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

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.

Histone Modifications Form Epigenetic Regulatory Networks to Regulate Abiotic Stress Response

Epigenetic modifiers such as erasers, readers, writers, and recruiters control abiotic stress response in flowering plants.

Functions and mechanisms of plant histone deacetylases

Lysine acetylation, one of the major types of post-translational modifications, plays critical roles in regulating gene expression and protein function. Histone deacetylases (HDACs) are responsible for removing acetyl groups from lysines of both histone and non-histone proteins. While tremendous progress has been made in understanding the function and mechanism of HDACs in animals in the past two decades, nearly half of the HDAC studies in plants were reported within the past five years. In this review, we summarize the major findings on plant HDACs, with a focus on the model plant Arabidopsis thaliana, and highlight the components, regulatory mechanisms, and biological functions of HDAC complexes.

Nitric oxide molecular targets: reprogramming plant development upon stress

Plants are sessile organisms that need to complete their life cycle by the integration of different abiotic and biotic environmental signals, tailoring developmental cues and defense concomitantly. Commonly, stress responses are detrimental to plant growth and, despite the fact that intensive efforts have been made to understand both plant development and defense separately, most of the molecular basis of this trade-off remains elusive. To cope with such a diverse range of processes, plants have developed several strategies including the precise balance of key plant growth and stress regulators [i.e. phytohormones, reactive nitrogen species (RNS), and reactive oxygen species (ROS)]. Among RNS, nitric oxide (NO) is a ubiquitous gasotransmitter involved in redox homeostasis that regulates specific checkpoints to control the switch between development and stress, mainly by post-translational protein modifications comprising S-nitrosation of cysteine residues and metals, and nitration of tyrosine residues. In this review, we have sought to compile those known NO molecular targets able to balance the crossroads between plant development and stress, with special emphasis on the metabolism, perception, and signaling of the phytohormones abscisic acid and salicylic acid during abiotic and biotic stress responses.

Histone Deacetylase HDT1 is Involved in Stem Vascular Development in Arabidopsis

Histone acetylation and deacetylation play essential roles in eukaryotic gene regulation. HD2 (HD-tuins) proteins were previously identified as plant-specific histone deacetylases. In this study, we investigated the function of the HDT1 gene in the formation of stem vascular tissue in Arabidopsis thaliana. The height and thickness of the inflorescence stems in the hdt1 mutant was lower than that of wild-type plants. Paraffin sections showed that the cell number increased compared to the wild type, while transmission electron microscopy showed that the size of individual tracheary elements and fiber cells significantly decreased in the hdt1 mutant. In addition, the cell wall thickness of tracheary elements and fiber cells increased. We also found that the lignin content in the stem of the hdt1 mutants increased compared to that of the wild type. Transcriptomic data revealed that the expression levels of many biosynthetic genes related to secondary wall components, including cellulose, lignin biosynthesis, and hormone-related genes, were altered, which may lead to the altered phenotype in vascular tissue of the hdt1 mutant. These results suggested that HDT1 is involved in development of the vascular tissue of the stem by affecting cell proliferation and differentiation.

Comprehensive genomic survey, structural classification and expression analysis of C2H2 zinc finger protein gene family in Brassica rapa L

C2H2 zinc finger protein (ZFP) genes have been extensively studied in many organisms and can function as transcription factors and be involved in many biological processes including plant growth and development and stress responses. In the current study, a comprehensive genomics analysis of the C2H2-ZFP genes in B. rapa was performed. A total of 301 B. rapa putative C2H2-ZFP (BrC2H2-ZFP) genes were identified from the available Brassica genome databases, and further characterized through analysis of conserved amino acid residues in C2H2-ZF domains and their organization, subcellular localization, phylogeny, additional domain, chromosomal location, synteny relationship, Ka/Ks ratio, and expression pattern. We also analyzed the expression patterns of eight B. rapa C2H2-ZFP genes under salt and drought stress conditions by using qRT-PCR technique. Our results showed that about one-third of these B. rapa C2H2-ZFP genes were originated from segmental duplication caused by the WGT around 13 to 17 MYA, one-third of them were highly and consecutively expressed in all tested tissues, and 92% of them were located in nucleus by prediction supporting then their functional roles as transcription factors, of which some may play important roles in plant growth and development. The Ka/Ks ratios of 264 orthologous C2H2-ZFP gene pairs between A. thaliana and B. rapa were all, except two, inferior to 1 (varied from 0.0116 to 1.4919, with an average value of 0.3082), implying that these genes had mainly experienced purifying selection during species evolution. The estimated divergence times of the same set of gene pairs ranged from 6.23 to 38.60 MY, with an average value of 18.29 MY, indicating that these gene members have undergone different selective pressures resulting in different evolutionary rates during species evolution. In addition, a few of these B. rapa C2H2-ZFPs were shown to be involved in stress responses in a similar way as their orthologs in A. thaliana. Comparison between A. thaliana and B. rapa orthologous C2H2-ZFP genes showed that the majority of these C2H2-ZFP gene members encodes proteins with conserved subcellular localization and functional domains between the two species but differed in their expression patterns in five tissues or organs. Thus, our study provides valuable information for further functional determination of each C2H2-ZFP gene across the Brassica species, and may help to select the appropriate gene targets for further in-depth studies, and genetic engineering and improvement of Brassica crops.

MYB96 recruits the HDA15 protein to suppress negative regulators of ABA signaling in Arabidopsis

Unlike activation of target genes in response to abscisic acid (ABA), how MYB96 transcription factor represses ABA-repressible genes to further enhance ABA responses remains unknown. Here, we show MYB96 interacts with the histone modifier HDA15 to suppress negative regulators of early ABA signaling. The MYB96-HDA15 complex co-binds to the promoters of a subset of RHO GTPASE OF PLANTS (ROP) genes, ROP6, ROP10, and ROP11, and represses their expression by removing acetyl groups of histone H3 and H4 from the cognate regions, particularly in the presence of ABA. In support, HDA15-deficient mutants display reduced ABA sensitivity and are susceptible to drought stress with derepression of the ROP genes, as observed in the myb96-1 mutant. Biochemical and genetic analyses show that MYB96 and HDA15 are interdependent in the regulation of ROP suppression. Thus, MYB96 confers maximal ABA sensitivity by regulating both positive and negative regulators of ABA signaling through distinctive molecular mechanisms.

