A plant-specific SWR1 chromatin-remodeling complex couples histone H2A.Z deposition with nucleosome sliding

Simultaneous profiling of chromatin accessibility and DNA methylation in complete plant genomes using long-read sequencing

Epigenetic regulations, including chromatin accessibility, nucleosome positioning and DNA methylation intricately shape genome function. However, current chromatin profiling techniques relying on short-read sequencing technologies fail to characterise highly repetitive genomic regions and cannot detect multiple chromatin features simultaneously. Here, we performed Simultaneous Accessibility and DNA Methylation Sequencing (SAM-seq) of purified plant nuclei. Thanks to the use of long-read nanopore sequencing, SAM-seq enables high-resolution profiling of m6A-tagged chromatin accessibility together with endogenous cytosine methylation in plants. Analysis of naked genomic DNA revealed significant sequence preference biases of m6A-MTases, controllable through a normalisation step. By applying SAM-seq to Arabidopsis and maize nuclei we obtained fine-grained accessibility and DNA methylation landscapes genome-wide. We uncovered crosstalk between chromatin accessibility and DNA methylation within nucleosomes of genes, TEs, and centromeric repeats. SAM-seq also detects DNA footprints over cis-regulatory regions. Furthermore, using the single-molecule information provided by SAM-seq we identified extensive cellular heterogeneity at chromatin domains with antagonistic chromatin marks, suggesting that bivalency reflects cell-specific regulations. SAM-seq is a powerful approach to simultaneously study multiple epigenetic features over unique and repetitive sequences, opening new opportunities for the investigation of epigenetic mechanisms.

Histone Chaperone Deficiency in Arabidopsis Plants Triggers Adaptive Epigenetic Changes in Histone Variants and Modifications

At the molecular scale, adaptive advantages during plant growth and development rely on modulation of gene expression, primarily provided by epigenetic machinery. One crucial part of this machinery is histone posttranslational modifications, which form a flexible system, driving transient changes in chromatin, and defining particular epigenetic states. Posttranslational modifications work in concert with replication-independent histone variants further adapted for transcriptional regulation and chromatin repair. However, little is known about how such complex regulatory pathways are orchestrated and interconnected in cells. In this work, we demonstrate the utility of mass spectrometry-based approaches to explore how different epigenetic layers interact in Arabidopsis mutants lacking certain histone chaperones. We show that defects in histone chaperone function (e.g., chromatin assembly factor-1 or nucleosome assembly protein 1 mutations) translate into an altered epigenetic landscape, which aids the plant in mitigating internal instability. We observe changes in both the levels and distribution of H2A.W.7, altogether with partial repurposing of H3.3 and changes in the key repressive (H3K27me1/2) or euchromatic marks (H3K36me1/2). These shifts in the epigenetic profile serve as a compensatory mechanism in response to impaired integration of the H3.1 histone in the fas1 mutants. Altogether, our findings suggest that maintaining genome stability involves a two-tiered approach. The first relies on flexible adjustments in histone marks, while the second level requires the assistance of chaperones for histone variant replacement.

Chromatin modifications and memory in regulation of stress-related polyphenols: finding new ways to control flavonoid biosynthesis

The influence of epigenetic factors on plant defense responses and the balance between growth and defense is becoming a central area in plant biology. It is believed that the biosynthesis of secondary metabolites can be regulated by epigenetic factors, but this is not associated with the formation of a "memory" to the previous biosynthetic status. This review shows that some epigenetic effects can result in epigenetic memory, which opens up new areas of research in secondary metabolites, in particular flavonoids. Plant-controlled chromatin modifications can lead to the generation of stress memory, a phenomenon through which information regarding past stress cues is retained, resulting in a modified response to recurring stress. How deeply are the mechanisms of chromatin modification and memory generation involved in the control of flavonoid biosynthesis? This article collects available information from the literature and interactome databases to address this issue. Visualization of the interaction of chromatin-modifying proteins with the flavonoid biosynthetic machinery is presented. Chromatin modifiers and "bookmarks" that may be involved in the regulation of flavonoid biosynthesis through memory have been identified. Through different mechanisms of chromatin modification, plants can harmonize flavonoid metabolism with: stress responses, developmental programs, light-dependent processes, flowering, and longevity programs. The available information points to the possibility of developing chromatin-modifying technologies to control flavonoid biosynthesis.

Mind the gap: Epigenetic regulation of chromatin accessibility in plants

Chromatin plays a crucial role in genome compaction and is fundamental for regulating multiple nuclear processes. Nucleosomes, the basic building blocks of chromatin, are central in regulating these processes, determining chromatin accessibility by limiting access to DNA for various proteins and acting as important signaling hubs. The association of histones with DNA in nucleosomes and the folding of chromatin into higher-order structures are strongly influenced by a variety of epigenetic marks, including DNA methylation, histone variants, and histone post-translational modifications. Additionally, a wide array of chaperones and ATP-dependent remodelers regulate various aspects of nucleosome biology, including assembly, deposition, and positioning. This review provides an overview of recent advances in our mechanistic understanding of how nucleosomes and chromatin organization are regulated by epigenetic marks and remodelers in plants. Furthermore, we present current technologies for profiling chromatin accessibility and organization.

Micropillar arrays, wide window acquisition and AI-based data analysis improve comprehensiveness in multiple proteomic applications

Comprehensive proteomic analysis is essential to elucidate molecular pathways and protein functions. Despite tremendous progress in proteomics, current studies still suffer from limited proteomic coverage and dynamic range. Here, we utilize micropillar array columns (µPACs) together with wide-window acquisition and the AI-based CHIMERYS search engine to achieve excellent proteomic comprehensiveness for bulk proteomics, affinity purification mass spectrometry and single cell proteomics. Our data show that µPACs identify ≤50% more peptides and ≤24% more proteins, while offering improved throughput, which is critical for large (clinical) proteomics studies. Combining wide precursor isolation widths of m/z 4-12 with the CHIMERYS search engine identified +51-74% and +59-150% more proteins and peptides, respectively, for single cell, co-immunoprecipitation, and multi-species samples over a conventional workflow at well-controlled false discovery rates. The workflow further offers excellent precision, with CVs <7% for low input bulk samples, and accuracy, with deviations <10% from expected fold changes for regular abundance two-proteome mixes. Compared to a conventional workflow, our entire optimized platform discovered 92% more potential interactors in a protein-protein interaction study on the chromatin remodeler Smarca5/Snf2h. These include previously described Smarca5 binding partners and undescribed ones including Arid1a, another chromatin remodeler with key roles in neurodevelopmental and malignant disorders.

