3D Chromatin Architecture of Large Plant Genomes Determined by Local A/B Compartments

Epigenetics in the modern era of crop improvements

Epigenetic mechanisms are integral to plant growth, development, and adaptation to environmental stimuli. Over the past two decades, our comprehension of these complex regulatory processes has expanded remarkably, producing a substantial body of knowledge on both locus-specific mechanisms and genome-wide regulatory patterns. Studies initially grounded in the model plant Arabidopsis have been broadened to encompass a diverse array of crop species, revealing the multifaceted roles of epigenetics in physiological and agronomic traits. With recent technological advancements, epigenetic regulations at the single-cell level and at the large-scale population level are emerging as new focuses. This review offers an in-depth synthesis of the diverse epigenetic regulations, detailing the catalytic machinery and regulatory functions. It delves into the intricate interplay among various epigenetic elements and their collective influence on the modulation of crop traits. Furthermore, it examines recent breakthroughs in technologies for epigenetic modifications and their integration into strategies for crop improvement. The review underscores the transformative potential of epigenetic strategies in bolstering crop performance, advocating for the development of efficient tools to fully exploit the agricultural benefits of epigenetic insights.

Cohesin organizes 3D DNA contacts surrounding active enhancers in C. elegans

In mammals, cohesin and CTCF organize the 3D genome into topologically associating domains (TADs) to regulate communication between cis-regulatory elements. Many organisms, including S. cerevisiae, C. elegans, and A. thaliana contain cohesin but lack CTCF. Here, we used C. elegans to investigate the function of cohesin in 3D genome organization in the absence of CTCF. Using Hi-C data, we observe cohesin-dependent features called "fountains," which have also been reported in zebrafish and mice. These are population average reflections of DNA loops originating from distinct genomic regions and are ∼20-40 kb in C. elegans Hi-C analysis upon cohesin and WAPL-1 depletion supports the idea that cohesin is preferentially loaded at sites bound by the C. elegans ortholog of NIPBL and loop extrudes in an effectively two-sided manner. ChIP-seq analyses show that cohesin translocation along the fountain trajectory depends on a fully intact complex and is extended upon WAPL-1 depletion. Hi-C contact patterns at individual fountains suggest that cohesin processivity is unequal on each side, possibly owing to collision with cohesin loaded from surrounding sites. The putative cohesin loading sites are closest to active enhancers, and fountain strength is associated with transcription. Compared with mammals, the average processivity of C. elegans cohesin is about 10-fold shorter, and the binding of NIPBL ortholog does not depend on cohesin. We propose that preferential loading and loop extrusion by cohesin is an evolutionarily conserved mechanism that regulates the 3D interactions of enhancers in animal genomes.

Topologically associating domains and the evolution of three-dimensional genome architecture in rice

We examined the nature and evolution of three-dimensional (3D) genome conformation, including topologically associating domains (TADs), in five genomes within the genus Oryza. These included three varieties from subspecies within domesticated Asian rice O. sativa as well as their closely related wild relatives O. rufipogon and O. meridionalis. We used the high-resolution chromosome conformation capture technique Micro-C, which we modified for use in rice. Our analysis of rice TADs shows that TAD boundaries have high transcriptional activity, low methylation levels, low transposable element (TE) content, and increased gene density. We also find a significant correlation of expression levels for genes within TADs, suggesting that they do function as genomic domains with shared regulatory features. Our findings indicate that animal and plant TADs may share more commonalities than were initially thought, as evidenced by similar genetic and epigenetic signatures associated with TADs and boundaries. To examine 3D genome divergence, we employed a computer vision-based algorithm for the comparison of chromatin contact maps and complemented this analysis by assessing the evolutionary conservation of individual TADs and their boundaries. We conclude that overall chromatin organization is conserved in rice, and 3D structural divergence correlates with evolutionary distance between genomes. We also note that individual TADs are not well conserved, even at short evolutionary timescales.

The Impact of Polycomb Group Proteins on 3D Chromatin Structure and Environmental Stresses in Plants

The two multi-subunit complexes, Polycomb Repressive Complex 1 and 2 (PRC1/2), act synergistically during development to maintain the gene silencing state among different species. In contrast with mammals and Drosophila melanogaster, the enzyme activities and components of the PRC1 complex in plants are not fully conserved. In addition, the mutual recruitment of PRC1 and PRC2 in plants differs from that observed in mammals and Drosophila. Polycomb Group (PcG) proteins and their catalytic activity play an indispensable role in transcriptional regulation, developmental processes, and the maintenance of cellular identity. In plants, PRC1 and PRC2 deposit H2Aub and H3K27me3, respectively, and also play an important role in influencing three-dimensional (3D) chromatin structure. With the development of high-throughput sequencing techniques and computational biology, remarkable progress has been made in the field of plant 3D chromatin structure, and PcG has been found to be involved in the epigenetic regulation of gene expression by mediating the formation of 3D chromatin structures. At the same time, some genetic evidence indicates that PcG enables plants to better adapt to and resist a wide range of stresses by dynamically regulating gene expression. In the following review, we focus on the recruitment relationship between PRC1 and PRC2, the crucial role of PcG enzyme activity, the effect of PcG on 3D chromatin structure, and the vital role of PcG in environmental stress in plants.

Constraint of accessible chromatins maps regulatory loci involved in maize speciation and domestication

Comparative genomic studies can identify genes under evolutionary constraint or specialized for trait innovation. Growing evidence suggests that evolutionary constraint also acts on non-coding regulatory sequences, exerting significant impacts on fitness-related traits, although it has yet to be thoroughly explored in plants. Using the assay for transposase-accessible chromatin by sequencing (ATAC-seq), we profile over 80,000 maize accessible chromatin regions (ACRs), revealing that ACRs evolve faster than coding genes, with about one-third being maize-specific and regulating genes associated with speciation. We highlight the role of transposable elements (TEs) in driving intraspecific innovation of ACRs and identify hundreds of candidate ACRs potentially involved in transcriptional rewiring during maize domestication. Additionally, we demonstrate the importance of accessible chromatin in maintaining subgenome dominance and controlling complex trait variations. This study establishes a framework for analyzing the evolutionary trajectory of plant regulatory sequences and offers candidate loci for downstream exploration and application in maize breeding.

Subfunctionalization and epigenetic regulation of a biosynthetic gene cluster in Solanaceae

Biosynthetic gene clusters (BGCs) are sets of often heterologous genes that are genetically and functionally linked. Among eukaryotes, BGCs are most common in plants and fungi and ensure the coexpression of the different enzymes coordinating the biosynthesis of specialized metabolites. Here, we report the identification of a withanolide BGC in Physalis grisea (ground-cherry), a member of the nightshade family (Solanaceae). A combination of transcriptomic, epigenomic, and metabolic analyses revealed that, following a duplication event, this BGC evolved two tissue-specifically expressed subclusters, containing several pairs of paralogs that contribute to related but distinct biochemical processes; this subfunctionalization is tightly associated with epigenetic features and the local chromatin environment. The two subclusters appear strictly isolated from each other at the structural chromatin level, each forming a highly self-interacting chromatin domain with tissue-dependent levels of condensation. This correlates with gene expression in either above- or below-ground tissue, thus spatially separating the production of different withanolide compounds. By comparative phylogenomics, we show that the withanolide BGC most likely evolved before the diversification of the Solanaceae family and underwent lineage-specific diversifications and losses. The tissue-specific subfunctionalization is common to species of the Physalideae tribe but distinct from other, independent duplication events outside of this clade. In sum, our study reports on an instance of an epigenetically modulated subfunctionalization within a BGC and sheds light on the biosynthesis of withanolides, a highly diverse group of steroidal triterpenoids important in plant defense and amenable to pharmaceutical applications due to their anti-inflammatory, antibiotic, and anticancer properties.

