I wish to first thank Dr. Wen-hui Shen and Dr. Alexandre Berr for accepting me in the laboratory and for kindly supervising my work along these almost four year of my PhD study. I greatly appreciate their support, guidance and suggestions provided throughout my studies. I am also grateful to their high level of expertise as scientists they exhibited that will undoubtedly benefit me for my future career. I owe sincere and earnest thankfulness to Dr. Alexandre Berr, for his valuable advice on my experiments and data analyzing. As a non-French speaker, I also benefit a lot from him in my daily life.

I am grateful to the European Commission -Research: The Seventh Framework Programme for financial support. I thank the selection committee of the Chromatin in Plants-European Training and Mobility (CHIP-ET) program for giving me the opportunity. In particular, I would like to thank Dr. Mieke Van Lijsebettens, the coordinator of CHIP-ET, for helpful scientific discussions and for hosting me for a month in her laboratory at VIB in Belgium. I also thank Pia Neyt from her laboratory for helping me to carry out the root and leaf analysis experiments. I thank all the PIs and students from the CHIP-ET network for all the great training meetings and fruitful discussions we shared together.

I am particularly grateful to all the members of the PhD jury: Prof. Anne-Catherine Schmit, Dr. Fredy Barneche, Dr. Célia Jager-Baroux and Dr. Franziska Turck, who have kindly accepted to evaluate my PhD work.

I thank the IBMP laboratory members Donghong Chen, Jing Feng, Qiannan Wang and Xue Zhang for their helps. I thank Nicolas Baumberger at the Platform of Protein Production and Purification in IBMP and Morgan TORCHY at the Platform of Genomics and Structural Biology in IGBMC for helping me to do the protein crystallography analysis. I am very grateful and thankful to my parents and to my wife for their enormous support, encouragement and understanding.

GENERAL INTRODUCTION 1. Epigenetics

In eukaryotic cells, genetic information is carried by the macromolecule: deoxyribonucleic acid (DNA) [START_REF] Avery | Studies on the chemical nature of the substance inducing transformation of pneumococcal types: induction of transformation by a desoxyribonucleic acid fraction isolated from pneumococcus type III[END_REF]. Indeed, many phenotypic traits are caused by the permanent alteration of the nucleotide sequence of DNA. Nevertheless, there also exist many phenomena that could not be explained by the classical genetics.

The term "epigenetics" had been put forward in the early 1940s by Conrad Waddington, and originally used to explain a genetic inexplicable phenomenon that how a fertilized zygote cell developed into a complex organism with distinct cell phenotypes and functions while the DNA is essentially the same in all cells (Dupont et al., 2009;Felsenfeld, 2014).

More recently, epigenetics has been defined and broadly accepted as "the study of mitotically and/or meiotically heritable changes in gene function that cannot be explained by changes in DNA sequence" [START_REF] Russo | Epigenetic Mechanisms of Gene Regulation[END_REF]. In nowadays, epigenetic regulation has been extensively investigated and found to play crucial roles in many developmental phenomena such as gene imprinting, cell differentiation and reprogramming in both plants and mammals (Feng et al., 2010); in the regulation of critical processes such as position effect variegation (PEV), paramutation, Xchromosome inactivation, parent-of origin effects and therefore also diseases like cancers in animals [START_REF] Poynter | Polycomb and trithorax opposition in development and disease[END_REF]; in many aspects of plant growth and development such as floral transition, gametogenesis, seed development, root growth, stem cell maintenance as well as plant responses to environmental stresses [START_REF] Pikaard | Epigenetic regulation in plants[END_REF].

At molecular level, epigenetic mechanisms primarily comprise covalent posttranslational histone modifications, DNA methylation, ATP-dependent chromatin remodeling, nucleosome assembly/disassembly, histone variant incorporation and non-coding RNAs regulation [START_REF] Yao | Epigenetic mechanisms in neurogenesis[END_REF]. These mechanisms can modulate chromatin structure, define genes expression state and maintain it through multiple rounds of cell division, or even to the next generation.

Here below, I will focus my introduction on covalent histone modifications.

Chromatin structure and histone modifications

In eukaryotic cells, genetic information encoded by DNA is packed into chromatin, a highly complex structure consisting of DNA together with all associated proteins that contribute to its structure and function. The basic structural unit of chromatin is the nucleosome. The nucleosome is made of 146-147 base pairs of DNA wrapped in about 1.65 turns around an octamer formed by two copies of each of the core histones H2A, H2B, H3 and H4 [START_REF] Luger | Crystal structure of the nucleosome core particle at 2.8 Å resolution[END_REF]; Figure 1). Individual nucleosomes are separated by linker DNA of 10-60 base pairs that is associated by histone H1 (linker histone), resulting in a ~10 nm 'beads-on-a-string chromatin fiber. The fiber can be further folded into more condensed, ~30 nm thick fibers and then 100-400 nm interphase fibers or further more highly compacted metaphase chromosome structures (during mitosis and meiosis) (Hansen, 2002;[START_REF] Peterson | Histones and histone modifications[END_REF]. The chromatin fibers can be more or less "open" or "closed" which influences fundamental DNA template processes like transcription, DNA replication, DNA-damage repair or DNA recombination.

Figure 1: Nucleosome core particle, DNA is bound to an octameric complex composed of two copies of each four core histones H2A, H2B, H3 and H4 [START_REF] Luger | Crystal structure of the nucleosome core particle at 2.8 Å resolution[END_REF].

Histones are highly alkaline proteins found within eukaryotic cell nuclei. The canonical four core histones H2A, H2B, H3 and H4 are highly evolutionary conserved in eukaryotes; each of them contains a C-terminal histone fold domain and a flexible N-terminal tail domain. The N-terminal and C-terminal tails protrude from the nucleosome and are exposed on the surface of the nucleosome. They are subject to multiple types of post-translation modifications (PTMs), including methylation, acetylation, phosphorylation, ubiquitination and others [START_REF] Kouzarides | Chromatin modifications and their function[END_REF].

In the histone code hypothesis, the histone modifications are proposed to act in a combinatorial or sequential manner specifically regulate chromatin structure and modify the way that genes are expressed and they can be recognized by other proteins (readers and effectors) and interpreted into specific biological outcomes [START_REF] Rea | Regulation of chromatin structure by site-specific histone H3 methyltransferases[END_REF][START_REF] Turner | Histone acetylation and an epigenetic code[END_REF].

Histone methylation

In 1964, RNA synthesis was reported to be an association of histone acetylation and methylation, opened the study of histone methylation (Allferey et al., 1964). Over the many last years, histone methylation has been one of the most extensively and best studied modifications in both animals and plants. It plays essential roles in epigenetic regulation of gene transcription and organism development as well as in response to environmental cues (Liu et al., 2010;Black et al., 2012).

Methylation can occur at different lysine (K) or arginine (R) residues of histone tails, and one residue can be methylated to different degrees depending on the number (up to three for K) of methyl groups added (Figure 2). In Arabidopsis, five lysine residues located on histone H3, i.e. (K4, K9, K23, K27 and K36) have been reported to be sites of methylation so far. In general, histone H3 di/tri-methylations on lysine 4 (H3K4me2/3) and 36 (H3K36me2/3) are associated with euchromatin and correspond to transcriptional activation; whereas methylations of H3K9 and H3K27 are associated with gene silencing (Johnson et al., 2004;[START_REF] Martin | The diverse functions of histone lysine methylation[END_REF]. Interestingly, the more recently identified H3K23 monomethylation (H3K23me1) is linked with CG DNA methylation but H3K23me1 enrichment along gene bodies does not correlate with transcription levels (Trejo-Arellano et al., 2017).

Genome-wide distribution of histone methylation

In Arabidopsis, H3K9me2 and H3K27me1 are heterochromatic marks, which are found essentially associated with transposons and centromeric repeat DNA sequences [START_REF] Roudier | Integrative epigenomic mapping defines four main chromatin states in Arabidopsis[END_REF]. Within euchromatin, H3K4me3 accumulates around the TSS (Transcription Start Site) regions and is thought to be involved in transcription initiation in both animals and plants [START_REF] Xiao | Tug of war: adding and removing histone lysine methylation in Arabidopsis[END_REF]; Figure 3). H3K36me2 and H3K36me3 are both enriched toward the 3' end of the gene body in animals. By contrast, in plants H3K36me2/3 are found across the transcribed regions, with H3K36me3 peaks in gene body toward the 5' end and H3K36me2 more distributed toward the 3' end of transcribed genes. Several functions of H3K36me2/3 have been described such as affecting alternative intron splicing, regulating Pol II elongation, and preventing aberrant transcription initiation within gene body [START_REF] Wagner | Understanding the language of Lys36 methylation at histone H3[END_REF].

H3K27me3 is at highest level in the promoter of inactive genes in animals. However, it is localized across the transcribed regions in Arabidopsis, with a remarkable bias towards the TSS mainly enriched in euchromatin regions (Figure 3). H3K27me3 is recognized as one of the primary gene silencing mechanisms and has broad functions in control of plant developmental processes [START_REF] Xiao | Polycomb repression in the regulation of growth and development in Arabidopsis[END_REF].

Writers of histone lysine methylation

Histone K methylation is catalyzed by histone lysine methyltransferases (HKMTs), 'writers' of histone code. The first discovered HKMTs are human and mouse SUV39H1 (KMT1A) and yeast CLR4 proteins [START_REF] Rea | Regulation of chromatin structure by site-specific histone H3 methyltransferases[END_REF]. Later, more and more HKMTs have been identified. Excepting Dot1, a histone H3K79 methyltransferase, all the other HKMTs contain a highly conserved SET domain (Dillon et al., 2005;[START_REF] Ng | Plant SET domain-containing proteins: structure, function and regulation[END_REF].

The SET domain, named after three HKMTs found in Drosophila: Suppressor variegation 3-9 (Su(var) [3][4][5][6][7][8][9], Enhancer of Zeste (E(z)) and Trithorax (Trx), is a 130 amino acid catalytic domain [START_REF] Schubert | Silencing by plant Polycomb-group genes requires dispersed trimethylation of histone H3 at lysine 27[END_REF]. It uses S-adenosylmethionine to transfer methyl groups to alkyl chain of the target lysine in a deep open hydrophobic channel, which is the key structural feature of the SET domain (Figure 4). This channel is also flexible and regulated by surrounding sequences to accommodate mono-, diand/or tri-methyl addition.

Figure 4: Representative overall structure of the SET domain of the human methyltransferase MLL1 [START_REF] Southall | Structural basis for the requirement of additional factors for MLL1 SET domain activity and recognition of epigenetic marks[END_REF].

In Arabidopsis, there are 47 SET Domain Group (SDG) proteins, which can be divided into distinct subgroups (Thorstensen et al., 2011;[START_REF] Ng | Plant SET domain-containing proteins: structure, function and regulation[END_REF]. Based on sequence similarity and phylogenetic analysis, SDGs of the five Trx homologs (ATX1/SET DOMAIN GROUP27/SDG27, ATX2/SDG30, ATX3/SDG14, ATX4/SDG16, and ATX5/SDG29), seven Trx related (ATXR1/SDG35, ATXR2/SDG36, ATXR3/SDG2, ATXR4/SDG38, ATXR5/SDG15, ATXR6/SDG34, and ATXR7/SDG25), and four ASH1 homologs (ASHH1/SDG26, ASHH2/SDG8, ASHH3/SDG7, and ASHH4/SDG24) are classified into the Trithorax group (TrxG) [START_REF] Berr | Histone modifications in transcriptional activation during plant development[END_REF]. They mainly catalyze H3K4 and/or H3K36 methylation and are associated with transcriptional gene activation. The E (z) subgroup SDGs (CLF/SDG1, MEA/SDG5, and SWN/SDG10) belong to Polycomb Group (PcG) and are responsible for H3K27 methylation, which is associated with transcriptional gene silencing [START_REF] Xiao | Tug of war: adding and removing histone lysine methylation in Arabidopsis[END_REF].

H3K4me3 writers

Primary writers of mono-, di-and trimethylation of H3K4 include histone methyltransferase Set1 in yeast and the mixed lineage leukaemia (MLL) family proteins of MLL1-MLL4, SET1A and SET1B in mammalian. These HKMTs exert catalyzing activity by forming complex with additional factors, called COMPASS (complex proteins associated with Set1) in yeast and COMPASS-like complexes in mammals [START_REF] Shilatifard | The COMPASS family of histone H3K4 methylases: mechanisms of regulation in development and disease pathogenesis[END_REF]. Later, COMPASS-like complexes were also identified in plants with highly conserved subunit compositions (Jiang et al., 2011). In Arabidopsis, deletion of the core subunit ASH2R (Arabidopsis Ash2 Relative) resulted in a global decrease of H3K4me3 without affecting H3K4me1/2 at flowering genes (Jiang et al., 2014).

Interestingly, two sequence-specific membrane-associated basic leucine zipper (bZIP) transcription factors bZIP28 and bZIP60 were demonstrated to physically interact with Arabidopsis COMPASS-like components both in vitro and in vivo, providing a possible mechanism for how the H3K4me2/3 writers be recruited to target loci (Song et al., 2015). In Arabidopsis, six SDGs in TrxG family were functionally characterized as H3K4 methyltransferase (Figure 5); they alone are able to methylate H3K4, which is in contrast to multi-protein complex requirement for enzyme activity of their homologs in animals. ARABIDOPSIS HOMOLOG of TRITHORAX1 (ATX1/SDG27), a close homolog of yeast Set1 and human MLL1, was the first TrxG member reported in plants [START_REF] Alvarez-Venegas | ATX-1, an Arabidopsis Homolog of Trithorax, Activates Flower Homeotic Genes[END_REF]. The atx1 mutant showed slightly early-flowering phenotypes caused by a decreased expression level of FLOWERING LOCUS C (FLC) associated with reduction of H3K4me3 but not H3K4me2 at the FLC locus, indicating the specific tri-methylation function of ATX1 [START_REF] Pien | ARABIDOPSIS TRITHORAX1 dynamically regulates FLOWERING LOCUS C activation via histone 3 lysine 4 trimethylation[END_REF]. In addition, mutations in ATX1 caused slightly smaller and serrated rosette leaves, perturbed resistance to pathogens and the plant's response to drought as well as dehydration stress signaling in both abscisic acid (ABA)-dependent and ABA-independent pathways [START_REF] Alvarez-Venegas | Epigenetic Control of a Transcription Factor at the Cross Section of Two Antagonistic Pathways[END_REF]Ding et al., 2009;Ding et al., 2011). ATX1 also plays roles in the regulation of flower development by activating flower homeotic genes and root development by maintaining root apical meristem activity as well as the transition to elongation [START_REF] Alvarez-Venegas | ATX-1, an Arabidopsis Homolog of Trithorax, Activates Flower Homeotic Genes[END_REF][START_REF] Napsucialy-Mendivil | Arabidopsis homolog of trithorax1 (ATX1) is required for cell production, patterning, and morphogenesis in root development[END_REF]. ATX2/SDG30, a paralog of ATX1, encodes a putative H3K4me2 methyltransferase [START_REF] Saleh | The highly similar Arabidopsis homologs of trithorax ATX1 and ATX2 encode proteins with divergent biochemical functions[END_REF]. However, atx2 single mutant did not show any obvious phenotype, while the atx1 atx2 double mutant displayed an enhanced earlyflowering phenotype compared to atx1, indicating some overlapping functions between ATX1 and ATX2 [START_REF] Saleh | The highly similar Arabidopsis homologs of trithorax ATX1 and ATX2 encode proteins with divergent biochemical functions[END_REF][START_REF] Pien | ARABIDOPSIS TRITHORAX1 dynamically regulates FLOWERING LOCUS C activation via histone 3 lysine 4 trimethylation[END_REF]. SDG25/ATXR7 is a floral repressor mainly through the activation of FLC expression via deposition of both H3K4me2/3 and H3K36me3, possibly through a different pathway from ATX1/ATX2 [START_REF] Berr | SET DOMAIN GROUP25 encodes a histone methyltransferase and is involved in FLOWERING LOCUS C activation and repression of flowering[END_REF][START_REF] Tamada | ARABIDOPSIS TRITHORAX-RELATED 7 is required for methylation of lysine 4 of histone H3 and for transcriptional activation of FLOWERING LOCUS C[END_REF][START_REF] Shafiq | Combinatorial functions of diverse histone methylations in Arabidopsis thaliana flowering time regulation[END_REF][START_REF] Berr | The trxG family histone methyltransferase SET DOMAIN GROUP 26 promotes flowering via a distinctive genetic pathway[END_REF]. In addition to control flowering time, SDG25 is also involved in plant defense by activating some key Resistance (R) genes via deposition of H3K4me3 at those loci [START_REF] Xia | Regulation of transcription of nucleotide-binding leucine-rich repeat-encoding genes SNC1 and RPP4 via H3K4 trimethylation[END_REF].

SDG26 was first identified as a flowering activator, but without knowing the underlying molecular mechanism (Xu et al., 2008). A more recent study demonstrated that loss of function of SDG26 caused a decrease of both H3K4me3 and H3K36me3 at a key flowering integrator gene SUPPRESSOR OF OVEREXPRESSION OF CONSTANS 1 (SOC1), leading to SOC1 repression and late flowering [START_REF] Berr | The trxG family histone methyltransferase SET DOMAIN GROUP 26 promotes flowering via a distinctive genetic pathway[END_REF]. SDG4/ASHR3 is potentially able to methylate H3K4me2/3 and H3K36me3, and it was reported to function in pollen, stamen and root development [START_REF] Cartagena | The Arabidopsis SDG4 contributes to the regulation of pollen tube growth by methylation of histone H3 lysines 4 and 36 in mature pollen[END_REF]Thorstensen et al., 2008;[START_REF] Kumpf | The ASH1-RELATED3 SET-domain protein controls cell division competence of the meristem and the quiescent center of the Arabidopsis primary root[END_REF].

So far, SDG2 is the methyltransferase identified to perform genome-wide trimethylation of H3K4. Consistently, sdg2 mutants showed decreased H3K4me3 on the global level and caused strong pleiotropic phenotypes including abnormal sporophytic and gametophytic formation, as well as root growth and development defects (Berr et al., 2010;[START_REF] Guo | SET DOMAIN GROUP2 is the major histone H3 lysine [corrected] 4 trimethyltransferase in Arabidopsis[END_REF][START_REF] Yao | SDG2-mediated H3K4 methylation is required for proper Arabidopsis root growth and development[END_REF][START_REF] Pinon | H3K4me3 is crucial for chromatin condensation and mitotic division during male gametogenesis[END_REF].

H3K36me3 writers

In yeast, the SET-domain protein Set2 catalyzes mono-(me1), di-(me2) and trimethylation (me3) at H3K36 [START_REF] Venkatesh | Set2 mediated H3 lysine 36 methylation: regulation of transcription elongation and implications in organismal development[END_REF]. The set2 homologue SETD2 in human specifically deposes H3K36me3 and is involved in transcriptional elongation [START_REF] Carvalho | Histone methyltransferase SETD2 coordinates FACT recruitment with nucleosome dynamics during transcription[END_REF]. Moreover, some of animal proteins, like human nuclear receptor SET domain containing 1 (NSD1; also known as KMT3B), have not only H3K36 mono-and dimethylase activity but also can methylate H4K20 or even non-histones like NF-κB (nuclear factor-κB); the NSD2 (also known as WHSC1) catalyzes methylation of H3K36 as well as H3K4, H3K27 and H4K20 sites [START_REF] Wagner | Understanding the language of Lys36 methylation at histone H3[END_REF].

In Arabidopsis, H3K36 methylation is primarily carried out by SDG8 [START_REF] Zhao | Prevention of early flowering by expression of FLOWERING LOCUS C requires methylation of histone H3 K36[END_REF]Xu et al., 2008), though the previously mentioned SDG4, SDG25 and SDG26 also exhibit H3K36 methylation in addition to their H3K4 methylation activity [START_REF] Cartagena | The Arabidopsis SDG4 contributes to the regulation of pollen tube growth by methylation of histone H3 lysines 4 and 36 in mature pollen[END_REF][START_REF] Shafiq | Combinatorial functions of diverse histone methylations in Arabidopsis thaliana flowering time regulation[END_REF][START_REF] Berr | The trxG family histone methyltransferase SET DOMAIN GROUP 26 promotes flowering via a distinctive genetic pathway[END_REF]. SDG8 has genome-wide catalyzing activity in H3K36me2/3 deposition [START_REF] Zhao | Prevention of early flowering by expression of FLOWERING LOCUS C requires methylation of histone H3 K36[END_REF]Xu et al., 2008;[START_REF] Ning | Two novel NAC transcription factors regulate gene expression and flowering time by associating with the histone demethylase JMJ14[END_REF]. It plays essential roles in prevention of floral transition by increasing H3K36m2/3 at FLC and thus elevating its expression level [START_REF] Zhao | Prevention of early flowering by expression of FLOWERING LOCUS C requires methylation of histone H3 K36[END_REF]. In addition, SDG8 is broadly involved in many other processes including regulation of shoot branching (Xu et al., 2008;[START_REF] Xu | Di-and tri-but not monomethylation on histone H3 lysine 36 marks active transcription of genes involved in flowering time regulation and other processes in Arabidopsis thaliana[END_REF][START_REF] Cazzonelli | Regulation of carotenoid composition and shoot branching in Arabidopsis by a chromatin modifying histone methyltransferase, SDG8[END_REF][START_REF] Bian | Combinatorial regulation of CLF and SDG8 during Arabidopsis shoot branching[END_REF], ovule and anther development [START_REF] Grini | The ASH1 HOMOLOG 2 (ASHH2) histone H3 methyltransferase is required for ovule and anther development in Arabidopsis[END_REF], carotenoid biosynthesis [START_REF] Cazzonelli | Regulation of carotenoid composition and shoot branching in Arabidopsis by a chromatin modifying histone methyltransferase, SDG8[END_REF][START_REF] Cazzonelli | Transcriptional control of SET DOMAIN GROUP 8 and CAROTENOID ISOMERASE during Arabidopsis development[END_REF], plant defense responses against necrotrophic fungal pathogens (Berr et al., 2010;De-La-Peña et al., 2012), innate immunity [START_REF] Palma | Autoimmunity in Arabidopsis acd11 is mediated by epigenetic regulation of an immune receptor[END_REF], brassinosteroid-regulated gene expression [START_REF] Wang | Histone lysine methyltransferase SDG8 is involved in brassinosteroidregulated gene expression in Arabidopsis thaliana[END_REF], seed gene repression and seed dormancy (Tang et al., 2012;Lee et al., 2014), mechanical stimulation response [START_REF] Cazzonelli | A chromatin modifying enzyme, SDG8, is involved in morphological, gene expression, and epigenetic responses to mechanical stimulation[END_REF], and high-level expression of light-and/or carbon-responsive genes [START_REF] Ning | Two novel NAC transcription factors regulate gene expression and flowering time by associating with the histone demethylase JMJ14[END_REF]. In spite of crucial functions of SDG8, how it is recruited to target loci still needs to be elucidated.

H3k27me3 writers

PcG proteins and TrxG proteins constitute two counteracting groups of chromatin regulators, providing an essential strategy in control of the temporal and spatial expression pattern of key developmental regulator genes in many multicellular organisms including animals and plants [START_REF] Schuettengruber | Trithorax group proteins: switching genes on and keeping them active[END_REF][START_REF] Pu | PcG and trxG in plants -friends or foes[END_REF]. PcG proteins, first identified in Drosophila, are nowadays known to act in multiprotein complexes for transcription repression in both plants and animals [START_REF] Margueron | The Polycomb complex PRC2 and its mark in life[END_REF]. Polycomb Repressive Complex 2 (PRC2) is among the best studied and is responsible for H3K27me3 deposition.

In Drosophila, PRC2 is composed of four core subunits, namely E(z), Su(z)12 (Suppressor of zeste 12), Esc (Extra sex combs), and N55 (a 55 kDa WD40 repeat protein) [START_REF] Simon | Occupying chromatin: Polycomb mechanisms for getting to genomic targets, stopping transcriptional traffic, and staying put[END_REF]. In Arabidopsis, there are three E(z) homologs: CURLY LEAF (CLF), MEDEA (MEA) and SWINGER (SWN); three Su(z)12 homologs: EMBRYONIC FLOWER 2 (EMF2), FERTILISATION INDEPENDENT SEED2 (FIS2) and VERNALIZATION2 (VRN2); a single Esc homolog: FERTILIZATION INDEPENDENT ENDOSPERM (FIE); and five p55 homologs: MULTICOPY SUPPRESSOR OF IRA 1 (MSI1), MSI2 to MSI5 (Hennig and Derkacheva, 2009;[START_REF] Grossniklaus | Transcriptional silencing by polycomb-group proteins[END_REF]. Different PRC2 sub-complexes likely have specialized functions, e.g. the FIS2-PRC2 complex (containing MEA, FIE, FIS2 and MSI1) is involved in sporophyte to gametophyte transition and early seed development, the EMF2-PRC2 complex (containing CLF or SWN, EMF2, MSI1 and FIE) is involved in embryonic to vegetative growth transition and in flowering time regulation, while the VRN2-PRC2 (containing CLF or SWN, VRN2, FIE and MSI1) is involved in winter memory (vernalization) in flowering time control (Hennig and Derkacheva, 2009;[START_REF] Pu | PcG and trxG in plants -friends or foes[END_REF][START_REF] Mozgova | The polycomb group protein regulatory network[END_REF]. Within PRC2, the SDG proteins CLF/SDG1, MEA/SDG5 and SWN/SDG10 are the enzyme cores for H3K27me3 deposition. CLF was the first identified plant PcG protein, which regulates leaf and flower development by repressing the floral homeotic genes AGAMOUS (AG) and the Class I KNOX gene SHOOTMERISTEMLESS (STM) [START_REF] Goodrich | A Polycomb-group gene regulates homeotic gene expression in Arabidopsis[END_REF]Katz et al., 2004;[START_REF] Schubert | Silencing by plant Polycomb-group genes requires dispersed trimethylation of histone H3 at lysine 27[END_REF]. CLF is also implicated in flowering time regulation by represses the floral repressor genes FLC, MAF4 and MAF5 (MADS AFFECTING FLOWERING 4/5) as well as the activator gene FT (FLOWERING LOCUS T) via H3K27me3 deposition (Jiang et al., 2008;[START_REF] Shafiq | Combinatorial functions of diverse histone methylations in Arabidopsis thaliana flowering time regulation[END_REF]. Lack of SWN function alone does not cause a clear mutant phenotype, but it can strongly enhance the clf phenotype. The swn clf double mutant plants flower after germination without a substantial vegetative growth phase, and develop embryo-or callus-like structures, in which H3K27me3 was totally lost [START_REF] Chanvivattana | Interaction of Polycomb-group proteins controlling flowering in Arabidopsis[END_REF][START_REF] Aichinger | CHD3 proteins and polycomb group proteins antagonistically determine cell identity in Arabidopsis[END_REF][START_REF] Roudier | Integrative epigenomic mapping defines four main chromatin states in Arabidopsis[END_REF][START_REF] Lafos | Dynamic regulation of H3K27 trimethylation during Arabidopsis differentiation[END_REF]. MEA is a self-regulated imprinting gene in the endosperm with maternal allele is expressed while the paternal allele is silent [START_REF] Baroux | Dynamic regulatory interactions of Polycomb group genes: MEDEA autoregulation is required for imprinted gene expression in Arabidopsis[END_REF]Jullien et al., 2006). DEMETER (DME), a DNA glycosylase, contributes to the MEA imprinting by specifically removes DNA methylation at MEA maternal-allele and thus activates its expression, which is in turn involved in silencing the paternal allele [START_REF] Gehring | DEMETER DNA glycosylase establishes MEDEA polycomb gene selfimprinting by allele-specific demethylation[END_REF]. Interestingly, it has been reported that MEA physically interact with DNA METHYLTRANSFERASE1 (MET1) and act together to regulate endosperm development in seeds, which also provides a link between histone modification and DNA methylation [START_REF] Schmidt | The Polycomb group protein MEDEA and the DNA methyltransferase MET1 interact to repress autonomous endosperm development in Arabidopsis[END_REF].

Erasers of histone methylation

Histone methylation is relatively more stable compared with other PTMs, and had been thought to be irreversible due to lacking of knowledge on histone demethylases, 'erasers'. It has been becoming clear that histone methylation is also dynamic since the finding of the first histone lysine demethylase (KDM1): human LYSINE SPECIFIC DEMETHYLASE 1 (LSD1) [START_REF] Shi | Histone demethylation mediated by the nuclear amine oxidase homolog LSD1[END_REF]. LSD1 is an amine oxidase that can demethylate H3K4me1/2 in a flavine adenine dinucleotide (FAD) dependent manner or demethylate H3K9me1/2 in the presence of the androgen receptor [START_REF] Shi | Histone demethylation mediated by the nuclear amine oxidase homolog LSD1[END_REF][START_REF] Metzger | LSD1 demethylates repressive histone marks to promote androgen-receptor-dependent transcription[END_REF]. Subsequently, another family of histone demethylases containing JmjC domain (JMJ) was discovered (Tsukada et al., 2005). All JMJ proteins contain a conserved JmjC domain that use a-ketoglutarate molecular oxygen and Fe(II) as cofactors to demethylate all forms of mono-/di-/trimethylated lysine residues (Tsukada et al., 2005, Klose et al., 2006a).

In plants, both types of demethylases have been identified: four homologues of LSD1 and 21 JMJs, which can be further divided into distinct subgroups based on sequence analysis (Jiang et al., 2007;[START_REF] Lu | Comparative analysis of JmjC domaincontaining proteins reveals the potential histone demethylases in Arabidopsis and rice[END_REF][START_REF] Spedaletti | Characterization of a lysine-specific histone demethylase from Arabidopsis thaliana[END_REF]Chen et al., 2011). Studies over the past decade have revealed that histone demethylases play fundamental roles in the regulation of histone methylation homeostasis and diverse chromatin-based processes as well as developmental pathways in plants [START_REF] Greer | Histone methylation: a dynamic mark in health, disease and inheritance[END_REF]Dimitrova et al., 2015;Liu et al., 2010;[START_REF] Chen | Epigenetic gene regulation by plant Jumonji group of histone demethylase[END_REF][START_REF] Prakash | Histone demethylases and control of gene expression in plants[END_REF]; Figure 6).

Erasers of H3K4 methylation

In Arabidopsis, LSD1-LIKE 1 (LDL1), LDL2, LDL3 and FLOWERING LOCUS D (FLD) are homologues of human LSD1. LDL1, LDL2 and FLD have been shown to control flowering time by repressing FLC expression. In both ldl1 ldl2 and ldl1 fld double mutants, an increase of H3K4me2 was found at FLC chromatin (He et al., 2003;Jiang et al., 2007;[START_REF] Shafiq | Combinatorial functions of diverse histone methylations in Arabidopsis thaliana flowering time regulation[END_REF]; furthermore, LDL1 showed specifically in vitro H3K4me1/2 demethylase activity [START_REF] Spedaletti | Characterization of a lysine-specific histone demethylase from Arabidopsis thaliana[END_REF]. LDL1 and LDL2 but not FLD are responsible for removal of H3K4me2 at floral repressor FWA chromatin and thus contribute to silencing of FWA during sporophytic development (Jiang et al., 2007).

More recently, LDL1 and LDL2 were reported to control seed dormancy via regulation of seed dormancy-related and ABA signaling-related genes (Zhao et al., 2015).

