DREAM complex suppresses DNA methylation maintenance genes and precludes DNA hypermethylation

Dynamic evolution of the heterochromatin sensing histone demethylase IBM1

Heterochromatin is critical for maintaining genome stability, especially in flowering plants, where it relies on a feedback loop involving the H3K9 methyltransferase, KRYPTONITE (KYP), and the DNA methyltransferase CHROMOMETHYLASE3 (CMT3). The H3K9 demethylase INCREASED IN BONSAI METHYLATION 1 (IBM1) counteracts the detrimental consequences of KYP-CMT3 activity in transcribed genes. IBM1 expression in Arabidopsis is uniquely regulated by methylation of the 7th intron, allowing it to monitor global H3K9me2 levels. We show the methylated intron is prevalent across flowering plants and its underlying sequence exhibits dynamic evolution. We also find extensive genetic and expression variations in KYP, CMT3, and IBM1 across flowering plants. We identify Arabidopsis accessions resembling weak ibm1 mutants and Brassicaceae species with reduced IBM1 expression or deletions. Evolution towards reduced IBM1 activity in some flowering plants could explain the frequent natural occurrence of diminished or lost CMT3 activity and loss of gene body DNA methylation, as cmt3 mutants in A. thaliana mitigate the deleterious effects of IBM1.

The dual role of the RETINOBLASTOMA-RELATED protein in the DNA damage response is coordinated by the interaction with LXCXE-containing proteins

Living organisms possess mechanisms to safeguard genome integrity. To avoid spreading mutations, DNA lesions are detected and cell division is temporarily arrested to allow repair mechanisms. Afterward, cells either resume division or respond to unsuccessful repair by undergoing programmed cell death (PCD). How the success rate of DNA repair connects to later cell fate decisions remains incompletely known, particularly in plants. The Arabidopsis thaliana RETINOBLASTOMA-RELATED1 (RBR) protein and its partner E2FA, play both structural and transcriptional functions in the DNA damage response (DDR). Here we provide evidence that distinct RBR protein interactions with LXCXE motif-containing proteins guide these processes. Using the N849F substitution in the RBR B-pocket domain, which specifically disrupts binding to the LXCXE motif, we show that these interactions are dispensable in unchallenging conditions. However, N849F substitution abolishes RBR nuclear foci and promotes PCD and growth arrest upon genotoxic stress. NAC044, which promotes growth arrest and PCD, accumulates after the initial recruitment of RBR to foci and can bind non-focalized RBR through the LXCXE motif in a phosphorylation-independent manner, allowing interaction at different cell cycle phases. Disrupting NAC044-RBR interaction impairs PCD, but their genetic interaction points to opposite independent roles in the regulation of PCD. The LXCXE-binding dependency of the roles of RBR in the DDR suggests a coordinating mechanism to translate DNA repair success to cell survival. We propose that RBR and NAC044 act in two distinct DDR pathways, but interact to integrate input from both DDR pathways to decide upon an irreversible cell fate decision.

Mechanistic insights into DNA damage recognition and checkpoint control in plants

The plant DNA damage response (DDR) pathway safeguards genomic integrity by rapid recognition and repair of DNA lesions that, if unrepaired, may cause genome instability. Most frequently, DNA repair goes hand in hand with a transient cell cycle arrest, which allows cells to repair the DNA lesions before engaging in a mitotic event, but consequently also affects plant growth and yield. Through the identification of DDR proteins and cell cycle regulators that react to DNA double-strand breaks or replication defects, it has become clear that these proteins and regulators form highly interconnected networks. These networks operate at both the transcriptional and post-transcriptional levels and include liquid-liquid phase separation and epigenetic mechanisms. Strikingly, whereas the upstream DDR sensors and signalling components are well conserved across eukaryotes, some of the more downstream effectors are diverged in plants, probably to suit unique lifestyle features. Additionally, DDR components display functional diversity across ancient plant species, dicots and monocots. The observed resistance of DDR mutants towards aluminium toxicity, phosphate limitation and seed ageing indicates that gaining knowledge about the plant DDR may offer solutions to combat the effects of climate change and the associated risk for food security.

