The Arabidopsis thaliana SNF2 homolog AtBRM controls shoot development and flowering

Abscisic acid-mediated autoregulation of the MYB41-BRAHMA module enhances drought tolerance in Arabidopsis

Drought stress poses a substantial challenge to plant growth and agricultural productivity worldwide. Upon water depletion, plants activate an abscisic acid (ABA) signaling pathway, leading to stomatal closure to reduce water loss. The MYB family of transcription factors plays diverse roles in growth, development, stress responses, and biosynthesis, yet their involvement in stomatal regulation remains unclear. Here, we demonstrate that ABA significantly upregulates the expression of MYB41, MYB74, and MYB102, with MYB41 serving as a key regulator that induces the expression of both MYB74 and MYB102. Through luciferase assays, chromatin immunoprecipitation (ChIP) assays, and electrophoretic mobility shift assays (EMSA), we reveal that MYB41 engages in positive feedback regulation by binding to its own promoter, thus amplifying its transcription in Arabidopsis (Arabidopsis thaliana). Furthermore, our investigation showed that MYB41 recruits BRAHMA (BRM), the core ATPase subunit of the SWI/SNF complex, to the MYB41 promoter, facilitating the binding of HISTONE DEACETYLASE 6 (HDA6). This recruitment triggers epigenetic modifications, resulting in reduced MYB41 expression characterized by elevated H3K27me3 levels and concurrent decreases in H3ac, H3K27ac, and H3K14ac levels in wild-type plants compared to brm knockout mutant plants. Our genetic and molecular analyses show that ABA mediates autoregulation of the MYB41-BRM module, which intricately modulates stomatal movement in A. thaliana. This discovery sheds light on a drought response mechanism with the potential to greatly enhance agricultural productivity.

BAPID suppresses the inhibition of BRM on Di19-PR module in response to drought

Drought is one of the most important abiotic stresses, and seriously threatens plant development and productivity. Increasing evidence indicates that chromatin remodelers are pivotal for plant drought response. However, molecular mechanisms of chromatin remodelers-mediated plant drought responses remain obscure. In this study, we found a novel interactor of BRM called BRM-associated protein involved in drought response (BAPID), which interacted with SWI/SNF chromatin remodeler BRM and drought-induced transcription factor Di19. Our findings demonstrated that BAPID acted as a positive drought regulator since drought tolerance was increased in BAPID-overexpressing plants, but decreased in BAPID-deficient plants, and physically bound to PR1, PR2, and PR5 promoters to mediate expression of PR genes to defend against dehydration stress. Genetic approaches demonstrated that BRM acted epistatically to BAPID and Di19 in drought response in Arabidopsis. Furthermore, the BAPID protein-inhibited interaction between BRM and Di19, and suppressed the inhibition of BRM on the Di19-PR module by mediating the H3K27me3 deposition at PR loci, thus changing nucleosome accessibility of Di19 and activating transcription of PR genes in response to drought. Our results shed light on the molecular mechanism whereby the BAPID-BRM-Di19-PRs pathway mediates plant drought responses. We provide data improving our understanding of chromatin remodeler-mediated plant drought regulation network.

Chromatin sensing: integration of environmental signals to reprogram plant development through chromatin regulators

Chromatin regulation in eukaryotes plays pivotal roles in controlling the developmental regulatory gene network. This review explores the intricate interplay between chromatin regulators and environmental signals, elucidating their roles in shaping plant development. As sessile organisms, plants have evolved sophisticated mechanisms to perceive and respond to environmental cues, orchestrating developmental programs that ensure adaptability and survival. A central aspect of this dynamic response lies in the modulation of versatile gene regulatory networks, mediated in part by various chromatin regulators. Here, we summarized current understanding of the molecular mechanisms through which chromatin regulators integrate environmental signals, influencing key aspects of plant development.

Molecular and structural basis of the chromatin remodeling activity by Arabidopsis DDM1

The histone H2A variant H2A.W occupies transposons and thus prevents access to them in Arabidopsis thaliana. H2A.W is deposited by the chromatin remodeler DDM1, which also promotes the accessibility of chromatin writers to heterochromatin by an unknown mechanism. To shed light on this question, we solve the cryo-EM structures of nucleosomes containing H2A and H2A.W, and the DDM1-H2A.W nucleosome complex. These structures show that the DNA end flexibility of the H2A nucleosome is higher than that of the H2A.W nucleosome. In the DDM1-H2A.W nucleosome complex, DDM1 binds to the N-terminal tail of H4 and the nucleosomal DNA and increases the DNA end flexibility of H2A.W nucleosomes. Based on these biochemical and structural results, we propose that DDM1 counters the low accessibility caused by nucleosomes containing H2A.W to enable the maintenance of repressive epigenetic marks on transposons and prevent their activity.

Self-organization underlies developmental robustness in plants

Development is a self-organized process that builds on cells and their interactions. Cells are heterogeneous in gene expression, growth, and division; yet how development is robust despite such heterogeneity is a fascinating question. Here, we review recent progress on this topic, highlighting how developmental robustness is achieved through self-organization. We will first discuss sources of heterogeneity, including stochastic gene expression, heterogeneity in growth rate and direction, and heterogeneity in division rate and precision. We then discuss cellular mechanisms that buffer against such noise, including Paf1C- and miRNA-mediated denoising, spatiotemporal growth averaging and compensation, mechanisms to improve cell division precision, and coordination of growth rate and developmental timing between different parts of an organ. We also discuss cases where such heterogeneity is not buffered but utilized for development. Finally, we highlight potential directions for future studies of noise and developmental robustness.

Strigolactone induces D14-dependent large-scale changes in gene expression requiring SWI/SNF chromatin remodellers

Strigolactones (SL) function as plant hormones in control of multiple aspects of plant development, mostly via the regulation of gene expression. Immediate early-gene regulation by SL remains unexplored due to difficulty in dissecting early from late gene expression responses to SL. We used synthetic SL, rac-GR24 treatment of protoplasts and RNA-seq to explore early SL-induced changes in gene expression over time (5-180 minutes) and discovered rapid, dynamic and SL receptor D14-dependent regulation of gene expression in response to rac-GR24. Importantly, we discovered a significant dependence of SL signalling on chromatin remodelling processes, as the induction of a key SL-induced transcription factor BRANCHED1 requires the SWI/SNF chromatin remodelling ATPase SPLAYED (SYD) and leads to upregulation of a homologue SWI/SNF ATPase BRAHMA. ATAC-seq profiling of genome-wide changes in chromatin accessibility in response to rac-GR24 identified large-scale changes, with over 1400 differentially accessible regions. These changes in chromatin accessibility often precede transcriptional changes and are likely to harbour SL cis-regulatory elements. Importantly, we discovered that this early and extensive modification of the chromatin landscape also requires SYD. This study, therefore, provides evidence that SL signalling requires regulation of chromatin accessibility, and it identifies genomic locations harbouring likely SL cis-regulatory sequences.