Three of Arabidopsis JMJ proteins, JMJ14, JMJ15 and JMJ18 have been shown to act as H3K4 demethylases and involved in flowering time control by distinct mechanisms. JMJ14 is able to demethylate H3K4me1/2/3 both in vivo and in vitro and repress flowering through down-regulation of several key floral integrator genes [START_REF] Lu | JMJ14 is an H3K4 demethylase regulating flowering time in Arabidopsis[END_REF]. JMJ15 is implicated in regulating flowering time by removal of H3K4me3 at FLC, and it is also involved in regulation of plant salt tolerance and female gametophyte development (Yang et al., 2012a;Shen et al., 2014;[START_REF] Lu | Comparative analysis of JmjC domaincontaining proteins reveals the potential histone demethylases in Arabidopsis and rice[END_REF]. JMJ18 accelerates flowering by repressing FLC expression via directly binding to FLC locus and removes H3K4me2/3 at its chromatin (Yang et al., 2012b). It is worthy to note that JMJ14 was recently found to target to flowering genes mediated by two NAC type transcription factors NAC50 and NAC52 through directly binding to their CTTGNNNNNCAAG consensus sequences [START_REF] Ning | Two novel NAC transcription factors regulate gene expression and flowering time by associating with the histone demethylase JMJ14[END_REF]Zhang et al., 2014), providing a possible mechanism for how histone demethylases are recruited to specific gene target loci.

Erasers of H3K36 methylation

The first reported H3K36 demethylase is human KDM4A that can remove both H3K36me2/3 and H3K9me2/3 (Klose et al., 2006b). Other erasers of H3K36 methylation in animals include KDM8/JMJD5 that remove 3K36me2, KDM2A and KDM2B that remove H3K36me1/2, as well as KDM4A, KDM4B, KDM4C, KDM4D and NO66 that remove H3K36me2/3 [START_REF] Kooistra | Molecular mechanisms and potential functions of histone demethylases[END_REF].

In plants, JMJ30 is the only H3K36 demethylase reported so far. It exhibits in vitro H3K36me2/3 demethylase activity and interacts with the MYB-family transcription factor EARLY FLOWERING MYB PROTEIN (EFM) to regulate H3K36me2 dynamics at FT to regulate flowering time [START_REF] Yan | A MYB-domain protein EFM mediates flowering responses to environmental cues in Arabidopsis[END_REF].

Erasers of H3K27 methylation

In animals, KDM6A and KDM6B enzymes are involved in demethylation of H3K27me2/3. The recruitment of these two demethylases is accomplished through interaction with sequence specific DNA-binding proteins (Bosselut et al., 2016).

Another histone demethylase JHDM1D, also known as KDM7A, was reported to remove H3K27me1/2 and to be involved in regulation of neural differentiation and early embryos and brain development [START_REF] Tsukada | KDM7 is a dual demethylase for histone H3 Lys 9 and Lys 27 and functions in brain development[END_REF][START_REF] Kooistra | Molecular mechanisms and potential functions of histone demethylases[END_REF].

In Arabidopsis, REF6 (JMJ12) specifically demethylates H3K27me2/3 and acts on FLC [START_REF] Noh | Divergent roles of a pair of homologous jumonji/zinc-finger-class transcription factor proteins in the regulation of Arabidopsis flowering time[END_REF][START_REF] Lu | Arabidopsis REF6 is a histone H3 lysine 27 demethylase[END_REF]. ELF6 (JMJ11), a closer homolog of REF6, acts as a flowering repressor in the photoperiod pathway [START_REF] Noh | Divergent roles of a pair of homologous jumonji/zinc-finger-class transcription factor proteins in the regulation of Arabidopsis flowering time[END_REF]. More recently, ELF6 was shown to remove H3K27me3 for resetting FLC expression during reproduction to ensure a vernalization requirement in the next generation [START_REF] Crevillen | Epigenetic reprogramming that prevents transgenerational inheritance of the vernalized state[END_REF]. Interestingly, REF6 was shown to be recruited to its target loci through its C-terminal C2H2 zinc-finger domain which associates with a CTCTGYTY motif on the genomic DNA [START_REF] Cui | REF6 recognizes a specific DNA sequence to demethylate H3K27me3 and regulate organ boundary formation in Arabidopsis[END_REF][START_REF] Bian | Combinatorial regulation of CLF and SDG8 during Arabidopsis shoot branching[END_REF]. JMJ30, which was shown to demethylate H3K36me2/3 in vitro [START_REF] Yan | A MYB-domain protein EFM mediates flowering responses to environmental cues in Arabidopsis[END_REF], was also a potential H3K27me2/3 demethylase. At elevated temperature, stabilized JMJ30 cooperates with JMJ32 to demethylate H3K27me3 at FLC locus to prevent premature flowering [START_REF] Gan | Jumonji demethylases moderate precocious flowering at elevated temperature via regulation of FLC in Arabidopsis[END_REF].

Readers of histone methylation

The methyl group does not affect the overall charge of the modified histone molecule, but it enhances the hydrophobic force that may alter intra-or intermolecular interactions or serve as a platform to recruit specific "readers" that will recognize the methylation signals and act as effectors for further downstream events (Liu et al., 2010;[START_REF] Bannister | Regulation of chromatin by histone modifications[END_REF]. Histone methylation reader proteins bind preferentially to the methylated sties through various functional domains, including PHD (Plant HomeoDomain), Chromo (Chromodomain), WD40 (Tryptophan-Aspartic dipeptide 40), MBT (Malignant Brain Tumor), PWWP (Pro-Trp-Trp-Pro), MRG (Morf Related Gene), and Tudor [START_REF] Yun | Readers of histone modifications[END_REF][START_REF] Musselman | Perceiving the epigenetic landscape through histone readers[END_REF]Liu and Min, 2016; Figure 7).

Readers of H3K4 methylation

In animals, several H3K4 methylation readers have been identified and characterized, e.g. the Chromo-containing ATPase protein CHD1; the PHD-containing proteins TAF3 (component of the transcriptional machinery), ING1/2 (components of Histone acetyltransferase complex) and PHF2/8 (histone demethylation related proteins); and the Tudor-containing histone demethylases JMJD2A/2C ( [START_REF] Yun | Readers of histone modifications[END_REF].

In Arabidopsis, ORC1a and ORC1b contain a PHD domain that is absent in the yeast and animal ORC1 (Origin Recognition Complex subunit 1). They can interact with H3K4me3 and are involved in transcriptional activation of target genes [START_REF] Sanchez | Arabidopsis ORC1 is a PHD-containing H3K4me3 effector that regulates transcription[END_REF]. AtING1/2 and Alfin1-like (AL1-7) are also PHD-containing proteins and are shown to interact with H3K4me2/3 in vitro [START_REF] Trimethylation-Kim | RAD6-Mediated transcription-coupled H2B ubiquitylation directly stimulates H3K4 methylation in human cells[END_REF]. More recently, AL6/7 were demonstrated to interact with some PcG proteins to silence seed maturation genes during germination by promoting a switch from H3K4me3 to H3K27me3 (Moliter et al., 2014). Two other PHD-containing proteins, EBS and SHL, were found to recognize H3K4me2/3 and to play an important role in preventing early flowering. EBS and SHL interact with the histone deacetylase HDA6 to repress the expression of flowering integrator genes FT and SOC1 by maintaining a low level of H3 acetylation at these loci (López-González et al., 2014). Lastly, WDR5a, a WD40 repeat containing proteins, is a conserved core component of COMPASS complex and can bind with H3K4me1/2/3 to regulate flowering time [START_REF] Gu | Repression of the floral transition via histone H2B monoubiquitination[END_REF].

Readers of H3K36 methylation

In animals, H3K36 methylation reader proteins include the Chromo-containing protein MRG15 (components of histone acetyltransferase complex), the PWWP-containing proteins DNMT3a (de novo DNA methylase), NSD1/2/3 (H3K36 histone methyltransferases) and BRPF1 (components of histone acetyltransferase complex) [START_REF] Yun | Readers of histone modifications[END_REF]. More recently, ZMYND11 (also known as BS69) was found to specifically recognize histone H3.3 lysine 36 trimethylation (H3.3K36me3) via its PWWP domain, providing a link to regulate transcription elongation and pre-mRNA processing [START_REF] Wen | ZMYND11 links histone H3.3K36me3 to transcription elongation and tumour suppression[END_REF][START_REF] Guo | BS69/ZMYND11 reads and connects histone H3.3 lysine 36[END_REF]. Lastly, the PcG protein PHF19 binds H3K36me3 via its Tudor domain and then recruits PRC2 as well as the H3K36me3 demethylase NO66 to embryonic stem cell genes during cell differentiation (Brien et al., 2012).

In Arabidopsis, MRG1 and MRG2, two homologs of animal MRG15, were shown to recognize both the H3K36me3 and H3K4me3 marks to regulate flowering time [START_REF] Bu | Regulation of Arabidopsis flowering by the histone mark readers MRG1/2 via interaction with CONSTANS to modulate FT expression[END_REF][START_REF] Xu | Arabidopsis MRG domain proteins bridge two histone modifications to elevate expression of flowering genes[END_REF]. MRG2 interacts with the transcription factor CONSTANS (CO) and enhances CO binding to the FT locus to activate its expression [START_REF] Bu | Regulation of Arabidopsis flowering by the histone mark readers MRG1/2 via interaction with CONSTANS to modulate FT expression[END_REF]. In addition, MRG1/2 physically interacts with the histone H4 acetyltransferases HAM1/2 and jointly regulates the expression of FLC and FT through bridging H3K36me3 and H4 acetylation [START_REF] Xu | Arabidopsis MRG domain proteins bridge two histone modifications to elevate expression of flowering genes[END_REF].

Readers of H3K27methylation

H3K27me2/3 deposited by PRC2 is read by PRC1. In animals, the PRC1 components Pc and CBX7 are Chromo-containing proteins responsible for H3K27me2/3 binding [START_REF] Bannister | Selective recognition of methylated lysine 9 on histone H3 by the HP1 chromo domain[END_REF][START_REF] Bernstein | Mouse polycomb proteins bind differentially to methylated histone H3 and RNA and are enriched in facultative heterochromatin[END_REF]. The PRC2 component EED also is able to recognize H3K27me3 via its WD40 domain [START_REF] Margueron | Role of the polycomb protein EED in the propagation of repressive histone marks[END_REF]. In Arabidopsis, homolog of Pc/CBX7 cannot be found, but the Chromo-containing protein LHP1 (LIKE HETEROCHROMATIN PROTEIN 1) can bind H3K27me3 and may play an analogous PRC1 function [START_REF] Zhang | Whole-genome analysis of histone H3 lysine 27 trimethylation in Arabidopsis[END_REF][START_REF] Turck | Arabidopsis TFL2/LHP1 Specifically Associates with Genes Marked by Trimethylation of Histone H3 Lysine 27[END_REF]. Loss of LHP1 causes pleiotropic defects, including early flowering, upward leaf curling, and reduced plant body and leaf size (Gaudin et al., 2003). More recently, genetic screens identified two Myb-family transcription factors TELOMEREREPEAT BINDING PROTEIN1 (TRB1) and TRB3 as the enhancer of the lhp1 mutant. Both LHP1 and TRBs bind telobox-motif enriched and H3K27me3-marked genomic regions and are involved in silencing of target genes [START_REF] Zhou | Complementary Activities of TELOMERE REPEAT BINDING Proteins and Polycomb Group Complexes in Transcriptional Regulation of Target Genes[END_REF].

Histone monoubiquitination

Ubiquitination is the covalent attachment of ubiquitin (Ub), a 76 amino acids and highly conserved protein in eukaryotes, to a substrate protein. It occurs via conjugation of the C-terminal residue of Ub to the side chain of a K residue of the substrate protein through three-step reactions (Hershko and Ciechanover, 1998;[START_REF] Weake | Histone ubiquitination: triggering gene activity[END_REF][START_REF] Voutsadakis | Ubiquitin, ubiquitination and the ubiquitin-proteasome system in cancer[END_REF]Figure 8). is recognized as a signal for targeting protein to degradation by the 26S proteasome, whereas monoubiquitination or polyubiquitination via the K63 residues of the Ub molecules is not efficiently recognized by the 26S proteasome but rather results in a modified substrate with changing activity, protein-protein interaction, subcellular localization or others (Hua and Vierstra, 2011;Braun and Madhani, 2012).

Histone ubiquitination is best studied with H2A and H2B monoubiquitination (H2Aub1 and H2Bub1), which are critical types of epigenetic marks in eukaryotes [START_REF] Zhang | Transcriptional regulation by histone ubiquitination and deubiquitination[END_REF]Feng and Shen, 2014). H2Aub1 is generally associated with transcription repression mediated by the PcG pathway, whereas H2Bub1 is involved in transcription activation (Hicke, 2001;[START_REF] Shilatifard | Chromatin modifications by methylation and ubiquitination: implications in the regulation of gene expression[END_REF]Feng and Shen, 2014).

H2B monoubiquitination

H2Bub1 is now widely accepted as an epigenetic mark involved in transcription activation. The ubiquitination site is located to a highly conserved K residue: H2BK123 in budding yeast, H2BK119 in fission yeast, H2BK120 in humans, and H2BK143 in Arabidopsis (Feng and Shen, 2014). From the genome-wide perspective, H2Bub1 is found distributed throughout the whole genome marking chromatin regions mainly in combination with H3K4me3 and/or H3K36me3 [START_REF] Roudier | Integrative epigenomic mapping defines four main chromatin states in Arabidopsis[END_REF].

Writers of H2B monoubiquitination

In yeast, the E2 enzyme Rad6 (radiation sensitivity protein 6) was firstly identified to act in conjunction with the E3 enzyme Bre1 (Brefeldin-A sensitivity protein 1) to monoubiquitinate H2B in vivo. Global H2Bub1 level was found decreased or undetectable in the loss-of-function mutants rad6 and bre1, causing severe cell growth defects [START_REF] Robzyk | Rad6-dependent ubiquitination of histone H2B in yeast[END_REF][START_REF] Wood | Bre1, an E3 ubiquitin ligase required for recruitment and substrate selection of Rad6 at a promoter[END_REF].

In Arabidopsis, three homologs of Rad6, namely UBC1, UBC2 and UBC3, and two homologs of Bre1, namely HISTONE MONOUBIQUITINATION1 (HUB1) and HUB2, were identified. UBC1 and UBC2 but not UBC3 have redundant functions and work together with HUB1/2 in H2Bub1 deposition [START_REF] Cao | Histone H2B monoubiquitination in the chromatin of FLOWERING LOCUS C regulates flowering time in Arabidopsis[END_REF][START_REF] Gu | Repression of the floral transition via histone H2B monoubiquitination[END_REF][START_REF] Xu | The E2 ubiquitinconjugating enzymes, AtUBC1 and AtUBC2, play redundant roles and are involved in activation of FLC expression and repression of flowering in Arabidopsis thaliana[END_REF]Fleury et al., 2007;Liu et al., 2007).

The hub1, hub2, hub1hub2 and ubc1ubc2 mutants display an early flowering phenotype and a reduced FLC expression, which is associated with reduced H2Bub1 levels at the FLC locus [START_REF] Cao | Histone H2B monoubiquitination in the chromatin of FLOWERING LOCUS C regulates flowering time in Arabidopsis[END_REF][START_REF] Gu | Repression of the floral transition via histone H2B monoubiquitination[END_REF][START_REF] Xu | The E2 ubiquitinconjugating enzymes, AtUBC1 and AtUBC2, play redundant roles and are involved in activation of FLC expression and repression of flowering in Arabidopsis thaliana[END_REF]. In addition, the hub mutants showed several other defects like pale leaf coloration, modified leaf shape, reduced rosette biomass and inhibited root growth, which could be largely explained by the down-regulation of some cell cycle genes that regulate the G2-to-M transition (Fleury et al., 2007). Seed dormancy was reduced associated with altered expression levels for several dormancy-related genes in hub mutants (Liu et al., 2007).

Several key circadian clock genes, including CCA1 (CIR-CADIAN CLOCK ASSOCIATED1), ELF4 (EARLYF LOWERING 4) and TOC1 (TIMING OF CAB EXPRESSION 1), were down-regulated and exhibited reduced H2Bub1 levels in the hub mutants (Himanenetal et al., 2012).

H2Bub1 also plays essential roles in response to light, as evidenced by the light sensitive phenotype of the hub1 mutant and by the finding that a large number of light responsive genes are impaired in light response during photomorphogenesis (Bourbousse et al., 2012). Moreover, HUB1 was reported as a regulatory factor in plant defense against necrotrophic fungal pathogens. The hub mutants show increase susceptibility to the necrotrophic fungal pathogens Botrytiscinerea and Alternaria brassicicola, which is associated in part with modifications of cell wall and surface structures, e.g. thinner cell walls and altered surface cutin and wax compositions, together with altered induction of some defense genes (Dhawan et al., 2009;Menard et al., 2014). It was also reported that H2Bub1 modulates the dynamics of microtubules (MTs) and acts as a positive regulator of expression of genes involved in the protein tyrosine phosphatases (PTPs)-mediated signaling pathway during the plants defense response to Verticillium dahliae toxins (Hu et al., 2014). Lastly, HUB1 and HUB2 were shown to impact plant immune responses through mediating H2Bub1 at the R gene SUPPRESSOR OF npr1-1 CONSTITUTIVE1 (SNC1) locus [START_REF] Zou | Monoubiquitination of histone 2B at the disease resistance gene locus regulates its expression and impacts immune responses in Arabidopsis[END_REF].

Erasers of H2B monoubiquitination

H2Bub1 homeostasis is regulated additionally by deubiquitination enzymes. In yeast, H2B monoubiquitination removal is achieved by two ubiquitin-specific proteases Ubp8 and Ubp10. Ubp8 is a component of the SAGA (Spt-Ada-Gcn5-acetyltransferase) complex and it deubiquitinates H2Bub1 during transcription activation (Henry et al., 2003), whereas Ubp10 functions independently from the SAGA complex but interacts with the silencing protein Sir4 to silence telomeric and rDNA regions (Emre et al., 2005).

In Arabidopsis, there are over 50 potential deubiquitinases comprising the UBP (UBIQUITIN-SPECIFIC PROTEASE) and OTU (OVARIAN TUMOR PROTEASE) families [START_REF] Yan | The ubiquitin-specific protease family from Arabidopsis. AtUBP1 and 2 are required for the resistance to the amino acid analog canavanine[END_REF][START_REF] Makarova | A novel superfamily of predicted cysteine proteases from eukaryotes, viruses and Chlamydia pneumoniae[END_REF]. Among the 27 UBPs, only UBP26 has been shown to deubiquitinate histone H2B so far. It was first reported that UBP26 deubiquitinates H2B and is involved in facilitating DNA methylation and heterochromatin formation at transgenes and transposons [START_REF] Sridhar | Control of DNA methylation and heterochromatic silencing by histone H2B deubiquitination[END_REF].

UBP26 also played essential functions in the repression of PHERES1 that is necessary for normal endosperm development [START_REF] Luo | UBIQUITIN-SPECIFIC PROTEASE 26 is required for seed development and the repression of PHERES1 in Arabidopsis[END_REF]. The ubp26 mutants display an early-flowering phenotype, and molecular analysis indicates that UBP26 is necessary for transcriptional activation of FLC via H2B deubiquitination (Schmitzet al., 2009).

From the OTU family, OTLD1 was reported to exhibit a histone deubiquitinase activity and to interact with the histone demethylase KDM1C to repress gene repression via removal of active euchromatin marks [START_REF] Krichevsky | Involvement of KDM1C histone demethylase-OTLD1 otubain-like histone deubiquitinase complexes in plant gene repression[END_REF]Keren et al., 2016).

Readers of H2Bub monoubiquitination

In contrast to the small size of methyl group, Ub is about half size of histone proteins. Monoubiquitination of H2B can directly impact on nucleosome stability and chromatin function (Fierz et al., 2011). Nonetheless, studies in yeast and animals have implicated that H2Bub1 may also exert its functions by recruiting various H2Bub1-specific readers [START_REF] Yun | Readers of histone modifications[END_REF][START_REF] Fuchs | Writing and reading H2B monoubiquitylation[END_REF]. More recently, BRG1/BAF155, a subunit of the SWI/SNF chromatin remodeling complex, was also reported to bind H2Bub1 to regulate transcription [START_REF] Shema-Yaacoby | Systematic identification of proteins binding to chromatin-embedded ubiquitylated H2B reveals recruitment of SWI/SNF to regulate transcription[END_REF]. So far, however, specific H2Bub1 readers in Arabidopsis are still unknown.

H2A monoubiquitination

H2Aub1 was not found in yeast but was generally regarded as transcription repressive mark in both animals and plants [START_REF] Weake | Histone ubiquitination: triggering gene activity[END_REF]Feng et al., 2014). The human PRC1 component Ring1B (also known as Ring2 and RNF2) was the first E3 identified as involved in catalyzing H2Aub1 formation [START_REF] Wang | Hierarchical recruitment of polycomb group silencing complexes[END_REF]. In Arabidopsis, five RING-domain proteins are identified as PRC1-like components involved in catalyzing H2Aub1 formation [START_REF] Molitor | The polycomb complex PRC1: composition and function in plants[END_REF][START_REF] Calonje | PRC1 marks the difference in plant PcG repression[END_REF].

Writers of H2A monoubiquitination

Based on sequence similarity, the five Arabidopsis PRC1-like RING-finger proteins can be subdivided into two clades: RING1 (AtRING1a and AtRING1b) and BMI1 (AtBMI1a, AtBMI1b and AtBMI1c) [START_REF] Sanchez-Pulido | RAWUL: a new ubiquitin-like domain in PRC1 ring finger proteins that unveils putative plant and worm PRC1 orthologs[END_REF]Xu et al., 2008). They contain two conserved domains: an N-terminal RING-domain and a C-terminal ubiquitin-like RAWUL domain. These RING-finger proteins have been demonstrated to catalyze H2Aub1formationin in vitro and their loss affects H2Aub1 levels at specific genes (Bratzel et al., 2010;Li et al., 2011;[START_REF] Yang | VAL-and AtBMI1mediated H2Aub initiate the switch from embryonic to postgerminative growth in Arabidopsis[END_REF]. Functional analyses have attributed crucial roles for these RING-finger proteins in plant development.

The Atring1a but not Atring1b single mutant was shown to display a late flowering phenotype, which is associated with increased expression of the FLC-analogous genes MAF4 and MAF5 (Shen et al., 2014). The double mutant Atring1aAtring1b displays an enlarged SAM, fascinated stem and ectopic meristem formation in cotyledons and leaves, unraveling redundant function of AtRING1a and AtRING1b in the regulation of SAM (Shoot Apical Meristem) activity (Xu et al., 2008). Accordingly, several key stem cell activity regulatory genes of the KNOX (Class IKNOTTED1-like homeobox), including STM (SHOOT-MERISTEMLESS), BP (BRE-VIPEDICELLUS)/KNAT1, KNAT2 and KNAT6, were found ectopically expressed in Atring1aAtring1b (Xu et al., 2008). A more recent study shows that ectopic expression of KNOX genes also severely impacts carpel and ovule development and that AtRING1b but not AtRING1a is an imprinted gene showing exclusively maternalpreferred expression in the endosperm [START_REF] Chen | Arabidopsis PRC1 core component AtRING1 regulates stem cell-determining carpel development mainly through repression of class I KNOX genes[END_REF]. Furthermore, the Atring1aAtring1b mutant exhibits perturbed cell-fate determinacy and derepression of embryonic traits during plant vegetative growth [START_REF] Chen | The Arabidopsis PRC1-like ring-finger proteins are necessary for repression of embryonic traits during vegetative growth[END_REF]. Consistently, the expression level of several key embryonic regulatory genes including ABI3 (ABSCISIC ACIDINSENSITIVE3), AGL15 (AGAMOUSLIKE 15), BBM (BABYBOOM), FUS3

(FUSCA 3), LEC1 (LEAFY COTYLEDON1) and LEC2 is drastically elevated in the mutant [START_REF] Chen | The Arabidopsis PRC1-like ring-finger proteins are necessary for repression of embryonic traits during vegetative growth[END_REF].

AtBMI1C is also a maternally endosperm imprinted gene but the single gene mutant of Atbmi1c does not show any obvious phenotype (Bratzel et al., 2012;[START_REF] Chen | Arabidopsis PRC1 core component AtRING1 regulates stem cell-determining carpel development mainly through repression of class I KNOX genes[END_REF]. However, overexpresion of AtBMI1c can cause H2Aub1 increase and can accelerate flowering time associated with reduced FLC expression (Li et al., 2011).

The Atbmi1aAtbmi1b double mutant has cell fate determinacy defects and shows ectopic expression of several seed developmental genes as observed in the Atring1aAtring1b mutant (Bratzel et al., 2010;[START_REF] Chen | The Arabidopsis PRC1-like ring-finger proteins are necessary for repression of embryonic traits during vegetative growth[END_REF]. Atbmi1c can enhance the Atbmi1aAtbmi1b mutant phenotype, unraveling thus redundant function of the three AtBMI1 genes [START_REF] Yang | VAL-and AtBMI1mediated H2Aub initiate the switch from embryonic to postgerminative growth in Arabidopsis[END_REF]. The mutant phenotype similarity and the detection of physical interaction between AtRING1 and AtBMI1 proteins strongly indicate that these RING-finger proteins act in common complexes in transcription repression.

Interestingly, Arabidopsis PHD-domain H3K4me3-binding AL proteins were found to physically interact with both AtBMI1 and AtRING1 proteins (Moliter et al., 2014). Both al6al7 and Atbmi1aAtbmi1b mutants show delayed seed germination and transcriptional derepression of seed developmental genes including ABI3 and DOG1.

Analyses of histone modifications indicate that the AL PHD-PRC1 complexes are involved in the H3K4me3-to-H3K27me3 chromatin state switch to repress seed developmental genes during seed germination (Moliter et al., 2014). A more recent study found that LHP1 interacts with the GAGA-motif binding factor protein BASIC PENTACYSTEINE6 (BPC6), pointing to important roles of plant GAGA binding factors in the recruitment of PRC1 components to PcG-responsive DNA element-like GAGA motifs (Hecker et al., 2015).

Erasers of H2A monoubiquitination

In animals, three specific H2A deubiquitinases have been identified: Ubp-M, 2A-DUB, and USP21 [START_REF] Nakagawa | Deubiquitylation of histone H2A activates transcriptional initiation via trans-histone cross-talk with H3K4 di-and trimethylation[END_REF]. In a recently published paper, Arabidopsis UBIQUITIN SPECIFIC PROTEASES12 and 13 (UBP12 and UBP13), homologs of UBP26, were shown to have H2Aub deubiquitination activity and involved in PcG protein gene silencing; loss of UBP12 and UBP13 leads to defects of fertilizationindependent endosperm development (Derkacheva et al., 2016).

Readers of H2A monoubiquitination

Human ZUOTIN-RELATED FACTOR1 (ZRF1) protein was first demonstrated to bind H2Aub1 via its UBD (ubiquitin-binding domain) domain [START_REF] Richly | Transcriptional activation of polycomb-repressed genes by ZRF1[END_REF]. ZRF1

homologs are broadly present in the green lineage [START_REF] Chen | Functional conservation and divergence of J-domain-containing ZUO1/ZRF orthologs throughout evolution[END_REF]. The Arabidopsis genome contains two ZRF1-encoding genes, AtZRF1a and AtZRF1b. Loss of both AtZRF1a and AtZRF1b affects plant growth and development in many aspects, such as delayed seed germination, abnormal flower development and defects in both male and female transmission as well as in embryogenesis (Guzmán-López et al., 2016;Feng et al., 2016). Moreover, Arabidopsis ZRF1 proteins potentially read H2Aub1 and enhance H2Aub1 as well as H3K27me3 levels in silencing of seed developmental genes during plant vegetative growth (Feng et al., 2016).

Crosstalks between histone modifications

During the last several decades, extensive studies and significant progress have been made for the distribution, physiological and biological function of individual histone modification marks. The diversity of histone modification related proteins including writers, erasers and readers have highlighted their considerable diversity of functions.

In recent years, accumulating evidence suggests that histone modifications can interplay with each other rather than only work independently, thus providing an extra level of complexity of crosstalk regulatory network that allow the reinforcement of different effects on histone modifications in the regulation of gene transcription and organism development (Fischle et al., 2003;[START_REF] Berger | The complex language of chromatin regulation during transcription[END_REF]Fischle, 2008;[START_REF] Wang | The roles of cross-talk epigenetic patterns in Arabidopsis thaliana[END_REF]Zhang et al., 2015).

Mechanically, crosstalk among modifications may happen at several different levels: (1) H3 more effectively when H3S10 is phosphorelated [START_REF] Kouzarides | Chromatin modifications and their function[END_REF]Suganuma and Workman 2008;[START_REF] Lee | Regulation and function of SOC1, a flowering pathway integrator[END_REF][START_REF] Bannister | Regulation of chromatin by histone modifications[END_REF].

The crosstalks among histone modifications in plants are also widely investigated.

Interestingly, Wang and her colleagues recently build a cross-talk network based on chromatin marks including histone modifications, DNA methylation, histone variants and noncoding RNAs into six classes of cross-talk interactions in Arabidopsis [START_REF] Wang | The roles of cross-talk epigenetic patterns in Arabidopsis thaliana[END_REF].

1 Crosstalk between methylated residues

High throughput characterization of combinatorial histone marks by LC-MS/MS analysis revealed that H3K4me3 and H3K36me2/3 rarely co-occur with H3K27me3 on the same histone H3 molecule [START_REF] Young | High throughput characterization of combinatorial histone codes[END_REF]. The in vitro experiments demonstrated that both H3K4me3 and H3K36me2/me3 inhibit PRC2-mediated H3k27me3 deposition [START_REF] Schmitges | Histone methylation by PRC2 is inhibited by active chromatin marks[END_REF][START_REF] Yuan | H3K36 methylation antagonizes PRC2-mediated H3K27 methylation[END_REF].

In Arabidopsis, CLF-PRC2 is the major writer for H3K27 methylation. In clf mutants, H3K27me3 is decreased, which is accompanied by elevated H3K4me3 at several flowering genes including FLC, MAF4, MAF5 and FT (Jiang et al., 2008). Reciprocally, reduction of H3K4me3 in the H3K4-writer mutants atx1, sdg25 or sdg26 results in an increase of H3K27me3 at FLC chromatin [START_REF] Pien | ARABIDOPSIS TRITHORAX1 dynamically regulates FLOWERING LOCUS C activation via histone 3 lysine 4 trimethylation[END_REF]Tamada et al., 2008;[START_REF] Berr | The trxG family histone methyltransferase SET DOMAIN GROUP 26 promotes flowering via a distinctive genetic pathway[END_REF]. In the H3K4-eraser mutant fld, H3K4me3 increase is accompanied by H3K27me3 decrease (Jiang et al., 2007). Bivalent H3K4me3 and H3K27me3 have been found at many genes in Arabidopsis seedlings by epigenome profiling analysis [START_REF] Roudier | Integrative epigenomic mapping defines four main chromatin states in Arabidopsis[END_REF]. A reciprocal antagonistic effect between H3K27me3 and H3K4me3 exists in plants as well as in animals. Remarkably, simultaneous loss of ATX1 and CLF restored the leaf and flowering phenotype caused by each single mutant, thus the atx1clf double mutant displayed a wild-type phenotype and levels of H3K4me3 and H3K27me3 were partially restored at some genes [START_REF] Saleh | The Arabidopsis homologs of trithorax (ATX1) and enhancer of zeste (CLF) establish 'bivalent chromatin marks' at the silent AGAMOUS locus[END_REF].

However, such restoration had not been detected with other double mutant combinations, e.g. the sdg25clf double mutant displayed enhanced phenotypes compared with the sdg25 or clf single mutant, and it showed an H3K27me3 reduction similar to that in clf and H3K4me3 reduction slightly less severe than that in sdg25 [START_REF] Shafiq | Combinatorial functions of diverse histone methylations in Arabidopsis thaliana flowering time regulation[END_REF].