An RRM domain protein SOE suppresses transgene silencing in rice

Gene expression is regulated at multiple levels, including RNA processing and DNA methylation/demethylation. How these regulations are controlled remains unclear. Here, through analysis of a suppressor for the OsEIN2 over-expressor, we identified an RNA recognition motif protein SUPPRESSOR OF EIN2 (SOE). SOE is localized in nuclear speckles and interacts with several components of the spliceosome. We find SOE associates with hundreds of targets and directly binds to a DNA glycosylase gene DNG701 pre-mRNA for efficient splicing and stabilization, allowing for subsequent DNG701-mediated DNA demethylation of the transgene promoter for proper gene expression. The V81M substitution in the suppressor mutant protein mSOE impaired its protein stability and binding activity to DNG701 pre-mRNA, leading to transgene silencing. SOE mutation enhances grain size and yield. Haplotype analysis in c. 3000 rice accessions reveals that the haplotype 1 (Hap 1) promoter is associated with high 1000-grain weight, and most of the japonica accessions, but not indica ones, have the Hap 1 elite allele. Our study discovers a novel mechanism for the regulation of gene expression and provides an elite allele for the promotion of yield potentials in rice.

MYB3R-mediated and cell cycle-dependent transcriptional regulation of a tobacco ortholog of SCARECROW-LIKE28 in synchronized cultures of BY-2 cells

Although it is well known that hierarchical transcriptional networks are essential for various aspects of plant development and environmental response, little has been investigated about whether and how they also regulate the plant cell cycle. Recent studies on cell cycle regulation in Arabidopsis thaliana identified SCARECROW-LIKE28 (SCL28), a GRAS-type transcription factor, that constitutes a hierarchical transcriptional pathway comprised of MYB3R, SCL28 and SIAMESE-RELATED (SMR). In this pathway, MYB3R family proteins regulate the G2/M-specific transcription of the SCL28 gene, of which products, in turn, positively regulate the transcription of SMR genes encoding a group of plant-specific inhibitor proteins of cyclin-dependent kinases. However, this pathway with a role in cell cycle inhibition is solely demonstrated in A. thaliana, thus leaving open the question of whether and to what extent this pathway is evolutionarily conserved in plants. In this study, we conducted differential display RT-PCR on synchronized Nicotiana tabacum (tobacco) BY-2 cells and identified several M-phase-specific cDNA clones, one of which turned out to be a tobacco ortholog of SCL28 and was designated NtSCL28. We showed that NtSCL28 is expressed specifically during G2/M and early G1 in the synchronized cultures of BY-2 cells. NtSCL28 contains MYB3R-binding promoter elements, so-called mitosis-specific activator elements, and is upregulated by a hyperactive form of NtmybA2, one of the MYB3R proteins from tobacco. Our study indicated that a part of the hierarchical pathway identified in A. thaliana is equally operating in tobacco cells, suggesting the conservation of this pathway across different families in evolution of angiosperm.

Short Interrupted Repeat Cassette (SIRC)-Novel Type of Repetitive DNA Element Found in Arabidopsis thaliana

Short interrupted repeat cassette (SIRC)-a novel DNA element found throughout the A. thaliana nuclear genome. SIRCs are represented by short direct repeats interrupted by diverse DNA sequences. The maxima of SIRC's distribution are located within pericentromeric regions. We suggest that originally SIRC was a special case of the complex internal structure of the miniature inverted repeat transposable element (MITE), and further MITE amplification, transposition, and loss of terminal inverted repeats gave rise to SIRC as an independent DNA element. SIRC sites were significantly enriched with several histone modifications associated with constitutive heterochromatin and mobile genetic elements. The majority of DNA-binding proteins, strongly associated with SIRC, are related to histone modifications for transcription repression. A part of SIRC was found to overlap highly inducible protein-coding genes, suggesting a possible regulatory role for these elements, yet their definitive functions need further investigation.

Exploring the crop epigenome: a comparison of DNA methylation profiling techniques

Epigenetic modifications play a vital role in the preservation of genome integrity and in the regulation of gene expression. DNA methylation, one of the key mechanisms of epigenetic control, impacts growth, development, stress response and adaptability of all organisms, including plants. The detection of DNA methylation marks is crucial for understanding the mechanisms underlying these processes and for developing strategies to improve productivity and stress resistance of crop plants. There are different methods for detecting plant DNA methylation, such as bisulfite sequencing, methylation-sensitive amplified polymorphism, genome-wide DNA methylation analysis, methylated DNA immunoprecipitation sequencing, reduced representation bisulfite sequencing, MS and immuno-based techniques. These profiling approaches vary in many aspects, including DNA input, resolution, genomic region coverage, and bioinformatics analysis. Selecting an appropriate methylation screening approach requires an understanding of all these techniques. This review provides an overview of DNA methylation profiling methods in crop plants, along with comparisons of the efficacy of these techniques between model and crop plants. The strengths and limitations of each methodological approach are outlined, and the importance of considering both technical and biological factors are highlighted. Additionally, methods for modulating DNA methylation in model and crop species are presented. Overall, this review will assist scientists in making informed decisions when selecting an appropriate DNA methylation profiling method.