MYB96 recruits the HDA15 protein to suppress negative regulators of ABA signaling in Arabidopsis

Unlike activation of target genes in response to abscisic acid (ABA), how MYB96 transcription factor represses ABA-repressible genes to further enhance ABA responses remains unknown. Here, we show MYB96 interacts with the histone modifier HDA15 to suppress negative regulators of early ABA signaling. The MYB96-HDA15 complex co-binds to the promoters of a subset of RHO GTPASE OF PLANTS (ROP) genes, ROP6, ROP10, and ROP11, and represses their expression by removing acetyl groups of histone H3 and H4 from the cognate regions, particularly in the presence of ABA. In support, HDA15-deficient mutants display reduced ABA sensitivity and are susceptible to drought stress with derepression of the ROP genes, as observed in the myb96-1 mutant. Biochemical and genetic analyses show that MYB96 and HDA15 are interdependent in the regulation of ROP suppression. Thus, MYB96 confers maximal ABA sensitivity by regulating both positive and negative regulators of ABA signaling through distinctive molecular mechanisms.

MYB96 can regulate both positive and negative regulators of ABA signaling to maximize plant drought tolerance. Here, the authors show that MYB96 represses expression of ABA negative regulators in Arabidopsis by interacting with HDA15 and promoting histone deacetylation at the cognate regions.

Molecular cloning and subcellular localization of six HDACs and their roles in response to salt and drought stress in kenaf (Hibiscus cannabinus L.)

BACKGROUND

Histone acetylation is an important epigenetic modification that regulates gene activity in response to stress. Histone acetylation levels are reversibly regulated by histone acetyltransferases (HATs) and histone deacetylases (HDACs). The imperative roles of HDACs in gene transcription, transcriptional regulation, growth and responses to stressful environment have been widely investigated in Arabidopsis. However, data regarding HDACs in kenaf crop has not been disclosed yet.

RESULTS

In this study, six HDACs genes (HcHDA2, HcHDA6, HcHDA8, HcHDA9, HcHDA19, and HcSRT2) were isolated and characterized. Phylogenetic tree revealed that these HcHDACs shared high degree of sequence homology with those of Gossypium arboreum. Subcellular localization analysis showed that GFP-tagged HcHDA2 and HcHDA8 were predominantly localized in the nucleus, HcHDA6 and HcHDA19 in nucleus and cytosol. The HcHDA9 was found in both nucleus and plasma membranes. Real-time quantitative PCR showed that the six HcHDACs genes were expressed with distinct expression patterns across plant tissues. Furthermore, we determined differential accumulation of HcHDACs transcripts under salt and drought treatments, indicating that these enzymes may participate in the biological process under stress in kenaf. Finally, we showed that the levels of histone H3 and H4 acetylation were modulated by salt and drought stress in kenaf.

CONCLUSIONS

We have isolated and characterized six HDACs genes from kenaf. These data showed that HDACs are imperative players for growth and development as well abiotic stress responses in kenaf.

Histone acetyltransferase GCN5 contributes to cell wall integrity and salt stress tolerance by altering the expression of cellulose synthesis genes

Excess soluble salts in soil are harmful to the growth and development of most plants. Evidence is emerging that the plant cell wall is involved in sensing and responding to salt stress, but the underlying mechanisms are not well understood. We reveal that the histone acetyltransferase General control non-repressed protein 5 (GCN5) is required for the maintenance of cell wall integrity and salt stress tolerance. The levels of GCN5 mRNA are increased in response to salt stress. The gcn5 mutants exhibited severe growth inhibition and defects in cell wall integrity under salt stress conditions. Combining RNA sequencing and chromatin immunoprecipitation assays, we identified the chitinase-like gene CTL1, polygalacturonase involved in expansion-3 (PGX3) and MYB domain protein-54 (MYB54) as direct targets of GCN5. Acetylation of H3K9 and H3K14 mediated by GCN5 is associated with activation of CTL1, PGX3 and MYB54 under salt stress. Moreover, constitutive expression of CTL1 in the gcn5 mutant restores salt tolerance and cell wall integrity. In addition, the expression of the wheat TaGCN5 gene in Arabidopsis gcn5 mutant plants complemented the salt tolerance and cell wall integrity phenotypes, suggesting that GCN5-mediated salt tolerance is conserved between Arabidopsis and wheat. Taken together, our data indicate that GCN5 plays a key role in the preservation of salt tolerance via versatile regulation in plants.

Histone Acetylation Dynamics Integrates Metabolic Activity to Regulate Plant Response to Stress

Histone lysine acetylation is an essential chromatin modification for epigenetic regulation of gene expression during plant response to stress. On the other hand, enzymes involved in histone acetylation homeostasis require primary metabolites as substrates or cofactors whose levels are greatly influenced by stress and growth conditions in plants. In addition, histone lysine acylation that requires similar enzymes for deposition and removal as histone acetylation has been recently characterized in plant. Results on understanding the intrinsic relationship between histone acetylation/acylation, metabolism and stress response in plants are accumulating. In this review, we summarize recent advance in the field and propose a model of interplay between metabolism and epigenetic regulation of genes expression in plant adaptation to stress.

Dynamic Changes in Genome-Wide Histone3 Lysine27 Trimethylation and Gene Expression of Soybean Roots in Response to Salt Stress

Soybean is an important economic crop for human diet, animal feeds and biodiesel due to high protein and oil content. Its productivity is significantly hampered by salt stress, which impairs plant growth and development by affecting gene expression, in part, through epigenetic modification of chromatin status. However, little is known about epigenetic regulation of stress response in soybean roots. Here, we used RNA-seq and ChIP-seq technologies to study the dynamics of genome-wide transcription and histone methylation patterns in soybean roots under salt stress. Eight thousand seven hundred ninety eight soybean genes changed their expression under salt stress treatment. Whole-genome ChIP-seq study of an epigenetic repressive mark, histone H3 lysine 27 trimethylation (H3K27me3), revealed the changes in H3K27me3 deposition during the response to salt stress. Unexpectedly, we found that most of the inactivation of genes under salt stress is strongly correlated with the de novo establishment of H3K27me3 in various parts of the promoter or coding regions where there is no H3K27me3 in control plants. In addition, the soybean histone modifiers were identified which may contribute to de novo histone methylation and gene silencing under salt stress. Thus, dynamic chromatin regulation, switch between active and inactive modes, occur at target loci in order to respond to salt stress in soybean. Our analysis demonstrates histone methylation modifications are correlated with the activation or inactivation of salt-inducible genes in soybean roots.

