VAL- and AtBMI1-Mediated H2Aub Initiate the Switch from Embryonic to Postgerminative Growth in Arabidopsis

Removal of H2Aub1 by ubiquitin-specific proteases 12 and 13 is required for stable Polycomb-mediated gene repression in Arabidopsis

BACKGROUND

Stable gene repression is essential for normal growth and development. Polycomb repressive complexes 1 and 2 (PRC1&2) are involved in this process by establishing monoubiquitination of histone 2A (H2Aub1) and subsequent trimethylation of lysine 27 of histone 3 (H3K27me3). Previous work proposed that H2Aub1 removal by the ubiquitin-specific proteases 12 and 13 (UBP12 and UBP13) is part of the repressive PRC1&2 system, but its functional role remains elusive.

RESULTS

We show that UBP12 and UBP13 work together with PRC1, PRC2, and EMF1 to repress genes involved in stimulus response. We find that PRC1-mediated H2Aub1 is associated with gene responsiveness, and its repressive function requires PRC2 recruitment. We further show that the requirement of PRC1 for PRC2 recruitment depends on the initial expression status of genes. Lastly, we demonstrate that removal of H2Aub1 by UBP12/13 prevents loss of H3K27me3, consistent with our finding that the H3K27me3 demethylase REF6 is positively associated with H2Aub1.

CONCLUSIONS

Our data allow us to propose a model in which deposition of H2Aub1 permits genes to switch between repression and activation by H3K27me3 deposition and removal. Removal of H2Aub1 by UBP12/13 is required to achieve stable PRC2-mediated repression.

Epigenetic Regulation of Auxin-Induced Somatic Embryogenesis in Plants

Somatic embryogenesis (SE) that is induced in plant explants in response to auxin treatment is closely associated with an extensive genetic reprogramming of the cell transcriptome. The significant modulation of the gene transcription profiles during SE induction results from the epigenetic factors that fine-tune the gene expression towards embryogenic development. Among these factors, microRNA molecules (miRNAs) contribute to the post-transcriptional regulation of gene expression. In the past few years, several miRNAs that regulate the SE-involved transcription factors (TFs) have been identified, and most of them were involved in the auxin-related processes, including auxin metabolism and signaling. In addition to miRNAs, chemical modifications of DNA and chromatin, in particular the methylation of DNA and histones and histone acetylation, have been shown to shape the SE transcriptomes. In response to auxin, these epigenetic modifications regulate the chromatin structure, and hence essentially contribute to the control of gene expression during SE induction. In this paper, we describe the current state of knowledge with regard to the SE epigenome. The complex interactions within and between the epigenetic factors, the key SE TFs that have been revealed, and the relationships between the SE epigenome and auxin-related processes such as auxin perception, metabolism, and signaling are highlighted.

Histone tales: lysine methylation, a protagonist in Arabidopsis development

Histone methylation plays a fundamental role in the epigenetic regulation of gene expression driven by developmental and environmental cues in plants, including Arabidopsis. Histone methyltransferases and demethylases act as 'writers' and 'erasers' of methylation at lysine and/or arginine residues of core histones, respectively. A third group of proteins, the 'readers', recognize and interpret the methylation marks. Emerging evidence confirms the crucial roles of histone methylation in multiple biological processes throughout the plant life cycle. In this review, we summarize the regulatory mechanisms of lysine methylation, especially at histone H3 tails, and focus on the recent advances regarding the roles of lysine methylation in Arabidopsis development, from seed performance to reproductive development, and in callus formation.

LEAFY COTYLEDONs: old genes with new roles beyond seed development

Seed development is a complex process and consists of two phases: embryo morphogenesis and seed maturation. LEAFY COTYLEDON (LEC) transcription factors, first discovered in Arabidopsis thaliana several decades ago, are master regulators of seed development. Here, we first summarize molecular genetic mechanisms underlying the control of embryogenesis and seed maturation by LECs and then provide a brief review of recent findings in the role of LECs in embryonic resetting of the parental ‘memory of winter cold’ in Arabidopsis. In addition, we discuss various chromatin-based mechanisms underlying developmental silencing of LEC genes throughout the post-embryonic development to terminate the embryonic developmental program.

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.

Arabidopsis ZUOTIN RELATED FACTOR1 Proteins Are Required for Proper Embryonic and Post-Embryonic Root Development

The H2A/UBIQUITIN-binding proteins AtZRF1a/b have been reported as key regulators involved in multiple processes of Arabidopsis plant growth and development. Yet, the cellular and molecular mechanisms underlying the mutant phenotype remain largely elusive. Here we show that loss-of-function of AtZRF1a/b causes defective root elongation and deformed root apical meristem organization in seedlings. The premature termination of the primary root in the atzrf1a;atzrf1b double mutant is associated with an advanced onset of endoreduplication and subsequent consumption of reservoir stem cells. Cytological analyses using cell type-specific markers and florescent dyes indicate that AtZRF1a/b are involved in maintenance of proper cell layer organization, determinacy of cell identity, and establishment of auxin gradient and maximum at the root tip. During embryogenesis AtZRF1a/b act dominantly in regulating the maintenance of ground tissue initial cells and production of lateral root cap. Lastly, quantitative real-time polymerase chain reaction analysis shows mis-expression of some key genes involved in regulating cell patterning, cell proliferation and/or hormone pathways. Our results provide important insight into AtZRF1a/b function in cell fate determinacy and in establishment and maintenance of proper stem cell reservoir during embryonic and post-embryonic root development.

Genome-wide association study of callus induction variation to explore the callus formation mechanism of rice

Rice (Oryza sativa) is one of the most widely cultivated food crops, worldwide. Tissue culture is extensively used in rice breeding and functional genome research. The ability to induce callus determines whether a particular rice variety can be subjected to tissue culture and Agrobacterium-mediated transformation. Over the past two decades, many quantitative trait loci (QTLs) related to callus induction traits have been identified; however, individual genes associated with rice callus induction have not been reported. In this study, we characterized three callus-induction traits in a global collection of 510 rice accessions. A genome-wide association study of the rice population in its entirety as well as subpopulations revealed 21 significant loci located in rice callus induction QTLs. We identified three candidate callus induction genes, namely CRL1, OsBMM1, and OsSET1, which are orthologs of Arabidopsis LBD17/LBD29, BBM, and SWN, respectively, which are known to affect callus formation. Furthermore, we predicted that 14 candidate genes might be involved in rice callus induction and showed that RNA interference (RNAi)-mediated disruption of OsIAA10 inhibited callus formation on tissue culture medium. Embryo growth in the OsIAA10 RNAi line was not inhibited by synthetic auxin (2,4-D) treatment, suggesting that OsIAA10 may perceive auxin and activate the expression of downstream genes, such as CRL1, to induce callus formation. The significant loci and candidate genes identified here may provide insight into the mechanism underlying callus formation in rice.