In vitro co-expression chromatin assembly and remodeling platform for plant histone variants

Histone variants play a central role in shaping the chromatin landscape in plants, yet, how their distinct combinations affect nucleosome properties and dynamics is still largely elusive. To address this, we developed a novel chromatin assembly platform for Arabidopsis thaliana, using wheat germ cell-free protein expression. Four canonical histones and five reported histone variants were used to assemble twelve A. thaliana nucleosome combinations. Seven combinations were successfully reconstituted and confirmed by supercoiling and micrococcal nuclease (MNase) assays. The effect of the remodeling function of the CHR11-DDR4 complex on these seven combinations was evaluated based on the nucleosome repeat length and nucleosome spacing index obtained from the MNase ladders. Overall, the current study provides a novel method to elucidate the formation and function of a diverse range of nucleosomes in plants.

Dual roles of the Arabidopsis PEAT complex in histone H2A deubiquitination and H4K5 acetylation

Histone H2A monoubiquitination is associated with transcriptional repression and needs to be removed by deubiquitinases to facilitate gene transcription in eukaryotes. However, the deubiquitinase responsible for genome-wide H2A deubiquitination in plants has yet to be identified. In this study, we found that the previously identified PWWP-EPCR-ARID-TRB (PEAT) complex components interact with both the ubiquitin-specific protease UBP5 and the redundant histone acetyltransferases HAM1 and HAM2 (HAM1/2) to form a larger version of PEAT complex in Arabidopsis thaliana. UBP5 functions as an H2A deubiquitinase in a nucleosome substrate-dependent manner in vitro and mediates H2A deubiquitination at the whole-genome level in vivo. HAM1/2 are shared subunits of the PEAT complex and the conserved NuA4 histone acetyltransferase complex, and are responsible for histone H4K5 acetylation. Within the PEAT complex, the PWWP components (PWWP1, PWWP2, and PWWP3) directly interact with UBP5 and are necessary for UBP5-mediated H2A deubiquitination, while the EPCR components (EPCR1 and EPCR2) directly interact with HAM1/2 and are required for HAM1/2-mediated H4K5 acetylation. Collectively, our study not only identifies dual roles of the PEAT complex in H2A deubiquitination and H4K5 acetylation but also illustrates how these processes collaborate at the whole-genome level to regulate the transcription and development in plants.

Replacement of Arabidopsis H2A.Z with human H2A.Z orthologs reveals extensive functional conservation and limited importance of the N-terminal tail sequence for Arabidopsis development

The incorporation of histone variants, distinct paralogs of core histones, into chromatin affects all DNA-templated processes in the cell, including the regulation of transcription. In recent years, much research has been focused on H2A.Z, an evolutionarily conserved H2A variant found in all eukaryotes. In order to investigate the functional conservation of H2A.Z histones during eukaryotic evolution we transformed h2a.z deficient plants with three human H2A.Z proteins to assess their ability to rescue the mutant defects. We discovered that human H2A.Z.1 and H2A.Z.2.1 fully complement the phenotypic abnormalities of h2a.z plants despite the fact that Arabidopsis and human H2A.Z N-terminal tail sequences are quite divergent. In contrast, the brain-specific splice variant H2A.Z.2.2 has a dominant-negative effect in wild-type plants. Furthermore, H2A.Z.1 almost completely re-establishes normal H2A.Z chromatin occupancy in h2a.z plants and restores the transcript levels of more than 84 % of misexpressed genes. Finally, our hypothesis that the N-terminal tail of Arabidopsis H2A.Z is not crucial for its developmental functions was supported by the ability of N-terminal end truncations of Arabidopsis HTA11 to largely rescue the defects of h2a.z mutants.

HapX-mediated H2B deub1 and SreA-mediated H2A.Z deposition coordinate in fungal iron resistance

Plant pathogens are challenged by host-derived iron starvation or excess during infection, but the mechanism through which pathogens counteract iron stress is unclear. Here, we found that Fusarium graminearum encounters iron excess during the colonization of wheat heads. Deletion of heme activator protein X (FgHapX), siderophore transcription factor A (FgSreA) or both attenuated virulence. Further, we found that FgHapX activates iron storage under iron excess by promoting histone H2B deubiquitination (H2B deub1) at the promoter of the responsible gene. Meanwhile, FgSreA is shown to inhibit genes mediating iron acquisition during iron excess by facilitating the deposition of histone variant H2A.Z and histone 3 lysine 27 trimethylation (H3K27 me3) at the first nucleosome after the transcription start site. In addition, the monothiol glutaredoxin FgGrx4 is responsible for iron sensing and control of the transcriptional activity of FgHapX and FgSreA via modulation of their enrichment at target genes and recruitment of epigenetic regulators, respectively. Taken together, our findings elucidated the molecular mechanisms for adaptation to iron excess mediated by FgHapX and FgSreA during infection in F. graminearum and provide novel insights into regulation of iron homeostasis at the chromatin level in eukaryotes.

Accessible gene borders establish a core structural unit for chromatin architecture in Arabidopsis

Three-dimensional (3D) chromatin structure is linked to transcriptional regulation in multicellular eukaryotes including plants. Taking advantage of high-resolution Hi-C (high-throughput chromatin conformation capture), we detected a small structural unit with 3D chromatin architecture in the Arabidopsis genome, which lacks topologically associating domains, and also in the genomes of tomato, maize, and Marchantia polymorpha. The 3D folding domain unit was usually established around an individual gene and was dependent on chromatin accessibility at the transcription start site (TSS) and transcription end site (TES). We also observed larger contact domains containing two or more neighboring genes, which were dependent on accessible border regions. Binding of transcription factors to accessible TSS/TES regions formed these gene domains. We successfully simulated these Hi-C contact maps via computational modeling using chromatin accessibility as input. Our results demonstrate that gene domains establish basic 3D chromatin architecture units that likely contribute to higher-order 3D genome folding in plants.

WRKY transcription factors and OBERON histone-binding proteins form complexes to balance plant growth and stress tolerance

WRKY transcription factors in plants are known to be able to mediate either transcriptional activation or repression, but the mechanism regulating their transcriptional activity is largely unclear. We found that group IId WRKY transcription factors interact with OBERON (OBE) proteins, forming redundant WRKY-OBE complexes in Arabidopsis thaliana. The coiled-coil domain of WRKY transcription factors binds to OBE proteins and is responsible for target gene selection and transcriptional repression. The PHD finger of OBE proteins binds to both histones and WRKY transcription factors. WRKY-OBE complexes repress the transcription of numerous stress-responsive genes and are required for maintaining normal plant growth. Several WRKY and OBE mutants show reduced plant size and increased drought tolerance, accompanied by increased expression of stress-responsive genes. Moreover, expression levels of most of these WRKY and OBE genes are reduced in response to drought stress, revealing a previously uncharacterized regulatory mechanism of the drought stress response. These results suggest that WRKY-OBE complexes repress transcription of stress-responsive genes, and thereby balance plant growth and stress tolerance.

