Chromatin regulation of flowering

Arabidopsis ABF3 and ABF4 Transcription Factors Act with the NF-YC Complex to Regulate SOC1 Expression and Mediate Drought-Accelerated Flowering

The drought-escape response accelerates flowering in response to drought stress, allowing plants to adaptively shorten their life cycles. Abscisic acid (ABA) mediates plant responses to drought, but the role of ABA-responsive element (ABRE)-binding factors (ABFs) in the drought-escape response is poorly understood. Here, we show that Arabidopsis thaliana ABF3 and ABF4 regulate flowering in response to drought through transcriptional regulation of the floral integrator SUPPRESSOR OF OVEREXPRESSION OF CONSTANS1 (SOC1). The abf3 abf4 mutant displayed ABA-insensitive late flowering under long-day conditions. Ectopic expression of ABF3 or ABF4 in the vasculature, but not in the shoot apex, induced early flowering, whereas expression of ABF3 fused with the SRDX transcriptional repressor domain delayed flowering. We identified SOC1 as a direct downstream target of ABF3/4, and found that SOC1 mRNA levels were lower in abf3 abf4 than in wild-type plants. Moreover, induction of SOC1 by ABA was hampered in abf3 abf4 mutants. ABF3 and ABF4 were enriched at the -1028- to -657-bp region of the SOC1 promoter, which does not contain canonical ABF-ABRE-binding motifs but has the NF-Y binding element. We found that ABF3 and ABF4 interact with nuclear factor Y subunit C (NF-YC) 3/4/9 in vitro and in planta, and induction of SOC1 by ABA was hampered in nf-yc3 yc4 yc9 mutants. Interestingly, the abf3 abf4, nf-yc3 yc4 yc9, and soc1 mutants displayed a reduced drought-escape response. Taken together, these results suggest that ABF3 and ABF4 act with NF-YCs to promote flowering by inducing SOC1 transcription under drought conditions. This mechanism might contribute to adaptation by enabling plants to complete their life cycles under drought stress.

The Arabidopsis H3K27me3 demethylase JUMONJI 13 is a temperature and photoperiod dependent flowering repressor

In plants, flowering time is controlled by environmental signals such as day-length and temperature, which regulate the floral pathway integrators, including FLOWERING LOCUS T (FT), by genetic and epigenetic mechanisms. Here, we identify an H3K27me3 demethylase, JUMONJI 13 (JMJ13), which regulates flowering time in Arabidopsis. Structural characterization of the JMJ13 catalytic domain in complex with its substrate peptide reveals that H3K27me3 is specifically recognized through hydrogen bonding and hydrophobic interactions. Under short-day conditions, the jmj13 mutant flowers early and has increased FT expression at high temperatures, but not at low temperatures. In contrast, jmj13 flowers early in long-day conditions regardless of temperature. Long-day condition and higher temperature induce the expression of JMJ13 and increase accumulation of JMJ13. Together, our data suggest that the H3K27me3 demethylase JMJ13 acts as a temperature- and photoperiod-dependent flowering repressor.

Diversity and redundancy of the ripening regulatory networks revealed by the fruitENCODE and the new CRISPR/Cas9 CNR and NOR mutants

Tomato is considered as the genetic model for climacteric fruits, in which three major players control the fruit ripening process: ethylene, ripening transcription factors, and DNA methylation. The fruitENCODE project has now shown that there are multiple transcriptional circuits regulating fruit ripening in different species, and H3K27me3, instead of DNA methylation, plays a conserved role in restricting these ripening pathways. In addition, the function of the core tomato ripening transcription factors is now being questioned. We have employed CRISPR/Cas9 genome editing to mutate the SBP-CNR and NAC-NOR transcription factors, both of which are considered as master regulators in the current tomato ripening model. These plants only displayed delayed or partial non-ripening phenotypes, distinct from the original mutant plants, which categorically failed to ripen, suggesting that they might be gain-of-function mutants. Besides increased DNA methylation genome-wide, the original mutants also have hyper-H3K27me3 in ripening gene loci such as ACS2, RIN, and TDR4. It is most likely that multiple genetic and epigenetic factors have contributed to their strong non-ripening phenotypes. Hence, we propose that the field should move beyond these linear and two-dimensional models and embrace the fact that important biological processes such as ripening are often regulated by highly redundant network with inputs from multiple levels.

Retrospective and perspective of plant epigenetics in China

Epigenetics refers to the study of heritable changes in gene function that do not involve changes in the DNA sequence. Such effects on cellular and physiological phenotypic traits may result from external or environmental factors or be part of normal developmental program. In eukaryotes, DNA wraps on a histone octamer (two copies of H2A, H2B, H3 and H4) to form nucleosome, the fundamental unit of chromatin. The structure of chromatin is subjected to a dynamic regulation through multiple epigenetic mechanisms, including DNA methylation, histone posttranslational modifications (PTMs), chromatin remodeling and noncoding RNAs. As conserved regulatory mechanisms in gene expression, epigenetic mechanisms participate in almost all the important biological processes ranging from basal development to environmental response. Importantly, all of the major epigenetic mechanisms in mammalians also occur in plants. Plant studies have provided numerous important contributions to the epigenetic research. For example, gene imprinting, a mechanism of parental allele-specific gene expression, was firstly observed in maize; evidence of paramutation, an epigenetic phenomenon that one allele acts in a single locus to induce a heritable change in the other allele, was firstly reported in maize and tomato. Moreover, some unique epigenetic mechanisms have been evolved in plants. For example, the 24-nt siRNA-involved RNA-directed DNA methylation (RdDM) pathway is plant-specific because of the involvements of two plant-specific DNA-dependent RNA polymerases, Pol IV and Pol V. A thorough study of epigenetic mechanisms is of great significance to improve crop agronomic traits and environmental adaptability. In this review, we make a brief summary of important progress achieved in plant epigenetics field in China over the past several decades and give a brief outlook on future research prospects. We focus our review on DNA methylation and histone PTMs, the two most important aspects of epigenetic mechanisms.