Leaf growth - complex regulation of a seemingly simple process

Understanding the underlying mechanisms of plant development is crucial to successfully steer or manipulate plant growth in a targeted manner. Leaves, the primary sites of photosynthesis, are vital organs for many plant species, and leaf growth is controlled by a tight temporal and spatial regulatory network. In this review, we focus on the genetic networks governing leaf cell proliferation, one major contributor to final leaf size. First, we provide an overview of six regulator families of leaf growth in Arabidopsis: DA1, PEAPODs, KLU, GRFs, the SWI/SNF complexes, and DELLAs, together with their surrounding genetic networks. Next, we discuss their evolutionary conservation to highlight similarities and differences among species, because knowledge transfer between species remains a big challenge. Finally, we focus on the increase in knowledge of the interconnectedness between these genetic pathways, the function of the cell cycle machinery as their central convergence point, and other internal and environmental cues.

BCL7A and BCL7B potentiate SWI/SNF-complex-mediated chromatin accessibility to regulate gene expression and vegetative phase transition in plants

Switch defective/sucrose non-fermentable (SWI/SNF) chromatin remodeling complexes are multi-subunit machineries that establish and maintain chromatin accessibility and gene expression by regulating chromatin structure. However, how the remodeling activities of SWI/SNF complexes are regulated in eukaryotes remains elusive. B-cell lymphoma/leukemia protein 7 A/B/C (BCL7A/B/C) have been reported as subunits of SWI/SNF complexes for decades in animals and recently in plants; however, the role of BCL7 subunits in SWI/SNF function remains undefined. Here, we identify a unique role for plant BCL7A and BCL7B homologous subunits in potentiating the genome-wide chromatin remodeling activities of SWI/SNF complexes in plants. BCL7A/B require the catalytic ATPase BRAHMA (BRM) to assemble with the signature subunits of the BRM-Associated SWI/SNF complexes (BAS) and for genomic binding at a subset of target genes. Loss of BCL7A and BCL7B diminishes BAS-mediated genome-wide chromatin accessibility without changing the stability and genomic targeting of the BAS complex, highlighting the specialized role of BCL7A/B in regulating remodeling activity. We further show that BCL7A/B fine-tune the remodeling activity of BAS complexes to generate accessible chromatin at the juvenility resetting region (JRR) of the microRNAs MIR156A/C for plant juvenile identity maintenance. In summary, our work uncovers the function of previously elusive SWI/SNF subunits in multicellular eukaryotes and provides insights into the mechanisms whereby plants memorize the juvenile identity through SWI/SNF-mediated control of chromatin accessibility.

PlantCHRs: A comprehensive database of plant chromatin remodeling factors

The Snf2 protein family is a group of ATP-dependent chromatin remodeling factors (CHRs) that play an essential role in gene expression regulation. In plants, Snf2 is involved in growth, development, as well as stress resistance. However, only a very limited number of experimentally validated Snf2 have been identified and reported, while the majority remaining undiscovered in most species . In this study, we predicted 3135 Snf2 proteins and 8398 chromatin remodeling complex (CRC) subunits in diverse plant species, and constructed the Plant Chromatin Remodeling Factors Database (PlantCHRs, http://www.functionalgenomics.cn/PlantCHRs/), which provide a comprehensive resource for researchers to access information about plant CHRs. We also developed an online tool capable of predicting CHRs and CRC subunits. Moreover, we investigated the distribution of Snf2 proteins in different species and observed a significant increase in the number of Snf2 proteins and the diversity of the Snf2 subfamily during the evolution, highlighting their evolutionary importance. By analyzing the expression patterns of the Snf2 genes in different tissues of maize and Arabidopsis, we found that the Snf2 proteins may show some conservation across different species in regulating plant growth and development. Over the all, we established a comprehensive database for plant CHRs, which will facilitate the researches on plant chromatin remodeling.

The regulation of chromatin configuration at AGAMOUS locus by LFR-SYD-containing complex is critical for reproductive organ development in Arabidopsis

Switch defective/sucrose non-fermentable (SWI/SNF) chromatin remodeling complexes are evolutionarily conserved, multi-subunit machinery that play vital roles in the regulation of gene expression by controlling nucleosome positioning and occupancy. However, little is known about the subunit composition of SPLAYED (SYD)-containing SWI/SNF complexes in plants. Here, we show that the Arabidopsis thaliana Leaf and Flower Related (LFR) is a subunit of SYD-containing SWI/SNF complexes. LFR interacts directly with multiple SWI/SNF subunits, including the catalytic ATPase subunit SYD, in vitro and in vivo. Phenotypic analyses of lfr-2 mutant flowers revealed that LFR is important for proper filament and pistil development, resembling the function of SYD. Transcriptome profiling revealed that LFR and SYD shared a subset of co-regulated genes. We further demonstrate that the LFR and SYD interdependently activate the transcription of AGAMOUS (AG), a C-class floral organ identity gene, by regulating the occupation of nucleosome, chromatin loop, histone modification, and Pol II enrichment on the AG locus. Furthermore, the chromosome conformation capture (3C) assay revealed that the gene loop at AG locus is negatively correlated with the AG expression level, and LFR-SYD was functional to demolish the AG chromatin loop to promote its transcription. Collectively, these results provide insight into the molecular mechanism of the Arabidopsis SYD-SWI/SNF complex in the control of higher chromatin conformation of the floral identity gene essential to plant reproductive organ development.

Beyond NPK: Mineral Nutrient-Mediated Modulation in Orchestrating Flowering Time

Flowering time in plants is a complex process regulated by environmental conditions such as photoperiod and temperature, as well as nutrient conditions. While the impact of major nutrients like nitrogen, phosphorus, and potassium on flowering time has been well recognized, the significance of micronutrient imbalances and their deficiencies should not be neglected because they affect the floral transition from the vegetative stage to the reproductive stage. The secondary major nutrients such as calcium, magnesium, and sulfur participate in various aspects of flowering. Micronutrients such as boron, zinc, iron, and copper play crucial roles in enzymatic reactions and hormone biosynthesis, affecting flower development and reproduction as well. The current review comprehensively explores the interplay between microelements and flowering time, and summarizes the underlying mechanism in plants. Consequently, a better understanding of the interplay between microelements and flowering time will provide clues to reveal the roles of microelements in regulating flowering time and to improve crop reproduction in plant industries.

Gibberellin signaling modulates flowering via the DELLA-BRAHMA-NF-YC module in Arabidopsis

Gibberellin (GA) plays a key role in floral induction by activating the expression of floral integrator genes in plants, but the epigenetic regulatory mechanisms underlying this process remain unclear. Here, we show that BRAHMA (BRM), a core subunit of the chromatin-remodeling SWItch/sucrose nonfermentable (SWI/SNF) complex that functions in various biological processes by regulating gene expression, is involved in GA-signaling-mediated flowering via the formation of the DELLA-BRM-NF-YC module in Arabidopsis (Arabidopsis thaliana). DELLA, BRM, and NF-YC transcription factors interact with one another, and DELLA proteins promote the physical interaction between BRM and NF-YC proteins. This impairs the binding of NF-YCs to SOC1, a major floral integrator gene, to inhibit flowering. On the other hand, DELLA proteins also facilitate the binding of BRM to SUPPRESSOR OF OVEREXPRESSION OF CONSTANS1 (SOC1). The GA-induced degradation of DELLA proteins disturbs the DELLA-BRM-NF-YC module, prevents BRM from inhibiting NF-YCs, and decreases the DNA-binding ability of BRM, which promote the deposition of H3K4me3 on SOC1 chromatin, leading to early flowering. Collectively, our findings show that BRM is a key epigenetic partner of DELLA proteins during the floral transition. Moreover, they provide molecular insights into how GA signaling coordinates an epigenetic factor with a transcription factor to regulate the expression of a flowering gene and flowering in plants.