CHROMATIN REMODELING 11-dependent nucleosome occupancy affects disease resistance in rice

Plant immune responses involve transcriptional reprograming of defense response genes, and chromatin remodeling is important for transcriptional regulation. However, nucleosome dynamics induced by pathogen infection and its association with gene transcription are largely unexplored in plants. Here, we investigated the role of the rice (Oryza sativa) gene CHROMATIN REMODELING 11 (OsCHR11) in nucleosome dynamics and disease resistance. Nucleosome profiling revealed that OsCHR11 is required for the maintaining of genome-wide nucleosome occupancy in rice. Nucleosome occupancy of 14% of the genome was regulated by OsCHR11. Infection of bacterial leaf blight Xoo (Xanthomonas oryzae pv. oryzae) repressed genome-wide nucleosome occupancy, and this process depended on OsCHR11 function. Furthermore, OsCHR11/Xoo-dependent chromatin accessibility correlated with gene transcript induction by Xoo. In addition, accompanied by increased resistance to Xoo, several defense response genes were differentially expressed in oschr11 after Xoo infection. Overall, this study reports the genome-wide effects of pathogen infection on nucleosome occupancy, its regulation, and its contribution to disease resistance in rice.

DDM1-mediated R-loop resolution and H2A.Z exclusion facilitates heterochromatin formation in Arabidopsis

Programmed constitutive heterochromatin silencing is essential for eukaryotic genome regulation, yet the initial step of this process is ambiguous. A large proportion of R-loops (RNA:DNA hybrids) had been unexpectedly identified within Arabidopsis pericentromeric heterochromatin with unknown functions. Through a genome-wide R-loop profiling screen, we find that DDM1 (decrease in DNA methylation 1) is the primary restrictor of pericentromeric R-loops via its RNA:DNA helicase activity. Low levels of pericentromeric R-loops resolved by DDM1 cotranscriptionally can facilitate constitutive heterochromatin silencing. Furthermore, we demonstrate that DDM1 physically excludes histone H2A variant H2A.Z and promotes H2A.W deposition for faithful heterochromatin initiation soon after R-loop clearance. The dual functions of DDM1 in R-loop resolution and H2A.Z eviction are essential for sperm nuclei structure maintenance in mature pollen. Our work unravels the cotranscriptional R-loop resolution coupled with accurate H2A variants deposition is the primary step of constitutive heterochromatin silencing in Arabidopsis, which might be conserved across eukaryotes.

Spotlight on Plant Bromodomain Proteins

Bromodomain-containing proteins (BRD-proteins) are the "readers" of histone lysine acetylation, translating chromatin state into gene expression. They act alone or as components of larger complexes and exhibit diverse functions to regulate gene expression; they participate in chromatin remodeling complexes, mediate histone modifications, serve as scaffolds to recruit transcriptional regulators or act themselves as transcriptional co-activators or repressors. Human BRD-proteins have been extensively studied and have gained interest as potential drug targets for various diseases, whereas in plants, this group of proteins is still not well investigated. In this review, we aimed to concentrate scientific knowledge on these chromatin "readers" with a focus on Arabidopsis. We organized plant BRD-proteins into groups based on their functions and domain architecture and summarized the published work regarding their interactions, activity and diverse functions. Overall, it seems that plant BRD-proteins are indispensable components and fine-tuners of the complex network plants have built to regulate development, flowering, hormone signaling and response to various biotic or abiotic stresses. This work will facilitate the understanding of their roles in plants and highlight BRD-proteins with yet undiscovered functions.

High ambient temperature impacts on flowering time in Brassica napus through both H2A.Z-dependent and independent mechanisms

Knowledge concerning the integration of genetic pathways mediating the responses to environmental cues controlling flowering initiation in crops is scarce. Here, we reveal the diversity in oilseed rape (OSR) flowering response to high ambient temperature. Using a set of different spring OSR varieties, we found a consistent flowering delay at elevated temperatures. Remarkably, one of the varieties assayed exhibited the opposite behaviour. Several FT-like paralogs are plausible candidates to be part of the florigen in OSR. We revealed that BnaFTA2 plays a major role in temperature-dependent flowering initiation. Analysis of the H2A.Z histone variant occupancy at this locus in different Brassica napus varieties produced contrasting results, suggesting the involvement of additional molecular mechanisms in BnaFTA2 repression at high ambient temperature. Moreover, BnARP6 RNAi plants showed little accumulation of H2A.Z at high temperature while maintaining temperature sensitivity and delayed flowering. Furthermore, we found that H3K4me3 present in BnaFTA2 under inductive flowering conditions is reduced at high temperature, suggesting a role for this hallmark of transcriptionally active chromatin in the OSR flowering response to warming. Our work emphasises the plasticity of flowering responses in B. napus and offers venues to optimise this process in crop species grown under suboptimal environmental conditions.

The dynamics of Arabidopsis H2A.Z on SMALL AUXIN UP RNAs regulates abscisic acid-auxin signaling crosstalk

Extreme environmental changes threaten plant survival and worldwide food production. In response to osmotic stresses, plant hormone ABA activates stress responses and restricts plant growth. However, the epigenetic regulation of the ABA signaling and ABA-auxin crosstalk are not well known. Here we report that the histone variant H2A.Z knockdown mutant in Arabidopsis Col-0 ecotype, h2a.z-kd, has altered ABA signaling and stress performances. RNA-sequencing data showed that a majority of stress related genes are activated in h2a.z-kd. In addition, we revealed that ABA directly promotes the deposition of H2A.Z on SMALL AUXIN UP RNAs (SAURs), which is involved in ABA-repressed SAUR expression. Moreover, we found that ABA represses the transcription of H2A.Z genes through suppressing ARF7/19-HB22/25 module. Our results shed light on a dynamic and reciprocal regulation hub through H2A.Z deposition on SAURs and ARF7/19-HB22/25-mediated H2A.Z transcription to integrate ABA/auxin signaling and regulate stress responses in Arabidopsis.