Copaifera langsdorffii Novel Putative Long Non-Coding RNAs: Interspecies Conservation Analysis in Adaptive Response to Different Biomes

Long non-coding RNAs (lncRNAs) are involved in multiple regulatory pathways and its versatile form of action has disclosed a new layer in gene regulation. LncRNAs have their expression levels modulated during plant development, and in response to stresses with tissue-specific functions. In this study, we analyzed lncRNA from leaf samples collected from the legume Copaifera langsdorffii Desf. (copaíba) present in two divergent ecosystems: Cerrado (CER; Ecological Station of Botanical Garden in Brasília, Brazil) and Atlantic Rain Forest (ARF; Rio de Janeiro, Brazil). We identified 8020 novel lncRNAs, and they were compared to seven Fabaceae genomes and transcriptomes, to which 1747 and 2194 copaíba lncRNAs were mapped, respectively, to at least one species. The secondary structures of the lncRNAs that were conserved and differentially expressed between the populations were predicted using in silico methods. A few selected lncRNA were confirmed by RT-qPCR in the samples from both biomes; Additionally, the analysis of the lncRNA sequences predicted that some might act as microRNA (miRNA) targets or decoys. The emerging studies involving lncRNAs function and conservation have shown their involvement in several types of biotic and abiotic stresses. Thus, the conservation of lncRNAs among Fabaceae species considering their rapid turnover, suggests they are likely to have been under functional conservation pressure. Our results indicate the potential involvement of lncRNAs in the adaptation of C. langsdorffii in two different biomes.

Genome encode analyses reveal the basis of convergent evolution of fleshy fruit ripening

Fleshy fruits using ethylene to regulate ripening have developed multiple times in the history of angiosperms, presenting a clear case of convergent evolution whose molecular basis remains largely unknown. Analysis of the fruitENCODE data consisting of 361 transcriptome, 71 accessible chromatin, 147 histone and 45 DNA methylation profiles reveals three types of transcriptional feedback circuits controlling ethylene-dependent fruit ripening. These circuits are evolved from senescence or floral organ identity pathways in the ancestral angiosperms either by neofunctionalisation or repurposing pre-existing genes. The epigenome, H3K27me3 in particular, has played a conserved role in restricting ripening genes and their orthologues in dry and ethylene-independent fleshy fruits. Our findings suggest that evolution of ripening is constrained by limited hormone molecules and genetic and epigenetic materials, and whole-genome duplications have provided opportunities for plants to successfully circumvent these limitations.

Epigenetic Environmental Memories in Plants: Establishment, Maintenance, and Reprogramming

Plants are immobile and must respond to or endure fluctuating surroundings and diverse environmental challenges. Environmental inputs often induce chromatin modifications at various responsive genes and consequent changes in their expression. Environment-induced chromatin marks at certain loci are transmittable through cell divisions after relief from the original external signals, leading to acquired 'memorization' of environmental experiences in plants, namely epigenetic environmental memories, which enable plants to adapt to environmental changes or to perform better when events recur. Here, we review recent progress in epigenetic or chromatin-mediated environmental memories in plants, including defense priming, stress memories, and 'epigenetic memory of winter cold' or vernalization. Various advances in epigenetic mechanisms underlying plant-environment interactions highlight that plant environmental epigenetics is emerging as an important area in plant biology.

Dual recognition of H3K4me3 and H3K27me3 by a plant histone reader SHL

The ability of a cell to dynamically switch its chromatin between different functional states constitutes a key mechanism regulating gene expression. Histone mark “readers” display distinct binding specificity to different histone modifications and play critical roles in regulating chromatin states. Here, we show a plant-specific histone reader SHORT LIFE (SHL) capable of recognizing both H3K27me3 and H3K4me3 via its bromo-adjacent homology (BAH) and plant homeodomain (PHD) domains, respectively. Detailed biochemical and structural studies suggest a binding mechanism that is mutually exclusive for either H3K4me3 or H3K27me3. Furthermore, we show a genome-wide co-localization of SHL with H3K27me3 and H3K4me3, and that BAH-H3K27me3 and PHD-H3K4me3 interactions are important for SHL-mediated floral repression. Together, our study establishes BAH-PHD cassette as a dual histone methyl-lysine binding module that is distinct from others in recognizing both active and repressive histone marks.

Histone mark reader proteins bind to particular histone modifications and regulate chromatin state. Here, Qian et al. show that the SHORT LIFE reader has a unique ability to recognize both activating and repressive histone marks and that these interactions enable SHORT LIFE to repress flowering in plants.

Regulation of Plant Growth and Development: A Review From a Chromatin Remodeling Perspective

In eukaryotes, genetic material is packaged into a dynamic but stable nucleoprotein structure called chromatin. Post-translational modification of chromatin domains affects the expression of underlying genes and subsequently the identity of cells by conveying epigenetic information from mother to daughter cells. SWI/SNF chromatin remodelers are ATP-dependent complexes that modulate core histone protein polypeptides, incorporate variant histone species and modify nucleotides in DNA strands within the nucleosome. The present review discusses the SWI/SNF chromatin remodeler family, its classification and recent advancements. We also address the involvement of SWI/SNF remodelers in regulating vital plant growth and development processes such as meristem establishment and maintenance, cell differentiation, organ initiation, flower morphogenesis and flowering time regulation. Moreover, the role of chromatin remodelers in key phytohormone signaling pathways is also reviewed. The information provided in this review may prompt further debate and investigations aimed at understanding plant-specific epigenetic regulation mediated by chromatin remodeling under continuously varying plant growth conditions and global climate change.

DNA Methylation and the Evolution of Developmental Complexity in Plants

All land plants so far examined use DNA methylation to silence transposons (TEs). DNA methylation therefore appears to have been co-opted in evolution from an original function in TE management to a developmental function (gene regulation) in both phenotypic plasticity and in normal development. The significance of DNA methylation to the evolution of developmental complexity in plants lies in its role in the management of developmental pathways. As such it is more important in fine tuning the presence, absence, and placement of organs rather than having a central role in the evolution of new organs. Nevertheless, its importance should not be underestimated as it contributes considerably to the range of phenotypic expression and complexity available to plants: the subject of the emerging field of epi-evodevo. Furthermore, changes in DNA methylation can function as a "soft" mutation that may be important in the early stages of major evolutionary novelty.

The PHD-containing protein EARLY BOLTING IN SHORT DAYS regulates seed dormancy in Arabidopsis

The Arabidopsis protein EARLY BOLTING IN SHORT DAYS (EBS), a plant-specific transcriptional regulator, is involved in the control of flowering time by repressing the floral integrator FT. The EBS protein binds the H3K4me3 histone mark and interacts with histone deacetylases to modulate gene expression. Here, we show that EBS also participates in the regulation of seed dormancy. ebs mutations cause a reduction in seed dormancy, and the concurrent loss of function of the EBS homologue SHORT LIFE (SHL) enhances this dormancy alteration. Transcriptomic analyses in ebs mutant seeds uncovered the misregulation of several regulators of seed dormancy including the MADS box gene AGAMOUS-LIKE67 (AGL67). AGL67 interacts genetically with EBS in seed dormancy regulation, indicating that both loci act in the same pathway. Interestingly, EBS functions independently of the master regulator gene of dormancy DELAY OF GERMINATION 1 (DOG1) and other genes encoding chromatin remodelling factors involved in the control of seed dormancy. Altogether, these data show that EBS is a central repressor of germination during seed dormancy and that SHL acts redundantly with EBS in the control of this developmental process. Our observations suggest that a tightly regulated crosstalk among histone modifications is necessary for a proper control of seed dormancy.