Cyclin A participates in the TSO1-MYB3R1 regulatory module to maintain shoot meristem size and fertility in Arabidopsis

The stem cell pools at the shoot apex and root tip give rise to all the above- and below-ground tissues of a plant. Previous studies in Arabidopsis identified a TSO1-MYB3R1 transcriptional module that controls the number and size of the stem cell pools at the shoot apex and root tip. As TSO1 and MYB3R1 are homologous to components of an animal cell cycle regulatory complex, DREAM, Arabidopsis mutants of TSO1 and MYB3R1 provide valuable tools for investigations into the link between cell cycle regulation and stem cell maintenance in plants. In this study, an Arabidopsis cyclin A gene, CYCA3;4, was identified as a member of the TSO1-MYB3R1 regulatory module and cyca3;4 mutations suppressed the tso1-1 mutant phenotype specifically in the shoot. The work reveals how the TSO1-MYB3R1 module is integrated with the cell cycle machinery to control cell division at the shoot meristem.

Arabidopsis Trithorax histone methyltransferases are redundant in regulating development and DNA methylation

Although the Trithorax histone methyltransferases ATX1-5 are known to regulate development and stress responses by catalyzing histone H3K4 methylation in Arabidopsis thaliana, it is unknown whether and how these histone methyltransferases affect DNA methylation. Here, we found that the redundant ATX1-5 proteins are not only required for plant development and viability but also for the regulation of DNA methylation. The expression and H3K4me3 levels of both RNA-directed DNA methylation (RdDM) genes (NRPE1, DCL3, IDN2, and IDP2) and active DNA demethylation genes (ROS1, DML2, and DML3) were downregulated in the atx1/2/4/5 mutant. Consistent with the facts that the active DNA demethylation pathway mediates DNA demethylation mainly at CG and CHG sites, and that the RdDM pathway mediates DNA methylation mainly at CHH sites, whole-genome DNA methylation analyses showed that hyper-CG and CHG DMRs in atx1/2/4/5 significantly overlapped with those in the DNA demethylation pathway mutant ros1 dml2 dml3 (rdd), and that hypo-CHH DMRs in atx1/2/4/5 significantly overlapped with those in the RdDM mutant nrpe1, suggesting that the ATX paralogues function redundantly to regulate DNA methylation by promoting H3K4me3 levels and expression levels of both RdDM genes and active DNA demethylation genes. Given that the ATX proteins function as catalytic subunits of COMPASS histone methyltransferase complexes, we also demonstrated that the COMPASS complex components function as a whole to regulate DNA methylation. This study reveals a previously uncharacterized mechanism underlying the regulation of DNA methylation.

Active DNA demethylation in plants: 20 years of discovery and beyond

Maintaining proper DNA methylation levels in the genome requires active demethylation of DNA. However, removing the methyl group from a modified cytosine is chemically difficult and therefore, the underlying mechanism of demethylation had remained unclear for many years. The discovery of the first eukaryotic DNA demethylase, Arabidopsis thaliana REPRESSOR OF SILENCING 1 (ROS1), led to elucidation of the 5-methylcytosine base excision repair mechanism of active DNA demethylation. In the 20 years since ROS1 was discovered, our understanding of this active DNA demethylation pathway, as well as its regulation and biological functions in plants, has greatly expanded. These exciting developments have laid the groundwork for further dissecting the regulatory mechanisms of active DNA demethylation, with potential applications in epigenome editing to facilitate crop breeding and gene therapy.