Organization, genomic targeting, and assembly of three distinct SWI/SNF chromatin remodeling complexes in Arabidopsis

Switch defective/sucrose nonfermentable (SWI/SNF) complexes are evolutionarily conserved multisubunit machines that play vital roles in chromatin architecture regulation for modulating gene expression via sliding or ejection of nucleosomes in eukaryotes. In plants, perturbations of SWI/SNF subunits often result in severe developmental disorders. However, the subunit composition, pathways of assembly, and genomic targeting of the plant SWI/SNF complexes are poorly understood. Here, we report the organization, genomic targeting, and assembly of 3 distinct SWI/SNF complexes in Arabidopsis thaliana: BRAHMA-Associated SWI/SNF complexes (BAS), SPLAYED-Associated SWI/SNF complexes (SAS), and MINUSCULE-Associated SWI/SNF complexes (MAS). We show that BAS complexes are equivalent to human ncBAF, whereas SAS and MAS complexes evolve in multiple subunits unique to plants, suggesting plant-specific functional evolution of SWI/SNF complexes. We further show overlapping and specific genomic targeting of the 3 plant SWI/SNF complexes on chromatin and reveal that SAS complexes are necessary for the correct genomic localization of the BAS complexes. Finally, we define the role of the core module subunit in the assembly of plant SWI/SNF complexes and highlight that ATPase module subunit is required for global complex stability and the interaction of core module subunits in Arabidopsis SAS and BAS complexes. Together, our work highlights the divergence of SWI/SNF chromatin remodelers during eukaryote evolution and provides a comprehensive landscape for understanding plant SWI/SNF complex organization, assembly, genomic targeting, and function.

Genome-wide identification of genes encoding SWI/SNF components in soybean and the functional characterization of GmLFR1 in drought-stressed plants

ATP-dependent SWI/SNF chromatin remodeling complexes (CRCs) are evolutionarily conserved multi-component machines that regulate transcription, replication, and genome stability in eukaryotes. SWI/SNF components play pivotal roles in development and various stress responses in plants. However, the compositions and biological functions of SWI/SNF complex subunits remain poorly understood in soybean. In this study, we used bioinformatics to identify 39 genes encoding SWI/SNF subunit distributed on the 19 chromosomes of soybean. The promoter regions of the genes were enriched with several cis-regulatory elements that are responsive to various hormones and stresses. Digital expression profiling and qRT-PCR revealed that most of the SWI/SNF subunit genes were expressed in multiple tissues of soybean and were sensitive to drought stress. Phenotypical, physiological, and molecular genetic analyses revealed that GmLFR1 (Leaf and Flower-Related1) plays a negative role in drought tolerance in soybean and Arabidopsis thaliana. Together, our findings characterize putative components of soybean SWI/SNF complex and indicate possible roles for GmLFR1 in plants under drought stress. This study offers a foundation for comprehensive analyses of soybean SWI/SNF subunit and provides mechanistic insight into the epigenetic regulation of drought tolerance in soybean.

SWI/SNF Chromatin Remodelers: Structural, Functional and Mechanistic Implications

The nuclear events of a eukaryotic cell, such as replication, transcription, recombination and repair etc. require the transition of the compactly arranged chromatin into an uncompacted state and vice-versa. This is mediated by post-translational modification of the histones, exchange of histone variants and ATP-dependent chromatin remodeling. The SWI/SNF chromatin remodeling complexes are one of the most well characterized families of chromatin remodelers. In addition to their role in modulating chromatin, they have also been assigned roles in cancer and health-related anomalies such as developmental, neurocognitive, and intellectual disabilities. Owing to their vital cellular and medical connotations, developing an understanding of the structural and functional aspects of the complex becomes imperative. However, due to the intricate nature of higher-order chromatin as well as compositional heterogeneity of the SWI/SNF complex, intra-species isoforms and inter-species homologs, this often becomes challenging. To this end, the present review attempts to present an amalgamated perspective on the discovery, structure, function, and regulation of the SWI/SNF complex.

Arabidopsis LFR, a SWI/SNF complex component, interacts with ICE1 and activates ICE1 and CBF3 expression in cold acclimation

Low temperatures restrict the growth and geographic distribution of plants, as well as crop yields. Appropriate transcriptional regulation is critical for cold acclimation in plants. In this study, we found that the mutation of Leaf and flower related (LFR), a component of SWI/SNF chromatin remodeling complex (CRC) important for transcriptional regulation in Arabidopsis (Arabidopsis thaliana), resulted in hypersensitivity to freezing stress in plants with or without cold acclimation, and this defect was successfully complemented by LFR. The expression levels of CBFs and COR genes in cold-treated lfr-1 mutant plants were lower than those in wild-type plants. Furthermore, LFR was found to interact directly with ICE1 in yeast and plants. Consistent with this, LFR was able to directly bind to the promoter region of CBF3, a direct target of ICE1. LFR was also able to bind to ICE1 chromatin and was required for ICE1 transcription. Together, these results demonstrate that LFR interacts directly with ICE1 and activates ICE1 and CBF3 gene expression in response to cold stress. Our work enhances our understanding of the epigenetic regulation of cold responses in plants.

A Green Light to Switch on Genes: Revisiting Trithorax on Plants

The Trithorax Group (TrxG) is a highly conserved multiprotein activation complex, initially defined by its antagonistic activity with the PcG repressor complex. TrxG regulates transcriptional activation by the deposition of H3K4me3 and H3K36me3 marks. According to the function and evolutionary origin, several proteins have been defined as TrxG in plants; nevertheless, little is known about their interactions and if they can form TrxG complexes. Recent evidence suggests the existence of new TrxG components as well as new interactions of some TrxG complexes that may be acting in specific tissues in plants. In this review, we bring together the latest research on the topic, exploring the interactions and roles of TrxG proteins at different developmental stages, required for the fine-tuned transcriptional activation of genes at the right time and place. Shedding light on the molecular mechanism by which TrxG is recruited and regulates transcription.

Comprehensive characterization of three classes of Arabidopsis SWI/SNF chromatin remodelling complexes

Although SWI/SNF chromatin remodelling complexes are known to regulate diverse biological functions in plants, the classification, compositions and functional mechanisms of the complexes remain to be determined. Here we comprehensively characterized SWI/SNF complexes by affinity purification and mass spectrometry in Arabidopsis thaliana, and found three classes of SWI/SNF complexes, which we termed BAS, SAS and MAS (BRM-, SYD- and MINU1/2-associated SWI/SNF complexes). By investigating multiple developmental phenotypes of SWI/SNF mutants, we found that three classes of SWI/SNF complexes have both overlapping and specific functions in regulating development. To investigate how the three classes of SWI/SNF complexes differentially regulate development, we mapped different SWI/SNF components on chromatin at the whole-genome level and determined their effects on chromatin accessibility. While all three classes of SWI/SNF complexes regulate chromatin accessibility at proximal promoter regions, SAS is a major SWI/SNF complex that is responsible for mediating chromatin accessibility at distal promoter regions and intergenic regions. Histone modifications are related to both the association of SWI/SNF complexes with chromatin and the SWI/SNF-dependent chromatin accessibility. Three classes of SWI/SNF-dependent accessibility may enable different sets of transcription factors to access chromatin. These findings lay a foundation for further investigation of the function of three classes of SWI/SNF complexes in plants.

