HD2C interacts with HDA6 and is involved in ABA and salt stress response in Arabidopsis

Gene regulation at transcriptional and post-transcriptional levels to combat salt stress in plants

Soil salinity is a major challenge that will be faced more and more by human population in the near future. Higher salt concentrations in the soil limit the growth and production of crops, which poses serious threats to global food production. Various plant breeding approaches have been followed in the past which are reported to reduce the effect of salt stress by inducing the level of protective metabolites like osmolytes and antioxidants. Conventional breeding approaches are time-consuming and not cost-effective. In recent times, genetic engineering has been largely followed to confer salt tolerance through introgressions of single transgenes or stacking multiple transgenes. However, most of such works are limited only at the laboratory level and field trials are still awaited to prove the long-term efficacy of such transgenics. In this review, we attempt to present a broad overview of the current strategies undertaken to develop halophytic and salt-tolerant crops. The salt-induced damages in the plants are highlighted, followed by representing the novel traits, associated with salt stress, which can be used for engineering salt tolerance in glycophytic crops. Additionally, the role of transcriptional and epigenetic regulation in plants for amelioration of salt-induced damages has been reviewed. The role of post-transcriptional mechanisms such as microRNA regulation, genome editing and alternative splicing, during salt stress, and their implications in the development of salt-tolerant crops are also discussed. Finally, we present a short overview about the role of ion transporters and rhizobacteria in the engineering of salt tolerance in crop species.

Molecular insights into the role of plant transporters in salt stress response

Salt stress disturbs the cellular osmotic and ionic balance, which then creates a negative impact on plant growth and development. The Na⁺ and Cl^- ions can enter into plant cells through various membrane transporters, including specific and non-specific Na⁺ , K⁺ , and Ca²⁺ transporters. Therefore, it is important to understand Na⁺ and K⁺ transport mechanisms in plants along with the isolation of genes, their characterization, the structural features, and their post-translation regulation under salt stress. This review summarizes the molecular insights of plant ion transporters, including non-selective cation transporters, cyclic nucleotide-gated cation transporters, glutamate-like receptors, membrane intrinsic proteins, cation proton antiporters, and sodium proton antiporter families. Further, we discussed the K⁺ transporter families such as high-affinity K⁺ transporters, HAK/KUP/KT transporters, shaker type K⁺ transporters, and K⁺ efflux antiporters. Besides the ion transport process, we have shed light on available literature on epigenetic regulation of transport processes under salt stress. Recent advancements of salt stress sensing mechanisms and various salt sensors within signaling transduction pathways are discussed. Further, we have compiled salt-stress signaling pathways, and their crosstalk with phytohormones.

Histone modifications and chromatin remodelling in plants in response to salt stress

In the face of global food security crises, it is necessary to boost agricultural production. One factor hampering the attempts to increase food production is elevated soil salinity, which can be due to salt that is naturally present in the soil or a consequence of excessive or prolonged irrigation or application of fertiliser. In response to environmental stresses, plants activate multiple molecular mechanisms, including the timely activation of stress-responsive transcriptional networks. However, in the case of salt stress, the combined effects of the initial osmotic shock and the subsequent ion-specific stress increase the complexity in the selective regulation of gene expressions involved in restoring or maintaining osmotic balance, ion homeostasis and reactive oxygen species scavenging. Histone modifications and chromatin remodelling are important epigenetic processes that regulate gene expressions by modifying the chromatin status and recruiting transcription regulators. In this review, we have specifically summarised the currently available knowledge on histone modifications and chromatin remodelling in relation to plant responses to salt stress. Current findings have revealed the functional importance of chromatin modifiers in regulating salt tolerance and identified the effector genes affected by epigenetic modifications, although counteraction between modifiers within the same family may occur. Emerging evidence has also illustrated the crosstalk between epigenetic modifications and hormone signalling pathways which involves formation of protein complexes. With an improved understanding of these processes, plant breeders will be able to develop alternative strategies using genome editing technologies for crop improvement.

The Epigenetic Mechanisms Underlying Thermomorphogenesis and Heat Stress Responses in Arabidopsis

Integration of temperature cues is crucial for plant survival and adaptation. Global warming is a prevalent issue, especially in modern agriculture, since the global rise in average temperature is expected to impact crop productivity worldwide. Hence, better understanding of the mechanisms by which plants respond to warmer temperatures is very important. This review focuses on the epigenetic mechanisms implicated in plant responses to high temperature and distinguishes the different epigenetic events that occur at warmer average temperatures, leading to thermomorphogenic responses, or subjected to extreme warm temperatures, leading to heat stress.

Nuclear factor Y subunit GmNFYA competes with GmHDA13 for interaction with GmFVE to positively regulate salt tolerance in soybean

Soybean is an important crop worldwide, but its production is severely affected by salt stress. Understanding the regulatory mechanism of salt response is crucial for improving the salt tolerance of soybean. Here, we reveal a role for nuclear factor Y subunit GmNFYA in salt tolerance of soybean likely through the regulation of histone acetylation. GmNFYA is induced by salt stress. Overexpression of GmNFYA significantly enhances salt tolerance in stable transgenic soybean plants by inducing salt-responsive genes. Analysis in soybean plants with transgenic hairy roots also supports the conclusion. GmNFYA interacts with GmFVE, which functions with putative histone deacetylase GmHDA13 in a complex for transcriptional repression possibly by reducing H3K9 acetylation at target loci. Under salt stress, GmNFYA likely accumulates and competes with GmHDA13 for interaction with GmFVE, leading to the derepression and maintenance of histone acetylation for activation of salt-responsive genes and finally conferring salt tolerance in soybean plants. In addition, a haplotype I GmNFYA promoter is identified with the highest self-activated promoter activity and may be selected during future breeding for salt-tolerant cultivars. Our study uncovers the epigenetic regulatory mechanism of GmNFYA in salt-stress response, and all the factors/elements identified may be potential targets for genetic manipulation of salt tolerance in soybean and other crops.

Label-Free Quantitative Proteome Analysis Reveals the Underlying Mechanisms of Grain Nuclear Proteins Involved in Wheat Water-Deficit Response

In this study, we performed the first nuclear proteome analysis of wheat developing grains under water deficit by using a label-free based quantitative proteomic approach. In total, we identified 625 unique proteins as differentially accumulated proteins (DAPs), of which 398 DAPs were predicted to be localized in nucleus. Under water deficit, 146 DAPs were up-regulated and mainly involved in the stress response and oxidation-reduction process, while 252 were down-regulated and mainly participated in translation, the cellular amino metabolic process, and the oxidation-reduction process. The cis-acting elements analysis of the key nuclear DAPs encoding genes demonstrated that most of these genes contained the same cis-acting elements in the promoter region, mainly including ABRE involved in abscisic acid response, antioxidant response element, MYB responsive to drought regulation and MYC responsive to early drought. The cis-acting elements related to environmental stress and hormones response were relatively abundant. The transcription expression profiling of the nuclear up-regulated DAPs encoding genes under different organs, developmental stages and abiotic stresses was further detected by RNA-seq and Real-time quantitative polymerase chain reaction, and more than 50% of these genes showed consistency between transcription and translation expression. Finally, we proposed a putative synergistic responsive network of wheat nuclear proteome to water deficit, revealing the underlying mechanisms of wheat grain nuclear proteome in response to water deficit.