A naturally-occurring phenomenon of flower color change during flower development in Xanthoceras sorbifolium

Xanthoceras sorbifolium (yellowhorn) is originated in China and is a unique tree in northern China. Yellowhorn is very popular because of the gradual color change of its flower at different flower developmental stages. During flowering development, the color at the base of yellowhorn flower petals gradually changes from yellow to purple. The mechanism of this miraculous phenomenon is still unclear. Here we show that anthocyanin accumulation during flowering development is the main reason for this color change. RT-PCR results show that the expression level of a variety of anthocyanin biosynthesis genes changes in different flower developmental stages. Realtime results show that the expression changes of these anthocyanin biosynthesis genes are positively regulated by a cluster of R2R3-MYB transcription factor genes, XsMYB113s. Furthermore, the DNA methylation analysis showed that CHH methylation status on the transposon element near the XsMYB113-1 influence its transcript level during flowering development. Our results suggest that dynamic epigenetic regulation of the XsMYB113-1 leads to the accumulation of anthocyanins during yellowhorn flower color change. These findings reemphasize the role of epigenetic regulation in flower development and provide a foundation for further studies of epigenetic regulation in long-lived woody perennials.

Conditional GWAS of non-CG transposon methylation in Arabidopsis thaliana reveals major polymorphisms in five genes

Genome-wide association studies (GWAS) have revealed that the striking natural variation for DNA CHH-methylation (mCHH; H is A, T, or C) of transposons has oligogenic architecture involving major alleles at a handful of known methylation regulators. Here we use a conditional GWAS approach to show that CHG-methylation (mCHG) has a similar genetic architecture—once mCHH is statistically controlled for. We identify five key trans-regulators that appear to modulate mCHG levels, and show that they interact with a previously identified modifier of mCHH in regulating natural transposon mobilization.

DNA methylation is an epigenetic mark common across eukaryotes. It is associated with transcriptional silencing, in particular of transposable elements. Multiple elements, including epigenetic inheritance, shape DNA methylation patterns, and the complexity makes it challenging to dissect the regulation. Our work on the 1001 Arabidopsis Epigenomes project led to the unexpected discovery that much of the natural variation for CHH methylation (mCHH; H is A, T, or C) on transposable elements could be attributed to allelic variation in three genes known to be involved in epigenetic regulation. However, our analysis of methylation in other sequence contexts (mCHG and mCG) revealed no genetic regulator.

Here we show that if mCHG variation is analyzed while taking mCHH into account, mCHG is also strongly influenced by allelic variation in a small number of genes with known or highly plausible direct roles in epigenetic regulation. The presence of common allelic variation of large effect is suggestive of some form of local adaptation. The nature of this adaptation remains obscure, but we present further evidence that allelic variation regulating DNA methylation influences transposon mobilization.

The Arabidopsis DREAM complex antagonizes WDR5A to modulate histone H3K4me2/3 deposition for a subset of genome repression

The master transcriptional repressor DREAM (dimerization partner, RB-like, E2F and multivulval class B) complex regulates the cell cycle in eukaryotes, but much remains unknown about how it transmits repressive signals on chromatin to the primary transcriptional machinery (e.g., RNA polymerase II [Pol II]). Through a forward genetic screen, we identified BTE1 (barrier of transcription elongation 1), a plant-specific component of the DREAM complex. The subsequent characterization demonstrated that DREAM complex containing BTE1 antagonizes the activity of Complex Proteins Associated with Set1 (COMPASS)-like complex to repress H3K4me3 occupancy and inhibits Pol II elongation at DREAM target genes. We showed that BTE1 is recruited to chromatin at the promoter-proximal regions of target genes by E2F transcription factors. DREAM target genes exhibit characteristic enrichment of H2A.Z and H3K4me2 modification on chromatin. We further showed that BTE1 directly interacts with WDR5A, a core component of COMPASS-like complex, repressing WDR5A chromatin binding and the elongation of transcription on DREAM target genes. H3K4me3 is known to correlate with the Pol II transcription activation and promotes efficient elongation. Thus, our study illustrates a transcriptional repression mechanism by which the DREAM complex dampens H3K4me3 deposition at a set of genes through its interaction with WDR5A.

A hierarchical transcriptional network activates specific CDK inhibitors that regulate G2 to control cell size and number in Arabidopsis

How cell size and number are determined during organ development remains a fundamental question in cell biology. Here, we identified a GRAS family transcription factor, called SCARECROW-LIKE28 (SCL28), with a critical role in determining cell size in Arabidopsis. SCL28 is part of a transcriptional regulatory network downstream of the central MYB3Rs that regulate G2 to M phase cell cycle transition. We show that SCL28 forms a dimer with the AP2-type transcription factor, AtSMOS1, which defines the specificity for promoter binding and directly activates transcription of a specific set of SIAMESE-RELATED (SMR) family genes, encoding plant-specific inhibitors of cyclin-dependent kinases and thus inhibiting cell cycle progression at G2 and promoting the onset of endoreplication. Through this dose-dependent regulation of SMR transcription, SCL28 quantitatively sets the balance between cell size and number without dramatically changing final organ size. We propose that this hierarchical transcriptional network constitutes a cell cycle regulatory mechanism that allows to adjust cell size and number to attain robust organ growth.