An antagonistic function between H3K36me3 and H3K27me3 was observed on FLC expression during vernalization [START_REF] Yang | Antagonistic roles for H3K36me3 and H3K27me3 in the cold-induced epigenetic switch at Arabidopsis FLC[END_REF]. Surprisingly, the sdg8clf double mutant plants showed a greater reduction in rosette size, more severely upward-curled leaves and earlier flowering time, compared with the clf or sdg8 single mutant. Molecular data indicated that the increase of H3K36me3 in clf was attenuated in sdg8clf, which showed a reduction of H3K36me3 similar to that in sdg8 but a reduction of H3K27me3 and an increase of H3K4me3 similar to those in clf [START_REF] Shafiq | Combinatorial functions of diverse histone methylations in Arabidopsis thaliana flowering time regulation[END_REF]. In addition, sdg8clf plants had more shoot branches with altered expression of shoot branching regulatory genes, several of which also showing reduction of H3K27me3 level as well as an elevated enrichment of H3K36me3 and H3K4me3 at those loci [START_REF] Bian | Combinatorial regulation of CLF and SDG8 during Arabidopsis shoot branching[END_REF]. Moreover, a drastic elevation of H3K4me3 at seed maturation loci was observed in the sdg8emf2 double mutant (Tang et al., 2012), but the molecular basis for this synergistic effect remains obscure. Taken together, while PcG (CLF) in transcription repression and TrxG (SDG8) in transcription activation are consistent with general knowledge, existence of a crosstalk between H3K27me3 and H3K36me3 deposition still remains elusive and needs future investigations.

In Arabidopsis H3K4me2/me3 and H3K36me3 depositions occur largely independently. It was shown that SDG8-mediated H3K36me3 overrides ATX1/ATX2mediated H3K4me2/me3 and LDL1/LDL2-mediated H3K4 demethylation in regulating FLC expression [START_REF] Shafiq | Combinatorial functions of diverse histone methylations in Arabidopsis thaliana flowering time regulation[END_REF]. This is in contrast to the study in yeast where Set1 (H3K4 methyltransferase) is required for Set2 to perform H3K36 methylation [START_REF] Biswas | Opposing roles for Set2 and yFACT in regulating TBP binding at promoters[END_REF].

2 Crosstalk between H3 methylation and H2A/H2B monoubiquitination

Crosstalk between different modifications can occur on the same histone (crosstalk in cis) or between different histones (crosstalk in trans). The best-studied crosstalk in trans is between H3K27me3 and H2Aub1 during PcG-mediated gene silencing. In general, the H3K27me3 deposited by PRC2 is read by PRC1, which in turn carries out H2Aub1 deposition [START_REF] Wang | Hierarchical recruitment of polycomb group silencing complexes[END_REF]. Another example of crosstalk in trans is between H3K4me3/H3K36me3 and H2Bub1 in transcription activation.

In yeast, the Rad6-mediated H2Bub1 was first found to be required for proper deposition of H3K4me3 catalyzed by Set1 within the COMPASS complex, in such H3K4me3 was almost undetected in the Rad6-depleted yeast mutant strain (Sun et al., 2002;Dover et al., 2002). Following this discovery, it was reported that Rad6-mediated H2Bub1 is also a prerequisite for the Dot1-mediated H3K79me3 deposition [START_REF] Ng | Ubiquitination of histone H2B by Rad6 is required for efficient Dot1-mediated methylation of histone H3 lysine 79[END_REF]. This so-called trans-tail histone modification pathway is also conserved in human [START_REF] Kim | The human homolog of yeast BRE1 functions as a transcriptional coactivator through direct activator interactions[END_REF][START_REF] Zhu | Monoubiquitination of human histone H2B: the factors involved and their roles in HOX gene regulation[END_REF][START_REF] Trimethylation-Kim | RAD6-Mediated transcription-coupled H2B ubiquitylation directly stimulates H3K4 methylation in human cells[END_REF].

There are several molecular mechanisms discovered for the crosstalk in yeast and human. Firstly, Cps35, an essential subunit of COMPASS complex, is required for COMPASS' catalytic activity in vivo. Interestingly, Cps35 only associates with the COMPASS in activating it for catalyzing H3K4me3 when the chromatin is decorated by H2Bub1 (Lee et al., 2007). The H3K79me3 deposition by Dot1 is also dependent on Cps35 (Lee et al., 2007). Secondly, in vitro experiments show that chemically ubiquitinated H2B, when incorporated into nucleosomes, could directly stimulate the catalytic activity of recombinant human Dot1, suggesting an additional mechanism for crosstalk between H2Bub1 and H3K79me3 [START_REF] Mcginty | Chemically ubiquitylated histone H2B stimulates hDot1L-mediated intranucleosomal methylation[END_REF]. Thirdly, H2Bub1 enhances activity of the yeast Set1C methyltransferase complex in vitro without affecting its assembly in vivo [START_REF] Racine | Histone H2B ubiquitylation promotes activity of the intact Set1 histone methyltransferase complex in fission yeast[END_REF]. Fourthly, n-SET domain within yeast Set1 as well as ASH2L in the human MLL complex is essential for H2Bub1-dependent H3K4 methylation [START_REF] Kim | The n-SET domain of Set1 regulates H2B ubiquitylation-dependent H3K4 methylation[END_REF][START_REF] Wu | ASH2L regulates ubiquitylation signaling to MLL: trans-regulation of H3 K4 methylation in higher eukaryotes[END_REF].

In addition to H2Bub1 deposition, H2B deubiquitination is also involved in regulation of trans crosstalk, in particular in relation with H3K36 methylation. In yeast, a mutation of the H2B deubiquitinase Ubp8 causes H2Bub1 increase together with H3K36me2 decrease, indicating that H2B deubiquitination is required for H3K36me2 deposition (Henry et al., 2003). Either substitution of the ubiquitination site in H2B (Lys123) or loss of Ubp8 can reduce specific genes expression levels, indicating that both ubiquitination and de-ubiquitination of H2B are required for gene activation, contrasting the opposite roles between histone acetylation and deacetylation in transcription regulation [START_REF] Wyce | H2B ubiquitylation and de-ubiquitylation in gene activation[END_REF]. It was shown that deubiquitination of H2B is important for recruitment of a complex containing the kinase Ctk1, which phosphorylates the RNA polymerase II (Pol II) C-terminal domain (CTD), and for subsequent recruitment of the Set2 methyltransferase in H3K36 methylation. Thus, the H2BK123ub1 acts as a ''barrier'' to block interaction of Ctk1 with histones, thereby reducing CTDS2ph and subsequent H3K36 methylation [START_REF] Wyce | H2B ubiquitylation acts as a barrier to Ctk1 nucleosomal recruitment prior to removal by Ubp8 within a SAGA-related complex[END_REF].

In summary, the studies in yeast and human indicate that H2B monoubiquitination is required for H3K4me3 deposition during transcription initiation, and its deubiquitination is necessary for H3K36me3 deposition during transcription elongation.

It was proposed that similar crosstalks may also exist in Arabidopsis (Figure 10). First, the key enzymes are found in Arabidopsis: HUB1/2 and UBC1/2 were identified as the homologue of Bre1 and Rad6, respectively, and were shown to be responsible for H2Bub1 deposition (Fleury et al., 2007;Liu et al., 2007;[START_REF] Cao | Histone H2B monoubiquitination in the chromatin of FLOWERING LOCUS C regulates flowering time in Arabidopsis[END_REF][START_REF] Xu | The E2 ubiquitinconjugating enzymes, AtUBC1 and AtUBC2, play redundant roles and are involved in activation of FLC expression and repression of flowering in Arabidopsis thaliana[END_REF][START_REF] Gu | Repression of the floral transition via histone H2B monoubiquitination[END_REF]; UBP26 was characterized as the H2B deubiquitination enzymes, similar to Ubp8/Ubp10 [START_REF] Sridhar | Control of DNA methylation and heterochromatic silencing by histone H2B deubiquitination[END_REF][START_REF] Schmitz | Histone H2B deubiquitination is required for transcriptional activation of FLOWERING LOCUS C and for proper control of flowering in Arabidopsis[END_REF]; SDG8 was identified as a major histone H3K36-methyltransferase, analogous to SET2 [START_REF] Zhao | Prevention of early flowering by expression of FLOWERING LOCUS C requires methylation of histone H3 K36[END_REF]Xu et al., 2008); several H3K4-methyltransferases were also identified [START_REF] Alvarez-Venegas | ATX-1, an Arabidopsis Homolog of Trithorax, Activates Flower Homeotic Genes[END_REF][START_REF] Berr | SET DOMAIN GROUP25 encodes a histone methyltransferase and is involved in FLOWERING LOCUS C activation and repression of flowering[END_REF]Berr et al., 2010;[START_REF] Guo | SET DOMAIN GROUP2 is the major histone H3 lysine [corrected] 4 trimethyltransferase in Arabidopsis[END_REF]. Second, loss of HUB1/2, UBC1/2 or UBP26 causes defects with similarity to sdg8 mutant, e.g.

early flowering, impaired resistance to pathogens [START_REF] Cao | Histone H2B monoubiquitination in the chromatin of FLOWERING LOCUS C regulates flowering time in Arabidopsis[END_REF][START_REF] Schmitz | Histone H2B deubiquitination is required for transcriptional activation of FLOWERING LOCUS C and for proper control of flowering in Arabidopsis[END_REF]Berr et al., 2010;[START_REF] Zou | Monoubiquitination of histone 2B at the disease resistance gene locus regulates its expression and impacts immune responses in Arabidopsis[END_REF]. Lastly, loss of H2Bub1 reduces the H3K4me3 level and the ubp26 mutant shows reduced H3K36me3 at FLC and some other genes [START_REF] Cao | Histone H2B monoubiquitination in the chromatin of FLOWERING LOCUS C regulates flowering time in Arabidopsis[END_REF][START_REF] Schmitz | Histone H2B deubiquitination is required for transcriptional activation of FLOWERING LOCUS C and for proper control of flowering in Arabidopsis[END_REF]. Nevertheless, more experimental data will be necessary to further verify/confirm/improve this model (Figure 9).

Flowering time regulation

Flowering represents the transition from the vegetative to reproductive development, commonly was referred as floral transition during plant life cycle (Figure 10). Correct timing to flowering is crucial for the reproductive success of plants, enabling completion of seed development in desirable environmental conditions. Each plant species has developed optimum systems for its flowering time regulation (Jung and Muller, 2009;Blumel et al., 2015;Bratzel and Turck, 2015). Arabidopsis thaliana has been the most used model plant in flowering time studies. et al., 2009;Huijser and Schmid, 2011;[START_REF] Poethig | Vegetative phase change and shoot maturation in plants[END_REF].

Arabidopsis thaliana as a model plant

Arabidopsis plant is small in size, thus can be easily grown in an indoor growth chamber or greenhouse. In addition, its short life cycle (6 weeks), reproduction primarily by self-fertilization, rapid flowering and production of a large number of seeds also have helped to make it an ideal model plant [START_REF] Dean | Advantages of Arabidopsis for Cloning Plant Genes[END_REF][START_REF] Meyerowitz | Arabidopsis[END_REF][START_REF] Meinke | Arabidopsis thaliana: A Model Plant for Genome Analysis[END_REF]. Arabidopsis thaliana is the first plant species to be sequenced at whole genome level (Arabidopsis Genome Initiative, 2000). It has been largely used to investigate the plant biological processes, plant physiology, molecular biology, genetics and epigenetic regulations. Genetic resources of this model organism are excellent, rich with ecotypes exhibiting different flowering times.

Genetic pathways in flowering time control

In Arabidopsis, the endogenous cues such as plant age and hormone (mainly giberellin acid, GA) coordinate with exogenous or environmental cues such as light (light intensity and duration of exposure) and temperature (winter vernalization) to determine when plants flower. Together, five major genetic pathways control flowering time (Figure 12). Different pathways converge to regulate the expression of a set of floral integrator genes, e.g. FLOWERING LOCUS T (FT), SUPPRESSOR OF OVEREXPRESSION OF CONSTANS 1/AGAMOUS-LIKE 20 (SOC1/AGL20). Floral integrators in turn activate some key meristem identity genes, such as LEAFY (LFY), APETALA1 (AP1), SEPAL-LATA3 (SEP3) and FRUITFULL (FUL) that irreversibly confer the transition from a vegetative to a floral meristem (He and Amasino, 2005;Farrona et al., 2008).

FT encodes a florigen, capable of long-distance moving from the leaves to shoot apices to promote flowering, preferentially through the photoperiod pathway [START_REF] Turck | Regulation and identity of florigen: FLOWERING LOCUS T moves center stage[END_REF]Andrés and Coupland, 2012;Song et al., 2015). It works as a parallel integrator as well as an upstream activator of SOC1 [START_REF] Lee | Regulation and function of SOC1, a flowering pathway integrator[END_REF].

The photoperiod pathway regulates flowering time in response to day length.

Arabidopsis is a long-day facultative plant, thus its flowering is promoted at long-day photoperiod growth conditions. The genes GIGANTEA (GI), CRYPTOCHROME2/ FHA (CRY2), FLAVIN KELCH F BOX 1 (FKF1) and CONSTANS (CO) have major roles in the photoperiod pathway, as evidenced by observation that mutations in these genes delay flowering in long days but not in short days [START_REF] Turck | Regulation and identity of florigen: FLOWERING LOCUS T moves center stage[END_REF]. The CO gene encodes a putative zinc finger transcription factor and is essential for photoperiod perception. GI encodes a plant specific protein that physically interacts with ubiquitin ligase KFK1; together the blue light receptor CRY2, they act upstream of CO to increase its mRNA level and also stabilize CO protein [START_REF] Turck | Regulation and identity of florigen: FLOWERING LOCUS T moves center stage[END_REF]Andrés and Coupland, 2012).

Vernalization is a process by which exposure to prolonged cold temperatures (e.g.

winter) accelerates flowering. There are two major types of Arabidopsis accessions, summer annuals (no vernalization requirement) and winter annuals (vernalization responsive). Vernalization requirement in winter annuals is conferred by dominant alleles of FRIGIDA (FRI) and FLC. FRI encodes a coiled-coil protein that elevates FLC expression to levels that inhibit flowering, resulting in the winter-annual growth habit [START_REF] Johanson | Molecular analysis of FRIGIDA, a major determinant of natural variation in Arabidopsis flowering time[END_REF]. Winter annuals only flower rapidly after exposure at a low temperature (4 °C) for 6 to 12 weeks (mimic the transition from winter to spring), during which FLC transcription is turn-off gradually and maintained stably later-on [START_REF] Amasino | Take a cold flower[END_REF]Sung 2005;Sung and Amasino, 2005;[START_REF] Trimethylation-Kim | RAD6-Mediated transcription-coupled H2B ubiquitylation directly stimulates H3K4 methylation in human cells[END_REF]. FLC encodes a MADS box transcription factor that blocks the floral transition through repression of FT and SOC1. FLC has five paralogs, namely MADS AFFECTING FLOWERING 1/FLOWERING LOCUS M (MAF1/FLM), and MAF2 to MAF5; they have redundant functions in repressing flowering under certain conditions [START_REF] Ratcliffe | Regulation of Flowering in Arabidopsis by an FLC Homologue[END_REF][START_REF] Scortecci | Genetic interactions between FLM and other flowering-time genes in Arabidopsis thaliana[END_REF][START_REF] Gu | Arabidopsis FLC clade members form flowering-repressor complexes coordinating responses to endogenous and environmental cues[END_REF].

The autonomous pathway was initially defined by a group of mutants that are late flowering under all photoperiods and are highly responsive to vernalization. This pathway regulatory genes include LUMINIDEPENDENS (LD), FCA, FY, FPA, FLOWERINGLOCUS D (FLD), FVE, and FLOWERING LOCUS K (FLK). Arabidopsis plants containing mutations in these genes have elevated levels of FLC expression.

Notably, their late-flowering phenotype can be overcome by vernalization treatment.

Thus, the autonomous pathway constitutively activates flowering by reduce FLC expression [START_REF] Mouradov | Control of flowering time interacting pathways as a basis for diversity[END_REF][START_REF] Srikanth | Regulation of flowering time: all roads lead to Rome[END_REF].

During the last decade, an endogenous pathway that adds plant age to the control et al., 2013;Teotia and Tang, 2015;Hyun et al., 2017).

Besides these major pathways, flowering time is also influenced by other plant hormones such as cytokinins, brassinosteroids, ethylene, salicylic acid, and abscisic acid, as well as by other environmental factors such as nutrients, ambient temperature, drought, salinity, chemicals, and pathogenic microbes (Tang et al., 2015;Kazan and Lyons, 2015;[START_REF] Cho | Floral dip: a simplified method for Agrobacterium-mediated transformation ofArabidopsis thaliana[END_REF]Ionescu et al., 2016).

Flowering time regulation via chromatin modifications

FLC served as target has been extensively studied during the functional characterization of writers, erasers and readers of histone modifications. While other flowering genes, e.g. FT and SOC1, are also regulated by histone modifications, epigenetic regulation of FLC remains the best elucidated (Farrona et al., 2008;He, 2012;Hepworth and Dean, 2015;[START_REF] Berry | Environmental perception and epigenetic memory: mechanistic insight through FLC[END_REF]. Below, I focus on both repression and activation of transcription to summarize the regulators involved in histone methylation and monoubiquitination in the regulation of FLC expression and flowering time.

Epigenetic repression of FLC by H3K27 methylation

H3K27me3 is a key epigenetic mark in silencing FLC during the process of vernalization. VERNALIZATION INSENSITIVE3 (VIN3), which is quickly up-regulated upon cold exposure, functions in a cold perception system that measure the time of cold exposure. VIN3 is a plant homeodomain (PHD) protein; it has three homologs VIN3-LIKE 1(VIL1)/VERNALIZATION5 (VRN5), VIL2/VIN3-LIKE1 (VEL1) and VIL3/VEL2. During prolonged cold, VIN3, VIL1 and VIL2 act as part of VRN2-PRC2 complex by physically interact with PRC2 core subunits [START_REF] Wood | The Arabidopsis thaliana vernalization response requires a polycomb-like protein complex that also includes VERNALIZATION INSENSITIVE 3[END_REF][START_REF] Lucia | A PHD-polycomb repressive complex 2 triggers the epigenetic silencing of FLC during vernalization[END_REF]. The newly formed PHD-PRC2 complex specifically targets to the nucleation region that is located just downstream of the FLC transcription start site and comprises about three nucleosomes (Figure 13B).

Two long noncoding RNAs (lncRNAs), COLDAIR and COOLAIR, generated during cold exposure from FLC, are also involved in vernalization-mediated FLC repression [START_REF] Swiezewski | Cold-induced silencing by long antisense transcripts of an Arabidopsis Polycomb target[END_REF][START_REF] Heo | Vernalization-mediated epigenetic silencing by a long intronic noncoding RNA[END_REF]. COOLAIR was proposed to be required to ensure removal of activating chromatin marks at the nucleation region and to mediate FLC transcriptional down-regulation [START_REF] Swiezewski | Cold-induced silencing by long antisense transcripts of an Arabidopsis Polycomb target[END_REF][START_REF] Berry | Environmental perception and epigenetic memory: mechanistic insight through FLC[END_REF]; while COLDAIR physically interact with CLF in vivo and was thought to recruit PHD-PRC2 complex to FLC [START_REF] Heo | Vernalization-mediated epigenetic silencing by a long intronic noncoding RNA[END_REF].

H3K27me3 is deposited by PHD-PRC2 complex at the nucleation region of FLC and its level increases at this region in proportion with the length of cold experienced (Figure 13B). Upon vernalization accomplishment, PHD-PRC2 spreads to cover the whole FLC locus and leads to high H3K27me3 along its entire length, resulting in epigenetically silenced state of FLC that is stable after return to warm temperature (Figure 13C). The H3K27me3 reader LHP1 also plays an important role in maintaining epigenetic silencing of FLC [START_REF] Mylne | LHP1, the Arabidopsis homologue of HETEROCHROMATIN PROTEIN1, is required for epigenetic silencing of FLC[END_REF]. Albeit an uncovered role of AtRING1a

Epigenetic activation of FLC by histone modifications

FLC expression is reset to high level during embryogenesis to ensure vegetative growth of the next generation [START_REF] Sheldon | Resetting of FLOWERING LOCUS C expression after epigenetic repression by vernalization[END_REF]. The H3K27me3 eraser ELF6 was found to be essential for FLC activation; in the next generation of elf6 mutants, H3K27me3 levels at the FLC locus induced by vernalization was found not fully removed and stayed high [START_REF] Crevillen | Epigenetic reprogramming that prevents transgenerational inheritance of the vernalized state[END_REF].

In addition to H3K27me3 removal, deposition of active epigenetic marks is essential for maintaining FLC expression. H3K4me3, H3K36me3, H2Bub1 as well as acetylated histones are broadly found to be enriched at activated FLC loci, indicating these marks are involved in FLC activation [START_REF] Berr | Histone modifications in transcriptional activation during plant development[END_REF][START_REF] Berry | Environmental perception and epigenetic memory: mechanistic insight through FLC[END_REF](Figure 12A). Enzymes that is responsible for deposition of active histone marks such as ATX1 and SDG25 in H3K4 methylation [START_REF] Pien | ARABIDOPSIS TRITHORAX1 dynamically regulates FLOWERING LOCUS C activation via histone 3 lysine 4 trimethylation[END_REF]Tamada et al., 2008;[START_REF] Shafiq | Combinatorial functions of diverse histone methylations in Arabidopsis thaliana flowering time regulation[END_REF], SDG8 in H3K36 methylation [START_REF] Zhao | Prevention of early flowering by expression of FLOWERING LOCUS C requires methylation of histone H3 K36[END_REF][START_REF] Shafiq | Combinatorial functions of diverse histone methylations in Arabidopsis thaliana flowering time regulation[END_REF][START_REF] Berr | The trxG family histone methyltransferase SET DOMAIN GROUP 26 promotes flowering via a distinctive genetic pathway[END_REF], UBC1/2 and HUB1/2 in H2B monoubiquitination [START_REF] Cao | Histone H2B monoubiquitination in the chromatin of FLOWERING LOCUS C regulates flowering time in Arabidopsis[END_REF] are all involved in generating an active FLC chromatin state for high levels of FLC expression.

Objectives of Thesis

Objectives of my PhD thesis focus to identify and characterize new writers/readers of histone methylations and to dissect crosstalk between H3K36 methylation and H2B monoubiquitination for an improved understanding of function of histone modifications in the regulation of gene transcription and plant growth and development.

In collaboration with the laboratory of Dr. Richard Amasino, we functionally characterized a new TrxG member, a homologue of SDG8, named SDG7. Genetic and molecular data demonstrated its fundamental role in the proper timing of the vernalization process. The major results are published and here included as Chapter I.

In animal and yeast, the crosstalk between histone H3K4me3/H3K36 methylation and H2B monoubiquitination in transcription activation is well documented. In plants, considerably less is known and studies were mainly concentrated on studying the functional relationship between H2Bub1 and H3K4me3. In order to fill this shortfall, I explored the functional interaction between H2Bub1 and H3K36me3 using a genetic approach based on crosses between different histone-modifying enzyme mutants. My studies mainly focused on the interplay of these different histone marks in regulation of plant growth and gene transcription. Results from this part are presented as Chapter II.

Compared to modifications like acetylation and monoubiquitination, histone methylation does not per se directly trigger changes in the chromatin structure but they require specific "readers" that will recognize one or more methylation signals and interpret it. In Arabidopsis, little is currently known about histone methylation readers, and especially regarding the domains involved in this binding as well as their specificity.

In the last part, I described a group of histone methylation readers with a particular attention to the binding affinity of their PWWP domain. Results from this part are presented as Chapter III.

(A) (B)

S

A methyltransferase required for proper timing of the vernalization response in Arabidopsis Joohyun Lee a , Jae-Young Yun a , Wei Zhao b , Wen-Hui Shen b , and Richard M. Amasino easonal timing of flowering is critical for reproductive success in many plant species. The timing of flowering is often strongly influenced by seasonal variables, such as day length and temperature. Vernalization, the process by which exposure to the prolonged cold of winter enables flowering in the spring, is an example of a temperature effect on flowering (1).

Arabidopsis thaliana contains both vernalization-requiring accessions (winter annual) and accessions that do not require vernalization (summer annual). The vernalization requirement is conferred by FRIGIDA (FRI)-mediated up-regulation of the potent flowering repressor FLOWERING LOCUS C (FLC) (2,3). The presence of active alleles of FRI and FLC ensures that flowering is repressed in the fall season. Summer annual accessions do not require vernalization to flower, because these accessions typically contain a mutant allele of FRI that is not able to up-regulate FLC (4). On perception of a sufficiently long period of cold, vernalization results in the chromatin-level suppression of FLC, which in turn provides competence to flower (3,5). Thus, in Arabidopsis, vernalization is an environmentally induced epigenetic switch (6).

Vernalization-mediated FLC silencing is associated with an increase of two chromatin modifications involving trimethylation of histone 3 at lysine 9 (H3K9me3) and at lysine 27 (H3K27me3) (7,8). The increase in H3K27me3 results from the action of the Polycomb Repressive Complex 2 (PRC2) on FLC chromatin (9). Many of the PRC2 components are conserved among plants, animals, and fungi [START_REF] Sawarkar | Interpretation of developmental signaling at chromatin: The Polycomb perspective[END_REF]11); however, a plant-specific PRC2associated protein, VERNALIZATION INSENSITIVE 3 (VIN3), is required for vernalizationand is uniquely expressed during cold (6). Thus, the cold-induced expression of VIN3 is a critical event in vernalization process (6,12). The system by which cold perception leads to induction of VIN3 is not well understood, however, and to date there is no evidence suggesting that it shares features with cold acclimation in Arabidopsis (13).

Here we report that SET DOMAIN GROUP 7 (SDG7), an extranuclear methyltransferase [START_REF] Sahr | O-carboxyl-and N-methyltransferases active on plant aquaporins[END_REF], is required for proper timing of VIN3 induction and of the vernalization process. SDG7 was isolated by a screening an ethyl methane sulfonate-mutagenized population for mutants in which VIN3 is induced more rapidly during cold exposure. Loss of SDG7 results in plants that exhibit a hypersensitive vernalization response, more rapid induction of VIN3 during cold exposure, higher levels of the FLCcomplementary noncoding RNAs COLDAIR [START_REF] Heo | Vernalization-mediated epigenetic silencing by a long intronic noncoding RNA[END_REF] and COOL-AIR ( 16) in the absence of cold, and increased H3K27me3 levels at FLC chromatin both with and without cold exposure.

Results and Discussion

A Mutation in SDG7 Alters VIN3 Expression and Flowering Behavior.

To explore the system by which cold exposure leads to the induction of VIN3 expression, we screened for mutants that exhibited altered timing of expression of a VIN3 reporter gene construct, VIN3p::GUS. Because VIN3 is induced by cold in both root and shoot apical meristems, we screened for mutants in which VIN3 is more rapidly induced in the root apical meristem, to enable the use of a viable GUS-staining system (17) (Fig. 1A). We characterized in detail a mutant line that exhibits detectable expression of the VIN3 transgene as well as the native VIN3 gene before cold exposure and rapid induction of VIN3 on cold exposure in both the root and shoot apices (Fig. 1 A-C). In a FRIcontaining background, in which the presence of FRI creates a vernalization requirement (2)(3)(4), the mutation results in a more rapid vernalization response, as well as more rapid flowering without cold exposure (Fig. 1D). The mutation had no visual phenotypes other than the flowering phenotypes. It behaved recessively and segregated in a 3:1 ratio (342:110; χ 2 = 0.74; P > 0.05) in an F2 population, indicating that the mutant phenotype is attributed to a single locus.

To identify the causative mutation, we carried out whole-genome sequencing (18)[START_REF] Hartwig | Fast isogenic mapping-by-sequencing of ethyl methanesulfonate-induced mutant bulks[END_REF][START_REF] Schneeberger | SHOREmap: Simultaneous mapping and mutation identification by deep sequencing[END_REF] on DNA pooled from mutants in an To whom correspondence should be addressed. Email: amasino@biochem.wisc.edu. This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10. 1073/pnas.1423585112/-/DCSupplement Significance Proper timing of flowering is a key factor in reproductive success. Certain plant species require prolonged cold exposure during winter to flower in the spring; however, how plants measure the duration of cold exposure is not well understood. In this study, we describe the identification of a methyltransferase that is required for proper measurement of the duration of cold exposure. F2 population; as controls, Col fri, Col FRI, the FRI-containing mutant, and VIN3p::GUS lines were sequenced in parallel. Analysis of the sequence data revealed a likely causative C-to-T mutation in the SDG7 gene on chromosome 2 (Fig. S1A). We refer to this mutation as sdg7-1. The mutation caused a missense mutation (Pro to Leu) in the SET domain, a domain conserved in certain methyltransferases in many eukaryotes (Fig. S1B). To evaluate whether the rapid flowering of the mutant is caused by mutation of SDG7, a FRI-containing line with the sdg7-1 mutation was crossed to T-DNA null allele of SDG7 (sdg7-2) (Fig. S1 A and C). F1 plants from this cross (which had the genotype FRI/-; sdg7-1/sdg7-2) and all FRI/-;sdg7/sdg7 plants in an F2 population derived from the F1 had the same flowering phenotype as the original sdg7-1 mutant in the FRI-containing background, indicating that SDG7 is indeed the causative gene (Fig. S1 D andE).

Vernalization Hypersensitivity of SDG7. To further characterize the effect of loss of SDG7 on the vernalization process in the FRI background, we measured the number of leaves formed before flowering as a function of the duration of cold exposure. The FRI sdg7-1 and FRI sdg7-2 mutant lines flowered more rapidly than the FRI control line without vernalization (Fig. 2A). Moreover, FRI sdg7-1 and FRI sdg7-2 exhibited saturation of the vernali-zation response after 20 d of cold exposure, whereas saturation of the control required 40 d of cold exposure (Fig. 2B).

We also characterized the effect of the duration of cold on FLC and VIN3 expression in the sdg7 mutant. sdg7 mutants had lower levels of FLC expression than controls before cold exposure (Fig. 2C), consistent with the more rapid flowering of sdg7 without vernalization. Furthermore, in sdg7 there was a decrease in FLC expression after 20 d of cold exposure, whereas in the control, a substantial decrease required 40 d of cold exposure (Fig. 2C). Similarly, VIN3 mRNA levels at 20 d of cold exposure in the mutant were similar to the levels at 40 d of cold exposure in WT (Fig. 2D). Thus, SDG7 is required for proper control of the timing of VIN3 and FLC expression changes during cold exposure.

VIN3 Dependence of the sdg7 Phenotype. As noted above (Fig. 2D), loss of SDG7 results in VIN3 expression without cold exposure and more rapid VIN3 induction during cold exposure. To determine whether the alteration of VIN3 expression is required for the sdg7 mutant phenotype, we generated sdg7-1 vin3 and sdg7-2 vin3 double mutants in the FRI-containing background and characterized their flowering phenotypes. The vin3 mutation partially suppressed the sdg7 phenotype in plants not exposed to cold (Fig. 3A). This is an interesting result, because in WT, VIN3 appears to function only during cold exposure; a vin3 single mutant has no flowering phenotype in the absence of cold exposure, and in WT, VIN3 is expressed only during cold exposure (6). That the vin3 mutant is epistatic to sdg7 in plants not exposed to cold, however, reveals a conditional "noncold" vin3 phenotype in the sdg7 mutant, and indicates that loss of SDG7 results in a partial vernalization in the absence of cold. As expected, vin3 also suppressed the more rapid vernalization phenotype of sdg7 (Fig. 3A). As controls, we combined the vin3 mutation with a range of other mutations that result in early flowering in the absence of cold [START_REF] He | PAF1 complex-mediated histone methylation of FLOWERING LOCUS C chromatin is required for the vernalization-responsive, winterannual habit in Arabidopsis[END_REF][START_REF] Noh | PIE1, an ISWI family gene, is required for FLC activation and floral repression in Arabidopsis[END_REF][START_REF] Choi | SUPPRESSOR OF FRIGIDA3 encodes a nuclear ACTIN-RELATED PROTEIN 6 required for floral repression in Arabidopsis[END_REF][START_REF] Bezerra | Lesions in the mRNA capbinding gene ABA HYPERSENSITIVE 1 suppress FRIGIDA-mediated delayed flowering in Arabidopsis[END_REF]. Unlike for sdg7, there was no interaction of vin3 with any of these mutations (Fig. 3A).