Nucleolar histone deacetylases HDT1, HDT2 and HDT3 regulate plant reproductive development

Nucleolus is a membrane-less organelle where ribosomes are assembled and rRNAs transcribed and processed. The assembled ribosomes composed of ribosomal proteins and rRNAs synthesize proteins for cell survival. In plants, the loss of nucleolar ribosomal proteins often causes gametophytically or embryonically lethality. The amount of rRNAs are under stringent regulation according to demand and partially switched off by epigenetic modifications. However, the molecular mechanism for the selective activation or silencing is still unclear, and the transcriptional coordination of rRNAs and ribosomal proteins is also unknown. Here we report the critical role of three Arabidopsis nucleolar protein HDT1, HDT2 and HDT3 in fertility and transcription of rDNAs and rRNA processing-related genes through histone acetylation. This study highlights the important roles of transcriptional repression of ribosome biogenesis-related genes for plant reproductive development.

Histone deacetylase SbHDT701 in Sorghum bicolor reveals functions in response to stress factors by enhancing acetylation

Histone acetylation plays important roles in eukaryotic chromatin modification and gene expression regulation. Acetylation levels are modulated by histone deacetylases (HDACs), which function as key epigenetic factors that regulate gene expression in response to various stresses. HDT701, a member of the HD2 subfamily of HDACs, plays crucial roles in plant responses to abiotic stress and pathogen infection. Here, we analysed the expression pattern of SbHDT701 in sorghum. Real-time fluorescence quantitative PCR (RT-qPCR) results showed that expression of SbHDT701 was tissue-specific, and up-regulated under drought (d-mannitol) and salt (NaCl) stresses. We also determined the optimal expression conditions for SbHDT701 protein accumulation, and successfully expressed and purified SbHDT701 protein. Besides, overexpression of SbHDT701 in could promote the growth of recombinant cells under abiotic stress. SbHDT701 expression in Escherichia coli also increased acetylation modification levels following treatment with 750 mM NaCl, and 100 mM or 300 mM d-mannitol. In summary, the sorghum HDAC SbHDT701 mediates stress responses by enhancing acetylation modification levels.

Rice histone deacetylase HDA704 positively regulates drought and salt tolerance by controlling stomatal aperture and density

HDA704 enhances drought and salt tolerance via stomata-regulated mechanism. HDA704 negatively regulates stomatal aperture and density, repressing the transcription of DST and ABIL2 by histone deacetylation modification. Drought and salinity can damage crop growth and reduce yield. Stomata play an important role in abiotic stress tolerance. In this study on rice, we identified the RPD3/HDA1-type histone deacetylase HDA704 as a positive regulatory factor in drought and salt tolerance. HDA704 was induced by drought and salt stresses. Overexpression of HDA704 in transgenic rice promoted stomatal closure, decreased the number of stomata and slowed down the rate of water loss, consequently resulting in increased drought and salt tolerance. By contrast, knockdown of HDA704 in transgenic rice decreased stomatal closure and accelerated the rate of water loss, leading to decrease drought and salt tolerance. We detected the transcript expression of DST (Drought and Salt Tolerance) and ABIL2 (Abscisic Acid-insensitive Like2), which positively regulate stomatal aperture and density in rice. Our results showed that HDA704 directly binds to DST and ABIL2, repressing their expression via histone deacetylation modification. Collectively, these findings reveal that HDA704 positively regulates drought and salt tolerance by repressing the expression of DST and ABIL2. Our findings provide a new insight into the molecular mechanisms of stomata-regulated abiotic stress tolerance of plants.

Epigenetic control of abiotic stress signaling in plants

BACKGROUND

Although plants may be regularly exposed to various abiotic stresses, including drought, salt, cold, heat, heavy metals, and UV-B throughout their lives, it is not possible to actively escape from such stresses due to the immobile nature of plants. To overcome adverse environmental stresses, plants have developed adaptive systems that allow appropriate responses to diverse environmental cues; such responses can be achieved by fine-tuning or controlling genetic and epigenetic regulatory systems. Epigenetic mechanisms such as DNA or histone modifications and modulation of chromatin accessibility have been shown to regulate the expression of stress-responsive genes in struggles against abiotic stresses.

OBJECTIVE

Herein, the current progress in elucidating the epigenetic regulation of abiotic stress signaling in plants has been summarized in order to further understand the systems plants utilize to effectively respond to abiotic stresses.

METHODS

This review focuses on the action mechanisms of various components that epigenetically regulate plant abiotic stress responses, mainly in terms of DNA methylation, histone methylation/acetylation, and chromatin remodeling.

CONCLUSIONS

This review can be considered a basis for further research into understanding the epigenetic control system for abiotic stress responses in plants. Moreover, the knowledge of such systems can be effectively applied in developing novel methods to generate abiotic stress resistant crops.

Cauliflower mosaic virus P6 Dysfunctions Histone Deacetylase HD2C to Promote Virus Infection

Histone deacetylases (HDACs) are vital epigenetic modifiers not only in regulating plant development but also in abiotic- and biotic-stress responses. Though to date, the functions of HD2C-an HD2-type HDAC-In plant development and abiotic stress have been intensively explored, its function in biotic stress remains unknown. In this study, we have identified HD2C as an interaction partner of the Cauliflower mosaic virus (CaMV) P6 protein. It functions as a positive regulator in defending against CaMV infection. The hd2c mutants show enhanced susceptibility to CaMV infection. In support, the accumulation of viral DNA, viral transcripts, and the deposition of histone acetylation on the viral minichromosomes are increased in hd2c mutants. P6 interferes with the interaction between HD2C and HDA6, and P6 overexpression lines have similar phenotypes with hd2c mutants. In further investigations, P6 overexpression lines, together with CaMV infection plants, are more sensitive to ABA and NaCl with a concomitant increasing expression of ABA/NaCl-regulated genes. Moreover, the global levels of histone acetylation are increased in P6 overexpression lines and CaMV infection plants. Collectively, our results suggest that P6 dysfunctions histone deacetylase HD2C by physical interaction to promote CaMV infection.

HD2-type histone deacetylases: unique regulators of plant development and stress responses

Plants have developed sophisticated and complex epigenetic regulation-based mechanisms to maintain stable growth and development under diverse environmental conditions. Histone deacetylases (HDACs) are important epigenetic regulators in eukaryotes that are involved in the deacetylation of lysine residues of histone H3 and H4 proteins. Plants have developed a unique HDAC family, HD2, in addition to the RPD3 and Sir2 families, which are also present in other eukaryotes. HD2s are well conserved plant-specific HDACs, which were first identified as nucleolar phosphoproteins in maize. The HD2 family plays important roles not only in fundamental developmental processes, including seed germination, root and leaf development, floral transition, and seed development but also in regulating plant responses to biotic and abiotic stresses. Some of the HD2 members coordinate with each other to function. The HD2 family proteins also show functional association with RPD3-type HDACs and other transcription factors as a part of repression complexes in gene regulatory networks involved in environmental stress responses. This review aims to analyse and summarise recent research progress in the HD2 family, and to describe their role in plant growth and development and in response to different environmental stresses.