Chromatin Topological Domains Associate With the Rapid Formation of Tandem Duplicates in Plants

In eukaryotes, chromatin is compacted within nuclei under the principle of compartmentalization. On top of that, condensin II establishes eukaryotic chromosome territories, while cohesin organizes the vertebrate genome by extruding chromatin loops and forming topologically associating domains (TADs). Thus far, the formation and roles of these chromatin structures in plants remain poorly understood. This study integrates Hi-C data from diverse plant species, demonstrating that nuclear DNA content influences large-scale chromosome conformation and affects the finer details of compartmental patterns. These contrasting compartmental patterns are distinguished by gene-to-gene loops and validated through cytological observations. Additionally, a novel chromatin domain type associated with tandem duplicate gene clusters is identified. These domains are independent of H3K27me3-mediated chromatin compartmentalization and exhibit evolutionary conservation across species. Gene pairs within TAD-like domains are younger and show higher levels of coexpression. These domains potentially promote the formation of tandem duplicates, a property appears unique to the Actinidia family. Overall, this study reveals functional chromatin domains in plants and provides evidence for the role of three-dimensional chromatin architecture in gene regulation and genome evolution.

Hi-GDT: A Hi-C-based 3D gene domain analysis tool for analyzing local chromatin contacts in plants

BACKGROUND

Three-dimensional (3D) chromatin organization is emerging as a key factor in gene regulation in eukaryotes. Recent studies using high-resolution Hi-C analysis have explored fine-scale local chromatin contact domains in plants, as exemplified by the basic contact domains established at accessible gene border regions in Arabidopsis (Arabidopsis thaliana). However, we lack effective tools to identify these contact domains and examine their structural dynamics.

RESULTS

We developed the Hi-C-based 3D Gene Domain analysis Tool (Hi-GDT) to identify fine-scale local chromatin contact domains in plants, with a particular focus on gene borders. Hi-GDT successfully identifies local contact domains, including single-gene and multigene domains, with high reproducibility. Hi-GDT can also be used to discover local contact domains that are differentially organized in association with differences in gene expression between tissue types, genotypes, or in response to environmental stimuli.

CONCLUSIONS

Hi-GDT is a valuable tool for identifying genes regulated by dynamic 3D conformational changes, expanding our understanding of the structural and functional relevance of local 3D chromatin organization in plants. Hi-GDT is publicly available at https://github.com/CDL-HongwooLee/Hi-GDT.

Nuclear Structure, Size Regulation, and Role in Cell Migration

The nucleus serves as a pivotal regulatory and control hub in the cell, governing numerous aspects of cellular functions, including DNA replication, transcription, and RNA processing. Therefore, any deviations in nuclear morphology, structure, or organization can strongly affect cellular activities. In this review, we provide an updated perspective on the structure and function of nuclear components, focusing on the linker of nucleoskeleton and cytoskeleton complex, the nuclear envelope, the nuclear lamina, and chromatin. Additionally, nuclear size should be considered a fundamental parameter for the cellular state. Its regulation is tightly linked to environmental changes, development, and various diseases, including cancer. Hence, we also provide a concise overview of different mechanisms by which nuclear size is determined, the emerging role of the nucleus as a mechanical sensor, and the implications of altered nuclear morphology on the physiology of diseased cells.

EMF1 functions as a 3D chromatin modulator in Arabidopsis

It is well known that genome organizers, like mammalian CCCTC-binding factor (CTCF) or Drosophila architectural proteins CP190 and BEAF-32, contribute to the three-dimensional (3D) organization of the genome and ensure normal gene transcription. However, bona fide genome organizers have not been identified in plants. Here, we show that EMBRYONIC FLOWER1 (EMF1) functions as a genome modulator in Arabidopsis. EMF1 interacts with the cohesin component SISTER CHROMATIN COHESION3 (SCC3), and both proteins are enriched at compartment domain (CD) boundaries. Accordingly, emf1 and scc3 show a strength decrease at the CD boundary in which these proteins colocalize. EMF1 maintains CD boundary strength, either independently or in cooperation with histone modifications. Moreover, EMF1 is required to maintain gene-resolution interactions and to block long-range aberrant chromatin loops. These data unveil a key role of EMF1 in regulating 3D chromatin structure.

A method for chromatin domain partitioning based on hypergraph clustering

For many years, multi-scale models of chromatin domains, such as A/B compartments, sub-compartments, topologically associated domains (TADs), sub-TADs, and loops have been popular. However, existing methods can only identify structures at a single scale and cannot partition multi-scale structures. In this paper, we proposed a method (TORNADOES) for chromatin domain partitioning based on hypergraph clustering. First, we use a density clustering algorithm to identify TADs at different scales based on Hi-C data with different resolutions. Then, by combining ChIP-seq data features and TAD results at different scales, we generate a hypergraph based on these TADs. Finally, we partition the chromatin domain structure at different scales, including A/B, A1, A2, B1, B2, and B3 based on the Laplacian matrix feature of the hypergraph. Similarity comparison experiments and ChIP-seq signal enrichment analysis are performed on the A/B region and sub-TAD levels, respectively, demonstrating that our method can identify chromatin domains with distinct features and provide a deeper understanding of the organizational patterns and functional differences in TADs at the genomic hierarchical structure. Comparative analysis of multiple cell line data shows that TORNADOES can better classify different numbers and types of compartments by changing the factors ChIP-seq data and clustering number used to characterize TAD compared to other methods. Source code for the TORNADOES method can be found at https://github.com/ghaiyan/TORNADOES.

Evolution and function of chromatin domains across the tree of life

The genome of all organisms is spatially organized to function efficiently. The advent of genome-wide chromatin conformation capture (Hi-C) methods has revolutionized our ability to probe the three-dimensional (3D) organization of genomes across diverse species. In this Review, we compare 3D chromatin folding from bacteria and archaea to that in mammals and plants, focusing on topology at the level of gene regulatory domains. In doing so, we consider systematic similarities and differences that hint at the origin and evolution of spatial chromatin folding and its relation to gene activity. We discuss the universality of spatial chromatin domains in all kingdoms, each encompassing one to several genes. We also highlight differences between organisms and suggest that similar features in Hi-C matrices do not necessarily reflect the same biological process or function. Furthermore, we discuss the evolution of domain boundaries and boundary-forming proteins, which indicates that structural maintenance of chromosome (SMC) proteins and the transcription machinery are the ancestral sculptors of the genome. Architectural proteins such as CTCF serve as clade-specific determinants of genome organization. Finally, studies in many non-model organisms show that, despite the ancient origin of 3D chromatin folding and its intricate link to gene activity, evolution tolerates substantial changes in genome organization.

