Global analysis of H3K4me3/H3K27me3 in Brachypodium distachyon reveals VRN3 as critical epigenetic regulation point in vernalization and provides insights into epigenetic memory

Chromatin loops gather targets of upstream regulators together for efficient gene transcription regulation during vernalization in wheat

BACKGROUND

Plants respond to environmental stimuli by altering gene transcription that is highly related with chromatin status, including histone modification, chromatin accessibility, and three-dimensional chromatin interaction. Vernalization is essential for the transition to reproductive growth for winter wheat. How wheat reshapes its chromatin features, especially chromatin interaction during vernalization, remains unknown.

RESULTS

Combinatory analysis of gene transcription and histone modifications in winter wheat under different vernalization conditions identifies 17,669 differential expressed genes and thousands of differentially enriched peaks of H3K4me3, H3K27me3, and H3K9ac. We find dynamic gene expression across the vernalization process is highly associated with H3K4me3. More importantly, the dynamic H3K4me3- and H3K9ac-associated chromatin-chromatin interactions demonstrate that vernalization leads to increased chromatin interactions and gene activation. Remarkably, spatially distant targets of master regulators like VRN1 and VRT2 are gathered together by chromatin loops to achieve efficient transcription regulation, which is designated as a "shepherd" model. Furthermore, by integrating gene regulatory network for vernalization and natural variation of flowering time, TaZNF10 is identified as a negative regulator for vernalization-related flowering time in wheat.

CONCLUSIONS

We reveal dynamic gene transcription network during vernalization and find that the spatially distant genes can be recruited together via chromatin loops associated with active histone mark thus to be more efficiently found and bound by upstream regulator. It provides new insights into understanding vernalization and response to environmental stimuli in wheat and other plants.

Distinct roles of H3K27me3 and H3K36me3 in vernalization response, maintenance, and resetting in winter wheat

Winter plants rely on vernalization, a crucial process for adapting to cold conditions and ensuring successful reproduction. However, understanding the role of histone modifications in guiding the vernalization process in winter wheat remains limited. In this study, we investigated the transcriptome and chromatin dynamics in the shoot apex throughout the life cycle of winter wheat in the field. Two core histone modifications, H3K27me3 and H3K36me3, exhibited opposite patterns on the key vernalization gene VERNALIZATION1 (VRN1), correlating with its induction during cold exposure. Moreover, the H3K36me3 level remained high at VRN1 after cold exposure, which may maintain its active state. Mutations in FERTILIZATION-INDEPENDENT ENDOSPERM (TaFIE) and SET DOMAIN GROUP 8/EARLY FLOWERING IN SHORT DAYS (TaSDG8/TaEFS), components of the writer complex for H3K27me3 and H3K36me3, respectively, affected flowering time. Intriguingly, VRN1 lost its high expression after the cold exposure memory in the absence of H3K36me3. During embryo development, VRN1 was silenced with the removal of active histone modifications in both winter and spring wheat, with selective restoration of H3K27me3 in winter wheat. The mutant of Tafie-cr-87, a component of H3K27me3 "writer" complex, did not influence the silence of VRN1 during embryo development, but rather attenuated the cold exposure requirement of winter wheat. Integrating gene expression with H3K27me3 and H3K36me3 patterns identified potential regulators of flowering. This study unveils distinct roles of H3K27me3 and H3K36me3 in controlling vernalization response, maintenance, and resetting in winter wheat.

Epigenomic identification of vernalization cis-regulatory elements in winter wheat

BACKGROUND

Winter wheat undergoes vernalization, a process activated by prolonged exposure to low temperatures. During this phase, flowering signals are generated and transported to the apical meristems, stimulating the transition to the inflorescence meristem while inhibiting tiller bud elongation. Although some vernalization genes have been identified, the key cis-regulatory elements and precise mechanisms governing this process in wheat remain largely unknown.

RESULTS

In this study, we construct extensive epigenomic and transcriptomic profiling across multiple tissues-leaf, axillary bud, and shoot apex-during the vernalization of winter wheat. Epigenetic modifications play a crucial role in eliciting tissue-specific responses and sub-genome-divergent expressions during vernalization. Notably, we observe that H3K27me3 primarily regulates vernalization-induced genes and has limited influence on vernalization-repressed genes. The integration of these datasets enables the identification of 10,600 putative vernalization-related regulatory elements including distal accessible chromatin regions (ACRs) situated 30Kb upstream of VRN3, contributing to the construction of a comprehensive regulatory network. Furthermore, we discover that TaSPL7/15, integral components of the aging-related flowering pathway, interact with the VRN1 promoter and VRN3 distal regulatory elements. These interactions finely regulate their expressions, consequently impacting the vernalization process and flowering.