The sdg7 Mutant Exhibits Elevated Levels of Noncoding RNAs Generated from FLC and Increased H3K27me3 of FLC Chromatin. As discussed above, VIN3 is necessary for the sdg7 phenotype; however, overexpression of VIN3 is not sufficient to cause the sdg7 mutant phenotype. In VIN3, overexpression lines that contain even higher VIN3 mRNA levels than the sdg7 mutant (Fig. S2A), there was no effect on flowering or FLC expression in the absence of cold, nor more rapid vernalization (Fig. 3 B andC). The VIN3 overexpression construct rescues vin3 mutants, indicating that the construct is functional (Fig. S2B). The finding that VIN3 overexpression does not affect flowering in a WT background was also noted in work using a different VIN3 transgene (6).

Because VIN3 overexpression is not sufficient to account for the sdg7 phenotype, loss of SDG7 may affect other factors involved in vernalization. Two long noncoding RNAs (lncRNAs), COLDAIR [START_REF] Heo | Vernalization-mediated epigenetic silencing by a long intronic noncoding RNA[END_REF] and COOLAIR (16), generated from FLC are up-regulated during cold exposure and are thought to be involved in vernalization-mediated FLC repression [START_REF] Heo | Vernalization-mediated epigenetic silencing by a long intronic noncoding RNA[END_REF]16). Therefore, we determined expression levels of these two lncRNAs in the sdg7 mutant. Interestingly, levels of these two lncRNAs were elevated in sdg7 in the absence of cold (Fig. 4A). Thus, all of the known cold-inducible components of the vernalization response required for FLC repression-VIN3, COOLAIR, and COLDAIR-are more highly expressed without cold in sdg7, consistent with the mutation causing partial vernalization without cold, as well as a more rapid vernalization response.

FLC repression during vernalization results from quantitatively increased H3K27me3 deposition [START_REF] Coustham | Quantitative modulation of polycomb silencing underlies natural variation in vernalization[END_REF]. Interestingly, in sdg7, all tested regions of FLC chromatin (Fig. 4B) have increased levels of H3K27me3 relative to WT in the absence of cold exposure (Fig. 4C). Furthermore, the sdg7 mutant exhibits enhanced H3K27me3 spreading at FLC during cold exposure (Fig. 4C). As noted above for VIN3, COLDAIR, and COOLAIR expression, the sdg7 H3K27me3 phenotype indicates that loss of SDG7 results in a partially vernalized state of FLC chromatin before cold exposure, and a more rapid chromatin-based repression during cold exposure.

Loss of SDG7 Does Not Affect Global Histone Methylations. Based on its overall structure and phylogenetic relationships, SDG7 is classified as a class II SET domain group protein. It resides in class II SET domain orthology group II-1 along with maize SDG110 and rice SDG724 [START_REF] Springer | Comparative analysis of SET domain proteins in maize and Arabidopsis reveals multiple duplications preceding the divergence of monocots and dicots[END_REF][START_REF] Ng | Plant SET domain-containing proteins: Structure, function and regulation[END_REF] (Fig. S3A). Members of this subgroup contain three conserved domains: an Associated with SET (AWS) domain, an SET domain, and a post-SET domain [START_REF] Springer | Comparative analysis of SET domain proteins in maize and Arabidopsis reveals multiple duplications preceding the divergence of monocots and dicots[END_REF][START_REF] Ng | Plant SET domain-containing proteins: Structure, function and regulation[END_REF]. A recent study proposed that rice SDG724 is a histone methyltransferase, because loss of SDG724 resulted in a decrease of global H3K36 methylation, and an in vitro assay revealed histone methyltransferase activity [START_REF] Sun | The histone methyltransferase SDG724 mediates H3K36me2/3 deposition at MADS50 and RFT1 and promotes flowering in rice[END_REF].

We determined whether loss of SDG7 affects certain histone methylations on a global level using antibodies that recognize H3K36me1, H3K36me3, H3K4me2, H3K4me3, or H3K27me3. As a control, we assessed the effect of loss of the histone H3K36 methyltransferase, SDG8 [START_REF] Xu | Di-and tri-but not monomethylation on histone H3 lysine 36 marks active transcription of genes involved in flowering time regulation and other pro-cesses in Arabidopsis thaliana[END_REF], which results in decreased global gL H3K36me3 levels; however, loss of SDG7 had no effect on the global levels of modifications of the histones examined (Fig. 5A). Consistent with a role of rice SDG724 in histone methylation, SDG724 and maize SDG110 contain a predicted nuclear localization signal in the C terminus [START_REF] Brameier | NucPred-predicting nuclear localization of proteins[END_REF]. However, the C-terminal regions of SDG7, SDG724, and SDG110 are quite divergent, and there are no predicted nuclear localization signals in SDG7 (Fig. S3B). Consistent with the lack of a predicted nuclear localization signal in SDG7, an SDG7-GFP fusion localizes to the cytosol [START_REF] Sahr | O-carboxyl-and N-methyltransferases active on plant aquaporins[END_REF]. Furthermore, although SDG7 has Lys methylation activity in vitro against an aquaporin substrate ( 14), we could not detect any histone methyltransferase activity toward individual histones in an in vitro assay (Fig. 5B). Thus, the biochemical role of SDG7 in the timing of the vernalization response is not clear. The role of SDG7 in vernalization could be via Lys methylation of a cytosolic protein or proteins. Recent studies indicate that Lys methylation of extranuclear proteins is widespread in human cells [START_REF] Cao | Large-scale global identification of protein lysine methylation in vivo[END_REF], although the role of much of this Lys methylation is not known [START_REF] Clarke | Protein methylation at the surface and buried deep: Thinking outside the histone box[END_REF]. The vernalization system of Arabidopsis provides a model system for exploring the role of a specific Lys-methyltransferase in the context of an environmentally induced developmental switch.

Conclusions

Because loss of SDG7 results in a more rapid vernalization response, as well as certain features of vernalization in the absence of cold, such as higher levels of VIN3, lncRNAs complementary to FLC, and elevated basal levels of H3K27me3 at FLC chromatin, SDG7 is a negative regulator of the vernalization response (Fig. S4). The timing of the vernalization response is critical. If plants were to vernalize too rapidly, then in temperate climates in the fall season, short periods of cold followed by warm could result in deleterious flowering before winter. Thus, it is likely that natural selection has resulted in vernalization systems that are well adapted to particular climates [START_REF] Li | Multiple FLC haplotypes defined by independent cis-regulatory variation underpin life history diversity in Arabidopsis thaliana[END_REF], and SDG7 is an example of a gene involved in the timing of upstream vernalization events.

Methods

Plant Materials and Growth Conditions. The mutant screen was done in the Columbia accession (Col) containing a VIN3p::GUS transgene. The VIN3p:: GUS line does not have FRI; thus, to test for a vernalization phenotype, the mutants were introgressed into the Col FRI line (3,[START_REF] Lee | Effect of vernalization, photoperiod, and light quality on the flowering phenotype of Arabidopsis plants containing the FRIGIDA gene[END_REF]. Seeds were sown without stratification and grown in Petri dishes containing 2 mM MES at pH 5.7, 0.8% (wt/vol) Phytoblend agar (Caisson Laboratories) with 0.65 -1 Peter's Excel 15-5-15 CalMag fertilizer. Young seedlings were incubated at 4 °C under short days for vernalization treatment or transferred to soil (Metro-Mix 360; Sun Gro Horticulture) and fertilized with Peters Excel CalMag at weekly intervals under long days (16-h light/8-h dark cycle).

DNA Constructs. The VIN3p::GUS construct was created by amplifying a VIN3 genomic fragment from 2 kb upstream of the start site of the VIN3 mRNA to end of the second exon and joining this fragment to GUS in a translational fusion. The fragments were cloned into the D-TOPO entry vector (Life Tech-nologies) and then cloned into the pMDC164 destination vector using the Gateway cloning system (Life Technologies). For VIN3 overexpression, a VIN3 genomic fragment from transcription start site to transcription end site was cloned into pMDC32 [START_REF] Curtis | A gateway cloning vector set for high-throughput functional analysis of genes in planta[END_REF], which drives VIN3 from the 35S promoter.

GUS Assays.

For viable GUS staining, seedlings were grown on filter paper in vertically placed Petri dishes for 7 d. The filter papers with seedlings were then incubated for 3-5 h in viable GUS staining solution [0.2 M NaPO4 pH 7.0, 50 mM K3[Fe(CN)6], 50 mM K4[Fe(CN)6], and 0.5 mM X-Gluc (5bromo-4-chloro-3-indolyl beta-D-glucuronide cyclohexamine salt)]. Mutants were then rinsed and transferred to soil to produce seed for further studies. For de-structive GUS assays, seedlings grown on vertical agar plates were collected and vacuum-infiltrated for 15 min in GUS staining solution, and then in-cubated at 37 °C for 3-5 h [START_REF] Jefferson | GUS fusions: Beta-glucuronidase as a sensitive and versatile gene fusion marker in higher plants[END_REF]. Gene Expression Assays. Total RNA was isolated from seedlings grown on Petri dishes using TRIzol (Life Technologies) according to the instructions provided from the manufacturer, and treated with DNase (RQ1 RNase-Free Dnase; Promega). cDNA was generated from DNA-free RNA using qScript cDNA SuperMix (Quanta Biosciences), and quantitative RT-PCR was performed with an Applied Biosystems 7500 Fast Real-Time PCR System (Life Technologies). RNA levels were normalized using Arabidopsis UBQ10 as an internal control. Because COLDAIR RNA is not polyadenylated, we used a COLDAIR-specific antisense primer, 5′-CTTCCATA-GAAGGAAGCGACTA-3′, for cDNA synthesis reactions.

Histone Extraction and Immunoblot Analysis. Histone extraction and

Western blot analyses were performed as described previously (29) using 3-wkold plants and the following antibodies: anti-histone H3 (trimethyl K36) antibody (ab9050; Abcam; www.abcam.com), anti-histone H3 (monomethyl K36) antibody (ab9048; Abcam), anti-trimethyl-histone H3 (Lys4) antibody (07-473; Merck Millipore; www. merckmillipore.com), anti-trimethyl-histone H3 (Lys27) antibody (07-449; Merck Millipore), and anti-histone H3 antibody (05-499; Merck Millipore).

Histone Methyltransferase Assays. The full-length SDG7 recombinant protein was produced using the Gateway pDEST vector and then purified using His-tag affinity chromatography according to the manufacturer's instructions (Invitrogen; www.lifetechnologies.com). The SDG7 protein was tested for methlytransferase activity using 14C-labeled S-adenosyl-Lmethionine (Per-kinElmer; www.perkinelmer.com) as a substrate in the histone methyl-transferase assay as described previously [START_REF] Xu | Di-and tri-but not monomethylation on histone H3 lysine 36 marks active transcription of genes involved in flowering time regulation and other pro-cesses in Arabidopsis thaliana[END_REF].

ChIP Assay. ChIP assays were performed using anti-H3K27me3 antibody (07-449; Merck Millipore) as described previously [START_REF] Tamada | ARABIDOPSIS TRITHORAX-RELATED 7 is required for methylation of lysine 4 of histone H3 and for transcriptional activation of FLOWERING LOCUS C[END_REF]. Quantitative RT-PCR was performed using Applied Biosystems 7500 Fast Real-Time PCR System (Life Technologies). The data were normalized to the level of enrichment of the AGAMOUS gene as described previously [START_REF] Heo | Vernalization-mediated epigenetic silencing by a long intronic noncoding RNA[END_REF].

PLANT BIOLOGY

FRI, At5g57380 for VIN3, and At2g44150 for SDG7). The following mutant lines were used: sdg7-2 (SALK_131218), sdg7-3 (SALK_072682), sdg8-1 [START_REF] Berr | Arabidopsis histone methyltransferase SET DOMAIN GROUP8 mediates induction of the jasmonate/ethylene pathway genes in plant defense re-sponse to necrotrophic fungi[END_REF], elf7-2 [START_REF] Tamada | ARABIDOPSIS TRITHORAX-RELATED 7 is required for methylation of lysine 4 of histone H3 and for transcriptional activation of FLOWERING LOCUS C[END_REF], elf8-1 [START_REF] He | PAF1 complex-mediated histone methylation of FLOWERING LOCUS C chromatin is required for the vernalization-responsive, winterannual habit in Arabidopsis[END_REF], pie1-3 [START_REF] Noh | PIE1, an ISWI family gene, is required for FLC activation and floral repression in Arabidopsis[END_REF], suf3-1 [START_REF] Choi | SUPPRESSOR OF FRIGIDA3 encodes a nuclear ACTIN-RELATED PROTEIN 6 required for floral repression in Arabidopsis[END_REF] and abh1-2 [START_REF] Kim | Two cap-binding proteins, CBP20 and CBP80, are involved in processing primary MicroRNAs[END_REF].

ACKNOWLEDGMENTS. We thank Donna Fernandez, Xuehua Zhong and members of R.M.A.'s laboratory for useful discussions, and Juan Gao for assistance with the HMTa se assays. Work in R.M.A.'s laboratory was supported by the In Arabidopsis, genetic interaction studies using different histone methyltransferase mutants, including atx1, atx2, sdg8, sdg25, sdg26 and clf as well as the double histone demethylase mutant ldl1ldl2, revealed that some complex functional connections exist between H3K4me3, H3K36me3 and H3K27me3, even if they appear to act largely independently in regulating the transcription of flowering time genes [START_REF] Shafiq | Combinatorial functions of diverse histone methylations in Arabidopsis thaliana flowering time regulation[END_REF][START_REF] Berr | The trxG family histone methyltransferase SET DOMAIN GROUP 26 promotes flowering via a distinctive genetic pathway[END_REF]. Further reinforcing this complexity, transcriptome analyses of various mutant combinations has strengthened the idea of their only partial functional overlapping [START_REF] Berr | The trxG family histone methyltransferase SET DOMAIN GROUP 26 promotes flowering via a distinctive genetic pathway[END_REF]Zhao et al., 2015).

H2Bub1 is an essential epigenetic mark mainly associated with active gene transcription. Studies in yeast and animals showed that H2Bub1 regulates transcription through trans-crosstalks with several histone methylation marks. In a simplified way, H2Bub1 first occurs, allowing histone H3 lysine 4 and 79 trimethylation (H3K4me3 and H3K79me3) and transcription initiation and early elongation. Latter, H2Bub1 is removed promoting H3K36 trimethylation to enable transcription elongation to proceed.

In yeast, H3K4 and H3K36 methylation are catalyzed by the SET domain lysine methyltransferases Set1 and Set2, respectively, while H3K79 methylation is mediated by Dot1, a non-SET domain-containing histone methyltransferase. It was proposed that similar crosstalks may also exist in Arabidopsis gene transcription regulation (Feng and Shen, 2014). However, while homologs of Set1 and Set2 exist in Arabidopsis, the Arabidopsis genome seems to be lacking H3K79 methylation as well as a homologue of Dot1. Based on the particular distribution profile of histone methylation mark along transcribed genes in Arabidopsis, it has been suggested that H3K36me3 may play a similar role as that of H3K79me3 in animal and yeast. Indeed, the distribution of H3K36me3 in Arabidopsis is very similar to that of H3K79me3, suggesting a role for H3K36me3 in transcription elongation.

However, experimental evidence for crosstalk between H2Bub1 and H3K36me3 in Arabidopsis is largely lacking so far since studies were mainly concentrated on the functional relationship between H2Bub and H3K4me3. In order to fill this shortfall, I will explore the functional interaction between H2Bub and H3K36me3 using a genetic approach based on crosses between different histone-modifying enzyme mutants. This study particularly focused on the interplay of these different histone marks in regulation of plant growth (at organism as well as at cellular levels), in maintenance of genome transcription, and in regulation of chromatin status associated with flowering time gene expression levels to control plant flowering time.

RESULTS

Production of sdg8-1 hub2-2 double mutants

In order to unravel how H3K36 methylation and H2Bub1 are functionally connected in the course of transcription, I tested the genetic interaction between the sdg8 mutant and a mutant defective in H2Bub1, hub1 or hub2.

T-DNA insertion lines sdg8-1 (Salk_065480) and hub2-2 (Salk_071289) or hub1-3 (GABI_276D08) in the Col background has been characterized previously [START_REF] Zhao | Prevention of early flowering by expression of FLOWERING LOCUS C requires methylation of histone H3 K36[END_REF]Fleury et al., 2007). Homozygous sdg8-1 was crossed with hub1-3 and hub2-2, respectively. Seeds (F1) from crosses were collected from individual silique on the parent plants and were then grown for self-pollinating to obtain the F2 generation. By phenotyping of F2 generation plants, 5 out of 96 plants were found to be the potential double sdg8-1 hub2-2 homozygous mutants, representing 5.2 % of the total, which is in agreement with the 6.25 % predicted for Mendelian segregation. Genotypes were further validated by genomic PCR (Figure 1). A similar approach was used to isolate double sdg8-1 hub1-3 homozygous plants (Supplemental Figure 1).

Phenotypes of the sdg8-1 hub2-2 double mutants

Under our growth conditions, sdg8-1 mutant showed pleiotropic phenotypes including early flowering, reduced organ size and increased shoot branching, which is consistent with previously reported data [START_REF] Zhao | Prevention of early flowering by expression of FLOWERING LOCUS C requires methylation of histone H3 K36[END_REF]Xu et al., 2008). As previously reported, the hub2-2 mutant bolted earlier, had pale leaf coloration and modified leaf shape (Fleury et al., 2007). Compared to sdg8-1 or hub2-2, the sdg8-1 hub2-2 double mutant displays worsened phenotypes, including smaller body and rosette sizes, and earlier flowering (Figure 2). Because the sdg8-1 hub1-3 mutant plants shows very similar phenotypes compared to sdg8-1 hub2-2 (Supplemental Figure 2), we hereafter concentrated on sdg8-1 hub2-2 for detailed analysis.

Global levels of histone modifications analysis in sdg8-1 hub2-2 double mutants

To get insight into the combinatorial role of SDG8 and HUB2 in histone modifications, global level changes of histone modifications were analyzed by western blots experiment and compared between wild type and mutant plants. Antibodies specifically recognizing histone H3, monomethyl-H3K36 (H3K36me1), dimethyl-H3K36 (H3K36me2), trimethyl-H3K36 (H3K36me3), and dimethyl-H3K4 (H3K4me2), trimethyl-H3K4(H3K4me3), and trimethyl-H3K27 (H3K27me3) as well as histone H2B monoubiquitination (H2Bub1) were used on nuclear protein extracted from 3-week-old wild-type (Col) and sdg8-1, hub2-2, sdg8-1 hub2-2 mutant plants (Figure 3). In sdg8-1, while H3K4me2, H3K4me3, H2Bub1 levels appeared unchanged, the level of H3K36me3 is significantly reduced whereas H3K36me1 is increased as compared to wild-type plants. In hub2-2, H2Bub1 is dramatically decreased to a level below our detection limit, while levels of H3K4me2, H3K4me3, H3K36me1 and To gether, our analysis of the global levels of H3K36me1/me3, H3K4me2/me3 and H2Bub1 is in agreement with the notion that SDG8 is the major H3K36me3 histone methyltransferase and HUB2 is a H2B monoubiquitinase. However, a hypothetical crosstalk between H2Bub1, H3K4me3 and H3K36me3 at the genomic scale is not detectable.

Diverse aspects of plant growth and development are dramatically affected in sdg8-1 hub2-2 double mutants

In order to get insight into the functional relation between SDG8 and HUB2 in controlling plant growth and development, we conducted an exhaustive analysis of the sdg8-1 hub2-2 double mutant phenotypes compared to those of sdg8-1, hub2-2 single mutant and wild-type plants.

Plant body size and weight are reduced in sdg8-1 hub2-2 mutants

Under standard laboratory growth conditions, the average plant body size of sdg8-1 hub2-2 double mutants remained smaller along different growth stages compared to the average size of wild-type and single mutant plants (Figure 4 A-C). The plant fresh weight was measured from at least 20 individual 5-week-old plants, grown under longday (LD; 16 h light and 8 h dark) photoperiod conditions. Consistent with the seedling phenotype, the average plant fresh weight appears extremely reduced in sdg8-1 hub2-2 compared to sdg8-1, hub2-2 and wild-type plants (Figure 4D). Therefore, with respect to plant size, our results suggest that the two mutations have additive or even synergistic effects.

Chlorophyll content is reduced in sdg8-1 hub2-2 mutants

Because a decrease in chlorophyll content is one of the many phenotypes characterizing the sdg8-1 single mutant, we next address it in our double mutant.

Visually, rosette leaves look slightly paler in sdg8-1 hub2-2 mutants as compared to the sdg8-1 single mutants (Figure 4 A-B). This observation was confirmed by measuring the chlorophyll content from over 10 individual 12-day-old plants grown under long-day (LD; 16 h light and 8 h dark) photoperiod conditions. Indeed the chlorophyll content was additively decreased in the double mutant as compared to the wild-type and single mutants (Figure 5), suggesting an additive effect of both mutations.

Siliques length is shortened in sdg8-1 hub2-2 mutants

The sdg8-1 hub2-2 double mutant plants are fertile, but only very few seeds can be harvested from each individual plant. Consistently, mild developmental defect of siliques was observed in sdg8-1 and hub2-2 single mutants, while it was much more pronounced in the double mutant (Figure 6A). The siliques length was measured from over 20 individual fully elongated siliques from plants, grown under long-day (LD; 16 h light and 8 h dark) photoperiod conditions. Consistent with our observation, the siliques length in the double mutant was additively shortened from the single mutants (Figure 6B).

Cortex cells number in primary root meristem is reduced in sdg8-1 hub2-2 mutants

To establish whether the reduction in root length of our double mutant derived from a reduction in meristem size, we measured the size of the root meristematic zone represented as the number of cortex cells spanning from the quiescent center to the first elongated cell in the transition zone. Six-day-old primary roots of each genotype were separately stained with propodium iodide for three minutes and then imaged by laser scanning confocal microscopy (Figure 8A). The number of cortex cells was counted from at least ten roots for each genotype (Figure 8B). For both single and double mutants, shortened root phenotypes appeared to be related to the reduced meristem size. However, the meristem size in sdg8-1 hub2-2 was not significantly smaller than in sdg8-1 and hub2-2 single mutants. Therefore, because the sdg8-1 hub2-2 root length was smaller than in sdg8-1 and hub2-2 single mutants, other factors such as the cell elongation might also contribute to the primary root growth defects observed in our double mutants.

Rosette leaf growth and development is impaired in sdg8-1 hub2-2 mutants

To further explore cellular parameters that may explain the drastic size decrease of our double mutant, cotyledons and rosette leaves were dissected from 23-day-old wildtype and mutant plants and analyzed in detail (Figure 9).

Quantification of leaf parameters in sdg8-1 hub2-2 double mutants

Visually, sdg8-1 hub2-2, sdg8-1 and hub2-2 rosette leaves were smaller than wild type ones, with leaves from the double mutant being the smallest ones. In addition, the sdg8-1 hub2-2 double mutant present a lower number of rosette leaves. Moreover, the double mutant present several additional leaf defects that were not described both in both single mutants such as dentate leaf margins, irregular leaf lamina surface, asymmetric lamina or reduced venation complexity (Figure 9A). Synchronous profiles were obtained for both single mutants, even if it was less pronounced in sdg8-1, thus indicating no retardation in leaf initiation (i.e. the lamina area increased from juvenile leaves 1, 2 and 3 to transition to adult leaf 4, then adult leaves 5 and 6, while it reduced for adult leaves 7, 8 and 9 and cauline leaf 10). Also, the lamina area of every leaf of the sdg8-1 and hub2-2 single mutant was significantly smaller than that of the wild type control, which was further exacerbated with the double

Analysis of ploidy levels in sdg8-1 hub2-2 mutants

To investigate cell cycle progression and evaluate any possible effect of SDG8 and/or HUB2 mutations on endoreduplication, we measured by flow cytometry the ploidy levels of nuclei extracted from the first pair of true leaves of wild-type and mutant plants at 10, 11, 12, 14, 17 and 21 days after stratification (Figure 11). Already visible precociously, the proportion of nuclei in sdg8-1 leaves with high ploidy levels (8C and 16C) was lower than in wild-type leaves, while the proportion of nuclei at the 2C and 4C levels stays abnormally high. These observations indicate that the entrance of cells in endoreduplication might be far less efficient in the mutant than in wild-type plants. Because both the cell number and the cell size were decreased in sdg8-1 compared to wild type, this low occurrence of endoreduplication might further indicate that the mitotic cell cycle in sdg8-1 is arrested either in the G1 phase or in the G2 phase of the diploid cell cycle.

At early stages, DNA ploidy profiles were largely similar between hub2-2 and wild type plants, however, with aging, the proportion of 2C and 4C nuclei progressively decreased to the benefit of higher ploidy levels (8C and 16C). This change in proportions indicate that the first endocycle occurs earlier and that subsequent endocycles succeed faster in hub2-2 leaves than in wild-type leaves, therefore supporting the lower number of cells and their bigger size in this mutant.

Together, our results indicate that SDG8 and HUB2 have different direct or indirect functions regarding the control of cell cycle, with SDG8 being involved in mitosis progression and HUB2 in repressing the escape from a normal cell cycle to endoreduplication. Finally, in the double mutant, nuclei proportion were in between that of sdg8-1 and hub2-2, thus resembling the proportion observed in wild type nuclei and further supporting the epistatic relationship that exist between this two allele regarding the control of cell cycle progression.

Flowering time analysis of sdg8-1 hub2-2 double mutants

The most obvious phenotype of the sdg8-1 hub2-2 (Figure 12) or sdg8-1 hub1-3 (Supplemental Figure 3) double mutant was its precocious flowering. Therefore, we further focused our work on exploring the molecular origins of this flowering phenotype.

Flowering time of sdg8-1 hub2-2 mutants

Classically, flowering time can be measured as the number of days to flowering and/or as the leaf number at bolting, which represents interest of measurement from a developmental perspective (Michaels and Amasino, 1999). Both measurements were used to compare the flowering time of the wild-type andsdg8-1, hub2-2, sdg8-1 hub2

Expression profiles of flowering time genes

Flowering represents the transition from the vegetative to reproductive development, and is mainly controlled by five flowering time pathways in Arabidopsis, including the photoperiod pathway, the vernalization pathway, the autonomous pathway, the gibberellins (GA) pathway and the age pathway (Teotia and Tang, 2015). FLC is a MADS-box transcription factor that acts as a central repressor of flowering, more specifically in the vernalization and autonomous pathways. FLC binds and represses the expression of several genes, including genes that promote flowering, the flowering time integrators FT and SOC1. The Arabidopsis genome contains five FLC homologs, named MAF1 to MAF5 that function redundantly in controlling flowering time. To investigate the molecular basis of the sdg8-1 hub2-2 early flowering phenotype, the transcript levels of several flowering genes including major flowering repressor genes (FLC, MAF1, MAF2, MAF3, MAF4 and MAF5) and flowering activator genes (FT and SOC1) were measured by quantitative RT-PCR and compared to that of sdg8-1 and hub2-2 single mutants (Figure 14).

In sdg8-1 mutants, transcript levels of MAF2 and MAF3 were only slightly reduced, while transcript levels of other flowering repressor genes FLC, MAF1, MAF4, and MAF5 were drastically reduced. More downstream, their target genes FT and SOC1 appeared increased compared to the wild-type control. In hub2-2, expressions of MAF2 and MAF3 were not significantly affected, while FLC and MAF1 transcript levels were only slightly reduced. Also, MAF4 and MAF5 expression levels in hub2-2 were reduced to similar levels as in sdg8-1 and SOC1 and FT expressions were increased.

Interestingly, expression levels of all the examined flowering time repressor genes were down-regulated in our sdg8-1 hub2-2 double mutants, resulting in an increase in both SOC1 and FT expressions. In details, FLC, MAF1 and MAF3 were similarly decreased in our double mutant as in sdg8-1, without any additive effect from the HUB2 mutation, indicating that SDG8 dominates over HUB2 in the regulation of the expression of these flowering time genes. MAF2 was reproducibly more downregulated in sdg8-1 hub2-2 than in sdg8-1, indicating a possible synergistic genetic interaction between SDG8 and HUB2. Interestingly, while the FT transcript level was synergistically increased in our double mutant compared to single ones, the SOC1 expression was not further increased in sdg8-1 hub2-2. The absence of additive effect on SOC1 expression might indicate that SDG8 and HUB2 act in the same pathway to activate the transcription of this locus.

ChIP analysis on flowering time genes

To gain insight into the molecular mechanism involved in the transcriptional misregulation of some flowering genes, ChIP (Chromatin ImmunoPrecipitation) assay was used to analyze and compare the level of H3K4me3, H3K36me3, H3K27me3 and H2Bub1 along the chromatin of FLC, MAF3, MAF4, MAF5, SOC1 and FT between wild-type and mutant seedlings (Figure 15).

ChIP analysis on FLC gene

Firstly, while comparing the different studied marks, it is interesting to notice that H3K4me3 formed a peak at the 5'-regions of FLC, around the Transcription Start Site (TSS), while H3K36me3, H2Bub1 and H3K27me3 were more spread along its chromatin. More precisely, in sdg8-1, the level of H3K4me3 was unchanged compared to the wild-type control, whereas levels of H3K36me3 and H2Bub1 were reduced all along FLC chromatin (Figure 16A, B and C). Probably benefiting from the decrease of HK36me3 and H2Bub1, the level of H3K27me3 was inversely increased all along FLC chromatin (Figure 16D), suggesting that H3K36me3 and/or H2Bub1 might contribute to prevent the spread of the H3K27me3 repressive mark. Overall, this set of results again confirms that SDG8 is per se an H3K36-specific methyltransferase essential for FLC transcription and suggest that at FLC, H3K36me3 is required for H2Bub1.

In hub2-2, the level of H2Bub1 was decreased, reinforcing the idea that HUB2 may function as an E3-ligase responsible for H2Bub1 deposition at FLC. Interestingly, both H3K4me3 and H3K36me3 levels were found decreased at FLC chromatin in hub2-2 mutants and surprisingly the decrease in H3K36me3 was similar as the one observed in sdg8-1 (Figure 16A, B andC). These observations indicate that at FLC H2Bub1 is required for both H3K4 and H3K36 methylation. Put into perspective with our Q-PCR analyses (Figure 14), a directional dependence might therefore exist regarding the impact of histone mark variations on transcription. Indeed, the requirement of H3K4 and H3K36 methylation over H2Bub1 seems to have a lower impact on FLC transcription than the requirement of H2Bub1 over H3K36me3. As in sdg8-1, the level of H3K27me3 was globally increased in hub2-2, although less markedly, especially at the 3' end of FLC (Figure 16D).

In sdg8-1 hub2-2, the level of H3K4me3 was similarly reduced as in hub2-2, and H2Bub1 was found decreased like in sdg8-1 or hub2-2 (Figure 16A andC). Thus, at FLC chromatin, the H3K4me3 deposition is dependent from HUB2 but independent from the SDG8-mediated H3K36me3 deposition. Surprisingly, the H3K36me3 level in sdg8-1 hub2-2 was additively reduced with respect to sdg8-1 and hub2-2 (Figure 16B).

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FLC-P1 FLC-P2 FLC-P3 FLC-P4 FLC-P5 FLC-P6 FLC-P7 FLC-P8 FLC-P9 FLC-P10

H3K4me3 level

FLC-P1 FLC-P2 FLC-P3 FLC-P4 FLC-P5 FLC-P6 FLC-P7 FLC-P8 FLC-P9 FLC-P10

H2Bub1

FLC-P1 FLC-P2 FLC-P3 FLC-P4 FLC-P5 FLC-P6 FLC-P7 FLC-P8 FLC-P9 FLC-P10

H3K27me3 level

Col sdg8-1 hub2-2 sdg8-1hub2-2

ChIP analysis on MAF3/4/5 genes

Overall, it is interesting to notice that levels of H3K4me3, H3K36me3 and H2Bub1 in wild type plants were higher on MAF3 than on MAF4 and MAF5, while it was the opposite for H3K27me3 (Figure 17). This feature will require further work, but it might indicate that in this gene cluster MAF3 is regulated differently than MAF4 and MAF5.