CRISPR-Cas9 mediated OsMIR168a knockout reveals its pleiotropy in rice

MicroRNA168 (MIR168) is a key miRNA that targets the main RNA-induced silencing complex component Argonaute 1 (AGO1) to regulate plant growth and environmental stress responses. However, the regulatory functions of MIR168 need to be further elucidated in rice. In this paper, we generated clean OsMIR168a deletion mutants by CRISPR-Cas9 strategy. We then phenotypically and molecularly characterized these mutants. The rice OsMIR168a mutants grew rapidly at the seedling stage, produced more tillers and matured early. Compared to the wild-type plants, the mutants were shorter at maturity and produced smaller spikelets and seeds. Analysis of gene expression showed that the transcription levels of OsMIR168a's target genes such as OsAGO1a, OsAGO1b and OsAGO1d were elevated significantly in the OsMIR168a mutants. Intriguingly, OsAGO18, a member of a new AGO clade that is conserved in monocots, was confirmed to be a target of OsMIR168a not only by informatic prediction but also by expression analysis and a cell-based cleavage assay in the OsMIR168a mutants. Many protein-coding genes and miRNAs showed differential expression in the OsMIR168a mutants, suggesting OsMIR168a exerts a major transcriptional regulatory role, likely through its potential target genes such as OsAGO1s and OsAGO18. KEGG enrichment analysis of these differentially expressed genes pointed to OsMIR168a's involvement in important processes such as plant hormone signalling transduction and plant-pathogen interaction. These data collectively support that the complex regulation module of OsMIR168a-OsAGO1/OsAGO18-miRNAs-target genes contributes to agronomically important traits, which sheds light on miRNA-mediated crop breeding.

Cycling in a crowd: Coordination of plant cell division, growth, and cell fate

The reiterative organogenesis that drives plant growth relies on the constant production of new cells, which remain encased by interconnected cell walls. For these reasons, plant morphogenesis strictly depends on the rate and orientation of both cell division and cell growth. Important progress has been made in recent years in understanding how cell cycle progression and the orientation of cell divisions are coordinated with cell and organ growth and with the acquisition of specialized cell fates. We review basic concepts and players in plant cell cycle and division, and then focus on their links to growth-related cues, such as metabolic state, cell size, cell geometry, and cell mechanics, and on how cell cycle progression and cell division are linked to specific cell fates. The retinoblastoma pathway has emerged as a major player in the coordination of the cell cycle with both growth and cell identity, while microtubule dynamics are central in the coordination of oriented cell divisions. Future challenges include clarifying feedbacks between growth and cell cycle progression, revealing the molecular basis of cell division orientation in response to mechanical and chemical signals, and probing the links between cell fate changes and chromatin dynamics during the cell cycle.

Nearby transposable elements impact plant stress gene regulatory networks: a meta-analysis in A. thaliana and S. lycopersicum

BACKGROUND

Transposable elements (TE) make up a large portion of many plant genomes and are playing innovative roles in genome evolution. Several TEs can contribute to gene regulation by influencing expression of nearby genes as stress-responsive regulatory motifs. To delineate TE-mediated plant stress regulatory networks, we took a 2-step computational approach consisting of identifying TEs in the proximity of stress-responsive genes, followed by searching for cis-regulatory motifs in these TE sequences and linking them to known regulatory factors. Through a systematic meta-analysis of RNA-seq expression profiles and genome annotations, we investigated the relation between the presence of TE superfamilies upstream, downstream or within introns of nearby genes and the differential expression of these genes in various stress conditions in the TE-poor Arabidopsis thaliana and the TE-rich Solanum lycopersicum.

RESULTS

We found that stress conditions frequently expressed genes having members of various TE superfamilies in their genomic proximity, such as SINE upon proteotoxic stress and Copia and Gypsy upon heat stress in A. thaliana, and EPRV and hAT upon infection, and Harbinger, LINE and Retrotransposon upon light stress in S. lycopersicum. These stress-specific gene-proximal TEs were mostly located within introns and more detected near upregulated than downregulated genes. Similar stress conditions were often related to the same TE superfamily. Additionally, we detected both novel and known motifs in the sequences of those TEs pointing to regulatory cooption of these TEs upon stress. Next, we constructed the regulatory network of TFs that act through binding these TEs to their target genes upon stress and discovered TE-mediated regulons targeted by TFs such as BRB/BPC, HD, HSF, GATA, NAC, DREB/CBF and MYB factors in Arabidopsis and AP2/ERF/B3, NAC, NF-Y, MYB, CXC and HD factors in tomato.