Arabidopsis Histone Methyltransferase SUVH5 Is a Positive Regulator of Light-Mediated Seed Germination

Plant lifecycle starts from seed germination, which is regulated by various environmental cues and endogenous hormones. Light promotes seed germination mainly by phytochrome B (PHYB) during the initial phase of imbibition, which involves genome-wide light-responsive transcription changes. Recent studies indicated an involvement of multiple epigenetic factors in the control of seed germination. However, few studies have been reported about the role of a histone methyltransferase in light-mediated seed germination process. Here, we identified SUVH5, a histone H3 lysine 9 methyltransferase, as a positive regulator in light-mediated seed germination in Arabidopsis. Loss of function of SUVH5 leads to decreased PHYB-dependent seed germination. RNA-sequencing analysis displayed that SUVH5 regulates 24.6% of light-responsive transcriptome in imbibed seeds, which mainly related to hormonal signaling pathways and developmental processes. Furthermore, SUVH5 represses the transcription of ABA biosynthesis and signal transduction-related genes, as well as a family of DELAY OF GERMINATION (DOG) genes via dimethylation of histone H3 at lysine 9 (H3K9me2) in imbibed seeds. Taken together, our findings revealed that SUVH5 is a novel positive regulator of light-mediated seed germination in Arabidopsis.

Structural and chemical biology of deacetylases for carbohydrates, proteins, small molecules and histones

Deacetylation is the removal of an acetyl group and occurs on a plethora of targets and for a wide range of biological reasons. Several pathogens deacetylate their surface carbohydrates to evade immune response or to support biofilm formation. Furthermore, dynamic acetylation/deacetylation cycles govern processes from chromatin remodeling to posttranslational modifications that compete with phosphorylation. Acetylation usually occurs on nitrogen and oxygen atoms and are referred to as N- and O-acetylation, respectively. This review discusses the structural prerequisites that enzymes must have to catalyze the deacetylation reaction, and how they adapted by formation of specific substrate and metal binding sites.

Retrospective and perspective of plant epigenetics in China

Epigenetics refers to the study of heritable changes in gene function that do not involve changes in the DNA sequence. Such effects on cellular and physiological phenotypic traits may result from external or environmental factors or be part of normal developmental program. In eukaryotes, DNA wraps on a histone octamer (two copies of H2A, H2B, H3 and H4) to form nucleosome, the fundamental unit of chromatin. The structure of chromatin is subjected to a dynamic regulation through multiple epigenetic mechanisms, including DNA methylation, histone posttranslational modifications (PTMs), chromatin remodeling and noncoding RNAs. As conserved regulatory mechanisms in gene expression, epigenetic mechanisms participate in almost all the important biological processes ranging from basal development to environmental response. Importantly, all of the major epigenetic mechanisms in mammalians also occur in plants. Plant studies have provided numerous important contributions to the epigenetic research. For example, gene imprinting, a mechanism of parental allele-specific gene expression, was firstly observed in maize; evidence of paramutation, an epigenetic phenomenon that one allele acts in a single locus to induce a heritable change in the other allele, was firstly reported in maize and tomato. Moreover, some unique epigenetic mechanisms have been evolved in plants. For example, the 24-nt siRNA-involved RNA-directed DNA methylation (RdDM) pathway is plant-specific because of the involvements of two plant-specific DNA-dependent RNA polymerases, Pol IV and Pol V. A thorough study of epigenetic mechanisms is of great significance to improve crop agronomic traits and environmental adaptability. In this review, we make a brief summary of important progress achieved in plant epigenetics field in China over the past several decades and give a brief outlook on future research prospects. We focus our review on DNA methylation and histone PTMs, the two most important aspects of epigenetic mechanisms.

An Effector from the Cyst Nematode Heterodera schachtii Derepresses Host rRNA Genes by Altering Histone Acetylation

Cyst nematodes are plant-pathogenic animals that secrete effector proteins into plant root cells to alter host gene expression and reprogram these cells to form specialized feeding sites, known as syncytia. The molecular mechanisms of these effectors are mostly unknown. We determined that the sugar beet cyst nematode (Heterodera schachtii) 32E03 effector protein strongly inhibits the activities of Arabidopsis thaliana histone deacetylases including the HDT1 enzyme, which has a known function in the regulation of rRNA gene expression through chromatin modifications. We determined that plants expressing the 32E03 coding sequence exhibited increased acetylation of histone H3 along the rDNA chromatin. At low 32E03 expression levels, these chromatin changes triggered the derepression of a subset of rRNA genes, which were conducive to H. schachtii parasitism. By contrast, high levels of 32E03 caused profound bidirectional transcription along the rDNA, which triggered rDNA-specific small RNA production leading to RNA-directed DNA methylation and silencing of rDNA, which inhibited nematode development. Our data show that the 32E03 effector alters plant rRNA gene expression by modulating rDNA chromatin in a dose-dependent manner. Thus, the 32E03 effector epigenetically regulates plant gene expression to promote cyst nematode parasitism.

Characterization and subcellular localization of histone deacetylases and their roles in response to abiotic stresses in soybean

BACKGROUND

Histone deacetylases (HDACs) function as key epigenetic factors in repressing the expression of genes in multiple aspects of plant growth, development and plant response to abiotic or biotic stresses. To date, the molecular function of HDACs is well described in Arabidopsis thaliana, but no systematic analysis of this gene family in soybean (Glycine max) has been reported.

RESULTS

In this study, 28 HDAC genes from soybean genome were identified, which were asymmetrically distributed on 12 chromosomes. Phylogenetic analysis demonstrated that GmHDACs fall into three major groups previously named RPD3/HDA1, SIR2, and HD2. Subcellular localization analysis revealed that YFP-tagged GmSRT4, GmHDT2 and GmHDT4 were predominantly localized in the nucleus, whereas GmHDA6, GmHDA13, GmHDA14 and GmHDA16 were found in both the cytoplasm and nucleus. Real-time quantitative PCR showed that GmHDA6, GmHDA13, GmHDA14, GmHDA16 and GmHDT4 were broadly expressed across plant tissues, while GmHDA8, GmSRT2, GmSRT4 and GmHDT2 showed differential expression across various tissues. Interestingly, we measured differential changes in GmHDACs transcripts accumulation in response to several abiotic cues, indicating that these epigenetic modifiers could potentially be part of a dynamic transcriptional response to stress in soybean. Finally, we show that the levels of histone marks previously reported to be associated with plant HDACs are modulated by cold and heat in this legume.

CONCLUSION

We have identified and classified 28 HDAC genes in soybean. Our data provides insights into the evolution of the HDAC gene family and further support the hypothesis that these genes are important for the plant responses to environmental stress.

