The histone methylation readers MRG1/MRG2 and the histone chaperones NRP1/NRP2 associate in fine-tuning Arabidopsis flowering time

Interplay between histone variants and chaperones in plants

Histone chaperones and histone variants play crucial roles in DNA replication, gene transcription, and DNA repair in eukaryotes. Histone chaperones reversibly promote nucleosome assembly and disassembly by incorporating or evicting histones and histone variants to modulate chromatin accessibility, thereby altering the chromatin states and modulating DNA-related biological processes. Cofactors assist histone chaperones to target specific chromatin regions to regulate the exchange of histones and histone variants. In this review, we summarize recent progress in the interplay between histone variants and chaperones in plants. We discuss the structural basis of chaperone-histone complexes and the mechanisms of their cooperation in regulating gene transcription and plant development.

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.

Histone methylation readers MRG1/2 interact with PIF4 to promote thermomorphogenesis in Arabidopsis

Warm ambient conditions induce thermomorphogenesis and affect plant growth and development. However, the chromatin regulatory mechanisms involved in thermomorphogenesis remain largely obscure. In this study, we show that the histone methylation readers MORF-related gene 1 and 2 (MRG1/2) are required to promote hypocotyl elongation in response to warm ambient conditions. A transcriptome sequencing analysis indicates that MRG1/2 and phytochrome interacting factor 4 (PIF4) coactivate a number of thermoresponsive genes, including YUCCA8, which encodes a rate-limiting enzyme in the auxin biosynthesis pathway. Additionally, MRG2 physically interacts with PIF4 to bind to thermoresponsive genes and enhances the H4K5 acetylation of the chromatin of target genes in a PIF4-dependent manner. Furthermore, MRG2 competes with phyB for binding to PIF4 and stabilizes PIF4 in planta. Our study indicates that MRG1/2 activate thermoresponsive genes by inducing histone acetylation and stabilizing PIF4 in Arabidopsis.

Histone methylation readers MRG1/MRG2 interact with the transcription factor TCP14 to positively modulate cytokinin sensitivity in Arabidopsis

Cytokinins influence many aspects of plant growth and development. Although cytokinin biosynthesis and signaling have been well studied in planta, little is known about the regulatory effects of epigenetic modifications on the cytokinin response. Here, we reveal that mutations to Morf Related Gene (MRG) proteins MRG1/MRG2, which are readers of trimethylated histone H3 lysine 4 and lysine 36 (H3K4me3 and H3K36me3), result in cytokinin hyposensitivity during various developmental processes, including callus induction and root and seedling growth inhibition. Similar to the mrg1 mrg2 mutant, plants with a defective AtTCP14, which belongs to the TEOSINTE BRANCHED, CYCLOIDEA, AND PROLIFERATING CELL FACTOR (TCP) transcription factor family, are insensitive to cytokinin. Furthermore, the transcription of several genes related to cytokinin signaling pathway is altered. Specifically, the expression of Arabidopsis thalianaHISTIDINE-CONTAINING PHOSPHOTRANSMITTER PROTEIN 2 (AHP2) decreases significantly in the mrg1 mrg2 and tcp14-2 mutants. We also confirm the interaction between MRG2 and TCP14 in vitro and in vivo. Thus, MRG2 and TCP14 can be recruited to AHP2 after recognizing H3K4me3/H3K36me3 markers and promote the histone-4 lysine-5 acetylation to further enhance AHP2 expression. In summary, our research elucidate a previously unknown mechanism mediating the effects of MRG proteins on the magnitude of the cytokinin response.

Rice OsMRG702 and Its Partner OsMRGBP Control Flowering Time through H4 Acetylation

MORF-RELATED GENE702 (OsMRG702) regulates flowering time genes in rice, but how it controls transcription is not well known. Here, we found that OsMRGBP can directly interact with OsMRG702. Both Osmrg702 and Osmrgbp mutants show the delayed flowering phenotype with the reduction in the transcription of multiple key flowering time genes, including Ehd1 and RFT1. Chromatin immunoprecipitation study showed that both OsMRG702 and OsMRGBP bind to the Ehd1 and RFT1 loci and the absence of either OsMRG702 or OsMRGBP leads to a decrease of H4K5 acetylation at these loci, indicating OsMRG702 and OsMRGBP cooperatively together to promote the H4K5 acetylation. In addition, whilst Ghd7 are upregulated in both Osmrg702 and Osmrgbp mutants, only OsMRG702 binds to the loci, together with the global increased and Ghd7 locus-specific increased H4K5ac levels in Osmrg702 mutants, suggesting an additional negative effect of OsMRG702 on H4K5 acetylation. In summary, OsMRG702 controls flowering gene regulation by altering H4 acetylation in rice; it works either together with OsMRGBP to enhance transcription by promoting H4 acetylation or with other unknown mechanisms to dampen transcription by preventing H4 acetylation.