Histone deacetylase HD2D is involved in regulating plant development and flowering time in Arabidopsis

HD2D is one of the 4 plant specific HD2 family histone deacetylases (HDAC) identified in Arabidopsis and it is distantly related to the other HD2 members. At this time, little is known about the function of HD2D in plants. Here we provide evidence that HD2D is involved in the control of flowering time. Flowering was delayed in transgenic plants overexpressing HD2D and hd2d mutant plants flowered earlier compared with wild-type in both long day and short day conditions. Expression of several floral identity genes was altered in these plants. Taken together, our findings suggest that HD2D is a negative regulator of flowering that modulates the transition of vegetative to reproductive growth in a photoperiod independent manner.

Arabidopsis stress associated protein 9 mediates biotic and abiotic stress responsive ABA signaling via the proteasome pathway

Arabidopsis thaliana Stress Associated Protein 9 (AtSAP9) is a member of the A20/AN1 zinc finger protein family known to play important roles in plant stress responses and in the mammalian immune response. Although SAPs of several plant species were shown to be involved in abiotic stress responses, the underlying molecular mechanisms are largely unknown, and little is known about the involvement of SAPs in plant disease responses. Expression of SAP9 in Arabidopsis is up-regulated in response to dehydration, cold, salinity and abscisic acid (ABA), as well as pathogen infection. Constitutive expression of AtSAP9 in Arabidopsis leads to increased sensitivity to ABA and osmotic stress during germination and post-germinative development. Plants that overexpress AtSAP9 also showed increased susceptibility to infection by non-host pathogen Pseudomonas syringae pv. phaseolicola, indicating a potential role of AtSAP9 in disease resistance. AtSAP9 was found to interact with RADIATION SENSITIVE23d (Rad23d), a shuttle factor for the transport of ubiquitinated substrates to the proteasome, and it is co-localized with Rad23d in the nucleus. Thus, AtSAP9 may promote the protein degradation process by mediating the interaction of ubiquitinated targets with Rad23d. Taken together, these results indicate that AtSAP9 regulates abiotic and biotic stress responses, possibly via the ubiquitination/proteasome pathway.

Functional characterization of Brassica napus DNA topoisomerase Iα-1 and its effect on flowering time when expressed in Arabidopsis thaliana

Previous studies have shown that DNA topoisomerase Iα (AtTOP1α) has specific developmental functions during growth and development in Arabidopsis thaliana. However, little is known about the roles of DNA topoisomerases in the closely related and commercially important plant, rapeseed (Brassica napus). Here, the full-length BnTOP1α-1 coding sequence was cloned from the A2 subgenome of the Brassica napus inbred line L111. We determine that all BnTOP1α paralogs showed differing patterns of expression in different organs of L111, and that when expressed in tobacco leaves as a fusion protein with green fluorescent protein, BnTOP1α-1 localized to the nucleus. We further showed that ectopic expression of BnTOP1α-1 in the A. thaliana top1α-7 mutant fully complemented the early flowering phenotype of the mutant. Moreover, altered expression levels in top1α-7 seedlings of several key genes controlling flowering time were restored to wild type levels by ectopic expression of BnTOP1α-1. These results provide valuable insights into the roles of rapeseed DNA topoisomerases in flowering time, and provide a promising target for genetic manipulation of this commercially significant process in rapeseed.

State of the Art: trxG Factor Regulation of Post-embryonic Plant Development

Multicellular organisms rely on the precise and consistent regulation of gene expression to direct their development in tissue- and cell-type specific patterns. This regulatory activity involves arrays of DNA-binding transcription factors and epigenetic factors that modify chromatin structure. Among the chromatin modifiers, trithorax (trxG) and Polycomb (PcG) group proteins play important roles in orchestrating the stable activation and repression of gene expression, respectively. These proteins have generally antagonistic functions in maintaining cell and tissue homeostasis as well as in mediating widespread transcriptional reprogramming during developmental transitions. Plants utilize multiple trxG factors to regulate gene transcription as they modulate their development in response to both endogenous and environmental cues. Here, I will discuss the roles of trxG factors and their associated proteins in post-embryonic plant development.

The Role of Long Noncoding RNAs in Plant Stress Tolerance

Plants must adapt to multiple biotic and abiotic stresses ; thus, sensing and responding to environmental signals is imperative for their survival. Moreover, understanding these responses is imperative for efforts to improve plant yield and consistency. Regulation of transcript levels is a key aspect of the plant response to environmental signals. Long noncoding RNAs (lncRNAs) have gained widespread attention in recent years with the advance of high-throughput sequencing technologies. As important biological regulators, lncRNAs have been implicated in a wide range of developmental processes and diseases in animals. However, knowledge of the role that lncRNAs play in plant stress tolerance remains limited. Here, we review recent studies on the identification, characteristics, classification, and biological functions of lncRNAs in response to various stresses, including bacterial pathogens, excess light, drought, salinity, hypoxia, extreme temperatures, and nitrogen/phosphate deficiency. We also discuss possible directions for future research.

APRF1 promotes flowering under long days in Arabidopsis thaliana

Arabidopsis thaliana flowering time mutants revealed the function of numerous genes that regulate the transition from vegetative to reproductive growth. Analyses of their loci have shown that many of them act as chromatin modifiers. In this study, a combination of molecular and genetic approaches have been implemented, to characterize the function of APRF1 (ANTHESIS POMOTING FACTOR 1) gene in A. thaliana and to investigate its role in plant development. APRF1 encodes for a low molecular weight nuclear WDR protein which displays functional homology to the Swd2 protein, an essential subunit of the yeast histone methylation COMPASS complex. Compared to WT plants, total loss-of-function aprf1 mutants exhibited shoot apical meristem (SAM) alterations and increased growth rates. However, the vegetative phase of aprf1 plants was prolonged and bolting was delayed, indicating an impairment in flowering under long days (LD). On the contrary, overexpression of APRF1 accelerates flowering. Consistent with the late flowering phenotype, the molecular data confirmed that FLC and SOC1 expression were significantly altered in the aprf1 mutants. Our data suggest that APRF1 acts upstream of FLC and promotes flowering under LD.