Delineating the epigenetic regulation of heat and drought response in plants

Being sessile in nature, plants cannot overlook the incursion of unfavorable environmental conditions, including heat and drought. Heat and drought severely affect plant growth, development, reproduction and therefore productivity which poses a severe threat to global food security. Plants respond to these hostile environmental circumstances by rearranging their genomic and molecular architecture. One such modification commonly known as epigenetic changes involves the perishable to inheritable changes in DNA or DNA-binding histone proteins leading to modified chromatin organization. Reversible epigenetic modifications include DNA methylation, exchange of histone variants, histone methylation, histone acetylation, ATP-dependent nucleosome remodeling, and others. These modifications are employed to regulate the spatial and temporal expression of genes in response to external stimuli or specific developmental requirements. Understanding the epigenetic regulation of stress-related gene expression in response to heat and drought would commence manifold avenues for crop improvement through molecular breeding or biotechnological approaches.

Poplar acetylome profiling reveals lysine acetylation dynamics in seasonal bud dormancy release

For perennials in boreal and temperate ecosystems, bud dormancy is crucial for survival in harsh winter. Dormancy is released by prolonged exposure to low temperatures and is followed by reactive growth in the spring. Lysine acetylation (Kac) is one of the major post-translational modifications (PTMs) that are involved in plant response to environmental signals. However, little information is available on the effects of Kac modification on bud dormancy release. Here, we report the dynamics of lysine acetylome in hybrid poplar (Populus tremula × Populus alba) dormant buds. A total of 7,594 acetyl-sites from 3,281 acetyl-proteins were identified, representing a large dataset of lysine acetylome in plants. Of them, 229 proteins were differentially acetylated during bud dormancy release and were mainly involved in the primary metabolic pathways. Site-directed mutagenesis enzymatic assays showed that Kac strongly modified the activities of two key enzymes of primary metabolism, pyruvate dehydrogenase (PDH) and isocitrate dehydrogenase (IDH). We thus propose that Kac of enzymes could be an important strategy for reconfiguration of metabolic processes during bud dormancy release. In all, our results reveal the importance of Kac in bud dormancy release and provide a new perspective to understand the molecular mechanisms of seasonal growth of trees.

Arabidopsis RPD3-like histone deacetylases form multiple complexes involved in stress response

The Arabidopsis thaliana RPD3-type histone deacetylases have been known to form conserved SIN3-type histone deacetylase complexes, but whether they form other types of complexes is unknown. Here, we perform affinity purification followed by mass spectrometry and demonstrate that the Arabidopsis RPD3-type histone deacetylases HDA6 and HDA19 interact with several previously uncharacterized proteins, thereby forming three types of plant-specific histone deacetylase complexes, which we named SANT, ESANT, and ARID. RNA-seq indicates that the newly identified components function together with HDA6 and HDA19 and coregulate the expression of a number of genes. HDA6 and HDA19 were previously thought to repress gene transcription by histone deacetylation. We find that the histone deacetylase complexes can repress gene expression via both histone deacetylation-dependent and -independent mechanisms. In the mutants of histone deacetylase complexes, the expression of a number of stress-induced genes is up-regulated, and several mutants of the histone deacetylase complexes show severe retardation in growth. Considering that growth retardation is thought to be a trade-off for an increase in stress tolerance, we infer that the histone deacetylase complexes identified in this study prevent overexpression of stress-induced genes and thereby ensure normal growth of plants under nonstress conditions.

Histone acetylation dynamics regulating plant development and stress responses

Crop productivity is directly dependent on the growth and development of plants and their adaptation during different environmental stresses. Histone acetylation is an epigenetic modification that regulates numerous genes essential for various biological processes, including development and stress responses. Here, we have mainly discussed the impact of histone acetylation dynamics on vegetative growth, flower development, fruit ripening, biotic and abiotic stress responses. Besides, we have also emphasized the information gaps which are obligatory to be examined for understanding the complete role of histone acetylation dynamics in plants. A comprehensive knowledge about the histone acetylation dynamics will ultimately help to improve stress resistance and reduce yield losses in different crops due to climate changes.

Perspectives for epigenetic editing in crops

Site-specific nucleases (SSNs) have drawn much attention in plant biotechnology due to their ability to drive precision mutagenesis, gene targeting or allele replacement. However, when devoid of its nuclease activity, the underlying DNA-binding activity of SSNs can be used to bring other protein functional domains close to specific genomic sites, thus expanding further the range of applications of the technology. In particular, the addition of functional domains encoding epigenetic effectors and chromatin modifiers to the CRISPR/Cas ribonucleoprotein complex opens the possibility to introduce targeted epigenomic modifications in plants in an easily programmable manner. Here we examine some of the most important agronomic traits known to be controlled epigenetically and review the best studied epigenetic catalytic effectors in plants, such as DNA methylases/demethylases or histone acetylases/deacetylases and their associated marks. We also review the most efficient strategies developed to date to functionalize Cas proteins with both catalytic and non-catalytic epigenetic effectors, and the ability of these domains to influence the expression of endogenous genes in a regulatable manner. Based on these new technical developments, we discuss the possibilities offered by epigenetic editing tools in plant biotechnology and their implications in crop breeding.

Identification, methylation profiling, and expression analysis of stress-responsive cytochrome P450 genes in rice under abiotic and phytohormones stresses

The cytochrome P450 (CYP) is a large and complex eukaryotic gene superfamily with enzymatic activities involved in several physiological and regulatory processes. As an objective, an in-silico genome-wide DNA methylation (5mC) analysis was performed in rice (Oryza sativa cv. Zhonghua11), and the epigenetic role of CYPs in two abiotic stresses was observed. Being a stable representative mark, DNA-methylation alters the gene expression under stressful environmental conditions. Rice plants under salinity and drought stresses were analyzed through MeDIP-chip hybridization, and 14 unique genes of the CYP family were identified in the rice genome with varying degrees of methylation. The gene structure, promoter sequences, and phylogenetic analysis were performed. Furthermore, the responses of CYPs to various abiotic stresses, including salinity, drought, and cold were revealed. Similarly, the expression profile of potential CYPs was also investigated under various phytohormone stresses, which revealed the potential involvement of CYPs to hormone regulations. Overall, the current study provides evidence for CYP's stress regulation and fundamental for further characterization and understanding their epigenetic roles in gene expression regulation and environmental stress regulation in higher plants.

Transcriptome profiling reveals histone deacetylase 1 gene overexpression improves flavonoid, isoflavonoid, and phenylpropanoid metabolism in Arachis hypogaea hairy roots

Background

The peanut (Arachis hypogaea) is a crop plant of high economic importance, but the epigenetic regulation of its root growth and development has not received sufficient attention. Research on Arabidopsis thaliana has shown that histone deacetylases (HDACs) are involved in cell growth, cell differentiation, and stress response. Few studies have focused on the role of HDACs in the root development of other plants, particularly crop plants. In earlier studies, we found large accumulations of A. hypogaea histone deacetylase 1 (AhHDA1) mRNA in peanut roots. However, we did not explore the role of AhHDA1 in peanut root development.