INDETERMINATE SPIKELET1 Recruits Histone Deacetylase and a Transcriptional Repression Complex to Regulate Rice Salt Tolerance

Perception and transduction of salt stress signals are critical for plant survival, growth, and propagation. Thus, identification of components of the salt stress-signaling pathway is important for rice (Oryza sativa) molecular breeding of salt stress resistance. Here, we report the identification of an apetala2/ethylene response factor transcription factor INDETERMINATE SPIKELET1 (IDS1) and its roles in the regulation of rice salt tolerance. By genetic screening and phenotype analysis, we demonstrated that IDS1 conferred transcriptional repression activity and acted as a negative regulator of salt tolerance in rice. To identify potential downstream target genes regulated by IDS1, we conducted chromatin immunoprecipitation (ChIP) sequencing and ChIP-quantitative PCR assays and found that IDS1 may directly associate with the GCC-box-containing motifs in the promoter regions of abiotic stress-responsive genes, including LEA1 (LATE EMBRYOGENESIS ABUNDANT PROTEIN1) and SOS1 (SALT OVERLY SENSITIVE1), which are key genes regulating rice salt tolerance. IDS1 physically interacted with the transcriptional corepressor topless-related 1 and the histone deacetylase HDA1, contributing to the repression of LEA1 and SOS1 expression. Analyses of histone H3 acetylation status and RNA polymerase II occupation on the promoters of LEA1 and SOS1 further defined the molecular foundation of the transcriptional repression activity of IDS1. Our findings illustrate an epigenetic mechanism by which IDS1 modulates salt stress signaling as well as salt tolerance in rice.

Histone Deacetylase 701 (HDT701) Induces Flowering in Rice by Modulating Expression of OsIDS1

Rice is a facultative short-day (SD) plant in which flowering is induced under SD conditions or by other environmental factors and internal genetic programs. Overexpression of Histone Deacetylase 701 (HDT701) accelerates flowering in hybrid rice. In this study, mutants defective in HDT701 flowered late under both SD and long-day conditions. Expression levels of florigens Heading date 3a (Hd3a) and Rice Flowering Locus T1 (RFT1), and their immediate upstream floral activator Early heading date 1 (Ehd1), were significantly decreased in the hdt701 mutants, indicating that HDT701 functions upstream of Ehd1 in controlling flowering time. Transcript levels of OsINDETERMINATE SPIKELET 1 (OsIDS1), an upstream repressor of Ehd1, were significantly increased in the mutants while those of OsGI and Hd1 were reduced. Chromatin-immunoprecipitation assays revealed that HDT701 directly binds to the promoter region of OsIDS1. These results suggest that HDT701 induces flowering by suppressing OsIDS1.

Epigenetic modifications preserve the hyperaccumulator Noccaea caerulescens from Ni geno-toxicity

The Ni hyperaccumulator Noccaea caerulescens has adapted to live in a naturally stressed environment, evolving a complex pattern of traits to cope with adverse conditions. Evidence is accumulating regarding the important role of epigenetic modifications in regulating plant responses to stress. In this study, we present data from the natural "open-field" adaptation of the Ni hyperaccumulator N. caerulescens to serpentine soil and provide the first evidence of the involvement of epigenetic changes in response to the high Ni content present in plant leaves. The alkaline comet assay revealed the integrity of the nuclei of leaf cells of N. caerulescens grown in a Ni-rich environment, while in the non-tolerant Arabidopsis thaliana exposed to Ni, the nuclei were severely damaged. DNA of N. caerulescens plants grown in situ were considerably hyper-methylated compared to A. thaliana plants exposed to Ni. In addition, qRT-PCR revealed that N. caerulescens MET1, DRM2, and HDA8 genes involved in epigenetic DNA and histone modification were up-regulated in the presence of high Ni content in leaves. Such epigenetic modifications may constitute a defense strategy that prevents genome instability and direct damage to the DNA structure by Ni ion, enabling plants to survive in an extreme environment. Further studies will be necessary to analyze in detail the involvement of DNA methylation and other epigenetic mechanisms in the complex process of metal hyperaccumulation and plants' adaptive response. Environ. Mol. Mutagen. 59:464-475, 2018. © 2018 Wiley Periodicals, Inc.

Genome-Wide Exon-Capture Approach Identifies Genetic Variants of Norway Spruce Genes Associated With Susceptibility to Heterobasidion parviporum Infection

Root and butt rot caused by members of the Heterobasidion annosum species complex is the most economically important disease of conifer trees in boreal forests. Wood decay in the infected trees dramatically decreases their value and causes considerable losses to forest owners. Trees vary in their susceptibility to Heterobasidion infection, but the genetic determinants underlying the variation in the susceptibility are not well-understood. We performed the identification of Norway spruce genes associated with the resistance to Heterobasidion parviporum infection using genome-wide exon-capture approach. Sixty-four clonal Norway spruce lines were phenotyped, and their responses to H. parviporum inoculation were determined by lesion length measurements. Afterwards, the spruce lines were genotyped by targeted resequencing and identification of genetic variants (SNPs). Genome-wide association analysis identified 10 SNPs located within 8 genes as significantly associated with the larger necrotic lesions in response to H. parviporum inoculation. The genetic variants identified in our analysis are potential marker candidates for future screening programs aiming at the differentiation of disease-susceptible and resistant trees.

Epigenetic switch from repressive to permissive chromatin in response to cold stress

Switching from repressed to active status in chromatin regulation is part of the critical responses that plants deploy to survive in an ever-changing environment. We previously reported that HOS15, a WD40-repeat protein, is involved in histone deacetylation and cold tolerance in Arabidopsis However, it remained unknown how HOS15 regulates cold responsive genes to affect cold tolerance. Here, we show that HOS15 interacts with histone deacetylase 2C (HD2C) and both proteins together associate with the promoters of cold-responsive COR genes, COR15A and COR47 Cold induced HD2C degradation is mediated by the CULLIN4 (CUL4)-based E3 ubiquitin ligase complex in which HOS15 acts as a substrate receptor. Interference with the association of HD2C and the COR gene promoters by HOS15 correlates with increased acetylation levels of histone H3. HOS15 also interacts with CBF transcription factors to modulate cold-induced binding to the COR gene promoters. Our results here demonstrate that cold induces HOS15-mediated chromatin modifications by degrading HD2C. This switches the chromatin structure status and facilitates recruitment of CBFs to the COR gene promoters. This is an apparent requirement to acquire cold tolerance.