Comparative Expression Profiling of Snf2 Family Genes During Reproductive Development and Stress Responses in Rice

Sucrose non-fermenting 2 (Snf2) protein family, as chromatin remodeling factors, is an enormous and the most diverse protein family, which contributes to biological processes of replication, transcription, and DNA repair using the energy of adenosine triphosphate (ATP) hydrolysis. The members of Snf2 family proteins have been well characterized in Arabidopsis, rice, and tomato. Although this family received significant attention, few genes were identified uniquely for their roles in mediating reproductive development and stress tolerance in rice. In the present study, we comprehensively analyzed the expression profiling of Snf2 genes during reproductive development and biotic/abiotic stresses. Our results showed that five proteins (OsCHR712/715/720/726/739) were mainly localized in the nucleus, while OsCHR715/739 were also slightly expressed in the cell membrane. There were abundant cis-acting elements in the putative promoter of Snf2 genes, including dehydration, MeJA, MYB binding site for drought, ABA-responsive, and stress-responsive element. Most of the genes were induced immediately after Magnaporthe oryzae infection at 12 h post-infection (hpi). About 55% of the total genes were upregulated under salt and drought stresses during the entire time, and 22-35% of the total genes were upregulated at 3 h. It was noteworthy that the seven genes (OsCHR705, OsCHR706, OsCHR710, OsCHR714, OsCHR721, OsCHR726, and OsCHR737) were upregulated, and one gene (OsCHR712) was downregulated under salt and drought stresses, respectively. The deficiency of OsCHR726 mutations displayed a hypersensitive phenotype under salt stress. These results will be significantly useful features for the validation of the rice Snf2 genes and facilitate understanding of the genetic engineering of crops with improved biotic and abiotic stresses.

OsDDM1b Controls Grain Size by Influencing Cell Cycling and Regulating Homeostasis and Signaling of Brassinosteroid in Rice

Snf2 family proteins are the crucial subunits of chromatin-remodeling complexes (CRCs), which contributes to the biological processes of transcription, replication, and DNA repair using ATP as energy. Some CRC subunits have been confirmed to be the critical regulators in various aspects of plant growth and development and in epigenetic mechanisms such as histone modification, DNA methylation, and histone variants. However, the functions of Snf2 family genes in rice were poorly investigated. In this study, the relative expression profile of 40 members of Snf2 family in rice was studied at certain developmental stages of seed. Our results revealed that OsCHR741/OsDDM1b (Decrease in DNA methylation 1) was accumulated highly in the early developmental stage of seeds. We further analyzed the OsDDM1b T-DNA insertion loss-of-function of mutant, which exhibited dwarfism, smaller organ size, and shorter and wider grain size than the wild type (Hwayoung, HY), yet no difference in 1,000-grain weight. Consistent with the grain size, the outer parenchyma cell layers of lemma in osddm1b developed more cells with decreased size. OsDDM1b encoded a nucleus, membrane-localized protein and was distributed predominately in young spikelets and seeds, asserting its role in grain size. Meanwhile, the osddm1b was less sensitive to brassinosteroids (BRs) while the endogenous BR levels increased. We detected changes in the expression levels of the BR signaling pathway and feedback-inhibited genes with and without exogenous BR application, and the alterations of expression were also observed in grain size-related genes in the osddm1b. Altogether, our results suggest that OsDDM1b plays a crucial role in grain size via influencing cell proliferation and regulating BR signaling and homeostasis.

The chromatin remodelling ATPase BRAHMA interacts with GATA-family transcription factor GNC to regulate flowering time in Arabidopsis

BRAHMA (BRM) is the ATPase of the SWItch/Sucrose Non-Fermentable (SWI/SNF) chromatin remodelling complex, which is indispensable for transcriptional inhibition and activation, associated with vegetative and reproductive development in Arabidopsis thaliana. Here, we show that BRM directly binds to the chromatin of SUPPRESSOR OF OVEREXPRESSION OF CONSTANS 1 (SOC1), which integrates multiple flowering signals to regulate floral transition, leading to flowering. In addition, genetic and molecular analysis showed that BRM interacts with GNC (GATA, NITRATE-INDUCIBLE, CARBON METABOLISM INVOLVED), a GATA transcription factor that represses flowering by directly repressing SOC1 expression. Furthermore, BRM is recruited by GNC to directly bind to the chromatin of SOC1. The transcript level of SOC1 is elevated in brm-3, gnc, and brm-3/gnc mutants, which is associated with increased histone H3 lysine 4 tri-methylation (H3K4Me3) but decreased DNA methylation. Taken together, our results indicate that BRM associates with GNC to regulate SOC1 expression and flowering time.

The integration of leaf-derived signals sets the timing of vegetative phase change in maize, a process coordinated by epigenetic remodeling

After germination, the maize shoot proceeds through a series of developmental stages before flowering. The first transition occurs during the vegetative phase where the shoot matures from the juvenile to the adult phase, called vegetative phase change (VPC). In maize, both phases exhibit easily-scored morphological characteristics, facilitating the elucidation of molecular mechanisms directing the characteristic gene expression patterns and resulting physiological features of each phase. miR156 expression is high during the juvenile phase, suppressing expression of squamosa promoter binding proteins/SBP-like transcription factors and miR172. The decline in miR156 and subsequent increase in miR172 expression marks the transition into the adult phase, where miR172 represses transcripts that confer juvenile traits. Leaf-derived signals attenuate miR156 expression and thus the duration of the juvenile phase. As found in other species, VPC in maize utilizes signals that consist of hormones, stress, and sugar to direct epigenetic modifiers. In this review we identify the intersection of leaf-derived signaling with components that contribute to the epigenetic changes which may, in turn, manage the distinct global gene expression patterns of each phase. In maize, published research regarding chromatin remodeling during VPC is minimal. Therefore, we identified epigenetic regulators in the maize genome and, using published gene expression data and research from other plant species, identify VPC candidates.

