ARABIDOPSIS TRITHORAX-RELATED7 is required for methylation of lysine 4 of histone H3 and for transcriptional activation of FLOWERING LOCUS C

Epigenetic memory in plants

Epigenetics refers to heritable changes in patterns of gene expression that occur without alterations in DNA sequence. The epigenetic mechanisms involve covalent modifications of DNA and histones, which affect transcriptional activity of chromatin. Since chromatin states can be propagated through mitotic and meiotic divisions, epigenetic mechanisms are thought to provide heritable 'cellular memory'. Here, we review selected examples of epigenetic memory in plants and briefly discuss underlying mechanisms.

Epigenetic regulation by long noncoding RNAs in plants

Many eukaryotes, including plants, produce a large number of long noncoding RNAs (lncRNAs).Growing number of lncRNAs are being reported to have regulatory roles in various developmental processes.Emerging mechanisms underlying the function of lncRNAs indicate that lncRNAs are versatile regulatory molecules. They function as potent cis- and trans-regulators of gene expression, including the formation of modular scaffolds that recruit chromatin-modifying complexes to target chromatin. LncRNAs have also been reported in plants. Here, we describe our current understanding on potential roles of lncRNA in plants.

Trithorax group protein Oryza sativa Trithorax1 controls flowering time in rice via interaction with early heading date3

Trithorax group proteins are chromatin-remodeling factors that activate target gene expression by antagonistically functioning against the Polycomb group. In Arabidopsis (Arabidopsis thaliana), Arabidopsis Trithorax protein1 (ATX1) regulates flowering time and floral organ identity. Here, we observed that suppression of Oryza sativa Trithorax1 (OsTrx1), an ortholog of ATX1, delayed flowering time in rice (Oryza sativa). Because the delay occurred only under long-day conditions, we evaluated the flowering signal pathways that specifically function under long-day conditions. Among them, the OsMADS50 and Heading date1 pathways were not affected by the mutation. However, the Grain number, plant height, and heading date7 (Ghd7) pathway was altered in ostrx1. Transcript levels of OsGI, phytochrome genes, and Early heading date3 (Ehd3), which function upstream of Ghd7, were unchanged in the mutant. Because Trx group proteins form a complex with other proteins to modify the chromatin structure of target genes, we investigated whether OsTrx1 interacts with a previously identified protein that functions upstream of Ghd7. We demonstrated that the plant homeodomain motif of OsTrx1 binds to native histone H3 from the calf thymus and that OsTrx1 binds to Ehd3 through the region between the plant homeodomain and SET domains. Finally, we showed that the SET domain at the C-terminal end of OsTrx1 has histone H3 methyltransferase activity when incubated with oligonucleosomes. Our results suggest that OsTrx1 plays an important role in regulating flowering time in rice by modulating chromatin structure.

Two FLX family members are non-redundantly required to establish the vernalization requirement in Arabidopsis

Studies of natural genetic variation for the vernalization requirement in Arabidopsis have revealed two genes, FRIGIDA (FRI) and FLOWERING LOCUS C (FLC), that are determinants of the vernalization-requiring, winter-annual habit. In this study, we show that FLC EXPRESSOR LIKE 4 (FLL4) is essential for up-regulation of FLC in winter-annual Arabidopsis accessions and establishment of a vernalization requirement. FLL4 is part of the FLC EXPRESSOR (FLX) gene family and both are non-redundantly involved in flowering-time control. Epistasis analysis among FRI, FLL4, FLX and autonomous-pathway genes reveals that FRI fve exhibits an extreme delay of flowering compared to fri fve, but mutants in other autonomous-pathway genes do not, indicating that FVE acts most antagonistically to FRI. FLL4 may represent a new member of a FRI-containing complex that activates FLC.

Combinatorial functions of diverse histone methylations in Arabidopsis thaliana flowering time regulation

Previous studies in Arabidopsis thaliana have identified several histone methylation enzymes, including Arabidopsis trithorax1 (ATX1)/set domain group 27 (SDG27), ATX2/SDG30, LSD1-LIKE1 (LDL1), LDL2, SDG8, SDG25, and curly leaf (CLF)/SDG1, as regulators of the key flowering repressor flowering locus C (FLC) and the florigen flowering locus T (FT). However, the combinatorial functions of these enzymes remain largely uninvestigated. Here, we investigated functional interplays of different histone methylation enzymes by studying higher order combinations of their corresponding gene mutants. We showed that H3K4me2/me3 and H3K36me3 depositions occur largely independently and that SDG8-mediated H3K36me3 overrides ATX1/ATX2-mediated H3K4me2/me3 or LDL1/LDL2-mediated H3K4 demethylation in regulating FLC expression and flowering time. By contrast, a reciprocal inhibition was observed between deposition of the active mark H3K4me2/me3 and/or H3K36me3 and deposition of the repressive mark H3K27me3 at both FLC and FT chromatin; and the double mutants sdg8 clf and sdg25 clf displayed enhanced early-flowering phenotypes of the respective single mutants. Collectively, our results provide important insights into the interactions of different types of histone methylation and enzymes in the regulation of FLC and FT expression in flowering time control.

Genetic and epigenetic mechanisms underlying vernalization

Plants have evolved a number of monitoring systems to sense their surroundings and to coordinate their growth and development accordingly. Vernalization is one example, in which flowering is promoted after plants have been exposed to a long-term cold temperature (i.e. winter). Vernalization results in the repression of floral repressor genes that inhibit the floral transition in many plant species. Here, we describe recent advances in our understanding of the vernalization-mediated promotion of flowering in Arabidopsis and other flowering plants. In Arabidopsis, the vernalization response includes the recruitment of chromatin-modifying complexes to floral repressors and thus results in the enrichment of repressive histone marks that ensure the stable repression of floral repressor genes. Changes in histone modifications at floral repressor loci are stably maintained after cold exposure, establishing the competence to flower the following spring. We also discuss similarities and differences in regulatory circuits in vernalization responses among Arabidopsis and other plants.

An RNA-seq transcriptome analysis of histone modifiers and RNA silencing genes in soybean during floral initiation process

Epigenetics has been recognised to play vital roles in many plant developmental processes, including floral initiation through the epigenetic regulation of gene expression. The histone modifying proteins that mediate these modifications involve the SET domain-containing histone methyltransferases, JmjC domain-containing demethylase, acetylases and deacetylases. In addition, RNA interference (RNAi)-associated genes are also involved in epigenetic regulation via RNA-directed DNA methylation and post-transcriptional gene silencing. Soybean, a major crop legume, requires a short day to induce flowering. How histone modifications regulate the plant response to external cues that initiate flowering is still largely unknown. Here, we used RNA-seq to address the dynamics of transcripts that are potentially involved in the epigenetic programming and RNAi mediated gene silencing during the floral initiation of soybean. Soybean is a paleopolyploid that has been subjected to at least two rounds of whole genome duplication events. We report that the expanded genomic repertoire of histone modifiers and RNA silencing genes in soybean includes 14 histone acetyltransferases, 24 histone deacetylases, 47 histone methyltransferases, 15 protein arginine methyltransferases, 24 JmjC domain-containing demethylases and 47 RNAi-associated genes. To investigate the role of these histone modifiers and RNA silencing genes during floral initiation, we compared the transcriptional dynamics of the leaf and shoot apical meristem at different time points after a short-day treatment. Our data reveal that the extensive activation of genes that are usually involved in the epigenetic programming and RNAi gene silencing in the soybean shoot apical meristem are reprogrammed for floral development following an exposure to inductive conditions.