3D chromatin maps of a brown alga reveal U/V sex chromosome spatial organization

Nuclear three dimensional (3D) folding of chromatin structure has been linked to gene expression regulation and correct developmental programs, but little is known about the 3D architecture of sex chromosomes within the nucleus, and how that impacts their role in sex determination. Here, we determine the sex-specific 3D organization of the model brown alga Ectocarpus chromosomes at 2 kb resolution, by mapping long-range chromosomal interactions using Hi-C coupled with Oxford Nanopore long reads. We report that Ectocarpus interphase chromatin exhibits a non-Rabl conformation, with strong contacts among telomeres and among centromeres, which feature centromere-specific LTR retrotransposons. The Ectocarpus chromosomes do not contain large local interactive domains that resemble TADs described in animals, but their 3D genome organization is largely shaped by post-translational modifications of histone proteins. We show that the sex determining region (SDR) within the U and V chromosomes are insulated and span the centromeres and we link sex-specific chromatin dynamics and gene expression levels to the 3D chromatin structure of the U and V chromosomes. Finally, we uncover the unique conformation of a large genomic region on chromosome 6 harboring an endogenous viral element, providing insights regarding the impact of a latent giant dsDNA virus on the host genome's 3D chromosomal folding.

Global nuclear reorganization during heterochromatin replication in the giant-genome plant Nigella damascena L

Among flowering plants, genome size varies remarkably, by >2200-fold, and this variation depends on the loss and gain of noncoding DNA sequences that form distinct heterochromatin complexes during interphase. In plants with giant genomes, most chromatin remains condensed during interphase, forming a dense network of heterochromatin threads called interphase chromonemata. Using super-resolution light and electron microscopy, we studied the ultrastructure of chromonemata during and after replication in root meristem nuclei of Nigella damascena L. During S-phase, heterochromatin undergoes transient decondensation locally at DNA replication sites. Due to the abundance of heterochromatin, the replication leads to a robust disassembly of the chromonema meshwork and a general reorganization of the nuclear morphology visible even by conventional light microscopy. After replication, heterochromatin recondenses, restoring the chromonema structure. Thus, we show that heterochromatin replication in interphase nuclei of giant-genome plants induces a global nuclear reorganization.

The chromatin tapestry as a framework for neurodevelopment

The neuronal nucleus houses a meticulously organized genome. Within this structure, genetic material is not simply compacted but arranged into a precise and functional 3D chromatin landscape essential for cellular regulation. This mini-review highlights the importance of this chromatin landscape in healthy neurodevelopment, as well as the diseases that occur with aberrant chromatin architecture. We discuss insights into the fundamental mechanistic relationship between histone modifications, DNA methylation, and genome organization. We then discuss findings that reveal how these epigenetic features change throughout normal neurodevelopment. Finally, we highlight single-gene neurodevelopmental disorders that illustrate the interdependence of epigenetic features, showing how disruptions in DNA methylation or genome architecture can ripple across the entire epigenome. As such, we emphasize the importance of measuring multiple chromatin architectural aspects, as the disruption of one mechanism can likely impact others in the intricate epigenetic network. This mini-review underscores the vast gaps in our understanding of chromatin structure in neurodevelopmental diseases and the substantial research needed to understand the interplay between chromatin features and neurodevelopment.

Drought-responsive dynamics of H3K9ac-marked 3D chromatin interactions are integrated by OsbZIP23-associated super-enhancer-like promoter regions in rice

BACKGROUND

In response to drought stress (DS), plants undergo complex processes that entail significant transcriptome reprogramming. However, the intricate relationship between the dynamic alterations in the three-dimensional (3D) genome and the modulation of gene co-expression in drought responses remains a relatively unexplored area.

RESULTS

In this study, we reconstruct high-resolution 3D genome maps based on genomic regions marked by H3K9ac, an active histone modification that dynamically responds to soil water variations in rice. We discover a genome-wide disconnection of 3D genome contact upon DS with over 10,000 chromatin loops lost, which are partially recovered in the subsequent re-watering. Loops integrating promoter-promoter interactions (PPI) contribute to gene expression in addition to basal H3K9ac modifications. Moreover, H3K9ac-marked promoter regions with high affinities in mediating PPIs, termed as super-promoter regions (SPRs), integrate spatially clustered PPIs in a super-enhancer-like manner. Interestingly, the knockout mutation of OsbZIP23, a well-defined DS-responsive transcription factor, leads to the disassociation of over 80% DS-specific PPIs and decreased expression of the corresponding genes under DS. As a case study, we show how OsbZIP23 integrates the PPI cluster formation and the co-expression of four dehydrin genes, RAB16A-D, through targeting the RAB16C SPR in a stress signaling-dependent manner.

CONCLUSIONS

Our high-resolution 3D genome maps unveil the principles and details of dynamic genome folding in response to water supply variations and illustrate OsbZIP23 as an indispensable integrator of the yet unique 3D genome organization that is essential for gene co-expression under DS in rice.

The MVPs (masterful versatile players): Chromatin factors as pivotal mediators between 3D genome organization and the response to environment

In recent years, the study of genome dynamics has become a prominent research field due to its influence on understanding the control of gene expression. The study of 3D genome organization has unveiled multiple mechanisms in orchestrating chromosome folding. Growing evidence reveals that these mechanisms are not only important for genome organization, but play a pivotal role in enabling plants to adapt to environmental stimuli. In this review, we provide an overview of the current knowledge concerning epigenetic factors and regulatory elements driving 3D genome dynamics and their responses to external stimuli. We discuss the most recent findings, previous evidence, and explore their implications for future research.

Cohesin organizes 3D DNA contacts surrounding active enhancers in C. elegans

In mammals, cohesin and CTCF organize the 3D genome into topologically associated domains (TADs) to regulate communication between cis-regulatory elements. Many organisms, including S. cerevisiae, C. elegans, and A. thaliana contain cohesin but lack CTCF. Here, we used C. elegans to investigate the function of cohesin in 3D genome organization in the absence of CTCF. Using Hi-C data, we observe cohesin-dependent features called "fountains", which are also reported in zebrafish and mice. These are population average reflections of DNA loops originating from distinct genomic regions and are ~20-40 kb in C. elegans. Hi-C analysis upon cohesin and WAPL depletion support the idea that cohesin is preferentially loaded at NIPBL occupied sites and loop extrudes in an effectively two-sided manner. ChIP-seq analyses show that cohesin translocation along the fountain trajectory depends on a fully intact complex and is extended upon WAPL-1 depletion. Hi-C contact patterns at individual fountains suggest that cohesin processivity is unequal on each side, possibly due to collision with cohesin loaded from surrounding sites. The putative cohesin loading sites are closest to active enhancers and fountain strength is associated with transcription. Compared to mammals, average processivity of C. elegans cohesin is ~10-fold shorter and NIPBL binding does not depend on cohesin. We propose that preferential loading and loop extrusion by cohesin is an evolutionarily conserved mechanism that regulates the 3D interactions of enhancers in animal genomes.

Genetic-epigenetic interplay in the determination of plant 3D genome organization

The 3D chromatin organization plays a major role in the control of gene expression. However, our comprehension of the governing principles behind nuclear organization remains incomplete. Particularly, the spatial segregation of loci with similar repressive transcriptional states in plants poses a significant yet poorly understood puzzle. In this study, employing a combination of genetics and advanced 3D genomics approaches, we demonstrated that a redistribution of facultative heterochromatin marks in regions usually occupied by constitutive heterochromatin marks disrupts the 3D genome compartmentalisation. This disturbance, in turn, triggers novel chromatin interactions between genic and transposable element (TE) regions. Interestingly, our results imply that epigenetic features, constrained by genetic factors, intricately mold the landscape of 3D genome organisation. This study sheds light on the profound genetic-epigenetic interplay that underlies the regulation of gene expression within the intricate framework of the 3D genome. Our findings highlight the complexity of the relationships between genetic determinants and epigenetic features in shaping the dynamic configuration of the 3D genome.