Inhibition of Polycomb Repressive Complex 2 activity reduces trimethylation of H3K27 and affects development in Arabidopsis seedlings

BACKGROUND

Polycomb repressive complex 2 (PRC2) is an epigenetic transcriptional repression system, whose catalytic subunit (ENHANCER OF ZESTE HOMOLOG 2, EZH2 in animals) is responsible for trimethylating histone H3 at lysine 27 (H3K27me3). In mammals, gain-of-function mutations as well as overexpression of EZH2 have been associated with several tumors, therefore making this subunit a suitable target for the development of selective inhibitors. Indeed, highly specific small-molecule inhibitors of EZH2 have been reported. In plants, mutations in some PRC2 components lead to embryonic lethality, but no trial with any inhibitor has ever been reported.

RESULTS

We show here that the 1,5-bis (3-bromo-4-methoxyphenyl)penta-1,4-dien-3-one compound (RDS 3434), previously reported as an EZH2 inhibitor in human leukemia cells, is active on the Arabidopsis catalytic subunit of PRC2, since treatment with the drug reduces the total amount of H3K27me3 in a dose-dependent fashion. Consistently, we show that the expression level of two PRC2 targets is significantly increased following treatment with the RDS 3434 compound. Finally, we show that impairment of H3K27 trimethylation in Arabidopsis seeds and seedlings affects both seed germination and root growth.

CONCLUSIONS

Our results provide a useful tool for the plant community in investigating how PRC2 affects transcriptional control in plant development.

Evolution and conservation of polycomb repressive complex 1 core components and putative associated factors in the green lineage

Background

Polycomb group (PcG) proteins play important roles in animal and plant development and stress response. Polycomb repressive complex 1 (PRC1) and PRC2 are the key epigenetic regulators of gene expression, and are involved in almost all developmental stages. PRC1 catalyzes H2A monoubiquitination resulting in transcriptional silencing or activation. The PRC1 components in the green lineage were identified and evolution and conservation was analyzed by bioinformatics techniques. RING Finger Protein 1 (RING1), B lymphoma Mo-MLV insertion region 1 homolog (BMI1), Like Heterochromatin Protein 1 (LHP1) and Embryonic Flower 1 (EMF1) are the PRC1 core components and Vernalization 1 (VRN1), VP1/ABI3-Like 1/2/3 (VAL1/2/3), Alfin-like 1–7 (AL1–7), Inhibitor of growth 1/2 (ING1/2), and Early Bolting in Short Days (EBS) / Short Life (SHL) are the associated factors.

Results

Each PRC1 subunit possesses special domain organizations, such as RING and the ring finger and WD40-associated ubiquitin-like (RAWUL) domains for RING1 and BMI1, chromatin organization modifier (CHROMO) and chromo shadow (ChSh) domains for LHP1, one or two B3 DNA binding domain(s) for VRN1, B3 and zf-CW domains for VAL1/2/3, Alfin and Plant HomeoDomain (PHD) domains for AL1–7, ING and PHD domains for ING1/2, Bromoadjacent homology (BAT) and PHD domains for EBS/SHL. Six new motifs are uncovered in EMF1.

The PRC1 core components RING1 and BMI1, and the associated factors VAL1/2/3, AL1–7, ING1/2, and EBS/SHL exist from alga to higher plants, whereas LHP1 only occurs in higher plants. EMF1 and VRN1 are present only in eudicots. PRC1 components undergo duplication in the plant evolution. Most of plants carry the homologous core component LHP1, the associated factor EMF1, and several homologs in RING1, BMI1, VRN1, AL1–7, ING1/2/3, and EBS/SHL. Cabbage, cotton, poplar, orange and maize often exhibit more gene copies than other species. Domain organization analysis shows that duplicated gene functions may be of diverse.

Conclusions

The PRC1 core components RING1 and BMI1, and the associated factors VAL1/2/3, AL1–7, ING1/2, and EBS/SHL originate from algae. The core component LHP1 is from moss and the associated factors EMF1 and VRN1 are from dicotyledon. PRC1 components are of functional redundancy and diversity in evolution.

Electronic supplementary material

The online version of this article (10.1186/s12864-019-5905-9) contains supplementary material, which is available to authorized users.

The repressive role of Arabidopsis H2A.Z in transcriptional regulation depends on AtBMI1 activity

H2A.Z variant has emerged as a critical player in regulating plant responses to environment; however, the mechanism by which H2A.Z mediates this regulation remains unclear. In Arabidopsis, H2A.Z has been proposed to have opposite effects on transcription depending on its localization within the gene. These opposite roles have been assigned by correlating gene expression and H2A.Z enrichment analyses but without considering the impact of possible H2A.Z post-translational modifications. Here, we show that H2A.Z can be monoubiquitinated by the PRC1 components AtBMI1A/B/C. The incorporation of this modification is required for H2A.Z-mediated transcriptional repression through a mechanism that does not require PRC2 activity. Our data suggest that the dual role of H2A.Z in regulating gene expression depends on the modification that it carries, while the levels of H2A.Z within genes depend on the transcriptional activity.

Arabidopsis H2A.Z plays an important role in regulating gene expression in response to stressors; however, the underlying mechanism is still puzzling. Here, the authors show that monoubiquitination of H2A.Z by AtBMI1 is required for H2A.Z-mediated transcriptional repression.

Embryonic resetting of the parental vernalized state by two B3 domain transcription factors in Arabidopsis

Some overwintering plants acquire competence to flower, after experiencing prolonged cold in winter, through a process termed vernalization. In the crucifer plant Arabidopsis thaliana, prolonged cold induces chromatin-mediated silencing of the potent floral repressor FLOWERING LOCUS C (FLC) by Polycomb proteins. This vernalized state is epigenetically maintained or 'memorized' in warm rendering plants competent to flower in spring, but is reset in the next generation. Here, we show that in early embryogenesis, two homologous B3 domain transcription factors LEAFY COTYLEDON 2 (LEC2) and FUSCA3 (FUS3) compete against two repressive B3-containing epigenome readers and Polycomb partners known as VAL1 and VAL2 for the cis-regulatory cold memory element (CME) of FLC to disrupt Polycomb silencing. Consistently, crystal structures of B3-CME complexes show that B3_FUS3, B3_LEC2 and B3_VAL1 employ a nearly identical binding interface for CME. We further found that LEC2 and FUS3 recruit the scaffold protein FRIGIDA in association with active chromatin modifiers to establish an active chromatin state at FLC, which results in resetting of the silenced FLC to active and erasing the epigenetic parental memory of winter cold in early embryos. Following embryo development, LEC2 and FUS3 are developmentally silenced throughout post-embryonic stages, enabling VALs to bind to the CME again at seedling stages at which plants experience winter cold. Our findings illustrate how overwintering crucifer annuals or biennials in temperate climates employ a subfamily of B3 domain proteins to switch on, off and on again the expression of a key flowering gene in the embryo-to-plant-to-embryo cycle, and thus to synchronize growth and development with seasonal temperature changes in their life cycles.