Methods

In this paper, we investigated the role of the peanut AhHDA1 gene and focused on the effect of altered AhHDA1 expression in hairy roots at both the phenotypic and transcriptional levels. We analyzed the transformation of A. hypogaea hairy roots using Agrobacterium rhizogenes and RNA sequencing to identify differentially expressed genes that were assigned to specific metabolic pathways. Transgenic hairy roots were used as experimental material to analyze the downstream genes expression and histone acetylation levels. To thoroughly understand AhHDA1 function, we also simultaneously screened the AhHDA1-interacting proteins using a yeast two-hybrid system.

Results

AhHDA1-overexpressing hairy roots were growth-retarded after 20 d in vitro cultivation, and they had a greater accumulation of superoxide anions and hydrogen peroxide than the control and RNAi groups. AhHDA1 overexpression in hairy roots accelerated flux through various secondary synthetic metabolic pathways, as well as inhibited the primary metabolism process. AhHDA1 overexpression also caused a significant upregulation of genes encoding the critical enzyme chalcone synthase (Araip.B8TJ0, CHS) in the flavonoid biosynthesis pathway, hydroxyisoflavanone synthase (Araip.0P3RJ) in the isoflavonoid biosynthesis pathway, and caffeoyl-CoA O-methyltransferase (Aradu.M62BY, CCoAOMT) in the phenylpropanoid biosynthesis pathway. In contrast, ferredoxin 1 (Araip.327XS), the polypeptide of the oxygen-evolving complex of photosystem II (Araip.N6ZTJ), and ribulose bisphosphate carboxylase (Aradu.5IY98) in the photosynthetic pathway were significantly downregulated by AhHDA1 overexpression. The expression levels of these genes had a positive correlation with histone acetylation levels.

Conclusion

Our results revealed that the relationship between altered gene metabolism activities and AhHDA1 overexpression was mainly reflected in flavonoid, isoflavonoid, and phenylpropanoid metabolism. AhHDA1 overexpression retarded the growth of transgenic hairy roots and may be associated with cell metabolism status. Future studies should focus on the function of AhHDA1-interacting proteins and their effect on root development.

Maintenance of Cell Wall Integrity under High Salinity

Cell wall biosynthesis is a complex biological process in plants. In the rapidly growing cells or in the plants that encounter a variety of environmental stresses, the compositions and the structure of cell wall can be dynamically changed. To constantly monitor cell wall status, plants have evolved cell wall integrity (CWI) maintenance system, which allows rapid cell growth and improved adaptation of plants to adverse environmental conditions without the perturbation of cell wall organization. Salt stress is one of the abiotic stresses that can severely disrupt CWI, and studies have shown that the ability of plants to sense and maintain CWI is important for salt tolerance. In this review, we highlight the roles of CWI in salt tolerance and the mechanisms underlying the maintenance of CWI under salt stress. The unsolved questions regarding the association between the CWI and salt tolerance are discussed.

Characteristic and evolution of HAT and HDAC genes in Gramineae genomes and their expression analysis under diverse stress in Oryza sativa

Comprehensive characterization of Gramineae HATs and HDACs reveals their conservation and variation. The recent WGD/SD gene pairs in the CBP and RPD/HDA1 gene family may confer specific adaptive evolutionary changes. Expression of OsHAT and OsHDAC genes provides a new vision in different aspects of development and response to diverse stress. The histone acetylase (HAT) and histone deacetylase (HDAC) have been proven to be tightly linked to play a crucial role in plant growth, development and response to abiotic stress by regulating histone acetylation levels. However, the evolutionary dynamics and functional differentiation of HATs and HDACs in Gramineae remain largely unclear. In the present study, we identified 37 HAT genes and 110 HDAC genes in seven Gramineae genomes by a detailed analysis. Phylogenetic trees of these HAT and HDAC proteins were constructed to illustrate evolutionary relationship in Gramineae. Gene structure, protein property and protein motif composition illustrated the conservation and variation of HATs and HDACs in Gramineae. Gene duplication analysis suggested that recent whole genome duplication (WGD)/segmental duplication (SD) events contributed to the diversification of the CBP and RPD3/HDA1 gene family in Gramineae. Furthermore, promoter cis-element prediction indicated that OsHATs and OsHDACs were likely functional proteins and involved in various signaling pathways. Expression analysis by RNA-seq data showed that all OsHAT and OsHDAC genes were expressed in different tissues or development stages, revealing that they were ubiquitously expressed. In addition, we found that their expression patterns were altered in response to cold, drought, salt, light, abscisic acid (ABA), and indole-3-acetic acid (IAA) treatments. These findings provide the basis for further identification of candidate OsHAT and OsHDAC genes that may be utilized in regulating growth and development and improving crop tolerance to abiotic stress.

A Systems Biology Approach to Identify Essential Epigenetic Regulators for Specific Biological Processes in Plants

Upon sensing developmental or environmental cues, epigenetic regulators transform the chromatin landscape of a network of genes to modulate their expression and dictate adequate cellular and organismal responses. Knowledge of the specific biological processes and genomic loci controlled by each epigenetic regulator will greatly advance our understanding of epigenetic regulation in plants. To facilitate hypothesis generation and testing in this domain, we present EpiNet, an extensive gene regulatory network (GRN) featuring epigenetic regulators. EpiNet was enabled by (i) curated knowledge of epigenetic regulators involved in DNA methylation, histone modification, chromatin remodeling, and siRNA pathways; and (ii) a machine-learning network inference approach powered by a wealth of public transcriptome datasets. We applied GENIE3, a machine-learning network inference approach, to mine public Arabidopsis transcriptomes and construct tissue-specific GRNs with both epigenetic regulators and transcription factors as predictors. The resultant GRNs, named EpiNet, can now be intersected with individual transcriptomic studies on biological processes of interest to identify the most influential epigenetic regulators, as well as predicted gene targets of the epigenetic regulators. We demonstrate the validity of this approach using case studies of shoot and root apical meristem development.

Nitric oxide signaling in the plant nucleus: the function of nitric oxide in chromatin modulation and transcription

Nitric oxide (NO) is involved in a vast number of physiologically important processes in plants, such as organ development, stress resistance, and immunity. Transduction of NO bioactivity is generally achieved by post-translational modification of proteins, with S-nitrosation of cysteine residues as the predominant form. While traditionally the subcellular location of the factors involved was of lesser importance, recent studies identified the connection between NO and transcriptional activity and thereby raised the question about the route of NO into the nuclear sphere. Identification of NO-affected transcription factors and chromatin-modifying histone deacetylases implicated the important role of NO signaling in the plant nucleus as a regulator of epigenetic mechanisms and gene transcription. Here, we discuss the relationship between NO and its directly regulated protein targets in the nuclear environment, focusing on S-nitrosated chromatin modulators and transcription factors.