Expression of AhDREB1, an AP2/ERF Transcription Factor Gene from Peanut, Is Affected by Histone Acetylation and Increases Abscisic Acid Sensitivity and Tolerance to Osmotic Stress in Arabidopsis

Drought stress negatively affects plant growth and development. An increasing number of reports have revealed the involvement of APETALA2/Ethylene Responsive Factor (AP2/ERF) transcription factors (TFs) in biotic and abiotic stress regulation in plants. However, research on these TFs in the peanut plant (Arachis hypogaea) has been limited. Here, we isolated a full-length coding sequence (CDS) of the AP2/ERF family gene AhDREB1 from the peanut plant and showed that its expression was induced by Polyethylene Glycol (PEG) 6000 and exogenous abscisic acid (ABA) treatment. When overexpressed in Arabidopsis, AhDREB1 increased both ABA levels and ABA sensitivity, affected the ABA signaling pathway and increased the expression of downstream drought stress-related genes RD29A, P5CS1, P5CS2 and NCED1. These results demonstrate that AhDREB1 can improve tolerance to drought via the ABA-dependent pathway in Arabidopsis. In the peanut plant, the specific histone deacetylases (HDACs) inhibitor trichostatin A (TSA) promotes AhDREB1 transcription and the enrichment level of H3ac was increased in regions of the AhDREB1 gene during TSA and PEG treatment. In summary, histone acetylation can affect the expression of AhDREB1 under osmotic stress conditions, thereby improving plant drought resistance.

Simulated microgravity, Mars gravity, and 2g hypergravity affect cell cycle regulation, ribosome biogenesis, and epigenetics in Arabidopsis cell cultures

Gravity is the only component of Earth environment that remained constant throughout the entire process of biological evolution. However, it is still unclear how gravity affects plant growth and development. In this study, an in vitro cell culture of Arabidopsis thaliana was exposed to different altered gravity conditions, namely simulated reduced gravity (simulated microgravity, simulated Mars gravity) and hypergravity (2g), to study changes in cell proliferation, cell growth, and epigenetics. The effects after 3, 14, and 24-hours of exposure were evaluated. The most relevant alterations were found in the 24-hour treatment, being more significant for simulated reduced gravity than hypergravity. Cell proliferation and growth were uncoupled under simulated reduced gravity, similarly, as found in meristematic cells from seedlings grown in real or simulated microgravity. The distribution of cell cycle phases was changed, as well as the levels and gene transcription of the tested cell cycle regulators. Ribosome biogenesis was decreased, according to levels and gene transcription of nucleolar proteins and the number of inactive nucleoli. Furthermore, we found alterations in the epigenetic modifications of chromatin. These results show that altered gravity effects include a serious disturbance of cell proliferation and growth, which are cellular functions essential for normal plant development.

Evolutionary diversification of type-2 HDAC structure, function and regulation in Nicotiana tabacum

Type-2 HDACs (HD2s) are plant-specific histone deacetylases that play diverse roles during development and in responses to biotic and abiotic stresses. In this study we characterized the six tobacco genes encoding HD2s that mainly differ by the presence or the absence of a typical zinc finger in their C-terminal part. Of particular interest, these HD2 genes exhibit a highly conserved intron/exon structure. We then further investigated the phylogenetic relationships among the HD2 gene family, and proposed a model of the genetic events that led to the organization of the HD2 family in Solanaceae. Absolute quantification of HD2 mRNAs in N. tabacum and in its precursors, N. tomentosiformis and N. sylvestris, did not reveal any pseudogenization of any of the HD2 genes, but rather specific regulation of HD2 expression in these three species. Functional complementation approaches in Arabidopsis thaliana demonstrated that the four zinc finger-containing HD2 proteins exhibit the same biological function in response to salt stress, whereas the two HD2 proteins without zinc finger have different biological function.

Fine-Tuning of Gene Expression by tRNA-Derived Fragments during Abiotic Stress Signal Transduction

When plants are subjected to unfavorable environmental conditions, overall gene expression in stressed cells is altered from a programmed pattern for normal development to an adaptive pattern for survival. Rapid changes in plant gene expression include production of stress responsive proteins for protection as well as reduction of irrelevant proteins to minimize energy consumption during growth. In addition to the many established mechanisms known to modulate gene expression in eukaryotes, a novel strategy involving tRNA-derived fragments (tRFs) was recently reported to control gene expression. In animals, tRFs are shown to play a certain role in infected or cancer cells. However, tRFs are expected to function in the regulation of gene expression against abiotic stress conditions in plants. Moreover, the underlying mechanism linking up-regulation of tRFs under stress conditions with the stress tolerant response remains unknown. In this review, the biogenesis and putative function of diverse tRFs in abiotic stress signaling are discussed with a focus on tRFs as a transcriptional/post-transcriptional/translational regulator.

Versatility of HDA19-deficiency in increasing the tolerance of Arabidopsis to different environmental stresses

Histone acetylation is controlled by HATs and HDACs, which are essential epigenetic elements that regulate plant response to environmental stresses. A previous study revealed that a deficiency in an HDAC isoform (HDA19) increases tolerance to high salinity stress in the Arabidopsis wild-type Col-0 background. Here, the increased tolerance of hda19 to drought and heat stresses is demonstrated. Results indicate that hda19 plants have greater tolerance than wild-type plants to stress conditions. The data indicate that the stress response pathway coordinated by HDA19 plays a pivotal role in increasing tolerance to a variety of different abiotic stresses in Arabidopsis, including salinity, drought, and heat. The greater level of tolerance of hda19 plants to several different environmental stresses suggests that HDA19 represents a promising target for pharmacological manipulation in order to enhance abiotic stress tolerance in plants.

ABBREVIATIONS

HAT, histone acetyltransferase; HDAC, histone deacetylase; HSF, heat shock transcription factor; RPD3, reduced potassium dependency 3; SIRT, Silent Information Regulator 2.

Elucidating the molecular mechanisms mediating plant salt-stress responses

Contents Summary 523 I. Introduction 523 II. Sensing salt stress 524 III. Ion homeostasis regulation 524 IV. Metabolite and cell activity responses to salt stress 527 V. Conclusions and perspectives 532 Acknowledgements 533 References 533 SUMMARY: Excess soluble salts in soil (saline soils) are harmful to most plants. Salt imposes osmotic, ionic, and secondary stresses on plants. Over the past two decades, many determinants of salt tolerance and their regulatory mechanisms have been identified and characterized using molecular genetics and genomics approaches. This review describes recent progress in deciphering the mechanisms controlling ion homeostasis, cell activity responses, and epigenetic regulation in plants under salt stress. Finally, we highlight research areas that require further research to reveal new determinants of salt tolerance in plants.