CONCLUSIONS

Overall, we map TE-mediated plant stress regulatory networks using numerous stress expression profile studies for two contrasting plant species to study the regulatory role TEs play in the response to stress. As TE-mediated gene regulation allows plants to adapt more rapidly to new environmental conditions, this study contributes to the future development of climate-resilient plants.

MethylScore, a pipeline for accurate and context-aware identification of differentially methylated regions from population-scale plant whole-genome bisulfite sequencing data

Whole-genome bisulfite sequencing (WGBS) is the standard method for profiling DNA methylation at single-nucleotide resolution. Different tools have been developed to extract differentially methylated regions (DMRs), often built upon assumptions from mammalian data. Here, we present MethylScore, a pipeline to analyse WGBS data and to account for the substantially more complex and variable nature of plant DNA methylation. MethylScore uses an unsupervised machine learning approach to segment the genome by classification into states of high and low methylation. It processes data from genomic alignments to DMR output and is designed to be usable by novice and expert users alike. We show how MethylScore can identify DMRs from hundreds of samples and how its data-driven approach can stratify associated samples without prior information. We identify DMRs in the A. thaliana 1,001 Genomes dataset to unveil known and unknown genotype-epigenotype associations .

The DREAM complex represses growth in response to DNA damage in Arabidopsis

The DNA of all organisms is constantly damaged by physiological processes and environmental conditions. Upon persistent damage, plant growth and cell proliferation are reduced. Based on previous findings that RBR1, the only Arabidopsis homolog of the mammalian tumor suppressor gene retinoblastoma, plays a key role in the DNA damage response in plants, we unravel here the network of RBR1 interactors under DNA stress conditions. This led to the identification of homologs of every DREAM component in Arabidopsis, including previously not recognized homologs of LIN52. Interestingly, we also discovered NAC044, a mediator of DNA damage response in plants and close homolog of the major DNA damage regulator SOG1, to directly interact with RBR1 and the DREAM component LIN37B. Consistently, not only mutants in NAC044 but also the double mutant of the two LIN37 homologs and mutants for the DREAM component E2FB showed reduced sensitivities to DNA-damaging conditions. Our work indicates the existence of multiple DREAM complexes that work in conjunction with NAC044 to mediate growth arrest after DNA damage.

DREAM On: Cell Cycle Control in Development and Disease

Perfectly orchestrated periodic gene expression during cell cycle progression is essential for maintaining genome integrity and ensuring that cell proliferation can be stopped by environmental signals. Genetic and proteomic studies during the past two decades revealed remarkable evolutionary conservation of the key mechanisms that control cell cycle-regulated gene expression, including multisubunit DNA-binding DREAM complexes. DREAM complexes containing a retinoblastoma family member, an E2F transcription factor and its dimerization partner, and five proteins related to products of Caenorhabditis elegans multivulva (Muv) class B genes lin-9, lin-37, lin-52, lin-53, and lin-54 (comprising the MuvB core) have been described in diverse organisms, from worms to humans. This review summarizes the current knowledge of the structure, function, and regulation of DREAM complexes in different organisms, as well as the role of DREAM in human disease. Expected final online publication date for the Annual Review of Genetics, Volume 55 is November 2021. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

Plant Epigenetics: Propelling DNA Methylation Variation across the Cell Cycle

DNA methylation is reconfigured during male reproduction in plants, but little is known regarding the mechanisms controlling these epigenetic dynamics. New research highlights how the cell cycle can influence DNA methylation dynamics observed during male gametogenesis and may induce epigenetic variation in clonally propagated plants.

