A warm temperature-released negative feedback loop fine-tunes PIF4-mediated thermomorphogenesis in Arabidopsis.

Plants can sense temperature changes and adjust their growth accordingly. In Arabidopsis, high ambient temperatures stimulate stem elongation by activating a key thermoresponsive regulator, PHYTOCHROME INTERACTING FACTOR 4 (PIF4). Here, we show that warmth promotes the nighttime transcription of GI, which is necessary for the high temperature-induced transcription of TOC1. Genetic analyses suggest that GI prevents excessive thermoresponsive growth by inhibiting PIF4, with this regulatory mechanism being partially reliant on TOC1. GI transcription is repressed by ELF3 and HY5, which concurrently inhibit PIF4 expression and activity. Temperature elevation causes the deactivation or degradation of ELF3 and HY5, leading to PIF4 activation and relief of GI transcriptional repression at high temperatures. This allows PIF4 to further activate GI transcription in response to elevated temperatures. GI, in turn, inhibits PIF4, establishing a negative feedback loop that fine-tunes PIF4 activity. In addition, we demonstrate that ELF3, HY5, and PIF4 regulate GI transcription by modulating the enrichment of histone variant H2A.Z at the GI locus. Together, our findings suggest that thermal release of a negative feedback loop finely adjusts plant thermomorphogenesis.

25 Years of thermomorphogenesis research: milestones and perspectives.

In 1998, Bill Gray and colleagues showed that warm temperatures trigger arabidopsis hypocotyl elongation in an auxin-dependent manner. This laid the foundation for a vibrant research discipline. With several active members of the 'thermomorphogenesis' community, we here reflect on 25 years of elevated ambient temperature research and look to the future.

Variation is important: Warranting chromatin function and dynamics by histone variants.

The chromatin of flowering plants exhibits a wide range of sequence variants of the core and linker histones. Recent studies have demonstrated that specific histone variant enrichment, combined with post-translational modifications (PTMs) of histones, defines distinct chromatin states that impact specific chromatin functions. Chromatin remodelers are emerging as key regulators of histone variant dynamics, contributing to shaping chromatin states and regulating gene transcription in response to environment. Recognizing the histone variants by their specific readers, controlled by histone PTMs, is crucial for maintaining genome and chromatin integrity. In addition, various histone variants have been shown to play essential roles in remodeling chromatin domains to facilitate important programmed transitions throughout the plant life cycle. In this review, we discuss recent findings in this exciting field of research, which holds immense promise for many surprising discoveries related to the evolution of complexity in plant organization through a seemingly simple protein family.

TONSOKU is required for the maintenance of repressive chromatin modifications in Arabidopsis.

The stability of eukaryotic genomes relies on the faithful transmission of DNA sequences and the maintenance of chromatin states through DNA replication. Plant TONSOKU (TSK) and its animal ortholog TONSOKU-like (TONSL) act as readers for newly synthesized histones and preserve DNA integrity via facilitating DNA repair at post-replicative chromatin. However, whether TSK/TONSL regulate the maintenance of chromatin states remains elusive. Here, we show that TSK is dispensable for global histone and nucleosome accumulation but necessary for maintaining repressive chromatin modifications, including H3K9me2, H2A.W, H3K27me3, and DNA methylation. TSK physically interacts with H3K9 methyltransferases and Polycomb proteins. Moreover, TSK mutation strongly enhances defects in Polycomb pathway mutants. TSK is intended to only associate with nascent chromatin until it starts to mature. We propose that TSK ensures the preservation of chromatin states by supporting the recruitment of chromatin modifiers to post-replicative chromatin in a critical short window of time following DNA replication.

Coordinated histone variant H2A.Z eviction and H3.3 deposition control plant thermomorphogenesis.

