Abiotic stress on parental plants during reproduction may have transgenerational effects on progeny. Here we investigated the transgenerational influence of pre-anthesis water-deficit stress by detailed analysis of the yield components, grain quality traits, and physiological traits in durum wheat. Next-generation sequencing analysis profiled the small RNA-omics, mRNA transcriptomics, and mRNA degradomics in first generation progeny. Parental water-deficit stress had positive impacts on the progeny for traits including harvest index and protein content in the less stress-tolerant variety. Small RNA-seq identified 1739 conserved and 774 novel microRNAs (miRNAs). Transcriptome-seq characterised the expression of 66,559 genes while degradome-seq profiled the miRNA-guided mRNA cleavage dynamics. Differentially expressed miRNAs and genes were identified, with significant regulatory patterns subject to trans- and inter-generational stress. Integrated analysis using three omics platforms revealed significant biological interactions between stress-responsive miRNA and targets, with transgenerational stress tolerance potentially contributed via pathways such as hormone signalling and nutrient metabolism. Our study provides the first confirmation of the transgenerational effects of water-deficit stress in durum wheat. New insights gained at the molecular level indicate that key miRNA-mRNA modules are candidates for transgenerational stress improvement.

DNA Demethylation in Response to Heat Stress in Arabidopsis thaliana

Environmental stress is one of the most important factors affecting plant growth and development. Recent studies have shown that epigenetic mechanisms, such as DNA methylation, play a key role in adapting plants to stress conditions. Here, we analyzed the dynamics of changes in the level of DNA methylation in Arabidopsis thaliana (L.) Heynh. (Brassicaceae) under the influence of heat stress. For this purpose, whole-genome sequencing of sodium bisulfite-treated DNA was performed. The analysis was performed at seven time points, taking into account the control conditions, heat stress, and recovery to control conditions after the stress treatment was discontinued. In our study we observed decrease in the level of DNA methylation under the influence of heat stress, especially after returning to control conditions. Analysis of the gene ontology enrichment and regulatory pathways showed that genes characterized by differential DNA methylation are mainly associated with stress response, including heat stress. These are the genes encoding heat shock proteins and genes associated with translation regulation. A decrease in the level of DNA methylation in such specific sites suggests that under the influence of heat stress we observe active demethylation phenomenon rather than passive demethylation, which is not locus specific.

Role of Chromatin Architecture in Plant Stress Responses: An Update

Sessile plants possess an assembly of signaling pathways that perceive and transmit environmental signals, ultimately resulting in transcriptional reprogramming. Histone is a key feature of chromatin structure. Numerous histone-modifying proteins act under different environmental stress conditions to help modulate gene expression. DNA methylation and histone modification are crucial for genome reprogramming for tissue-specific gene expression and global gene silencing. Different classes of chromatin remodelers including SWI/SNF, ISWI, INO80, and CHD are reported to act upon chromatin in different organisms, under diverse stresses, to convert chromatin from a transcriptionally inactive to a transcriptionally active state. The architecture of chromatin at a given promoter is crucial for determining the transcriptional readout. Further, the connection between somatic memory and chromatin modifications may suggest a mechanistic basis for a stress memory. Studies have suggested that there is a functional connection between changes in nuclear organization and stress conditions. In this review, we discuss the role of chromatin architecture in different stress responses and the current evidence on somatic, intergenerational, and transgenerational stress memory.