RUVBL proteins are involved in plant gametophyte development

The proper development of male and female gametophytes is critical for successful sexual reproduction and requires a carefully regulated series of events orchestrated by a suite of various proteins. RUVBL1 and RUVBL2, plant orthologues of human Pontin and Reptin, respectively, belong to the evolutionarily highly conserved AAA⁺ family linked to a wide range of cellular processes. Previously, we found that RUVBL1 and RUVBL2A mutations are homozygous lethal in Arabidopsis. Here, we report that RUVBL1 and RUVBL2A play roles in reproductive development. We show that mutant plants produce embryo sacs with an abnormal structure or with various numbers of nuclei. Although pollen grains of heterozygous mutant plants exhibit reduced viability and reduced pollen tube growth in vitro, some of the ruvbl pollen tubes are capable of targeting ovules in vivo. Similarly, some ruvbl ovules retain the ability to attract wild-type pollen tubes but fail to develop further. The activity of the RUVBL1 and RUVBL2A promoters was observed in the embryo sac, pollen grains, and tapetum cells and, for RUVBL2A, also in developing ovules. In summary, we show that the RUVBL proteins are essential for the proper development of both male and particularly female gametophytes in Arabidopsis.

PMT6 Is Required for SWC4 in Positively Modulating Pepper Thermotolerance

High temperature stress (HTS), with growth and development impairment, is one of the most important abiotic stresses frequently encountered by plants, in particular solanacaes such as pepper, that mainly distribute in tropical and subtropical regions. Plants activate thermotolerance to cope with this stress; however, the underlying mechanism is currently not fully understood. SWC4, a shared component of SWR1- and NuA4 complexes implicated in chromatin remodeling, was previously found to be involved in the regulation of pepper thermotolerance, but the underlying mechanism remains poorly understood. Herein, PMT6, a putative methyltranferase was originally found to interact with SWC4 by co-immunoprecipitation (Co-IP)-combined LC/MS assay. This interaction was further confirmed by bimolecular fluorescent complimentary (BiFC) and Co-IP assay, and PMT6 was further found to confer SWC4 methylation. By virus-induced gene silencing, it was found that PMT6 silencing significantly reduced pepper basal thermotolerance and transcription of CaHSP24 and significantly reduced the enrichment of chromatin-activation-related H3K9ac, H4K5ac, and H3K4me3 in TSS of CaHSP24, which was previously found to be positively regulated by CaSWC4. By contrast, the overexpression of PMT6 significantly enhanced basal thermotolerance of pepper plants. All these data indicate that PMT6 acts as a positive regulator in pepper thermotolerance, likely by methylating SWC4.

Contribution of the histone variant H2A.Z to expression of responsive genes in plants

The histone variant H2A.Z plays a critical role in chromatin-based processes such as transcription, replication, and repair in eukaryotes. Although many H2A.Z-associated processes and features are conserved in plants and animals, a distinguishing feature of plant chromatin is the enrichment of H2A.Z in the bodies of genes that exhibit dynamic expression, particularly in response to differentiation and the environment. Recent work sheds new light on the plant machinery that enables dynamic changes in H2A.Z enrichment and identifies additional chromatin-based pathways that contribute to transcriptional properties of H2A.Z-enriched chromatin. In particular, analysis of a variety of responsive loci reveals a repressive role for H2A.Z in expression of responsive genes and identifies roles for SWR1 and INO80 chromatin remodelers in enabling dynamic regulation of H2A.Z levels and transcription. These studies lay the groundwork for understanding how this ancient histone variant is harnessed by plants to enable responsive and dynamic gene expression (Graphical Abstract).

AtMCM10 promotes DNA replication-coupled nucleosome assembly in Arabidopsis

Minichromosome Maintenance protein 10 (MCM10) is essential for DNA replication initiation and DNA elongation in yeasts and animals. Although the functions of MCM10 in DNA replication and repair have been well documented, the detailed mechanisms for MCM10 in these processes are not well known. Here, we identified AtMCM10 gene through a forward genetic screening for releasing a silenced marker gene. Although plant MCM10 possesses a similar crystal structure as animal MCM10, AtMCM10 is not essential for plant growth or development in Arabidopsis. AtMCM10 can directly bind to histone H3-H4 and promotes nucleosome assembly in vitro. The nucleosome density is decreased in Atmcm10, and most of the nucleosome density decreased regions in Atmcm10 are also regulated by newly synthesized histone chaperone Chromatin Assembly Factor-1 (CAF-1). Loss of both AtMCM10 and CAF-1 is embryo lethal, indicating that AtMCM10 and CAF-1 are indispensable for replication-coupled nucleosome assembly. AtMCM10 interacts with both new and parental histones. Atmcm10 mutants have lower H3.1 abundance and reduced H3K27me1/3 levels with releasing some silenced transposons. We propose that AtMCM10 deposits new and parental histones during nucleosome assembly, maintaining proper epigenetic modifications and genome stability during DNA replication.

Comprehensive Survey of ChIP-Seq Datasets to Identify Candidate Iron Homeostasis Genes Regulated by Chromatin Modifications

Vital biochemical reactions including photosynthesis to respiration require iron, which should be tightly regulated. Although increasing evidence reveals the importance of epigenetic regulation in gene expression and signaling, the role of histone modifications and chromatin remodeling in plant iron homeostasis is not well understood. In this study, we surveyed publicly available ChIP-seq datasets of Arabidopsis wild-type and mutants defective in key enzymes of histone modification and chromatin remodeling and compared the deposition of epigenetic marks on loci of genes involved in iron regulation. Based on the analysis, we compiled a comprehensive list of iron homeostasis genes with differential enrichment of various histone modifications. This report will provide a resource for future studies to investigate epigenetic regulatory mechanisms of iron homeostasis in plants.

Histone variants and modifications during abiotic stress response

Plants have developed multiple mechanisms as an adaptive response to abiotic stresses, such as salinity, drought, heat, cold, and oxidative stress. Understanding these regulatory networks is critical for coping with the negative impact of abiotic stress on crop productivity worldwide and, eventually, for the rational design of strategies to improve plant performance. Plant alterations upon stress are driven by changes in transcriptional regulation, which rely on locus-specific changes in chromatin accessibility. This process encompasses post-translational modifications of histone proteins that alter the DNA-histones binding, the exchange of canonical histones by variants that modify chromatin conformation, and DNA methylation, which has an implication in the silencing and activation of hypervariable genes. Here, we review the current understanding of the role of the major epigenetic modifications during the abiotic stress response and discuss the intricate relationship among them.