Supporting this hypothesis, CT values obtained during our Q-PCR analysis were systematically lower for MAF3 (around 25.3) than for MAF4 and MAF5 (from 26.4 to 28.3), therefore reflecting a stronger level of transcript and probably a stronger expression. Moreover, one should also notice that unlike what we previously described for FLC chromatin, H3K4me3, H3K36me3 and H2Bub1 profiles were mostly overlapped.

When comparing mutants to wild type profiles, the H3K4me3 behavior along the chromatin of MAF3, 4 and 5 in sdg8-1, hub2-2 and sdg8-1 hub2-2 appeared very similar as what we described for FLC (Figure 17A). However, while an additive effect was observed at FLC in our double mutant, the level of H3K36me3 was decreased similarly in sdg8-1, hub2-2 and sdg8-1 hub2-2 (Figure 17B). Another difference between FLC and this gene cluster was observable for the level of H2Bub1. Indeed, while the H2Bub1 decrease along FLC was very similar between sdg8-1, hub2-2 and sdg8-1 hub2-2, the level of H2Bub1 in sdg8-1 was intermediate between the wild type level and the hub2-2 and sdg8-1 hub2-2 ones (Figure 17C). Regarding H3K27me3, its level was increased in both single and double mutants, but the increase was lower in hub2-2 than in sdg8-1 and sdg8-1 hub2-2 and was limited to MAF4 and MAF5 chromatins (Figure 17D). As a result of the comparison between histone marks profiles and transcript levels, we propose that the decrease in MAF3 mRNA observed for sdg8-1 and the double mutants cannot be only explained by the decrease in the loading of the studied active marks or the increase in the H3K27me3 repressive one that we address in this work. Indeed, other marks, such as H3K36me2 that is known to be affected in sdg8-1 (Xu et al., 2008), might explain with despite very similar histone profiles, MAF3 is downregulated in sdg8-1 and unchanged in hub2-2. Because changes in active marks were weaker in MAF4 compared to MAF3 and to a lower extent to MAF5, while the level of H3K27me3 was the highest, we further propose that the decrease in MAF4 mRNA observed for all mutant combinations might be essentially due to the increase in the loading of the studied repressive mark. Finally, because MAF5 can be roughly defined as being half-way between MAF3 and MAF4 in terms of histone profiles, we propose that the decrease in MAF5 mRNA observed for all mutant combinations might be due to the combined effect of the increase and the decrease in the loading of the studied repressive and active marks, respectively.

Together, the here reported differences between MAF3, MAF4 and MAF5 strengthen the above-mentioned hypothesis that while being on a gene cluster, MAF3, MAF4 and MAF5 might be transcriptionally regulated differently by histone modifications. Chromatin was prepared from 16-day-old plants grown under medium-day (MD; 12 h light and 12 h dark) photoperiod conditions. Data were normalized to the input and presented relative to Tubulin 2 (Tub2) in the wild type control.

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M A F -P 1 M A F -P 2 M A F -P 3 M A F -P 4 M A F -P 5 M A F -P 6 M A F -P 7 M A F -P 8 M A F -P 9 M A F -P 1 0 M A F -P 1 1 M A F -P 1 2 M A F -P 1 3 M A F -P 1 4 M A F -P 1 5

H3K4me3 level

M A F -P 1 M A F -P 2 M A F -P 3 M A F -P 4 M A F -P 5 M A F -P 6 M A F -P 7 M A F -P 8 M A F -P 9 M A F -P 1 0 M A F -P 1 1 M A F -P 1 2 M A F -P 1 3 M A F -P 1 4 M A F -P 1 5

H3K36me3 level

ChIP analysis on SOC1 gene

Like FLC, a H3K4me3 peak was visible around the SOC1 TSS and the H2Bub1 mark was more dispersed along its chromatin (Figure 18). However, unlike FLC but similarly to MAF3, the H3K36me3 peak was almost overlapping with H3K4me3 one.

M A F -P 1 M A F -P 2 M A F -P 3 M A F -P 4 M A F -P 5 M A F -P 6 M A F -P 7 M A F -P 8 M A F -P 9 M A F -P 1 0 M A F -P 1 1 M A F -P 1 2 M A F -P 1 3 M A F -P 1 4 M A F -P 1 5

H3K27me3 level

Col sdg8-1 hub2-2 sdg8-1hub2-2
In general, the chromatin behavior of the three studied active marks along SOC1

was very similar to that of FLC in our different mutant combinations (Figure 18A, B and C). In details, H3K4me3 was unchanged in sdg8-1, while its level was decreased similarly in hub2-2 and sdg8-1 hub2-2 (Figure 18A). Next, the level of H3K36me3 along SOC1 chromatin was equally decreased in sdg8-1 and hub2-2 (Figure 18B). In sdg8-1 hub2-2, a subtle and less-than-additive effect was detectable, especially around the Transcription Start Site (TSS), where the H3K36me3 peaks. This observation might indicate that, like for FLC chromatin but less pronounced, part of the H3K36me3 deposition is dependent on the presence of H2Bub1, while the other part seems inversely required for H2B monoubiquitination. Another feature that distinguishes SOC1 from FLC or the MAF gene cluster is regarding the H3K27me3 enrichment observed in hub2-2 which reach a similar level as the ones observed with either sdg8-1 or sdg8-1 hub2-2. We believe that this might be due to the less pronounced additive effect observed between sdg8-1 and hub2-2 for the level of H3K36me3 in our double mutant. Together, the mutations of SDG8 and/or HUB2 contribute to roughly similar effect on the level of H3K4me3, H3K36me3, H2Bub1 and H3K27me3, while the impacts of these mutations on transcription were opposite, with FLC, MAF3, MAF4 and MAF5 being downregulated and SOC1 being upregulated. As a possible explanation, we proposed that the expected inhibitory effect on SOC1 expression that would have resulted from the decrease in H3K4me3, H3K36me3 and H2Bub1 and the increase in H3K27me3 observed in our mutants might have been over-compensated by the low expression levels of its repressors FLC, MAF3, MAF4 and MAF5.

SOC1-P1 SOC1-P2 SOC1-P3 SOC1-P4 SOC1-P5 SOC1-P6 SOC1-P7 SOC1-P8 SOC1-P9 SOC1-P10 SOC1-P11 SOC1-P12 SOC1-P13

H3K27me3 level

Col sdg8-1 hub2-2 sdg8-1hub2-2

ChIP analysis on FT gene

Another floral integrator gene, FT, was also analyzed by ChIP. Our choice to present ChIP results as relative to our reference gene in the wild type control and not relative to wild type at each primer pair enabled us to explore variations of histone mark profiles along each gene, and made it also possible to compare profiles between the studied genes. Thus, when comparing the chromatin of FT with the one of other flowering genes, it becomes evident that levels of H3K4me3, H3K36me3, H2Bub1 and H3K27me3 were dramatically lower (Figure 16,17,18 and 19). Unfortunately, our ChIP procedure does not enable us to address specifically chromatin profiles in cells were the gene of interest is expressed. Therefore, chromatin profiles of cells where a gene is active get "diluted" in chromatin profiles from cells where the gene is silenced or inactive. This technical limit might therefore explain, at least partially, the distinctive observation we made for the FT chromatin profile, especially when considering that prior flowering FT is far less widely expressed than FLC and SOC1 (Searle et al.,2006;Farrona et al., 2012).

Of course, a supplementary explanation would simply be that FT transcription is weakly influenced by the variability of histone modification levels, but is more under the direct control of its repressors (i.e. FLC, MAF3, MAF4 and MAF5). Supporting this last hypothesis, it is interesting to notice that despite the low levels of active marks along FT chromatin and their slight variations in our mutants, the FT expression was found upregulated as the result of the low expression levels of its repressors (Figure 14).

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FT-P1 FT-P2 FT-P3 FT-P4 FT-P5 FT-P6 FT-P7 FT-P8 FT-P9 FT-P10 FT-P11 FT-P12

H3K4me3 level

FT-P1 FT-P2 FT-P3 FT-P4 FT-P5 FT-P6 FT-P7 FT-P8 FT-P9 FT-P10 FT-P11 FT-P12

H2Bub1

FT-P1 FT-P2 FT-P3 FT-P4 FT-P5 FT-P6 FT-P7 FT-P8 FT-P9 FT-P10 FT-P11 FT-P12

H3K27me3 level

Transcriptome analysis of sdg8-1 hub2-2 double mutants

In order to investigate the role of SDG8 and HUB2 in genome expression pattern, transcriptome analysis was performed for the sdg8 -1, hub2-2, andsdg8-1 hub2-2 mutants. Genes showing greater than 1.5-fold changes and with a P values less than 0.05 (P≤0.05) were considered as differentially expressed genes as compared to our wild-type control.

In sdg8-1, 1094 genes were found as differentially expressed (Supplemental Figure 4), with 721 (65.9%) and 373 (34.1%) genes being down-regulated and upregulated, respectively (Figure 20). In hub2-2, 300 genes were differentially expressed (Supplemental Figure 4), with 156 (52%) and 144 (48%) genes being down-regulated and up-regulated, respectively (Figure 20). Remarkably, SDG8 and HUB2 have both common and different target genes in the genome. Comparison of differentially expressed genes between the sdg8-1 and hub2-2 mutants revealed an overlap of 108 genes, a number relatively small but significantly above the random value expected for two independent regulators (Representation Factor (RF) = 5.8, P<2.46E54; RF and statistical significance was measured with a hyper-geometric test using the web-based calculator http://nemates.org/MA/progs/overlap_stats.html. An RF>1 indicates more overlap than expected between two groups at random). Also, it is interesting to notice that the large majority (87%) of the 108 overlapping genes were mis-regulated in the same direction, with 66 genes being up-regulated and 28 genes being down-regulated in both sdg8-1 and hub2-2 (Figure 20).

In the sdg8-1 hub2-2 double mutant, 1472 genes in total were found as differentially expressed (Supplementary Table 5), with 909 (61.7) and 563 (38.3%) genes being down-regulated and up-regulated, respectively (Figure 20). Among downregulated genes, about 30%, 4.9% and 3.1% are overlapping with sdg8-1, hub2-2, or both, respectively (Figure 20A). Among up-regulated genes, about 22%, 3.0% and 4.1% are overlapping with sdg8-1, hub2-2, or both, respectively (Figure 20B). The remaining differentially expressed genes (about 62% and 71% for down-regulated and upregulated genes, respectively) are specifically found in the sdg8-1 hub2-2 double

Discussion and conclusions

SDG8 encodes a major H3K36me2/3 methyltransferase as in sdg8 mutants, H3K36me2/3 level was globally decreased [START_REF] Zhao | Prevention of early flowering by expression of FLOWERING LOCUS C requires methylation of histone H3 K36[END_REF]Xu et al., 2008;Liu et al., 2016;this study). H2B monoubiquitination is catalyzed by the E2 ubiquitinconjugating enzymes UBC1 and UBC2 together with the E3 ubiquitin-ligation enzymes HUB1 and HUB2. H2Bub1 levels are globally lost in hub1 and hub2 single mutants as well as in the hub1 hub2 and ubc1 ubc2 double mutants, but are unaffected in the ubc1, ubc2 single mutants [START_REF] Cao | Histone H2B monoubiquitination in the chromatin of FLOWERING LOCUS C regulates flowering time in Arabidopsis[END_REF][START_REF] Gu | Repression of the floral transition via histone H2B monoubiquitination[END_REF][START_REF] Xu | The E2 ubiquitinconjugating enzymes, AtUBC1 and AtUBC2, play redundant roles and are involved in activation of FLC expression and repression of flowering in Arabidopsis thaliana[END_REF]. To investigate the possible crosstalk between H2Bub1 and H3K36me3 in Arabidopsis we generated the sdg8-1 hub2-2 double mutant by classical genetic cross between sdg8-1 and hub2-2. The plant size is intrinsically determined by cell division and cell expansion.

Crosstalks among

Because no retardation in leaf initiation was observed, we measured the cell size and number parameters in rosette leaf to uncover the mechanisms underlying the growth defect of our mutants. Interestingly, we observed that the decrease of the cell number per leaf was enhanced in the double mutants, suggesting that SDG8 and HUB2

synergistically regulate cell proliferation. The cell area was decreased in sdg8-1 and sdg8-1 hub2-2, while it was increased in hub2-2, indicating that SDG8 and HUB2 may act in opposite manner along the same regulatory pathway, with SDG8 acting upstream of HUB2. However, our flow cytometry data demonstrates that cell cycle progression and endoreduplication are differently affected in our mutants, suggesting that the number of cells is indeed decreased in both sdg8-1 and hub2-2, but for different reasons (i.e. a delay in mitosis for sdg8-1 and a precocious entry in endoreduplication for hub2-2).

Altogether, these results reveal that while plant growth is synergistically affected by the two mutations, SDG8 and HUB2 act separately, with SDG8 being more involved in the mitosis progression and HUB2 the mitosis-to-endoreplication transition.

SDG8 and HUB2 synergistically regulate Arabidopsis flowering time.

Since the most obvious phenotype of sdg8-1 and hub2-2 mutants is their early flowering, it is evident that H3K36me2/3 and H2Bub1 play together or not an important role in preventing precocious flowering. It is therefore of great interest to unveil if a crosstalk exist between these marks to control flowering time. Previously, some genetic interactions were tested regarding the control of flowering time in which the SDG8 mutation was combined with different mutants encoding different histone methylation enzymes (i.e. comprising the histone methyltransferase ATX1/2, SDG25, SDG26, CLF)

and the histone demethylase LDL1/2 [START_REF] Shafiq | Combinatorial functions of diverse histone methylations in Arabidopsis thaliana flowering time regulation[END_REF][START_REF] Berr | The trxG family histone methyltransferase SET DOMAIN GROUP 26 promotes flowering via a distinctive genetic pathway[END_REF]Liu et al., 2016). These combined mutants showed different flowering behavior with sdg8 atx1, sdg8 atx2, sdg8 sdg25 or sdg8 sdg26, sdg8 ldl1ldl2 displaying a sdg8-like earlyflowering phenotype, while sdg8 clf was flowering even earlier than either sdg8 or clf single mutant.

In this vein, we here addressed the question of the flowering control in our sdg8-1 hub2-2 double mutant. We observed that sdg8-1 hub2-2 was flowering earlier than each single mutant, at a similar level as what was previously published for sdg8 clf,

suggesting that an additive or even synergistic interaction occurred between SDG8 and HUB2 to control flowering.

Crosstalk between histone methylation and monoubiquitination during transcription.

To address the molecular mechanism underlying the additive or synergistic interaction that exist between SDG8 and HUB2 to control flowering, we explored transcriptional and chromatin changes at different flowering time genes. Previously, SDG8, SDG25, SDG26, ATX1/2, CLF and LDL1/2 were reported to regulate largely independently flowering genes [START_REF] Shafiq | Combinatorial functions of diverse histone methylations in Arabidopsis thaliana flowering time regulation[END_REF][START_REF] Berr | The trxG family histone methyltransferase SET DOMAIN GROUP 26 promotes flowering via a distinctive genetic pathway[END_REF]Liu et al., 2016).

In this study, we found that SDG8 and HUB2 play related roles in the transcriptional regulation of flowering time genes. Indeed, FLC, MAF1, MAF3, MAF4, and MAF5 transcript levels were decreased in sdg8-1 and FLC, MAF1, MAF4 and MAF5 transcript levels were decreased in hub2-2, which is broadly in line with previous reports. Interestingly, all these flowering repressor genes were downregulated in the sdg8-1 hub2-2 double mutant to levels at or below the sdg8-1 level. These data indicate that both SDG8 and HUB2 are involved in the transcriptional regulation of FLC, MAF1, MAF2, MAF3, MAF4 and MAF5, with SDG8 playing a stronger role than HUB2.

Correspondingly, transcript levels of the downstream gene targets FT and SOC1 were increased in sdg8-1, hub2-2 and sdg8-1 hub2-2. Interestingly, the transcript level of FT was further increased in sdg8-1 hub2-2 double mutant compared to single ones, which can largely explain the flowering behavior of our double mutant. Interestingly, the SOC1 expression is not only being repressed by FLC, but is also activated by FT [START_REF] Yoo | CONSTANS activates SUPPRESSOR OF OVEREXPRESSION OF CONSTANS 1 through FLOWERING LOCUS T to promote flowering in Arabidopsis[END_REF]. Because SOC1 is up-regulated to a similar level in both our single and double mutants, we proposed that a decrease in the transcription of one of its repressor might be enough to provoke an increase in the expression of SOC1 to a maximum that cannot be overcome.

H3K36me2/3 and H2Bub1 in regulation of flowering time gene chromatin status.

Previous studies have demonstrated that SDG8 and HUB2 are respectively required for H3K36me2/3 and H2Bub1 deposition at FLC chromatin and thus promoted its gene transcription. This is consistent with the notion that H3K36me2/3 and H2Bub1 are important in establishing an active chromatin state. During transcription, H2Bub1 was proposed to be coupled to H3K4me3 and H3K36me2 in yeast and humans. In Arabidopsis, H3K4me3 and H3K36me2 were found to be decreased on FLC in hub2-2 mutant [START_REF] Cao | Histone H2B monoubiquitination in the chromatin of FLOWERING LOCUS C regulates flowering time in Arabidopsis[END_REF].

In this study, we found the H3K4me3 level on FLC, MAF3/4/5 and SOC1 was unchanged in sdg8-1, but decreased to a similar level in sdg8-1 hub2-2 as in hub2-2.

These results strongly indicate that the H3K4me3 deposition is dependent from H2Bub1 but independent from the SDG8-mediated H3K36me3 deposition.

The level of H3K36me3 was decreased in sdg8-1 along FLC, MAF3/4/5 and SOC1 chromatin. In hub2-2, H3K36me3 was reduced to a similar level as in sdg8-1 at these genes loci. Interestingly, at MAF3/4/5, the H3K36me3 decrease in sdg8-1 hub2-2 double mutant was similar as in sdg8-1, hub2-2 single ones, while an additive effect at FLC and to a less extent at SOC1 was observed in our double mutant. This result might indicate that the deposition of the H3K36me3 mark first strongly depend on SDG8 and second can be subdivided in two categories, (i) one that is required for H2Bub1 deposition and (ii) another one, in line with the hierarchical model of histone modification during transcription in animal and yeast, which dependent on H2Bub1 deposition. In this context, it is noteworthy that the H3K36me3 in Arabidopsis has been proposed to play an equal role as H3K79me3 in yeast during transcription elongation due to their similar genome-wide distribution [START_REF] Roudier | Integrative epigenomic mapping defines four main chromatin states in Arabidopsis[END_REF]. Given that the deposition of H3K36me3 on some specific genes loci is rely on H2Bub1, similar to that of H3K79me3 in yeast, thus provides additional evidence to support this hypothesis.

Intriguingly, H2Bub1 was reduced in sdg8-1 and to a similar extent in hub2-2 and sdg8-1 hub2-2 along the chromatin of FLC and SOC1. While at MAF3/4/5, the level of H2Bub1 in sdg8-1 was intermediate between the hub2-2 and sdg8-1 hub2-2 ones.

These data uncovered that H3K36 me3 is inversely required for H2Bub1.

FT transcription seems to be less affected by the variability of histone modification levels, because the levels of H3K4me3, H3K36me3, H2Bub1 and H3K27me3 on FT Collectively, on the basis of these results, we propose a model for the interplay of H3K4me3, H3K36me3 and H2Bub1 in the regulation of plant flowering genes transcription (Figure 21B). In this model, H3K4me3 is fully dependent on the loading of H2Bub1 as in animals (Figure 21). However, and in contrast with animal and yeast models (Figure 21A), there is not a fully exclusive correlation between H3K36me3 and H2Bub1 as it was not possible to clearly establish a hierarchical dependency.

Therefore, SDG8 and HUB2 might mutually favor each other in establishing an active chromatin state on flowering genes (Figure 21B). Considering a weak but significant overlap of mis-regulated genes in sdg8-1, hub2-2 and sdg8-1 hub2-2 mutants revealed by our transcriptome analysis and the additive and/or even synergistic other plant growth defects in the double mutant, it is reasonable to speculate this model likely act beyond flowering genes transcription, but also involved in genes that control many other aspects of plant growth and

INTRODUCTION

SET DOMAIN GROUP 8 (SDG8)-mediated H3K36me2/3 is required for multiple developmental processes in Arabidopsis. Consistently, the sdg8 mutant plants show globally reduced H3K36me2/3 level and thus has pleiotropic phenotypes [START_REF] Zhao | Prevention of early flowering by expression of FLOWERING LOCUS C requires methylation of histone H3 K36[END_REF]Xu et al., 2008). However, the mechanism of H3K36me2/3 function is largely not known in plants. Compared to acetylation that changes the charge and thus reduces the interaction between histone and DNA, histone methylation does not per se directly trigger changes in the chromatin structure but it requires specific "readers" that will recognize the methylation signal and interpret it [START_REF] Yun | Readers of histone modifications[END_REF]. In Arabidopsis, little is currently known about histone methylation readers, and especially regarding the domains involved in this binding as well as their specificity. Previously in the team, yeast two-hybrid screen was carried out using the SDG8 as bait by a colleague in our lab to investigate how SDG8 be recruited to target loci. One of the potential SDG8 partner identified in the screen was the HULK2 protein (HUA2-LIKE 2), a homolog of HUA2 (ENHANCER OF AG-4 2) (Jali et al., 2014). The most prominent feature of the HUA2 family proteins is the amino-terminal PWWP (named after the central core motif

Pro-Trp-Trp-Pro) domain, which is implicated in epigenetic regulation through differentially bind methylated and/or unmethylated histones in both animals and yeast [START_REF] Qin | Structure and function of the nucleosome-binding PWWP domain[END_REF]. Both HUA2 and HULK2 have been reported to regulate flowering time, however, their PWWP domain was not explored (Doyle et al., 2005;Jali et al., 2014). Firstly, despite that it was failed to detect the interaction between SDG8 and HULK2 observed in yeast using other method such as pull-down assay. Nonetheless, we demonstrated that the PWWP domain of HUA2/HULK2 is able to bind H3K36 methylation by in vitro binding assay. To further explore this binding, we determined the crystal structure of the plant HUA2 PWWP domain. Finally, to better understand the role of PWWP domain in vivo, genetic interaction between HUA2 and SDG8 was analyzed.

RESULTS

Test HULK2 and SDG8 interaction by pull-down assay

To identify new partners of SDG8, my colleague performed yeast two-hybrid screen and found that a PWWP-domain protein, HULK2, could be a potential partner of SDG8.

To verify this by pull-down experiment, the potential interaction domain proteins of His tagged SDG8-N terminal (1-816aa) and GST tagged HULK2-C terminal (1068-1366aa)

were expressed and purified. However, we failed to detect the protein-protein interaction between SDG8 and HULK2 in pull-down assays (Figure 1). were analyzed by Western blot using antibodies against the His tag.

Sequence alignment of the HUA2/HULK2 PWWP-domain

By sequence similarity, multiple alignment, and tree reconstruction approaches, the existence of three additional genes with high sequence similarity to HUA2 was identified. These genes were named HULK (HUA2 LIKE) genes and together with HUA2, the family is referred to as the HULK gene family (Jali et al., 2014). All these four genes encode proteins containing a conserved PWWP domain; potentially has the ability to bind methylated histones. A sequence alignment of the HULK family PWWP domain with those of the mammalian PSIP1, MUM1, MSH6, DNMT3A, HDGF, NSD3_C, PWWP2B, and RBBP1 is shown in Figure 2. The overall sequence of the four HULK family proteins with human and yeast proteins showed low similarity.

However, the four core residues PAWP (in black rectangle box) showed highly conservation across all the species, especially the fourth residue Pro is absolutely conserved. Thus, these residues were chosen for site directed mutagenesis to functionally analyze the HUA2 and HULK2 PWWP domain in in vitro histone binding assays and for complementation studies in this study.

The HUA2/ HULK2 PWWP-domain exhibits in vitro histone peptide binding capacity

To analyze the potential histone binding capacity of the HUA2 and HULK2 PWWPdomain in vitro, they were fused to 6×His tag. His-HUA2-PWWP and His-HULK2-PWWP proteins were expressed in E.coli and were purified to incubate with synthetic biotinylated histone peptides H3K4me1, H3K4me2, H3K4me3 and H3K36me1, H3K36me2, H3K36me3, respectively. Proteins bound to the histone peptides were precipitated by Ni-NTA Agarose beads. Following western blots analysis with a alphabiotin antibody revealed both the His-HUA2-PWWP and His-HULK2-PWWP proteins specifically bind to mono-, di-and tri-methylated H3K36 (K36me1/me2/me3) peptides, but not the H3K4me1/2/3 (Figure 3). A point mutation of the conserved proline 16 to alanine HUA2-PWWP (P16A) did not alter the binding properties towards H3K36 methylation; whereas point mutated proteins HUA2-PWWP (W18A) and HUA2-PWWP (P19A) fully abolished the binding ability, suggesting that the mutation of the W18 and P19 residue alone is sufficient to alter the binding specificity of the HUA2 PWWP-domain. It is worthy to note that none of these proteins bind to H3K27me3 and thus reinforced the binding specificity.

Protein crystallography analysis of HUA2 PWWP-domain

To obtain the first crystal structure of functionally important PWWP domains of HUA2 family proteins in plants, His-HUA2-PWWP and His-HULK2-PWWP fusion proteins were further purified by removal of the His tag and followed by chromatography IMAC.

Highly pure proteins were set up for crystal screening. Crystals for HUA2 were got in two conditions but none of crystals for HULK2 (Figure 5A).

The crystals were diffracted to 1.85 Å by X-ray, and showed a primitive tetragonal space group with unit cell dimensions a=b=39.81 Å, c=116.63 Å. The structure was solved by molecular replacement with the structure of the PWWP domain of human PSIP1 (PDB ID: 4FU6) used as a search model. The HUA2 PWWP-domain contains a β-barrel-like fold formed by four anti-parallel β-strands (β1-β4), in which a short alpha helix is inserted between β2 and β3. It is slight different with reported human PWWP domains structure, which contains a complete β-barrel that has five anti-parallel βstrands (β1-β5) and a short alpha helix is inserted between β4 and β5 [START_REF] Qin | Structure and function of the nucleosome-binding PWWP domain[END_REF]. Nevertheless, the β-sheet core in HUA2 PWWP-domain is conserved and superimposed well with the structure of the human PSIP1 PWWP-domain (RSMD=1.5003 over 57 residues, 1-57 amino acid in HUA2 and 4-61 amino acid in PSIP1). A unique structural feature of the PWWP domain is the presence of a helix bundle of one to six a-helices following the b-barrel, which is also shared in HUA2

PWWP-domain structure (Figure 5B). However, whereas in PSIP1 the two C-terminal helices fold over the core domain, in HUA2 PWWP they maintain an extended structure.

This extended helix makes contact with a symmetry related molecule and the protein appears to form a dimer (Figure 5C).

From structural perspective, we found that the P16 in the P-W-W-P motif located on a flexible loop outside the β-barrel-like fold, while W18 and P19 resides in the second β-sheet of the barrel fold (Figure 5D), which could be a reason for these residues have different effect on PWWP domain binding activity.

To provide mechanistic insight about HUA2 PWWP domain binding with H3K36me1/2/3, we aimed at to get HUA2 PWWP bound to methylated H3K36 peptides complex structure. To this end, we screened the co-crystallization of HUA2 PWWP with H3K36me2 and H3K36me3, respectively. Crystals of HUA2 with H3K36me2, H3K36me3 have been screened and got crystals in several conditions (Figure 5E).

A B

Genetic interaction between SDG8 and HUA2

We have identified the HUA2 PWWP-domain has the ability to bind methylated H3K36.

Because SDG8 is the major H3K36 methyltransferase in Arabidopsis [START_REF] Zhao | Prevention of early flowering by expression of FLOWERING LOCUS C requires methylation of histone H3 K36[END_REF]Xu et al., 2008) and because its corresponding mutant is early flowering like the hua2 mutant, thus the genetic interaction analysis between sdg8-1 and hua2-7 was carried out.

Production of sdg8-1 hua2-7 mutants

T-DNA insertion lines sdg8-1 (Salk_065480) and hua2-7 (reference number:

314_A08.b.1a.Lb3Fa) have been characterized previously [START_REF] Zhao | Prevention of early flowering by expression of FLOWERING LOCUS C requires methylation of histone H3 K36[END_REF]Wang et al., 2007). Seeds (F1) from crosses were collected from individual siliques on the parent plants and were then grown and self-pollinated to obtain the F2 generation (Figure 6A). These F2 plants were genotyped by PCR. A total of 192 F2 plants were genotyped to identify double sdg8-1 hua2-7 homozygous mutants (Figure 6B).

Phenotypes of the sdg8-1 hua2-7 mutants

Under our growth conditions, hua2-7 mutant bolted earlier that wild type and did not show other obvious phenotypes, as previously reported [START_REF] Wang | HUA2 caused natural variation in shoot morphology of A. thaliana[END_REF]; sdg8-1 mutant had pleiotropic phenotypes including early flowering, reduced organ size, and increased shoot branching. Interestingly, sdg8-1 hua2-7 double mutant showed a reduced size as compared to single mutant, flowering time was early as compared to wild-type.

Flowering time analysis of sdg8-1 hua2-7 mutants

To explore the functional interaction between SDG8 and HUA2 in the flowering time regulation, flowering time was measurement by total number of rosette leaves at the time of bolting and days to bolting under long-day (LD: 16 h light: 8 h dark), mediumday (MD; 12 h light: 12 h dark) and short-day (SD; 8 h light: 16 h dark) photoperiod conditions (Figure 8). The sdg8-1 hua2-7 double mutants showed an early flowering time compared to wild-type and was in between sdg8-1 and hua2-7.

A B 12 h dark) or short-day (SD; 16 h light : 8 h dark) photoperiod conditions.

Expression profiles of flowering time genes

To investigate the molecular mechanism underlying the flowering behavior of sdg8-1 hua2-7 mutant plants, the expression level of major flowering repressor genes FLC, MAF1-5 and flowering activator genes FT and SOC1 was compared in the sdg8-1 hua2-7 double mutants and sdg8-1 and hua2-7 single ones by quantitative RT-PCR analysis. Interestingly, the FLC mRNA level was similar between both single and double mutants indicating a redundancy between SDG8 and HUA2. More downstream along the flowering genetic pathway, the FT transcript level in the double mutant was in between the one observed in sdg8-1 and hua2-7 single mutants indicating that at this level SDG8 and HUA2 regulate FT independently. Together, our work indicates that, at least for FLC, SDG8 and HUA2 may functionally interact to regulate gene transcription.

Discussion and conclusions

H3K36 di-and tri-methylation (H3K36me2/3) play critical roles in epigenetic activation of gene expression in both animals and plants. Although Arabidopsis MRG has been reported as H3K36me2/3 reader protein, however, it binds to H3K4me2/3 as well [START_REF] Bu | Regulation of Arabidopsis flowering by the histone mark readers MRG1/2 via interaction with CONSTANS to modulate FT expression[END_REF][START_REF] Xu | Arabidopsis MRG domain proteins bridge two histone modifications to elevate expression of flowering genes[END_REF]. It is thus of great interest to identify H3K36 methylation specific readers to elucidate the mechanism of roles of H3K36me2/3 in Arabidopsis.

Functional and structural analysis of the HUA2/HULK2 PWWP domain

We performed in vitro binding assay and found that HUA2/HULK2 PWWP domain specifically binds to H3K36me1/2/3 but not H3K4me1/2/3 or H3K27me3. This result demonstrates that HUA2 and HULK2 as novel H3K36 methylation specific binding proteins via their PWWP domain. However, we have failed to verify the specificity of the binding by isothermal titration calorimetry (ITC) so far, suggesting that the interaction between PWWP domain and H3K36me1/2/3 peptide is likely weak. Indeed, many histone reader proteins show very weak affinity to histone peptide in ITC and sometimes even make it undetectable by ITC [START_REF] Bu | Regulation of Arabidopsis flowering by the histone mark readers MRG1/2 via interaction with CONSTANS to modulate FT expression[END_REF].