Chloroplast retrograde signal regulates flowering

Light is a major environmental factor regulating flowering time, thus ensuring reproductive success of higher plants. In contrast to our detailed understanding of light quality and photoperiod mechanisms involved, the molecular basis underlying high light-promoted flowering remains elusive. Here we show that, in Arabidopsis, a chloroplast-derived signal is critical for high light-regulated flowering mediated by the FLOWERING LOCUS C (FLC). We also demonstrate that PTM, a PHD transcription factor involved in chloroplast retrograde signaling, perceives such a signal and mediates transcriptional repression of FLC through recruitment of FVE, a component of the histone deacetylase complex. Thus, our data suggest that chloroplasts function as essential sensors of high light to regulate flowering and adaptive responses by triggering nuclear transcriptional changes at the chromatin level.

Histone variants in plant transcriptional regulation

Chromatin based organization of eukaryotic genome plays a profound role in regulating gene transcription. Nucleosomes form the basic subunits of chromatin by packaging DNA with histone proteins, impeding the access of DNA to transcription factors and RNA polymerases. Exchange of histone variants in nucleosomes alters the properties of nucleosomes and thus modulates DNA exposure during transcriptional regulation. Growing evidence indicates the important function of histone variants in programming transcription during developmental transitions and stress response. Here we review how histone variants and their deposition machineries regulate the nucleosome stability and dynamics, and discuss the link between histone variants and transcriptional regulation in plants. This article is part of a Special Issue entitled: Plant Gene Regulatory Mechanisms and Networks, edited by Dr. Erich Grotewold and Dr. Nathan Springer.

Structure and function of histone methylation-binding proteins in plants

Post-translational modifications of histones play important roles in modulating many essential biological processes in both animals and plants. These covalent modifications, including methylation, acetylation, phosphorylation, ubiquitination, SUMOylation and so on, are laid out and erased by histone-modifying enzymes and read out by effector proteins. Recent studies have revealed that a number of developmental processes in plants are under the control of histone post-translational modifications, such as floral transition, seed germination, organogenesis and morphogenesis. Therefore, it is critical to identify those protein domains, which could specifically recognize these post-translational modifications to modulate chromatin structure and regulate gene expression. In the present review, we discuss the recent progress in understanding the structure and function of the histone methylation readers in plants, by focusing on Arabidopsis thaliana proteins.

MtVRN2 is a Polycomb VRN2-like gene which represses the transition to flowering in the model legume Medicago truncatula

Optimising the timing of flowering contributes to successful sexual reproduction and yield in agricultural plants. FLOWERING LOCUS T (FT) genes, first identified in Arabidopsis thaliana (Arabidopsis), promote flowering universally, but the upstream flowering regulatory pathways can differ markedly among plants. Flowering in the model legume, Medicago truncatula (Medicago) is accelerated by winter cold (vernalisation) followed by long day (LD) photoperiods leading to elevated expression of the floral activator, FT-like gene FTa1. However, Medicago, like some other plants, lacks the activator CONSTANS (CO) and the repressor FLOWERING LOCUS C (FLC) genes which directly regulate FT and are key to LD and vernalisation responses in Arabidopsis. Conversely, Medicago has a VERNALISATION2-LIKE VEFS-box gene (MtVRN2). In Arabidopsis AtVRN2 is a key member of a Polycomb complex involved in stable repression of Arabidopsis FLC after vernalisation. VRN2-like genes have been identified in other eudicot plants, but their function has never been reported. We show that Mtvrn2 mutants bypass the need for vernalisation for early flowering in LD conditions in Medicago. Investigation of the underlying mechanism by transcriptome analysis reveals that Mtvrn2 mutants precociously express FTa1 and other suites of genes including floral homeotic genes. Double-mutant analysis indicates that early flowering is dependent on functional FTa1. The broad significance of our study is that we have demonstrated a function for a VRN2-like VEFS gene beyond the Brassicaceae. In particular, MtVRN2 represses the transition to flowering in Medicago by regulating the onset of expression of the potent floral activator, FTa1.

Molecular regulation of phenology in trees-because the seasons they are a-changin'

The perennial trees, in contrast to the much more studied annual plants, have to adapt their vegetative growth and development to the sometimes extremely contrasting environmental conditions that occur over the different seasons. Recently, studies of the molecular framework underlying this adaptation in Populus trees is reinforcing the notion that the genetic pathways controlling growth and dormancy cycles have a remarkable conservation with the pathways controlling the regulation of flowering time in annual plants. Insight into these mechanisms will be important for our understanding of how trees will respond to various future global climate scenarios.

Chromatin Immunoprecipitation Assay for the Identification of Arabidopsis Protein-DNA Interactions In Vivo

Intricate gene regulatory networks orchestrate biological processes and developmental transitions in plants. Selective transcriptional activation and silencing of genes mediate the response of plants to environmental signals and developmental cues. Therefore, insights into the mechanisms that control plant gene expression are essential to gain a deep understanding of how biological processes are regulated in plants. The chromatin immunoprecipitation (ChIP) technique described here is a procedure to identify the DNA-binding sites of proteins in genes or genomic regions of the model species Arabidopsis thaliana. The interactions with DNA of proteins of interest such as transcription factors, chromatin proteins or posttranslationally modified versions of histones can be efficiently analyzed with the ChIP protocol. This method is based on the fixation of protein-DNA interactions in vivo, random fragmentation of chromatin, immunoprecipitation of protein-DNA complexes with specific antibodies, and quantification of the DNA associated with the protein of interest by PCR techniques. The use of this methodology in Arabidopsis has contributed significantly to unveil transcriptional regulatory mechanisms that control a variety of plant biological processes. This approach allowed the identification of the binding sites of the Arabidopsis chromatin protein EBS to regulatory regions of the master gene of flowering FT. The impact of this protein in the accumulation of particular histone marks in the genomic region of FT was also revealed through ChIP analysis.

ABSCISIC ACID-INSENSITIVE 4 negatively regulates flowering through directly promoting Arabidopsis FLOWERING LOCUS C transcription

During the life cycle of a plant, one of the major biological processes is the transition from the vegetative to the reproductive stage. In Arabidopsis, flowering time is precisely controlled by extensive environmental and internal cues. Gibberellins (GAs) promote flowering, while abscisic acid (ABA) is considered as a flowering suppressor. However, the detailed mechanism through which ABA inhibits the floral transition is poorly understood. Here, we report that ABSCISIC ACID-INSENSITIVE 4 (ABI4), a key component in the ABA signalling pathway, negatively regulates floral transition by directly promoting FLOWERING LOCUS C (FLC) transcription. The abi4 mutant showed the early flowering phenotype whereas ABI4-overexpressing (OE-ABI4) plants had delayed floral transition. Consistently, quantitative reverse transcription-PCR (qRT-PCR) assay revealed that the FLC transcription level was down-regulated in abi4, but up-regulated in OE-ABI4. The change in FT level was consistent with the pattern of FLC expression. Chromatin immunoprecipitation-qPCR (ChIP-qPCR), electrophoretic mobility shift assay (EMSA), and tobacco transient expression analysis showed that ABI4 promotes FLC expression by directly binding to its promoter. Genetic analysis demonstrated that OE-ABI4::flc-3 could not alter the flc-3 phenotype. OE-FLC::abi4 showed a markedly delayed flowering phenotype, which mimicked OE-FLC::WT, and suggested that ABI4 acts upstream of FLC in the same genetic pathway. Taken together, these findings suggest that ABA inhibits the floral transition by activating FLC transcription through ABI4.