Transcription elongator SPT6L regulates the occupancies of the SWI2/SNF2 chromatin remodelers SYD/BRM and nucleosomes at transcription start sites in Arabidopsis

Chromatin remodelers have been thought to be crucial in creating an accessible chromatin environment before transcription activation. However, it is still unclear how chromatin remodelers recognize and bind to the active regions. In this study, we found that chromatin remodelers SPLAYED (SYD) and BRAHMA (BRM) interact and co-occupy with Suppressor of Ty6-like (SPT6L), a core subunit of the transcription machinery, at thousands of the transcription start sites (TSS). The association of SYD and BRM to chromatin is dramatically reduced in spt6l and can be restored mainly by SPT6LΔtSH2, which binds to TSS in a RNA polymerase II (Pol II)-independent manner. Furthermore, SPT6L and SYD/BRM are involved in regulating the nucleosome and Pol II occupancy around TSS. The presence of SPT6L is sufficient to restore the association of the chromatin remodeler SYD to chromatin and maintain normal nucleosome occupancy. Our findings suggest that the two chromatin remodelers can form protein complexes with the core subunit of the transcription machinery and regulate nucleosome occupancy in the early transcription stage.

Feedback regulation of auxin signaling through the transcription of H2A.Z and deposition of H2A.Z to SMALL AUXIN UP RNAs in Arabidopsis

Auxin is a critical phytohormone that is involved in the regulation of most plant growth and developmental responses. In particular, epigenetic mechanisms, like histone modifications and DNA methylation, were reported to affect auxin biosynthesis and transport. However, the involvement of other epigenetic factors, such as histone variant H2A.Z, in the auxin-related developmental regulation remains unclear. We report that the histone variant H2A.Z knockdown mutant in Arabidopsis Col-0 ecotype, h2a.z-kd, has more lateral roots and weak gravitational responses related to auxin-regulated growth performances. Further study revealed that auxin promotes the eviction of H2A.Z from the auxin-responsive genes SMALL AUXIN-UP RNAs (SAURs) to activate their transcriptions. We found that IAA promotes the transcription of H2A.Z genes through HOMEOBOX PROTEIN 22/25 (AtHB22/25) transcription factors which work as downstream targets of ARF7/19 in auxin signaling. Double mutant of hb22 hb25 showed similar lateral root and gravitropism phenotypes to h2a.z-kd. Our results shed light on a reciprocal regulation hub through INOSITOL AUXOTROPHY 80-mediated H2A.Z eviction and ARF7/19-HB22/25-mediated H2A.Z transcription to modulate the activation of SAURs and plant growth in Arabidopsis.

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.

Histone modification and chromatin remodeling in plant response to pathogens

As sessile organisms, plants are constantly exposed to changing environments frequently under diverse stresses. Invasion by pathogens, including virus, bacterial and fungal infections, can severely impede plant growth and development, causing important yield loss and thus challenging food/feed security worldwide. During evolution, plants have adapted complex systems, including coordinated global gene expression networks, to defend against pathogen attacks. In recent years, growing evidences indicate that pathogen infections can trigger local and global epigenetic changes that reprogram the transcription of plant defense genes, which in turn helps plants to fight against pathogens. Here, we summarize up plant defense pathways and epigenetic mechanisms and we review in depth current knowledge's about histone modifications and chromatin-remodeling factors found in the epigenetic regulation of plant response to biotic stresses. It is anticipated that epigenetic mechanisms may be explorable in the design of tools to generate stress-resistant plant varieties.

MS1/MMD1 homologues in the moss Physcomitrium patens are required for male and female gametogenesis

The Arabidopsis Plant HomeoDomain (PHD) proteins AtMS1 and AtMMD1 provide chromatin-mediated transcriptional regulation essential for tapetum-dependent pollen formation. This pollen-based male gametogenesis is a derived trait of seed plants. Male gametogenesis in the common ancestors of land plants is instead likely to have been reminiscent of that in extant bryophytes where flagellated sperms are produced by an elaborate gametophyte generation. Still, also bryophytes possess MS1/MMD1-related PHD proteins. We addressed the function of two MS1/MMD1-homologues in the bryophyte model moss Physcomitrium patens by the generation and analysis of reporter and loss-of-function lines. The two genes are together essential for both male and female fertility by providing functions in the gamete-producing inner cells of antheridia and archegonia. They are furthermore expressed in the diploid sporophyte generation suggesting a function during sporogenesis, a process proposed related by descent to pollen formation in angiosperms. We propose that the moss MS1/MMD1-related regulatory network required for completion of male and female gametogenesis, and possibly for sporogenesis, represent a heritage from ancestral land plants.

Deposition and eviction of histone variants define functional chromatin states in plants

The organization of DNA with histone proteins into chromatin is fundamental for the regulation of gene expression. Incorporation of different histone variants into the nucleosome together with post-translational modifications of these histone variants allows modulating chromatin accessibility and contributes to the establishment of functional chromatin states either permissive or repressive for transcription. This review highlights emerging mechanisms required to deposit or evict histone variants in a timely and locus-specific manner. This review further discusses how assembly of specific histone variants permits to reinforce transmission of chromatin states during replication, to maintain heterochromatin organization and stability and to reprogram existing epigenetic information.

Systematic histone H4 replacement in Arabidopsis thaliana reveals a role for H4R17 in regulating flowering time

Despite the broad array of roles for epigenetic mechanisms on regulating diverse processes in eukaryotes, no experimental system is currently available in plants for the direct assessment of histone function. In this work, we present the development of a genetic strategy in Arabidopsis (Arabidopsis thaliana) whereby modified histone H4 transgenes can completely replace the expression of endogenous histone H4 genes. Accordingly, we established a collection of plants expressing different H4 point mutants targeting residues that may be post-translationally modified in vivo. To demonstrate its utility, we screened this new H4 mutant collection to uncover substitutions in H4 that alter flowering time. We identified different mutations in the H4 tail (H4R17A) and the H4 globular domain (H4R36A, H4R39K, H4R39A, and H4K44A) that strongly accelerate the floral transition. Furthermore, we identified a conserved regulatory relationship between H4R17 and the ISWI chromatin remodeling complex in plants: As with other biological systems, H4R17 regulates nucleosome spacing via ISWI. Overall, this work provides a large set of H4 mutants to the plant epigenetics community that can be used to systematically assess histone H4 function in Arabidopsis and a roadmap to replicate this strategy for studying other histone proteins in plants.