Canonical and Noncanonical Actions of Arabidopsis Histone Deacetylases in Ribosomal RNA Processing

Ribosome biogenesis is a fundamental process required for all cellular activities. Histone deacetylases play critical roles in many biological processes including transcriptional repression and rDNA silencing. However, their function in pre-rRNA processing remains poorly understood. Here, we discovered a previously uncharacterized role of Arabidopsis thaliana histone deacetylase HD2C in pre-rRNA processing via both canonical and noncanonical manners. HD2C interacts with another histone deacetylase HD2B and forms homo- and/or hetero-oligomers in the nucleolus. Depletion of HD2C and HD2B induces a ribosome-biogenesis deficient phenotype and aberrant accumulation of 18S pre-rRNA intermediates. Our genome-wide analysis revealed that HD2C binds and represses the expression of key genes involved in ribosome biogenesis. Using RNA immunoprecipitation and sequencing, we further uncovered a noncanonical mechanism of HD2C directly associating with pre-rRNA and small nucleolar RNAs to regulate rRNA methylation. Together, this study reveals a multifaceted role of HD2C in ribosome biogenesis and provides mechanistic insights into how histone deacetylases modulate rRNA maturation at the transcriptional and posttranscriptional levels.

Biology in the Dry Seed: Transcriptome Changes Associated with Dry Seed Dormancy and Dormancy Loss in the Arabidopsis GA-Insensitive sleepy1-2 Mutant

Plant embryos can survive years in a desiccated, quiescent state within seeds. In many species, seeds are dormant and unable to germinate at maturity. They acquire the capacity to germinate through a period of dry storage called after-ripening (AR), a biological process that occurs at 5-15% moisture when most metabolic processes cease. Because stored transcripts are among the first proteins translated upon water uptake, they likely impact germination potential. Transcriptome changes associated with the increased seed dormancy of the GA-insensitive sly1-2 mutant, and with dormancy loss through long sly1-2 after-ripening (19 months) were characterized in dry seeds. The SLY1 gene was needed for proper down-regulation of translation-associated genes in mature dry seeds, and for AR up-regulation of these genes in germinating seeds. Thus, sly1-2 seed dormancy may result partly from failure to properly regulate protein translation, and partly from observed differences in transcription factor mRNA levels. Two positive regulators of seed dormancy, DELLA GAI (GA-INSENSITIVE) and the histone deacetylase HDA6/SIL1 (MODIFIERS OF SILENCING1) were strongly AR-down-regulated. These transcriptional changes appeared to be functionally relevant since loss of GAI function and application of a histone deacetylase inhibitor led to decreased sly1-2 seed dormancy. Thus, after-ripening may increase germination potential over time by reducing dormancy-promoting stored transcript levels. Differences in transcript accumulation with after-ripening correlated to differences in transcript stability, such that stable mRNAs appeared AR-up-regulated, and unstable transcripts AR-down-regulated. Thus, relative transcript levels may change with dry after-ripening partly as a consequence of differences in mRNA turnover.

The Distinct Roles of Class I and II RPD3-Like Histone Deacetylases in Salinity Stress Response

Histone acetylation is an essential process in the epigenetic regulation of diverse biological processes, including environmental stress responses in plants. Previously, our research group identified a histone deacetylase (HDAC) inhibitor (HDI) that confers salt tolerance in Arabidopsis (Arabidopsis thaliana). In this study, we demonstrate that class I HDAC (HDA19) and class II HDACs (HDA5/14/15/18) control responses to salt stress through different pathways. The screening of 12 different selective HDIs indicated that seven newly reported HDIs enhance salt tolerance. Genetic analysis, based on a pharmacological study, identified which HDACs function in salinity stress tolerance. In the wild-type Columbia-0 background, hda19 plants exhibit tolerance to high-salinity stress, while hda5/14/15/18 plants exhibit hypersensitivity to salt stress. Transcriptome analysis revealed that the effect of HDA19 deficiency on the response to salinity stress is distinct from that of HDA5/14/15/18 deficiencies. In hda19 plants, the expression levels of stress tolerance-related genes, late embryogenesis abundant proteins that prevent protein aggregation and positive regulators such as ABI5 and NAC019 in abscisic acid signaling, were induced strongly relative to the wild type. Neither of these elements was up-regulated in the hda5/14/15/18 plants. The mutagenesis of HDA19 by genome editing in the hda5/14/15/18 plants enhanced salt tolerance, suggesting that suppression of HDA19 masks the phenotype caused by the suppression of class II HDACs in the salinity stress response. Collectively, our results demonstrate that HDIs that inhibit class I HDACs allow the rescue of plants from salinity stress regardless of their selectivity, and they provide insight into the hierarchal regulation of environmental stress responses through HDAC isoforms.

Expression of a populus histone deacetylase gene 84KHDA903 in tobacco enhances drought tolerance

Histone deacetylases (HDACs) play a key role in regulating plant growth, development and stress responses. However, functions of HDACs in woody plants are largely unknown. In this study, a novel gene encoding a RPD3/HDA1-type histone deacetylase was cloned from 84K poplar (Populus alba×Populus glandulosa) and designated as 84KHDA903. The 84KHDA903 encodes a protein composed of 500 amino acid residues, which contains a conserved HDAC domain. Transient expression of 84KHDA903 in onion epidermal cells suggested that it was exclusively localized in nucleus. The 84KHDA903 exhibited different expression patterns under drought, salt and ABA treatments. The expression of 84KHDA903 was responsive to drought and ABA but not to salt. To understand the function of 84KHDA903 in stress responses, the 84KHDA903 gene was transformed into tobacco. The expression of 84KHDA903 in tobacco increased the tolerance of transgenic seeds to mannitol but not to salt. In adult stage, the 84KHDA903-expressing tobacco exhibited drought tolerance and showed strong capacity to recover after drought. During the recovery period, the stress-responsive genes including NtDREB4, NtDREB3 and NtLEA5 were induced to be highly expressed in the 84KHDA903 transgenic plants in contrast to wild-type plants. Taken together, for the first time, we reported a RPD3/HDA1-type histone deacetylase from poplar, 84KHDA903, which acted as a positive regulator in drought stress responses.

HISTONE DEACETYLASE 6 represses pathogen defence responses in Arabidopsis thaliana

Plant defence mechanisms are suppressed in the absence of pathogen attack to prevent wasted energy and growth inhibition. However, how defence responses are repressed is not well understood. Histone deacetylase 6 (HDA6) is a negative regulator of gene expression, and its role in pathogen defence response in plants is not known. In this study, a novel allele of hda6 (designated as shi5) with spontaneous defence response was isolated from a forward genetics screening in Arabidopsis. The shi5 mutant exhibited increased resistance to hemibiotrophic bacterial pathogen Pst DC3000, constitutively activated expression of pathogen-responsive genes including PR1, PR2, etc. and increased histone acetylation levels at the promoters of most tested genes that were upregulated in shi5. In both wild type and shi5 plants, the expression and histone acetylation of these genes were upregulated by pathogen infection. HDA6 was found to bind to the promoters of these genes under both normal growth conditions and pathogen infection. Our research suggests that HDA6 is a general repressor of pathogen defence response and plays important roles in inhibiting and modulating the expression of pathogen-responsive genes in Arabidopsis.