Bromodomain-containing proteins BRD1, BRD2, and BRD13 are core subunits of SWI/SNF complexes and vital for their genomic targeting in Arabidopsis

Switch defective/sucrose non-fermentable (SWI/SNF) chromatin remodeling complexes are multi-subunit machines that play vital roles in the regulation of chromatin structure and gene expression. However, the mechanisms by which SWI/SNF complexes recognize their target loci in plants are not fully understood. Here, we show that the Arabidopsis thaliana bromodomain-containing proteins BRD1, BRD2, and BRD13 are core subunits of SWI/SNF complexes and critical for SWI/SNF genomic targeting. These three BRDs interact directly with multiple SWI/SNF subunits, including the BRAHMA (BRM) catalytic subunit. Phenotypic and transcriptomic analyses of the brd1 brd2 brd13 triple mutant revealed that these BRDs act largely redundantly to control gene expression and developmental processes that are also regulated by BRM. Genome-wide occupancy profiling demonstrated that these three BRDs extensively colocalize with BRM on chromatin. Simultaneous loss of function of three BRD genes results in reduced BRM protein levels and decreased occupancy of BRM on chromatin across the genome. Furthermore, we demonstrated that the bromodomains of BRDs are essential for genomic targeting of the BRD subunits of SWI/SNF complexes to their target sites. Collectively, these results demonstrate that BRD1, BRD2, and BRD13 are core subunits of SWI/SNF complexes and reveal their biological roles in facilitating genomic targeting of BRM-containing SWI/SNF complexes in plants.

Genome-wide occupancy of Arabidopsis SWI/SNF chromatin remodeler SPLAYED provides insights into its interplay with its close homolog BRAHMA and Polycomb proteins

SPLAYED (SYD) is a SWItch/Sucrose Non-Fermentable (SWI/SNF)-type chromatin remodeler identified in Arabidopsis thaliana (Arabidopsis). It is believed to play both redundant and differential roles with its closest homolog BRAHMA (BRM) in diverse plant growth and development processes. To better understand how SYD functions, we profiled the genome-wide occupancy of SYD and its impact on the global transcriptome and trimethylation of histone H3 on lysine 27 (H3K27me3). To map the global occupancy of SYD, we generated a GFP-tagged transgenic line and used it for chromatin immunoprecipitation experiments followed by next-generation sequencing, by which more than 6000 SYD target genes were identified. Through integrating SYD occupancy and transcriptome profiles, we found that SYD preferentially targets to nucleosome-free regions of expressed genes. Further analysis revealed that SYD occupancy peaks exhibit five distinct patterns, which were also shared by BRM and BAF60, a conserved SWI/SNF complex component, indicating the common target sites of these SWI/SNF chromatin remodelers and the functional relevance of such distinct patterns. To investigate the interplay between SYD and Polycomb-group (PcG) proteins, we performed a genome-wide analysis of H3K27me3 in syd-5. We observed both increases and decreases in H3K27me3 levels at a few hundred genes in syd-5 compared to wild type. Our results imply that SYD can act antagonistically or synergistically with PcG at specific genes. Together, our SYD genome-wide occupancy data and the transcriptome and H3K27me3 profiles provide a much-needed resource for dissecting SYD's crucial roles in the regulation of plant growth and development.

BRAHMA-interacting proteins BRIP1 and BRIP2 are core subunits of Arabidopsis SWI/SNF complexes

Switch defective/sucrose non-fermentable (SWI/SNF) chromatin remodelling complexes are multi-protein machineries that control gene expression by regulating chromatin structure in eukaryotes. However, the full subunit composition of SWI/SNF complexes in plants remains unclear. Here we report that in Arabidopsis thaliana, two homologous glioma tumour suppressor candidate region domain-containing proteins, named BRAHMA-interacting proteins 1 (BRIP1) and BRIP2, are core subunits of plant SWI/SNF complexes. brip1 brip2 double mutants exhibit developmental phenotypes and a transcriptome remarkably similar to those of BRAHMA (BRM) mutants. Genetic interaction tests indicated that BRIP1 and BRIP2 act together with BRM to regulate gene expression. Furthermore, BRIP1 and BRIP2 physically interact with BRM-containing SWI/SNF complexes and extensively co-localize with BRM on chromatin. Simultaneous mutation of BRIP1 and BRIP2 results in decreased BRM occupancy at almost all BRM target loci and substantially reduced abundance of the SWI/SNF assemblies. Together, our work identifies new core subunits of BRM-containing SWI/SNF complexes in plants and uncovers the essential role of these subunits in maintaining the abundance of SWI/SNF complexes in plants.

CHR721, interacting with OsRPA1a, is essential for both male and female reproductive development in rice

CHR721 functions as a chromatin remodeler and interacts with a known single-stranded binding protein, OsRPA1a, to regulate both male and female reproductive development in rice. Reproductive development and fertility are important for seed production in rice. Here, we identified a sterile rice mutant, chr721, that exhibited defects in both male and female reproductive development. Approximately 5% of the observed defects in chr721, such as asynchronous dyad division, occurred during anaphase II of meiosis. During the mitotic stage, approximately 80% of uninucleate microspores failed to develop into tricellular pollen, leading to abnormal development. In addition, defects in megaspore development were detected after functional megaspore formation. CHR721, which encodes a nuclear protein belonging to the SNF2 subfamily SMARCAL1, was identified by map-based cloning. CHR721 was expressed in various tissues, especially in spikelets. CHR721 was found to interact with replication protein A (OsRPA1a), which is involved in DNA repair. The expressions of genes involved in DNA repair and cell-cycle checkpoints were consistently upregulated in chr721. Although numerous genes involved in male and female development have been identified, the mode of participation of chromatin-remodeling factors in reproductive development is still not well understood. Our results suggest that CHR721, a novel gene cloned from rice, plays a vital role in both male and female reproductive development.

Genetic and molecular basis of floral induction in Arabidopsis thaliana

Many plants synchronize their life cycles in response to changing seasons and initiate flowering under favourable environmental conditions to ensure reproductive success. To confer a robust seasonal response, plants use diverse genetic programmes that integrate environmental and endogenous cues and converge on central floral regulatory hubs. Technological advances have allowed us to understand these complex processes more completely. Here, we review recent progress in our understanding of genetic and molecular mechanisms that control flowering in Arabidopsis thaliana.

SWI3B and HDA6 interact and are required for transposon silencing in Arabidopsis

Although the interplay of covalent histone acetylation/deacetylation and ATP-dependent chromatin remodelling is crucial for the regulation of chromatin structure and gene expression in eukaryotes, the underlying molecular mechanism in plants remains largely unclear. Here we show a direct interaction between Arabidopsis SWI3B, an essential subunit of the SWI/SNF chromatin-remodelling complex, and the RPD3/HDA1-type histone deacetylase HDA6 both in vitro and in vivo. Furthermore, SWI3B and HDA6 co-repress the transcription of a subset of transposons. Both SWI3B and HDA6 maintain transposon silencing by decreasing histone H3 lysine 9 acetylation, but increasing histone H3 lysine 9 di-methylation, DNA methylation and nucleosome occupancy. Our findings reveal that SWI3B and HDA6 may act in the same co-repressor complex to maintain transposon silencing in Arabidopsis.