Evolution of Polycomb-group function in the green lineage

Epigenetic gene regulation ensures the mitotically or meiotically stable heritability (or both) of gene expression or protein activity states and maintains repetitive element repression and cellular identities. The repressive Polycomb-group (PcG) proteins consist of several large complexes that control cellular memory by acting on chromatin and are antagonized by the Trithorax-group proteins. Especially, Polycomb repressive complex 2 (PRC2) is highly conserved in plants and animals but its function in unicellular eukaryotes and during land plant evolution is less understood. Additional PcG complexes and associated proteins are only partially conserved and have evolved in a lineage-specific manner. In this review, I will focus on recent advances in the understanding of PcG function in the green lineage and its contribution to land plant evolution.

Insight into the B3Transcription Factor Superfamily and Expression Profiling of B3 Genes in Axillary Buds after Topping in Tobacco(Nicotiana tabacum L.)

Members of the plant-specific B3 transcription factor superfamily play important roles in various growth and developmental processes in plants. Even though there are many valuable studies on B3 genes in other species, little is known about the B3 superfamily in tobacco. We identified 114 B3 proteins from tobacco using comparative genome analysis. These proteins were classified into four subfamilies based on their phylogenetic relationships, and include the ARF, RAV, LAV, and REM subfamilies. The chromosomal locations, gene structures, conserved protein motifs, and sub-cellular localizations of the tobacco B3 proteins were analyzed. The patterns of exon-intron numbers and arrangement and the protein structures of the tobacco B3 proteins were in general agreement with their phylogenetic relationships. The expression patterns of 114 B3 genes revealed that many B3 genes show tissue-specific expression. The expression levels of B3 genes in axillary buds after topping showed that the REM genes are mainly up-regulated in response to topping, while the ARF genes are down-regulated after topping.

Regulation of FUSCA3 Expression During Seed Development in Arabidopsis

FUSCA3 (FUS3) is a master regulator of seed development important in establishing and maintaining embryonic identity whose expression is tightly regulated at genetic and epigenetic levels. Despite this prominent role, the control of FUS3 expression remains poorly understood. Promoter and functional complementation analyses provided insight into the regulation of FUS3. W-boxes present in the promoter proximal to the start of transcription are recognized by WRKY type-1 factors which are necessary for the activation of FUS3 expression. The RY motif, the binding site of B3 factors, is important for the activation of FUS3 in the embryo proper but not in the suspensor. The loss of a negative regulatory sequence (NRS) leads to preferential expression of FUS3 in the vasculature of vegetative tissues. Since the NRS includes the RY motif, mechanisms of activation and repression target adjacent or overlapping regions. These findings discriminate the regulation of FUS3 from that of LEAFY COTYLEDON2 by the control exerted by WRKY factors and by the presence of the RY motif, yet also confirm conservation of certain regulatory elements, thereby implicating potential regulation by BASIC PENTACYSTEINE (BPC) factors and POLYCOMB REPRESSIVE COMPLEX2 (PRC2).

E2FA and E2FB transcription factors coordinate cell proliferation with seed maturation

The E2F transcription factors and the RETINOBLASTOMA-RELATED repressor protein are principal regulators coordinating cell proliferation with differentiation, but their role during seed development is little understood. We show that in fully developed Arabidopsis thaliana embryos, cell number was not affected either in single or double mutants for the activator-type E2FA and E2FB. Accordingly, these E2Fs are only partially required for the expression of cell cycle genes. In contrast, the expression of key seed maturation genes LEAFY COTYLEDON 1/2 (LEC1/2), ABSCISIC ACID INSENSITIVE 3, FUSCA 3 and WRINKLED 1 is upregulated in the e2fab double mutant embryo. In accordance, E2FA directly regulates LEC2, and mutation at the consensus E2F-binding site in the LEC2 promoter de-represses its activity during the proliferative stage of seed development. In addition, the major seed storage reserve proteins, 12S globulin and 2S albumin, became prematurely accumulated at the proliferating phase of seed development in the e2fab double mutant. Our findings reveal a repressor function of the activator E2Fs to restrict the seed maturation programme until the cell proliferation phase is completed.

Highlighted Article: During seed and embryo development the E2FA and E2FB transcription factors coordinate cell proliferation with differentiation and accumulation of seed reserves; however, they are not essential for sustaining cell proliferation.

Somatic Embryogenesis Induction in Woody Species: The Future After OMICs Data Assessment

Very early somatic embryogenesis has been recognized as a powerful method to propagate plants in vitro. For some woody species and in particular for some coniferous trees, somatic embryogenesis induction has become a routine procedure. For the majority, the application of this technology presents yet many limitations especially due to the genotype, the induction conditions, the number of embryos produced, maturation, and conversion, among other factors that compromise the systematic use of somatic embryogenesis for commercial purposes especially of woody species and forest trees in particular. The advancements obtained on somatic embryogenesis in Arabidopsis and the development of OMIC technologies allowed the characterization of genes and the corresponding proteins that are conserved in woody species. This knowledge will help in understanding the molecular mechanisms underlying the complex regulatory networks that control somatic embryogenesis in woody plants. In this revision, we report on developments of OMICs (genomics, transcriptomics, metabolomics, and proteomics) applied to somatic embryogenesis induction and its contribution for understanding the change of fate giving rise to the expression of somatic embryogenesis competence.