To further explore this binding, we determined the crystal structure of the HUA2 PWWP domain by protein crystallography method. It contains a classical β-barrel-like fold formed by four anti-parallel β-strands (β1-β4) that also present in animal and yeast PWWP domain. Thus, this domain appeared highly conserved among plant, animal and yeast, albeit their amino acid sequence have less similarities. However, animals and yeast PWWP domain was shown to be able to bind diverse methylated histone peptides. For example, human BRPF1-PWWP binds H3K36me3, HDGF2-PWWP binds both H4K20me3 and H3K79me3, and yeast Pdp1-PWWP could specifically recognize H4K20me [START_REF] Qin | Structure and function of the nucleosome-binding PWWP domain[END_REF]. Interestingly, we found that HUA2-PWWP structure superimposed best with the structure of human PISP-PWWP, which also bounds with H3K36me3. Structural analysis of the PWWP bounds with histone peptide complex structures identified a conserved hydrophobic pocket for methyl-lysine binding, which is composed of mainly aromatic residues. Consistently, in this work, we found that the interaction between HUA2-PWWP domain and H3K36me1/me2/me3 is fully abolished by mutation of the third W (W18) and the fourth P (P19), but not the first P (P16) in the P-W-W-P motif (PAWP in HUA2 and other HULK2 family members). This bias can be very nicely explained from the structural point of view, the P16 located on a flexible loop outside the β-barrel-like fold, indicating it is not functionally important; on the contrary, W18 and P19 resides in the second β-sheet of the barrel fold, with high possibility to constitute the aromatic cage that makes it indispensable to fulfill its binding ability. Indeed, several human proteins such as RBBP1 and NSD1, the PWWP domain contains an incomplete aromatic cage that does not show any binding to methylated histone peptides.

Because the dimension of these cages decides their selectivity toward mono-, di-or tri-methylated lysines, and given the fact that the HUA2-PWWP present as a dimer, it is very unlikely that the HUA2-PWWP domain uses a unique or flexible mechanism to accommodate the different size of H3K36me1/2/3. It will be very important to solve the HUA2-PWWP domain bounds with H3K36 peptide complex structure that we are currently working on and reasonable to expect that will offer evidence to support these ideas.

Furthermore, we aimed to test if the P19 residue is important for in vivo HUA2 function, we constructed P19A substitution in the HUA2 genomic DNA fragment introduced them into the hua2 mutant. We will test if the transgenetic line be able to rescue the hua2 mutant early flowering phenotype.

SDG8 and HUA2 function related in regulation of flowering time

HUA2 has been reported to regulate flowering time (Jali et al., 2014), however, little is known about the underlying mechanisms. The hua2 mutant is early flowering like sdg8 mutant, which is defective in H3K36 di-and tri-methylation [START_REF] Zhao | Prevention of early flowering by expression of FLOWERING LOCUS C requires methylation of histone H3 K36[END_REF].

By flowering time measurement, we found that sdg8-1 hua2-7 double mutant flowered in between sdg8-1 and hua2-7 single mutant, suggesting SDG8 and HUA2 are genetically related in flowering time regulation. Consistent with the flowering phenotype, our molecular data showed that FT transcription level is intermediary between that in sdg8-1 and hua2-7. Interestingly, FLC mRNA level was decreased to the similar level in both single and double mutants, indicating SDG8 and HUA2 act in a same pathway to regulate FLC transcription. In this context, we would like to investigate if the disruption of the PWWP domain abolishes or reduces H3K36me3 levels at FLC chromatin and further lead to its depression. Therefore, transgenic lines expressing tagged HUA2 protein has been generating and will be used for ChIP-Seq analysis to test histone methylation level changes in vivo.

All together, this work is important for mechanistic understanding of SDG8 catalyzed H3K36 methylation function in flowering time control.

GENERAL CONCLUSIONS

AND PERSPECTIVE

In this work, we present results from diverse aspects of studying Arabidopsis TrxG family proteins.

Firstly, in collaboration with the laboratory of Dr. Richard Amasino, a novel role for TrxG proteins was identified. By providing genetic and molecular data, we demonstrated that SDG7, a close homolog of the H3K36 methyltransferase SDG8, plays crucial role in repression of vernalization process. And this novel function seemed to be independent its methyltransferase activity, as evidenced by our western blot analysis, in which we were not able to detect any significant changes in the level of different histone methylation marks in sdg7 mutant plants, and recombinant SDG7 protein could not methylate histones. Thus, the biochemical role of SDG7 in the timing of the vernalization response still needs to be investigated. A possibility is SDG7 fulfills its role in vernalization by methylating a cytosolic protein or proteins, supported by the fact that SDG7 lacks a nuclear localization signal and was demonstrated to methylate aquaporins in vitro (Sahr et al., 2010). To verify this, other potential substrates of SDG7 should be further explored. Furthermore, the Arabidopsis genome encoding around 12 TrxG members, and some of them already have been shown

to play distinct biological roles. Thus, molecular and functional characterization of other TrxG genes are desperately necessary to broad our knowledge on their diverse functions in regulation of plant growth and development.

Secondly, by the phenotype and molecular analysis of the combined Arabidopsis mutants defective in histone H3K36me3 methyltransferase SDG8 and H2B monoubiquitinase HUB2, we found that an ordered dependency between H2Bub1 and H3K36me3 depositions cannot be clearly established; rather SDG8 and HUB2 mutually favor each other in establishing active chromatin state. This contrasts the yeast model where H3K36me3 deposition occurs at downstream and in a dependent manner of H2Bub1. Because H3K36 methylation and H2B deubiquitination are also functionally connected in the course of transcription in yeast, it is thus of great interest to test the conservation of this crosstalk in Arabidopsis. Based on previously reported largely independent role between histone methyltransferases and demethylases and the results present here, it is reasonable to conclude that connections between histone marks in Arabidopsis are numerous, varied and extremely sophisticated, rendering difficult, if not impossible, to establish a simple functional network with clear hierarchical interconnections.

Such complexity might reflect a certain degree of flexibility, which is in line with the modular concept of plant developmental plasticity.

Finally, we found Arabidopsis HUA2/HULK2 proteins act as histone methylation reader proteins via their PWWP domain. Our in vitro binding data revealed that the PWWP domain of HUA2 preferably interacts with H3K36 methylation (H3K36me1/me2/m3). In Arabidopsis, there are at least 16 PWWP-domain containing proteins, the binding ability and/or specificity of other PWWP domains towards modified histone peptides still needs to be investigated. Because the PWWP domain in human and yeast showed binding activity to differently methylated histones despite of their overall structure of this domain is conserved across many spices, we speculate the role of Arabidopsis PWWP domain played both

Plant Materials

Arabidopsis thaliana is a flowering plant used as a model species for basic research in genetic and molecular biology. The Columbia (Col-0) ecotype was used as genetic background for wild-type and all the mutants listed below are in the Col-0 background.

Arabidopsis thaliana mutants carrying a T-DNA insertion in this study are listed in Table 1-1. Seeds were ordered from the European Arabidopsis Stock Center (NASC). Plasmids amplification E. coli strain TOP10 Transformation BL21 (DE3) Protein expression

Constructs and Vectors

All vectors were created using the GATEWAY® cloning system (Invitrogen™) or by using restriction endonucleases and ligation.

Bent , 1998). All the transgenic Arabidopsis thaliana lines generated are listed in Table 1234.

FLC-F At5g10140 FLC CCTAATTTGATCCTCAGGTTTGGG FLC-R At5g10140 FLC CCGACGAAGAAAAAGTAGATAGGCAC MAF1-F At1g77080 MAF1 GGAAAGAATACGTTGCTGGCAACA MAF1-R At1g77080 MAF1 CCGTTGATGATGGTGGCTAATTGA MAF2-F At5g65050 MAF2 GGCTCCGGAAAACTCTACAA MAF2-R At5g65050 MAF2 TTCTGCAAGATCTAAGGCTTCA MAF3-F At5g65060 MAF3 ACAGAACTAATGATGGAGGATATGAA MAF3-R At5g65060 MAF3 CTTCTTCCCCACCTGGCTA MAF4-F At5g65070 MAF4 GAGCAATGTCACCGGAAAGTAG MAF4-R At5g65070 MAF4 CAGTCGTTGGTGATGGTGGTTA MAF5-F At5g65080 MAF5 AAGAGCAGTAATGTCACCGGAA MAF5-R At5g65080 MAF5 ACTTGAGAAGCGGGAGAGTC SOC1-F At2g45660 SOC1 GCTCTCAGTGCTTTGTGATGC SOC1-R At2g45660 SOC1 A AGAACGTACTTGGAGCTGGC FT-F At1g65480 FT GAGACCCTCTTATAGTAAGCAGAGTTG FT-R At1g65480 FT GGGAGTTCAAGTGAAAGAACCAAAGT Tip 4.1-F At2g25810 Tip 4.1 GTGAAAACTGTTGGAGAGAAGCAA Tip4.1-R At2g25810 Tip 4.1 TCAACTGGATACCCTTTCGCA EXP-F EXP GAGCTGAAGTGGCTTCCATGA EXP-R EXP GGATCATGGGTATGTCGGACC GADPH-F At1g13440 GADPH TTGGTGACAACAGGTCAAGCA GADPH-R At1g13440
FLC1-F At5g10140 FLC ATTTAGCAACGAAAGTGAAAACTAAGG FLC1-R At5g10140 FLC GCCACGTGTACCGCATGAC FLC2-F At5g10140 FLC AGAAATCAAGCGAATTGAGAACAA FLC2-R At5g10140 FLC CGTTGCGACGTTTGGAGAA FLC3-F At5g10140 FLC AATTGCATGTCATTCACGATTTG FLC3-R At5g10140 FLC TGAAACTTCACTCAACAACATCGA FLC4-F At5g10140 FLC TTATCGCCCTTAATCTTATCATCGT FLC4-R At5g10140 FLC AAGATTGAGGTTGTGGATTGTCAA FLC5-F At5g10140 FLC CCGCCACATCATCATTATCATC FLC5-R At5g10140 FLC ACAAGGTTTTTTCCAGCGATAGA FLC6-F At5g10140 FLC AAGCCAGCGCTATCACTAAACTTT FLC6-R At5g10140 FLC TCGGCAGATTGAAAATGACATT FLC7-F At5g10140 FLC CATCTCTCCAGCCTGGTCAAG FLC7-R At5g10140 FLC GGCTTTAAGATCATCAGCATGCT FLC8-F At5g10140 FLC AGCCAGGTAACGAAAGCTACATTT FLC8-R At5g10140 FLC ACATGGACATTGGACACACAACA FLC9-F At5g10140 FLC CATCATGTGGGAGCAGAAGCT FLC9-R At5g10140 FLC CGGAAGATTGTCGGAGATTTG FLC10-F At5g10140 FLC TTTTTGGGCCTATGTCGGTCA FLC10-R At5g10140 FLC GGTCGGTCACGTTAACAGCA MAF4-F1 At5g65070 MAF4 CTTCCTCGCTTCAAAACGGC MAF4-R1 At5g65070 MAF4 TGATCCTAACCACCGGGGAC MAF4-F2 At5g65070 MAF4 TCCACATTGATGCATCCCCA MAF4-R2 At5g65070 MAF4 ATGGTACACACCGAAAAGGGT MAF4-F3 At5g65070 MAF4 ATGTAGGCGCATGCTTCAGT MAF4-R3 At5g65070 MAF4 TCGGTTGGAGGAATTTATAGAGTGA MAF4-F4 At5g65070 MAF4 AGTCACTTCAAGAAAGGGTACG MAF4-R4 At5g65070 MAF4 TCAGCATTACCCTACAATATGGCT MAF4-F5 At5g65070 MAF4 AAAAGTACGGATTGGATTCAAAAGA MAF4-R5 At5g65070 MAF4 GGAAAACCAAAGGAATTAAGAAGCT MAF4-F6 At5g65070 MAF4 GACCAACGCGCCACAAG MAF4-R6 At5g65070 MAF4 CGGTGCGTTTTTAATAGGAGTTTAG MAF5-F1 At5g65080 MAF5 TGGTGTCCAAAGTCCAGAGATG MAF5-R1 At5g65080 MAF5 CCTTGCAACAAATGAAAACAATAGA MAF5-F2 At5g65080 MAF5 CCGAATTGAGACCTTGTAGAAGTAGA MAF5-R2 At5g65080 MAF5 ATCAAGCATTTGTGGTGTTAAGTATGA MAF5-F3 At5g65080 MAF5 GTTGTTTTCCTTTTCTGTTGTTTATCTATG MAF5-R3 At5g65080 MAF5 ATACTTACATTATCGCTTTTCGCTTCT MAF5-F4 At5g65080 MAF5 GCGGAAAGCCGGTAAAAGAC MAF5-R4 At5g65080 MAF5 CGAATCTGGGCTTAACAGTAACAGT SOC1-1-F At2g45660 SOC1 GGGCATATAGGTACGAGAGAGTG SOC1-1-R At2g45660 SOC1 CTAGTTGGATGGAAATGCCTGT SOC1-2-F At2g45660 SOC1 AGCAGAGAGAGAAGAGACG SOC1-2-R At2g45660 SOC1 GAAGTAGCTTTCCTCGTTTCAT SOC1-3-F At2g45660 SOC1 CTTTCTTTCTTCTTCTCCCTCCAGT SOC1-3-R At2g45660 SOC1 CCTAACCAGGAGGAAGCTTTCG SOC1-4-F At2g45660 SOC1 GCATCCTTCAATTAAACCGATAAC SOC1-4-R At2g45660 SOC1 AAGTCAACGAAAGATTAAGTACCC SOC1-5-F At2g45660 SOC1 TCTGTGTGCAAGGGAAATTAAC SOC1-5-R At2g45660 SOC1 GAGAAAGTCACTTGTCTGCT SOC1-6-F At2g45660 SOC1 TGGATTTGATTGGCCTTTTGTGGAA SOC1-6-R At2g45660 SOC1 AGCCCTAATTTTGCAGAAACCAAG SOC1-7-F At2g45660 SOC1 CCCTCATGCTTCATTTCATGCTC SOC1-7-R At2g45660 SOC1 GGTTTGGCTAGCTAGCTTTGAGT SOC1-8-F At2g45660 SOC1 ATCTCAAGGTCACAGGATCAAGTC SOC1-8-R At2g45660 SOC1 TGCTGCTTCATATTTCAAATGCTGT SOC1-9-F At2g45660 SOC1 GCAGCTCAAGCAAAAGGTAAAGTAG SOC1-9-R At2g45660 SOC1 GCACAAGAGGCTTACTTACTTGGAA SOC1-10F At2g45660 SOC1 CATGAAAGCGAAGTTTGGTC SOC1-10R At2g45660 SOC1 GTAACCCAATGAACAATTGCG FT-1-F At1g65480 FT GCATGCGAAAATCTAGTGGAAGA FT-1-R At1g65480 FT GTCGCATAATGTTCGCAACCT FT-2-F At1g65480 FT GCGACTGCGACCTATTTTTTTC FT-2-R At1g65480 FT GCTATATGCACTTTTTAACGACTAGC FT-3-F At1g65480 FT GCTAGTCGTTAAAAAGTGCATATAG FT-3-R At1g65480 FT CCACTGTTCTACACGTCCATAG FT-4-F At1g65480 FT TCCTTTATTTTCCAGTTTGGACAG FT-4-R At1g65480 FT ACCATAGCCTAACAACTGTAGGAA FT-5-F At1g65480 FT CCTACAGTTGTTAGGCTATGGT FT-5-R At1g65480 FT CTAACCATCCATTTGCACGAC FT-6-F At1g65480 FT TCTAACCTGAAGGATCCCTTG FT-6-R At1g65480 FT AATTCGAAAGCGAAAACGTTC FT-7-F At1g65480 FT GAACGTTTTCGCTTTCGAATT FT-7-R At1g65480 FT GAAAAAAGTAGGGTACCGCC FT-8-F At1g65480 FT AGTGTGGTGGGTTTGGAATAC FT-8-R At1g65480 FT GCATTAACTCGGGTCGGTGA FT-9-F At1g65480 FT AGAGGGTTCATGCCTATGATAC FT-9-R At1g65480 FT CTTTGATCTTGAACAAACAGGTG FT-10-F At1g65480 FT GAGACCCTCTTATAGTAAGCAGA FT-10-R At1g65480 FT GTATAGAAGTTCCTGAGGTCTTC FT-11-F At1g65480 FT GACAATGTGTGATGTACGTAGAATCAGT FT-11-R At1g65480 FT CGGTGAAATCATAACCACAATCTT FT-12-F At1g65480 FT GCCAGCCTTTAAGATACTCTCTGCTA FT-12-R At1g65480 FT TGAGATAACACAAGAAAGAAGAAGAAA ACT
Sequences of primers used for GATEWAY® cloning system (Invitrogen™) or restriction endonucleases and ligation system are listed in Table 1-

Plant Methods

Seeds sterilization

Seeds were sterilized by using chlorine gas. Few hundreds of Arabidopsis seeds were disposed in an opened 1.5 ml eppendorf tube in a desiccator in a fume hood. A beaker containing 100ml of bleach (FLOREAL Haagen) was placed in the desiccator next to the seeds. Then, 5 ml of 37% HCl was added to the bleach and the lid of the desiccator was immediately closed tightly to keep the seeds dry.

Seeds of Arabidopsis thaliana were either surface sterilized by treatment with 70% ethanol for 10 min followed by 5 min with 96% ethanol, and then dried in a clean bench.

Plant growth conditions

Arabidopsis thaliana plants were grown on soil under long day (LD; in 16 h light: 8 h dark), medium day (MD; 12 h light: 12 h dark) or short day (SD; 8 h light: 16 h dark) photoperiod conditions in greenhouse. Seeds were either sowed on 1/2 MS medium; after stratification for 48 hours at 4°C to synchronize the germination time, the seeds were transferred into a 21°C, 16/8 h light-dark cycle growth chamber.

Crossing Arabidopsis plants

In inflorescences of a recipient" / "female" plant (4 to 6 weeks old), mature opened flowers as well as buds that have already a white tip or buds that are too small were removed; the flower buds have the right size and contain short immature stamens with anthers should remain. The anthers in the selected buds were then removed using fine forceps. Two days after emasculation, stigmas were manually pollinated with pollens from a "donor" / "male" plant. Following a successful pollination, the pistils will elongate as the seeds in siliques developed. After dried, seeds (F1) in separate silique were harvested.

Arabidopsis transformation by floral dip method

Arabidopsis plants were grown under long day (16 h light/ 8 h dark) conditions until flowering. The first bolts were clipped to induce proliferation of more secondary bolts.

Plants generated a lot of immature flower clusters and less fertilized siliques after about 1 week of clipping, and will be ready for Agrobacterium tumefaciens mediated transformation.

Agrobacterium tumefaciens strain carrying gene of interest on a binary vector will be prepared. A single colony was inoculated in 3 ml LB with antibiotics and incubated at 28 °C overnight. The overnight culture was then diluted in 300 ml LB supplements with the same antibiotics followed by incubated at 28 °C for 16-24 hours. The bacteria cells were harvested by centrifugation at 5000 rpm for 10 min, and resuspended to OD600 value close to 0.8 in 5% sucrose solution. Silwet L-77 was added to the solution to a final concentration of 0.05% (500 L /L) and mixed well. Above-ground parts of plant were dipped into 100-200 ml Agrobacterium solution for less than1 min with gentle agitation. Dipped plants were placed either under a dome or cover to maintain high humidity for 16 to 24 hours. Plants were grown in greenhouse under normal growth conditions until seeds were harvested.

Genotyping

Seeds (F1) from crosses were grown and self-pollinated to obtain the F2 generation. The DNA extraction buffer: 50 mM Tris-HCl pH 7.2; 300 mM NaCl; 300 mM sucrose.

Arabidopsis total RNA isolation

100-200 mg of 16-day-old young seedlings grown at 22 °C under long day (16 h light/ 8 h dark) photoperiod conditions were harvested into an eppendorf tube containing glass beads (diameter 1 mm) and frozen in liquid nitrogen. Samples were grinded using the Silama S5 apparatus (Ivoclar, Vivadent). Total RNA was extracted by using the Nucleospin RNA kit (Macherey-Nagel) according to the manufacturer's instructions. RNA concentration was evaluated using nanodrop 2000 by measuring the absorbance at 260 nm (1 unit of OD260 = 40μg /ml of RNA). The threshold cycle value (CT) was set so that the fluorescent signal was above the baseline noise but as low as possible in the exponential amplification phase.

Microarray analysis

The microarray analysis was performed in Shanghai Biotechnology Corporation (China).

Total RNA was isolated from a pool of about 100 individual 6-day-old wild-type and sdg8- Raw data were normalized by MAS 5.0 algorithm, Affy packages in R. Gene expression analysis was performed using two independent experiments obtained each with three independently derived sets of RNA for each plant genotype. Genes with at least a 1.5-fold expression change and the P-values inferior to 0.05 in the three independent biological replicates were considered as significantly perturbed in the mutant compared to the wildtype control.

Nuclear protein extraction

3-week-old of Arabidopsis seedlings grown under medium day (MD; 16 h light and 8 h dark)

were harvested in liquid nitrogen to a quantity of 3-5 grams. Then nuclear protein extraction steps are as listed below.

1. Grind the tissues to a fine powder in liquid nitrogen (3-5 ml materials) using cold mortar and pestle. The powder was then put into a 50 mL Falcon tube.

2. Add 30 mL cold Lysis buffer into the powder, vortex and place on a rotation wheel for 30 min at 4 °C.

Chromatin ImmunoPrecipitation (ChIP)

Day 1: Arabidopsis 16 days old seedlings were harvested in 100 ml fixation buffer followed cross-linking by vacuum infiltration for 10 minutes. Cross-linking was quenched by adding 5 ml of 2.5 M glycine and continued to infiltrate in vacuum for 5 minutes. Seedlings were rinsed with water 5 times and dried as much as possible. The dried seedlings were subsequently grinded in liquid nitrogen to a fine powder and kept at -80 °C.

Bacterial techniques

Preparation of competent cells for heat shock using CaCl2

A single bacterial colony was inoculated in 5 ml LB medium and incubated with shaking at

Gateway cloning

The LR reaction: Entry clone (≈150 ng), destination vector (pGWB604 and pGWB616 ≈150 ng)

and TE buffer was added to a final volume of 8 μL in 1.5 ml tubes at 25 °C. After thawed in ice for 2 minutes, 2 μL of the LR clonase TM II enzyme was added to the reaction and mixed well by vortex briefly. The reaction was incubated at 25 °C for 2 hours or overnight followed by adding 1 μL of proteinase K to stop the reaction by incubating at 37 °C for 10 minutes.

Extraction of plasmid DNA

1.5 ml bacterial culture was incubated at 37 °C overnight and cells were harvested by centrifugation 30 seconds at 11000 rpm. Plasmid DNA was extracted using the NucleoSpin® Plasmid QuickPure kit (MACHEREY -NAGEL, Germany) according to manufacturers' instructions.

Protein expression and purification in E. coli

Constructs of pGEX4T1-HULK2-C terminal, pET28a-SUMO-SDG8-N terminal, pET28a-SUMO-HULK2-PWWP, pET28a -SUMO-HUA2-PWWP, pET28a-SUMO-HUA2-PWWP (P16A), pET28a-SUMO-HUA2-PWWP (W18A) and pET28a-SUMO-HUA2-PWWP (P19A)

were transformed into BL21 (DE3) competent cells. The transformed cells were grown at 37 °C overnight in 5 ml of liquid LB medium supplemented with antibiotics (100 mg/ ml ampicilin for pGEX4T1 fusion construct and 50 mg/ ml kanamycin for pET28a fusion construct). 1 ml of the bacterial cell suspension from each culture was used inoculated for 100 ml of LB medium supplemented with antibiotics. The bacteria cultures were grown at 37 °C with shaken until the value of OD600 reaches 0.6, then 1 mM IPTG was added, and the cultures were vigorously shaken for an additional 5 hours at room temperature. The cultures were harvested by centrifugation at 5000 × g for 20 min at 4 °C. The supernatants were decanted, and the pellets were frozen and stored at -80 °C, until further analysis.

Purification of 6x His-tagged proteins for crystallization

His-tagged proteins expressed cells were resuspended in

GST fusion protein purification

GST-tagged proteins expressed cells were resuspended in 20 ml of ice-cold GST lysis buffer with 100 μg/ ml lysozyme and protease inhibitor cocktail (Roche). After disrupting the cells by sonication, the cell lysate was centrifuged at 13200 rpm for 20 min at 4 °C.

Then the supernatant was mixed with settled glutathion-Sepharose-4B beads (Amersham Bioscences), which was pre-washed 3 times with water and 3 times with 1X PBS buffer.

The suspension was incubated on a rotation wheel for 2 hours or overnight at 4 °C. Beads were spun down and washed once in 1X PBS with 0.1% Triton X-100 buffer followed by 3 times in 1X PBS with 10% glycerol buffer. Finally, 1X PBS with10% glycerol buffer was added to make 10% slurry. GST-tagged proteins were fixed to the beads and determined by SDS-PAGE for further use.

GST lysis buffer: 1X PBS buffer; 0.1% Triton X-100; In eukaryotes, nuclear DNA containing the genetic information assembles with histone proteins to form chromatin. On the one hand, the chromatin compaction protects DNA from damage, but on the other hand this structure also impedes the access of enzymes involved in gene transcription. Therefore, several mechanisms have evolved to dynamically modulate the chromatin structure in order to properly regulate gene expression, thus providing an extra layer of control. Among other processes, chromatin remodeling is exercised through a plethora of covalent post-translational modifications of histone proteins, including acetylation, methylation, phosphorylation, ubiquitinylation, and others. Compare to acetylation that modifies the charge and thus the interaction between histone and DNA, histone methylation does not per se directly trigger changes in the chromatin structure but it requires specific "readers" that will recognize the methylation signal and interpret it. In Arabidopsis, little is currently known about histone methylation readers, and especially regarding the domains involved in this binding as well as their specificity. In the first part of my thesis, I will describe my results on a recently identified group of histone methylation readers with a particular attention to the binding affinity of their PWWP domain. Also, histone methylation does not act alone but in combination with other histone marks to define different chromatin states throughout the entire genome. This is particularly true during transcription. Indeed, the actual model of active transcription is built around a precise chronology of histone modifications. In a simplified way, histone H2B monoubiquitination (H2Bub1) first occurs, allowing histone H3 lysine 4 trimethylation (H3K4me3) and transcription initiation. Latter, H2Bub is removed enabling H3K36me3 to occur at the onset of elongation. While in animal and yeast, the above described model is well documented, in plants, considerably less is known and studies were mainly concentrated on studying the functional relationship between H2Bub and H3K4me3. In order to fill this shortfall, I will explore, in the second part of my thesis, the functional interaction between H2Bub and H3K36me3 using a genetic approach based on crosses between different histone-modifying enzyme mutants. Among histone-modifying enzymes, the SET-domain group (SDG) protein superfamily includes proteins known to methylate histones on lysine residues. Within this large family, the evolutionarily conserved Trithorax group (TrxG) proteins are known to mediate methylation on H3K4 and/or H3K36 in order to promote transcription in response to diverse developmental and environmental cues. In Arabidopsis, the TrxG group was involved in the control of multiple fundamental processes and comprises around 12 members, with less than half being functionally characterized. The H3K4 methyltransferase SDG27/ATX1 and the H3K36 methyltransferase SDG8 are the most studied. In the last part of my thesis, I will describe a homologue of ATX1 and SDG8 named SDG7 and demonstrate its fundamental role in the proper timing of the vernalization process.

HUA2 and HULK2 function as H3K36 methylation readers

Previously in the team, HULK2 (HUA2-LIKE 2) was identified as a prey using a yeast two-hybrid system screen with SDG8 as the bait protein. HULK2 contain a conserved PWWP domain known to indifferently bind methylated/unmethylated histones in animal. In Arabidopsis, the PWWP domain proteins HUA2 (ENHANCER OF AG-4 2) and HULK2 have been reported to regulate flowering time, however, their PWWP domain was not explored (Jali et al., 2014). Firstly, despite that it was not possible to demonstrate the interaction observed in yeast using other methods, my work aims at testing the binding affinity of the PWWP domain to modified histones.

The His tagged HUA2-PWWP and HULK2-PWWP fusion proteins were purified and separately incubated with synthetic modified histone peptides H3K4me1, H3K4me2, H3K4me3 as well as H3K36me1, H3K36me2 and H3K36me3. Using western blots analysis, I demonstrated the binding of the His-HUA2-PWWP and His-HULK2-PWWP fusion proteins specifically to H3K36me1/me2/me3 peptides, and not H3K4me1/2/3 ones. To further explore this binding, we determined the crystal structure of the plant HUA2 PWWP domain. This domain contains aβ-barrel-like fold formed by four anti-parallelβ-strands (β1-β4) and appeared highly conserved among plant, animal and yeast. Because SDG8 is the major H3K36 methyltransferase in Arabidopsis [START_REF] Zhao | Prevention of early flowering by expression of FLOWERING LOCUS C requires methylation of histone H3 K36[END_REF] and because its corresponding mutant is early flowering like the hua2 mutant, I next generated the sdg8 hua2 double mutant and characterized its phenotype. While the sdg8 hua2 double mutant showed a reduced size as compared to single mutants, flowering time was in between sdg8 and hua2. To further explore this flowering phenotype, the transcript level of several flowering genes was analyzed using quantitative PCR. Interestingly, the FLC mRNA level was similar between both single and double mutants indicating a redundancy between SDG8 and HUA2. More downstream along the flowering genetic pathway, the FT transcript level in the double mutant was in between the one observed in sdg8 and hua2 single mutants indicating that at this level SDG8 and HUA2 regulate FT independently. Together my work

indicates that, at least for FLC, SDG8 and HUA2 may functionally interact to regulate gene transcription.

Crosstalk between histone methylation and monoubiquitination during transcription

In order to unravel how H3K36 methylation and H2Bub1 are functionally for FLC, the level of H3K36me3 in sdg8 hub2 was additively decreased from sdg8 and hub2, indicating that for this gene, H3K36 methylation might present both a dependent and independent loading from H2Bub1. Also, while H3K4me3 was decreased in hub2 and unchanged in sdg8, H3K36me3 and H2Bub1 were similarly decreased between sdg8, hub2 and sdg8 hub2 at all other genes except FT. Indeed, FT seems to be regulated slightly differently from the other analyzed flowering genes as levels of H3K4me3 and H3K36me3 were lower and H2Bub was almost absent.

Together, my ChIP data indicate that like in animal H3K4me3 is fully dependent on the loading of H2Bub1. However, and in contrast with animal and yeast models, there is not a fully exclusive correlation between H3K36me3 and H2Bub1 as it was not possible to clearly establish a hierarchical dependency. Therefore, SDG8 and HUB2 might mutually favor each other in establishing an active chromatin state.

SDG7, an atypical methyltransferase in Arabidopsis

Vernalization is a process by which exposure to prolonged cold temperatures accelerates of flowering in many plant species. In Arabidopsis, vernalization is

Résumé en anglais

Histone methylation is one of the keys epigenetic marks evolutionarily conserved in eukaryotes. My study focuses on the characterization of factors potentially involved in the deposition and reading of lysine (K) methylation to appreciate its role and underlying mechanisms in the regulation of transcription and plant development, using Arabidopsis thaliana as a model organism. In the first part of my thesis, I report on our study of SET DOMAIN GROUP7 (SDG7), a protein containing the evolutionarily conserved SET domain, which is generally recognized as a signature of Kmethyltransferases. We found that SDG7 plays an important role in the regulation of VIN3 induction associated with cold duration measure during vernalization treatment. Intriguingly, levels of several different histone methylations were found unchanged in the sdg7 mutant plants and the recombinant SDG7 protein failed to show a histone-methyltransferase activity in vitro. We thus conclude that SDG7 might methylate a yet unknown non-histone protein to regulate transcription and proper measurement of the duration of cold exposure in the vernalization process. In the second part, I studied interaction between SDG8 and HISTONE MONOUBIQUITINATION1 (HUB1) and HUB2. My results unravel that H3K36me3 and H2Bub1 are deposited largely independently in Arabidopsis, which is in contrast to the dependent crosstalk of these two different epigenetic marks previously reported in yeast. In the last part of my thesis, I report on the identification of the PWWP-domain proteins HUA2/HULK2 as readers of H3K36me3 and demonstrate that sdg8 and hua2 genetically interacts in the regulation of flowering time. : Epigenetics, Chromatin, Histone Modification, Transcription, Flowering, Arabidopsis.