MYBD employed by HY5 increases anthocyanin accumulation via repression of MYBL2 in Arabidopsis

Photomorphogenesis is an essential program in plant development. This process is effected by the balanced cooperation of many factors under light and dark conditions. In a previous study, we showed that MYB hypocotyl elongation-related (MYBH) is involved in cell elongation. To expand our understanding of MYBH function, we performed a yeast two-hybrid assay and identified an MYB-like Domain transcription factor (MYBD). In this study, we investigated the function of MYBD, which is an MYBH homolog involved in anthocyanin accumulation. MYBD expression increased in response to light or cytokinin, and MYBD enhanced anthocyanin biosynthesis via repression of MYBL2, which encodes a transcription factor that has a negative effect on this process. In addition, MYBD binding in vivo to the MYBL2 promoter and the lower level of histone H3K9 acetylation at the upstream region of MYBL2 in MYBD over-expressing plants in comparison with wild-type plants imply that MYBD represses MYBL2 expression via an epigenetic mechanism. HY5 directly binds to the MYBD promoter, which indicates that MYBD acts on HY5-downstream in light- or cytokinin-triggered signaling pathways, leading to anthocyanin accumulation. Our results suggest that, although MYBD and MYBH are homologs, they act in opposite ways during plant photomorphogenesis, and these functions should be examined in further studies.

CONSTITUTIVE PHOTOMORPHOGENIC 10 (COP10) Contributes to Floral Repression under Non-Inductive Short Days in Arabidopsis

In Arabidopsis, CONSTITUTIVE PHOTOMORPHOGENIC/DE-ETIOLATED/FUSCA (COP/DET/FUS) genes act in repression of photomorphogenesis in darkness, and recent reports revealed that some of these genes, such as COP1 and DET1, also have important roles in controlling flowering time and circadian rhythm. The COP/DET/FUS protein COP10 interacts with DET1 and DNA DAMAGE-BINDING PROTEIN 1 (DDB1) to form a CDD complex and represses photomorphogenesis in darkness. The cop10-4 mutants flower normally in inductive long days (LD) but early in non-inductive short days (SD) compared with wild type (WT); however, the role of COP10 remains unknown. Here, we investigate the role of COP10 in SD-dependent floral repression. Reverse transcription-quantitative PCR revealed that in SD, expression of the LD-dependent floral inducers GI, FKF1, and FT significantly increased in cop10-4 mutants, compared with WT. This suggests that COP10 mainly regulates FT expression in a CO-independent manner. We also show that COP10 interacts with GI in vitro and in vivo, suggesting that COP10 could also affect GI function at the posttranslational level. Moreover, FLC expression was repressed drastically in cop10-4 mutants and COP10 interacts with MULTICOPY SUPPRESSOR OF IRA1 4 (MSI4)/FVE (MSI4/FVE), which epigenetically inhibits FLC expression. These data suggest that COP10 contributes to delaying flowering in the photoperiod and autonomous pathways by downregulating FT expression under SD.

MORF-RELATED GENE702, a Reader Protein of Trimethylated Histone H3 Lysine 4 and Histone H3 Lysine 36, Is Involved in Brassinosteroid-Regulated Growth and Flowering Time Control in Rice

The methylation of histone H3 lysine 36 (H3K36) plays critical roles in brassinosteroid (BR)-related processes and is involved in controlling flowering time in rice (Oryza sativa). Although enzymes that catalyze this methylation reaction have been described, little is known about the recognition mechanisms to decipher H3K36 methylation information in rice. In this study, biochemical characterizations showed that MORF-RELATED GENE702 (MRG702) binds to trimethylated H3K4 and H3K36 (H3K4me3 and H3K36me3) in vitro. Similar to the loss-of-function mutants of the rice H3K36 methyltransferase gene SET DOMAIN GROUP725 (SDG725), the MRG702 knockdown mutants displayed typical BR-deficient mutant and late-flowering phenotypes. Gene transcription analyses showed that MRG702 knockdown resulted in the down-regulation of BR-related genes, including DWARF11, BRASSINOSTEROD INSENSITIVE1, and BRASSINOSTEROID UPREGULATED1, and several flowering genes, including Early heading date1 (Ehd1), Ehd2, Ehd3, OsMADS50, Heading date 3a, and RICE FLOWERING LOCUS T1. A binding analysis showed that MRG702 directly binds to the chromatin at target gene loci. This binding is dependent on the level of trimethylated H3K36, which is mediated by SDG725. Together, our results demonstrate that MRG702 acts as a reader protein of H3K4me3 and H3K36me3 and deciphers the H3K36 methylation information set by SDG725. Therefore, the role of MRG702 in the BR pathway and in controlling flowering time in rice is to function as a reader protein to decipher methylation information.

Keeping the gate closed: functions of the polycomb repressive complex PRC2 in development

Plant ontogeny relies on the correct timing and sequence of transitions between individual developmental phases. These are specified by gene expression patterns that are established by the balanced action of activators and repressors. Polycomb repressive complexes (PRCs) represent an evolutionarily conserved system of epigenetic gene repression that governs the establishment and maintenance of cell, tissue and organ identity, contributing to the correct execution of the developmental programs. PRC2 is a four-subunit histone methyltransferase complex that catalyzes trimethylation of lysine 27 on histone H3 (H3K27me3), which contributes to the change of chromatin structure and long-lasting gene repression. Here, we review the composition and molecular function of the different known PRC2 complexes in plants, and focus on the role of PRC2 in mediating the establishment of different developmental phases and transitions between them.

H2A.Z mediates different aspects of chromatin function and modulates flowering responses in Arabidopsis

Eukaryotic organisms have canonical histones and a number of histone variants that perform specialized functions and confer particular structural properties to the nucleosomes that contain them. The histone H2A family comprises several variants, with H2A.Z being the most evolutionarily conserved. This variant is essential in eukaryotes and has emerged as a key player in chromatin function, performing an essential role in gene transcription and genome stability. During recent years, biochemical, genetic and genomic studies have begun to uncover the role of several ATP-dependent chromatin-remodeling complexes in H2A.Z deposition and removal. These ATPase complexes are widely conserved from yeast to mammals. In Arabidopsis there are homologs for most of the subunits of these complexes, and their functions are just beginning to be unveiled. In this review, we discuss the major contributions made in relation to the biology of the H2A.Z in plants, and more specifically concerning the function of this histone variant in the transition from vegetative to reproductive development. Recent advances in the understanding of the molecular mechanisms underlying the H2A.Z-mediated modulation of the floral transition, and thermosensory flowering responses in particular, are discussed. The emerging picture shows that plants contain chromatin-remodeling complexes related to those involved in modulating the dynamics of H2A.Z in other eukaryotes, but their precise biochemical nature remains elusive.