Trichostatin A and 5-Aza-2'-Deoxycytidine influence the expression of cold-induced genes in Arabidopsis

The expression of cold-induced genes is critical for plants to survive under freezing stress. However, the underlying mechanisms for the decision of when, where, and which genes to express are unclear when a plant meets a sudden temperature drop. Previous studies have demonstrated epigenetics to play a central role in the regulation of gene expression in plant responses to environmental stress. DNA methylation and histone deacetylation are the two most important epigenetic modifications. This study was conducted to investigate the effects of inhibiting DNA methylation and histone deacetylation on gene expression, and to explore the potential role of epigenetics in plant responses to cold stress. The results revealed that histone deacetylase inhibitors (trichostatin A) and DNA methylation inhibitors (5-Aza-2'-deoxycytosine) treatment enhanced cold tolerance. DNA microarray analysis and the gene ontology method revealed 76 cold-induced differently expressed genes in Arabidopsis thaliana seedlings that were treated to 0°C for 24 h following Trichostatin A and 5-Aza-2'-Deoxycytidine. Furthermore, analyses of metabolic pathways and transcription factors of 3305 differentially expressed genes were performed. Each four metabolic pathways were significantly affected (p < 0.01) by Trichostatin A and 5-Aza-2'-Deoxycytidine. Finally, 10 genes were randomly selected and verified via qPCR analysis. Our study indicated that Trichostatin A and 5-Aza-2'-Deoxycytidine can improve the plant cold resistance and influence the expression of the cold-induced gene in A. thaliana. This result will advance our understanding of plant freezing responses and may provide a helpful strategy for cold tolerance improvement in crops.

Expression of the SIN3 homologue from banana, MaSIN3, suppresses ABA responses globally during plant growth in Arabidopsis

The SIN3 family of co-repressors is a family of highly conserved eukaryotic repressor proteins that regulates diverse functions in yeasts and animals but remains largely uncharacterized functionally even in plants like Arabidopsis. The sole SIN3 homologue in banana, MaSIN3, was identified as a 1408 amino acids, nuclear localized protein conserved to other SIN3s in the PAH, HID and HCR domains. Interestingly, MaSIN3 over-expression in Arabidopsis mimics a state of reduced ABA responses throughout plant development affecting growth processes such as germination, root growth, stomatal closure and water loss, flowering and senescence. The reduction in ABA responses is not due to reduced ABA levels but due to suppression of expression of several transcription factors mediating ABA responses. Transcript levels of negative regulators of germination (ABI3, ABI5, PIL5, RGL2 and RGL3) are reduced post-imbibition while those responsible for GA biosynthesis are up-regulated in transgenic MaSIN3 over-expressers. ABA-associated transcription factors are also down-regulated in response to ABA treatment. The HDAC inhibitors, SAHA and sodium butyrate, in combination with ABA differentially suppress germination in control and transgenic lines suggesting the recruitment by MaSIN3 of HDACs involved in suppression of ABA responses in different processes. The studies provide an insight into the ability of MaSIN3 to specifically affect a subset of developmental processes governed largely by ABA.

Multilevel Regulation of Abiotic Stress Responses in Plants

The sessile lifestyle of plants requires them to cope with stresses in situ. Plants overcome abiotic stresses by altering structure/morphology, and in some extreme conditions, by compressing the life cycle to survive the stresses in the form of seeds. Genetic and molecular studies have uncovered complex regulatory processes that coordinate stress adaptation and tolerance in plants, which are integrated at various levels. Investigating natural variation in stress responses has provided important insights into the evolutionary processes that shape the integrated regulation of adaptation and tolerance. This review primarily focuses on the current understanding of how transcriptional, post-transcriptional, post-translational, and epigenetic processes along with genetic variation orchestrate stress responses in plants. We also discuss the current and future development of computational tools to identify biologically meaningful factors from high dimensional, genome-scale data and construct the signaling networks consisting of these components.

Plant-Specific Histone Deacetylases HDT1/2 Regulate GIBBERELLIN 2-OXIDASE2 Expression to Control Arabidopsis Root Meristem Cell Number

Root growth is modulated by environmental factors and depends on cell production in the root meristem (RM). New cells in the meristem are generated by stem cells and transit-amplifying cells, which together determine RM cell number. Transcription factors and chromatin-remodeling factors have been implicated in regulating the switch from stem cells to transit-amplifying cells. Here, we show that two Arabidopsis thaliana paralogs encoding plant-specific histone deacetylases, HDT1 and HDT2, regulate a second switch from transit-amplifying cells to expanding cells. Knockdown of HDT1/2 (hdt1,2i) results in an earlier switch and causes a reduced RM cell number. Our data show that HDT1/2 negatively regulate the acetylation level of the C₁₉-GIBBERELLIN 2-OXIDASE2 (GA2ox2) locus and repress the expression of GA2ox2 in the RM and elongation zone. Overexpression of GA2ox2 in the RM phenocopies the hdt1,2i phenotype. Conversely, knockout of GA2ox2 partially rescues the root growth defect of hdt1,2i These results suggest that by repressing the expression of GA2ox2, HDT1/2 likely fine-tune gibberellin metabolism and they are crucial for regulating the switch from cell division to expansion to determine RM cell number. We propose that HDT1/2 function as part of a mechanism that modulates root growth in response to environmental factors.

HISTONE DEACETYLASE6 Acts in Concert with Histone Methyltransferases SUVH4, SUVH5, and SUVH6 to Regulate Transposon Silencing

Histone deacetylases (HDACs) play important roles in regulating gene expression. In yeast and animals, HDACs act as components of multiprotein complexes that modulate transcription during various biological processes. However, little is known about the interacting proteins of plant HDACs. To identify the plant HDAC complexes and interacting proteins, we developed an optimized workflow using immunopurification coupled to mass spectrometry-based proteomics in Arabidopsis thaliana We found that the histone deacetylase HDA6 can interact with the histone methyltransferases SUVH4, SUVH5, and SUVH6 (SUVH4/5/6). Domain analysis revealed that the C-terminal regions of HDA6 and SUVH5 are important for their interaction. Furthermore, HDA6 interacts with SUVH4/5/6 and coregulates a subset of transposons through histone H3K9 methylation and H3 deacetylation. In addition, two phosphorylated serine residues, S427 and S429, were unambiguously identified in the C-terminal region of HDA6. Phosphomimetics (amino acid substitutions that mimic a phosphorylated protein) of HDA6 resulted in increased enzymatic activity, whereas the mutation of S427 to alanine in HDA6 abolished its interaction with SUVH5 and SUVH6, suggesting that the phosphorylation of HDA6 is important for its activity and function.