CONCLUSIONS

Our study offers critical insights into wheat vernalization's epigenomic dynamics and identifies the putative regulatory elements crucial for developing wheat germplasm with varied vernalization characteristics. It also establishes a vernalization-related transcriptional network, and uncovers that TaSPL7/15 from the aging pathway participates in vernalization by directly binding to the VRN1 promoter and VRN3 distal regulatory elements.

Epigenetic control on transcription of vernalization genes and whole-genome gene expression profile induced by vernalization in common wheat

Vernalization is necessary for winter wheat to flower. However, it is unclear whether vernalization is also required for spring wheat, which is frequently sown in fall, and what molecular mechanisms underlie the vernalization response in wheat varieties. In this study, we examined the molecular mechanisms that regulate vernalization response in winter and spring wheat varieties. For this purpose, we determined how major vernalization genes (VRN1, VRN2, and VRN3) respond to vernalization in these varieties and whether modifications to histones play a role in changes in gene expression. We also identified genes that are differentially regulated in response to vernalization in winter and spring wheat varieties. We found that in winter wheat, but not in spring wheat, VRN1 expression decreases when returned to warm temperature following vernalization. This finding may be associated with differences between spring and winter wheat in the levels of tri-methylation of lysine 27 on histone H3 (H3K27me3) and tri-methylation of lysine 4 on histone H3 (H3K4me3) at the VRN1 gene. Analysis of winter wheat transcriptomes before and after vernalization revealed that vernalization influences the expression of several genes, including those involved in leucine catabolism, cysteine biosynthesis, and flavonoid biosynthesis. These findings provide new candidates for further study on the mechanism of vernalization regulation in wheat.

A molecular mechanism for embryonic resetting of winter memory and restoration of winter annual growth habit in wheat

The staple food crop winter bread wheat (Triticum aestivum) acquires competence to flower in late spring after experiencing prolonged cold in temperate winter seasons, through the physiological process of vernalization. Prolonged cold exposure results in transcriptional repression of the floral repressor VERNALIZATION 2 (TaVRN2) and activates the expression of the potent floral promoter VERNALIZATION 1 (TaVRN1). Cold-induced TaVRN1 activation and TaVRN2 repression are maintained in post-cold vegetative growth and development, leading to an epigenetic 'memory of winter cold', enabling spring flowering. When and how the cold memory is reset in wheat is essentially unknown. Here we report that the cold-induced TaVRN1 activation is inherited by early embryos, but reset in subsequent embryo development, whereas TaVRN2 remains silenced through seed development, but is reactivated rapidly by light during seed germination. We further found that a chromatin reader mediates embryonic resetting of TaVRN1 and that chromatin modifications play an important role in the regulation of TaVRN1 expression and thus the floral transition, in response to developmental state and environmental cues. The findings define a two-step molecular mechanism for re-establishing vernalization requirement in common wheat, ensuring that each generation must experience winter cold to acquire competence to flower in spring.

INDETERMINATE1-mediated expression of FT family genes is required for proper timing of flowering in Brachypodium distachyon

The transition to flowering is a major developmental switch in plants. In many temperate grasses, perception of indicators of seasonal change, such as changing day-length and temperature, leads to expression of FLOWERING LOCUS T1 (FT1) and FT-Like (FTL) genes that are essential for promoting the transition to flowering. However, little is known about the upstream regulators of FT1 and FTL genes in temperate grasses. Here, we characterize the monocot-specific gene INDETERMINATE1 (BdID1) in Brachypodium distachyon and demonstrate that BdID1 is a regulator of FT family genes. Mutations in ID1 impact the ability of the short-day (SD) vernalization, cold vernalization, and long-day (LD) photoperiod pathways to induce certain FTL genes. BdID1 is required for upregulation of FTL9 (FT-LIKE9) expression by the SD vernalization pathway, and overexpression of FTL9 in an id1 background can partially restore the delayed flowering phenotype of id1. We show that BdID1 binds in vitro to the promoter region of FTL genes suggesting that ID1 directly activates FTL expression. Transcriptome analysis shows that BdID1 is required for FT1, FT2, FTL12, and FTL13 expression under inductive LD photoperiods, indicating that BdID1 is a regulator of the FT gene family. Moreover, overexpression of FT1 in the id1 background results in rapid flowering similar to overexpressing FT1 in the wild type, demonstrating that BdID1 is upstream of FT family genes. Interestingly, ID1 negatively regulates a previously uncharacterized FTL gene, FTL4, and we show that FTL4 is a repressor of flowering. Thus, BdID1 is critical for proper timing of flowering in temperate grasses.