JMJ30-mediated demethylation of H3K9me3 drives tissue identity changes to promote callus formation in Arabidopsis

Plant somatic cells can be reprogrammed by in vitro tissue culture methods, and massive genome-wide chromatin remodeling occurs, particularly during callus formation. Since callus tissue resembles root primordium, conversion of tissue identity is essentially required when leaf explants are used. Consistent with the fact that the differentiation state is defined by chromatin structure, which permits limited gene profiles, epigenetic changes underlie cellular reprogramming for changes to tissue identity. Although a histone methylation process suppressing leaf identity during leaf-to-callus transition has been demonstrated, the epigenetic factor involved in activation of root identity remains elusive. Here, we report that JUMONJI C DOMAIN-CONTAINING PROTEIN 30 (JMJ30) stimulates callus formation by promoting expression of a subset of LATERAL ORGAN BOUNDARIES-DOMAIN (LBD) genes that establish root primordia. The JMJ30 protein binds to promoters of the LBD16 and LBD29 genes along with AUXIN RESPONSE FACTOR 7 (ARF7) and ARF19 and activates LBD expression. Consistently, the JMJ30-deficient mutant displays reduced callus formation with low LBD transcript levels. The ARF-JMJ30 complex catalyzes the removal of methyl groups from H3K9me3, especially at the LBD16 and LBD29 loci to activate their expression during leaf-to-callus transition. Moreover, the ARF-JMJ30 complex further recruits ARABIDOPSIS TRITHORAX-RELATED 2 (ATXR2), which promotes deposition of H3K36me3 at the LBD16 and LBD29 promoters, and the tripartite complex ensures stable LBD activation during callus formation. These results indicate that the coordinated epigenetic modifications promote callus formation by establishing root primordium identity.

Molecular and epigenetic regulations and functions of the LAFL transcriptional regulators that control seed development

The LAFL (i.e. LEC1, ABI3, FUS3, and LEC2) master transcriptional regulators interact to form different complexes that induce embryo development and maturation, and inhibit seed germination and vegetative growth in Arabidopsis. Orthologous genes involved in similar regulatory processes have been described in various angiosperms including important crop species. Consistent with a prominent role of the LAFL regulators in triggering and maintaining embryonic cell fate, their expression appears finely tuned in different tissues during seed development and tightly repressed in vegetative tissues by a surprisingly high number of genetic and epigenetic factors. Partial functional redundancies and intricate feedback regulations of the LAFL have hampered the elucidation of the underpinning molecular mechanisms. Nevertheless, genetic, genomic, cellular, molecular, and biochemical analyses implemented during the last years have greatly improved our knowledge of the LALF network. Here we summarize and discuss recent progress, together with current issues required to gain a comprehensive insight into the network, including the emerging function of LEC1 and possibly LEC2 as pioneer transcription factors.

Structural Analysis of the Arabidopsis AL2-PAL and PRC1 Complex Provides Mechanistic Insight into Active-to-Repressive Chromatin State Switch

Polycomb group proteins play essential roles in transcriptional gene repression during both animal and plant development. Polycomb repression complex 1 (PRC1) is one of the key functional modules in polycomb group silencing. It acts as both a reader of H3K27me3 (histone H3 lysine 27 trimethylation) and a writer of H2Aub1 (histone H2A monoubiquitination) in establishing stable repression chromatin state. Intriguingly, a recent study showed that Arabidopsis PRC1 contains the H3K4me3-binding proteins of the ALFIN-like (AL) family, pointing to a chromatin state switch from active to repressive transcription of embryonic genes required for vegetative plant development. However, molecular and structural basis of AL-PRC1 complexes are lacking, which harmed insightful mechanistic understanding of AL-PRC1 complex function. In the present study, we report the crystal structures of the PAL domain (DUF3594 domain) of AL2 and AL7 proteins as well as their mechanistic binding to the PRC1 ring-finger proteins (RING1 and BMI1). We found that the PAL domain exists as a homodimer and represents a novel protein fold. We further determined the crystal structures of the PAL domain of AL2 (AL2-PAL) in complex with AtRING1a and AtBMI1b, the two core components of Arabidopsis PRC1. Interestingly, two PAL-binding sites were found on AtRING1a. Each of them can bind AL but with different affinities and distinct structural bases. Based on our results, we propose a mechanistic model to understand how AL proteins target PRC1 to active chromatin to undergo the transition from H3K4me3 to H2Aub1/H3K27me3 in establishing gene silencing.

Chromatin modulation and gene regulation in plants: insight about PRC1 function

In plant and metazoan, Polycomb Group (PcG) proteins play key roles in regulating developmental processes by repression of gene expression. PcG proteins function as multi-protein complexes; among them the best characterized ones are Polycomb Repressive Complex 1 (PRC1) and PRC2. PRC2 catalyzes histone H3 lysine 27 trimethylation (H3K27me3), and PRC1 can bind H3K27me3 and catalyzes H2A monoubiquitination. While the PRC2 components and molecular functions are evolutionarily conserved, varied PRC1 complexes are found and they show high divergences between animals and plants. In addition to the core subunits, an exponentially increasing number of PRC1-associated factors have been identified in Arabidopsis thaliana. Recent studies have also unraveled cross-component interactions and intertwined roles of PRC1 and PRC2 in chromatin modulation. In addition, complexities of interactions and functions between PcG and Trithorax Group proteins have been observed. This short review summarizes up current knowledge to provide insight about repressive functional mechanism of PRC1 and its interplay with other factors.

Trithorax Group Proteins Act Together with a Polycomb Group Protein to Maintain Chromatin Integrity for Epigenetic Silencing during Seed Germination in Arabidopsis

Polycomb group (PcG) and trithorax group (trxG) proteins have been shown to act antagonistically to epigenetically regulate gene expression in eukaryotes. The trxG proteins counteract PcG-mediated floral repression in Arabidopsis, but their roles in other developmental processes are poorly understood. We investigated the interactions between the trxG genes, ARABIDOPSIS HOMOLOG OF TRITHORAX1 (ATX1) and ULTRAPETALA1 (ULT1), and the PcG gene EMBRYONIC FLOWER 1 (EMF1) during early development. Unexpectedly, we found that mutations in the trxG genes failed to rescue the early-flowering phenotype of emf1 mutants. Instead, emf1 atx1 ult1 seedlings showed a novel swollen root phenotype and massive deregulation of gene expression. Greater ectopic expression of seed master regulatory genes in emf1 atx1 ult1 triple than in emf1 single mutants indicates that PcG and trxG factors together repress seed gene expression after germination. Furthermore, we found that the widespread gene derepression is associated with reduced levels of H3K27me3, an epigenetic repressive mark of gene expression, and with globally altered chromatin organization. EMF1, ATX1, and ULT1 are able to bind the chromatin of seed genes and ULT1 can physically interact with ATX1 and EMF1, suggesting that the trxG and EMF1 proteins directly associate at target gene loci for EMF1-mediated gene silencing. Thus, while ATX1, ULT1, and EMF1 interact antagonistically to regulate flowering, they work together to maintain chromatin integrity and prevent precocious seed gene expression after germination.