The ULT1 and ULT2 trxG genes play overlapping roles in Arabidopsis development and gene regulation

The epigenetic regulation of gene expression is critical for ensuring the proper deployment and stability of defined genome transcription programs at specific developmental stages. The cellular memory of stable gene expression states during animal and plant development is mediated by the opposing activities of Polycomb group (PcG) factors and trithorax group (trxG) factors. Yet, despite their importance, only a few trxG factors have been characterized in plants and their roles in regulating plant development are poorly defined. In this work, we report that the closely related Arabidopsis trxG genes ULTRAPETALA1 (ULT1) and ULT2 have overlapping functions in regulating shoot and floral stem cell accumulation, with ULT1 playing a major role but ULT2 also making a minor contribution. The two genes also have a novel, redundant activity in establishing the apical–basal polarity axis of the gynoecium, indicating that they function in differentiating tissues. Like ULT1 proteins, ULT2 proteins have a dual nuclear and cytoplasmic localization, and the two proteins physically associate in planta. Finally, we demonstrate that ULT1 and ULT2 have very similar overexpression phenotypes and regulate a common set of key development target genes, including floral MADS-box genes and class I KNOX genes. Our results reveal that chromatin remodeling mediated by the ULT1 and ULT2 proteins is necessary to control the development of meristems and reproductive organs. They also suggest that, like their animal counterparts, plant trxG proteins may function in multi-protein complexes to up-regulate the expression of key stage- and tissue-specific developmental regulatory genes.

Regulation of transcription of nucleotide-binding leucine-rich repeat-encoding genes SNC1 and RPP4 via H3K4 trimethylation

Plant nucleotide-binding leucine-rich repeat (NB-LRR) proteins serve as intracellular sensors to detect pathogen effectors and trigger immune responses. Transcription of the NB-LRR-encoding Resistance (R) genes needs to be tightly controlled to avoid inappropriate defense activation. How the expression of the NB-LRR R genes is regulated is poorly understood. The Arabidopsis (Arabidopsis thaliana) suppressor of npr1-1, constitutive 1 (snc1) mutant carries a gain-of-function mutation in a Toll/Interleukin1 receptor-like (TIR)-NB-LRR-encoding gene, resulting in the constitutive activation of plant defense responses. A snc1 suppressor screen identified modifier of snc1,9 (mos9), which partially suppresses the autoimmune phenotypes of snc1. Positional cloning revealed that MOS9 encodes a plant-specific protein of unknown function. Expression analysis showed that MOS9 is required for the full expression of TIR-NB-LRR protein-encoding RECOGNITION OF PERONOSPORA PARASITICA 4 (RPP4) and SNC1, both of which reside in the RPP4 cluster. Coimmunoprecipitation and mass spectrometry analyses revealed that MOS9 associates with the Set1 class lysine 4 of histone 3 (H3K4) methyltransferase Arabidopsis Trithorax-Related7 (ATXR7). Like MOS9, ATXR7 is also required for the full expression of SNC1 and the autoimmune phenotypes in the snc1 mutant. In atxr7 mutant plants, the expression of RPP4 is similarly reduced, and resistance against Hyaloperonospora arabidopsidis Emwa1 is compromised. Consistent with the attenuated expression of SNC1 and RPP4, trimethylated H3K4 marks are reduced around the promoters of SNC1 and RPP4 in mos9 plants. Our data suggest that MOS9 functions together with ATXR7 to regulate the expression of SNC1 and RPP4 through H3K4 methylation, which plays an important role in fine-tuning their transcription levels and functions in plant defense.

EMBRYONIC FLOWER1 and ULTRAPETALA1 Act Antagonistically on Arabidopsis Development and Stress Response

Epigenetic regulation of gene expression is of fundamental importance for eukaryotic development. EMBRYONIC FLOWER1 (EMF1) is a plant-specific gene that participates in Polycomb group-mediated transcriptional repression of target genes such as the flower MADS box genes AGAMOUS, APETALA3, and PISTILLATA. Here, we investigated the molecular mechanism underlying the curly leaf and early flowering phenotypes caused by reducing EMF1 activity in the leaf primordia of LFYasEMF1 transgenic plants and propose a combined effect of multiple flower MADS box gene activities on these phenotypes. ULTRAPETALA1 (ULT1) functions as a trithorax group factor that counteracts Polycomb group action in Arabidopsis (Arabidopsis thaliana). Removing ULT1 activity rescues both the abnormal developmental phenotypes and most of the misregulated gene expression of LFYasEMF1 plants. Reducing EMF1 activity increases salt tolerance, an effect that is diminished by introducing the ult1-3 mutation into the LFYasEMF1 background. EMF1 is required for trimethylating lysine-27 on histone 3 (H3K27me3), and ULT1 associates with ARABIDOPSIS TRITHORAX1 (ATX1) for trimethylating lysine-3 on histone 4 (H3K4me3) at flower MADS box gene loci. Reducing EMF1 activity decreases H3K27me3 marks and increases H3K4me3 marks on target gene loci. Removing ULT1 activity has the opposite effect on the two histone marks. Removing both gene activities restores the active and repressive marks to near wild-type levels. Thus, ULT1 acts as an antirepressor that counteracts EMF1 action through modulation of histone marks on target genes. Our analysis indicates that, instead of acting as off and on switches, EMF1 and ULT1 mediate histone mark deposition and modulate transcriptional activities of the target genes.

SDG2-mediated H3K4 methylation is required for proper Arabidopsis root growth and development

Trithorax group (TrxG) proteins are evolutionarily conserved in eukaryotes and play critical roles in transcriptional activation via deposition of histone H3 lysine 4 trimethylation (H3K4me3) in chromatin. Several Arabidopsis TrxG members have been characterized, and among them SET DOMAIN GROUP 2 (SDG2) has been shown to be necessary for global genome-wide H3K4me3 deposition. Although pleiotropic phenotypes have been uncovered in the sdg2 mutants, SDG2 function in the regulation of stem cell activity has remained largely unclear. Here, we investigate the sdg2 mutant root phenotype and demonstrate that SDG2 is required for primary root stem cell niche (SCN) maintenance as well as for lateral root SCN establishment. Loss of SDG2 results in drastically reduced H3K4me3 levels in root SCN and differentiated cells and causes the loss of auxin gradient maximum in the root quiescent centre. Elevated DNA damage is detected in the sdg2 mutant, suggesting that impaired genome integrity may also have challenged the stem cell activity. Genetic interaction analysis reveals that SDG2 and CHROMATIN ASSEMBLY FACTOR-1 act synergistically in root SCN and genome integrity maintenance but not in telomere length maintenance. We conclude that SDG2-mediated H3K4me3 plays a distinctive role in the regulation of chromatin structure and genome integrity, which are key features in pluripotency of stem cells and crucial for root growth and development.

Coordination of the vernalization response through a VIN3 and FLC gene family regulatory network in Arabidopsis

Vernalization is an environmentally induced epigenetic switch in which winter cold triggers epigenetic silencing of floral repressors and thus provides competence to flower in spring. Vernalization triggers the recruitment of chromatin-modifying complexes to a clade of flowering repressors that are epigenetically silenced via chromatin modifications. In Arabidopsis thaliana, VERNALIZATION INSENSITIVE3 (VIN3) and its related plant homeodomain finger proteins act together with Polycomb Repressive Complex 2 to increase repressive histone marks at floral repressor loci, including FLOWERING LOCUS C (FLC) and its related genes, by vernalization. Here, we show that VIN3 family of proteins nonredundantly functions to repress different subsets of the FLC gene family during the course of vernalization. Each VIN3 family protein binds to modified histone peptides in vitro and directly associates with specific sets of FLC gene family chromatins in vivo to mediate epigenetic silencing. In addition, members of the FLC gene family are also differentially regulated during the course of vernalization to mediate proper vernalization response. Our results show that these two gene families cooperated during the course of evolution to ensure proper vernalization response through epigenetic changes.