GSK-J4 Inhibition of KDM6B Histone Demethylase Blocks Adhesion of Mantle Cell Lymphoma Cells to Stromal Cells by Modulating NF-κB Signaling

Multiple signaling pathways facilitate the survival and drug resistance of malignant B-cells by regulating their migration and adhesion to microenvironmental niches. NF-κB pathways are commonly dysregulated in mantle cell lymphoma (MCL), but the exact underlying mechanisms are not well understood. Here, using a co-culture model system, we show that the adhesion of MCL cells to stromal cells is associated with elevated levels of KDM6B histone demethylase mRNA in adherent cells. The inhibition of KDM6B activity, using either a selective inhibitor (GSK-J4) or siRNA-mediated knockdown, reduces MCL adhesion to stromal cells. We showed that KDM6B is required both for the removal of repressive chromatin marks (H3K27me3) at the promoter region of NF-κB encoding genes and for inducing the expression of NF-κB genes in adherent MCL cells. GSK-J4 reduced protein levels of the RELA NF-κB subunit and impaired its nuclear localization. We further demonstrated that some adhesion-induced target genes require both induced NF-κB and KDM6B activity for their induction (e.g., IL-10 cytokine gene), while others require induction of NF-κB but not KDM6B (e.g., CCR7 chemokine gene). In conclusion, KDM6B induces the NF-κB pathway at different levels in MCL, thereby facilitating MCL cell adhesion, survival, and drug resistance. KDM6B represents a novel potential therapeutic target for MCL.

DNA hypermethylation promotes the flowering of orchardgrass during vernalization

Vernalization, influenced by environmental factors, is an essential process associated with the productivity of temperate crops, during which epigenetic regulation of gene expression plays an important role. Although DNA methylation is one of the major epigenetic mechanisms associated with the control of gene expression, global changes in DNA methylation in the regulation of gene expression during vernalization-induced flowering of temperate plants remain largely undetermined. To characterize vernalization-associated DNA methylation dynamics, we performed whole-genome bisulfite-treated sequencing and transcriptome sequencing in orchardgrass (Dactylis glomerata) during vernalization. The results revealed that increased levels of genome DNA methylation during the early vernalization of orchardgrass were associated with transcriptional changes in DNA methyltransferase and demethylase genes. Upregulated expression of vernalization-related genes during early vernalization was attributable to an increase in mCHH in the promoter regions of these genes. Application of an exogenous DNA methylation accelerator or overexpression of orchardgrass NUCLEAR POLY(A) POLYMERASE (DgPAPS4) promoted earlier flowering, indicating that DNA hypermethylation plays an important role in vernalization-induced flowering. Collectively, our findings revealed that vernalization-induced hypermethylation is responsible for floral primordium initiation and development. These observations provide a theoretical foundation for further studies on the molecular mechanisms underlying the control of vernalization in temperate grasses.

Wheat genomic study for genetic improvement of traits in China

Bread wheat (Triticum aestivum L.) is a major crop that feeds 40% of the world's population. Over the past several decades, advances in genomics have led to tremendous achievements in understanding the origin and domestication of wheat, and the genetic basis of agronomically important traits, which promote the breeding of elite varieties. In this review, we focus on progress that has been made in genomic research and genetic improvement of traits such as grain yield, end-use traits, flowering regulation, nutrient use efficiency, and biotic and abiotic stress responses, and various breeding strategies that contributed mainly by Chinese scientists. Functional genomic research in wheat is entering a new era with the availability of multiple reference wheat genome assemblies and the development of cutting-edge technologies such as precise genome editing tools, high-throughput phenotyping platforms, sequencing-based cloning strategies, high-efficiency genetic transformation systems, and speed-breeding facilities. These insights will further extend our understanding of the molecular mechanisms and regulatory networks underlying agronomic traits and facilitate the breeding process, ultimately contributing to more sustainable agriculture in China and throughout the world.

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.

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.

Moving Beyond DNA Sequence to Improve Plant Stress Responses

Plants offer a habitat for a range of interactions to occur among different stress factors. Epigenetics has become the most promising functional genomics tool, with huge potential for improving plant adaptation to biotic and abiotic stresses. Advances in plant molecular biology have dramatically changed our understanding of the molecular mechanisms that control these interactions, and plant epigenetics has attracted great interest in this context. Accumulating literature substantiates the crucial role of epigenetics in the diversity of plant responses that can be harnessed to accelerate the progress of crop improvement. However, harnessing epigenetics to its full potential will require a thorough understanding of the epigenetic modifications and assessing the functional relevance of these variants. The modern technologies of profiling and engineering plants at genome-wide scale provide new horizons to elucidate how epigenetic modifications occur in plants in response to stress conditions. This review summarizes recent progress on understanding the epigenetic regulation of plant stress responses, methods to detect genome-wide epigenetic modifications, and disentangling their contributions to plant phenotypes from other sources of variations. Key epigenetic mechanisms underlying stress memory are highlighted. Linking plant response with the patterns of epigenetic variations would help devise breeding strategies for improving crop performance under stressed scenarios.