Interactions between transcription factors and chromatin regulators in the control of flower development

Chromatin modifiers and remodelers are involved in generating dynamic changes at the chromatin, which allow differential and specific readouts of the genome. While genetic evidence indicates that several chromatin factors play a key role in controlling basic developmental programs for inflorescence and flower morphogenesis, it remained unknown until recently how they exert their specificity toward gene expression, both temporally and spatially. An emerging topic is the recruitment or eviction of chromatin factors through the activity of sequence-specific DNA-binding domains, present in the chromatin factors themselves or in partnering transcription factors. Here we summarize recent progress that has been made in this regard in the model plant Arabidopsis thaliana. We further outline the different possible modes through which chromatin complexes specifically target genes involved in flower development.

Chromatin-Based Regulation of Plant Root Development

Plant is endowed with sessile habit and nutrient acquisition mainly through the root organ, which also provides an excellent model to study stem cell fate and asymmetric division due to well-organized cell layers and relatively simple cell types in root meristem. Besides genetic material DNA wrapped around histone octamer, chromatin structure determined by chromatin modification including DNA methylation, histone modification and chromatin remodeling also contributes greatly to the regulation of gene expression. In this review, we summarize the current progresses on the molecular mechanisms of chromatin modification in regulating root development.

Unraveling the Complex Epigenetic Mechanisms that Regulate Gene Activity

Our understanding of the epigenetic mechanisms that regulate gene expression has been largely increased in recent years by the development and refinement of different techniques. This has revealed that gene transcription is highly influenced by epigenetic mechanisms, i.e., those that do not involve changes in the genome sequence, but rather in nuclear architecture, chromosome conformation and histone and DNA modifications. Our understanding of how these different levels of epigenetic regulation interact with each other and with classical transcription-factor based gene regulation to influence gene transcription has just started to emerge. This review discusses the latest advances in unraveling the complex interactions between different types of epigenetic regulation and transcription factor activity, with special attention to the approaches that can be used to study these interactions.

Epigenetic Regulation of Juvenile-to-Adult Transition in Plants

Epigenetic regulation is referred to as changes in gene function that do not involve changes in the DNA sequence, it is usually accomplished by DNA methylation, histone modifications (repressive marks such as H3K9me, H3K27me, H2Aub, or active marks such as H3K4me, H3K36me, H3Ac), and chromatin remodeling (nucleosome composition, occupancy, and location). In plants, the shoot apex produces different lateral organs during development to give rise to distinguishable phases of a juvenile, an adult and a reproductive phase after embryogenesis. The juvenile-to-adult transition is a key developmental event in plant life cycle, and it is regulated by a decrease in the expression of a conserved microRNA-miR156/157, and a corresponding increase in the expression of its target genes encoding a set of plant specific SQUAMOSA PROMOTER BINDING PROTEIN-LIKE (SPL) proteins. Recent work has revealed that the miR156/157-SPL pathway is the master regulator of juvenile-to-adult transition in plants, and genes in this pathway are subjected to epigenetic regulation, such as DNA methylation, histone modifications, and chromatin remodeling. In this review, we summarized the recent progress in understanding the epigenetic regulation of the miR156/157-SPL pathway during juvenile-to-adult transition and bring forward some perspectives of future research in this field.

A transcriptional view on somatic embryogenesis

Somatic embryogenesis is a form of induced plant cell totipotency where embryos develop from somatic or vegetative cells in the absence of fertilization. Somatic embryogenesis can be induced in vitro by exposing explants to stress or growth regulator treatments. Molecular genetics studies have also shown that ectopic expression of specific embryo‐ and meristem‐expressed transcription factors or loss of certain chromatin‐modifying proteins induces spontaneous somatic embryogenesis. We begin this review with a general description of the major developmental events that define plant somatic embryogenesis and then focus on the transcriptional regulation of this process in the model plant Arabidopsis thaliana (arabidopsis). We describe the different somatic embryogenesis systems developed for arabidopsis and discuss the roles of transcription factors and chromatin modifications in this process. We describe how these somatic embryogenesis factors are interconnected and how their pathways converge at the level of hormones. Furthermore, the similarities between the developmental pathways in hormone‐ and transcription‐factor‐induced tissue culture systems are reviewed in the light of our recent findings on the somatic embryo‐inducing transcription factor BABY BOOM.

Conservation and diversification of polycomb repressive complex 2 (PRC2) proteins in the green lineage

The polycomb group (PcG) proteins are key epigenetic regulators of gene expression in animals and plants. They act in multiprotein complexes, of which the best characterized is the polycomb repressive complex 2 (PRC2), which catalyses the trimethylation of histone H3 at lysine 27 (H3K27me3) at chromatin targets. In Arabidopsis thaliana, PRC2 proteins are involved in the regulation of diverse developmental processes, including cell fate determination, vegetative growth and development, flowering time control and embryogenesis. Here, we systematically analysed the evolutionary conservation and diversification of PRC2 components in lower and higher plants. We searched for and identified PRC2 homologues from the sequenced genomes of several green lineage species, from the unicellular green alga Ostreococcus lucimarinus to more complicated angiosperms. We found that some PRC2 core components, e.g. E(z), ESC/FIE and MSI/p55, are ancient and have multiplied coincidently with multicellular evolution. For one component, some members are newly formed, especially in the Cruciferae. During evolution, higher plants underwent copy number multiplication of various PRC2 components, which occurred independently for each component, without any obvious co-amplification of PRC2 members. Among the amplified members, usually one was well-conserved and the others were more diversified. Gene amplification occurred at different times for different PcG members during green lineage evolution. Certain PRC2 core components or members of them were highly conserved. Our study provides an insight into the evolutionary conservation and diversification of PcG proteins and may guide future functional characterization of these important epigenetic regulators in plants other than Arabidopsis.