Plants can sense temperature changes and adjust their development and morphology accordingly in a process called thermomorphogenesis. This phenotypic plasticity implies complex mechanisms regulating gene expression reprogramming in response to environmental alteration. Histone variants often associate with specific chromatin states; yet, how their deposition/eviction modulates transcriptional changes induced by environmental cues remains elusive. In Arabidopsis thaliana, temperature elevation-induced transcriptional activation at thermo-responsive genes entails the chromatin eviction of a histone variant H2A.Z by INO80, which is recruited to these loci via interacting with a key thermomorphogenesis regulator PIF4. Here, we show that both INO80 and the deposition chaperones of another histone variant H3.3 associate with ELF7, a critical component of the transcription elongator PAF1 complex. H3.3 promotes thermomorphogenesis and the high temperature-enhanced RNA Pol II transcription at PIF4 targets, and it is broadly required for the H2A.Z removal-induced gene activation. Reciprocally, INO80 and ELF7 regulate H3.3 deposition, and are necessary for the high temperature-induced H3.3 enrichment at PIF4 targets. Our findings demonstrate close coordination between H2A.Z eviction and H3.3 deposition in gene activation induced by high temperature, and pinpoint the importance of histone variants dynamics in transcriptional regulation.

A REF6-dependent H3K27me3-depleted state facilitates gene activation during germination in Arabidopsis.

Seed germination is a critical developmental switch from a quiescent state to active growth, which involves extensive changes in metabolism, gene expression, and cellular identity. However, our understanding of epigenetic and transcriptional reprogramming during this process is limited. The histone H3 lysine 27 trimethylation (H3K27me3) plays a key role in regulating gene repression and cell fate specification. Here, we profile H3K27me3 dynamics and dissect the function of H3K27 demethylation during germination in Arabidopsis. Our temporal genome-wide profiling of H3K27me3 and transcription reveals delayed H3K27me3 reprogramming compared with transcriptomic changes during germination, with H3K27me3 changes mainly occurring when the embryo is entering into vegetative development. RELATIVE OF EARLY FLOWERING 6 (REF6)-mediated H3K27 demethylation is necessary for robust germination but does not significantly contribute to H3K27me3 dynamics during germination, but rather stably establishes an H3K27me3-depleted state that facilitates the activation of hormone-related and expansin-coding genes important for germination. We also show that the REF6 chromatin occupancy is gradually established during germination to counteract increased Polycomb repressive complex 2 (PRC2). Our study provides key insights into the H3K27me3 dynamics during germination and suggests the function of H3K27me3 in facilitating cell fate switch. Furthermore, we reveal the importance of H3K27 demethylation-established transcriptional competence in gene activation during germination and likely other developmental processes.

Histone H3.3 deposition in seed is essential for the post-embryonic developmental competence in Arabidopsis.

The acquisition of germination and post-embryonic developmental ability during seed maturation is vital for seed vigor, an important trait for plant propagation and crop production. How seed vigor is established in seeds is still poorly understood. Here, we report the crucial function of Arabidopsis histone variant H3.3 in endowing seeds with post-embryonic developmental potentials. H3.3 is not essential for seed formation, but loss of H3.3 results in severely impaired germination and post-embryonic development. H3.3 exhibits a seed-specific 5' gene end distribution and facilitates chromatin opening at regulatory regions in seeds. During germination, H3.3 is essential for proper gene transcriptional regulation. Moreover, H3.3 is constantly loaded at the 3' gene end, correlating with gene body DNA methylation and the restriction of chromatin accessibility and cryptic transcription at this region. Our results suggest a fundamental role of H3.3 in initiating chromatin accessibility at regulatory regions in seed and licensing the embryonic to post-embryonic transition.

Hierarchical deployment of Tbx3 dictates the identity of hypothalamic KNDy neurons to control puberty onset.

The neuroendocrine system consists of a heterogeneous collection of neuropeptidergic neurons in the brain, among which hypothalamic KNDy neurons represent an indispensable cell subtype controlling puberty onset. Although neural progenitors and neuronal precursors along the cell lineage hierarchy adopt a cascade diversification strategy to generate hypothalamic neuronal heterogeneity, the cellular logic operating within the lineage to specify a subtype of neuroendocrine neurons remains unclear. As human genetic studies have recently established a link between TBX3 mutations and delayed puberty onset, we systematically studied Tbx3-derived neuronal lineage and Tbx3-dependent neuronal specification and found that Tbx3 hierarchically established and maintained the identity of KNDy neurons for triggering puberty. Apart from the well-established lineage-dependent fate determination, we uncovered rules of interlineage interaction and intralineage retention operating through neuronal differentiation in the absence of Tbx3. Moreover, we revealed that human TBX3 mutations disturbed the phase separation of encoded proteins and impaired transcriptional regulation of key neuropeptides, providing a pathological mechanism underlying TBX3-associated puberty disorders.