Light control of three-dimensional chromatin organization in soybean

Higher-order chromatin structure is critical for regulation of gene expression. In plants, light profoundly affects the morphogenesis of emerging seedlings as well as global gene expression to ensure optimal adaptation to environmental conditions. However, the changes and functional significance of chromatin organization in response to light during seedling development are not well documented. We constructed Hi-C contact maps for the cotyledon, apical hook and hypocotyl of soybean subjected to dark and light conditions. The resulting high-resolution Hi-C contact maps identified chromosome territories, A/B compartments, A/B sub-compartments, TADs (Topologically Associated Domains) and chromatin loops in each organ. We observed increased chromatin compaction under light and we found that domains that switched from B sub-compartments in darkness to A sub-compartments under light contained genes that were activated during photomorphogenesis. At the local scale, we identified a group of TADs constructed by gene clusters consisting of different numbers of Small Auxin-Upregulated RNAs (SAURs), which exhibited strict co-expression in the hook and hypocotyl in response to light stimulation. In the hypocotyl, RNA polymerase II (RNAPII) regulated the transcription of a SAURs cluster under light via TAD condensation. Our results suggest that the 3D genome is involved in the regulation of light-related gene expression in a tissue-specific manner.

GenomicLinks: deep learning predictions of 3D chromatin interactions in the maize genome

Gene regulation in eukaryotes is partly shaped by the 3D organization of chromatin within the cell nucleus. Distal interactions between cis-regulatory elements and their target genes are widespread, and many causal loci underlying heritable agricultural traits have been mapped to distal non-coding elements. The biology underlying chromatin loop formation in plants is poorly understood. Dissecting the sequence features that mediate distal interactions is an important step toward identifying putative molecular mechanisms. Here, we trained GenomicLinks, a deep learning model, to identify DNA sequence features predictive of 3D chromatin interactions in maize. We found that the presence of binding motifs of specific transcription factor classes, especially bHLH, is predictive of chromatin interaction specificities. Using an in silico mutagenesis approach we show the removal of these motifs from loop anchors leads to reduced interaction probabilities. We were able to validate these predictions with single-cell co-accessibility data from different maize genotypes that harbor natural substitutions in these TF binding motifs. GenomicLinks is currently implemented as an open-source web tool, which should facilitate its wider use in the plant research community.

Chromosome compartmentalization: causes, changes, consequences, and conundrums

The spatial segregation of the genome into compartments is a major feature of 3D genome organization. New data on mammalian chromosome organization across different conditions reveal important information about how and why these compartments form and change. A combination of epigenetic state, nuclear body tethering, physical forces, gene expression, and replication timing (RT) can all influence the establishment and alteration of chromosome compartments. We review the causes and implications of genomic regions undergoing a 'compartment switch' that changes their physical associations and spatial location in the nucleus. About 20-30% of genomic regions change compartment during cell differentiation or cancer progression, whereas alterations in response to a stimulus within a cell type are usually much more limited. However, even a change in 1-2% of genomic bins may have biologically relevant implications. Finally, we review the effects of compartment changes on gene regulation, DNA damage repair, replication, and the physical state of the cell.

An atlas of the tomato epigenome reveals that KRYPTONITE shapes TAD-like boundaries through the control of H3K9ac distribution

In recent years, the exploration of genome three-dimensional (3D) conformation has yielded profound insights into the regulation of gene expression and cellular functions in both animals and plants. While animals exhibit a characteristic genome topology defined by topologically associating domains (TADs), plants display similar features with a more diverse conformation across species. Employing advanced high-throughput sequencing and microscopy techniques, we investigated the landscape of 26 histone modifications and RNA polymerase II distribution in tomato (Solanum lycopersicum). Our study unveiled a rich and nuanced epigenetic landscape, shedding light on distinct chromatin states associated with heterochromatin formation and gene silencing. Moreover, we elucidated the intricate interplay between these chromatin states and the overall topology of the genome. Employing a genetic approach, we delved into the role of the histone modification H3K9ac in genome topology. Notably, our investigation revealed that the ectopic deposition of this chromatin mark triggered a reorganization of the 3D chromatin structure, defining different TAD-like borders. Our work emphasizes the critical role of H3K9ac in shaping the topology of the tomato genome, providing valuable insights into the epigenetic landscape of this agriculturally significant crop species.

Identification of Highly Repetitive Enhancers with Long-range Regulation Potential in Barley via STARR-seq

Enhancers are DNA sequences that can strengthen transcription initiation. However, the global identification of plant enhancers is complicated due to uncertainty in the distance and orientation of enhancers, especially in species with large genomes. In this study, we performed self-transcribing active regulatory region sequencing (STARR-seq) for the first time to identify enhancers across the barley genome. A total of 7323 enhancers were successfully identified, and among 45 randomly selected enhancers, over 75% were effective as validated by a dual-luciferase reporter assay system in the lower epidermis of tobacco leaves. Interestingly, up to 53.5% of the barley enhancers were repetitive sequences, especially transposable elements (TEs), thus reinforcing the vital role of repetitive enhancers in gene expression. Both the common active mark H3K4me3 and repressive mark H3K27me3 were abundant among the barley STARR-seq enhancers. In addition, the functional range of barley STARR-seq enhancers seemed much broader than that of rice or maize and extended to ±100 kb of the gene body, and this finding was consistent with the high expression levels of genes in the genome. This study specifically depicts the unique features of barley enhancers and provides available barley enhancers for further utilization.

A pan-TE map highlights transposable elements underlying domestication and agronomic traits in Asian rice

Transposable elements (TEs) are ubiquitous genomic components and hard to study due to being highly repetitive. Here we assembled 232 chromosome-level genomes based on long-read sequencing data. Coupling the 232 genomes with 15 existing assemblies, we developed a pan-TE map comprising both cultivated and wild Asian rice. We detected 177 084 high-quality TE variations and inferred their derived state using outgroups. We found TEs were one source of phenotypic variation during rice domestication and differentiation. We identified 1246 genes whose expression variation was associated with TEs but not single-nucleotide polymorphisms (SNPs), such as OsRbohB, and validated OsRbohB's relative expression activity using a dual-Luciferase (LUC) reporter assays system. Our pan-TE map allowed us to detect multiple novel loci associated with agronomic traits. Collectively, our findings highlight the contributions of TEs to domestication, differentiation and agronomic traits in rice, and there is massive potential for gene cloning and molecular breeding by the high-quality Asian pan-TE map we generated.