Capturing Environmental Plant Memories in DNA, with a Little Help from Chromatin

Plants are eukaryotes living mostly immotile in harsh environments. On occasion, it is beneficial for their survival to maintain a transcriptional response to an environmental stress longer than the stress lasts (transcriptional memory) and even to reiterate such a response more quickly or more strongly when the same stress is re-encountered (priming memory). In eukaryotes, transcription takes place in the context of chromatin, the packaging material of DNA. Chromatin regulation is often invoked when it comes to environmental transcriptional and priming memory in plants, but rarely chromatin-based regulation can be accurately assigned to a given aspect of transcription in vivo. The conserved eukaryotic chromatin-modifying system Polycomb/Trithorax can support both long-term stability and flexibility of gene expression in Drosophila. The main principles of Polycomb/Trithorax regulation will be outlined and illustrated with the best-studied case of environmental memory from Arabidopsis. Despite being complex, the Polycomb/Trithorax system relies on experimentally tractable elements in the form of DNA, termed Polycomb/Trithorax Responsive Elements. PREs/TREs are essentially memory DNA elements. Here, relevant information to identify PRE/TRE-like elements in plants is highlighted. Examples of priming memory in plants are discussed in relation to the first two reported putative memory DNA elements. Arguably, similar cases from plants can be conducive in dissecting the contribution of DNA-based from chromatin-based regulation of transcription, when two types of DNA elements are defined: those representing binding sites for the transcription factors determining the environmental response and those controlling memory by regulating chromatin modification dynamics, ultimately maintaining the corresponding transcriptional state.

Developmental transitions: integrating environmental cues with hormonal signaling in the chromatin landscape in plants

Plant development is predominantly postembryonic and tuned in to respond to environmental cues. All living plant cells can be triggered to de-differentiate, assume different cell identities, or form a new organism. This developmental plasticity is thought to be an adaptation to the sessile lifestyle of plants. Recent discoveries have advanced our understanding of the orchestration of plant developmental switches by transcriptional master regulators, chromatin state changes, and hormone response pathways. Here, we review these recent advances with emphasis on the earliest stages of plant development and on the switch from pluripotency to differentiation in different plant organ systems.

Ctf4-related protein recruits LHP1-PRC2 to maintain H3K27me3 levels in dividing cells in Arabidopsis thaliana

Polycomb Repressive Complex (PRC) 2 catalyzes the H3K27me3 modification that warrants inheritance of a repressive chromatin structure during cell division, thereby assuring stable target gene repression in differentiated cells. It is still under investigation how H3K27me3 is passed on from maternal to filial strands during DNA replication; however, cell division can reinforce H3K27me3 coverage at target regions. To identify novel factors involved in the Polycomb pathway in plants, we performed a forward genetic screen for enhancers of the like heterochromatin protein 1 (lhp1) mutant, which shows relatively mild phenotypic alterations compared with other plant PRC mutants. We mapped enhancer of lhp1 (eol) 1 to a gene related to yeast Chromosome transmission fidelity 4 (Ctf4) based on phylogenetic analysis, structural similarities, physical interaction with the CMG helicase component SLD5, and an expression pattern confined to actively dividing cells. A combination of eol1 with the curly leaf (clf) allele, carrying a mutation in the catalytic core of PRC2, strongly enhanced the clf phenotype; furthermore, H3K27me3 coverage at target genes was strongly reduced in eol1 clf double mutants compared with clf single mutants. EOL1 physically interacted with CLF, its partially redundant paralog SWINGER (SWN), and LHP1. We propose that EOL1 interacts with LHP1-PRC2 complexes during replication and thereby participates in maintaining the H3K27me3 mark at target genes.

H2A monoubiquitination in Arabidopsis thaliana is generally independent of LHP1 and PRC2 activity

BACKGROUND

Polycomb group complexes PRC1 and PRC2 repress gene expression at the chromatin level in eukaryotes. The classic recruitment model of Polycomb group complexes in which PRC2-mediated H3K27 trimethylation recruits PRC1 for H2A monoubiquitination was recently challenged by data showing that PRC1 activity can also recruit PRC2. However, the prevalence of these two mechanisms is unknown, especially in plants as H2AK121ub marks were examined at only a handful of Polycomb group targets.

RESULTS

By using genome-wide analyses, we show that H2AK121ub marks are surprisingly widespread in Arabidopsis thaliana, often co-localizing with H3K27me3 but also occupying a set of transcriptionally active genes devoid of H3K27me3. Furthermore, by profiling H2AK121ub and H3K27me3 marks in atbmi1a/b/c, clf/swn, and lhp1 mutants we found that PRC2 activity is not required for H2AK121ub marking at most genes. In contrast, loss of AtBMI1 function impacts the incorporation of H3K27me3 marks at most Polycomb group targets.

CONCLUSIONS

Our findings show the relationship between H2AK121ub and H3K27me3 marks across the A. thaliana genome and unveil that ubiquitination by PRC1 is largely independent of PRC2 activity in plants, while the inverse is true for H3K27 trimethylation.

The AFL subfamily of B3 transcription factors: evolution and function in angiosperm seeds

Seed development follows zygotic embryogenesis; during the maturation phase reserves accumulate and desiccation tolerance is acquired. This is tightly regulated at the transcriptional level and the AFL (ABI3/FUS3/LEC2) subfamily of B3 transcription factors (TFs) play a central role. They alter hormone biosynthesis, mainly in regards to abscisic acid and gibberellins, and also regulate the expression of other TFs and/or modulate their downstream activity via protein-protein interactions. This review deals with the origin of AFL TFs, which can be traced back to non-vascular plants such as Physcomitrella patens and achieves foremost expansion in the angiosperms. In green algae, like the unicellular Chlamydomonas reinhardtii or the pluricellular Klebsormidium flaccidum, a single B3 gene and four B3 paralogous genes are annotated, respectively. However, none of them present with the structural features of the AFL subfamily, with the exception of the B3 DNA-binding domain. Phylogenetic analysis groups the AFL TFs into four Major Clusters of Ortologous Genes (MCOGs). The origin and function of these genes is discussed in view of their expression patterns and in the context of major regulatory interactions in seeds of monocotyledonous and dicotyledonous species.

Governing the Silencing State of Chromatin: The Roles of Polycomb Repressive Complex 1 in Arabidopsis

Polycomb group proteins form multiple protein complexes such as Polycomb Repressive Complex (PRC) 1 and PRC2, which repress the expression of thousands of genes. PRC1 and PRC2 are essential for normal development in Arabidopsis. Recently, significant progress has been made in understanding the functions and regulatory mechanisms of PRC1. In this review, we focus on the discovery of the composition of PRC1, functions of its components, the recruitment of PRC1 to target genes and the control of PRC1 function in Arabidopsis. Perspectives on dissecting the roles of PRC1 in plant gene expression and development are also given.