Keys words

2. 5

 5 In vitro binding assay (using peptide) and dot western ......................................................... 139 REFERENCE ............................................................................................................................... 141 ABBREVIATIONS……………………………………………………………………………………….168

Figure 2 :

 2 Figure 2: Schematic representation of histone lysine residue is mono, di and tri-methylated.

Figure 3 :

 3 Figure 3: Histone lysine methylation distribution in animals and plants (Adapted from Xiao et al., 2016).

Figure 5 :

 5 Figure 5: Writers of histone methylation in Arabidopsis, TrxG and PcG proteins.

Figure 6 :

 6 Figure 6: Erasers of histone methylation in Arabidopsis, LDL and JMJ family proteins.

Figure 7 :

 7 Figure 7: Readers of histone methylation in Arabidopsis, PHD domain-, WD40 repeat-, MRG domain-and Chromodomain-containing proteins.

Figure 8 :

 8 Figure 8: Enzymatic cascade of ubiquitination (Adapted from Voutsadakis, 2010).

Figure 9 :

 9 Figure 9: Writers, erasers and readers of histone H2A and H2B monoubiquitination in Arabidopsis.

Figure 10 :

 10 Figure 10: A proposed model for deposition and removal of histone H2B monoubiquitination in transcriptional activation of FLC and MAFs in flowering time regulation (adapted from Feng and Shen, 2014).

Figure 11 :

 11 Figure 11: Phases of plant development. After fertilization, root apical meristem (RAM) and the shoot apical meristem (SAM) (red) is formed in the embryo. Upon germination, plants generally undergo two major developmental transitions, one is juvenile-to-adult phase transition (vegetative stage), followed by a vegetative to reproductive phase transition (reproductive flowering stage) and eventually seed set and senescence (Adapted from Liuet al., 2009; Huijser and Schmid, 2011;

Figure 12 :

 12 Figure 12: A simplified diagram of five independent flowering pathways in Arabidopsis: vernalization, photoperiod, autonomous, gibberellin, and aging (Adapted from Khan et al., 2013).

  of flowering time has been extensively studied. The Arabidopsis age pathway is mainly mediated by two key miRNAs, miR156 and miR172. MiR156 is highly expressed in the juvenile stage and decreases during the following adult stage, which negatively regulate several SQUAMOSA PROMOTER BINDING PROTEIN-LIKE (SPL) genes that promotes flowering even under a non-inductive photoperiod, mainly through activating the floral meristem identity gene LEAFY (LFY). MiR172 is an activator of flowering; it has an expression pattern opposing MiR156. MiR172 directly repress its target AP2-like genes, including AP2, SCHLAFMUTZE (SMZ), SCHNARCHZAPFEN (SNZ), TARGET OF EAT1 (TOE1), TOE2, and TOE3 that normally repress flowering by down-regulating SOC1. Therefore, miR156 and miR172 form a complementary relationship in the aging pathway to fine-tune the flowering time in Arabidopsis (Khan

  in flowering time regulation through repression of MAF4/5 (Shen et al., 2014), whether PRC1-mediated H2Aub1 plays a function in repression of MAF4/5 and/or FLC remains currently unclear.

Figure 13 :

 13 Figure 13: FLC chromatin state before (A), during (B) and after (C) cold in vernalization process (Adapted from Berry and Dean, 2015).

Figure

  Figure (A) Phylogenetic analysis of TrxG family proteins in animals and plants. It showed that Arabidopsis proteins SDG7 and SDG8 belong to the same clade. (B) Schematic diagram of the SDG7 and SDG8 proteins. Both proteins contain the conserved domains of AWS, SET and post-SET.

  Author contributions: J.L., J.-Y.Y., and R.M.A. designed research; J.L., W.Z., and W.-H.S. performed research; J.L. analyzed data; and J.L. and R.M.A. wrote the paper. Reviewers: G.C., Max Planck Institute for Plant Breeding Research; and B.T., Commonwealth Science and Industry Research Organization.The authors declare no conflict of interest.

1

 1

Fig. 1 .

 1 Fig. 1. Loss of SDG7 results in increased VIN3 expression and more rapid flowering. (A) VIN3 reporter gene expression in sdg7-1 using a viable GUS staining assay. The arrow indicates VIN3p::GUS expression in the root tip. (B) VIN3p::GUS expression in the shoot apex region using destructive GUS staining. (C) Elevated VIN3 expression in sdg7-1 without cold exposure (NV) and more rapid induction of VIN3 after 5 d of cold exposure (5V). (D) Flowering behavior, represented as the number of rosette leaves generated on the primary shoot before the transition to flowering, of sdg7 in the FRIGIDA (FRI) background (FRI sdg7) and the Col FRI reference line, which is FRI introgressed into Col (34), in long day (16 h light/8 h dark) without cold exposure (NV) and with 5 d cold exposure (5V). Expression data are presented as mean values of three biological replicates, and flowering time data are presented as mean values of 12 individual plants. Error bars indicate SD.

Fig. 2 .

 2 Fig. 2. Loss of SDG7 results in a more rapid vernalization response and altered timing of VIN3 and FLC expression. (A) Flowering phenotypes of FRI, FRI sdg7-1, and FRI sdg7-2 without (NV) or with 20 d (20V) or 40 d (40V) of cold exposure. (B) Flowering behavior, measured as number of primary leaves formed, as a function of length of cold exposure. 5V, 10V, 20V, 30V, and 40V indicate the number of days of cold exposure. (C) FLC repression in sdg7-1 and sdg7-2 as a function of duration of cold exposure compared with Col FRI. 40VT7 represents 40 d of cold treatment followed by 7 d of growth at 22 °C. (D) VIN3 expression as a function of duration of cold exposure in the sdg7 mutants and WT. Flowering time data are presented as mean values of 12 individual plants, and expression data are presented as mean values of three biological replicates. Error bars indicate SD.

Fig. 3 .

 3 Fig. 3. VIN3 dependence of the sdg7 phenotype. (A) VIN3-dependent flowering in sdg7 was evaluated by determining the primary shoot leaf number of a FRI sdg7 vin3 mutant without vernalization (NV) and after 40 d of vernalization (40V) in long days. As a control, epistasis analysis was performed with rapidflowering mutants in the following genes: EARLY FLOWERING7 (ELF7) and ELF8, components of RNA polymerase II-associated factor 1 complex (Paf1c), which catalyzes H3K4 methylation; PHOTOPERIOD INDEPENDENT EARLY1 (PIE1) and SUPPRESSOR OF FRIGIDA3 (SUF3)/ACTIN-RELATED PROTEIN6 (ARP6)/EARLY IN SHORT DAYS1 (ESD1), which are components of an AtSWR1 complex involved in H2A.Z replacement; and ABA HYPERSENSITIVE 1(ABH1)/CAP BINDING PROTEIN 80 (CBP80). (B) Flowering behavior was monitored in three individual lines of VIN3 overexpressors with and without cold treatment. (C) FLC expression in the VIN3 overexpressors. Flowering time data are presented as mean values of 12 individual, plants and expression data are presented as mean values of three biological replicates. Error bars indicate SD. Symbols for the duration of cold exposure are as in the legend to Fig. 2.

Fig. 4 .

 4 Fig. 4. Loss of SDG7 results in increased levels of lncRNAs and increased H3K27me3 density without cold exposure. (A) COLDAIR and COOLAIR expression without cold exposure and after various durations of cold exposure in sdg7 and WT. (B) Schematic representation of the FLC genomic region and probes used for ChIP analysis. (C) H3K27me3 density of the indicated regions of FLC chromatin in sdg7 and WT after various durations of cold exposure. Expression and ChIP data are presented as mean values of three biological replicates. Error bars indicate SD. Symbols for the duration of cold exposure are as in the legend to Fig. 2.

  Fig. 5. SDG7 does not affect global histone modifications and lacks detectable HMTase activity. (A) Analysis of global histone methylation in the FRI-containing sdg7 mutant (FRI sdg7-1). Col, Col FRI, and fri sdg8-1 served as controls. The sdg8-1 control exhibits reduced H3K36me3 and increased H3K36me1, whereas sdg7-1 did not exhibit any changes in histone methylation. (B) Histone methyltransferase assay of SDG7. (Upper) Assay for incorporation 14C-labeled S-adenosyl-L-methionine into histones. SDG714 and mock control were used as positive and negative controls, respectively. (Lower) Coomassie blue staining of the same gel. Arrows indicate the bands corresponding to SDG7 and SDG714 proteins. Gel migration positions are indicated for the four nucleosomal core histones H2A, H2B, H3, and H4. Histones were assayed as oligonucleosomes, mononucleosomes, and free histones. Only results obtained on oligonucleosomes are shown; however, none of these different forms of histones were substrates for SDG7.

Fig

  Fig. S2. VIN3 expression and flowering phenotype in VIN3 overexpressors. (A) VIN3 expression level in VIN3 overexpressors. (B) Flowering phenotype of FRI, FRI vin3, and three individual VIN3 overexpressors in a FRI-containing background and in a FRI vin3 background without (NV) or after 40 d (40V) of cold treatment.

Fig

  Fig. S3. Phylogenetic analysis and sequence alignments of SDG7 homologs. (A) Phylogenetic analysis was performed using class II-SDG7 homologs (SDG7, SDG24, SDG4, SDG26, and SDG8 in Arabidopsis; SDG110 and SDG102 in maize; SDG724, SDG736, SDG708, and SDG725 in rice). Orthology group 1-4 was also indicated. (B) Sequence alignments were represented between the orhology group 1 proteins. Blue, black, and red lines represent AWS, SET, and post-SET domains, respectively. The square box represents nuclear localization signals predicted by NucPred software (www.sbc.su.se/∼maccallr/nucpred/).

Fig

  Fig. S4. Schematic representation of the role of SDG7 in the vernalization process. In WT, SDG7 is a negative regulator of VIN3 and two lncRNAs. In the sdg7 mutant, VIN3 and the lncRNAs exhibit increased expression without cold, and FLC is more rapidly suppressed.

  Correct timing to flowering is crucial for the reproductive success of plants and thus is fine-tuned by multiple genetic and molecular pathways. Histone methylation is involved in flowering time control by regulating the expression of several flowering time genes. Among them, FLOWERING LOCUS C (FLC) encodes a MADS box transcription factor that blocks the floral transition by directly repressing the transcription of the flowering time integrators FLOWERING LOCUS T (FT) and SUPPRESSOR OF OVEREXPRESSION OF CO 1 (SOC1). It is now well-known that histone methylation does not occur alone but in combination with other histone marks to define different chromatin states throughout the entire genome. This is particularly true during transcription.

Figure 1 :

 1 Figure 1: Identification of the double sdg8-1 hub2-2 homozygous mutant. (A) Scheme of sdg8-1 hub2-2 double mutant identification. (B) Genotype verification of wild-type (WT) and the sdg8-1, hub2-2, and sdg8-1 hub2-2 mutants by PCR amplification using the specific gene/T-DNA primers as reported. PCR products separated by electrophoresis on agarose gels are shown.

Figure 2 :

 2 Figure 2: Representative 23-day-old seedlings of Col, sdg8-1, hub2-2, and sdg8-1 hub2-2 grown under mid-day (MD; 12 h light and 12 h dark) photoperiod conditions. Scale bars represent 1 cm (upper) and 0.5 cm in (lower).

Figure 3 :

 3 Figure 3: Western blot analysis of global levels of histone modifications in wild-type (WT) and the sdg8-1, hub2-2, and sdg8-1 hub2-2 mutant plants. Nuclear proteins were extracted from 3-weekold plants grown under mid-day (MD; 12 h light and 12 h dark) photoperiod conditions, and were analyzed by using specific antibodies that recognize the indicated form of histones. Numbers indicate "mean (±standard error)" of densitometry values measured from at least 3 independent experiments, normalized to H3, and shown as relative to WT (set as 1).

  H3K36me3 are unchanged as compared to those detected in wild-type plants. The sdg8-1 hub2-2 double mutants show a reduced level of H3K36me3 and an increased level of H3K36me1 like the single sdg8-1 mutant, and a reduced level of H2Bub1 like the single hub2-2 mutant. Like in single mutants, levels of H3K4me2 and H3K4me3 are unchanged in our double mutant. Interestingly, the repressive mark H3K27me3 level is only significantly increased in the sdg8-1 hub2-2 double mutant, suggesting a combined antagonistic relationship between both H3K36me3 and H2Bub1 active marks and the repressive H3K27me3 one.

Figure 4 :

 4 Figure 4: Representative 7-day-old (A), 18-day-old (B) and 7-week-old (C) of wild-type and mutant seedlings grown under long-day (LD; 16h light/8 h dark) photoperiod conditions, are shown. (D) Mean values ± SD of plant fresh weight are measured from over 20 individual 5-week-old plants, grown under long-day (LD; 16 h light and 8 h dark) photoperiod conditions, and are shown as relative to WT control (set as 1). SD means standard deviation. Scale bars represent 2 mm in (A) and (B), 3 cm in (C).

Figure 5 :

 5 Figure 5: Mean values ± SD of the chlorophyll content measured from at least 10 individual 12-dayold plants grown under long-day (LD; 16 h light and 8 h dark) photoperiod conditions. Values are presented relative to the WT control set as 1. SD means standard deviation.

Figure 6 :

 6 Figure 6: (A) Representative siliques from 7-week-old of wild-type and mutant plants grown under long-day (LD; 16 h light and 8 h dark) photoperiod conditions are shown. (B) Mean values ± SD of silique length measured from at least 20 individual fully elongated siliques dissected from 7-weekold plants grown under long-day (LD; 16 h light and 8 h dark) photoperiod conditions. Values are presented relative to the WT control set as 1. SD means standard deviation. Scale bar represents 0.5 cm.

BFigure 7 :

 7 Figure 7: (A) Phenotypic appearance of 11-day-old wild-type and mutant plants grown vertically on MS under continuous-light (24h light and 0h dark). (B) Mean values ± SD of primary root length measured from at least 15 individual roots within a window of 5 to 11 days after stratification on wild-type and mutant plants grown under continuous-light (24 h light and 0 h dark). SD means standard deviation. Scale bar represents 1 cm.

Figure 8 :

 8 Figure 8: (A) Longitudinal view of the Arabidopsis root meristem in 6-day-old wild-type and mutant plants grown under continuous-light (24h light and 0h dark). Blue arrows indicate the transition zone, where cells leave the meristem and enter the elongation-differentiation zone. White arrows indicate the quiescent center (QC). (B) Average number of cortex cells ± SD in primary root meristem measured from at least 10 individual roots from 6-day-old plants grown under continuouslight. SD means standard deviation.

Figure 10 :

 10 Figure 10: Mean values ± SD of leaf area (A), cell area (B) and cell number (C) measured on leaf-3 dissected from at least 5 23-day-old wild-type and mutant plants grown under long-day (LD; 16 h light and 8 h dark) photoperiod conditions. SD means standard deviation.

Figure 11 :

 11 Figure 11: Mean values ± SD of the percentage of 2C, 4C, 8C and 16C nuclei extracted from the first pairs of leaves of 10, 11, 12, 14, 17, 21-day-old plants grown on ½ MS medium under long-day (LD; 16 h light and 8 h dark) photoperiod conditions. SD means standard deviation.

Figure 12 :

 12 Figure 12: Representative flowering phenotype of plants grown under long-day (LD; 16 h light and 8 h dark) photoperiod conditions. Scale bar represents 1 cm.

Figure 13 :

 13 Figure 13: Flowering time measured by (A) rosette leaf number at bolting and (B) days to bolting in wild-type and mutant plants grown under long-day (LD;16 h light and 8 h dark), medium-day (MD; 12 h light and 12 h dark) or short-day (SD; 8 h light and 16 h dark) photoperiod conditions. Mean values are shown together with standard deviation bars, based on 3 independent biological replicates with each replicate comprising over 15 plants for each genotype.

Figure 14 :

 14 Figure 14: Quantitative RT-PCR analysis of flowering genes FLOWERING LOCUS C (FLC), MADS AFFECTING FLOWERING 1-5 (MAF1-5), SUPPRESSOR OF OVEREXPRESSION OF CO1 (SOC1) and FLOWERING LOCUS T (FT) in different mutants. A pool (over 10 seedlings) of 16day-old seedlings was used for RNA extraction. Data were normalized using internal reference genes (Tip4.1, Exp and GADPH). Values are presented as relative to levels the wild type control (set as 1). Mean values are shown together with standard deviation bars.

Figure 15 :

 15 Figure 15: The different regions are numbered from the 5′-end to the 3′-end of the gene and their positions are indicated under the schematic presentation of FLC (A), MAF3, MAF4 and MAF5 (B), SOC1 (C) and FT (D) gene structure showing black boxes for exons, black lines for promoters and introns, and arrows for transcription start sites.

  This result might indicate that the deposition of the H3K36me3 mark first strongly depend on SDG8 and second can be subdivided in two categories, (i) one that is required for H2Bub1 deposition and (ii) another one, in line with the hierarchical model of histone modification during transcription in animal and yeast, which dependent on H2Bub1 deposition. Finally, the H3K27me3 enrichment was similar between sdg8-1 and sdg8-1 hub2-2. This last observation, together with the difference observed between sdg8-1 and hub2-2 regarding the enrichment of H3K27me3, indicate that the H3K36 trimethylation arising from the H2Bub1 deposition might directly or even indirectly less well restrain the spreading of H3K27me3.

Figure 16 :

 16 Figure 16: ChIP analyses of H3K4me3 (A), H3K36me3 (B), H2Bub1 (C) and H3K27me3 (D) levels in sdg8-1, hub2-2 and sdg8-1 hub2-2 mutant plants along the chromatin of FLC. Chromatin was prepared from 16-day-old plants grown under medium-day (MD; 12 h light and 12 h dark) photoperiod conditions. Data were normalized to the input and presented relative to Tubulin 2 (Tub2) in the wild-type control.

Figure 17 :

 17 Figure 17: ChIP analyses of H3K4me3 (A), H3K36me3 (B) and H2Bub1 (C) and H3K27me3 (D) levels in sdg8-1, hub2-2 and sdg8-1 hub2-2 mutant plants along the chromatins of MAF3/4/5.

Figure 18 :

 18 Figure 18: ChIP analyses of H3K4me3 (A), H3K36me3 (B) and H2Bub1 (C) and H3K27me3 (D) levels in sdg8-1, hub2-2 and sdg8-1 hub2-2 mutant plants along the chromatins of SOC1.Chromatin was prepared from 16-day-old plants grown under medium-day (MD; 12 h light and 12 h dark) photoperiod conditions. Data were normalized to the input and presented relative to Tubulin 2 (Tub2) in the wild type control.

Figure 19 :

 19 Figure 19: ChIP analyses of H3K4me3 (A), H3K36me3 (B) and H2Bub1 (C) and H3K27me3 (D) levels in sdg8-1, hub2-2 and sdg8-1 hub2-2 mutant plants along the chromatins of FT. Chromatin was prepared from 16-day-old plants grown under medium-day (MD; 12 h light and 12 h dark) photoperiod conditions. Data were normalized to the input and presented relative to Tubulin 2 (Tub2) in the wild type control.

3 . 2

 32 and H2Bub1 are major active histone marks in yeast, animals and plants. Studies in yeast and animals have uncovered intriguing crosstalks among these marks in concert to control organism development and particularly focused on gene transcription. In yeast, loss-of-function of Bre1, the homologue of HUB2, causes a total loss of H3K4me3(Hwang et al., 2003;[START_REF] Zhu | Monoubiquitination of human histone H2B: the factors involved and their roles in HOX gene regulation[END_REF] and loss-of-function of Ubp8, the H2B deubiquitinase, causes a decreased in H3K36me2 at specific loci. A model of active transcription was built with histone modifications occurring in a sequential manner. In this model, H2Bub1 is required for H3K4me3 deposition during transcription initiation, the removal of H2Bub1 is latter necessary for H3K36me3 deposition during transcription elongation. It is thus of great interest to investigate the conservation of such a crosstalk in Arabidopsis.By western blots, we did not observe any changes of H3K4me3 in hub2-2 as well as in sdg8-1 hub2-2. Moreover, global level of H3K36 methylation and H2Bub1 in the double mutant was similar as in sdg8-1 and hub2-2 single ones, respectively. These data strongly indicate that a clear crosstalk between H2Bub1, H3K4me3 and H3K36me3 is not evident in Arabidopsis as in animal or yeast at a global genomicscale. However, a significant increase of the global level of the repressive mark H3K27me3 was observed in our double mutant, indicating that SDG8 and HUB2 might together contribute to limit the spread of this repressive mark. Because CLF is one of the H3K27me3 methyltransferase in Arabidopsis of which the mutation is causing a global decrease of H3K27me3(Jiang et al., 2008), it will be very interesting to investigate the H3K27me3 level in sdg8-1 hub2-2 clf triple mutants. SDG8 and HUB2 act synergistically to regulate Arabidopsis plant growth.Since the depletion of either SDG8 or HUB2 leads to smaller plant size, H3K36me2/3 and H2Bub1 might be positively linked to plant growth. It is thus of great interest to uncover possible functional relationships between histone methylation and monoubiquitination during plant growth. To this end, we have thoroughly analyzed phenotypes of sdg8-1 hub2-2 mutants and found that double mutant plants display an enhanced reduction of silique length, chlorophyll content, root length, plant body size and rosette leave size compared to the sdg8-1 and hub2-2 single mutants.

  were dramatically lower compared to those other flowering genes and only slight change can be observed among different genotypes. It is reasonable to conclude that the increase of FT expression is caused by reduced expression level of its repressors (i.e. FLC, MAF3, MAF4 and MAF5), possibly regardless of the deposition and changes of these transcriptional histone marks.

ABFigure 21 :

 21 Figure 21: Model of interplay among H2Bub1, H3K4me3 and H3K36me3 on flowering genes transcription in yeast and animals (A) and Arabidopsis (B). In yeast animals and plants, H2Bub1 is necessary for the deposition of H3K4me3 during transcriptional initiation; H2Bub1 and H3K36me3 are mutually favor each other during transcriptional elongation in plants, while H2Bub1 is required for H3K36me3 in animals and yeast. It is yet not known the relationship between H2B deubiquitination and H3K36me3 deposition in plants.

3 :

 3 Flowering time measured by (a) rosette leaf number at bolting and (b) days to bolting in wild-type and mutant plants grown under long-day (LD;16 h light and 8 h dark), mediumday (MD; 12 h light and 12 h dark) or short-day (SD; 8 h light and 16 h dark) photoperiod conditions.

Figure 1 :

 1 Figure 1: Pull down assay. Agarose beads coated with GST or GST-HULK2C (1068-1366aa) were incubated with an equal aliquot of His-SDG8N (1-816aa) protein. The pull down fractions and inputs

Figure 2 :

 2 Figure 2: Alignment of HULK family PWWP domain with analyzed domains from mammalian and S. pombe proteins. Alignment of PWWP domains from following proteins: HUA2, HULK1, HULK2, HULK3, PSIP1, MUM1, MSH6, DNMT3A, HDGF, NSD3_C, PWWP2B, and RBBP1. Most conserved amino acids shaded in red, secondly conserved amino acids were shaded in blue and the less conserved amino acids were shaded in green. The four feature residues of PWWP domain was in black rectangle.

Figure 3 :

 3 Figure 3: In vitro binding assays of N terminal His-tagged PWWP domains of HUA2 (His-HUA2-PWWP) and HULK2 (His-HULK2-PWWP) with H3 peptides containing mono-, di-or trimethylation on K4 or K36.

Figure 4 :

 4 Figure 4: In vitro binding assay of His-HUA2-PWWP, His-HULK2PWWP and point mutated HUA2-PWWP-P16A, HUA2-PWWP-W18A, and HUA2-PWWP-W18A with H3 peptides containing mono-, di-or trimethylation on K36 and trimethylation on K27.

Figure 6 :

 6 Figure 6: (A) Scheme of sdg8-1 hua2-7 double mutants' identification. (B) Example of genotyping results from 4 plants of wild-type, sdg8-1, hua2-7 and sdg8-1 hua2-7.

Figure 7 :

 7 Figure 7: Representative 18-day-old seedlings of Col, sdg8-1, hua2-7, and sdg8-1 hua2-7 grown under long-day (LD; 16 h light and 8 h dark) photoperiod conditions.

Figure 8 :

 8 Figure 8: Flowering time measured by (A) rosette leaf number at bolting and (B) days to bolting in wildtype and mutant plants grown under long-day (LD;16 h light : 8 h dark), medium-day (MD; 12 h light :

Figure 9 :

 9 Figure 9: Quantitative RT-PCR analysis of flowering genes expression in different mutants. A pool of 16 days old seedlings was used for RNA extraction and each experiment was normalized to internal reference genes (Tip4.1, Exp and GADPH) expression. Values are presented as relative to levels in Col-0 (set as 1), error bar indicates standard deviation (SD).

  redundant and divergent roles in reading methylation. The further work is necessary to gain insight into the mechanisms their binding specificity. Because HUA2 and SDG8 were genetically linked to control flowering time by redundantly repress FLC transcription. It is also interestingly to detect the H3K36 methylation changes on FLC chromatin, which will provide us mechanistic sight to understand the role of H3K36me2/3 in Arabidopsis flowering time regulation.Collectively, results from my thesis work contribute to significant progresses of our understanding on molecular mechanisms about deposition and reading of histone methylation marks in transcription and regulation of plant development.

F2

  plants were genotyped by PCR. Genomic DNA was extracted from F2 plants by the DNA extraction buffer and then used as PCR template for genotyping. The primers for PCR genotyping were used as listed. For wild type alleles, PCR was carried out using a pair of gene specific primers, named Left and Right genomic primer (LP and RP) flanking the T-DNA insertion site on the genomic sequence. For the detection of mutant alleles, PCR was carried out using the gene specific right primer RP and the Left Border primer of the T-DNA insertion (i.e. primer LBb1.3 for SALK T-DNA insertion lines).

2. 1 . 7 2 . 1 . 8

 17218 Reverse transcription 10 μL RNA (≤5 µg), 5 μL of reverse transcription 10X buffer (Promega) and 5 μL of 25 mM MgCl2 were mixed in a 200 μL PCR tube and treated with 1 μL of RQ1 Dnase (Promega) at 37 °C for 10 min to eliminate genomic DNA from RNA samples, and followed by at 65°C for 10 min to inactivate the Dnase activity. 1 μL of 100 mM oligo dT was added to the reaction and incubate it at 70 °C for 5 min, then put it on ice immediately for 5 min. 2.5 μL of 10 mM dNTPs and 1 μL of Impro II Reverse Transcriptase (Promega) were added into the mixture and incubated at 42 °C for 70 min, followed inactivation by heating at 70 °C for 15 min. The synthesized cDNA was used as template for quantitative PCR. Quantitative PCR Quantitative PCR was used to detect relative or absolute gene expression level. It was performed in 384 wells optical plate on a light cycler 480 II (Roche) apparatus, according to the manufacturer's instructions. PCR total volume was scaled to 10 μl for each reaction, including 2 μL of mixed primers (containing 2.5 mM forward and reverse gene specific primers), 5 μL of 480 SYBER Green fluorescent reporter, 2 μL water and 1 μL cDNA template. For each sample, PCR was performed in triplicate using fixed amounts of cDNA template. GADPH, EXP and Tip4.1 were used as internal reference genes. PCR was carried out by following procedures: pre-heating at 95 °C for 10 min, followed by 40 cycles of 15 sec at 95 °C, 30 sec at 60 °C and 15 sec at 72 °C. Melting curves of PCR reactions were checked to ensure the quality of PCR reaction and to avoid any DNA contamination.

Day 2 :

 2 30 ml of ChIP lysis buffer was added to 5 ml powder of each sample, and incubated on a rotation wheel for 40 min at 4°C. The mixture was filtered through 100 μm nylon meshes and centrifuged at 4000 rpm at 4°C for 20 min. After removing supernatant, the pellet may appear grey; otherwise the pellet was resuspended again in ChIP lysis buffer and centrifuged again. The pellet was resuspended in 570 μL of ChIP lysis buffer and transferred to a new 1.5 ml tube. Centrifuged solution at 4000 rpm at 4 °C for 20 min. Removed supernatant as much as possible and resuspended pellet in 570 μL lysis buffer plus 30 μL of 10% SDS. Chromatin was sonicated four times with a Bioruptor (Diagenode) in ice water, 30 seconds ON and 30 seconds OFF at high power during 5 minutes for each time. The sonicated chromatin sample was centrifuged at 13000 rpm at 4 °C for 10 min. Supernatant was transferred to a new tube and added up 2 ml with ChIP lysis buffer. To check the sonication efficiency, 40 μL of solution was taken and 360 μl ChIP elution buffer and 16 μl of 5 M NaCl were added and incubated at 65 °C least 6 hour or overnight. DNA was recovered by NucleoSpin® Gel and PCR Clean-up kit (MACHEREY-NAGEL, Germany) and eluted in 30μl H2O. Then 5μl DNA was treated with 1μl RNase and used for agarose gel electrophoresis to visualize the quality of the sonication (DNA fragment should be 100-500 bp). The residue DNA was used later as input for qPCR analysis. After confirming the sonication was well done, 300 μL of chromatin was added up to 2 ml by antibody binding buffer and used for ImmunoPrecipitation by incubating with 1 to 3 μL desired antibodies overnight at 4 °C under rotation. Parallel, chromatin without any antibody was used as a mock control. Day 3: Chromatin with antibody complex was collected by adding 20 μl of slurry magnetic Protein A beads (Millipore) for each reaction and incubated at 4 °C for 1-3 hours under Ploidy Analysis PA-1 (Partec) and a total of 20000 nuclei per sample were counted. Ploidy levels data was analyzed by Flowjo (TreeStar).