Multiple strategies, including 6mA methylation, affecting plant alternative splicing in allopolyploid peanut

Alternative splicing (AS), an important post-transcriptional regulation mechanism in eukaryotes, can significantly increase transcript diversity and contribute to gene expression regulation and many other complicated developmental processes. While plant gene AS events are well described, few studies have investigated the comprehensive regulation machinery of plant AS. Here, we use multi-omics to analyse peanut AS events. Using long-read isoform sequencing, 146 464 full-length non-chimeric transcripts were obtained, resulting in annotation corrections for 1782 genes and the identification of 4653 new loci. Using Iso-Seq RNA sequences, 271 776 unique splice junctions were identified, 82.49% of which were supported by transcriptome data. We characterized 50 977 polyadenylation sites for 23 262 genes, 12 369 of which had alternative polyadenylation sites. AS allows differential regulation of the same gene by miRNAs at the isoform level coupled with polyadenylation. In addition, we identified many long non-coding RNAs and fusion transcripts. There is a suppressed effect of 6mA on AS and gene expression. By analysis of chromatin structures, the genes located in the boundaries of topologically associated domains, proximal chromosomal telomere regions, inter- or intra-chromosomal loops were found to have more unique splice isoforms, higher expression, lower 6mA and more transposable elements (TEs) in their gene bodies than the other genes, indicating that chromatin interaction, 6mA and TEs play important roles in AS and gene expression. These results greatly refine the peanut genome annotation and contribute to the study of gene expression and regulation in peanuts. This work also showed AS is associated with multiple strategies for gene regulation.

Establishing an optimized ATAC-seq protocol for the maize

The advent of next-generation sequencing in crop improvement offers unprecedented insights into the chromatin landscape closely linked to gene activity governing key traits in plant development and adaptation. Particularly in maize, its dynamic chromatin structure is found to collaborate with massive transcriptional variations across tissues and developmental stages, implying intricate regulatory mechanisms, which highlights the importance of integrating chromatin information into breeding strategies for precise gene controls. The depiction of maize chromatin architecture using Assay for Transposase Accessible Chromatin with high-throughput sequencing (ATAC-seq) provides great opportunities to investigate cis-regulatory elements, which is crucial for crop improvement. In this context, we developed an easy-to-implement ATAC-seq protocol for maize with fewer nuclei and simple equipment. We demonstrate a streamlined ATAC-seq protocol with four key steps for maize in which nuclei purification can be achieved without cell sorting and using only a standard bench-top centrifuge. Our protocol, coupled with the bioinformatic analysis, including validation by read length periodicity, key metrics, and correlation with transcript abundance, provides a precise and efficient assessment of the maize chromatin landscape. Beyond its application to maize, our testing design holds the potential to be applied to other crops or other tissues, especially for those with limited size and amount, establishing a robust foundation for chromatin structure studies in diverse crop species.

Insight into chromatin compaction and spatial organization in rice interphase nuclei

Chromatin organization and its interactions are essential for biological processes, such as DNA repair, transcription, and DNA replication. Detailed cytogenetics data on chromatin conformation, and the arrangement and mutual positioning of chromosome territories in interphase nuclei are still widely missing in plants. In this study, level of chromatin condensation in interphase nuclei of rice (Oryza sativa) and the distribution of chromosome territories (CTs) were analyzed. Super-resolution, stimulated emission depletion (STED) microscopy showed different levels of chromatin condensation in leaf and root interphase nuclei. 3D immuno-FISH experiments with painting probes specific to chromosomes 9 and 2 were conducted to investigate their spatial distribution in root and leaf nuclei. Six different configurations of chromosome territories, including their complete association, weak association, and complete separation, were observed in root meristematic nuclei, and four configurations were observed in leaf nuclei. The volume of CTs and frequency of their association varied between the tissue types. The frequency of association of CTs specific to chromosome 9, containing NOR region, is also affected by the activity of the 45S rDNA locus. Our data suggested that the arrangement of chromosomes in the nucleus is connected with the position and the size of the nucleolus.

The jet-like chromatin structure defines active secondary metabolism in fungi

Eukaryotic genomes are spatially organized within the nucleus in a nonrandom manner. However, fungal genome arrangement and its function in development and adaptation remain largely unexplored. Here, we show that the high-order chromosome structure of Fusarium graminearum is sculpted by both H3K27me3 modification and ancient genome rearrangements. Active secondary metabolic gene clusters form a structure resembling chromatin jets. We demonstrate that these jet-like domains, which can propagate symmetrically for 54 kb, are prevalent in the genome and correlate with active gene transcription and histone acetylation. Deletion of GCN5, which encodes a core and functionally conserved histone acetyltransferase, blocks the formation of the domains. Insertion of an exogenous gene within the jet-like domain significantly augments its transcription. These findings uncover an interesting link between alterations in chromatin structure and the activation of fungal secondary metabolism, which could be a general mechanism for fungi to rapidly respond to environmental cues, and highlight the utility of leveraging three-dimensional genome organization in improving gene transcription in eukaryotes.

Maintenance and dynamic reprogramming of chromatin organization during development

Controlled transcription of genes is critical for cell differentiation and development. Gene expression regulation therefore involves a multilayered control from nucleosome composition in histone variants and their post-translational modifications to higher-order folding of chromatin fibers and chromatin interactions in nuclear space. Recent technological advances have allowed gaining insight into these mechanisms, the interplay between local and higher-order chromatin organization, and the dynamic changes that occur during stress response and developmental transitions. In this review, we will discuss chromatin organization from the nucleosome to its three-dimensional structure in the nucleus, and consider how these different layers of organization are maintained during the cell cycle or rapidly reprogrammed during development.

Genome organization and botanical diversity

The rich diversity of angiosperms, both the planet's dominant flora and the cornerstone of agriculture, is integrally intertwined with a distinctive evolutionary history. Here, we explore the interplay between angiosperm genome organization and botanical diversity, empowered by genomic approaches ranging from genetic linkage mapping to analysis of gene regulation. Commonality in the genetic hardware of plants has enabled robust comparative genomics that has provided a broad picture of angiosperm evolution and implicated both general processes and specific elements in contributing to botanical diversity. We argue that the hardware of plant genomes-both in content and in dynamics-has been shaped by selection for rather substantial differences in gene regulation between plants and animals such as maize and human, organisms of comparable genome size and gene number. Their distinctive genome content and dynamics may reflect in part the indeterminate development of plants that puts strikingly different demands on gene regulation than in animals. Repeated polyploidization of plant genomes and multiplication of individual genes together with extensive rearrangement and differential retention provide rich raw material for selection of morphological and/or physiological variations conferring fitness in specific niches, whether natural or artificial. These findings exemplify the burgeoning information available to employ in increasing knowledge of plant biology and in modifying selected plants to better meet human needs.

Chromosome-scale genome assembly of bread wheat's wild relative Triticum timopheevii

Wheat (Triticum aestivum) is one of the most important food crops with an urgent need for increase in its production to feed the growing world. Triticum timopheevii (2n = 4x = 28) is an allotetraploid wheat wild relative species containing the At and G genomes that has been exploited in many pre-breeding programmes for wheat improvement. In this study, we report the generation of a chromosome-scale reference genome assembly of T. timopheevii accession PI 94760 based on PacBio HiFi reads and chromosome conformation capture (Hi-C). The assembly comprised a total size of 9.35 Gb, featuring a contig N50 of 42.4 Mb and included the mitochondrial and plastid genome sequences. Genome annotation predicted 166,325 gene models including 70,365 genes with high confidence. DNA methylation analysis showed that the G genome had on average more methylated bases than the At genome. In summary, the T. timopheevii genome assembly provides a valuable resource for genome-informed discovery of agronomically important genes for food security.