Role of Chromatin Architecture in Plant Stress Responses: An Update

Sessile plants possess an assembly of signaling pathways that perceive and transmit environmental signals, ultimately resulting in transcriptional reprogramming. Histone is a key feature of chromatin structure. Numerous histone-modifying proteins act under different environmental stress conditions to help modulate gene expression. DNA methylation and histone modification are crucial for genome reprogramming for tissue-specific gene expression and global gene silencing. Different classes of chromatin remodelers including SWI/SNF, ISWI, INO80, and CHD are reported to act upon chromatin in different organisms, under diverse stresses, to convert chromatin from a transcriptionally inactive to a transcriptionally active state. The architecture of chromatin at a given promoter is crucial for determining the transcriptional readout. Further, the connection between somatic memory and chromatin modifications may suggest a mechanistic basis for a stress memory. Studies have suggested that there is a functional connection between changes in nuclear organization and stress conditions. In this review, we discuss the role of chromatin architecture in different stress responses and the current evidence on somatic, intergenerational, and transgenerational stress memory.

Overexpression of the HDA15 Gene Confers Resistance to Salt Stress by the Induction of NCED3, an ABA Biosynthesis Enzyme

Salt stress constitutes a major form of abiotic stress in plants. Histone modification plays an important role in stress tolerance, with particular reference to salt stress resistance. In the current study, we found that HDA15 overexpression confers salt stress resistance to young seedling stages of transgenic plants. Furthermore, salt stress induces HDA15 overexpression. Transcription levels of stress-responsive genes were increased in transgenic plants overexpressing HDA15 (HDA15 OE). NCED3, an abscisic acid (ABA) biosynthetic gene, which is highly upregulated in HDA15 transgenic plants, enhanced the accumulation of ABA, which promotes adaptation to salt stress. ABA homeostasis in HDA15 OE plants is maintained by the induction of CYP707As, which optimize endogenous ABA levels. Lastly, we found that the double-mutant HDA15 OE/hy5 ko plants are sensitive to salt stress, indicating that interaction between HDA15 and ELONGATED HYPOCOTYL 5 (HY5) is crucial to salt stress tolerance shown by HDA15 OE plants. Thus, our findings indicate that HDA15 is crucial to salt stress tolerance in Arabidopsis.

Histone deacetylase HDA710 controls salt tolerance by regulating ABA signaling in rice

Plants have evolved numerous mechanisms that assist them in withstanding environmental stresses. Histone deacetylases (HDACs) play crucial roles in plant stress responses; however, their regulatory mechanisms remain poorly understood. Here, we explored the function of HDA710/OsHDAC2, a member of the HDAC RPD3/HDA1 family, in stress tolerance in rice (Oryza sativa). We established that HDA710 localizes to both the nucleus and cytoplasm and is involved in regulating the acetylation of histone H3 and H4, specifically targeting H4K5 and H4K16 under normal conditions. HDA710 transcript accumulation levels were strongly induced by abiotic stresses including drought and salinity, as well as by the phytohormones jasmonic acid (JA) and abscisic acid (ABA). hda710 knockout mutant plants showed enhanced salinity tolerance and reduced ABA sensitivity, whereas transgenic plants overexpressing HDA710 displayed the opposite phenotypes. Moreover, ABA- and salt-stress-responsive genes, such as OsLEA3, OsABI5, OsbZIP72, and OsNHX1, were upregulated in hda710 compared with wild-type plants. These expression differences corresponded with higher levels of histone H4 acetylation in gene promoter regions in hda710 compared with the wild type under ABA and salt-stress treatment. Collectively, these results suggest that HDA710 is involved in regulating ABA- and salt-stress-responsive genes by altering H4 acetylation levels in their promoters. This article is protected by copyright. All rights reserved.

Histone deacetylase HDA19 interacts with histone methyltransferase SUVH5 to regulate seed dormancy in Arabidopsis

Seed dormancy controls the timing of germination and plays a significant role in adaptation and evolution of seed plants. In this study, a yeast two-hybrid, pull-down assay and co-immunoprecipitation assay were used to ascertain the protein relationship of SUVH5 and HDA19. Both qRT-PCR and ChIP-qPCR were used to examine the molecular mechanism of how HDA19 and SUVH5 regulate seed dormancy. The results demonstrated that histone methyltransferase SUVH5 interacted with histone deacetylase HDA19 in vivo and in vitro. In addition, they showed that mutants of HDA19 could deepen seed dormancy, and that SUVH5 had the same effect. The hda19 suvh5 double mutant displayed a higher level of seed dormancy than the single mutants hda19 or suvh5. Moreover, the expression of seed dormancy-related genes increased in hda19, suvh5 and in hda19 suvh5 double mutant plants, which was associated with increased histone H3 acetylation (H3ac), but decreased histone H3 Lys 9 dimethylation (H3K9me2). ChIP assays proved that HDA19 could directly integrate into the chromatin of genes regulating seed dormancy. Taken together, our results show that HDA19 and SUVH5 work together and have a negative role in seed dormancy.

Epigenetics and epigenomics: underlying mechanisms, relevance, and implications in crop improvement

Epigenetics is defined as changes in gene expression that are not associated with changes in DNA sequence but due to the result of methylation of DNA and post-translational modifications to the histones. These epigenetic modifications are known to regulate gene expression by bringing changes in the chromatin state, which underlies plant development and shapes phenotypic plasticity in responses to the environment and internal cues. This review articulates the role of histone modifications and DNA methylation in modulating biotic and abiotic stresses, as well as crop improvement. It also highlights the possibility of engineering epigenomes and epigenome-based predictive models for improving agronomic traits.

Genome-wide identification of the HDAC family proteins and functional characterization of CsHD2C, a HD2-type histone deacetylase gene in tea plant (Camellia sinensis L. O. Kuntze)

The histone deacetylases (HDACs) are involved in growth, development and stress responses in many plants. However, the functions of HDACs in tea plant (Camellia sinensis L. O. Kuntze) and other woody plants remain unclear. Here, 18 CsHDAC genes were identified by genome-wide analysis in tea plant. The phylogenetic analysis demonstrated that the CsHDAC proteins were divided into three subfamilies, namely, the RPD3/HDA1 subfamily (8 members), the SIR2 subfamily (4 members) and the plant specific HD2 subfamily (6 members). The expression patterns showed that most members of CsHDACs family were regulated by different abiotic stress. High correlation was found between the expression of the CsHDACs and the accumulation of theanine, catechin, EGCG and other metabolites in tea plant. Most of the CsHDAC proteins were negative regulators. We further studied a specific gene CsHD2C (NCBI-ID: KY364373) in tea plant, which is the homolog of AtHD2C, encoded a protein of 306 aa. CsHD2C was highly expressed in leaves, young buds and stems. The transcription of CsHD2C was inhibited by ABA, NaCl and low temperature. It was found localized in the nucleus when fused with a YFP reporter gene. Overexpression of CsHD2C can rescue the phenotype related to different abiotic stresses in the mutant of AtHD2C in Arabidopsis. The stress-responsive genes RD29A, RD29B, ABI1 and ABI2 were also investigated to understand the regulating role of CsHD2C under abiotic stresses. We also found that CsHD2C could renew the change of acetylation level for histone H4 and the RNAP-II occupancy accumulation in the promoter of abiotic stress responses gene in the hd2c Arabidopsis mutant. Together, our results suggested that CsHD2C may act as a positive regulator in abiotic stress responses in tea plant.