Molecular networks regulating cell division during Arabidopsis leaf growth

Leaves are the primary organs for photosynthesis, and as such have a pivotal role for plant growth and development. Leaf development is a multifactorial and dynamic process involving many genes that regulate size, shape, and differentiation. The processes that mainly drive leaf development are cell proliferation and cell expansion, and numerous genes have been identified that, when ectopically expressed or down-regulated, increase cell number and/or cell size during leaf growth. Many of the genes regulating cell proliferation are functionally interconnected and can be grouped into regulatory modules. Here, we review our current understanding of six important gene regulatory modules affecting cell proliferation during Arabidopsis leaf growth: ubiquitin receptor DA1-ENHANCER OF DA1 (EOD1), GROWTH REGULATING FACTOR (GRF)-GRF-INTERACTING FACTOR (GIF), SWITCH/SUCROSE NON-FERMENTING (SWI/SNF), gibberellin (GA)-DELLA, KLU, and PEAPOD (PPD). Furthermore, we discuss how post-mitotic cell expansion and these six modules regulating cell proliferation make up the final leaf size.

Regulation of Shoot Apical Meristem and Axillary Meristem Development in Plants

Plants retain the ability to produce new organs throughout their life cycles. Continuous aboveground organogenesis is achieved by meristems, which are mainly organized, established, and maintained in the shoot apex and leaf axils. This paper will focus on reviewing the recent progress in understanding the regulation of shoot apical meristem and axillary meristem development. We discuss the genetics of plant meristems, the role of plant hormones and environmental factors in meristem development, and the impact of epigenetic factors on meristem organization and function.

Activities of Chromatin Remodeling Factors and Histone Chaperones and Their Effects in Root Apical Meristem Development

Eukaryotic genes are packaged into dynamic but stable chromatin structures to deal with transcriptional reprogramming and inheritance during development. Chromatin remodeling factors and histone chaperones are epigenetic factors that target nucleosomes and/or histones to establish and maintain proper chromatin structures during critical physiological processes such as DNA replication and transcriptional modulation. Root apical meristems are vital for plant root development. Regarding the well-characterized transcription factors involved in stem cell proliferation and differentiation, there is increasing evidence of the functional implications of epigenetic regulation in root apical meristem development. In this review, we focus on the activities of chromatin remodeling factors and histone chaperones in the root apical meristems of the model plant species Arabidopsis and rice.

The SWI/SNF ATP-Dependent Chromatin Remodeling Complex in Arabidopsis Responds to Environmental Changes in Temperature-Dependent Manner

SWI/SNF ATP-dependent chromatin remodeling complexes (CRCs) play important roles in the regulation of transcription, cell cycle, DNA replication, repair, and hormone signaling in eukaryotes. The core of SWI/SNF CRCs composed of a SWI2/SNF2 type ATPase, a SNF5 and two of SWI3 subunits is sufficient for execution of nucleosome remodeling in vitro. The Arabidopsis genome encodes four SWI2/SNF2 ATPases, four SWI3, a single SNF5 and two SWP73 subunits. Genes of the core SWI/SNF components have critical but not fully overlapping roles during plant growth, embryogenesis, and sporophyte development. Here we show that the Arabidopsis swi3c mutant exhibits a phenotypic reversion when grown at lower temperature resulting in partial restoration of its embryo, root development and fertility defects. Our data indicates that the swi3c mutation alters the expression of several genes engaged in low temperature responses. The location of SWI3C-containing SWI/SNF CRCs on the ICE1, MYB15 and CBF1 target genes depends on the temperature conditions, and the swi3c mutation thus also influences the transcription of several cold-responsive (COR) genes. These findings, together with genetic analysis of swi3c/ice1 double mutant and enhanced freezing tolerance of swi3c plants illustrate that SWI/SNF CRCs contribute to fine-tuning of plant growth responses to different temperature regimes.

Unwinding BRAHMA Functions in Plants

The ATP-dependent Switch/Sucrose non-fermenting (SWI/SNF) chromatin remodeling complex (CRC) regulates the transcription of many genes by destabilizing interactions between DNA and histones. In plants, BRAHMA (BRM), one of the two catalytic ATPase subunits of the complex, is the closest homolog of the yeast and animal SWI2/SNF2 ATPases. We summarize here the advances describing the roles of BRM in plant development as well as its recently reported chromatin-independent role in pri-miRNA processing in vitro and in vivo. We also enlighten the roles of plant-specific partners that physically interact with BRM. Three main types of partners can be distinguished: (i) DNA-binding proteins such as transcription factors which mostly cooperate with BRM in developmental processes, (ii) enzymes such as kinases or proteasome-related proteins that use BRM as substrate and are often involved in response to abiotic stress, and (iii) an RNA-binding protein which is involved with BRM in chromatin-independent pri-miRNA processing. This overview contributes to the understanding of the central position occupied by BRM within regulatory networks controlling fundamental biological processes in plants.

Identification and Expression Analysis of Snf2 Family Proteins in Tomato (Solanum lycopersicum)

As part of chromatin-remodeling complexes (CRCs), sucrose nonfermenting 2 (Snf2) family proteins alter chromatin structure and nucleosome position by utilizing the energy of ATP, which allows other regulatory proteins to access DNA. Plant genomes encode a large number of Snf2 proteins, and some of them have been shown to be the key regulators at different developmental stages in Arabidopsis. Yet, little is known about the functions of Snf2 proteins in tomato (Solanum lycopersicum). In this study, 45 Snf2s were identified by the homologous search using representative sequences from yeast (S. cerevisiae), fruit fly (D. melanogaster), and Arabidopsis (A. thaliana) against the tomato genome annotation dataset. Tomato Snf2 proteins (also named SlCHRs) could be clustered into 6 groups and distributed on 11 chromosomes. All SlCHRs contained a helicase-C domain with about 80 amino acid residues and a SNF2-N domain with more variable amino acid residues. In addition, other conserved motifs were also identified in SlCHRs by using the MEME program. Expression profile analysis indicated that tomato Snf2 family genes displayed a wide range of expressions in different tissues and some of them were regulated by the environmental stimuli such as salicylic acid, abscisic acid, salt, and cold. Taken together, these results provide insights into the functions of SlCHRs in tomato.

Floral regulators FLC and SOC1 directly regulate expression of the B3-type transcription factor TARGET OF FLC AND SVP 1 at the Arabidopsis shoot apex via antagonistic chromatin modifications

Integration of environmental and endogenous cues at plant shoot meristems determines the timing of flowering and reproductive development. The MADS box transcription factor FLOWERING LOCUS C (FLC) of Arabidopsis thaliana is an important repressor of floral transition, which blocks flowering until plants are exposed to winter cold. However, the target genes of FLC have not been thoroughly described, and our understanding of the mechanisms by which FLC represses transcription of these targets and how this repression is overcome during floral transition is still fragmentary. Here, we identify and characterize TARGET OF FLC AND SVP1 (TFS1), a novel target gene of FLC and its interacting protein SHORT VEGETATIVE PHASE (SVP). TFS1 encodes a B3-type transcription factor, and we show that tfs1 mutants are later flowering than wild-type, particularly under short days. FLC and SVP repress TFS1 transcription leading to deposition of trimethylation of Iysine 27 of histone 3 (H3K27me3) by the Polycomb Repressive Complex 2 at the TFS1 locus. During floral transition, after downregulation of FLC by cold, TFS1 transcription is promoted by SUPPRESSOR OF OVEREXPRESSION OF CONSTANS1 (SOC1), a MADS box protein encoded by another target of FLC/SVP. SOC1 opposes PRC function at TFS1 through recruitment of the histone demethylase RELATIVE OF EARLY FLOWERING 6 (REF6) and the SWI/SNF chromatin remodeler ATPase BRAHMA (BRM). This recruitment of BRM is also strictly required for SQUAMOSA PROMOTER BINDING PROTEIN-LIKE 9 (SPL9) binding at TFS1 to coordinate RNAPII recruitment through the Mediator complex. Thus, we show that antagonistic chromatin modifications mediated by different MADS box transcription factor complexes play a crucial role in defining the temporal and spatial patterns of transcription of genes within a network of interactions downstream of FLC/SVP during floral transition.