Nuclear translocation of MTL5 from cytoplasm requires its direct interaction with LIN9 and is essential for male meiosis and fertility

Meiosis is essential for the generation of gametes and sexual reproduction, yet the factors and underlying mechanisms regulating meiotic progression remain largely unknown. Here, we showed that MTL5 translocates into nuclei of spermatocytes during zygotene-pachytene transition and ensures meiosis advances beyond pachytene stage. MTL5 shows strong interactions with MuvB core complex components, a well-known transcriptional complex regulating mitotic progression, and the zygotene-pachytene transition of MTL5 is mediated by its direct interaction with the component LIN9, through MTL5 C-terminal 443–475 residues. Male Mtl5^c-mu/c-mu mice expressing the truncated MTL5 (p.Ser445Arg fs*3) that lacks the interaction with LIN9 and is detained in cytoplasm showed male infertility and spermatogenic arrest at pachytene stage, same as that of Mtl5 knockout mice, indicating that the interaction with LIN9 is essential for the nuclear translocation and function of MTL5 during meiosis. Our data demonstrated MTL5 translocates into nuclei during the zygotene-pachytene transition to initiate its function along with the MuvB core complex in pachytene spermatocytes, highlighting a new mechanism regulating the progression of male meiosis.

Meiosis is essential for spermatogenesis and male fertility. However, the factors regulating the progression of meiosis remain largely unknown. We reported the testis specific protein MTL5 translocated into the nuclei of spermatocytes at the zygotene-pachytene transition by direct interaction with LIN9, which is an essential component of MuvB core complex, to promote meiotic progression beyond the pachytene stage. We also showed that MTL5 pulls down MYBL1 and all of the MuvB core complex (except LIN54) in spermatocytes. Given the known role of the MuvB core complex as a cell cycle regulator in mitotic cells, we suggested that MTL5 promotes meiotic progression along with the MuvB core complex to ensure male fertility. Our results indicated a novel function of the MuvB complex in male meiosis and also shed light on the master regulator proteins that control meiotic progression at the pachytene stage. MTL5 is a novel and germ-cell specific regulator of cell cycle progression to function at a specific stage by nuclear translocation in meiosis.

Repression of CHROMOMETHYLASE 3 prevents epigenetic collateral damage in Arabidopsis

DNA methylation has evolved to silence mutagenic transposable elements (TEs) while typically avoiding the targeting of endogenous genes. Mechanisms that prevent DNA methyltransferases from ectopically methylating genes are expected to be of prime importance during periods of dynamic cell cycle activities including plant embryogenesis. However, virtually nothing is known regarding how DNA methyltransferase activities are precisely regulated during embryogenesis to prevent the induction of potentially deleterious and mitotically stable genic epimutations. Here, we report that microRNA-mediated repression of CHROMOMETHYLASE 3 (CMT3) and the chromatin features that CMT3 prefers help prevent ectopic methylation of thousands of genes during embryogenesis that can persist for weeks afterwards. Our results are also consistent with CMT3-induced ectopic methylation of promoters or bodies of genes undergoing transcriptional activation reducing their expression. Therefore, the repression of CMT3 prevents epigenetic collateral damage on endogenous genes. We also provide a model that may help reconcile conflicting viewpoints regarding the functions of gene-body methylation that occurs in nearly all flowering plants.

Identification of two tobacco genes encoding MYB3R proteins with repressor function and showing cell cycle-regulated transcript accumulation

MYB3R family transcription factors play a central role in the regulation of G2/M-specific gene transcription in Arabidopsis thaliana. Among the members of this family, MYB3R3 and MYB3R5 are structurally closely related and are involved in the transcriptional repression of target genes in both proliferating and quiescent cells. This type of MYB3R repressor is widespread in plants; however, apart from the studies on MYB3Rs in Arabidopsis thaliana, little information about them is available. Here we isolated tobacco cDNA clones encoding two closely related MYB3R proteins designated as NtmybC1 and NtmybC2 and determined the nucleotide sequences of the entire coding regions. Phylogenetic analysis suggested that NtmybC1 and NtmybC2 can be grouped into a conserved subfamily of plant MYB3Rs that also contains MYB3R3 and MYB3R5. When transiently expressed in protoplasts prepared from tobacco BY-2 cells, NtmybC1 and NtmybC2 repressed the activity of target promoters and blocked promoter activation mediated by NtmybA2, a MYB3R activator from tobacco. Unlike MYB3R3 and MYB3R5, NtmybC1 and NtmybC2 showed cell cycle-regulated transcript accumulation. In synchronized cultures of BY-2 cells, mRNAs for both NtmybC1 and NtmybC2 were preferentially expressed during the G2 and M phases, coinciding with the expression of NtmybA2 and G2/M-specific target genes. These results not only broadly confirm our fundamental view that this type of MYB3R protein acts as transcriptional repressor of G2/M-specific genes but also suggest a possible divergence of MYB3R repressors in terms of the mechanisms of their action and regulation.