Histone Variants in the Specialization of Plant Chromatin

The basic unit of chromatin, the nucleosome, is an octamer of four core histone proteins (H2A, H2B, H3, and H4) and serves as a fundamental regulatory unit in all DNA-templated processes. The majority of nucleosome assembly occurs during DNA replication when these core histones are produced en masse to accommodate the nascent genome. In addition, there are a number of nonallelic sequence variants of H2A and H3 in particular, known as histone variants, that can be incorporated into nucleosomes in a targeted and replication-independent manner. By virtue of their sequence divergence from the replication-coupled histones, these histone variants can impart unique properties onto the nucleosomes they occupy and thereby influence transcription and epigenetic states, DNA repair, chromosome segregation, and other nuclear processes in ways that profoundly affect plant biology. In this review, we discuss the evolutionary origins of these variants in plants, their known roles in chromatin, and their impacts on plant development and stress responses. We focus on the individual and combined roles of histone variants in transcriptional regulation within euchromatic and heterochromatic genome regions. Finally, we highlight gaps in our understanding of plant variants at the molecular, cellular, and organismal levels, and we propose new directions for study in the field of plant histone variants.

The Arabidopsis NuA4 histone acetyltransferase complex is required for chlorophyll biosynthesis and photosynthesis

Although two Enhancer of Polycomb-like proteins, EPL1A and EPL1B (EPL1A/B), are known to be conserved and characteristic subunits of the NuA4-type histone acetyltransferase complex in Arabidopsis thaliana, the biological function of EPL1A/B and the mechanism by which EPL1A/B function in the complex remain unknown. Here, we report that EPL1A/B are required for the histone acetyltransferase activity of the NuA4 complex on the nucleosomal histone H4 in vitro and for the enrichment of histone H4K5 acetylation at thousands of protein-coding genes in vivo. Our results suggest that EPL1A/B are required for linking the NuA4 catalytic subunits HISTONE ACETYLTRANSFERASE OF THE MYST FAMILY 1（HAM1) and HAM2 with accessory subunits in the NuA4 complex. EPL1A/B function redundantly in regulating plant development especially in chlorophyll biosynthesis and de-etiolation. The EPL1A/B-dependent transcription and H4K5Ac are enriched at genes involved in chlorophyll biosynthesis and photosynthesis. We also find that EAF6, another characteristic subunit of the NuA4 complex, contributes to de-etiolation. These results suggest that the Arabidopsis NuA4 complex components function as a whole to mediate histone acetylation and transcriptional activation specifically at light-responsive genes and are critical for photomorphogenesis.

Unraveling Microbial Volatile Elicitors Using a Transparent Methodology for Induction of Systemic Resistance and Regulation of Antioxidant Genes at Expression Levels in Chili against Bacterial Wilt Disease

Microbial volatiles benefit the agricultural ecological system by promoting plant growth and systemic resistance against diseases without harming the environment. To explore the plant growth-promoting efficiency of VOCs produced by Pseudomonas fluorescens PDS1 and Bacillus subtilis KA9 in terms of chili plant growth and its biocontrol efficiency against Ralstonia solanacearum, experiments were conducted both in vitro and in vivo. A closure assembly was designed using a half-inverted plastic bottle to demonstrate plant–microbial interactions via volatile compounds. The most common volatile organic compounds were identified and reported; they promoted plant development and induced systemic resistance (ISR) against wilt pathogen R. solanacearum. The PDS1 and KA9 VOCs significantly increased defensive enzyme activity and overexpressed the antioxidant genes PAL, POD, SOD, WRKYa, PAL1, DEF-1, CAT-2, WRKY40, HSFC1, LOX2, and NPR1 related to plant defense. The overall gene expression was greater in root tissue as compared to leaf tissue in chili plant. Our findings shed light on the relationship among rhizobacteria, pathogen, and host plants, resulting in plant growth promotion, disease suppression, systemic resistance-inducing potential, and antioxidant response with related gene expression in the leaf and root tissue of chili.

Epigenetics: a catalyst of plant immunity against pathogens

The plant immune system protects against pests and diseases. The recognition of stress-related molecular patterns triggers localised immune responses, which are often followed by longer-lasting systemic priming and/or up-regulation of defences. In some cases, this induced resistance (IR) can be transmitted to following generations. Such transgenerational IR is gradually reversed in the absence of stress at a rate that is proportional to the severity of disease experienced in previous generations. This review outlines the mechanisms by which epigenetic responses to pathogen infection shape the plant immune system across expanding time scales. We review the cis- and trans-acting mechanisms by which stress-inducible epigenetic changes at transposable elements (TEs) regulate genome-wide defence gene expression and draw particular attention to one regulatory model that is supported by recent evidence about the function of AGO1 and H2A.Z in transcriptional control of defence genes. Additionally, we explore how stress-induced mobilisation of epigenetically controlled TEs acts as a catalyst of Darwinian evolution by generating (epi)genetic diversity at environmentally responsive genes. This raises questions about the long-term evolutionary consequences of stress-induced diversification of the plant immune system in relation to the long-held dichotomy between Darwinian and Lamarckian evolution.

Chromatin enrichment for proteomics in plants (ChEP-P) implicates the histone reader ALFIN-LIKE 6 in jasmonate signalling

Background

Covalent modifications of core histones govern downstream DNA-templated processes such as transcription by altering chromatin structure and function. Previously, we reported that the plant homeodomain protein ALFIN-LIKE 6 (AL6), a bona fide histone reader that preferentially binds trimethylated lysin 4 on histone 3 (H3K4me3), is critical for recalibration of cellular phosphate (Pi) homeostasis and root hair elongation under Pi-deficient conditions.

Results

Here, we demonstrate that AL6 is also involved in the response of Arabidopsis seedlings to jasmonic acid (JA) during skotomorphogenesis, possibly by modulating chromatin dynamics that affect the transcriptional regulation of JA-responsive genes. Dark-grown al6 seedlings showed a compromised reduction in hypocotyl elongation upon exogenously supplied JA, a response that was calibrated by the availability of Pi in the growth medium. A comparison of protein profiles between wild-type and al6 mutant seedlings using a quantitative Chromatin Enrichment for Proteomics (ChEP) approach, that we modified for plant tissue and designated ChEP-P (ChEP in Plants), yielded a comprehensive suite of chromatin-associated proteins and candidates that may be causative for the mutant phenotype.

Conclusions

Altered abundance of proteins involved in chromatin organization in al6 seedlings suggests a role of AL6 in coordinating the deposition of histone variants upon perception of internal or environmental stimuli. Our study shows that ChEP-P is well suited to gain holistic insights into chromatin-related processes in plants. Data are available via ProteomeXchange with identifier PXD026541.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12864-021-08160-6.