The initiation of flowering in plants is exquisitely sensitive to environmental signals, ensuring that reproduction occurs at the appropriate time of year. The sensitivity of these responses depends upon strong repression of flowering under inappropriate conditions. FLOWERING LOCUS C (FLC) and SHORT VEGETATIVE PHASE (SVP) are related transcription factors that act in concert to strongly inhibit flowering in crucifer plants through repressing transcription of their target genes. Many direct FLC/ SVP targets have been identified in genome-wide studies, however few of these genes have been characterized for their roles in regulating flowering time or other aspects of reproductive development. Here, we characterize TARGET OF FLC AND SVP1 (TFS1) as a novel target of FLC and SVP, and demonstrate that TFS1 contributes to proper flowering-time control. Moreover, we provide a detailed mechanistic view of how TFS1 transcription is controlled during reproductive development through the repressive activity of FLC/SVP being overcome by the transcriptional activator SUPPRESSOR OF OVEREXPRESSION OF CONSTANS 1. Thus we further elucidate the network of genes repressed by FLC/SVP to block flowering and determine mechanisms by which their repressive activity is overcome during the initiation of flowering.

Selective auxin agonists induce specific AUX/IAA protein degradation to modulate plant development

Auxin phytohormones control most aspects of plant development through a complex and interconnected signaling network. In the presence of auxin, AUXIN/INDOLE-3-ACETIC ACID (AUX/IAA) transcriptional repressors are targeted for degradation by the SKP1-CULLIN1-F-BOX (SCF) ubiquitin-protein ligases containing TRANSPORT INHIBITOR RESISTANT 1/AUXIN SIGNALING F-BOX (TIR1/AFB). CULLIN1-neddylation is required for SCF^TIR1/AFB functionality, as exemplified by mutants deficient in the NEDD8-activating enzyme subunit AUXIN-RESISTANT 1 (AXR1). Here, we report a chemical biology screen that identifies small molecules requiring AXR1 to modulate plant development. We selected four molecules of interest, RubNeddin 1 to 4 (RN1 to -4), among which RN3 and RN4 trigger selective auxin responses at transcriptional, biochemical, and morphological levels. This selective activity is explained by their ability to consistently promote the interaction between TIR1 and a specific subset of AUX/IAA proteins, stimulating the degradation of particular AUX/IAA combinations. Finally, we performed a genetic screen using RN4, the RN with the greatest potential for dissecting auxin perception, which revealed that the chromatin remodeling ATPase BRAHMA is implicated in auxin-mediated apical hook development. These results demonstrate the power of selective auxin agonists to dissect auxin perception for plant developmental functions, as well as offering opportunities to discover new molecular players involved in auxin responses.

BRUSHY1/TONSOKU/MGOUN3 is required for heat stress memory

Plants encounter biotic and abiotic stresses many times during their life cycle and this limits their productivity. Moderate heat stress (HS) primes a plant to survive higher temperatures that are lethal in the naïve state. Once temperature stress subsides, the memory of the priming event is actively retained for several days preparing the plant to better cope with recurring HS. Recently, chromatin regulation at different levels has been implicated in HS memory. Here, we report that the chromatin protein BRUSHY1 (BRU1)/TONSOKU/MGOUN3 plays a role in the HS memory in Arabidopsis thaliana. BRU1 is also involved in transcriptional gene silencing and DNA damage repair. This corresponds with the functions of its mammalian orthologue TONSOKU-LIKE/NFΚBIL2. During HS memory, BRU1 is required to maintain sustained induction of HS memory-associated genes, whereas it is dispensable for the acquisition of thermotolerance. In summary, we report that BRU1 is required for HS memory in A. thaliana, and propose a model where BRU1 mediates the epigenetic inheritance of chromatin states across DNA replication and cell division.

The SWI/SNF subunit SWI3B regulates IAMT1 expression via chromatin remodeling in Arabidopsis leaf development

The SWI/SNF complex is crucial to chromatin remodeling in various biological processes in different species, but the distinct functions of its components in plant development remain unclear. Here we uncovered the role of SWI3B, a subunit of the Arabidopsis thaliana SWI/SNF complex, via RNA interference. Knockdown of SWI3B resulted in an upward-curling leaf phenotype. Further investigation showed that the RNA level of IAA carboxyl methyltransferase 1 (IAMT1), encoding an enzyme involved in auxin metabolism, was dramatically elevated in the knockdown (SWI3B-RNAi) plants. In addition, activation of IAMT1 produced a leaf-curling phenotype similar to that of the SWI3B-RNAi lines. Database analysis suggested that the last intron of IAMT contains a site of polymerase V (Pol V) stabilized nucleosome, which may be associated with SWI3B. Data from a micrococcal nuclease (MNase) digestion assay showed that nucleosome occupancy around this site was downregulated in the leaves of SWI3B-RNAi plants. In addition, knockdown of IAMT1 in the SWI3B-RNAi background repressed the abnormal leaf development. Thus, SWI3B-mediated chromatin remodeling is critical in regulating the expression of IAMT1 in leaf development.

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.

Modify the Histone to Win the Battle: Chromatin Dynamics in Plant-Pathogen Interactions

Relying on an immune system comes with a high energetic cost for plants. Defense responses in these organisms are therefore highly regulated and fine-tuned, permitting them to respond pertinently to the attack of a microbial pathogen. In recent years, the importance of the physical modification of chromatin, a highly organized structure composed of genomic DNA and its interacting proteins, has become evident in the research field of plant-pathogen interactions. Several processes, including DNA methylation, changes in histone density and variants, and various histone modifications, have been described as regulators of various developmental and defense responses. Herein, we review the state of the art in the epigenomic aspects of plant immunity, focusing on chromatin modifications, chromatin modifiers, and their physiological consequences. In addition, we explore the exciting field of understanding how plant pathogens have adapted to manipulate the plant epigenomic regulation in order to weaken their immune system and thrive in their host, as well as how histone modifications in eukaryotic pathogens are involved in the regulation of their virulence.

Mechanisms of auxin signaling

The plant hormone auxin triggers complex growth and developmental processes. Its underlying molecular mechanism of action facilitates rapid switching between transcriptional repression and gene activation through the auxin-dependent degradation of transcriptional repressors. The nuclear auxin signaling pathway consists of a small number of core components. However, in most plants each component is represented by a large gene family. The modular construction of the pathway can thus produce diverse transcriptional outputs depending on the cellular and environmental context. Here, and in the accompanying poster, we outline the current model for TIR1/AFB-dependent auxin signaling with an emphasis on recent studies.