Evolutionarily conserved histone methylation dynamics during seed life-cycle transitions

Plants have a remarkable ability to react to seasonal changes by synchronizing life-cycle transitions with environmental conditions. We addressed the question of how transcriptional re-programming occurs in response to an environmental cue that triggers the major life cycle transition from seed dormancy to germination and seedling growth. We elucidated an important mechanistic aspect of this process by following the chromatin dynamics of key regulatory genes with a focus on the two antagonistic marks, H3K4me3 and H3K27me3. Histone methylation patterns of major dormancy regulators changed during the transition to germination and seedling growth. We observed a switch from H3K4me3 and high transcription levels to silencing by the repressive H3K27me3 mark when dormancy was broken through exposure to moist chilling, underscoring that a functional PRC2 complex is necessary for this transition. Moreover, this reciprocal regulation by H3K4me3 and H3K27me3 is evolutionarily conserved from gymnosperms to angiosperms.

The catalytic subunit of Arabidopsis DNA polymerase α ensures stable maintenance of histone modification

Mitotic inheritance of identical cellular memory is crucial for development in multicellular organisms. The cell type-specific epigenetic state should be correctly duplicated upon DNA replication to maintain cellular memory during tissue and organ development. Although a role of DNA replication machinery in maintenance of epigenetic memory has been proposed, technical limitations have prevented characterization of the process in detail. Here, we show that INCURVATA2 (ICU2), the catalytic subunit of DNA polymerase α in Arabidopsis, ensures the stable maintenance of repressive histone modifications. The missense mutant allele icu2-1 caused a defect in the mitotic maintenance of vernalization memory. Although neither the recruitment of CURLY LEAF (CLF), a SET-domain component of Polycomb Repressive Complex 2 (PRC2), nor the resultant deposition of the histone mark H3K27me3 required for vernalization-induced FLOWERING LOCUS C (FLC) repression were affected, icu2-1 mutants exhibited unstable maintenance of the H3K27me3 level at the FLC region, which resulted in mosaic FLC de-repression after vernalization. ICU2 maintains the repressive chromatin state at additional PRC2 targets as well as at heterochromatic retroelements. In icu2-1 mutants, the subsequent binding of LIKE-HETEROCHROMATIN PROTEIN 1 (LHP1), a functional homolog of PRC1, at PRC2 targets was also reduced. We demonstrated that ICU2 facilitates histone assembly in dividing cells, suggesting a possible mechanism for ICU2-mediated epigenetic maintenance.

Chromatin regulation of flowering

The transition to flowering is a major developmental switch in the life cycle of plants. In Arabidopsis (Arabidopsis thaliana), chromatin mechanisms play critical roles in flowering-time regulation through the expression control of key flowering-regulatory genes. Various conserved chromatin modifiers, plant-specific factors, and long noncoding RNAs are involved in chromatin regulation of FLOWERING LOCUS C (FLC, a potent floral repressor). The well-studied FLC regulation has provided a paradigm for chromatin-based control of other developmental genes. In addition, chromatin modification plays an important role in the regulation of FLOWERING LOCUS T (FT, encoding florigen), which is widely conserved in angiosperm species. The chromatin mechanisms underlying FT regulation in Arabidopsis are likely involved in the regulation of FT relatives and, therefore, flowering-time control in other plants.

The histone methyltransferase SDG724 mediates H3K36me2/3 deposition at MADS50 and RFT1 and promotes flowering in rice

Chromatin modifications affect flowering time in the long-day plant Arabidopsis thaliana, but the role of histone methylation in flowering time regulation of rice (Oryza sativa), a short-day plant, remains to be elucidated. We identified a late-flowering long vegetative phase1 (lvp1) mutant in rice and used map-based cloning to reveal that lvp1 affects the SET domain group protein 724 (SDG724). SDG724 functions as a histone methyltransferase in vitro and contributes to a major fraction of global histone H3 lysine 36 (H3K36) methylation in vivo. Expression analyses of flowering time genes in wild-type and lvp1 mutants revealed that Early heading date1, but not Heading date1, are misregulated in lvp1 mutants. In addition, the double mutant of lvp1 with photoperiod sensitivity5 (se5) flowered later than the se5 single mutant, indicating that lvp1 delays flowering time irrespective of photoperiod. Chromatin immunoprecipitation assays showed that lvp1 had reduced levels of H3K36me2/3 at MADS50 and RFT1. This suggests that the divergent functions of paralogs RFT1 and Hd3a, and of MADS50 and MADS51, are in part due to differential H3K36me2/3 deposition, which also correlates with higher expression levels of MADS50 and RFT1 in flowering promotion in rice.

Flowering time control: another window to the connection between antisense RNA and chromatin

A high proportion of all eukaryotic genes express antisense RNA (asRNA), which accumulates to varying degrees at different loci. Whether there is a general function for asRNA is unknown, but its widespread occurrence and frequent regulation by stress suggest an important role. The best-characterized plant gene exhibiting a complex antisense transcript pattern is the Arabidopsis floral regulator FLOWERING LOCUS C (FLC). Changes occur in the accumulation, splicing, and polyadenylation of this antisense transcript, termed COOLAIR, in different environments and genotypes. These changes are associated with altered chromatin regulation and differential FLC expression, provoking mechanistic comparisons with many well-studied loci in yeast and mammals. Detailed analysis of these specific examples may shed light on the complex interplay between asRNA and chromatin modifications in different genomes.

Overexpression of a histone H3K4 demethylase, JMJ15, accelerates flowering time in Arabidopsis

The methylation of histone 3 lysine 4 (H3K4) is essential for gene activation. Flowering Locus C (FLC), an important flowering repressor, quantitatively regulates flowering time in Arabidopsis and its expression level is coincident with H3K4 trimethylation (H3K4me3) dynamics. The methylation state of FLC chromatin is determined by the balance between methylation and demethylation, which is mediated by histone methyltransferases and demethylases, respectively. However, little is known about the role of histone demethylase(s) in FLC regulation. Here, we characterized the biochemical activity and biological function of a novel JmjC domain-containing H3K4 demethylase, JMJ15, in Arabidopsis. JMJ15, which is a member of the H3K4 demethylase JARID1 family, displayed H3K4me3 demethylase activity both in vitro and in vivo. The mutation of JMJ15 did not produce an obvious phenotype; however, overexpression JMJ15 resulted in an obvious early flowering phenotype, which was associated with the repression of FLC level and reduction in H3K4me3 at the FLC locus, resulting in increased FT expression. Our results suggest that JMJ15 is a novel H3K4 demethylase, involved in the control of flowering time by demethylating H3K4me3 at FLC chromatin when it was overexpressed in Arabidopsis.

KEY MESSAGE

Overexpression of a histone H3K4 demethylase, JMJ15, represses FLC expression by decreasing its chromatin H3K4me3 level, thereby controlling flowering time in Arabidopsis.

Vernalization-mediated chromatin changes

Proper flowering time is vital for reproductive fitness in flowering plants. In Arabidopsis, vernalization is mediated primarily through the repression of a MADS box transcription factor, FLOWERING LOCUS C (FLC). The induction of a plant homeodomain-containing protein, VERNALIZATION INSENSITIVE 3 (VIN3), by vernalizing cold is required for proper repression of FLC. One of a myriad of changes that occurs after VIN3 is induced is the establishment of FLC chromatin at a mitotically repressed state due to the enrichment of repressive histone modifications. VIN3 induction by cold is the earliest known event during the vernalization response and includes changes in histone modifications at its chromatin. Here, the current understanding of the vernalization-mediated chromatin changes in Arabidopsis is discussed, with a focus on the roles of shared chromatin-modifying machineries in regulating VIN3 and FLC gene family expression during the course of vernalization.