COMPASS functions as a module of the INO80 chromatin remodeling complex to mediate histone H3K4 methylation in Arabidopsis

In the INO80 chromatin remodeling complex, all of the accessory subunits are assembled on the following three domains of INO80: N-terminal domain (NTD), HSA domain, and ATPase domain. Although the ATPase and HSA domains and their interacting accessory subunits are known to be responsible for chromatin remodeling, it is largely unknown how the accessory subunits that interact with the INO80 NTD regulate chromatin status. Here, we identify both conserved and nonconserved accessory subunits that interact with the three domains in the INO80 complex in Arabidopsis thaliana. While the accessory subunits that interact with all the three INO80 domains can mediate transcriptional repression, the INO80 NTD and the accessory subunits interact with it can contribute to transcriptional activation even when the ATPase domain is absent, suggesting that INO80 has an ATPase-independent role. A subclass of the COMPASS histone H3K4 methyltransferase complexes interact with the INO80 NTD in the INO80 complex and function together with the other accessory subunits that interact with the INO80 NTD, thereby facilitating H3K4 trimethylation and transcriptional activation. This study suggests that the opposite effects of the INO80 complex on transcription are required for the balance between vegetative growth and flowering under diverse environmental conditions.

Three functionally redundant plant-specific paralogs are core subunits of the SAGA histone acetyltransferase complex in Arabidopsis

The SAGA (Spt-Ada-Gcn5 acetyltransferase) complex is an evolutionarily conserved histone acetyltransferase complex that has a critical role in histone acetylation, gene expression, and various developmental processes in eukaryotes. However, little is known about the composition and function of the SAGA complex in plants. In this study, we found that the SAGA complex in Arabidopsis thaliana contains not only conserved subunits but also four plant-specific subunits: three functionally redundant paralogs, SCS1, SCS2A, and SCS2B (SCS1/2A/2B), and a TAF-like subunit, TAFL. Mutations in SCS1/2A/2B lead to defective phenotypes similar to those caused by mutations in the genes encoding conserved SAGA subunits HAG1 and ADA2B, including delayed juvenile-to-adult phase transition, late flowering, and increased trichome density. Furthermore, we demonstrated that SCS1/2A/2B are required for the function of the SAGA complex in histone acetylation, thereby promoting the transcription of development-related genes. These results together suggest that SCS1/2A/2B are core subunits of the SAGA complex in Arabidopsis. Compared with SAGA complexes in other eukaryotes, the SAGA complexes in plants have evolved unique features that are necessary for normal growth and development.

Spatiotemporal control of miR398 biogenesis, via chromatin remodeling and kinase signaling, ensures proper ovule development

The coordinated development of sporophytic and gametophytic tissues is essential for proper ovule patterning and fertility. However, the mechanisms regulating their integrated development remain poorly understood. Here, we report that the Swi2/Snf2-Related1 (SWR1) chromatin-remodeling complex acts with the ERECTA receptor kinase-signaling pathway to control female gametophyte and integument growth in Arabidopsis thaliana by inhibiting transcription of the microRNA gene MIR398c in early-stage megagametogenesis. Moreover, pri-miR398c is transcribed in the female gametophyte but is then translocated to and processed in the ovule sporophytic tissues. Together, SWR1 and ERECTA also activate ARGONAUTE10 (AGO10) expression in the chalaza; AGO10 sequesters miR398, thereby ensuring the expression of three AGAMOUS-LIKE (AGL) genes (AGL51, AGL52, and AGL78) in the female gametophyte. In the context of sexual organ morphogenesis, these findings suggest that the spatiotemporal control of miRNA biogenesis, resulting from coordination between chromatin remodeling and cell signaling, is essential for proper ovule development in Arabidopsis.

NF-YCs modulate histone variant H2A.Z deposition to regulate photomorphogenic growth in Arabidopsis

In plants, light signals trigger a photomorphogenic program involving transcriptome changes, epigenetic regulation, and inhibited hypocotyl elongation. The evolutionarily conserved histone variant H2A.Z, which functions in transcriptional regulation, is deposited in chromatin by the SWI2/SNF2-RELATED 1 complex (SWR1c). However, the role of H2A.Z in photomorphogenesis and its deposition mechanism remain unclear. Here, we show that in Arabidopsis thaliana, H2A.Z deposition at its target loci is induced by light irradiation via NUCLEAR FACTOR-Y, subunit C (NF-YC) proteins, thereby inhibiting photomorphogenic growth. NF-YCs physically interact with ACTIN-RELATED PROTEIN6 (ARP6), a key component of the SWR1c that is essential for depositing H2A.Z, in a light-dependent manner. NF-YCs and ARP6 function together as negative regulators of hypocotyl growth by depositing H2A.Z at their target genes during photomorphogenesis. Our findings reveal an important role for the histone variant H2A.Z in photomorphogenic growth and provide insights into a novel transcription regulatory node that mediates H2A.Z deposition to control plant growth in response to changing light conditions.

ERECTA signaling regulates plant immune responses via chromatin-mediated promotion of WRKY33 binding to target genes

The signaling pathway mediated by the receptor-like kinase ERECTA (ER) plays important roles in plant immune responses, but the underlying mechanism is unclear. Genetic interactions between ER signaling and the chromatin remodeling complex SWR1 in the control of plant immune responses were studied. Electrophoretic mobility shift assay and yeast one-hybrid analysis were applied to identify ER-WRKY33 downstream components. Chromatin immunoprecipitation analyses were further investigated. In this study, we show that the chromatin remodeling complex SWR1 enhances resistance to the white mold fungus Sclerotinia sclerotiorum in Arabidopsis thaliana via a process mediated by ER signaling. We identify a series of WRKY33 target YODA DOWNSTREAM (YDD) genes and demonstrate that SWR1 and ER signaling are required to enrich H2A.Z histone variant and H3K4me3 histone modification at YDDs and the binding of WRKY33 to YDD promoters upon S. sclerotiorum infection. We also reveal that the binding of WRKY33 to YDD promoters in turn promotes the enrichment of H2A.Z and H3K4me3 at YDD genes, thereby forming a positive regulatory loop to activate YDDs expression. Our study reveals how H2A.Z, H3K4me3 and ER signaling mutually regulate YDDs gene expression upon pathogen infection, highlighting the critical role of chromatin structure in ER-signaling-mediated plant immune responses.

Chromatin remodeling factors regulate environmental stress responses in plants

Environmental stress from climate change and agricultural activity threatens global plant biodiversity as well as crop yield and quality. As sessile organisms, plants must maintain the integrity of their genomes and adjust gene expression to adapt to various environmental changes. In eukaryotes, nucleosomes are the basic unit of chromatin around which genomic DNA is packaged by condensation. To enable dynamic access to packaged DNA, eukaryotes have evolved Snf2 (sucrose nonfermenting 2) family proteins as chromatin remodeling factors (CHRs) that modulate the position of nucleosomes on chromatin. During plant stress responses, CHRs are recruited to specific genomic loci, where they regulate the distribution or composition of nucleosomes, which in turn alters the accessibility of these loci to general transcription or DNA damage repair machinery. Moreover, CHRs interplay with other epigenetic mechanisms, including DNA methylation, histone modifications, and deposition of histone variants. CHRs are also involved in RNA processing at the post-transcriptional level. In this review, we discuss major advances in our understanding of the mechanisms by which CHRs function during plants' response to environmental stress.