The Importance of Networking: Plant Polycomb Repressive Complex 2 and Its Interactors

Polycomb Repressive Complex 2 (PRC2) is arguably the best-known plant complex of the Polycomb Group (PcG) pathway, formed by a group of proteins that epigenetically represses gene expression. PRC2-mediated deposition of H3K27me3 has amply been studied in Arabidopsis and, more recently, data from other plant model species has also been published, allowing for an increasing knowledge of PRC2 activities and target genes. How PRC2 molecular functions are regulated and how PRC2 is recruited to discrete chromatin regions are questions that have brought more attention in recent years. A mechanism to modulate PRC2-mediated activity is through its interaction with other protein partners or accessory proteins. Current evidence for PRC2 interactors has demonstrated the complexity of its protein network and how far we are from fully understanding the impact of these interactions on the activities of PRC2 core subunits and on the formation of new PRC2 versions. This review presents a list of PRC2 interactors, emphasizing their mechanistic action upon PRC2 functions and their effects on transcriptional regulation.

Polycomb Repressive Complex 2 in Eukaryotes-An Evolutionary Perspective

Polycomb repressive complex 2 (PRC2) represents a group of evolutionarily conserved multi-subunit complexes that repress gene transcription by introducing trimethylation of lysine 27 on histone 3 (H3K27me3). PRC2 activity is of key importance for cell identity specification and developmental phase transitions in animals and plants. The composition, biochemistry, and developmental function of PRC2 in animal and flowering plant model species are relatively well described. Recent evidence demonstrates the presence of PRC2 complexes in various eukaryotic supergroups, suggesting conservation of the complex and its function. Here, we provide an overview of the current understanding of PRC2-mediated repression in different representatives of eukaryotic supergroups with a focus on the green lineage. By comparison of PRC2 in different eukaryotes, we highlight the possible common and diverged features suggesting evolutionary implications and outline emerging questions and directions for future research of polycomb repression and its evolution.

Seed 'primeomics': plants memorize their germination under stress

Seed priming is a pre-germination treatment administered through various chemical, physical and biological agents, which induce mild stress during the early phases of germination. Priming facilitates synchronized seed germination, better seedling establishment, improved plant growth and enhanced yield, especially in stressful environments. In parallel, the phenomenon of 'stress memory' in which exposure to a sub-lethal stress leads to better responses to future or recurring lethal stresses has gained widespread attention in recent years. The versatility and realistic yield gains associated with seed priming and its connection with stress memory make a critical examination useful for the design of robust approaches for maximizing future yield gains. Herein, a literature review identified selenium, salicylic acid, poly-ethylene glycol, CaCl₂ and thiourea as the seed priming agents (SPRs) for which the most studies have been carried out. The average priming duration for SPRs generally ranged from 2 to 48 h, i.e. during phase I/II of germination. The major signalling events for regulating early seed germination, including the DOG1 (delay of germination 1)-abscisic acid (ABA)-heme regulatory module, ABA-gibberellic acid antagonism and nucleus-organelle communication are detailed. We propose that both seed priming and stress memory invoke a 'bet-hedging' strategy in plants, wherein their growth under optimal conditions is compromised in exchange for better growth under stressful conditions. The molecular basis of stress memory is explained at the level of chromatin reorganization, alternative transcript splicing, metabolite accumulation and autophagy. This provides a useful framework to study similar mechanisms operating during seed priming. In addition, we highlight the potential for merging findings on seed priming with those of stress memory, with the dual benefit of advancing fundamental research and boosting crop productivity. Finally, a roadmap for future work, entailing identification of SPR-responsive varieties and the development of dual/multiple-benefit SPRs, is proposed for enhancing SPR-mediated agricultural productivity worldwide.