Arabidopsis trithorax-related3/SET domain GROUP2 is required for the winter-annual habit of Arabidopsis thaliana

The winter-annual habit of Arabidopsis thaliana requires active alleles of flowering locus C (FLC), which encodes a potent flowering repressor, and FRIGIDA (FRI), an activator of FLC. FLC activation by FRI is accompanied by an increase in specific histone modifications, such as tri-methylation of histone H3 at lysine 4 (H3K4me3), and requires three H3K4 methyltransferases, the Drosophila Trithorax-class Arabidopsis trithorax1 (ATX1) and ATX2, and yeast Set1-class ATX-related7/set domain group25 (ATXR7/SDG25). However, lesions in all of these genes failed to suppress the enhanced FLC expression caused by FRI completely, suggesting that another H3K4 methyltransferase may participate in the FLC activation. Here, we show that ATXR3/SDG2, which is a member of a novel class of H3K4 methyltransferases, also contributes to FLC activation. An ATXR3 lesion suppressed the enhanced FLC expression and delayed flowering caused by an active allele of FRI in non-vernalized plants. The decrease in FLC expression in atxr3 mutants was accompanied by reduced H3K4me3 levels at FLC chromatin. We also found that the rapid flowering of atxr3 was epistatic to that of atxr7, suggesting that ATXR3 functions in FLC activation in sequence with ATXR7. Our results indicate that the novel-class H3K4 methyltransferase, ATXR3, is a transcriptional activator that plays a role in the FLC activation and establishing the winter-annual habit. In addition, ATXR3 also contributes to the activation of other FLC clade members, such as flowering locus M/MADS affecting flowering1 (FLM/MAF1) and MAF5, at least partially explaining the ATXR3 function in delayed flowering caused by non-inductive photoperiods.

FLC: a hidden polycomb response element shows up in silence

A sizeable fraction of eukaryotic genomes is regulated by Polycomb group (PcG) and trithorax group (trxG) proteins, which play key roles in epigenetic repression and activation, respectively. In Drosophila melanogaster, homeotic genes are well-documented PcG targets; they are known to contain cis-acting elements termed Polycomb response elements (PREs), which bind PcG proteins and satisfy three defined criteria, and also often contain binding sites for the trithorax (trx) protein. However, the presence of PREs, or an alternative mode for PcG/trxG interaction with the genome, has not been well documented outside Drosophila. In Arabidopsis thaliana, PcG/trxG regulation has been studied extensively for the flowering repressor gene FLOWERING LOCUS C (FLC). Here we evaluate how PRE-like activities that reside within the FLC locus may satisfy the defined Drosophila criteria, by analyzing four FLC transcription states. When the FLC locus is not transcribed, the intrinsic PcG recruitment ability of the coding region can be attributed to two redundant cis-acting elements (Modules IIA and IIB). When FLC is highly expressed, trxG recruitment is to a region overlapping the transcription start site (Module I). Exposure to prolonged cold converts the active FLC state into a repressed state that is maintained after the cold period finishes. These two additional transcriptional states also rely on the same three modules for PcG/trxG regulation. We conclude that each of Modules I, IIA and IIB partially fulfills the PRE function criteria, and that together they represent the functional FLC PRE, which differs structurally from canonical PREs in Drosophila.

A companion cell-dominant and developmentally regulated H3K4 demethylase controls flowering time in Arabidopsis via the repression of FLC expression

Flowering time relies on the integration of intrinsic developmental cues and environmental signals. FLC and its downstream target FT are key players in the floral transition in Arabidopsis. Here, we characterized the expression pattern and function of JMJ18, a novel JmjC domain-containing histone H3K4 demethylase gene in Arabidopsis. JMJ18 was dominantly expressed in companion cells; its temporal expression pattern was negatively and positively correlated with that of FLC and FT, respectively, during vegetative development. Mutations in JMJ18 resulted in a weak late-flowering phenotype, while JMJ18 overexpressors exhibited an obvious early-flowering phenotype. JMJ18 displayed demethylase activity toward H3K4me3 and H3K4me2, and bound FLC chromatin directly. The levels of H3K4me3 and H3K4me2 in chromatins of FLC clade genes and the expression of FLC clade genes were reduced, whereas FT expression was induced and the protein expression of FT increased in JMJ18 overexpressor lines. The early-flowering phenotype caused by the overexpression of JMJ18 was mainly dependent on the functional FT. Our findings suggest that the companion cell–dominant and developmentally regulated JMJ18 binds directly to the FLC locus, reducing the level of H3K4 methylation in FLC chromatin and repressing the expression of FLC, thereby promoting the expression of FT in companion cells to stimulate flowering.

Flowering is an important developmental transition during plant life cycle and the key process for production of the next generation. Flowering time is controlled by both intrinsic developmental and environmental signals. FLC and its target FT work as repressor and activator, respectively, to regulate flowering time in Arabidopsis; thus the regulation of FLC and FT expression is the key for the control of floral transition. Epigenetic modifications are critical for transcription regulation. Here, we show that a novel JmjC domain-containing histone H3K4 demethylase, JMJ18, is a key regulator for the expression of FLC and FT in companion cells and flowering time. JMJ18 is dominantly expressed in vascular tissue; its temporal expression pattern was developmentally regulated, and negatively and positively correlated with FLC and FT, respectively. JMJ18 mutation leads to weak late-flowering, while JMJ18 overexpressor exhibited an obvious early-flowering phenotype. JMJ18 binds to chromatin of FLC, represses its expression, and releases expression of FT in companion cells. Our results suggest that JMJ18 is a developmentally regulated companion cell–dominantly expressed signal to control flowering time by binding to FLC—reducing level of H3K4 methylation in FLC and repressing expression of FLC—thereby promoting expression of FT in companion cells during vegetative development in Arabidopsis.

Expansion and diversification of the SET domain gene family following whole-genome duplications in Populus trichocarpa

Background

Histone lysine methylation modifies chromatin structure and regulates eukaryotic gene transcription and a variety of developmental and physiological processes. SET domain proteins are lysine methyltransferases containing the evolutionarily-conserved SET domain, which is known to be the catalytic domain.

Results

We identified 59 SET genes in the Populus genome. Phylogenetic analyses of 106 SET genes from Populus and Arabidopsis supported the clustering of SET genes into six distinct subfamilies and identified 19 duplicated gene pairs in Populus. The chromosome locations of these gene pairs and the distribution of synonymous substitution rates showed that the expansion of the SET gene family might be caused by large-scale duplications in Populus. Comparison of gene structures and domain architectures of each duplicate pair indicated that divergence took place at the 3'- and 5'-terminal transcribed regions and at the N- and C-termini of the predicted proteins, respectively. Expression profile analysis of Populus SET genes suggested that most Populus SET genes were expressed widely, many with the highest expression in young leaves. In particular, the expression profiles of 12 of the 19 duplicated gene pairs fell into two types of expression patterns.

Conclusions

The 19 duplicated SET genes could have originated from whole genome duplication events. The differences in SET gene structure, domain architecture, and expression profiles in various tissues of Populus suggest that members of the SET gene family have a variety of developmental and physiological functions. Our study provides clues about the evolution of epigenetic regulation of chromatin structure and gene expression.

Phylogenetics and evolution of Trx SET genes in fully sequenced land plants

Plant Trx SET proteins are involved in H3K4 methylation and play a key role in plant floral development. Genes encoding Trx SET proteins constitute a multigene family in which the copy number varies among plant species and functional divergence appears to have occurred repeatedly. To investigate the evolutionary history of the Trx SET gene family, we made a comprehensive evolutionary analysis on this gene family from 13 major representatives of green plants. A novel clustering (here named as cpTrx clade), which included the III-1, III-2, and III-4 orthologous groups, previously resolved was identified. Our analysis showed that plant Trx proteins possessed a variety of domain organizations and gene structures among paralogs. Additional domains such as PHD, PWWP, and FYR were early integrated into primordial SET-PostSET domain organization of cpTrx clade. We suggested that the PostSET domain was lost in some members of III-4 orthologous group during the evolution of land plants. At least four classes of gene structures had been formed at the early evolutionary stage of land plants. Three intronless orphan Trx SET genes from the Physcomitrella patens (moss) were identified, and supposedly, their parental genes have been eliminated from the genome. The structural differences among evolutionary groups of plant Trx SET genes with different functions were described, contributing to the design of further experimental studies.