Integrative multiomics profiling of passion fruit reveals the genetic basis for fruit color and aroma

Passion fruit (Passiflora edulis) possesses a complex aroma and is widely grown in tropical and subtropical areas. Here, we conducted the de novo assembly, annotation, and comparison of PPF (P. edulis Sims) and YPF (P. edulis f. flavicarpa) reference genomes using PacBio, Illumina, and Hi-C technologies. Notably, we discovered evidence of recent whole-genome duplication events in P. edulis genomes. Comparative analysis revealed 7.6∼8.1 million single nucleotide polymorphisms, 1 million insertions/deletions, and over 142 Mb presence/absence variations among different P. edulis genomes. During the ripening of yellow passion fruit, metabolites related to flavor, aroma, and color were substantially accumulated or changed. Through joint analysis of genomic variations, differentially expressed genes, and accumulated metabolites, we explored candidate genes associated with flavor, aroma, and color distinctions. Flavonoid biosynthesis pathways, anthocyanin biosynthesis pathways, and related metabolites are pivotal factors affecting the coloration of passion fruit, and terpenoid metabolites accumulated more in PPF. Finally, by heterologous expression in yeast (Saccharomyces cerevisiae), we functionally characterized 12 terpene synthases. Our findings revealed that certain TPS homologs in both YPF and PPF varieties produce identical terpene products, while others yield distinct compounds or even lose their functionality. These discoveries revealed the genetic and metabolic basis of unique characteristics in aroma and flavor between the 2 passion fruit varieties. This study provides resources for better understanding the genome architecture and accelerating genetic improvement of passion fruits.

HiCrayon reveals distinct layers of multi-state 3D chromatin organization

The co-visualization of chromatin conformation with 1D 'omics data is key to the multi-omics driven data analysis of 3D genome organization. Chromatin contact maps are often shown as 2D heatmaps and visually compared to 1D genomic data by simple juxtaposition. While common, this strategy is imprecise, placing the onus on the reader to align features with each other. To remedy this, we developed HiCrayon, an interactive tool that facilitates the integration of 3D chromatin organization maps and 1D datasets. This visualization method integrates data from genomic assays directly into the chromatin contact map by coloring interactions according to 1D signal. HiCrayon is implemented using R shiny and python to create a graphical user interface (GUI) application, available in both web or containerized format to promote accessibility. HiCrayon is implemented in R, and includes a graphical user interface (GUI), as well as a slimmed-down web-based version that lets users quickly produce publication-ready images. We demonstrate the utility of HiCrayon in visualizing the effectiveness of compartment calling and the relationship between ChIP-seq and various features of chromatin organization. We also demonstrate the improved visualization of other 3D genomic phenomena, such as differences between loops associated with CTCF/cohesin vs. those associated with H3K27ac. We then demonstrate HiCrayon's visualization of organizational changes that occur during differentiation and use HiCrayon to detect compartment patterns that cannot be assigned to either A or B compartments, revealing a distinct 3rd chromatin compartment. Overall, we demonstrate the utility of co-visualizing 2D chromatin conformation with 1D genomic signals within the same matrix to reveal fundamental aspects of genome organization. Local version: https://github.com/JRowleyLab/HiCrayon Web version: https://jrowleylab.com/HiCrayon.

Increased long-distance and homo-trans interactions related to H3K27me3 in Arabidopsis hybrids

In plants, the genome structure of hybrids changes compared with their parents, but the effects of these changes in hybrids remain elusive. Comparing reciprocal crosses between Col × C24 and C24 × Col in Arabidopsis using high-throughput chromosome conformation capture assay (Hi-C) analysis, we found that hybrid three-dimensional (3D) chromatin organization had more long-distance interactions relative to parents, and this was mainly located in promoter regions and enriched in genes with heterosis-related pathways. The interactions between euchromatin and heterochromatin were increased, and the compartment strength decreased in hybrids. In compartment domain (CD) boundaries, the distal interactions were more in hybrids than their parents. In the hybrids of CURLY LEAF (clf) mutants clfCol  × clfC24 and clfC24  × clfCol , the heterosis phenotype was damaged, and the long-distance interactions in hybrids were fewer than in their parents with lower H3K27me3. ChIP-seq data revealed higher levels of H3K27me3 in the region adjacent to the CD boundary and the same interactional homo-trans sites in the wild-type (WT) hybrids, which may have led to more long-distance interactions. In addition, the differentially expressed genes (DEGs) located in the boundaries of CDs and loop regions changed obviously in WT, and the functional enrichment for DEGs was different between WT and clf in the long-distance interactions and loop regions. Our findings may therefore propose a new epigenetic explanation of heterosis in the Arabidopsis hybrids and provide new insights into crop breeding and yield increase.

Conserved chromatin and repetitive patterns reveal slow genome evolution in frogs

Frogs are an ecologically diverse and phylogenetically ancient group of anuran amphibians that include important vertebrate cell and developmental model systems, notably the genus Xenopus. Here we report a high-quality reference genome sequence for the western clawed frog, Xenopus tropicalis, along with draft chromosome-scale sequences of three distantly related emerging model frog species, Eleutherodactylus coqui, Engystomops pustulosus, and Hymenochirus boettgeri. Frog chromosomes have remained remarkably stable since the Mesozoic Era, with limited Robertsonian (i.e., arm-preserving) translocations and end-to-end fusions found among the smaller chromosomes. Conservation of synteny includes conservation of centromere locations, marked by centromeric tandem repeats associated with Cenp-a binding surrounded by pericentromeric LINE/L1 elements. This work explores the structure of chromosomes across frogs, using a dense meiotic linkage map for X. tropicalis and chromatin conformation capture (Hi-C) data for all species. Abundant satellite repeats occupy the unusually long (~20 megabase) terminal regions of each chromosome that coincide with high rates of recombination. Both embryonic and differentiated cells show reproducible associations of centromeric chromatin and of telomeres, reflecting a Rabl-like configuration. Our comparative analyses reveal 13 conserved ancestral anuran chromosomes from which contemporary frog genomes were constructed.

3D genome structural variations play important roles in regulating seed oil content of Brassica napus

Dissecting the complex regulatory mechanism of seed oil content (SOC) is one of the main research goals in Brassica napus. Increasing evidence suggests that genome architecture is linked to multiple biological functions. However, the effect of genome architecture on SOC regulation remains unclear. Here, we used high-throughput chromatin conformation capture to characterize differences in the three-dimensional (3D) landscape of genome architecture of seeds from two B. napus lines, N53-2 (with high SOC) and Ken-C8 (with low SOC). Bioinformatics analysis demonstrated that differentially accessible regions and differentially expressed genes between N53-2 and Ken-C8 were preferentially enriched in regions with quantitative trait loci (QTLs)/associated genomic regions (AGRs) for SOC. A multi-omics analysis demonstrated that expression of SOC-related genes was tightly correlated with genome structural variations in QTLs/AGRs of B. napus. The candidate gene BnaA09g48250D, which showed structural variation in a QTL/AGR on chrA09, was identified by fine-mapping of a KN double-haploid population derived from hybridization of N53-2 and Ken-C8. Overexpression and knockout of BnaA09g48250D led to significant increases and decreases in SOC, respectively, in the transgenic lines. Taken together, our results reveal the 3D genome architecture of B. napus seeds and the roles of genome structural variations in SOC regulation, enriching our understanding of the molecular mechanisms of SOC regulation from the perspective of spatial chromatin structure.