DNA polymerase epsilon binds histone H3.1-H4 and recruits MORC1 to mediate meiotic heterochromatin condensation.

Heterochromatin is essential for genomic integrity and stability in eukaryotes. The mechanisms that regulate meiotic heterochromatin formation remain largely undefined. Here, we show that the catalytic subunit (POL2A) of Arabidopsis DNA polymerase epsilon (POL ε) is required for proper formation of meiotic heterochromatin. The POL2A N terminus interacts with the GHKL adenosine triphosphatase (ATPase) MORC1 (Microrchidia 1), and POL2A is required for MORC1's localization on meiotic heterochromatin. Mutations affecting the POL2A N terminus cause aberrant morphology of meiotic heterochromatin, which is also observed in morc1. Moreover, the POL2A C-terminal zinc finger domain (ZF1) specifically binds to histone H3.1-H4 dimer or tetramer and is important for meiotic heterochromatin condensation. Interestingly, we also found similar H3.1-binding specificity for the mouse counterpart. Together, our results show that two distinct domains of POL2A, ZF1 and N terminus bind H3.1-H4 and recruit MORC1, respectively, to induce a continuous process of meiotic heterochromatin organization. These activities expand the functional repertoire of POL ε beyond its classic role in DNA replication and appear to be conserved in animals and plants.

Cap-binding complex assists RNA polymerase II transcription in plant salt stress response.

Adaptive response to stress involves an extensive reprogramming of gene expression. Under stressful conditions, the induction of efficient changes in messenger RNA (mRNA) production is crucial for maximized plant survival. Transcription and pre-mRNA processing are two closely related steps in mRNA biogenesis, yet how they are controlled in plant stress response remains elusive. Here, we show that the Arabidopsis nuclear cap-binding complex (CBC) component CBP20 directly interacts with ELF7, a subunit of the transcription elongation factor RNA Pol II-associated factor 1 complex (PAF1c) to promote RNA Pol II transcription in plant response to salt stress. CBP20 and ELF7 coregulate the expression of a large number of genes including those crucial for salt tolerance. Both CBP20 and ELF7 are required for enhanced RNA Pol II elongation at salt-activated genes. Though CBP20 also regulates intron splicing, this function is largely independent of ELF7. Our study reveals the function of an RNA processing regulator CBC in assisting efficient RNA Pol II transcription and pinpoints the complex roles of CBC on mRNA production in plant salt stress resistance.

The histone variant H3.3 promotes the active chromatin state to repress flowering in Arabidopsis.

The histone H3 family in animals and plants includes replicative H3 and nonreplicative H3.3 variants. H3.3 preferentially associates with active transcription, yet its function in development and transcription regulation remains elusive. The floral transition in Arabidopsis (Arabidopsis thaliana) involves complex chromatin regulation at a central flowering repressor FLOWERING LOCUS C (FLC). Here, we show that H3.3 upregulates FLC expression and promotes active histone modifications histone H3 lysine 4 trimethylation (H3K4me3) and histone H3 lysine 36 trimethylation (H3K36me3) at the FLC locus. The FLC activator FRIGIDA (FRI) directly mediates H3.3 enrichment at FLC, leading to chromatin conformation changes and further induction of active histone modifications at FLC. Moreover, the antagonistic H3.3 and H2A.Z act in concert to activate FLC expression, likely by forming unstable nucleosomes ideal for transcription processing. We also show that H3.3 knockdown leads to H3K4me3 reduction at a subset of particularly short genes, suggesting the general role of H3.3 in promoting H3K4me3. The finding that H3.3 stably accumulates at FLC in the absence of H3K36me3 indicates that the H3.3 deposition may serve as a prerequisite for active histone modifications. Our results reveal the important function of H3.3 in mediating the active chromatin state for flowering repression.