Functions of plants long non-coding RNAs

Long non-coding RNAs (lncRNAs) have been emerged as important players for various biological pathways, including dosage compensation, genomic imprinting, chromatin regulation, alternative splicing and nuclear organization. A large number of lncRNAs had already been identified by different approaches in plants, while the functions of only a few of them have been investigated. This review will summarize our current understanding of a wide range of plant lncRNAs functions, and highlight their roles in the regulation of diverse pathways in plants. This article is part of a Special Issue entitled: Clues to long noncoding RNA taxonomy1, edited by Dr. Tetsuro Hirose and Dr. Shinichi Nakagawa.

The chromatin-remodeling factor AtINO80 plays crucial roles in genome stability maintenance and in plant development

INO80 is a conserved chromatin-remodeling factor in eukaryotes. While a previous study reported that the Arabidopsis thaliana INO80 (AtINO80) is required for somatic homologous recombination (HR), the role of AtINO80 in plant growth and development remains obscure. Here, we identified and characterized two independent atino80 mutant alleles, atino80-5 and atino80-6, which display similar and pleiotropic phenotypes, including smaller plant and organ size, and late flowering. Under standard growth conditions, atino80-5 showed decreased HR; however, after genotoxic treatment, HR in the mutant increased, accompanied by more DNA double-strand breaks and stronger cellular responses. Transcription analysis showed that many developmental and environmental responsive genes are overrepresented in the perturbed genes in atino80-5. These genes significantly overlapped with the category of H2A.Z body-enriched genes. AtINO80 also interacts with H2A.Z, and facilitates the enrichment of H2A.Z at the ends of the key flowering repressor genes FLC and MAF4/5. Our characterization of the atino80-5 and atino80-6 mutants confirms and extends the previous AtINO80 study, and provides perspectives for linking studies of epigenetic mechanisms involved in plant chromatin stability with plant response to developmental and environmental cues.

Long non-coding RNAs and their biological roles in plants

With the development of genomics and bioinformatics, especially the extensive applications of high-throughput sequencing technology, more transcriptional units with little or no protein-coding potential have been discovered. Such RNA molecules are called non-protein-coding RNAs (npcRNAs or ncRNAs). Among them, long npcRNAs or ncRNAs (lnpcRNAs or lncRNAs) represent diverse classes of transcripts longer than 200 nucleotides. In recent years, the lncRNAs have been considered as important regulators in many essential biological processes. In plants, although a large number of lncRNA transcripts have been predicted and identified in few species, our current knowledge of their biological functions is still limited. Here, we have summarized recent studies on their identification, characteristics, classification, bioinformatics, resources, and current exploration of their biological functions in plants.

Long noncoding RNA transcriptome of plants

Since their discovery more than two decades ago, animal long noncoding RNAs (lncRNAs) have emerged as important regulators of many biological processes. Recently, a large number of lncRNAs have also been identified in higher plants, and here, we review their identification, classification and known regulatory functions in various developmental events and stress responses. Knowledge gained from a deeper understanding of this special group of noncoding RNAs may lead to biotechnological improvement of crops. Some possible examples in this direction are discussed.

The Arabidopsis SWI2/SNF2 chromatin Remodeler BRAHMA regulates polycomb function during vegetative development and directly activates the flowering repressor gene SVP

The chromatin remodeler BRAHMA (BRM) is a Trithorax Group (TrxG) protein that antagonizes the functions of Polycomb Group (PcG) proteins in fly and mammals. Recent studies also implicate such a role for Arabidopsis (Arabidopsis thaliana) BRM but the molecular mechanisms underlying the antagonism are unclear. To understand the interplay between BRM and PcG during plant development, we performed a genome-wide analysis of trimethylated histone H3 lysine 27 (H3K27me3) in brm mutant seedlings by chromatin immunoprecipitation followed by next generation sequencing (ChIP-seq). Increased H3K27me3 deposition at several hundred genes was observed in brm mutants and this increase was partially supressed by removal of the H3K27 methyltransferase CURLY LEAF (CLF) or SWINGER (SWN). ChIP experiments demonstrated that BRM directly binds to a subset of the genes and prevents the inappropriate association and/or activity of PcG proteins at these loci. Together, these results indicate a crucial role of BRM in restricting the inappropriate activity of PcG during plant development. The key flowering repressor gene SHORT VEGETATIVE PHASE (SVP) is such a BRM target. In brm mutants, elevated PcG occupancy at SVP accompanies a dramatic increase in H3K27me3 levels at this locus and a concomitant reduction of SVP expression. Further, our gain- and loss-of-function genetic evidence establishes that BRM controls flowering time by directly activating SVP expression. This work reveals a genome-wide functional interplay between BRM and PcG and provides new insights into the impacts of these proteins in plant growth and development.

Photoperiodic flowering: time measurement mechanisms in leaves

Many plants use information about changing day length (photoperiod) to align their flowering time with seasonal changes to increase reproductive success. A mechanism for photoperiodic time measurement is present in leaves, and the day-length-specific induction of the FLOWERING LOCUS T (FT) gene, which encodes florigen, is a major final output of the pathway. Here, we summarize the current understanding of the molecular mechanisms by which photoperiodic information is perceived in order to trigger FT expression in Arabidopsis as well as in the primary cereals wheat, barley, and rice. In these plants, the differences in photoperiod are measured by interactions between circadian-clock-regulated components, such as CONSTANS (CO), and light signaling. The interactions happen under certain day-length conditions, as previously predicted by the external coincidence model. In these plants, the coincidence mechanisms are governed by multilayered regulation with numerous conserved as well as unique regulatory components, highlighting the breadth of photoperiodic regulation across plant species.

PHDs govern plant development

Posttranslational modifications present in the amino-terminal tails of histones play a pivotal role in the chromatin-mediated regulation of gene expression patterns that control plant developmental transitions. Therefore, the function of protein domains that specifically recognize these histone covalent modifications and recruit chromatin remodeling complexes and the transcriptional machinery to modulate gene expression is essential for a proper control of plant development. Plant HomeoDomain (PHD) motifs act as effectors that can specifically bind a number of histone modifications and mediate the activation or repression of underlying genes. In this review we summarize recent findings that emphasize the crucial role of this versatile family of chromatin "reader" domains in the transcriptional regulation of plant developmental processes such as meiosis and postmeiotic events during pollen maturation, embryo meristem initiation and root development, germination as well as flowering time.