MYB3R-mediated active repression of cell cycle and growth under salt stress in Arabidopsis thaliana

Under environmental stress, plants are believed to actively repress their growth to save resource and alter its allocation to acquire tolerance against the stress. Although a lot of studies have uncovered precise mechanisms for responding to stress and acquiring tolerance, the mechanisms for regulating growth repression under stress are not as well understood. It is especially unclear which particular genes related to cell cycle control are involved in active growth repression. Here, we showed that decreased growth in plants exposed to moderate salt stress is mediated by MYB3R transcription factors that have been known to positively and negatively regulate the transcription of G2/M-specific genes. Our genome-wide gene expression analysis revealed occurrences of general downregulation of G2/M-specific genes in Arabidopsis under salt stress. Importantly, this downregulation is significantly and universally mitigated by the loss of MYB3R repressors by mutations. Accordingly, the growth performance of Arabidopsis plants under salt stress is significantly recovered in mutants lacking MYB3R repressors. This growth recovery involves improved cell proliferation that is possibly due to prolonging and accelerating cell proliferation, which were partly suggested by enlarged root meristem and increased number of cells positive for CYCB1;1-GUS. Our ploidy analysis further suggested that cell cycle progression at the G2 phase was delayed under salt stress, and this delay was recovered by loss of MYB3R repressors. Under salt stress, the changes in expression of MYB3R activators and repressors at both the mRNA and protein levels were not significant. This observation suggests novel mechanisms underlying MYB3R-mediated growth repression under salt stress that are different from the mechanisms operating under other stress conditions such as DNA damage and high temperature.

Convergent Epigenetic Mechanisms Avoid Constitutive Expression of Immune Receptor Gene Subsets

The gene pool encoding PRR and NLR immune receptors determines the ability of a plant to resist microbial infections. Basal expression of these genes is prevented by diverse mechanisms since their hyperactivity can be harmful. To approach the study of epigenetic control of PRR/NLR genes we here analyzed their expression in mutants carrying abnormal repressive 5-methyl cytosine (5-mC) and histone 3 lysine 9 dimethylation (H3K9me2) marks, due to lack of MET1, CMT3, MOM1, SUVH4/5/6, or DDM1. At optimal growth conditions, none of the mutants showed basal expression of the defense gene marker PR1, but all of them had greater resistance to Pseudomonas syringae pv. tomato than wild type plants, suggesting they are primed to stimulate immune cascades. Consistently, analysis of available transcriptomes indicated that all mutants showed activation of particular PRR/NLR genes under some growth conditions. Under low defense activation, 37 PRR/NLR genes were expressed in these plants, but 29 of them were exclusively activated in specific mutants, indicating that MET1, CMT3, MOM1, SUVH4/5/6, and DDM1 mediate basal repression of different subsets of genes. Some epigenetic marks present at promoters, but not gene bodies, could explain the activation of these genes in the mutants. As expected, suvh4/5/6 and ddm1 activated genes carrying 5-mC and H3K9me2 marks in wild type plants. Surprisingly, all mutants expressed genes harboring promoter H2A.Z/H3K27me3 marks likely affected by the chromatin remodeler PIE1 and the histone demethylase REF6, respectively. Therefore, MET1, CMT3, MOM1, SUVH4/5/6, and DDM1, together with REF6, seemingly contribute to the establishment of chromatin states that prevent constitutive PRR/NLR gene activation, but facilitate their priming by modulating epigenetic marks at their promoters.

Roles of plant retinoblastoma protein: cell cycle and beyond

The human retinoblastoma (RB1) protein is a tumor suppressor that negatively regulates cell cycle progression through its interaction with members of the E2F/DP family of transcription factors. However, RB-related (RBR) proteins are an early acquisition during eukaryote evolution present in plant lineages, including unicellular algae, ancient plants (ferns, lycophytes, liverworts, mosses), gymnosperms, and angiosperms. The main RBR protein domains and interactions with E2Fs are conserved in all eukaryotes and not only regulate the G1/S transition but also the G2/M transition, as part of DREAM complexes. RBR proteins are also important for asymmetric cell division, stem cell maintenance, and the DNA damage response (DDR). RBR proteins play crucial roles at every developmental phase transition, in association with chromatin factors, as well as during the reproductive phase during female and male gametes production and embryo development. Here, we review the processes where plant RBR proteins play a role and discuss possible avenues of research to obtain a full picture of the multifunctional roles of RBR for plant life.