Salinity stress induces epigenetic alterations to the promoter of MsMYB4 encoding a salt-induced MYB transcription factor

The transcriptomic response of plants to salinity stress is regulated in part by epigenetic alterations to gene promoter sequences. The transcription factor MsMYB4 is an important component of the response of alfalfa to salinity stress, but the involvement of epialleles of its encoding gene has not as yet been explored. Here, the MsMYB4 promoter was isolated using a genome walking approach in order to perform a deletion analysis to identify the region harboring the elements required for its stress inducibility. The analysis showed that these reside in the sequence lying between 739 and 336 nt up stream of the MsMYB4 translation start codon. The methylation status of the sequence around the MsMYB4 translation start site was altered by the imposition of salinity stress. The activation of MsMYB4 was associated with an increased level of histone H3K4 trimethylation and H3K9 acetylation in specific regions of the promoter sequence. Our results suggest a critical role for MsMYB4's activation by DNA methylation and/or histone modifications in response to salinity stress in alfalfa.

The Histone-Modifying Complex PWR/HOS15/HD2C Epigenetically Regulates Cold Tolerance

Cold stress is a major environmental stress that severely affects plant growth and crop productivity. Arabidopsis (Arabidopsis thaliana) HIGH EXPRESSION OF OSMOTICALLY RESPONSIVE GENE15 (HOS15) is a substrate receptor of the CULLIN4-based CLR4 ubiquitin E3 ligase complex, which epigenetically regulates cold tolerance by degrading HISTONE DEACETYLASE2C (HD2C) to switch from repressive to permissive chromatin structure in response to cold stress. In this study, we characterized a HOS15-binding protein, POWERDRESS (PWR), and analyzed its function in the cold stress response. PWR loss-of-function plants (pwr) showed lower expression of cold-regulated (COR) genes and sensitivity to freezing. PWR interacts with HD2C through HOS15, and cold-induced HD2C degradation by HOS15 is diminished in the pwr mutant. The association of HOS15 and HD2C to promoters of cold-responsive COR genes was dependent on PWR. Consistent with these observations, the high acetylation levels of histone H3 by cold-induced and HOS15-mediated HD2C degradation were significantly reduced in pwr under cold stress. PWR also interacts with C-repeat element-binding factor transcription factors to modulate their cold-induced binding to the promoter of COR genes. Collectively, our data signify that the PWR-HOS15-HD2C histone-modifying complex regulates the expression of COR genes and the freezing tolerance of plants.

The Interplay between Toxic and Essential Metals for Their Uptake and Translocation Is Likely Governed by DNA Methylation and Histone Deacetylation in Maize

The persistent nature of lead (Pb) and cadmium (Cd) in the environment severely affects plant growth and yield. Conversely, plants acquire zinc (Zn) from the soil for their vital physiological and biochemical functions. However, the interplay and coordination between essential and toxic metals for their uptake and translocation and the putative underlying epigenetic mechanisms have not yet been investigated in maize. Here, we report that the presence of Zn facilitates the accumulation and transport of Pb and Cd in the aerial parts of the maize plants. Moreover, the Zn, Pb, and Cd interplay specifically interferes with the uptake and translocation of other divalent metals, such as calcium and magnesium. Zn, Pb, and Cd, individually and in combinations, differentially regulate the expression of DNA methyltransferases, thus alter the DNA methylation levels at the promoter of Zinc-regulated transporters, Iron-regulated transporter-like Protein (ZIP) genes to regulate their expression. Furthermore, the expression of histone deacetylases (HDACs) varies greatly in response to individual and combined metals, and HDACs expression showed a negative correlation with ZIP transporters. Our study highlights the implication of DNA methylation and histone acetylation in regulating the metal stress tolerance dynamics through Zn transporters and warns against the excessive use of Zn fertilizers in metal contaminated soils.

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.

A histone deacetylase, GhHDT4D, is positively involved in cotton response to drought stress

Acetylation and deacetylation of histones are important for regulating a series of biological processes in plants. Histone deacetylases (HDACs) control the histone deacetylation that plays an important role in plant response to abiotic stress. In our study, we show the evidence that GhHDT4D (a member of the HD2 subfamily of HDACs) is involved in cotton (Gossypium hirsutum) response to drought stress. Overexpression of GhHDT4D in Arabidopsis increased plant tolerance to drought, whereas silencing GhHDT4D in cotton resulted in plant sensitivity to drought. Simultaneously, the H3K9 acetylation level was altered in the GhHDT4D silenced cotton, compared with the controls. Further study revealed that GhHDT4D suppressed the transcription of GhWRKY33, which plays a negative role in cotton defense to drought, by reducing its H3K9 acetylation level. The expressions of the stress-related genes, such as GhDREB2A, GhDREB2C, GhSOS2, GhRD20-1, GhRD20-2 and GhRD29A, were significantly decreased in the GhHDT4D silenced cotton, but increased in the GhWRKY33 silenced cotton. Given these data together, our findings suggested that GhHDT4D may enhance drought tolerance by suppressing the expression of GhWRKY33, thereby activating the downstream drought response genes in cotton.

Linking signaling pathways to histone acetylation dynamics in plants

As sessile organisms, plants face versatile environmental challenges and require proper responses at multiple levels for survival. Epigenetic modification of DNA and histones is a conserved gene-regulatory mechanism and plays critical roles in diverse aspects of biological processes, ranging from genome defense and imprinting to development and physiology. In recent years, emerging studies have revealed the interplay between signaling transduction pathways, epigenetic modifications, and chromatin cascades. Specifically, histone acetylation and deacetylation dictate plant responses to environmental cues by modulating chromatin dynamics to regulate downstream gene expression as signaling outputs. In this review, we summarize current understandings of the link between plant signaling pathways and epigenetic modifications with a focus on histone acetylation and deacetylation.

Chromatin Regulation in the Response of Ethylene: Nuclear Events in Ethylene Signaling

Plant hormones, produced in response to environmental stimuli, regulate almost all aspects of plant growth and development. Ethylene is a gaseous plant hormone that plays pleotropic roles in plant growth, plant development, fruit ripening, stress responses, and pathogen defenses. After decades of research, the key components of ethylene signaling have been identified and characterized. Although the molecular mechanisms of the sensing of ethylene signal and the transduction of ethylene signaling have been studied extensively, how chromatin influences ethylene signaling and ethylene response is a new area of research. This review describes the current understanding of how chromatin modifications, specifically histone acetylation, regulate ethylene signaling and the ethylene response.

Transcriptional and Epigenetic Regulation of Autophagy in Plants

Autophagy, a highly conserved quality control mechanism, is essential for maintaining cellular homeostasis and healthy growth of plants. Compared with extensive research in the cytoplasmic control of autophagy, studies regarding the nuclear events involved in the regulation of plant autophagy are just beginning to emerge. Accumulating evidence reveals a coordinated expression of plant autophagy genes in response to diverse developmental states and growth conditions. Here, we summarize recent progress in the identification of tightly controlled transcription factors and histone marks associated with the autophagic process in plants, and propose several modules, consisting of transcription regulators and epigenetic modifiers, as important nuclear players that could contribute to both short-term and long-term controls of plant autophagy at the transcriptional and post-transcriptional levels.