Refinement of bamboo genome annotations through integrative analyses of transcriptomic and epigenomic data

Bamboo, one of the most crucial nontimber forest resources worldwide, has the capacity for rapid growth. In recent years, the genome of moso bamboo (Phyllostachys edulis) has been decoded, and a large amount of transcriptome data has been published. In this study, we generated the genome-wide profiles of the histone modification H3K4me3 in leaf, stem, and root tissues of bamboo. The trends in the distribution patterns were similar to those in rice. We developed a processing pipeline for predicting novel transcripts to refine the structural annotation of the genome using H3K4me3 ChIP-seq data and 29 RNA-seq datasets. As a result, 12,460 novel transcripts were predicted in the bamboo genome. Compared with the transcripts in the newly released version 2.0 of the bamboo genome, these novel transcripts are tissue-specific and shorter, and most have a single exon. Some representative novel transcripts were validated by semiquantitative RT-PCR and qRT-PCR analyses. Furthermore, we put these novel transcripts back into the ChIP-seq analysis pipeline and discovered that the percentages of H3K4me3 in genic elements were increased. Overall, this work integrated transcriptomic data and epigenomic data to refine the annotation of the genome in order to discover more functional genes and study bamboo growth and development, and the application of this predicted pipeline may help refine the structural annotation of the genome in other species.

Insights into the evo-devo of plant reproduction using next-generation sequencing approaches

The development of plant model organisms has traditionally been analyzed using resource-heavy, tailored applications that are not easily transferable to distantly related non-model taxa. Thus, our understanding of plant development has been limited to a subset of traits, and evolutionary studies conducted most effectively either across very wide [e.g. Arabidopsis thaliana and Oryza sativa (rice)] or narrow (i.e. population level) phylogenetic distances. As plant biologists seek to capitalize on natural diversity for crop improvement, enhance ecosystem functioning, and better understand plant responses to climate change, high-throughput and broadly applicable forms of existing molecular biology assays are becoming an invaluable resource. Next-generation sequencing (NGS) is increasingly becoming a powerful tool in evolutionary developmental biology (evo-devo) studies, particularly through its application to understanding trait evolution at different levels of gene regulation. Here, I review some of the most common and emerging NGS-based methods, using exemplar studies in reproductive plant evo-devo to illustrate their potential.

Transcriptional memories mediate the plasticity of cold stress responses to enable morphological acclimation in Brachypodium distachyon

Plants that successfully acclimate to stress can resume growth under stressful conditions. The grass Brachypodium distachyon can grow a cold-adaptive morphology during cold acclimation. Studies on transcriptional memory (TM) have revealed that plants can be primed for stress by adjusting their transcriptional responses, but the function of TM in stress acclimation is not well understood. We investigated the function of TM during cold acclimation in B. distachyon. Quantitative polymerase chain reaction (qPCR), RNA-seq and chromatin immunoprecipitation qPCR analyses were performed on plants exposed to repeated episodes of cold to characterize the presence and stability of TM during the stress and growth responses of cold acclimation. Transcriptional memory mainly dampened stress responses as growth resumed and as B. distachyon became habituated to cold stress. Although permanent on vernalization gene VRN1, TMs were short-term and reversible on cold-stress genes. Growing under cold conditions also coincided with the acquisition of new and targeted cold-induced transcriptional responses. Overall, TM provided plasticity to cold stress responses during cold acclimation in B. distachyon, leading to stress habituation, acquired stress responses, and resumed growth. Our study shows that chromatin-associated TMs are involved in tuning plant responses to environmental change and, as such, regulate both stress and developmental components that characterize cold-climate adaptation in B. distachyon.

Histone H3K27 dimethylation landscapes contribute to genome stability and genetic recombination during wheat polyploidization

Bread wheat (Triticum aestivum) is an allohexaploid that was formed via two allopolyploidization events. Growing evidence suggests histone modifications are involved in the response to 'genomic shock' and environmental adaptation during polyploid formation and evolution. However, the role of histone modifications, especially histone H3 lysine-27 dimethylation (H3K27me2), in genome evolution remains elusive. Here we analyzed H3K27me2 and H3K27me3 profiles in hexaploid wheat and its tetraploid and diploid relatives. Although H3K27me3 levels were relatively stable among wheat species with different ploidy levels, H3K27me2 intensities increased concurrent with increased ploidy levels, and H3K27me2 peaks were colocalized with massively amplified DTC transposons (CACTA family) in euchromatin, which may silence euchromatic transposons to maintain genome stability during polyploid wheat evolution. Consistently, the distribution of H3K27me2 is mutually exclusive with another repressive histone mark, H3K9me2, that mainly silences transposons in heterochromatic regions. Remarkably, the regions with low H3K27me2 levels (named H3K27me2 valleys) were associated with the formation of DNA double-strand breaks in genomes of wheat, maize (Zea mays) and Arabidopsis. Our results provide a comprehensive view of H3K27me2 and H3K27me3 distributions during wheat evolution, which support roles for H3K27me2 in silencing euchromatic transposons to maintain genome stability and in modifying genetic recombination landscapes. These genomic insights may empower breeding improvement of crops.