Histone deacetylase HD2D is involved in regulating plant development and flowering time in Arabidopsis

HD2D is one of the 4 plant specific HD2 family histone deacetylases (HDAC) identified in Arabidopsis and it is distantly related to the other HD2 members. At this time, little is known about the function of HD2D in plants. Here we provide evidence that HD2D is involved in the control of flowering time. Flowering was delayed in transgenic plants overexpressing HD2D and hd2d mutant plants flowered earlier compared with wild-type in both long day and short day conditions. Expression of several floral identity genes was altered in these plants. Taken together, our findings suggest that HD2D is a negative regulator of flowering that modulates the transition of vegetative to reproductive growth in a photoperiod independent manner.

Metabolomic and Proteomic Analysis of Maize Embryonic Callus induced from immature embryo

The low ratio of embryonic callus (EC) induction has inhibited the rapid development of maize genetic engineering. Still, little is known to explain the genotype-dependence of EC induction. Here, we performed a large-scale, quantitative analysis of the maize EC metabolome and proteome at three typical induction stages in two inbred lines with a range of EC induction capabilities. Comparison of the metabolomes and proteomes suggests that the differential molecular responses begin at an early stage of development and continue throughout the process of EC formation. The two inbred lines show different responses under various conditions, such as metal ion binding, cell enlargement, stem cell formation, meristematic activity maintenance, somatic embryogenesis, cell wall synthesis, and hormone signal transduction. Furthermore, the differences in hormone (auxin, cytokinin, gibberellin, salicylic acid, jasmonic acid, brassinosteroid and ethylene) synthesis and transduction ability could partially explain the higher EC induction ratio in the inbred line 18-599R. During EC formation, repression of the "histone deacetylase 2 and ERF transcription factors" complex in 18-599R activated the expression of downstream genes, which further promoted EC induction. Together, our data provide new insights into the molecular regulatory mechanism responsible for efficient EC induction in maize.

Arabidopsis NF-YCs Mediate the Light-Controlled Hypocotyl Elongation via Modulating Histone Acetylation

Light is a crucial environmental signal that promotes photomorphogenesis, the developmental process with a series of light-dependent alterations for plants to adapt various external challenges. Chromatin modification has been proposed to be involved in such light-mediated growth, but the underlying mechanism is still elusive. In this study, we identified four Arabidopsis thaliana Nuclear Factor-YC homologs, NF-YC1, NF-YC3, NF-YC4, and NF-YC9 (NF-YCs), which function redundantly as repressors of light-controlled hypocotyl elongation via histone deacetylation. Obvious etiolation phenotypes are observed in NF-YCs loss-of-function mutant seedlings grown under light conditions, including significant elongated hypocotyls and fewer opened cotyledons. We found that NF-YCs interact with histone deacetylase HDA15 in the light, co-target the promoters of a set of hypocotyl elongation-related genes, and modulate the levels of histone H4 acetylation on the associated chromatins, thus repressing gene expression. In contrast, NF-YC-HDA15 complex is dismissed from the target genes in the dark, resulting in increased level of H4 acetylation and consequent etiolated growth. Further analyses revealed that transcriptional repression activity of NF-YCs on the light-controlled hypocotyl elongation partially depends on the deacetylation activity of HDA15, and loss of HDA15 function could rescue the short-hypocotyl phenotype of NF-YCs overexpression plants. Taken together, our results indicate that NF-YC1, NF-YC3, NF-YC4, and NF-YC9 function as transcriptional co-repressors by interacting with HDA15 to inhibit hypocotyl elongation in photomorphogenesis during the early seedling stage. Our findings highlight that NF-YCs can modulate plant development in response to environmental cues via epigenetic regulation.

Nitric Oxide Modulates Histone Acetylation at Stress Genes by Inhibition of Histone Deacetylases

Histone acetylation, which is an important mechanism to regulate gene expression, is controlled by the opposing action of histone acetyltransferases and histone deacetylases (HDACs). In animals, several HDACs are subjected to regulation by nitric oxide (NO); in plants, however, it is unknown whether NO affects histone acetylation. We found that treatment with the physiological NO donor S-nitrosoglutathione (GSNO) increased the abundance of several histone acetylation marks in Arabidopsis (Arabidopsis thaliana), which was strongly diminished in the presence of the NO scavenger 2-4-carboxyphenyl-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide. This increase was likely triggered by NO-dependent inhibition of HDAC activity, since GSNO and S-nitroso-N-acetyl-dl-penicillamine significantly and reversibly reduced total HDAC activity in vitro (in nuclear extracts) and in vivo (in protoplasts). Next, genome-wide H3K9/14ac profiles in Arabidopsis seedlings were generated by chromatin immunoprecipitation sequencing, and changes induced by GSNO, GSNO/2-4-carboxyphenyl-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide or trichostatin A (an HDAC inhibitor) were quantified, thereby identifying genes that display putative NO-regulated histone acetylation. Functional classification of these genes revealed that many of them are involved in the plant defense response and the abiotic stress response. Furthermore, salicylic acid, which is the major plant defense hormone against biotrophic pathogens, inhibited HDAC activity and increased histone acetylation by inducing endogenous NO production. These data suggest that NO affects histone acetylation by targeting and inhibiting HDAC complexes, resulting in the hyperacetylation of specific genes. This mechanism might operate in the plant stress response by facilitating the stress-induced transcription of genes.

Genome-Wide Identification of Histone Modifiers and Their Expression Patterns during Fruit Abscission in Litchi

Modifications to histones, including acetylation and methylation processes, play crucial roles in the regulation of gene expression in plant development as well as in stress responses. However, limited information on the enzymes catalyzing histone acetylation and methylation in non-model plants is currently available. In this study, several histone modifier (HM) types, including six histone acetyltransferases (HATs), 11 histone deacetylases (HDACs), 48 histone methyltransferases (HMTs), and 22 histone demethylases (HDMs), are identified in litchi (Litchi chinensis Sonn. cv. Feizixiao) based on similarities in their sequences to homologs in Arabidopsis (A. thaliana), tomato (Solanum lycopersicum), and rice (Oryza sativa). Phylogenetic analyses reveal that HM enzymes can be grouped into four HAT, two HDAC, two HMT, and two HDM subfamilies, respectively, while further expression profile analyses demonstrate that 17 HMs were significantly altered during fruit abscission in two field treatments. Analyses reveal that these genes exhibit four distinct patterns of expression in response to fruit abscission, while an in vitro assay was used to confirm the HDAC activity of LcHDA2, LcHDA6, and LcSRT2. Our findings are the first in-depth analysis of HMs in the litchi genome, and imply that some are likely to play important roles in fruit abscission in this commercially important plant.