The theoretical mechanism of Parkinson's oscillation frequency bands: a computational model study.

Excessive synchronous oscillation activities appear in the brain is a key pathological feature of Parkinson's disease, the mechanism of which is still unclear. Although some previous studies indicated that ß oscillation (13-30 Hz) may directly originate in the network composed of the subthalamic nucleus (STN) and external globus pallidus (GPe) neurons, specific onset mechanisms of which are unclear, especially theoretical evidences in numerical simulation are still little. In this paper, we employ a STN-GPe mean-field model to explore the onset mechanism of Parkinson's oscillation. In addition to ß oscillation, we find that some other common oscillation frequency bands can appear in this network, such as the α oscillation band (8-12 Hz), the θ oscillation band (4-7 Hz) and δ oscillation band (1-3 Hz). In addition to the coupling weight between the STN and GPe, the delay is also a critical factor to affect oscillatory activities, which can not be neglected compared to other parameters. Through simulation and analysis, we propose two possible theories may induce the system to transfer from the stable state to the oscillatory state in this model: (1). The oscillation activity can be induced when the firing activation level of the population increases to large enough; (2). In some special cases, the population may stay in the high firing rate stable state and the mean discharge rate of which is too large to induce oscillations, then oscillation activities may be induced as the MD decreases to moderate value. In most situations, the change trends of the MD and oscillation dominant frequency are contrary, which may be an important physiological phenomenon shown in this model. The delays and firing rates were obtained by simulating, which may be verified in the experiment in the future.

The INO80 chromatin remodeling complex promotes thermomorphogenesis by connecting H2A.Z eviction and active transcription in Arabidopsis.

Global warming poses a major threat to plant growth and crop production. In some plants, including Arabidopsis thaliana, elevated temperatures induce a series of morphological and developmental adjustments termed thermomorphogenesis, which facilitates plant cooling under high-temperature conditions. Plant thermal response is suppressed by histone variant H2A.Z. At warm temperatures, H2A.Z is evicted from nucleosomes at thermoresponsive genes, resulting in changes in their expression. However, the mechanisms that regulate H2A.Z eviction and subsequent transcriptional changes are largely unknown. Here, we show that the INO80 chromatin remodeling complex (INO80-C) promotes thermomorphogenesis and activates the expression of thermoresponsive and auxin-related genes. INO80-C associates with PHYTOCHROME-INTERACTING FACTOR 4 (PIF4), a potent regulator of thermomorphogenesis, and mediates temperature-induced H2A.Z eviction at PIF4 targets. Moreover, INO80-C directly interacts with COMPASS-like and transcription elongation factors to promote active histone modification, histone H3 lysine 4 trimethylation, and RNA polymerase II elongation, leading to the thermal induction of transcription. Notably, the transcription elongation factors SPT4 and SPT5 are required for H2A.Z eviction at PIF4 targets, suggesting the cooperation of INO80-C and transcription elongation in H2A.Z removal. Taken together, these results suggest that the (PIF4)-(INO80-C)-(COMPASS-like)-(transcription elongator) module controls plant thermal response, thereby establishing a link between H2A.Z eviction and active transcription.

Histone variants take center stage in shaping the epigenome.

The dynamic properties of the nucleosome are central to genomic activity. Variants of the core histones that form the nucleosome play a pivotal role in modulating nucleosome structure and function. Despite often small differences in sequence, histone variants display remarkable diversity in genomic deposition and post-translational modification. Here, we summarize the roles played by histone variants in the establishment, maintenance and reprogramming of plant chromatin landscapes, with a focus on histone H3 variants. Deposition of replicative H3.1 during DNA replication controls epigenetic inheritance, while local replacement of H3.1 with H3.3 marks cells undergoing terminal differentiation. Deposition of specialized H3 variants in specific cell types is emerging as a novel mechanism of selective epigenetic reprogramming during the plant life cycle.