GmFLD, a soybean homolog of the autonomous pathway gene FLOWERING LOCUS D, promotes flowering in Arabidopsis thaliana

BACKGROUND

Flowering at an appropriate time is crucial for seed maturity and reproductive success in all flowering plants. Soybean (Glycine max) is a typical short day plant, and both photoperiod and autonomous pathway genes exist in soybean genome. However, little is known about the functions of soybean autonomous pathway genes. In this article, we examined the functions of a soybean homolog of the autonomous pathway gene FLOWERING LOCUS D (FLD), GmFLD in the flowering transition of A. thaliana.

RESULTS

In soybean, GmFLD is highly expressed in expanded cotyledons of seedlings, roots, and young pods. However, the expression levels are low in leaves and shoot apexes. Expression of GmFLD in A. thaliana (Col) resulted in early flowering of the transgenic plants, and rescued the late flowering phenotype of the A. thaliana fld mutant. In GmFLD transgenic plants (Col or fld background), the FLC (FLOWERING LOCUS C) transcript levels decreased whereas the floral integrators, FT and SOC1, were up-regulated when compared with the corresponding non-transgenic genotypes. Furthermore, chromatin immuno-precipitation analysis showed that in the transgenic rescued lines (fld background), the levels of both tri-methylation of histone H3 Lys-4 and acetylation of H4 decreased significantly around the transcriptional start site of FLC. This is consistent with the function of GmFLD as a histone demethylase.

CONCLUSIONS

Our results suggest that GmFLD is a functional ortholog of the Arabidopsis FLD and may play an important role in the regulation of chromatin state in soybean. The present data provides the first evidence for the evolutionary conservation of the components in the autonomous pathway in soybean.

Chromatin-dependent repression of the Arabidopsis floral integrator genes involves plant specific PHD-containing proteins

The interplay among histone modifications modulates the expression of master regulatory genes in development. Chromatin effector proteins bind histone modifications and translate the epigenetic status into gene expression patterns that control development. Here, we show that two Arabidopsis thaliana paralogs encoding plant-specific proteins with a plant homeodomain (PHD) motif, SHORT LIFE (SHL) and EARLY BOLTING IN SHORT DAYS (EBS), function in the chromatin-mediated repression of floral initiation and play independent roles in the control of genes regulating flowering. Previous results showed that repression of the floral integrator FLOWERING LOCUS T (FT) requires EBS. We establish that SHL is necessary to negatively regulate the expression of SUPPRESSOR OF OVEREXPRESSION OF CO1 (SOC1), another floral integrator. SHL and EBS recognize di- and trimethylated histone H3 at lysine 4 and bind regulatory regions of SOC1 and FT, respectively. These PHD proteins maintain an inactive chromatin conformation in SOC1 and FT by preventing high levels of H3 acetylation, bind HISTONE DEACETYLASE6, and play a central role in regulating flowering time. SHL and EBS are widely conserved in plants but are absent in other eukaryotes, suggesting that the regulatory module mediated by these proteins could represent a distinct mechanism for gene expression control in plants.

Regulation of arabidopsis flowering by the histone mark readers MRG1/2 via interaction with CONSTANS to modulate FT expression

Day-length is important for regulating the transition to reproductive development (flowering) in plants. In the model plant Arabidopsis thaliana, the transcription factor CONSTANS (CO) promotes expression of the florigen FLOWERING LOCUS T (FT), constituting a key flowering pathway under long-day photoperiods. Recent studies have revealed that FT expression is regulated by changes of histone modification marks of the FT chromatin, but the epigenetic regulators that directly interact with the CO protein have not been identified. Here, we show that the Arabidopsis Morf Related Gene (MRG) group proteins MRG1 and MRG2 act as H3K4me3/H3K36me3 readers and physically interact with CO to activate FT expression. In vitro binding analyses indicated that the chromodomains of MRG1 and MRG2 preferentially bind H3K4me3/H3K36me3 peptides. The mrg1 mrg2 double mutant exhibits reduced mRNA levels of FT, but not of CO, and shows a late-flowering phenotype under the long-day but not short-day photoperiod growth conditions. MRG2 associates with the chromatin of FT promoter in a way dependent of both CO and H3K4me3/H3K36me3. Vice versa, loss of MRG1 and MRG2 also impairs CO binding at the FT promoter. Crystal structure analyses of MRG2 bound with H3K4me3/H3K36me3 peptides together with mutagenesis analysis in planta further demonstrated that MRG2 function relies on its H3K4me3/H3K36me3-binding activity. Collectively, our results unravel a novel chromatin regulatory mechanism, linking functions of MRG1 and MRG2 proteins, H3K4/H3K36 methylations, and CO in FT activation in the photoperiodic regulation of flowering time in plants.

The photoperiodic flowering in Arabidopsis requires the key regulator CO and its target gene FT. However, how CO regulates FT expression in the context of chromatin remains largely obscure. In this work, we present Arabidopsis MRG1/2 as novel chromatin effectors directly involved in the CO-FT photoperiodic flowering. Firstly, MRG1/2 proteins are identified as recognition factors of H3K4 and H3K36 methylation via their chromodomains. The mrg1 mrg2 double mutant shows a late-flowering phenotype only under long-day conditions through down-regulation of FT but not of CO. MRG2 can directly target in vivo the FT promoter chromatin in a H3K4me3/H3K36me3-level dependent manner. More importantly, MRG2 and CO physically interact and enhance each other's binding to the FT promoter in planta. Determination of co-crystal structures of MRG2 with H3K4me3/H3K36me3 peptides and mutagenesis of a key amino acid residue involved in structural interaction demonstrate that MRG2 reader activity is essential for in planta function. Taken together, our findings uncover a novel mechanism of FT activation in flowering promotion and provide a striking example of mutual interplay between a transcription factor and a histone methylation reader in transcription regulation.