Photoperiodic flowering in Arabidopsis: Multilayered regulatory mechanisms of CONSTANS and the florigen FLOWERING LOCUS T

The timing of flowering affects the success of sexual reproduction. This developmental event also determines crop yield, biomass, and longevity. Therefore, this mechanism has been targeted for improvement along with crop domestication. The underlying mechanisms of flowering are highly conserved in angiosperms. Central to these mechanisms is how environmental and endogenous conditions control transcriptional regulation of the FLOWERING LOCUS T (FT) gene, which initiates floral development under long-day conditions in Arabidopsis. Since the identification of FT as florigen, efforts have been made to understand the regulatory mechanisms of FT expression. Although many transcriptional regulators have been shown to directly influence FT, the question of how they coordinately control the spatiotemporal expression patterns of FT still requires further investigation. Among FT regulators, CONSTANS (CO) is the primary one whose protein stability is tightly controlled by phosphorylation and ubiquitination/proteasome-mediated mechanisms. In addition, various CO interaction partners, some of them previously identified as FT transcriptional regulators, positively or negatively modulate CO protein activity. The FT promoter possesses several transcriptional regulatory "blocks," highly conserved regions among Brassicaceae plants. Different transcription factors bind to specific blocks and affect FT expression, often causing topological changes in FT chromatin structure, such as the formation of DNA loops. We discuss the current understanding of the regulation of FT expression mainly in Arabidopsis and propose future directions related to this topic.

Arabidopsis γ-H2A.X-INTERACTING PROTEIN participates in DNA damage response and safeguards chromatin stability

Upon the occurrence of DNA double strand breaks (DSB), the proximal histone variant H2A.X is phosphorylated as γ-H2A.X, a critical signal for consequent DSB signaling and repair pathways. Although γ-H2A.X-triggered DNA damage response (DDR) has been well-characterized in yeast and animals, the corresponding pathways in plant DDR are less well understood. Here, we show that an Arabidopsis protein γ-H2A.X-INTERACTING PROTEIN (XIP) can interact with γ-H2A.X. Its C-terminal dual-BRCT-like domain contributes to its specific interaction with γ-H2A.X. XIP-deficient seedlings display smaller meristems, inhibited growth, and higher sensitivity to DSB-inducing treatment. Loss-of-function in XIP causes transcriptome changes mimicking wild-type plants subject to replicative or genotoxic stresses. After genotoxic bleomycin treatment, more proteins with upregulated phosphorylation modifications, more DNA fragments and cell death were found in xip mutants. Moreover, XIP physically interacts with RAD51, the key recombinase in homologous recombination (HR), and somatic HR frequency is significantly reduced in xip mutants. Collectively, XIP participates in plant response to DSB and contributes to chromatin stability.

Methylation in DNA, histone, and RNA during flowering under stress condition: A review

Flowering is the most critical transition period in the whole lifecycle of plants, and it is a highly sensitive period to stress. New combinations of temperature, drought stress, carbon dioxide and other abiotic/biotic conditions resulting from contemporary climate change affect the flowering process. Plants have evolved several strategies to deal with environmental stresses, including epigenetic modifications. Numerous studies show that environmental stresses trigger methylation/demethylation during flowering to preserve/accelerate plant lifecycle. What's more, histone and DNA methylation can be induced to respond to stresses, resulting in changes of flowering gene expression and enhancing stress tolerance in plants. Furthermore, RNA methylation may influence stress-regulated flowering by regulating mRNA stability and antioxidant mechanism. Our review presents the involvement of methylation in stress-repressed and stress-induced flowering. The crosstalk between methylation and small RNAs, phytohormones and exogenous substances (such as salicylic acid, nitric oxide) during flowering under different stresses were discussed. The latest regulatory evidence of RNA methylation in stress-regulated flowering was collected for the first time. Meanwhile, the limited evidences of methylation in biotic stress-induced flowering were summarized. Thus, the review provides insights into understanding of methylation mechanism in stress-regulated flowering and makes use for the development of regulating plant flowering at epigenetic level in the future.

Active and adaptive plasticity in a changing climate

Better understanding of the mechanistic basis of plant plasticity will enhance efforts to breed crops resilient to predicted climate change. However, complexity in plasticity's conceptualisation and measurement may hinder fruitful crossover of concepts between disciplines that would enable such advances. We argue active adaptive plasticity is particularly important in shaping the fitness of wild plants, representing the first line of a plant's defence to environmental change. Here, we define how this concept may be applied to crop breeding, suggest appropriate approaches to measure it in crops, and propose a refocussing on active adaptive plasticity to enhance crop resilience. We also discuss how the same concept may have wider utility, such as in ex situ plant conservation and reintroductions.

The genomic basis of the plant island syndrome in Darwin's giant daisies

The repeated, rapid and often pronounced patterns of evolutionary divergence observed in insular plants, or the ‘plant island syndrome’, include changes in leaf phenotypes, growth, as well as the acquisition of a perennial lifestyle. Here, we sequence and describe the genome of the critically endangered, Galápagos-endemic species Scalesia atractyloides Arnot., obtaining a chromosome-resolved, 3.2-Gbp assembly containing 43,093 candidate gene models. Using a combination of fossil transposable elements, k-mer spectra analyses and orthologue assignment, we identify the two ancestral genomes, and date their divergence and the polyploidization event, concluding that the ancestor of all extant Scalesia species was an allotetraploid. There are a comparable number of genes and transposable elements across the two subgenomes, and while their synteny has been mostly conserved, we find multiple inversions that may have facilitated adaptation. We identify clear signatures of selection across genes associated with vascular development, growth, adaptation to salinity and flowering time, thus finding compelling evidence for a genomic basis of the island syndrome in one of Darwin’s giant daisies.