Environmentally coordinated epigenetic silencing of FLC by protein and long noncoding RNA components

In Arabidopsis, the role of the vernalization pathway is to repress expression of a potent floral repressor, FLOWERING LOCUS C (FLC), after a sufficient period of winter cold has been perceived. Following winter, the lack of FLC expression allows unimpeded operation of the photoperiod pathway and hence rapid flowering of vernalized plants in spring via the activation of floral integrator genes. Molecular studies revealed that regulation of the key floral repressor, FLC, is under the control of the interplay between Trithorax group (TrxG)-mediated activation and Polycomb group (PcG)-mediated repression. On-off switch of genes by TrxG and PcG is an evolutionarily conserved mechanism to coordinate cellular identity in eukaryotes. Regulation of FLC by external cues provides an excellent model system to study mechanisms in which cell identity is influenced by environment. In this review, we discuss coordinated contributions by protein and long noncoding RNA components to this environmentally induced epigenetic switch of a developmental program in plants.

Phylogenetic analysis and classification of the Brassica rapa SET-domain protein family

BACKGROUND

The SET (Su(var)3-9, Enhancer-of-zeste, Trithorax) domain is an evolutionarily conserved sequence of approximately 130-150 amino acids, and constitutes the catalytic site of lysine methyltransferases (KMTs). KMTs perform many crucial biological functions via histone methylation of chromatin. Histone methylation marks are interpreted differently depending on the histone type (i.e. H3 or H4), the lysine position (e.g. H3K4, H3K9, H3K27, H3K36 or H4K20) and the number of added methyl groups (i.e. me1, me2 or me3). For example, H3K4me3 and H3K36me3 are associated with transcriptional activation, but H3K9me2 and H3K27me3 are associated with gene silencing. The substrate specificity and activity of KMTs are determined by sequences within the SET domain and other regions of the protein.

RESULTS

Here we identified 49 SET-domain proteins from the recently sequenced Brassica rapa genome. We performed sequence similarity and protein domain organization analysis of these proteins, along with the SET-domain proteins from the dicot Arabidopsis thaliana, the monocots Oryza sativa and Brachypodium distachyon, and the green alga Ostreococcus tauri. We showed that plant SET-domain proteins can be grouped into 6 distinct classes, namely KMT1, KMT2, KMT3, KMT6, KMT7 and S-ET. Apart from the S-ET class, which has an interrupted SET domain and may be involved in methylation of nonhistone proteins, the other classes have characteristics of histone methyltransferases exhibiting different substrate specificities: KMT1 for H3K9, KMT2 for H3K4, KMT3 for H3K36, KMT6 for H3K27 and KMT7 also for H3K4. We also propose a coherent and rational nomenclature for plant SET-domain proteins. Comparisons of sequence similarity and synteny of B. rapa and A. thaliana SET-domain proteins revealed recent gene duplication events for some KMTs.

CONCLUSION

This study provides the first characterization of the SET-domain KMT proteins of B. rapa. Phylogenetic analysis data allowed the development of a coherent and rational nomenclature of this important family of proteins in plants, as in animals. The results obtained in this study will provide a base for nomenclature of KMTs in other plant species and facilitate the functional characterization of these important epigenetic regulatory genes in Brassica crops.

The RPT2 subunit of the 26S proteasome directs complex assembly, histone dynamics, and gametophyte and sporophyte development in Arabidopsis

The regulatory particle (RP) of the 26S proteasome contains a heterohexameric ring of AAA-ATPases (RPT1-6) that unfolds and inserts substrates into the core protease (CP) for degradation. Through genetic analysis of the Arabidopsis thaliana gene pair encoding RPT2, we show that this subunit plays a critical role in 26S proteasome assembly, histone dynamics, and plant development. rpt2a rpt2b double null mutants are blocked in both male and female gamete transmission, demonstrating that the subunit is essential. Whereas rpt2b mutants are phenotypically normal, rpt2a mutants display a range of defects, including impaired leaf, root, trichome, and pollen development, delayed flowering, stem fasciation, hypersensitivity to mitomycin C and amino acid analogs, hyposensitivity to the proteasome inhibitor MG132, and decreased 26S complex stability. The rpt2a phenotype can be rescued by both RPT2a and RPT2b, indicative of functional redundancy, but not by RPT2a mutants altered in ATP binding/hydrolysis or missing the C-terminal hydrophobic sequence that docks the RPT ring onto the CP. Many rpt2a phenotypes are shared with mutants lacking the chromatin assembly factor complex CAF1. Like caf1 mutants, plants missing RPT2a or reduced in other RP subunits contain less histones, thus implicating RPT2 specifically, and the 26S proteasome generally, in plant nucleosome assembly.

Identification of regulators required for the reactivation of FLOWERING LOCUS C during Arabidopsis reproduction

FLOWERING LOCUS C (FLC) is a central floral repressor for the determination of flowering time in Arabidopsis. FLC expression is reactivated upon fertilization and regulated during seed development to ensure the appropriate floral behavior; however, the molecular mechanism for this process is largely unknown. Here, we report the identification of crucial regulators for FLC reactivation during embryogenesis by analyzing FLC::GUS and endogenous FLC expression. We newly define that the full reactivation of FLC requires a FRIGIDA (FRI)-containing protein complex throughout embryogenesis. Mutations in EARLY FLOWERING 7 (ELF7) and VERNALIZATION INDEPENDENCE4 (VIP4) showed severe defects in the reactivation of FLC transcription, suggesting that both of the genes, Arabidopsis homologs of the members of the yeast RNA polymerase II-associated factor 1 (Paf1) complex, are indispensable for FLC reactivation. actin-related protein 6 (arp6), arabidopsis trithorax 1 (atx1), arabidopsis trithorax-related 7 (atxr7), and atx1 atxr7 double mutants also caused the downregulation of FLC during seed development, but the defects were less severe than those in mutants for the FRI- and Paf1-complexes. These results suggest that the ARP6-containing Swr1-complex and FLC-specific histone methyltransferases, ATX1 and ATXR7, have relatively partial roles in FLC reactivation. In contrast to the roles of the histone modifiers, factors in the DNA methylation pathway and biogenesis of small RNAs are not involved in FLC regulation during reproduction. Taken together, our results demonstrate that adjustment by select FLC activators is critical for the re-establishment of an FLC expression state after fertilization to ensure competence for optimal flowering in the next generation.

Myo-inositol and beyond--emerging networks under stress

Myo-inositol is a versatile compound that generates diversified derivatives upon phosphorylation by lipid-dependent and -independent pathways. Phosphatidylinositols form one such group of myo-inositol derivatives that act both as membrane structural lipid molecules and as signals. The significance of these compounds lies in their dual functions as signals as well as key metabolites under stress. Several stress- and non-stress related pathways regulated by phosphatidylinositol isoforms and associated enzymes, kinases and phosphatases, appear to function in parallel to coordinatively adapt growth and stress responses in plants. Recent evidence also postulates their crucial roles in nuclear functions as they interact with the key players of chromatin structure, yet other nuclear functions remain largely unknown. Phosphatidylinositol monophosphate 5-kinase interacts with and represses a cytosolic neutral invertase, a key enzyme of sugar metabolism suggesting a crosstalk between lipid and sugar signaling. Besides phosphatidylinositol, myo-inositol derived galactinol and associated raffinose-family oligosaccharides are emerging as antioxidants and putative signaling compounds too. Importantly, myo-inositol polyphosphate 5-phosphatase (5PTase) acts, depending on sugar status, as a positive or negative regulator of a global energy sensor, SnRK1. This implies that both myo-inositol- and sugar-derived (e.g. trehalose 6-phosphate) molecules form part of a broad regulatory network with SnRK1 as the central regulator. Recently, it was shown that the transcription factor bZIP11 also takes part in this network. Moreover, a functional coordination between neutral invertase and hexokinase is emerging as a sweet network that contributes to oxidative stress homeostasis in plants. In this review, we focus on myo-inositol, its direct and more downstream derivatives (galactinol, raffinose), and the contribution of their associated networks to plant stress tolerance.