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

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

The Core Subunit of A Chromatin-Remodeling Complex, ZmCHB101, Plays Essential Roles in Maize Growth and Development

ATP-dependent chromatin remodeling complexes play essential roles in the regulation of diverse biological processes by formulating a DNA template that is accessible to the general transcription apparatus. Although the function of chromatin remodelers in plant development has been studied in A. thaliana, how it affects growth and development of major crops (e.g., maize) remains uninvestigated. Combining genetic, genomic and bioinformatic analyses, we show here that the maize core subunit of chromatin remodeling complex, ZmCHB101, plays essential roles in growth and development of maize at both vegetative and reproductive stages. Independent ZmCHB101 RNA interference plant lines displayed abaxially curling leaf phenotype due to increase of bulliform cell numbers, and showed impaired development of tassel and cob. RNA-seq-based transcriptome profiling revealed that ZmCHB101 dictated transcriptional reprogramming of a significant set of genes involved in plant development, photosynthesis, metabolic regulation, stress response and gene expressional regulation. Intriguingly, we found that ZmCHB101 was required for maintaining normal nucleosome density and 45 S rDNA compaction. Our findings suggest that the SWI3 protein, ZmCHB101, plays pivotal roles in maize normal growth and development via regulation of chromatin structure.

Asymmetric cell division in plants: mechanisms of symmetry breaking and cell fate determination

Asymmetric cell division is a fundamental mechanism that generates cell diversity while maintaining self-renewing stem cell populations in multicellular organisms. Both intrinsic and extrinsic mechanisms underpin symmetry breaking and differential daughter cell fate determination in animals and plants. The emerging picture suggests that plants deal with the problem of symmetry breaking using unique cell polarity proteins, mobile transcription factors, and cell wall components to influence asymmetric divisions and cell fate. There is a clear role for altered auxin distribution and signaling in distinguishing two daughter cells and an emerging role for epigenetic modifications through chromatin remodelers and DNA methylation in plant cell differentiation. The importance of asymmetric cell division in determining final plant form provides the impetus for its study in the areas of both basic and applied science.

The Role of SWI/SNF Chromatin Remodeling Complexes in Hormone Crosstalk

SWI/SNF-type ATP-dependent chromatin remodeling complexes (CRCs) are evolutionarily conserved multiprotein machineries controlling DNA accessibility by regulating chromatin structure. We summarize here recent advances highlighting the role of SWI/SNF in the regulation of hormone signaling pathways and their crosstalk in Arabidopsis thaliana. We discuss the functional interdependences of SWI/SNF complexes and key elements regulating developmental and hormone signaling pathways by indicating intriguing similarities and differences in plants and humans, and summarize proposed mechanisms of SWI/SNF action on target loci. We postulate that, given their viability, several plant SWI/SNF mutants may serve as an attractive model for searching for conserved functions of SWI/SNF CRCs in hormone signaling, cell cycle control, and other regulatory pathways.

Arabidopsis DNA polymerase ϵ recruits components of Polycomb repressor complex to mediate epigenetic gene silencing

Arabidopsis ESD7 locus encodes the catalytic subunit of the DNA Pol ϵ involved in the synthesis of the DNA leading strand and is essential for embryo viability. The hypomorphic allele esd7-1 is viable but displays a number of pleiotropic phenotypic alterations including an acceleration of flowering time. Furthermore, Pol ϵ is involved in the epigenetic silencing of the floral integrator genes FT and SOC1, but the molecular nature of the transcriptional gene silencing mechanisms involved remains elusive. Here we reveal that ESD7 interacts with components of the PRC2 such as CLF, EMF2 and MSI1, and that mutations in ESD7 cause a decrease in the levels of the H3K27me3 mark present in the chromatin of FT and SOC1. We also demonstrate that a domain of the C-terminal region of ESD7 mediates the binding to the different PRC2 components and this interaction is necessary for the proper recruitment of PRC2 to FT and SOC1 chromatin. We unveil the existence of interplay between the DNA replication machinery and the PcG complexes in epigenetic transcriptional silencing. These observations provide an insight into the mechanisms ensuring that the epigenetic code at pivotal loci in developmental control is faithfully transmitted to the progeny of eukaryotic cells.

A Direct Link between Abscisic Acid Sensing and the Chromatin-Remodeling ATPase BRAHMA via Core ABA Signaling Pathway Components

Optimal response to drought is critical for plant survival and will affect biodiversity and crop performance during climate change. Mitotically heritable epigenetic or dynamic chromatin state changes have been implicated in the plant response to the drought stress hormone abscisic acid (ABA). The Arabidopsis SWI/SNF chromatin-remodeling ATPase BRAHMA (BRM) modulates response to ABA by preventing premature activation of stress response pathways during germination. We show that core ABA signaling pathway components physically interact with BRM and post-translationally modify BRM by phosphorylation/dephosphorylation. Genetic evidence suggests that BRM acts downstream of SnRK2.2/2.3 kinases, and biochemical studies identified phosphorylation sites in the C-terminal region of BRM at SnRK2 target sites that are evolutionarily conserved. Finally, the phosphomimetic BRM(S1760D S1762D) mutant displays ABA hypersensitivity. Prior studies showed that BRM resides at target loci in the ABA pathway in the presence and absence of the stimulus, but is only active in the absence of ABA. Our data suggest that SnRK2-dependent phosphorylation of BRM leads to its inhibition, and PP2CA-mediated dephosphorylation of BRM restores the ability of BRM to repress ABA response. These findings point to the presence of a rapid phosphorylation-based switch to control BRM activity; this property could be potentially harnessed to improve drought tolerance in plants.

Determining Nucleosome Position at Individual Loci After Biotic Stress Using MNase-qPCR

Nucleosome occupancy in promoter and genic regions can severely influence the transcription levels. Few methods have been established to investigate the nucleosome occupancy along the DNA. In this chapter we describe a detailed protocol to analyze the nucleosome occupancy at a specific locus using MNase-pPCR.

Regulation of plant growth and development by the GROWTH-REGULATING FACTOR and GRF-INTERACTING FACTOR duo

Transcription factors are key regulators of gene expression and play pivotal roles in all aspects of living organisms. Therefore, identification and functional characterization of transcription factors is a prerequisite step toward understanding life. This article reviews molecular and biological functions of the two transcription regulator families, GROWTH-REGULATING FACTOR (GRF) and GRF-INTERACTING FACTOR (GIF), which have only recently been recognized. A myriad of experimental evidence clearly illustrates that GRF and GIF are bona fide partner proteins and form a plant-specific transcriptional complex. One of the most conspicuous outcomes from this research field is that the GRF-GIF duo endows the primordial cells of vegetative and reproductive organs with a meristematic specification state, guaranteeing the supply of cells for organogenesis and successful reproduction. It has recently been shown that GIF1 proteins, also known as ANGUSTIFOLIA3, recruit chromatin remodelling complexes to target genes, and that AtGRF expression is directly activated by the floral identity factors, APETALA1 and SEPALLATA3, providing an important insight into understanding of the action of GRF-GIF. Moreover, GRF genes are extensively subjected to post-transcriptional control by microRNA396, revealing the presence of a complex regulatory circuit in regulation of plant growth and development by the GRF-GIF duo.