The molecular mechanism of vernalization in Arabidopsis and cereals: role of Flowering Locus C and its homologs

Winter varieties of plants can flower only after exposure to prolonged cold. This phenomenon is known as vernalization and has been widely studied in the model plant Arabidopsis thaliana as well as in monocots. Through the repression of floral activator genes, vernalization prevents flowering in winter. In Arabidopsis, FLOWERING LOCUS C or FLC is the key repressor during vernalization, while in monocots vernalization is regulated through VRN1, VRN2 and VRN3 (or FLOWERING LOCUS T). Interestingly, VRN genes are not homologous to FLC but FLC homologs are found to have a significant role in vernalization response in cereals. The presence of FLC homologs in monocots opens new dimensions to understand, compare and retrace the evolution of vernalization pathways between monocots and dicots. In this review, we discuss the molecular mechanism of vernalization-induced flowering along with epigenetic regulations in Arabidopsis and temperate cereals. A better understanding of cold-induced flowering will be helpful in crop breeding strategies to modify the vernalization requirement of economically important temperate cereals.

Genome-wide analysis of H3K4me3 and H3K27me3 modifications due to Lr28 for leaf rust resistance in bread wheat (Triticum aestivum)

Present study revealed a complex relationship among histone H3 methylation (examined using H3K4/K27me3 marks), cytosine DNA methylation and differential gene expression during Lr28 mediated leaf rust resistance in wheat. During the present study, genome-wide histone modifications were examined in a pair of near isogenic lines (NILs) (with and without Lr28 in the background of cv. HD2329). The two histone marks used included H3K4me3 (an activation mark) and H3K27me3 (a repression mark). The results were compared with levels of expression (using RNA-seq) and DNA methylation (MeDIP) data obtained using the same pair of NILs. Some of the salient features of the present study include the following: (i) large scale differential binding sites (DBS) were available for only H3K4me3 in the susceptible cultivar, but for both H3K4me3 and H3K27me3 in its resistant NIL; (ii) DBSs for H3K27me3 mark were more abundant (> 80%) in intergenic regions, whereas DBSs for H3K4me3 were distributed in all genomic regions including exons, introns, intergenic, TTS (transcription termination sites) and promoters; (iii) fourteen (14) genes associated with DBSs showed co-localization for both the marks; (iv) only a small fraction (7% for H3K4me3 and 12% for H3K27me3) of genes associated with DBSs matched with the levels of gene expression inferred from RNA-seq data; (v) validation studies using qRT-PCR were conducted on 26 selected representative genes; results for only 11 genes could be validated. The proteins encoded by important genes involved in promoting infection included domains generally carried by R gene proteins such as Mlo like protein, protein kinases and purple acid phosphatase. Similarly, proteins encoded by genes involved in resistance included those carrying domains for lectin kinase, R gene, aspartyl protease, etc. Overall, the results suggest a very complex network of downstream genes that are expressed during compatible and incompatible interactions; some of the genes identified during the present study may be used in future validation studies involving RNAi/overexpression approaches.

Chromatin regulation of somatic abiotic stress memory

In nature, plants are often subjected to periods of recurrent environmental stress that can strongly affect their development and productivity. To cope with these conditions, plants can remember a previous stress, which allows them to respond more efficiently to a subsequent stress, a phenomenon known as priming. This ability can be maintained at the somatic level for a few days or weeks after the stress is perceived, suggesting that plants can store information of a past stress during this recovery phase. While the immediate responses to a single stress event have been extensively studied, knowledge on priming effects and how stress memory is stored is still scarce. At the molecular level, memory of a past condition often involves changes in chromatin structure and organization, which may be maintained independently from transcription. In this review, we will summarize the most recent developments in the field and discuss how different levels of chromatin regulation contribute to priming and plant abiotic stress memory.

Molecular Regulation of Plant Responses to Environmental Temperatures

Temperature is a key factor governing the growth and development, distribution, and seasonal behavior of plants. The entire plant life cycle is affected by environmental temperatures. Plants grow rapidly and exhibit specific changes in morphology under mild average temperature conditions, a response termed thermomorphogenesis. When exposed to chilling or moist chilling low temperatures, flowering or seed germination is accelerated in some plant species; these processes are known as vernalization and cold stratification, respectively. Interestingly, once many temperate plants are exposed to chilling temperatures for some time, they can acquire the ability to resist freezing stress, a process termed cold acclimation. In the face of global climate change, heat stress has emerged as a frequent challenge, which adversely affects plant growth and development. In this review, we summarize and discuss recent progress in dissecting the molecular mechanisms regulating plant thermomorphogenesis, vernalization, and responses to extreme temperatures. We also discuss the remaining issues that are crucial for understanding the interactions between plants and temperature.