Timing is everything in plant development. The central role of floral repressors

Progress in understanding the molecular basis of flowering time control has revealed that floral repressors play a central role in modulating the floral transition and are essential to prevent the precocious onset of flowering. A number of cellular processes including chromatin remodeling, selective protein degradation, and transcriptional regulation mediated by transcription factors are involved in repressing the initiation of flowering. Floral repressors interact at different levels with floral inductive pathways and prevent the premature onset of flowering that could impact negatively on the reproductive success of plants. Despite recent advances, further studies will be needed to understand how the interactions between floral repressors and the regulatory networks involved in the control of flowering time have evolved in different species. Recent data suggest that a diversity of regulatory proteins act as central floral repressors in different plants, and even in those species where regulatory modules are conserved new elements that modulate the function of these pathways have been recruited to mediate specific adaptive responses. The development of genomic tools and predictive models that can integrate large datasets related to the flowering behavior of plant species will facilitate the characterization of the repressor mechanisms underlying flowering responses, a trait with implications in the yield of crop species. In a scenario of global climate change, an in depth understanding of these gene circuits will be essential for the development of crop varieties with improved yield.

SET domain proteins in plant development

Post-translational methylation of lysine residues on histone tails is an epigenetic modification crucial for regulation of chromatin structure and gene expression in eukaryotes. The majority of the histone lysine methyltransferases (HKMTases) conferring such modifications are proteins with a conserved SET domain responsible for the enzymatic activity. The SET domain proteins in the model plant Arabidopsis thaliana can be assigned to evolutionarily conserved classes with different specificities allowing for different outcomes on chromatin structure. Here we review the present knowledge of the biochemical and biological functions of plant SET domain proteins in developmental processes. This article is part of a Special Issue entitled: Epigenetic control of cellular and developmental processes in plants.

Transcript profiling of crown rootless1 mutant stem base reveals new elements associated with crown root development in rice

Background

In rice, the major part of the post-embryonic root system is made of stem-derived roots named crown roots (CR). Among the few characterized rice mutants affected in root development, crown rootless1 mutant is unable to initiate crown root primordia. CROWN ROOTLESS1 (CRL1) is induced by auxin and encodes an AS2/LOB-domain transcription factor that acts upstream of the gene regulatory network controlling CR development.

Results

To identify genes involved in CR development, we compared global gene expression profile in stem bases of crl1 mutant and wild-type (WT) plants. Our analysis revealed that 250 and 236 genes are down- and up-regulated respectively in the crl1 mutant. Auxin induces CRL1 expression and consequently it is expected that auxin also alters the expression of genes that are early regulated by CRL1. To identify genes under the early control of CRL1, we monitored the expression kinetics of a selected subset of genes, mainly chosen among those exhibiting differential expression, in crl1 and WT following exogenous auxin treatment. This analysis revealed that most of these genes, mainly related to hormone, water and nutrient, development and homeostasis, were likely not regulated directly by CRL1. We hypothesized that the differential expression for these genes observed in the crl1 mutant is likely a consequence of the absence of CR formation. Otherwise, three CRL1-dependent auxin-responsive genes: FSM (FLATENNED SHOOT MERISTEM)/FAS1 (FASCIATA1), GTE4 (GENERAL TRANSCRIPTION FACTOR GROUP E4) and MAP (MICROTUBULE-ASSOCIATED PROTEIN) were identified. FSM/FAS1 and GTE4 are known in rice and Arabidopsis to be involved in the maintenance of root meristem through chromatin remodelling and cell cycle regulation respectively.

Conclusion

Our data showed that the differential regulation of most genes in crl1 versus WT may be an indirect consequence of CRL1 inactivation resulting from the absence of CR in the crl1 mutant. Nevertheless some genes, FAS1/FSM, GTE4 and MAP, require CRL1 to be induced by auxin suggesting that they are likely directly regulated by CRL1. These genes have a function related to polarized cell growth, cell cycle regulation or chromatin remodelling. This suggests that these genes are controlled by CRL1 and involved in CR initiation in rice.

Identification of the Arabidopsis REDUCED DORMANCY 2 gene uncovers a role for the polymerase associated factor 1 complex in seed dormancy

The life of a plant is characterized by major phase transitions. This includes the agriculturally important transitions from seed to seedling (germination) and from vegetative to generative growth (flowering induction). In many plant species, including Arabidopsis thaliana, freshly harvested seeds are dormant and incapable of germinating. Germination can occur after the release of dormancy and the occurrence of favourable environmental conditions. Although the hormonal control of seed dormancy is well studied, the molecular mechanisms underlying the induction and release of dormancy are not yet understood.

In this study, we report the cloning and characterization of the mutant reduced dormancy 2-1 (rdo2-1). We found that RDO2 is allelic to the recently identified dormancy gene TFIIS, which is a transcription elongation factor. HUB1, which was previously called RDO4, was identified in the same mutagenesis screen for reduced dormancy as rdo2-1 and was also shown to be involved in transcription elongation. The human homologues of RDO2 and HUB1 interact with the RNA Polymerase II Associated Factor 1 Complex (PAF1C). Therefore, we investigated the effect of other Arabidopsis PAF1C related factors; VIP4, VIP5, ELF7, ELF8 and ATXR7 on seed dormancy. Mutations in these genes resulted in reduced dormancy, similar to hub1-2 and rdo2-1. Consistent with a role at the end of seed maturation, we found that HUB1, RDO2 and VIP5 are upregulated during this developmental phase. Since mutants in PAF1C related factors are also described to be early flowering, we conclude that these components are involved in the regulation of both major developmental transitions in the plant.

Histone modifications in transcriptional activation during plant development

In eukaryotic cell nuclei, chromatin states dictated by different combinations of post-translational modifications of histones, such as acetylation, methylation and monoubiquitination of lysine residues, are part of the multitude of epigenomes involved in the fine-tuning of all genetic functions and in particular transcription. During the past decade, an increasing number of 'writers', 'readers' and 'erasers' of histone modifications have been identified. Characterization of these factors in Arabidopsis has unraveled their pivotal roles in the regulation of essential processes, such as floral transition, cell differentiation, gametogenesis, and plant response/adaptation to environmental stresses. In this review we focus on histone modification marks associated with transcriptional activation to highlight current knowledge on Arabidopsis 'writers', 'readers' and 'erasers' of histone modifications and to discuss recent findings on molecular mechanisms of integration of histone modifications with the RNA polymerase II transcriptional machinery during transcription of the flowering repressor gene FLC.

Genome-wide analysis of the SET DOMAIN GROUP family in grapevine

The SET DOMAIN GROUP (SDG) proteins represent an evolutionarily-conserved family of epigenetic regulators present in eukaryotes and are putative candidates for the catalysis of lysine methylation in histones. Plant genomes analyses of this family have been performed in arabidopsis, maize, and rice and functional studies have shown that SDG genes are involved in the control of plant development. In this work, we describe the identification and structural characterization of SDG genes in the Vitis vinifera genome. This analysis revealed the presence of 33 putative SDG genes that can be grouped into different classes, as it has been previously described for plants. In addition to the SET domain, the proteins identified possessed other domains in the different classes. As part of our study regarding the growth and development of grapevine, we selected eight genes and their expression levels were analyzed in representative vegetative and reproductive organs of this species. The selected genes showed different patterns of expression during inflorescence and fruit development, suggesting that they participate in these processes. Furthermore, we showed that the expression of selected SDGs changes during viral infection, using as a model Grapevine Leafroll Associated Virus 3-infected symptomatic grapevine leaves and fruits. Our results suggest that developmental changes caused by this virus could be the result of alterations in SDG expression.