The evolution and functional divergence of the histone H2B family in plants.

Chromatin regulation of eukaryotic genomes depends on the formation of nucleosome complexes between histone proteins and DNA. Histone variants, which are diversified by sequence or expression pattern, can profoundly alter chromatin properties. While variants in histone H2A and H3 families are well characterized, the extent of diversification of histone H2B proteins is less understood. Here, we report a systematic analysis of the histone H2B family in plants, which have undergone substantial divergence during the evolution of each major group in the plant kingdom. By characterising Arabidopsis H2Bs, we substantiate this diversification and reveal potential functional specialization that parallels the phylogenetic structure of emergent clades in eudicots. In addition, we identify a new class of highly divergent H2B variants, H2B.S, that specifically accumulate during chromatin compaction of dry seed embryos in multiple species of flowering plants. Our findings thus identify unsuspected diverse properties among histone H2B proteins in plants that has manifested into potentially novel groups of histone variants.

Histone modifications and their regulatory roles in plant development and environmental memory.

Plants grow in dynamic environments where they receive diverse environmental signals. Swift and precise control of gene expression is essential for plants to align their development and metabolism with fluctuating surroundings. Modifications on histones serve as "histone code" to specify chromatin and gene activities. Different modifications execute distinct functions on the chromatin, promoting either active transcription or gene silencing. Histone writers, erasers, and readers mediate the regulation of histone modifications by catalyzing, removing, and recognizing modifications, respectively. Growing evidence indicates the important function of histone modifications in plant development and environmental responses. Histone modifications also serve as environmental memory for plants to adapt to environmental changes. Here we review recent progress on the regulation of histone modifications in plants, the impact of histone modifications on environment-controlled developmental transitions including germination and flowering, and the role of histone modifications in environmental memory.

FRIGIDA establishes a local chromosomal environment for FLOWERING LOCUS C mRNA production.

FRIGIDA (FRI) upregulates the expression of the potent floral repressor FLOWERING LOCUS C (FLC) to confer the winter-annual growth habit in Arabidopsis thaliana: accelerated transition to flowering after prolonged cold exposure (vernalization). Here, we show that FRI, histone acetyltransferases, the histone methyltransferase COMPASS-like and other chromatin modifiers are part of a FRI-containing supercomplex enriched in a region around the FLC transcription start site (TSS) to promote its expression. Several FRI partners are also enriched in a 3' region flanking FLC and, together with FRI, they function to increase the frequency of physical association of the region around TSS with the 3' region and promote the expression of both sense FLC and antisense non-coding RNAs. Our results show that the FRI supercomplex establishes a local chromosomal environment at FLC with active chromatin modifications and topology to promote transcriptional activation, fast elongation and efficient pre-messenger RNA splicing, leading to a high-level production of FLC mRNAs.

LHP1 Interacts with ATRX through Plant-Specific Domains at Specific Loci Targeted by PRC2.

Heterochromatin Protein 1 (HP1) is a major regulator of chromatin structure and function. In animals, the network of proteins interacting with HP1 is mainly associated with constitutive heterochromatin marked by H3K9me3. HP1 physically interacts with the putative ortholog of the SNF2 chromatin remodeler ATRX, which controls deposition of histone variant H3.3 in mammals. In this study, we show that the Arabidopsis thaliana ortholog of ATRX participates in H3.3 deposition and possesses specific conserved domains in plants. We found that plant Like HP1 (LHP1) protein interacts with ATRX through domains that evolved specifically in land plant ancestors. Loss of ATRX function in Arabidopsis affects the expression of a limited subset of genes controlled by PRC2 (POLYCOMB REPRESSIVE COMPLEX 2), including the flowering time regulator FLC. The function of ATRX in regulation of flowering time requires novel LHP1-interacting domain and ATPase activity of the ATRX SNF2 helicase domain. Taken together, these results suggest that distinct evolutionary pathways led to the interaction between ATRX and HP1 in mammals and its counterpart LHP1 in plants, resulting in distinct modes of transcriptional regulation.