Enhanced Relative Electron Transport Rate Contributes to Increased Photosynthetic Capacity in Autotetraploid Pak Choi

Autopolyploids often show growth advantages over their diploid progenitors because of their increased photosynthetic activity; however, the underlying molecular basis of such mechanism remains elusive. In this study, we aimed to characterize autotetraploid pak choi (Brassica rapa ssp. chinensis) at the physiological, cellular and molecular levels. Autotetraploid pak choi has thicker leaves than its diploid counterparts, with relatively larger intercellular spaces and cell size and greater grana thylakoid height. Photosynthetic data showed that the relative electron transport rate (rETR) was markedly higher in autotetraploid than in diploid pak choi. Transcriptomic data revealed that the expressions of genes involved in 'photosynthesis' biological process and 'thylakoids' cellular component were mainly regulated in autotetraploids. Overall, our findings suggested that the increased rETR in the thylakoids contributed to the increased photosynthetic capacity of autotetraploid leaves. Furthermore, we found that the enhanced rETR is associated with increased BrPetC expression, which is likely altered by histone modification. The ectopic expression of BrPetC in Arabidopsis thaliana led to increased rETR and biomass, which were decreased in BrPetC-silenced pak choi. Autotetraploid pak choi also shows altered hormone levels, which was likely responsible for the increased drought resistance and the impaired powdery mildew resistance of this lineage. Our findings further our understanding on how autotetraploidy provides growth advantages to plants.

Current research frontiers in plant epigenetics: an introduction to a Virtual Issue

http://www.newphytologist.com/virtualissues

Epigenetic signatures of stress adaptation and flowering regulation in response to extended drought and recovery in Zea mays

During their lifespan, plants respond to a multitude of stressful factors. Dynamic changes in chromatin and concomitant transcriptional variations control stress response and adaptation, with epigenetic memory mechanisms integrating environmental conditions and appropriate developmental programs over the time. Here we analyzed transcriptome and genome-wide histone modifications of maize plants subjected to a mild and prolonged drought stress just before the flowering transition. Stress was followed by a complete recovery period to evaluate drought memory mechanisms. Three categories of stress-memory genes were identified: i) "transcriptional memory" genes, with stable transcriptional changes persisting after the recovery; ii) "epigenetic memory candidate" genes in which stress-induced chromatin changes persist longer than the stimulus, in absence of transcriptional changes; iii) "delayed memory" genes, not immediately affected by the stress, but perceiving and storing stress signal for a delayed response. This last memory mechanism is described for the first time in drought response. In addition, applied drought stress altered floral patterning, possibly by affecting expression and chromatin of flowering regulatory genes. Altogether, we provided a genome-wide map of the coordination between genes and chromatin marks utilized by plants to adapt to a stressful environment, describing how this serves as a backbone for setting stress memory.

Experiencing winter for spring flowering: A molecular epigenetic perspective on vernalization

Many over-wintering plants, through vernalization, overcome a block to flowering and thus acquire competence to flower in the following spring after experiencing prolonged cold exposure or winter cold. The vernalization pathways in different angiosperm lineages appear to have convergently evolved to adapt to temperate climates. Molecular and epigenetic mechanisms for vernalization regulation have been well studied in the crucifer model plant Arabidopsis thaliana. Here, we review recent progresses on the vernalization pathway in Arabidopsis. In addition, we summarize current molecular and genetic understandings of vernalization regulation in temperate grasses including wheat and Brachypodium, two monocots from Pooideae, followed by a brief discussion on divergence of the vernalization pathways between Brassicaceae and Pooideae.

The Role of FLOWERING LOCUS C Relatives in Cereals

FLOWERING LOCUS C (FLC) is one of the best characterized genes in plant research and is integral to vernalization-dependent flowering time regulation. Yet, despite the abundance of information on this gene and its relatives in Arabidopsis thaliana, the role FLC genes play in other species, in particular cereal crops and temperate grasses, remains elusive. This has been due in part to the comparative reduced availability of bioinformatic and mutant resources in cereals but also on the dominant effect in cereals of the VERNALIZATION (VRN) genes on the developmental process most associated with FLC in Arabidopsis. The strong effect of the VRN genes has led researchers to believe that the entire process of vernalization must have evolved separately in Arabidopsis and cereals. Yet, since the confirmation of the existence of FLC-like genes in monocots, new light has been shed on the roles these genes play in both vernalization and other mechanisms to fine tune development in response to specific environmental conditions. Comparisons of FLC gene function and their genetic and epigenetic regulation can now be made between Arabidopsis and cereals and how they overlap and diversify is coming into focus. With the advancement of genome editing techniques, further study on these genes is becoming increasingly easier, enabling us to investigate just how essential FLC-like genes are to modulating flowering time behavior in cereals.