Chromatin remodeling analysis reveals the RdDM pathway responds to low-phosphorus stress in maize

Chromatin in eukaryotes folds into a complex three-dimensional (3D) structure that is essential for controlling gene expression and cellular function and is dynamically regulated in biological processes. Studies on plant phosphorus signaling have concentrated on single genes and gene interactions. It is critical to expand the existing signaling pathway in terms of its 3D structure. In this study, low-Pi treatment led to greater chromatin volume. Furthermore, low-Pi stress increased the insulation score and the number of TAD-like domains, but the effects on the A/B compartment were not obvious. The methylation levels of target sites (hereafter as RdDM levels) peaked at specific TAD-like boundaries, whereas RdDM peak levels at conserved TAD-like boundaries shifted and decreased sharply. The distribution pattern of RdDM sites originating from the Helitron transposons matched that of genome-wide RdDM sites near TAD-like boundaries. RdDM pathway genes were upregulated in the middle or early stages and downregulated in the later stages under low-Pi conditions. The RdDM pathway mutant ddm1a showed increased tolerance to low-Pi stress, with shortened and thickened roots contributing to higher Pi uptake from the shallow soil layer. ChIP-seq results revealed that ZmDDM1A could bind to Pi- and root development-related genes. Strong associations were found between interacting genes in significantly different chromatin-interaction regions and root traits. These findings not only expand the mechanisms by which plants respond to low-Pi stress through the RdDM pathway but also offer a crucial framework for the analysis of biological issues using 3D genomics.

MINI-AC: inference of plant gene regulatory networks using bulk or single-cell accessible chromatin profiles

Gene regulatory networks (GRNs) represent the interactions between transcription factors (TF) and their target genes. Plant GRNs control transcriptional programs involved in growth, development, and stress responses, ultimately affecting diverse agricultural traits. While recent developments in accessible chromatin (AC) profiling technologies make it possible to identify context-specific regulatory DNA, learning the underlying GRNs remains a major challenge. We developed MINI-AC (Motif-Informed Network Inference based on Accessible Chromatin), a method that combines AC data from bulk or single-cell experiments with TF binding site (TFBS) information to learn GRNs in plants. We benchmarked MINI-AC using bulk AC datasets from different Arabidopsis thaliana tissues and showed that it outperforms other methods to identify correct TFBS. In maize, a crop with a complex genome and abundant distal AC regions, MINI-AC successfully inferred leaf GRNs with experimentally confirmed, both proximal and distal, TF-target gene interactions. Furthermore, we showed that both AC regions and footprints are valid alternatives to infer AC-based GRNs with MINI-AC. Finally, we combined MINI-AC predictions from bulk and single-cell AC datasets to identify general and cell-type specific maize leaf regulators. Focusing on C4 metabolism, we identified diverse regulatory interactions in specialized cell types for this photosynthetic pathway. MINI-AC represents a powerful tool for inferring accurate AC-derived GRNs in plants and identifying known and novel candidate regulators, improving our understanding of gene regulation in plants.

Evolutionary insights into 3D genome organization and epigenetic landscape of Vigna mungo

Eukaryotic genomes show an intricate three-dimensional (3D) organization within the nucleus that regulates multiple biological processes including gene expression. Contrary to animals, understanding of 3D genome organization in plants remains at a nascent stage. Here, we investigate the evolution of 3D chromatin architecture in legumes. By using cutting-edge PacBio, Illumina, and Hi-C contact reads, we report a gap-free, chromosome-scale reference genome assembly of Vigna mungo, an important minor legume cultivated in Southeast Asia. We spatially resolved V. mungo chromosomes into euchromatic, transcriptionally active A compartment and heterochromatic, transcriptionally-dormant B compartment. We report the presence of TAD-like-regions throughout the diagonal of the HiC matrix that resembled transcriptional quiescent centers based on their genomic and epigenomic features. We observed high syntenic breakpoints but also high coverage of syntenic sequences and conserved blocks in boundary regions than in the TAD-like region domains. Our findings present unprecedented evolutionary insights into spatial 3D genome organization and epigenetic patterns and their interaction within the V. mungo genome. This will aid future genomics and epigenomics research and breeding programs of V. mungo.

Genome-wide DNase I-hypersensitive site assay reveals distinct genomic distributions and functional features of open chromatin in autopolyploid sugarcane

The characterization of cis-regulatory DNA elements (CREs) is essential for deciphering the regulation of gene expression in eukaryotes. Although there have been endeavors to identify CREs in plants, the properties of CREs in polyploid genomes are still largely unknown. Here, we conducted the genome-wide identification of DNase I-hypersensitive sites (DHSs) in leaf and stem tissues of the auto-octoploid species Saccharum officinarum. We revealed that DHSs showed highly similar distributions in the genomes of these two S. officinarum tissues. Notably, we observed that approximately 74% of DHSs were located in distal intergenic regions, suggesting considerable differences in the abundance of distal CREs between S. officinarum and other plants. Leaf- and stem-dependent transcriptional regulatory networks were also developed by mining the binding motifs of transcription factors (TFs) from tissue-specific DHSs. Four TEOSINTE BRANCHED 1, CYCLOIDEA, and PCF1 (TCP) TFs (TCP2, TCP4, TCP7, and TCP14) and two ethylene-responsive factors (ERFs) (ERF109 and ERF03) showed strong causal connections with short binding distances from each other, pointing to their possible roles in the regulatory networks of leaf and stem development. Through functional validation in transiently transgenic protoplasts, we isolate a set of tissue-specific promoters. Overall, the DHS maps presented here offer a global view of the potential transcriptional regulatory elements in polyploid sugarcane and can be expected to serve as a valuable resource for both transcriptional network elucidation and genome editing in sugarcane breeding.

Three-dimensional chromatin architecture in plants - General features and novelties

Research on the three-dimensional (3D) structure of the genome and its distribution within the nuclear space has made a big leap in the last two decades. Work in the animal field has led to significant advances in our general understanding on eukaryotic genome organization. This did not only bring along insights into how the 3D genome interacts with the epigenetic landscape and the transcriptional machinery but also how 3D genome architecture is relevant for fundamental developmental processes, such as cell differentiation. In parallel, the 3D organization of plant genomes have been extensively studied, which resulted in both congruent and novel findings, contributing to a more complete view on how eukaryotic genomes are organized in multiple dimensions. Plant genomes are remarkably diverse in size, composition, and ploidy. Furthermore, as intrinsically sessile organisms without the possibility to relocate to more favorable environments, plants have evolved an elaborate epigenetic repertoire to rapidly respond to environmental challenges. The diversity in genome organization and the complex epigenetic programs make plants ideal study subjects to acquire a better understanding on universal features and inherent constraints of genome organization. Furthermore, considering a wide range of species allows us to study the evolutionary crosstalk between the various levels of genome architecture. In this article, we aim at summarizing important findings on 3D genome architecture obtained in various plant species. These findings cover many aspects of 3D genome organization on a wide range of levels, from gene loops to topologically associated domains and to global 3D chromosome configurations. We present an overview on plant 3D genome organizational features that resemble those in animals and highlight facets that have only been observed in plants to date.

HSFA1a: the quarterback of heat stress response and 3D-chromatin organization

In eukaryotes, the highly complex chromatin exhibits structural dynamism to modulate cellular responses. Recently, Huang et al. have shown a novel role of the heat stress master regulator, HSFA1a, in transiently reorganizing the 3D chromatin to promote distal and proximal enhancer-promoter contacts to regulate stress-responsive genes in tomato.