Overexpression of a methyl-CpG-binding protein gene OsMBD707 leads to larger tiller angles and reduced photoperiod sensitivity in rice

BACKGROUND

Methyl-CpG-binding domain (MBD) proteins play important roles in epigenetic gene regulation, and have diverse molecular, cellular, and biological functions in plants. MBD proteins have been functionally characterized in various plant species, including Arabidopsis, wheat, maize, and tomato. In rice, 17 sequences were bioinformatically predicted as putative MBD proteins. However, very little is known regarding the function of MBD proteins in rice.

RESULTS

We explored the expression patterns of the rice OsMBD family genes and identified 13 OsMBDs with active expression in various rice tissues. We further characterized the function of a rice class I MBD protein OsMBD707, and demonstrated that OsMBD707 is constitutively expressed and localized in the nucleus. Transgenic rice overexpressing OsMBD707 displayed larger tiller angles and reduced photoperiod sensitivity-delayed flowering under short day (SD) and early flowering under long day (LD). RNA-seq analysis revealed that overexpression of OsMBD707 led to reduced photoperiod sensitivity in rice and to expression changes in flowering regulator genes in the Ehd1-Hd3a/RFT1 pathway.

CONCLUSION

The results of this study suggested that OsMBD707 plays important roles in rice growth and development, and should lead to further studies on the functions of OsMBD proteins in growth, development, or other molecular, cellular, and biological processes in rice.

The Histone Acetyltransferase GCN5 and the Associated Coactivators ADA2: From Evolution of the SAGA Complex to the Biological Roles in Plants

Transcription of protein-encoding genes starts with forming a pre-initiation complex comprised of RNA polymerase II and several general transcription factors. To activate gene expression, transcription factors must overcome repressive chromatin structure, which is accomplished with multiprotein complexes. One such complex, SAGA, modifies the nucleosomal histones through acetylation and other histone modifications. A prototypical histone acetyltransferase (HAT) known as general control non-repressed protein 5 (GCN5), was defined biochemically as the first transcription-linked HAT with specificity for histone H3 lysine 14. In this review, we analyze the components of the putative plant SAGA complex during plant evolution, and current knowledge on the biological role of the key components of the HAT module, GCN5 and ADA2b in plants, will be summarized.

Gene co-expression analysis reveals transcriptome divergence between wild and cultivated chickpea under drought stress

Ancestral adaptations in crop wild relatives can provide a genetic reservoir for crop improvement. Here we document physiological changes to mild and severe drought stress, and the associated transcriptome dynamics in both wild and cultivated chickpea. Over 60% of transcriptional changes were related to metabolism, indicating that metabolic plasticity is a core and conserved drought response. In addition, changes in RNA processing and protein turnover were predominant in the data, suggestive of broad restructuring of the chickpea proteome in response to drought. While 12% of the drought-responsive transcripts have similar dynamics in cultivated and wild accessions, numerous transcripts had expression patterns unique to particular genotypes, or that distinguished wild from cultivated genotypes and whose divergence may be a consequence of domestication. These and other comparisons provide a transcriptional correlate of previously described species' genetic diversity, with wild accessions well differentiated from each other and from cultivars, and cultivars essentially indistinguishable at the broad transcriptome level. We identified metabolic pathways such as phenylpropanoid metabolism, and biological processes such as stomatal development, which are differentially regulated across genotypes with potential consequences on drought tolerance. These data indicate that wild Cicer reticulatum may provide both conserved and divergent mechanisms as a resource in breeding for drought tolerance in cultivated chickpea.

FHA2 is a plant-specific ISWI subunit responsible for stamen development and plant fertility

Imitation Switch (ISWI) chromatin remodelers are known to function in diverse multi-subunit complexes in yeast and animals. However, the constitution and function of ISWI complexes in Arabidopsis thaliana remain unclear. In this study, we identified forkhead-associated domain 2 (FHA2) as a plant-specific subunit of an ISWI chromatin-remodeling complex in Arabidopsis. By in vivo and in vitro analyses, we demonstrated that FHA2 directly binds to RLT1 and RLT2, two redundant subunits of the ISWI complex in Arabidopsis. The stamen filament is shorter in the fha2 and rlt1/2 mutants than in the wild type, whereas their pistil lengths are comparable. The shorter filament, which is due to reduced cell size, results in insufficient pollination and reduced fertility. The rlt1/2 mutant shows an early-flowering phenotype, whereas the phenotype is not shared by the fha2 mutant. Consistent with the functional specificity of FHA2, our RNA-seq analysis indicated that the fha2 mutant affects a subset of RLT1/2-regulated genes that does not include genes involved in the regulation of flowering time. This study demonstrates that FHA2 functions as a previously uncharacterized subunit of the Arabidopsis ISWI complex and is exclusively involved in regulating stamen development and plant fertility.

A plant-specific SWR1 chromatin-remodeling complex couples histone H2A.Z deposition with nucleosome sliding

Deposition of H2A.Z in chromatin is known to be mediated by a conserved SWR1 chromatin-remodeling complex in eukaryotes. However, little is known about whether and how the SWR1 complex cooperates with other chromatin regulators. Using immunoprecipitation followed by mass spectrometry, we found all known components of the Arabidopsis thaliana SWR1 complex and additionally identified the following three classes of previously uncharacterized plant-specific SWR1 components: MBD9, a methyl-CpG-binding domain-containing protein; CHR11 and CHR17 (CHR11/17), ISWI chromatin remodelers responsible for nucleosome sliding; and TRA1a and TRA1b, accessory subunits of the conserved NuA4 histone acetyltransferase complex. MBD9 directly interacts with CHR11/17 and the SWR1 catalytic subunit PIE1, and is responsible for the association of CHR11/17 with the SWR1 complex. MBD9, TRA1a, and TRA1b function as canonical components of the SWR1 complex to mediate H2A.Z deposition. CHR11/17 are not only responsible for nucleosome sliding but also involved in H2A.Z deposition. These results indicate that the association of the SWR1 complex with CHR11/17 may facilitate the coupling of H2A.Z deposition with nucleosome sliding, thereby co-regulating gene expression, development, and flowering time.