Histone variants and modifications in plant gene regulation

Genomes are packaged by complexing DNA with histone proteins, which provides an opportunity to regulate gene expression by dynamically impeding access of transcriptional regulatory proteins and RNA polymerases to DNA. The incorporation of histone variants into nucleosomes and addition of post-translational modifications to histones can alter the physical properties of nucleosomes and thereby serve as a mechanism for regulating DNA exposure. Chromatin-based gene regulation has profound effects on developmental processes including regulation of the vegetative to reproductive transition, as well as responses to pathogens and abiotic factors. Incorporation of the histone variant H2A.Z and methylation of histone H3 lysine residues 4 and 27 have emerged as key elements in the regulation of genes involved in each of these processes.

Arabidopsis COMPASS-like complexes mediate histone H3 lysine-4 trimethylation to control floral transition and plant development

Histone H3 lysine-4 (H3K4) methylation is associated with transcribed genes in eukaryotes. In Drosophila and mammals, both di- and tri-methylation of H3K4 are associated with gene activation. In contrast to animals, in Arabidopsis H3K4 trimethylation, but not mono- or di-methylation of H3K4, has been implicated in transcriptional activation. H3K4 methylation is catalyzed by the H3K4 methyltransferase complexes known as COMPASS or COMPASS-like in yeast and mammals. Here, we report that Arabidopsis homologs of the COMPASS and COMPASS-like complex core components known as Ash2, RbBP5, and WDR5 in humans form a nuclear subcomplex during vegetative and reproductive development, which can associate with multiple putative H3K4 methyltransferases. Loss of function of ARABIDOPSIS Ash2 RELATIVE (ASH2R) causes a great decrease in genome-wide H3K4 trimethylation, but not in di- or mono-methylation. Knockdown of ASH2R or the RbBP5 homolog suppresses the expression of a crucial Arabidopsis floral repressor, FLOWERING LOCUS C (FLC), and FLC homologs resulting in accelerated floral transition. ASH2R binds to the chromatin of FLC and FLC homologs in vivo and is required for H3K4 trimethylation, but not for H3K4 dimethylation in these loci; overexpression of ASH2R causes elevated H3K4 trimethylation, but not H3K4 dimethylation, in its target genes FLC and FLC homologs, resulting in activation of these gene expression and consequent late flowering. These results strongly suggest that H3K4 trimethylation in FLC and its homologs can activate their expression, providing concrete evidence that H3K4 trimethylation accumulation can activate eukaryotic gene expression. Furthermore, our findings suggest that there are multiple COMPASS-like complexes in Arabidopsis and that these complexes deposit trimethyl but not di- or mono-methyl H3K4 in target genes to promote their expression, providing a molecular explanation for the observed coupling of H3K4 trimethylation (but not H3K4 dimethylation) with active gene expression in Arabidopsis.

Histones can be covalently modified and histone modifications regulate chromatin structure and gene transcription. One such modification is histone H3 lysine-4 (H3K4) methylation, which can be mono-, di-, or tri-methylated. In animals such as fruitfly and mammals, both di- and tri-methylation of H3K4 are associated with active gene expression. In contrast to animals, in the flowering plant Arabidopsis only H3K4 trimethylation has been implicated in gene transcriptional activation. H3K4 methylation is catalyzed by the H3K4 methyltransferase complexes known as COMPASS-like in mammals. Here, we report that COMPASS-like H3K4 methyltransferase complexes exist in Arabidopsis. Loss of function of a core complex protein causes a great decrease in Arabidopsis genome-wide H3K4 trimethylation, but not in di- or mono-methylation. Our analyses of several direct target genes of these COMPASS-like complexes show that they mediate deposition of trimethyl but not dimethyl H3K4 in these loci to activate their expression, providing concrete evidence for the notion that H3K4 trimethylation accumulation can activate eukaryotic gene expression. Furthermore, our findings provide a molecular explanation for the observed coupling of trimethylation but not dimethylation of H3K4 with active gene expression in Arabidopsis. In addition, we found that H3K4 trimethylation regulates leaf growth and development, flowering, and embryo development.

Polycomb repressive complex 2 controls the embryo-to-seedling phase transition

Polycomb repressive complex 2 (PRC2) is a key regulator of epigenetic states catalyzing histone H3 lysine 27 trimethylation (H3K27me3), a repressive chromatin mark. PRC2 composition is conserved from humans to plants, but the function of PRC2 during the early stage of plant life is unclear beyond the fact that it is required for the development of endosperm, a nutritive tissue that supports embryo growth. Circumventing the requirement of PRC2 in endosperm allowed us to generate viable homozygous null mutants for FERTILIZATION INDEPENDENT ENDOSPERM (FIE), which is the single Arabidopsis homolog of Extra Sex Combs, an indispensable component of Drosophila and mammalian PRC2. Here we show that H3K27me3 deposition is abolished genome-wide in fie mutants demonstrating the essential function of PRC2 in placing this mark in plants as in animals. In contrast to animals, we find that PRC2 function is not required for initial body plan formation in Arabidopsis. Rather, our results show that fie mutant seeds exhibit enhanced dormancy and germination defects, indicating a deficiency in terminating the embryonic phase. After germination, fie mutant seedlings switch to generative development that is not sustained, giving rise to neoplastic, callus-like structures. Further genome-wide studies showed that only a fraction of PRC2 targets are transcriptionally activated in fie seedlings and that this activation is accompanied in only a few cases with deposition of H3K4me3, a mark associated with gene activity and considered to act antagonistically to H3K27me3. Up-regulated PRC2 target genes were found to act at different hierarchical levels from transcriptional master regulators to a wide range of downstream targets. Collectively, our findings demonstrate that PRC2-mediated regulation represents a robust system controlling developmental phase transitions, not only from vegetative phase to flowering but also especially from embryonic phase to the seedling stage.

Polycomb proteins regulate the quantitative induction of VERNALIZATION INSENSITIVE 3 in response to low temperatures

Vernalization, the promotion of flowering in response to low temperatures, is one of the best characterized examples of epigenetic regulation in plants. The promotion of flowering is proportional to the duration of the cold period, but the mechanism by which plants measure time at low temperatures has been a long-standing mystery. We show that the quantitative induction of the first gene in the Arabidopsis vernalization pathway, VERNALIZATION INSENSITIVE 3 (VIN3), is regulated by the components of Polycomb Response Complex 2, which trimethylates histone H3 lysine 27 (H3K27me3). In differentiated animal cells, H3K27me3 is mostly associated with long-term gene repression, whereas, in pluripotent embyonic stem cells, many cell lineage-specific genes are inactive but exist in bivalent chromatin that carries both active (H3K4me3) and repressive (H3K27me3) marks on the same molecule. During differentiation, bivalent domains are generally resolved to an active or silent state. We found that H3K27me3 maintains VIN3 in a repressed state prior to cold exposure; this mark is not removed during VIN3 induction. Instead, active VIN3 is associated with bivalently marked chromatin. The continued presence of H3K27me3 ensures that induction of VIN3 is proportional to the duration of the cold, and that plants require prolonged cold to promote the transition to flowering. The observation that Polycomb proteins control VIN3 activity defines a new role for Polycomb proteins in regulating the rate of gene induction.

Regulation of the floral repressor gene FLC: the complexity of transcription in a chromatin context

The genetic pathways regulating the floral transition in Arabidopsis are becoming increasingly well understood. The ease with which mutant phenotypes can be quantified has led to many suppressor screens and the molecular identification of the underlying genes. One focus has been on the pathways that regulate the gene encoding the floral repressor FLC. This has revealed a set of antagonistic pathways comprising evolutionary conserved activities that link chromatin regulation, transcription level and co-transcriptional RNA metabolism. Here we discuss our current understanding of the transcriptional activation of FLC, how different activities are integrated at this one locus and why FLC regulation seems so sensitive to mutation in these conserved gene regulatory pathways.