Arabidopsis RRP6L1 and RRP6L2 function in FLOWERING LOCUS C silencing via regulation of antisense RNA synthesis

The exosome complex functions in RNA metabolism and transcriptional gene silencing. Here, we report that mutations of two Arabidopsis genes encoding nuclear exosome components AtRRP6L1 and AtRRP6L2, cause de-repression of the main flowering repressor FLOWERING LOCUS C (FLC) and thus delay flowering in early-flowering Arabidopsis ecotypes. AtRRP6L mutations affect the expression of known FLC regulatory antisense (AS) RNAs AS I and II, and cause an increase in Histone3 K4 trimethylation (H3K4me3) at FLC. AtRRP6L1 and AtRRP6L2 function redundantly in regulation of FLC and also act independently of the exosome core complex. Moreover, we discovered a novel, long non-coding, non-polyadenylated antisense transcript (ASL, for Antisense Long) originating from the FLC locus in wild type plants. The AtRRP6L proteins function as the main regulators of ASL synthesis, as these mutants show little or no ASL transcript. Unlike ASI/II, ASL associates with H3K27me3 regions of FLC, suggesting that it could function in the maintenance of H3K27 trimethylation during vegetative growth. AtRRP6L mutations also affect H3K27me3 levels and nucleosome density at the FLC locus. Furthermore, AtRRP6L1 physically associates with the ASL transcript and directly interacts with the FLC locus. We propose that AtRRP6L proteins participate in the maintenance of H3K27me3 at FLC via regulating ASL. Furthermore, AtRRP6Ls might participate in multiple FLC silencing pathways by regulating diverse antisense RNAs derived from the FLC locus.

Arabidopsis MRG domain proteins bridge two histone modifications to elevate expression of flowering genes

Trimethylation of lysine 36 of histone H3 (H3K36me3) is found to be associated with various transcription events. In Arabidopsis, the H3K36me3 level peaks in the first half of coding regions, which is in contrast to the 3'-end enrichment in animals. The MRG15 family proteins function as 'reader' proteins by binding to H3K36me3 to control alternative splicing or prevent spurious intragenic transcription in animals. Here, we demonstrate that two closely related Arabidopsis homologues (MRG1 and MRG2) are localised to the euchromatin and redundantly ensure the increased transcriptional levels of two flowering time genes with opposing functions, FLOWERING LOCUS C and FLOWERING LOCUS T (FT). MRG2 directly binds to the FT locus and elevates the expression in an H3K36me3-dependent manner. MRG1/2 binds to H3K36me3 with their chromodomain and interact with the histone H4-specific acetyltransferases (HAM1 and HAM2) to achieve a high expression level through active histone acetylation at the promoter and 5' regions of target loci. Together, this study presents a mechanistic link between H3K36me3 and histone H4 acetylation. Our data also indicate that the biological functions of MRG1/2 have diversified from their animal homologues during evolution, yet they still maintain their conserved H3K36me3-binding molecular function.

Research on floral timing by ambient temperature comes into blossom

The floral transition is an essential process in the life cycle of flower-bearing plants, because their reproductive success depends on it. To determine the right moment of flowering, plants respond to many environmental signals, including day length, light quality, and temperature. Small changes in ambient temperature also affect the flowering process, although our knowledge of the genetic and molecular mechanisms underlying this flowering pathway is limited. However, recent advances in Arabidopsis (Arabidopsis thaliana) have uncovered multiple molecular mechanisms controlling ambient temperature regulation of flowering, which modulate both repressing and activating factors of flowering time. At a time when temperatures are rising worldwide, understanding how plants integrate ambient temperature signals can be crucial for crop production.

VP16 fusion induces the multiple-knockout phenotype of redundant transcriptional repressors partly by Med25-independent mechanisms in Arabidopsis

Biological functions of only some plant transcriptional repressors are known owing to the lack of knockout lines or unclear phenotypes because of redundancy. Here we show that strong viral activation domain VP16 fusion to the transcriptional repressor FLOWERING LOCUS C reversed its function and caused a stronger phenotype than that of the multiple-knockout line of redundant genes, suggesting the potential of this technique to identify transcription factor function that cannot be detected in a single-knockout line. Loss-of-function of transcriptional coactivator Mediator25 did not affect VP16 activity despite their in vivo interaction, suggesting the existence of other key mechanism(s) in plants.

Beyond ubiquitination: proteolytic and nonproteolytic roles of HOS1

The E3 ubiquitin ligase HIGH EXPRESSION OF OSMOTICALLY RESPONSIVE GENES 1 (HOS1) functions as a cold signaling attenuator by degrading the INDUCER OF CBF EXPRESSION 1 transcription factor, which is a key regulator of the cold-induced transcriptome and freezing tolerance in plants. Recent studies demonstrate that HOS1 also plays nonproteolytic roles in gene expression regulation. HOS1 acts as a chromatin remodeling factor that modulates FLOWERING LOCUS C chromatin in cold regulation of flowering time. It associates with the nuclear pore complex to facilitate nucleocytoplasmic mRNA export to maintain circadian periodicity over a range of light and temperature conditions. In this review, we summarize recent advances in molecular mechanisms underlying HOS1 function during plant development in response to fluctuating environmental conditions.

Unique and conserved features of floral evocation in legumes

Legumes, with their unique ability to fix atmospheric nitrogen, play a vital role in ensuring future food security and mitigating the effects of climate change because they use less fossil energy and produce less greenhouse gases compared with N-fertilized systems. Grain legumes are second only to cereal crops as a source of human and animal food, and they contribute approximately one third of the protein consumed by the human population. The productivity of seed crops, such as grain legumes, is dependent on flowering. Despite the genetic variation and importance of flowering in legume production, studies of the molecular pathways that control flowering in legumes are limited. Recent advances in genomics have revealed that legume flowering pathways are divergent from those of such model species as Arabidopsis thaliana. Here, we discuss the current understanding of flowering time regulation in legumes and highlight the unique and conserved features of floral evocation in legumes.

Quantitative trait loci affecting reproductive phenology in peach

BACKGROUND

The reproductive phenology of perennial plants in temperate climates is largely conditioned by the duration of bud dormancy, and fruit developmental processes. Bud dormancy release and bud break depends on the perception of cumulative chilling and heat during the bud development. The objective of this work was to identify new quantitative trait loci (QTLs) associated to temperature requirements for bud dormancy release and flowering and to fruit harvest date, in a segregating population of peach.

RESULTS

We have identified QTLs for nine traits related to bud dormancy, flowering and fruit harvest in an intraspecific hybrid population of peach in two locations differing in chilling time accumulation. QTLs were located in a genetic linkage map of peach based on single nucleotide polymorphism (SNP) markers for eight linkage groups (LGs) of the peach genome sequence. QTLs for chilling requirements for dormancy release and blooming clustered in seven different genomic regions that partially coincided with loci identified in previous works. The most significant QTL for chilling requirements mapped to LG1, close to the evergrowing locus. QTLs for heat requirement related traits were distributed in nine genomic regions, four of them co-localizing with QTLs for chilling requirement trait. Two major loci in LG4 and LG6 determined fruit harvest time.

CONCLUSIONS

We identified QTLs associated to nine traits related to the reproductive phenology in peach. A search of candidate genes for these QTLs rendered different genes related to flowering regulation, chromatin modification and hormone signalling. A better understanding of the genetic factors affecting crop phenology might help scientists and breeders to predict changes in genotype performance in a context of global climate change.