Peroxynitrite induced signaling pathways in plant response to non-proteinogenic amino acids

Nitro/oxidative modifications of proteins and RNA nitration resulted from altered peroxynitrite generation are elements of the indirect mode of action of canavanine and meta-tyrosine in plants Environmental conditions and stresses, including supplementation with toxic compounds, are known to impair reactive oxygen (ROS) and reactive nitrogen species (RNS) homeostasis, leading to modification in production of oxidized and nitrated derivatives. The role of nitrated and/or oxidized biotargets differs depending on the stress factors and developmental stage of plants. Canavanine (CAN) and meta-tyrosine (m-Tyr) are non-proteinogenic amino acids (NPAAs). CAN, the structural analog of arginine, is found mostly in seeds of Fabaceae species, as a storage form of nitrogen. In mammalian cells, CAN is used as an anticancer agent due to its inhibitory action on nitric oxide synthesis. m-Tyr is a structural analogue of phenylalanine and an allelochemical found in root exudates of fescues. In animals, m-Tyr is recognized as a marker of oxidative stress. Supplementation of plants with CAN or m-Tyr modify ROS and RNS metabolism. Over the last few years of our research, we have collected the complex data on ROS and RNS metabolism in tomato (Solanum lycopersicum L.) plants exposed to CAN or m-Tyr. In addition, we have shown the level of nitrated RNA (8-Nitro-guanine) in roots of seedlings, stressed by the tested NPAAs. In this review, we describe the model of CAN and m-Tyr mode of action in plants based on modifications of signaling pathways induced by ROS/RNS with a special focus on peroxynitrite induced RNA and protein modifications.

Mechanisms of Plant Responses and Adaptation to Soil Salinity

Soil salinity is a major environmental stress that restricts the growth and yield of crops. Understanding the physiological, metabolic, and biochemical responses of plants to salt stress and mining the salt tolerance-associated genetic resource in nature will be extremely important for us to cultivate salt-tolerant crops. In this review, we provide a comprehensive summary of the mechanisms of salt stress responses in plants, including salt stress-triggered physiological responses, oxidative stress, salt stress sensing and signaling pathways, organellar stress, ion homeostasis, hormonal and gene expression regulation, metabolic changes, as well as salt tolerance mechanisms in halophytes. Important questions regarding salt tolerance that need to be addressed in the future are discussed.

Over-Expression of HDA710 Delays Leaf Senescence in Rice (Oryza sativa L.)

Histone deacetylases (HDACs) influence chromatin state and gene expression. Eighteen HDAC genes with important biological functions have been identified in rice. In this study, we surveyed the gene presence frequency of all 18 rice HDAC genes in 3,010 rice accessions. HDA710/OsHDAC2 showed insertion/deletion (InDel) polymorphisms in almost 98.8% japonica accessions but only 1% indica accessions. InDel polymorphism association analysis showed that accessions with partial deletions in HDA710 tended to display early leaf senescence. Further transgenic results confirmed that HDA710 delayed leaf senescence in rice. The over-expression of HDA710 delayed leaf senescence, and the knock-down of HDA710 accelerated leaf senescence. Transcriptome analysis showed that photosynthesis and chlorophyll biosynthesis related genes were up-regulated in HDA710 over-expression lines, while some programmed cell death and disease resistance related genes were down-regulated. Co-expression network analysis with gene expression view revealed that HDA710 was co-expressed with multiple genes, particularly OsGSTU12, which was significantly up-regulated in 35S::HDA710-sense lines. InDels in the promoter region of OsGSTU12 and in the gene region of HDA710 occurred coincidentally among more than 90% accessions, and we identified multiple W-box motifs at the InDel position of OsGSTU12. Over-expression of OsGSTU12 also delayed leaf senescence in rice. Taken together, our results suggest that both HDA710 and OsGSTU12 are involved in regulating the process of leaf senescence in rice.

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

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

Epigenetic regulation in plant abiotic stress responses

In eukaryotic cells, gene expression is greatly influenced by the dynamic chromatin environment. Epigenetic mechanisms, including covalent modifications to DNA and histone tails and the accessibility of chromatin, create various chromatin states for stress-responsive gene expression that is important for adaptation to harsh environmental conditions. Recent studies have revealed that many epigenetic factors participate in abiotic stress responses, and various chromatin modifications are changed when plants are exposed to stressful environments. In this review, we summarize recent progress on the cross-talk between abiotic stress response pathways and epigenetic regulatory pathways in plants. Our review focuses on epigenetic regulation of plant responses to extreme temperatures, drought, salinity, the stress hormone abscisic acid, nutrient limitations and ultraviolet stress, and on epigenetic mechanisms of stress memory.

Homeostasis of histone acetylation is critical for auxin signaling and root morphogenesis

The auxin signaling and root morphogenesis are harmoniously controlled by two counteracted teams including (1) auxin/indole-3-acetic acid (AUX/IAA)-histone deacetylase (HDA) and (2) auxin response factor (ARF)-histone acetyltransferase (HAT). The involvement of histone acetylation in the regulation of transcription was firstly reported a few decades ago. In planta, auxin is the first hormone group that was discovered and it is also the most studied phytohormone. Current studies have elucidated the functions of histone acetylation in the modulation of auxin signaling as well as in the regulation of root morphogenesis under both normal and stress conditions. Based on the recent outcomes, this review is to provide a hierarchical view about the functions of histone acetylation in auxin signaling and root morphogenesis. In this report, we suggest that the auxin signaling must be controlled harmoniously by two counteracted teams including (1) auxin/indole-3-acetic acid (AUX/IAA)-histone deacetylase (HDA) and (2) auxin response factor (ARF)-histone acetyltransferase (HAT). Moreover, the balance in auxin signaling is very critical to contribute to normal root morphogenesis.

Histone Deacetylase TaHDT701 Functions in TaHDA6-TaHOS15 Complex to Regulate Wheat Defense Responses to Blumeria graminis f.sp. tritici

Powdery mildew disease caused by Blumeria graminis f.sp. tritici (Bgt) leads to severe economic losses in bread wheat (Triticum aestivum L.). To date, only a few epigenetic modulators have been revealed to regulate wheat powdery mildew resistance. In this study, the histone deacetylase 2 (HD2) type histone deacetylase TaHDT701 was identified as a negative regulator of wheat defense responses to Bgt. Using multiple approaches, we demonstrated that TaHDT701 associates with the RPD3 type histone deacetylase TaHDA6 and the WD40-repeat protein TaHOS15 to constitute a histone deacetylase complex, in which TaHDT701 could stabilize the TaHDA6-TaHOS15 association. Furthermore, knockdown of TaHDT701, TaHDA6, and TaHOS15 resulted in enhanced wheat powdery mildew resistance, suggesting that the TaHDT701-TaHDA6-TaHOS15 histone deacetylase complex negatively regulates wheat defense responses to Bgt. Moreover, chromatin immunoprecipitation assays revealed that TaHDT701 could function in concert with TaHOS15 to recruit TaHDA6 to the promoters of defense-related genes such as TaPR1, TaPR2, TaPR5, and TaWRKY45. In addition, silencing of TaHDT701, TaHDA6, and TaHOS15 resulted in the up-regulation of TaPR1, TaPR2, TaPR5, and TaWRKY45 accompanied with increased histone acetylation and methylation, as well as reduced nucleosome occupancy, at their promoters, suggesting that the TaHDT701-TaHDA6-TaHOS15 histone deacetylase complex suppresses wheat powdery mildew resistance by modulating chromatin state at defense-related genes.