Regulation of transcription in plants: mechanisms controlling developmental switches

Unlike animals, plants produce new organs throughout their life cycle using pools of stem cells that are organized in meristems. Although many key regulators of meristem and organ identities have been identified, it is still not well understood how they function at the molecular level and how they can switch an entire developmental programme in which thousands of genes are involved. Recent advances in the genome-wide identification of target genes controlled by key plant transcriptional regulators and their interactions with epigenetic factors provide new insights into general transcriptional regulatory mechanisms that control switches of developmental programmes and cell fates in complex organisms.

A membrane-tethered transcription factor ANAC089 negatively regulates floral initiation in Arabidopsis thaliana

The plant-specific NAC (NAM, ATAF1/2, and CUC2) transcription factors have a regulatory function in developmental processes and stress responses. Notably a group of NAC members named NTLs (NTM1-Like) are membrane-tethered, ensuring plants rapidly respond to developmental changes and environmental stimuli. Our results indicated that ANAC089 was a membrane-tethered transcription factor and its truncated form was responsible for the physiological function in flowering time control.

SET DOMAIN GROUP2 is the major histone H3 lysine [corrected] 4 trimethyltransferase in Arabidopsis

Posttranslational modifications of histones play important roles in modulating chromatin structure and regulating gene expression. We have previously shown that more than two thirds of Arabidopsis genes contain histone H3 methylation at lysine 4 (H3K4me) and that trimethylation of H3K4 (H3K4me3) is preferentially located at actively transcribed genes. In addition, several Arabidopsis mutants with locus-specific loss of H3K4me have been found to display various developmental abnormalities. These findings suggest that H3K4me3 may play important roles in maintaining the normal expression of a large number of genes. However, the major enzyme(s) responsible for H3K4me3 has yet to be identified in plants, making it difficult to address questions regarding the mechanisms and functions of H3K4me3. Here we described the characterization of SET DOMAIN GROUP 2 (SDG2), a large Arabidopsis protein containing a histone lysine methyltransferase domain. We found that SDG2 homologs are highly conserved in plants and that the Arabidopsis SDG2 gene is broadly expressed during development. In addition, the loss of SDG2 leads to severe and pleiotropic phenotypes, as well as the misregulation of a large number of genes. Consistent with our finding that SDG2 is a robust and specific H3K4 methyltransferase in vitro, the loss of SDG2 leads to a drastic decrease in H3K4me3 in vivo. Taken together, these results suggest that SDG2 is the major enzyme responsible for H3K4me3 in Arabidopsis and that SDG2-dependent H3K4m3 is critical for regulating gene expression and plant development.

Vernalization-mediated VIN3 Induction Overcomes the LIKE-HETEROCHROMATIN PROTEIN1/POLYCOMB REPRESSION COMPLEX2-mediated epigenetic repression

VERNALIZATION INSENSITIVE3 (VIN3) induction by vernalization is one of the earliest events in the vernalization response of Arabidopsis (Arabidopsis thaliana). However, the mechanism responsible for vernalization-mediated VIN3 induction is poorly understood. Here, we show that the constitutive repression of VIN3 in the absence of the cold is due to multiple repressive components, including a transposable element-derived sequence, LIKE-HETEROCHROMATIN PROTEIN1 and POLYCOMB REPRESSION COMPLEX2. Furthermore, the full extent of VIN3 induction by vernalization requires activating complex components, including EARLY FLOWERING7 and EARLY FLOWERING IN SHORT DAYS. In addition, we observed dynamic changes in the histone modifications present at VIN3 chromatin during the course of vernalization. Our results show that the induction of VIN3 includes dynamic changes at the level of chromatin triggered by long-term cold exposure.

Arabidopsis SET DOMAIN GROUP2 is required for H3K4 trimethylation and is crucial for both sporophyte and gametophyte development

Histone H3 lysine 4 trimethylation (H3K4me3) is abundant in euchromatin and is in general associated with transcriptional activation in eukaryotes. Although some Arabidopsis thaliana SET DOMAIN GROUP (SDG) genes have been previously shown to be involved in H3K4 methylation, they are unlikely to be responsible for global genome-wide deposition of H3K4me3. Most strikingly, sparse knowledge is currently available about the role of histone methylation in gametophyte development. In this study, we show that the previously uncharacterized SDG2 is required for global H3K4me3 deposition and its loss of function causes wide-ranging defects in both sporophyte and gametophyte development. Transcriptome analyses of young flower buds have identified 452 genes downregulated by more than twofold in the sdg2-1 mutant; among them, 11 genes, including SPOROCYTELESS/NOZZLE (SPL/NZZ) and MALE STERILITY1 (MS1), have been previously shown to be essential for male and/or female gametophyte development. We show that both SPL/NZZ and MS1 contain bivalent chromatin domains enriched simultaneously with the transcriptionally active mark H3K4me3 and the transcriptionally repressive mark H3K27me3 and that SDG2 is specifically required for the H3K4me3 deposition. Our data suggest that SDG2-mediated H3K4me3 deposition poises SPL/NZZ and MS1 for transcriptional activation, forming a key regulatory mechanism in the gene networks responsible for gametophyte development.

Growth habit determination by the balance of histone methylation activities in Arabidopsis

In Arabidopsis, the rapid-flowering summer-annual versus the vernalization-requiring winter-annual growth habit is determined by natural variation in FRIGIDA (FRI) and FLOWERING LOCUS C (FLC). However, the biochemical basis of how FRI confers a winter-annual habit remains elusive. Here, we show that FRI elevates FLC expression by enhancement of histone methyltransferase (HMT) activity. EARLY FLOWERING IN SHORT DAYS (EFS), which is essential for FRI function, is demonstrated to be a novel dual substrate (histone H3 lysine 4 (H3K4) and H3K36)-specific HMT. FRI is recruited into FLC chromatin through EFS and in turn enhances EFS activity and engages additional HMTs. At FLC, the HMT activity of EFS is balanced by the H3K4/H3K36- and H3K4-specific histone demethylase (HDM) activities of autonomous-pathway components, RELATIVE OF EARLY FLOWERING 6 and FLOWERING LOCUS D, respectively. Loss of HDM activity in summer annuals results in dominant HMT activity, leading to conversion to a winter-annual habit in the absence of FRI. Thus, our study provides a model of how growth habit is determined through the balance of the H3K4/H3K36-specific HMT and HDM activities.

The Arabidopsis Paf1c complex component CDC73 participates in the modification of FLOWERING LOCUS C chromatin

FLOWERING LOCUS C (FLC) is a key repressor of flowering in Arabidopsis (Arabidopsis thaliana) and is regulated, both positively and negatively, by posttranslational histone modifications. For example, vernalization (the promotion of flowering by cold temperatures) epigenetically silences FLC expression through repressive histone modifications such as histone H3 lysine-9 dimethylation (H3K9me2) and H3K27me3. In contrast, an RNA polymerase II-associated complex (Paf1c) activates FLC expression through increased H3K4 and H3K36 methylation. As a result of this regulation, FLC has become a useful model for the study of chromatin structure in Arabidopsis. Here we show that At3g22590 is the Arabidopsis homolog of the yeast (Saccharomyces cerevisiae) Paf1c component CDC73 and is enriched at FLC chromatin. In contrast to other Paf1c component mutants that exhibit pleiotropic developmental phenotypes, the effects of cdc73 mutations are primarily limited to flowering time, suggesting that CDC73 may only be required for Paf1c function at a subset of target genes. In rapid-cycling strains, cdc73 mutants showed reduced FLC mRNA levels and decreased H3K4me3 at the FLC locus. Interestingly, in late-flowering autonomous-pathway mutants, which contain higher levels of FLC, cdc73 mutations only suppressed FLC in a subset of mutants. H3K4me3 was uniformly reduced in all autonomous-pathway cdc73 double mutants tested; however, those showing reduced FLC expression also showed an increase in H3K27me3. Thus, CDC73 is required for high levels of FLC expression in a subset of autonomous-pathway-mutant backgrounds and functions both to promote activating histone modifications (H3K4me3) as well as preventing repressive ones (e.g. H3K27me3).