LHP1 Could Act as an Activator and a Repressor of Transcription in Plants

Polycomb group (PcG) proteins within the polycomb repressive complex 1 (PRC1) and PRC2 are significant epigenetic regulatory factors involved in important cellular and developmental processes in eukaryotes. In Arabidopsis, LIKE HETEROCHROMATIN PROTEIN 1 (LHP1), also known as TERMINAL FLOWER 2, has been proposed as a plant specific subunit of PRC1 that could bind the trimethylated lysine 27 of histone H3 (H3K27me3), which is established by PRC2 and is required for a functional plant PcG system. LHP1 not only interacts with PRC1 to catalyze monoubiquitination at lysine 119 of histone H2A but also functions with PRC2 to establish H3K27me3. This review is about the interaction of LHP1 with PRC1 and PRC2, in which LHP1 may act as a bridge between the two. Meantime, this review highlights that LHP1 could act as an activator and a repressor of transcription.

Polycomb Group Proteins RING1A and RING1B Regulate the Vegetative Phase Transition in Arabidopsis

Polycomb group (PcG) protein-mediated gene silencing is a major regulatory mechanism in higher eukaryotes that affects gene expression at the transcriptional level. Here, we report that two conserved homologous PcG proteins, RING1A and RING1B (RING1A/B), are required for global H2A monoubiquitination (H2Aub) in Arabidopsis. The mutation of RING1A/B increased the expression of members of the SQUAMOSA PROMOTER BINDING PROTEIN-LIKE (SPL) gene family and caused an early vegetative phase transition. The early vegetative phase transition observed in ring1a ring1b double mutant plants was dependent on an SPL family gene, and the H2Aub status of the chromatin at SPL locus was dependent on RING1A/B. Moreover, mutation in RING1A/B affected the miRNA156a-mediated vegetative phase transition, and RING1A/B and the AGO7-miR390-TAS3 pathway were found to additively regulate this transition in Arabidopsis. Together, our results demonstrate that RING1A/B regulates the vegetative phase transition in Arabidopsis through the repression of SPL family genes.

Characterization of the Polycomb-Group Mark H3K27me3 in Unicellular Algae

Polycomb Group (PcG) proteins mediate chromatin repression in plants and animals by catalyzing H3K27 methylation and H2AK118/119 mono-ubiquitination through the activity of the Polycomb repressive complex 2 (PRC2) and PRC1, respectively. PcG proteins were extensively studied in higher plants, but their function and target genes in unicellular branches of the green lineage remain largely unknown. To shed light on PcG function and modus operandi in a broad evolutionary context, we demonstrate phylogenetic relationship of core PRC1 and PRC2 proteins and H3K27me3 biochemical presence in several unicellular algae of different phylogenetic subclades. We focus then on one of the species, the model red alga Cyanidioschizon merolae, and show that H3K27me3 occupies both, genes and repetitive elements, and mediates the strength of repression depending on the differential occupancy over gene bodies. Furthermore, we report that H3K27me3 in C. merolae is enriched in telomeric and subtelomeric regions of the chromosomes and has unique preferential binding toward intein-containing genes involved in protein splicing. Thus, our study gives important insight for Polycomb-mediated repression in lower eukaryotes, uncovering a previously unknown link between H3K27me3 targets and protein splicing.

BLISTER Regulates Polycomb-Target Genes, Represses Stress-Regulated Genes and Promotes Stress Responses in Arabidopsis thaliana

HIGHLIGHTS The PRC2 interacting protein BLISTER likely acts downstream of PRC2 to silence Polycomb target genes and is a key regulator of specific stress responses in Arabidopsis. Polycomb group (PcG) proteins are key epigenetic regulators of development. The highly conserved Polycomb repressive complex 2 (PRC2) represses thousands of target genes by trimethylating H3K27 (H3K27me3). Plant specific PcG components and functions are largely unknown, however, we previously identified the plant-specific protein BLISTER (BLI) as a PRC2 interactor. BLI regulates PcG target genes and promotes cold stress resistance. To further understand the function of BLI, we analyzed the transcriptional profile of bli-1 mutants. Approximately 40% of the up-regulated genes in bli are PcG target genes, however, bli-1 mutants did not show changes in H3K27me3 levels at all tested genes, indicating that BLI regulates PcG target genes downstream of or in parallel to PRC2. Interestingly, a significant number of BLI regulated H3K27me3 target genes is regulated by the stress hormone absciscic acid (ABA). We further reveal an overrepresentation of genes responding to abiotic stresses such as drought, high salinity, or heat stress among the up-regulated genes in bli mutants. Consistently, bli mutants showed reduced desiccation stress tolerance. We conclude that the PRC2 associated protein BLI is a key regulator of stress-responsive genes in Arabidopsis: it represses ABA-responsive PcG target genes, likely downstream of PRC2, and promotes resistance to several stresses such as cold and drought.

Arabidopsis PRC1 core component AtRING1 regulates stem cell-determining carpel development mainly through repression of class I KNOX genes

Background

Polycomb repressive complex 2 (PRC2)-catalyzed H3K27me3 marks are tightly associated with the WUS-AG negative feedback loop to terminate floral stem cell fate to promote carpel development, but the roles of Polycomb repressive complex 1 (PRC1) in this event remain largely uncharacterized.

Results

Here we show conspicuous variability in the morphology and number of carpels among individual flowers in the absence of the PRC1 core components AtRING1a and AtRING1b, which contrasts with the wild-type floral meristem consumed by uniform carpel production in Arabidopsis thaliana. Promoter-driven GUS reporter analysis showed that AtRING1a and AtRING1b display a largely similar expression pattern, except in the case of the exclusively maternal-preferred expression of AtRING1b, but not AtRING1a, in the endosperm. Indeterminate carpel development in the atring1a;atring1b double mutant is due to replum/ovule-to-carpel conversion in association with ectopic expression of class I KNOX (KNOX-I) genes. Moreover, AtRING1a and AtRING1b also play a critical role in ovule development, mainly through promoting the degeneration of non-functional megaspores and proper integument formation. Genetic interaction analysis indicates that the AtRING1a/b-regulated KNOX-I pathway acts largely in a complementary manner with the WUS-AG pathway in controlling floral stem cell maintenance and proper carpel development.

Conclusions

Our study uncovers a novel mechanistic pathway through which AtRING1a and AtRING1b repress KNOX-I expression to terminate floral stem cell activities and establish carpel cell fate identities.

Electronic supplementary material

The online version of this article (doi:10.1186/s12915-016-0336-4) contains supplementary material, which is available to authorized users.