Genome-wide analysis of the H3K27me3 epigenome and transcriptome in Brassica rapa

BACKGROUND

Genome-wide maps of histone modifications have been obtained for several plant species. However, most studies focus on model systems and do not enforce FAIR data management principles. Here we study the H3K27me3 epigenome and associated transcriptome of Brassica rapa, an important vegetable cultivated worldwide.

FINDINGS

We performed H3K27me3 chromatin immunoprecipitation followed by high-throughput sequencing and transcriptomic analysis by 3'-end RNA sequencing from B. rapa leaves and inflorescences. To analyze these data we developed a Reproducible Epigenomic Analysis pipeline using Galaxy and Jupyter, packaged into Docker images to facilitate transparency and reuse. We found that H3K27me3 covers roughly one-third of all B. rapa protein-coding genes and its presence correlates with low transcript levels. The comparative analysis between leaves and inflorescences suggested that the expression of various floral regulatory genes during development depends on H3K27me3. To demonstrate the importance of H3K27me3 for B. rapa development, we characterized a mutant line deficient in the H3K27 methyltransferase activity. We found that braA.clf mutant plants presented pleiotropic alterations, e.g., curly leaves due to increased expression and reduced H3K27me3 levels at AGAMOUS-like loci.

CONCLUSIONS

We characterized the epigenetic mark H3K27me3 at genome-wide levels and provide genetic evidence for its relevance in B. rapa development. Our work reveals the epigenomic landscape of H3K27me3 in B. rapa and provides novel genomics datasets and bioinformatics analytical resources. We anticipate that this work will lead the way to further epigenomic studies in the complex genome of Brassica crops.

Crop Improvement Through Temperature Resilience

Abnormal environmental temperature affects plant growth and threatens crop production. Understanding temperature signal sensing and the balance between defense and development in plants lays the foundation for improvement of temperature resilience. Here, we summarize the current understanding of cold signal perception/transduction as well as heat stress response. Dissection of plant responses to different levels of cold stresses (chilling and freezing) illustrates their common and distinct signaling pathways. Axillary bud differentiation in response to chilling is presented as an example of the trade-off between defense and development. Vernalization is a cold-dependent development adjustment mediated by O-GlcNAcylation and phosphorylation to sense long-term cold. Recent progress on major quantitative trait loci genes for heat tolerance has been summarized. Molecular mechanisms in utilizing temperature-sensitive sterility in super hybrid breeding in China are revealed. The way to improve crop temperature resilience using integrative knowledge of omics as well as systemic and synthetic biology, especially the molecular module program, is summarized.

VERNALIZATION1 controls developmental responses of winter wheat under high ambient temperatures

Low temperatures are required to regulate the transition from vegetative to reproductive growth via a pathway called vernalization. In wheat, vernalization predominantly involves the cold upregulation of the floral activator VERNALIZATION1 (VRN1). Here, we have used an extreme vernalization response, identified through studying ambient temperature responses, to reveal the complexity of temperature inputs into VRN-A1, with allelic inter-copy variation at a gene expansion of VRN-A1 modulating these effects. We find that the repressors of the reproductive transition, VERNALIZATION2 (VRN2) and ODDSOC2, are re-activated when plants experience high temperatures during and after vernalization. In addition, this re-activation is regulated by photoperiod for VRN2 but was independent of photoperiod for ODDSOC2 We also find this warm temperature interruption affects flowering time and floret number and is stage specific. This research highlights the important balance between floral activators and repressors in coordinating the response of a plant to temperature, and that the absence of warmth is essential for the completion of vernalization. This knowledge can be used to develop agricultural germplasm with more predictable vernalization responses that will be more resilient to variable growth temperatures.

Remembering winter through vernalisation

Vernalisation is the programmed physiological process in which prolonged cold-exposure provides competency to flower in plants; widely found in winter and biennial species, such as Arabidopsis, fruit trees, vegetables and wheat. This phenomenon is regulated by diverse genetic networks, and memory of vernalisation in a life cycle mainly depends on epigenetic mechanisms. However, less is known about how to count winter-dosage for flowering in plants. Here, we compare the vernalisation genetic framework between the dicots Arabidopsis, temperate grasses, wheat, barley and Brachypodium. We discuss vernalisation mechanisms involving crosstalk between phosphorylation and O-GlcNAcylation modification of key proteins, and epigenetic modifications of the key gene VRN1 in wheat. We also highlight the potential evolutionary origins of vernalisation in various species. Current progress toward understanding the regulation of vernalisation requirements provides insight that will inform the design of molecular breeding strategies for winter crops.