Transcriptional analyses of two soybean cultivars under salt stress

Soybean is an economically important plant, and its production is affected in soils with high salinity levels. It is important to understand the adaptive mechanisms through which plants overcome this kind of stress and to identify potential genes for improving abiotic stress tolerance. RNA-Seq data of two Glycine max cultivars, a drought-sensitive (C08) and a tolerant (Conquista), subjected to different periods of salt stress were analyzed. The transcript expression profile was obtained using a transcriptogram approach, comparing both cultivars and different times of treatment. After 4 h of salt stress, Conquista cultivar had 1400 differentially expressed genes, 647 induced and 753 repressed. Comparative expression revealed that 719 genes share the same pattern of induction or repression between both cultivars. Among them, 393 genes were up- and 326 down-regulated. Salt stress also modified the expression of 54 isoforms of miRNAs in Conquista, by the maturation of 39 different pre-miRNAs. The predicted targets for 12 of those mature miRNAs also have matches with 15 differentially expressed genes from our analyses. We found genes involved in important pathways related to stress adaptation. Genes from both ABA and BR signaling pathways were modulated, with possible crosstalk between them, and with a likely post-transcriptional regulation by miRNAs. Genes related to ethylene biosynthesis, DNA repair, and plastid translation process were those that could be regulated by miRNA.

In silico analysis of maize HDACs with an emphasis on their response to biotic and abiotic stresses

Histone deacetylases (HDACs) are key epigenetic factors in regulating chromatin structure and gene expression in multiple aspects of plant growth, development, and response to abiotic or biotic stresses. Many studies on systematic analysis and molecular function of HDACs in Arabidopsis and rice have been conducted. However, systematic analysis of HDAC gene family and gene expression in response to abiotic and biotic stresses has not yet been reported. In this study, a systematic analysis of the HDAC gene family in maize was performed and 18 ZmHDACs distributed on nine chromosomes were identified. Phylogenetic analysis of ZmHDACs showed that this gene family could be divided into RPD3/HDA1, SIR2, and HD2 groups. Tissue-specific expression results revealed that ZmHDACs exhibited diverse expression patterns in different tissues, indicating that these genes might have diversified functions in growth and development. Expression pattern of ZmHDACs in hormone treatment and inoculation experiment suggested that several ZmHDACs might be involved in jasmonic acid or salicylic acid signaling pathway and defense response. Interestingly, HDAC genes were downregulated under heat stress, and immunoblotting results demonstrated that histones H3K9ac and H4K5ac levels were increased under heat stress. These results provide insights into ZmHDACs, which could help to reveal their functions in controlling maize development and responses to abiotic or biotic stresses.

The Sucrose Non-Fermenting 1-Related Protein Kinase 2 (SnRK2) Genes Are Multifaceted Players in Plant Growth, Development and Response to Environmental Stimuli

Reversible protein phosphorylation orchestrated by protein kinases and phosphatases is a major regulatory event in plants and animals. The SnRK2 subfamily consists of plant-specific protein kinases in the Ser/Thr protein kinase superfamily. Early observations indicated that SnRK2s are mainly involved in response to abiotic stress. Recent evidence shows that SnRK2s are multifarious players in a variety of biological processes. Here, we summarize the considerable knowledge of SnRK2s, including evolution, classification, biological functions and regulatory mechanisms at the epigenetic, post-transcriptional and post-translation levels.

Role of chromatin modification and remodeling in stem cell regulation and meristem maintenance in Arabidopsis

In higher plants, pluripotent stem cells reside in the specialized microenvironment called stem cell niches (SCNs) harbored at the shoot apical meristem (SAM) and root apical meristem (RAM), which give rise to the aerial and underground parts of a plant, respectively. The model plant Arabidopsis thaliana (Arabidopsis) has been extensively studied to decipher the intricate regulatory mechanisms involving some key transcriptions factors and phytohormones that play pivotal roles in stem cell homeostasis, meristem maintenance, and organ formation. However, there is increasing evidence to show the epigenetic regulation of the chromatin architecture, gene expression exerting an influence on an innate balance between the self-renewal of stem cells, and differentiation of the progeny cells to a specific tissue type or organ. Post-translational histone modifications, ATP-dependent chromatin remodeling, and chromatin assembly/disassembly are some of the key features involved in the modulation of chromatin architecture. Here, we discuss the major epigenetic regulators and illustrate their roles in the regulation of stem cell activity, meristem maintenance, and related organ patterning in Arabidopsis.

Histone deacetylase gene PtHDT902 modifies adventitious root formation and negatively regulates salt stress tolerance in poplar

Histone deacetylases (HDACs) regulate gene transcription, and play a critical role in plant growth, development and stress responses. HD2 proteins are plant specific histone deacetylases. In woody plants, functions of HD2s are not known. In this study, we cloned an HD2 gene PtHDT902 from Populus trichocarpa and investigated its sequence, expression, subcellular localization, and functions in root development and salt stress responses. Our findings indicated that PtHDT902 was a nuclear protein and its expression was regulated by abiotic stresses. The over-expression of PtHDT902 in both Arabidopsis and poplar increased the expression levels of gibberellin (GA) biosynthetic genes. The expression of PtHDT902 in Arabidopsis enhanced primary root growth, and its over-expression in poplar inhibited adventitious root formation. These phenotypes resulted from over-expression of PtHDT902 were consistent with the GA-overproduction phenotypes. In addition, the poplar plants over-expressing PtHDT902 exhibited lower tolerance to salt than non-transgenic plants. These findings indicated that PtHDT902 worked as an important regulator in adventitious root formation and salt stress tolerance in poplar.

Histone Deacetylase HDA9 With ABI4 Contributes to Abscisic Acid Homeostasis in Drought Stress Response

Drought stress, a major environmental factor, significantly affects plant growth and reproduction. Plants have evolved complex molecular mechanisms to tolerate drought stress. In this study, we investigated the function of the Arabidopsis thaliana RPD3-type HISTONE DEACETYLASE 9 (HDA9) in response to drought stress. The loss-of-function mutants hda9-1 and hda9-2 were insensitive to abscisic acid (ABA) and sensitive to drought stress. The ABA content in the hda9-1 mutant was reduced in wild type (WT) plant. Most histone deacetylases in animals and plants form complexes with other chromatin-remodeling components, such as transcription factors. In this study, we found that HDA9 interacts with the ABA INSENSITIVE 4 (ABI4) transcription factor using a yeast two-hybrid assay and coimmunoprecipitation. The expression of CYP707A1 and CYP707A2, which encode (+)-ABA 8'-hydroxylases, key enzymes in ABA catabolic pathways, was highly induced in hda9-1, hda9-2, abi4, and hda9-1 abi4 mutants upon drought stress. Chromatin immunoprecipitation and quantitative PCR showed that the HDA9 and ABI4 complex repressed the expression of CYP707A1 and CYP707A2 by directly binding to their promoters in response to drought stress. Taken together, these data suggest that HDA9 and ABI4 form a repressive complex to regulate the expression of CYP707A1 and CYP707A2 in response to drought stress in Arabidopsis.