Accessible gene borders establish a core structural unit for chromatin architecture in Arabidopsis

Three-dimensional (3D) chromatin structure is linked to transcriptional regulation in multicellular eukaryotes including plants. Taking advantage of high-resolution Hi-C (high-throughput chromatin conformation capture), we detected a small structural unit with 3D chromatin architecture in the Arabidopsis genome, which lacks topologically associating domains, and also in the genomes of tomato, maize, and Marchantia polymorpha. The 3D folding domain unit was usually established around an individual gene and was dependent on chromatin accessibility at the transcription start site (TSS) and transcription end site (TES). We also observed larger contact domains containing two or more neighboring genes, which were dependent on accessible border regions. Binding of transcription factors to accessible TSS/TES regions formed these gene domains. We successfully simulated these Hi-C contact maps via computational modeling using chromatin accessibility as input. Our results demonstrate that gene domains establish basic 3D chromatin architecture units that likely contribute to higher-order 3D genome folding in plants.

Chromatin dynamics and subnuclear gene positioning for transcriptional regulation

Plants have been found to exhibit diverse characteristics and functions of chromatin organization, showing both similarities and differences to animals. It is becoming clear how chromatin organization is linked to transcriptional regulation in response to environmental stresses. Regulation of specific chromatin positions in the nuclear space is important for transcription, and the mechanisms that enable such chromatin dynamics are gradually being unveiled. Genes move between subdomains responsible for transcriptional activation or suppression in the subnuclear space in a gene repositioning cycle. We propose a model of localized chromatin interaction in nuclear subdomains, in which the dynamics of local chromatin interactions have a more important impact on the regulation of gene expression than large-scale chromatin organization. In this mini-review, we highlight recent findings on chromatin dynamics, particularly involving transcriptional regulation, and discuss future directions in the study of chromatin organization in plants.

Chromatin architecture of two different strains of Artemisia annua reveals the alterations in interaction and gene expression

MAIN CONCLUSION

The hierarchical architecture of chromatins affects the gene expression level of glandular secreting trichomes and the artemisinin biosynthetic pathway-related genes, consequently bringing on huge differences in the content of artemisinin and its derivatives of A. annua. The plant of traditional Chinese medicine "Qinghao" is called Artemisia annua L. in Chinese Pharmacopoeia. High content and the total amount of artemisinin is the main goal of A. annua breeding, nevertheless, the change of chromatin organization during the artemisinin synthesis process has not been discovered yet. This study intended to find the roles of chromatin structure in the production of artemisinin through bioinformatics and experimental validation. Chromosome conformation capture analysis was used to scrutinize the interactions among chromosomes and categorize various scales of chromatin during artemisinin synthesis in A. annua. To confirm the effect of the changes in chromatin structure, Hi-C and RNA-sequencing were performed on two different strains to find the correlation between chromatin structure and gene expression levels on artemisinin synthesis progress and regulation. Our results revealed that the frequency of intra-chromosomal interactions was higher in the inter-chromosomal interactions between the root and leaves on a high artemisinin production strain (HAP) compared to a low artemisinin production strain (LAP). We found that compartmental transition was connected with interactions among different chromatins. Interestingly, glandular secreting trichomes (GSTs) and the artemisinin biosynthetic pathway (ABP) related genes were enriched in the areas which have the compartmental transition, reflecting the regulation of artemisinin synthesis. Topologically associated domain boundaries were associated with various distributions of genes and expression levels. Genes associated with ABP and GST in the adjacent loop were highly expressed, suggesting that epigenetic regulation plays an important role during artemisinin synthesis and glandular secreting trichomes production process. Chromatin structure could show an important status in the mechanisms of artemisinin synthesis process in A. annua.

A multi-omic Nicotiana benthamiana resource for fundamental research and biotechnology

Nicotiana benthamiana is an invaluable model plant and biotechnology platform with a ~3 Gb allotetraploid genome. To further improve its usefulness and versatility, we have produced high-quality chromosome-level genome assemblies, coupled with transcriptome, epigenome, microRNA and transposable element datasets, for the ubiquitously used LAB strain and a related wild accession, QLD. In addition, single nucleotide polymorphism maps have been produced for a further two laboratory strains and four wild accessions. Despite the loss of five chromosomes from the ancestral tetraploid, expansion of intergenic regions, widespread segmental allopolyploidy, advanced diploidization and evidence of recent bursts of Copia pseudovirus (Copia) mobility not seen in other Nicotiana genomes, the two subgenomes of N. benthamiana show large regions of synteny across the Solanaceae. LAB and QLD have many genetic, metabolic and phenotypic differences, including disparate RNA interference responses, but are highly interfertile and amenable to genome editing and both transient and stable transformation. The LAB/QLD combination has the potential to be as useful as the Columbia-0/Landsberg errecta partnership, utilized from the early pioneering days of Arabidopsis genomics to today.

Low temperature-induced regulatory network rewiring via WRKY regulators during banana peel browning

Banana (Musa spp.) fruits, as typical tropical fruits, are cold sensitive, and lower temperatures can disrupt cellular compartmentalization and lead to severe browning. How tropical fruits respond to low temperature compared to the cold response mechanisms of model plants remains unknown. Here, we systematically characterized the changes in chromatin accessibility, histone modifications, distal cis-regulatory elements, transcription factor binding, and gene expression levels in banana peels in response to low temperature. Dynamic patterns of cold-induced transcripts were generally accompanied by concordant chromatin accessibility and histone modification changes. These upregulated genes were enriched for WRKY binding sites in their promoters and/or active enhancers. Compared to banana peel at room temperature, large amounts of banana WRKYs were specifically induced by cold and mediated enhancer-promoter interactions regulating critical browning pathways, including phospholipid degradation, oxidation, and cold tolerance. This hypothesis was supported by DNA affinity purification sequencing, luciferase reporter assays, and transient expression assay. Together, our findings highlight widespread transcriptional reprogramming via WRKYs during banana peel browning at low temperature and provide an extensive resource for studying gene regulation in tropical plants in response to cold stress, as well as potential targets for improving cold tolerance and shelf life of tropical fruits.

Genome-wide high-throughput chromosome conformation capture analysis reveals hierarchical chromatin interactions during early somatic embryogenesis

Somatic embryogenesis (SE), like zygotic embryo development, is a progressive process. Early SE is the beginning of a switch from a somatic to an embryogenic state and is an important stage for initiating chromatin reprogramming of SE. Previous studies suggest that changes in chromatin accessibility occur during early SE, although information on the 3D structure of chromatin is not yet available. Here, we present a chromosome-level genome assembly of longan (Dimocarpus longan) using PacBio combined with high-through chromosome conformation capture scaffolding, which resulted in a 446 Mb genome assembly anchored onto 15 scaffolds. During early SE, chromatin was concentrated and then decondensed, and a large number of long terminal repeat retrotransposons (LTR-RTs) were enriched in the local chromatin interaction region, suggesting LTR-RTs were involved in chromatin reorganization. Early SE was accompanied by the transformation from A to B compartments, and the interactions between B compartments were enhanced. Results from chromatin accessibility, monomethylation of histone H3 at lysine 4 (H3K4me1) modification, and transcription analyses further revealed a gene regulatory network for cell wall thickening during SE. Particularly, we found that the H3K4me1 differential peak binding motif showed abnormal activation of ethylene response factor transcription factors and participation in SE. The chromosome-level genomic and multiomics analyses revealed the 3D conformation of chromatin during early SE, providing insight into the molecular mechanisms underlying cell wall thickening and the potential regulatory networks of TFs during early SE in D. longan. These results provide additional clues for revealing the molecular mechanisms of plant SE.