  Gateway® Cloning Technology (Invitrogen Corporation) is an excellent cloning system and provides a rapid and highly efficient way to clone or sub-clone DNA segments and thus facilitates gene functional analysis, protein expression. It is achieved by two recombination reactions (BP and LR reactions) for transferring DNA segments between different cloning vectors. The BP reaction involves the transfer of PCR product of interest flanked by attB1 and attB2 sites into a donor (pDONR™) vector which has attP1 and attP sites flanking the cloning site to create an entry clone. The LR reaction is able to transfer the PCR product in the Entry Clone to one or more destination vectors with attR1 and attR2 sites and yields expression clone(s).BP reaction: The gene or DNA fragment was amplified with flanking sequence of attB sites by PCR. Purified attB-PCR products (≈150 ng), donor vector (pDONR207 or pDONR) and TE buffer were added to a final volume of 8 μl in 1.5 ml tubes at 25 °C. The BP clonase TM II enzyme was kept in ice for 2 minutes to saw, and then 2 μl of enzyme was put into the reaction. After mixing well by brief vortex twice, the reaction was incubated at 25 °C for 2 hours or overnight. Finally, 1 μL of proteinase K was added and incubated at 37 °C for 10 min to stop the reaction.

  j'ai utilisé la technique d'immunoprécipitation de la chromatine (ChIP) pour adresser la question de l'état chromatinien des gènes FLC, MAF4, MAF5, FT et SOC1 à l'aide d'anticorps anti-H3, -H3K4me3, -H3K36me3 et -H3K27me3 et d'amorces couvrant la majeure partie de leurs unités de transcription. En général, les profils observés chez le double mutant apparaissent similaires à ceux observés pour H3K4me3 chez hub2 et H2Bub1 chez sdg8 et hub2. Spécifiquement pour FLC, le niveau d'H3K36me3 chez sdg8 hub2 est diminué de manière additive par rapport aux diminutions observées chez sdg8 et hub2, ce qui indique que pour ce gène, la méthylation d'H3K36 pourrait être mise en place à la fois de manière dépendante et indépendante d'H2Bub1. De plus, alors que le niveau d'H3K4me3 est diminué chez hub2 et inchangé chez sdg8, H3K36me3 et H2Bub1 sont diminués de manière identique chez sdg8, hub2 et sdg8 hub2 chez tous les autres gènes étudiés à l'exception de FT. En effet, FT semble être moins sous le control de variations au niveau de sa chromatine que les autres gènes de floraison analysés, les niveaux de H3K4me3 et H3K36me3 étant beaucoup plus faibles et de H2Bub presque absents. L'ensemble de mes données de ChIP indiquent que, comme chez l'animal, H3K4me3 dépend entièrement de H2Bub1. Cependant, contrairement au modèle animal et à la levure, il n'y a pas de corrélation exclusive entre H3K36me3 et H2Bub1, rendant impossible d'établir une dépendance hiérarchique claire. Par conséquent, il semble que SDG8 et HUB2 soient capable de se promouvoir mutuellement dans l'établissement d'un état chromatinien actif. SDG7, une méthyltransferase atypique chez Arabidopsis La vernisation est un processus par lequel l'exposition prolongées à des températures froides accélère la floraison dans de nombreuses espèces végétales. Chez Arabidopsis, la vernalisation est initiée par l'induction de VERNALISATION INSENSITIVE3 (VIN3) lors de l'exposition au froid. VIN3 forme un complexe avec le Complexe Répressif Polycomb 2 (PRC2) et est nécessaire pour que le PRC2 puisse induire l'extinction épigénétique de FLC suite au dépôt de la marque d'histone répressive H3K27me3 sur la chromatine de FLC. Cependant, la façon dont l'exposition au froid conduit à l'induction du VIN3 est encore largement inconnue. En collaboration avec le laboratoire du Dr Richard Amasino, nous avons montré que SET DOMAIN GROUP 7 (SDG7) est nécessaire au bon déroulement de l'induction de VIN3 et du processus de vernalisation. Des données génétiques et moléculaires nous ont permis de démontrer que la mutation de SDG7 entraîne une réaction de vernalisation plus rapide que chez le contrôle sauvage, voir même un état partiellement vernalisé sans exposition au froid, impliquant SDG7 comme régulateur négatif de la vernalisation. Par western blot, j'ai ainsi observé que les niveaux de différentes marques de méthylation des histones étaient inchangés chez le mutant sdg7. De plus, la mesure de l'activité méthyltransférase in vitro nous a permis de démontrer que la protéine SDG7 recombinante est incapable de méthyler les histones.Dans leur ensemble, nos résultats indiquent que SDG7 pourrait méthyler une protéine non histone encore inconnue dans la régulation de la transcription et le contrôle de la durée de l'exposition au froid nécessaire au processus de vernalisation(Lee et al., 2015).Dans leur ensemble, les résultats de mon travail de thèse ont permis de réaliser des progrès significatifs de notre compréhension des mécanismes moléculaires de dépôt et de lecture des marques de méthylation d'histone lors de la transcription et de la régulation du développement des plantes. En mettant l'accent sur la famille TrxG, mon travail a également permis de démontrer que les connexions entre les marques d'histones chez Arabidopsis sont nombreuses, variées et extrêmement sophistiquées, rendant difficile, sinon impossible, d'établir un réseau fonctionnel simple avec des interconnexions hiérarchiques claires. Une telle complexité peut parfaitement refléter un certain degré de flexibilité, ce qui est conforme au concept de modularité et de plasticité du développement des plantes.

  connected in the course of transcription, I tested the genetic interaction between the sdg8 mutant and a mutant defective in H2Bub1, hub1-3 and hub2-2(Fleury et al., 2007). After being generated by crosses, the phenotype of the double mutant was thoroughly analyzed. In general, compared to each single mutant, the double mutant displayed an enhanced phenotype, with plants being smaller and flowering occurring earlier under all tested photoperiod conditions. Using western blot, I further observed that the global level of H3K36 methylation and H2Bub1 in the double mutant were similar as in sdg8 and hub2 single ones, respectively. However, a significant increase in the global level of the H3K27me3 repressive mark was only detected in the double mutant, thus suggesting that SDG8 and HUB2 might together limit the spread of this repressive mark. Analyses of the plant growth phenotype indicate that SDG8 and HUB2 act largely in a synergistic manner in the regulation of plant body and organ size. Because early flowering was the most obvious phenotype of my different mutant I next focus my molecular analyses on several flowering genes including the flowering repressor FLC, MAF1/FLM, MAF2, MAF3, MAF4 and MAF5 as well as the flowering activator FT and SOC1. My quantitative RT-PCR analysis indicate that in sdg8 and as previously published, the transcript level of MAF2 and MAF3 remained unchanged, while the FLC, MAF1, MAF4, and MAF5 transcript levels were drastically decreased,and FT and SOC1 transcript levels were increased as compared to the wild-type. In hub2, the transcript level of MAF4 and MAF5 was significantly reduced, while it was increased for FT. Interestingly, the transcript level of FT was increased in sdg8 hub2 double mutant compared to single ones to a level in between the ones observed in sdg8 and hub2 single mutants. Next, I used Chromatin Immuno Precipitation (ChIP) to address the question of the chromatin state of these genes using antibodies for H3, H3K4me3, H3K36me3 and H3K27me3 and primers covering FLC, MAF4, MAF5, FT and SOC1 genes. In general, profiles observed in the double mutant were similar to the ones observed for H3K4me3 in hub2 and H2Bub1 in sdg8 and hub2. Specifically

  initiated by the induction of VERNALIZATION INSENSITIVE3 (VIN3) upon cold exposure. VIN3 forms a complex with Polycomb Repressive Complex 2 (PRC2) and is necessary for PRC2 to mediate the epigenetic silencing of FLC through the deposition of the repressive histone mark H3K27me3 at FLC chromatin. However, how cold exposure leads to the induction of VIN3 is largely unknown. In collaboration with the laboratory of Dr. Richard Amasino, we showed that SET DOMAINGROUP 7 (SDG7) is required for proper timing of VIN3 induction and duration of the vernalization process. Genetic and molecular data demonstrated that the SDG7 loss-of-function mutant results in a more rapid vernalization response or even creates a partially vernalized state without cold exposure, implying SDG7 as a negative regulator of vernalization. Using western blot analyses, I showed that levels of different histone methylation marks remain unchanged in sdg7 mutant plants. Furthermore, recombinant SDG7 protein could not methylate histones in in vitro histone methyltransferase assays. Taken together, our results indicate that SDG7 might methylate a yet unknown non-histone protein in regulation of transcription and proper measurement of the duration of cold exposure in the vernalization process(Lee et al., 2015). As a whole, results from my thesis work contribute to significant progresses of our understanding on molecular mechanisms about deposition and reading of histone methylation marks in transcription and regulation of plant development. By focusing on the TrxG family, my work also has demonstrated that connections between histone marks in Arabidopsis are numerous, varied and extremely sophisticated, rendering difficult, if not impossible, to establish a simple functional network with clear hierarchical interconnections. Such complexity might reflect a certain degree of flexibility, which is in line with the modular concept of plant developmental plasticity. Wei ZHAO Caractérisation moléculaire et fonctionnelle des gènes impliqués dans la mise en place et la lecture de la méthylation d'histones chez l'Arabidopsis thaliana Résumé La méthylation des histones constitue un niveau important de contrôle épigénétique chez les eucaryotes. Mes études portent sur la caractérisation des facteurs potentiellement intervenant dans la mise en place et la lecture de la méthylation pour mieux apprécier son rôle et des mécanismes sous-jacents dans la régulation de la transcription et du développement des plantes chez l'Arabidopsis thaliana. Ainsi, la première partie de mes travaux de thèse a contribué à l'étude d'une protéine à domaine SET (SET DOMAIN GROUP7, SDG7) et à montrer que SDG7 est nécessaire au bon déroulement de l'induction de VIN3 et du processus de vernalisation pour la floraison. Nos résultats suggèrent que SDG7 pourrait méthyler une protéine non-histone encore inconnue dans la régulation de la transcription et le contrôle de la durée de vernalisation. La deuxième partie de ma thèse porte sur l'étude de SDG8 et les H2B-UBIQUITIN-ligases HUB1/HUB2 pour examiner un cross-talk éventuel entre la triméthylation de H3K36 (H3K36me3) et la monoubiquitination d'H2B (H2Bub1). Nous avons montré que H3K36me3 et H2Bub1 sont déposés largement indépendamment, qui diffère d'une dépendance hiérarchique de déposition préalablement observée chez la levure. La dernière partie de ma thèse a permis l'identification des protéines HUA2/HULK2 à domaine PWWP comme lecteurs éventuels de H3K36me3 dans la régulation de la floraison et du développement des plantes. Mots clés: Epigénétique, Chromatine, Modification d'histones, Transcription, Floraison, Arabidopsis.

  Prolonged exposure to winter cold enables flowering in many plant species through a process called vernalization. In Arabidopsis, vernalization results from the epigenetic silencing of the floral repressor FLOWERING LOCUS C (FLC) via a Polycomb Repressive Complex 2 (PRC2)-mediated increase in the density of the epigenetic silencing mark H3K27me3 at FLC chromatin. During cold exposure, a gene encoding a unique, cold-specific PRC2 component,VERNALIZATION INSENSITIVE 3 (VIN3), which is necessary for PRC2-mediated silencing of FLC, is induced. Here we

a,1 a Department of Biochemistry, University of Wisconsin-Madison, Madison, WI 53706; and b Institut de Biologie Moléculaire des Plantes du CNRS, Université de Strasbourg, Strasbourg 67084, France

Contributed by Richard M. Amasino, December 10, 2014 (sent for review

October 2, 2014; reviewed by George Coupland and Ben Trevaskis)

show that SET DOMAIN GROUP 7 (SDG7) is required for proper timing of VIN3 induction and of the vernalization process. Loss of SDG7 results in a vernalization-hyper-sensitive phenotype, as well as more rapid cold-mediated up-regulation of VIN3. In the absence of cold, loss of SDG7 results in elevated levels of long noncoding RNAs, which are thought to participate in epigenetic repression of FLC. Furthermore, loss of SDG7 results in increased H3K27me3 deposition on FLC chromatin in the absence of cold exposure and enhanced H3K27me3 spreading during cold treatment. Thus, SDG7 is a negative regulator of vernalization, and loss of SDG7 creates a partially vernalized state without cold exposure. vernalization | flowering time | SET DOMAIN GROUP 7

Table 1 -1 List of mutants and transgenic lines.

 1

	Name	Locus	Insertion/Description	Seed Source
	sdg8-1	AT5G23150	SALK_065480	Zhao et al., 2005
	hub2-2	AT1G55250	SALK_071289	Liu et al., 2007
	hub1-3	AT 2G44950	GABI_276D08	Liu et al., 2007
	ubp26-4	AT3G49600	SALK_024392	NASC
	hua2-7	AT 5G23150	SAIL_314_A08	Wang et al., 2007
	1.2 Bacterial strains
	All bacterial strains are listed in

Table 1 -2. Table 1-2 List of bacteria strains used in this study

 1

	Bacteria Strains	Application
	E. coli strain DH5α

Table 1 -4 List of created transgenetic lines.

 1

	Name	Selection in medium/soil	description
	hua2-7::HUA2-GFP	BASTA	transgentic lines with GFP tag
	hua2-7::HUA2-MYC	BASTA	transgentic lines with MYC tag
	hua2-7::HUA2	Kanamycin	transgentic lines for complementation
	hua2-7::HUA2△PAWP	Kanamycin	mutated (P to A) transgentic lines for complementation
	hua2-7::HUA2△PAWP	Kanamycin	mutated (W to A) transgentic lines for complementation
	1.5 Antibodies and peptides

Table 1 -5 Antibodies and peptides

 1

	Name		Company (Cat. #)	Dilution	Host	Purpose
	Primary antibodies
	anti-H3		Merck Millipore (05-499) 1:5000	rabbit western blot, ChIP
	anti-H3K36me3		Abcam (ab9050)	1:5000	rabbit western blot, ChIP
	anti-H3K36me1		Abcam (ab9048)	1:5000	rabbit western blot
	anti-H3K4me3		Merck Millipore (07-473) 1:5000	rabbit western blot, ChIP
	anti-H3K4me2		Merck Millipore (04-790) 1:5000	rabbit	western blot
	anti-H2Bub1	Merck Millipore (05-1312) 1:5000	rabbit western blot, ChIP
	anti-H3K27me3	Merck Millipore (07-449)	1:5000	rabbit	western blot, ChIP
	anti-6x-His Tag	Thermofisher (MA1-21315) 1:5000	rabbit	western blot
	Secondary antibodies
	anti-rabbit-HRP conjugated Sigma-Aldrich (A9169)	1:10000	goat	western blot
	anti-mouse-HRP conjugated Thermofisher (62-6520) 1:10000	goat	western blot
	Biotinylated peptides
	H3K4me1 (residues 1-21)		synthesized by Scilight Biotechnology	2μg	in-vitro IP
	H3K4me2 (residues 1-21)		synthesized by Scilight Biotechnology 2μg	in-vitro IP
	H3K4me3 (residues 1-21)		synthesized by Scilight Biotechnology 2μg	in-vitro IP
	H3K36me1 (residues 21-44)	synthesized by Scilight Biotechnology 2μg	in-vitro IP
	H3K36me2 (residues 21-44)	synthesized by Scilight Biotechnology 2μg	in-vitro IP
	H3K36me3 (residues 21-44)	synthesized by Scilight Biotechnology 2μg	in-vitro IP
	H3K27me3 (residues 21-44)	synthesized by Scilight Biotechnology 2μg	in-vitro IP
	Ni-NTA Agarose		QIAGEN (No.1000632)			6×His-tagged protein expression
	Glutathione Sepharose 4B		GE Healthcare (No. 17075601)		GST tagged protein expression
	PureProteom TM Protein G		Millipore (CA 925590)		ChIP

Primers Sequences of primers used for genotyping are listed in Table 1-6 (below).

	Primer	AGI ID	Gene	Sequence (5' 3')
			name
	SDG8.1-	AT5G23150 SDG8 CCTTCATCGCAATCGTAAATC
	LP
	SDG8.1-	AT5G23150 SDG8 TTTTGCGCTAAACTAGTTGGG
	RP
	HUB1.3-	AT2G44950 HUB1 GGATTTTTTAATTGGCATTG
	LP
	HUB1.3-	AT2G44950 HUB1 GCCGTTATCAACCCAAAACT
	RP
	HUB2.2-	AT1G55250 HUB2 CATGGTACCACATCCAAGGTC
	LP
	HUB2.2-	AT1G55250 HUB2 CCTCTTTAGGCCGATCAAAAC
	RP
	HUA2.7-	AT5G23150 HUA2 TCGTCGAATTGAGCTCTAGTGT
	LP
	HUA2.7-	AT5G23150 HUA2 GAAACACCTGATAACCGAGGTC
	RP
	UBP26.4-	AT3G49600 UBP26 TTGTGGAAACACCCCACAAAA
	LP
	UBP26.4-	AT3G49600 UBP26 TTGGCTTCGTCTATGGGCTGA
	RP
	GABI LB			ATATTGACCATCATACTCATTGC
	SAIL LB			TAGCATCTGAATTTCATAACC
				AATCTCGATACAC
	LBb1.3			ATTTTGCCGATTTCGGAAC

Sequences of primers used for quantitative RT-PCR analyses are listed in Table 1-7 (below).

	Primer	AGI ID	Gene	Sequence (5'	3')
			name

GADPH AAACTTGTCGCTCAATGCAATC Sequences of primers used for Chromatin ImmunoPrecipitation (ChIP) analyses are listed in Table 1-8 (below).

	Primer	AGI ID	Gene	Sequence (5'	3')
			name

3 Agrobacterium Transformation by electroporation

  37 °C overnight. The culture was used to inoculate 500 ml LB (1: 100) and incubated by shaking at 37 °C until OD600 value reaches 0.4-0.6. Cells were harvested by centrifugation at 3000 rpm for 10 minutes at 4 °C and resuspended in 50 ml of ice-cold 0.1 M CaCl2 solution on ice. After centrifugation at 4000 rpm for 10 minutes at 4 °C, supernatant was poured off and cells were resuspended gently in 3 ml of ice-cold 50 mM CaCl2/ 15% glycerol on ice. The competent cells were then dispensed into 100 μL aliquots, frozen in liquid nitrogen and stored in -80 °C. After keeping on ice for 30 minutes, the tube was put on a rack and transferred to preheated 42 °C circulating water bath for 45 seconds to 60 seconds. The tube was then chilled in ice for 3-5 minutes and added into 400 μL LB medium. The culture was incubated t at 37 °C 1 hour with gentle shaking to recover the bacteria. Proper volume of transformed competent cells was transferred onto LB agar medium plate with appropriate antibiotic. The plate was incubated at 37 °C and colonies should appear in 12-16 hours. The cuvette outside was dried and was put in a white plastic holder of Biorad apparatus that delivering an electrical pulse of 25 uF capacitance, 2.5 kV and 200Ω to carry out the transformation. The transformation was finished after hearing a high constant tone, and then immediately added 1ml of LB medium to the cuvette. Cells were transferred from the cuvette into a 1.5ml eppendorf tube and incubated at 28 °C for 1-3 hours to recover the bacteria. Proper volume of transformed culture was transferred onto LB agar medium plate with appropriate antibiotic. The plate was incubated at 37 °C and colonies should appear in 48 hours.

	2.2.

LB medium: 10 g/ L Tryptone; 5 g/ L Yeast extract; 10 g/ L NaCl

2.2.2 Transformation by heat shock

1μL plasmids, 1-5 μL of DNA ligation or BP/ LR recombination reaction products were added into 100 μL competent cells in a sterile tube and mixed well by swirling gently.

2 μL LR recombination reaction plasmids mixed with 50μL Agrobacterium competent cells in a sterile tube was transferred into a cold electroporation cuvette.

  [START_REF] Kim | Two cap-binding proteins, CBP20 and CBP80, are involved in processing primary MicroRNAs[END_REF] ml loading buffer and disrupted by sonication. After centrifugation at 15000 rpm for 1 hour, the supernatant was loaded onto a His Trap column (GE) for affinity chromatography. Proteins were eluted by AKTA system using elution buffer. The 6x His SUMO tag was cleavage by 50 μL of 5 mg/ ml ULP1 enzyme at 16 °C for 5 hours. Then Ni NTA Qiagen resin was added and after several times wash by wash buffer the 6x His SUMO tag was removed. Protein was further

	purified by molecular sieve chromatography using Superdex G 75 16/60 column. Pure
	protein was concentrated by ultra filtration with Amicon ULTRA cutoff 3 kDa at 4 °C and
	frozen in liquid nitrogen and stored at 80 °C for crystallization use.
	Loading buffer: 50 mM Tris-HCl pH 8.8; NaCl 300 mM; 5%Glycerol
	Elution buffer: 50 mM Tris-HCl pH 8.8; 500 mM Imidazole; 300 mM NaCl; 5% glycerol
	Wash buffer: 50 mM Tris-HCl pH 8.8; 10 mM Imidazole; 300 mM NaCl; 5% glycerol
	Gel filtration buffer: 25 mM Tris-HCl pH 8.8; 100 mM NaCl

moléculaire et fonctionnelle des gènes impliqués dans la mise en place et la lecture de la méthylation d'histones chez l'Arabidopsis thaliana

  Afin de combler cette lacune, j'explorerais, dans une deuxième partie, l'interaction fonctionnelle entre H2Bub1 et H3K36me3 en utilisant une approche génétique basée sur des croisements entre différents mutants d'enzymes de modification les histones. Parmi les enzymes modifiant les histones, la superfamille Afin d'explorer davantage cette liaison, j'ai déterminé la structure cristalline du domaine HUA2 PWWP d'Arabidopsis. Ce domaine contient un pli de type tonneau ß (ß-barrel) formé par quatre brins anti-parallèles (β1-β4) très conservé chez les plantes, les animaux et la levure. Parce que SDG8 est la principale méthyltransférase d'H3K36 chez Arabidopsis (Zhao et al., 2005) et parce que son mutant présente un phénotype de floraison précoce tout comme le mutant hua2, j'ai ensuite généré le double mutant sdg8 hua2 et caractérisé son phénotype. Alors que le double mutant sdg8 hua2 présente une taille réduite par rapport aux mutants simples, sa floraison est comprise entre celles de sdg8 et de hua2. Afin d'explorer davantage ce phénotype de floraison, les niveaux de transcription de plusieurs gènes de floraison ont ensuite été analysés par PCR quantitative. De manière intéressante, le niveau d'ARNm de FLC apparait similaire entre les mutants simples et doubles indiquant une redondance fonctionnelle entre SDG8 et HUA2. Plus en aval le long de la voie génétique de régulation de la floraison, le niveau de transcription de FT dans le double mutant est lui exactement entre celui observé chez sdg8 et hua2, indiquant qu'à ce niveau, SDG8 et HUA2 régulent FT indépendamment. Dans son ensemble, cette partie de ma thèse indique que, au moins pour FLC, SDG8 et HUA2 peuvent interagir génétiquement pour réguler la transcription génique.

	AP ATP Amp bp cm PBS PCR PI PMSF PVDF de protéines du groupe à domaine SET (SDG) comprend des protéines connues pour Ammonium persulfate Adenosine Triphosphate Ampicillin base pair Phosphate Buffer Saline Polymerase Chain Reaction Propidium Iodide Phenyl Methyl Sulphony Fluoride Caractérisation Chez les eucaryotes, l'ADN nucléaire contenant l'information génétique méthyler les histones sur différents résidus de lysine. Au sein de cette grande famille, PolyVinylidene DiFluoride s'assemble avec des protéines de type histones pour former la chromatine. D'une nous savons que les protéines du groupe Trithorax (TrxG), conservées chez la centimeter Col qPCR quantitative Polymerase Chain Reaction part, le compactage de la chromatine protège l'ADN des dommages, mais d'autre part plupart des eukaryotes, sont à l'origine de la méthylation de H3K4 et / ou H3K36 Columbia ChIP RNA RiboNucleic Acid cette structure empêche également l'accès des enzymes impliquées dans la permettant de favoriser la transcription en réponse à divers signaux Chromatin Immunoprecipitation rpm rotations per minute transcription des gènes. Par conséquent, plusieurs mécanismes ont évolué pour développementaux et environnementaux. Chez Arabidopsis, le groupe TrxG a été
	Da RT moduler dynamiquement la structure de la chromatine afin de réguler correctement Dalton room temperature impliqué dans le contrôle de plusieurs processus fondamentaux et comprend environ
	DAPI DAS DNA DTT EDTA TE RT PCR l'expression génique, constituant ainsi un niveau supplémentaire de contrôle. Parmi 4, 6-diamidino-2-phenylindole Days After Stratification DeoxyriboNucleic Acid dithiothreitol Tris-EDTA buffer Ethylene Diamine Tetraacetic Acid 12 membres, dont moins de la moitié a été fonctionnellement caractérisée. La Reverse Transcription Polymerase Chain Reaction SD divers processus, le remodelage de la chromatine est possible au travers d'une méthyltransférase SDG27 / ATX1 et la H3K4 méthyltransférase SDG8 sont les plus Short Day SDS pléthore de modifications covalentes post-traductionnelles des histones, comprenant étudiées. Dans la dernière partie de ma thèse, je décrirais un homologue d'ATX1 et Sodium Dodecyl Sulphate T-DNA l'acétylation, la méthylation, la phosphorylation, l'ubiquitinylation, et d'autres encore. SDG8 nommé SDG7 et je démontrerais son rôle fondamental dans le contrôle du Transfer-DNA processus de vernalisation.
	GST TEMED HUA2 et HULK2, des lecteurs de H3K36 methylée Glutathione-S-Transferase N, N, N', N'-tetramethyl-ethylenediamine. 1,2-diamine
	1 mM DTT; 10% glycerol 1X PBS buffer: 140 mM NaCl; 2.7 mM KCl; 10 mM Na2HPO4; 1.8 mM KH2PO4; pH 7.4 2.3 Protein crystallization g gram h hour His histidine Kana Kanamycin kb kilo base pairs LD Long Day μ micro M Molar (mole/ liter) MD Medium Day min minutes MS Murashige and Skoog MW molecular weight nm nanometer OD600 Optical Density, measured at 600 nm PAGE PolyAcrylamide Gel Electrophoresis Tris Tris(hydroxymethyl) aminomethane TTBS Triton-Tris Buffer Saline Ub ubiquitin pour définir différents états chromatiniens tout au long du génome. Ceci est particulièrement vrai lors de la transcription. En effet, le modèle actuel de transcription active est construit autour d'une chronologie précise de modifications d'histones. De manière simplifiée, la monoubiquitination de l'histone H2B (H2Bub1) se produit en premier, permettant ensuite la triméthylation de l'histone H3 sur la lysine 4 (H3K4me3) et l'initiation de la transcription. Enfin, H2Bub1 est éliminé permettant à la triméthylation de H3K36 (H3K36me3) de se produire en début d'élongation. Bien que Précédemment, HULK2 (HUA2-LIKE 2) a été identifié dans l'équipe comme un partenaire protéique potentiel de SDG8 par la technique de double hybride chez la levure. HULK2 contient un domaine PWWP conservé connu pour lier indifféremment des histones méthylées ou non méthylées chez l'animal. Chez Arabidopsis, les protéines de domaine PWWP HUA2 (AGENT DE AG-4 2) et HULK2 ont été récemment impliquées dans la régulation de la floraison, cependant, le(s) rôle(s) de leur domaine PWWP n'a pas été exploré(s) (Jali et al., 2014). Malgré l'impossibilité de démontrer l'interaction observée chez la levure à l'aide d'autres méthodes, mon travail s'est focalisé dans un premier temps à tester l'affinité de liaison du domaine PWWP à différentes histones modifiées. Les protéines de fusion marquées HUA2-PWWP et HULK2-PWWP ont été purifiées et incubées séparément avec les peptides histones modifiés synthétiques H3K4me1, H3K4me2, H3K4me3 ainsi que H3K36me1, H3K36me2 et H3K36me3. Ensuite, par western blot, j'ai pu démontrer la liaison des protéines de fusion His-HUA2-PWWP et His-HULK2-PWWP spécifiquement aux peptides H3K36me1/me2/me3, et non aux peptides H3K4me1/2/3. Collaboration

For crystallization, purified HUA2 PWWP-domain protein was concentrated to round 130 μL at 5 mg/ mL (in total of 650 μg) in 50 mM Tris-HCl pH 8.0, 300 mM NaCl, 5% glycerol, Comparé à l'acétylation qui modifie la charge et donc l'interaction entre les histones et l'ADN, la méthylation des histones ne provoque pas en soi de changement direct de la structure chromatinienne, mais nécessite des « lecteurs » spécifiques qui reconnaîtront le signal de méthylation et l'interpréteront. Chez Arabidopsis, peu de choses sont actuellement connues sur ces lecteurs de méthylation des histones, et en particulier sur les domaines protéiques impliqués dans cette liaison ainsi que sur leur spécificité. Dans la première partie de ma thèse, je vais décrire mes résultats sur un groupe récemment identifié de lecteurs de méthylation d'histone avec une attention particulière pour l'affinité de liaison de leur domaine PWWP. De plus, la méthylation des histones n'agit pas seule mais en association avec d'autres marques d'histones chez les animaux et la levure le modèle décrit ci-dessus soit bien documenté, chez les plantes, beaucoup moins d'informations sont disponibles et les quelques études ont été principalement concentrées sur la relation fonctionnelle entre H2Bub1 et H3K4me3.

entre histone méthylation et monoubiquitination lors de la transcription

  manière synergique dans la régulation de la taille des organes de la plante. La floraison précoce étant le phénotype le plus évident de mon double mutant, j'ai ensuite concentré mes analyses moléculaires sur plusieurs gènes de floraison, tels que les répresseurs de floraison FLC, MAF1/FLM, MAF2, MAF3, MAF4 et MAF5 ainsi que les activateurs de floraison FT et SOC1. Par rapport au control sauvage, mes analyses de RT-PCR quantitative indiquent que chez sdg8, et comme précédemment publié (Zhao et al., 2005), les niveaux de transcription de MAF2 et MAF3 demeurent inchangés, tandis que ceux de FLC, MAF1, MAF4 et MAF5 sont drastiquement diminués et de FT et SOC1 sont augmentés. Chez hub2, le niveau de transcription de MAF4 et MAF5 est lui considérablement réduit, alors qu'il est augmenté pour FT. Fait intéressant, le niveau de transcription de FT est également augmenté chez le double mutant sdg8 hub2 à un niveau compris entre celui de sdg8 et celui de hub2. Ensuite,

	Afin de déterminer si la méthylation de H3K36 et la monoubiquitination d'H2B
	interagissent fonctionnellement lors de la transcription, j'ai testé une éventuelle
	interaction génétique entre le mutant sdg8 et un mutant défectueux dans l'H2Bub1,
	hub1-3 et hub2-2 (Fleury et al., 2007). Après avoir été généré par croisements, les
	phénotypes du double mutant ont été soigneusement analysés. En général, par
	rapport à chaque mutant simple, le double mutant présente un phénotype accentué,
	les plantes étant plus petites et la floraison se produisant plus tôt dans toutes les
	conditions de photopériode testées. Par western blot, j'ai également observé que les
	niveaux globaux de méthylation d'H3K36 et d'H2Bub1 dans le double mutant étaient
	similaires à ceux observés chez sdg8 et chez hub2, respectivement. Cependant, une
	augmentation significative du niveau global de la marque répressive H3K27me3 n'a

été détectée que chez le double mutant, suggérant que SDG8 et HUB2 pourraient ensemble limiter la propagation de cette marque répressive. L'analyse de la croissance du double mutant indique de plus que SDG8 et HUB2 agissent en grande partie de

PLANT BIOLOGY

Acknowledgements

Primary root growth and development is impaired in sdg8-1 hub2-2 mutants

The root is a very important plant organ required for plant nutrients and water absorption from the soil. Compared to wild-type, root growth is slightly affected in sdg8-1 and hub2-2 but severely impaired in sdg8-1 hub2-2 (Figure 7A).

Kinetic analysis of primary root growth

The root growth was studied by measuring root length within a window of 5 to 11 days after stratification (DAS) of seedlings grown in vertical position under continuous light.

The average root length of sdg8-1 and hub2-2 single mutants was slightly reduced compared to that of wild type plants, while the sdg8-1 hub2-2 double mutant showed an enhanced phenotype (Figure 7B). Together, the shortened root length of the double mutant does not simply reflect an additive phenotype but indicate a synergistic effect of sdg8-1 and hub2-2.

Quantification of leaf cellular parameters in sdg8-1 hub2-2 mutants

To get more insight into leaf growth defects at the cellular level, leaf number 3 was taken as representative for cellular analysis because it is fully developed at 23 days with completed cell proliferation and expansion. Leaf 3 area was significantly reduced in the sdg8-1 and hub2-2 single mutant compared to wild type and the reduction was further enhanced in the double mutant (Figure 10A). Epidermal cell area was similar between the double mutant, sdg8-1 and wild type, but was significantly increased in hub2-2 (Figure 10B). Finally, the number of epidermal per leaf 3 was significantly reduced in both single mutants compared to wild type and additively reduced in our double mutant compared to sdg8-1 and hub2-2 (Figure 10C). Hence, it seems that the lamina area, width and length in the double mutant were reduced most likely because mutant but not in the sdg8-1 and hub2-2 single mutants, suggesting the synergistic/redundant co-regulation of these genes by SDG8 and HUB2 (Figure 20).

A B

Supplementary data

Supplemental rotation. The beads were pelleted using a Magana GrIP racks (Millipore) and washed successively with 500 μL of low salt wash buffer, high salt wash buffer, LiCl wash buffer under rotation at 4 °C for 10 min and finally 500 μL TE buffer under rotation 25 °C for 10 minutes, then removed TE buffer as much as possible. The beads-antibody chromatin complex was eluted in 100 μl elution buffer with 1 μL of protease K and incubated at 65 °C overnight under agitation.