Tug of war: adding and removing histone lysine methylation in Arabidopsis

Histone lysine methylation plays a fundamental role in the epigenetic regulation of gene expression in multicellular eukaryotes, including plants. It shapes plant developmental and growth programs as well as responses to the environment. The methylation status of certain amino-acids, in particular of the histone 3 (H3) lysine tails, is dynamically controlled by opposite acting histone methyltransferase 'writers' and histone demethylase 'erasers'. The methylation status is interpreted by a third set of proteins, the histone modification 'readers', which specifically bind to a methylated amino-acid on the H3 tail. Histone methylation writers, readers, and erasers themselves are regulated by intrinsic or extrinsic stimuli; this forms a feedback loop that contributes to development and environmental adaptation in Arabidopsis and other plants. Recent studies have expanded our knowledge regarding the biological roles and dynamic regulation of histone methylation. In this review, we will discuss recent advances in understanding the regulation and roles of histone methylation in plants and animals.

Transcription factors AS1 and AS2 interact with LHP1 to repress KNOX genes in Arabidopsis

Polycomb group proteins are important repressors of numerous genes in higher eukaryotes. However, the mechanism by which Polycomb group proteins are recruited to specific genes is poorly understood. In Arabidopsis, LIKE HETEROCHROMATIN PROTEIN 1 (LHP1), also known as TERMINAL FLOWER 2, was originally proposed as a subunit of polycomb repressive complex 1 (PRC1) that could bind the tri-methylated lysine 27 of histone H3 (H3K27me3) established by the PRC2. In this work, we show that LHP1 mainly functions with PRC2 to establish H3K27me3, but not with PRC1 to catalyze monoubiquitination at lysine 119 of histone H2A. Our results show that complexes of the transcription factors ASYMMETRIC LEAVES 1 (AS1) and AS2 could help to establish the H3K27me3 modification at the chromatin regions of Class-I KNOTTED1-like homeobox (KNOX) genes BREVIPEDICELLUS and KNAT2 via direct interactions with LHP1. Additionally, our transcriptome analysis indicated that there are probably more common target genes of AS1 and LHP1 besides Class-I KNOX genes during leaf development in Arabidopsis.

Deciphering and modifying LAFL transcriptional regulatory network in seed for improving yield and quality of storage compounds

Increasing yield and quality of seed storage compounds in a sustainable way is a key challenge for our societies. Genome-wide analyses conducted in both monocot and dicot angiosperms emphasized drastic transcriptional switches that occur during seed development. In Arabidopsis thaliana, a reference species, genetic and molecular analyses have demonstrated the key role of LAFL (LEC1, ABI3, FUS3, and LEC2) transcription factors (TFs), in controlling gene expression programs essential to accomplish seed maturation and the accumulation of storage compounds. Here, we summarize recent progress obtained in the characterization of these LAFL proteins, their regulation, partners and target genes. Moreover, we illustrate how these evolutionary conserved TFs can be used to engineer new crops with altered seed compositions and point out the current limitations. Last, we discuss about the interest of investigating further the environmental and epigenetic regulation of this network for the coming years.

H2A deubiquitinases UBP12/13 are part of the Arabidopsis polycomb group protein system

Polycomb group (PcG) proteins form an epigenetic memory system in plants and animals, but interacting proteins are poorly known in plants. Here, we have identified Arabidopsis UBIQUITIN SPECIFIC PROTEASES (USP; UBP in plant and yeasts) 12 and 13 as partners of the plant-specific PcG protein LIKE HETEROCHROMATIN PROTEIN 1 (LHP1). UBP12 binds to chromatin of PcG target genes and is required for histone H3 lysine 27 trimethylation and repression of a subset of PcG target genes. Plants lacking UBP12 and UBP13 developed autonomous endosperm in the absence of fertilization. We have identified UBP12 and UBP13 as new proteins in the plant PcG regulatory network. UBP12 and UBP13 belong to an ancient gene family and represent plant homologues of metazoan USP7. We have found that Drosophila USP7 shares a function in heterochromatic gene repression with UBP12/13 and their homologue UBP26. In summary, we demonstrate that USP7-like proteins are essential for gene silencing in diverse genomic contexts.

HSI2 Repressor Recruits MED13 and HDA6 to Down-Regulate Seed Maturation Gene Expression Directly During Arabidopsis Early Seedling Growth

Arabidopsis HSI2 (HIGH-LEVEL EXPRESSION OF SUGAR-INDUCIBLE GENE 2) which carries a EAR (ERF-associated amphiphilic repression) motif acts as a repressor of seed maturation genes and lipid biosynthesis, whereas MEDIATOR (MED) is a conserved multiprotein complex linking DNA-bound transcription factors to RNA polymerase II transcription machinery. How HSI2 executes its repressive function through MED is hitherto unknown. Here, we show that HSI2 and its homolog, HSI2-lik (HSL1), are able to form homo- and heterocomplexes. Both factors bind to the TRAP240 domain of MED13, a subunit of the MED CDK8 module. Mutant alleles of the med13 mutant show elevated seed maturation gene expression and increased lipid accumulation in cotyledons; in contrast, HSI2- or MED13-overexpressing plants display the opposite phenotypes. The overexpression phenotypes of HSI2 and MED13 are abolished in med13 and hsi2 hsl1, respectively, indicating that HSI2 and MED13 together are required for these functions. The HSI2 C-terminal region interacts with HDA6, whose overexpression also reduces seed maturation gene expression and lipid accumulation. Moreover, HSI2, MED13 and HDA6 bind to the proximal promoter and 5'-coding regions of seed maturation genes. Taken together, our results suggest that HSI2 recruits MED13 and HDA6 to suppress directly a subset of seed maturation genes post-germination.

LHP1 Regulates H3K27me3 Spreading and Shapes the Three-Dimensional Conformation of the Arabidopsis Genome

Precise expression patterns of genes in time and space are essential for proper development of multicellular organisms. Dynamic chromatin conformation and spatial organization of the genome constitute a major step in this regulation to modulate developmental outputs. Polycomb repressive complexes (PRCs) mediate stable or flexible gene repression in response to internal and environmental cues. In Arabidopsis thaliana, LHP1 co-localizes with H3K27me3 epigenetic marks throughout the genome and interacts with PRC1 and PRC2 members as well as with a long noncoding RNA. Here, we show that LHP1 is responsible for the spreading of H3K27me3 towards the 3' end of the gene body. We also identified a subset of LHP1-activated genes and demonstrated that LHP1 shapes local chromatin topology in order to control transcriptional co-regulation. Our work reveals a general role of LHP1 from local to higher conformation levels of chromatin configuration to determine its accessibility to define gene expression patterns.