Many island plant species share a syndrome of characteristic phenotype and life history. Cerca et al. find the genomic basis of the plant island syndrome in one of Darwin’s giant daisies, while separating ancestral genomes in a chromosome-resolved polyploid assembly.

Molecular Genetic Understanding of Photoperiodic Regulation of Flowering Time in Arabidopsis and Soybean

The developmental switch from a vegetative phase to reproduction (flowering) is essential for reproduction success in flowering plants, and the timing of the floral transition is regulated by various environmental factors, among which seasonal day-length changes play a critical role to induce flowering at a season favorable for seed production. The photoperiod pathways are well known to regulate flowering time in diverse plants. Here, we summarize recent progresses on molecular mechanisms underlying the photoperiod control of flowering in the long-day plant Arabidopsis as well as the short-day plant soybean; furthermore, the conservation and diversification of photoperiodic regulation of flowering in these two species are discussed.

A MRG-operated chromatin switch at SOC1 attenuates abiotic stress responses during the floral transition

Plants react to environmental challenges by integrating external cues with endogenous signals to optimize survival and reproductive success. However, the mechanisms underlying this integration remain obscure. While stress conditions are known to impact plant development, how developmental transitions influence responses to adverse conditions has not been addressed. Here, we reveal a molecular mechanism of stress response attenuation during the onset of flowering in Arabidopsis (Arabidopsis thaliana). We show that Arabidopsis MORF-RELATED GENE (MRG) proteins, components of the NuA4 histone acetyltransferase complex that bind trimethylated-lysine 36 in histone H3 (H3K36me3), function as a chromatin switch on the floral integrator SUPPRESSOR OF OVEREXPRESSION OF CONSTANS 1 (SOC1) to coordinate flowering initiation with plant responsiveness to hostile environments. MRG proteins are required to activate SOC1 expression during flowering induction by promoting histone H4 acetylation. In turn, SOC1 represses a broad array of genes that mediate abiotic stress responses. We propose that during the transition from vegetative to reproductive growth, the MRG-SOC1 module constitutes a central hub in a mechanism that tunes down stress responses to enhance the reproductive success and plant fitness at the expense of costly efforts for adaptation to challenging environments.

MRG1/2 histone methylation readers and HD2C histone deacetylase associate in repression of the florigen gene FT to set a proper flowering time in response to day-length changes

Day-length changes represent an important cue for modulating flowering time. In Arabidopsis, the expression of the florigen gene FLOWERING LOCUS T (FT) exhibits a 24-h circadian rhythm under long-day (LD) conditions. Here we focus on the chromatin-based mechanism regarding the control of FT expression. We conducted co-immunoprecipitation assays along with LC-MS/MS analysis and identified HD2C histone deacetylase as the binding protein of the H3K4/H3K36 methylation reader MRG2. HD2C and MRG1/2 regulate flowering time under LD conditions, but not under short-day conditions. Moreover, HD2C functions as an effective deacetylase in planta, mainly targeting H3K9ac, H3K23ac and H3K27ac. At dusk, HD2C is recruited to FT to deacetylate histones and repress transcription in an MRG1/2-dependent manner. More importantly, HD2C competes with CO for the binding of MRG2, and the accumulation of HD2C at the FT locus occurs at the end of the day. Our findings not only reveal a histone deacetylation mechanism contributing to prevent FT overexpression and precocious flowering, but also support the model in which the histone methylation readers MRG1/2 provide a platform on chromatin for connecting regulatory factors involved in activating FT expression in response to daylight and decreasing FT expression around dusk under long days.

The Crosstalk Between Epigenetic Mechanisms and Alternative RNA Processing Regulation

As a co-transcriptional process, RNA processing, including alternative splicing and alternative polyadenylation, is crucial for the generation of multiple mRNA isoforms. RNA processing mechanisms are widespread across all higher eukaryotes and play critical roles in cell differentiation, organ development and disease response. Recently, significant progresses have been made in understanding the mechanism of RNA processing. RNA processing is regulated by trans-acting factors such as splicing factors, RNA-binding proteins and cis-sequences in pre-mRNA, and increasing evidence suggests that epigenetic mechanisms, which are important for the dynamic regulation and state of specific chromatic regions, are also involved in co-transcriptional RNA processing. In contrast, recent studies also suggest that alternative RNA processing also has a feedback regulation on epigenetic mechanisms. In this review, we discuss recent studies and summarize the current knowledge on the epigenetic regulation of alternative RNA processing. In addition, a feedback regulation of RNA processing on epigenetic regulators is also discussed.