Arabidopsis thaliana transcriptional co-activators ADA2b and SGF29a are implicated in salt stress responses

Histone deacetylase 9 interacts with SiHAT3.1 and SiHDA19 to repress dehydration responses through H3K9 deacetylation in foxtail millet

Climate change inflicts several stresses on plants, of which dehydration stress severely affects growth and productivity. C4 plants possess better adaptability to dehydration stress; however, the role of epigenetic modifications underlying this trait is unclear. In particular, the molecular links between histone modifiers and their regulation remain elusive. In this study, genome-wide H3K9 acetylation (H3K9ac) enrichment using ChIP-sequencing was performed in two foxtail millet cultivars with contrasting dehydration tolerances (IC403579, cv. IC4-tolerant, and IC480117, cv. IC41-sensitive). It revealed that a histone deacetylase, SiHDA9, was significantly up-regulated in the sensitive cultivar. Further characterization indicated that SiHDA9 interacts with SiHAT3.1 and SiHDA19 to form a repressor complex. SiHDA9 might be recruited through the SiHAT3.1 recognition sequence onto the upstream of dehydration-responsive genes to decrease H3K9 acetylation levels. The silencing of SiHDA9 resulted in the up-regulation of crucial genes, namely, SiRAB18, SiRAP2.4, SiP5CS2, SiRD22, SiPIP1;4, and SiLHCB2.3, which imparted dehydration tolerance in the sensitive cultivar (IC41). Overall, the study provides mechanistic insights into SiHDA9-mediated regulation of dehydration stress response in foxtail millet.

The SCL30a SR protein regulates ABA-dependent seed traits and germination under stress

SR proteins are conserved RNA-binding proteins best known as splicing regulators that have also been implicated in other steps of gene expression. Despite mounting evidence for a role in plant development and stress responses, the molecular pathways underlying SR protein regulation of these processes remain poorly understood. Here we show that the plant-specific SCL30a SR protein negatively regulates ABA signaling to control seed traits and stress responses during germination in Arabidopsis. Transcriptome-wide analyses revealed that loss of SCL30a function barely affects splicing, but largely induces ABA-responsive gene expression and genes repressed during germination. Accordingly, scl30a mutant seeds display delayed germination and hypersensitivity to ABA and high salinity, while transgenic plants overexpressing SCL30a exhibit reduced ABA and salt stress sensitivity. An ABA biosynthesis inhibitor rescues the enhanced mutant seed stress sensitivity, and epistatic analyses confirm that this hypersensitivity requires a functional ABA pathway. Finally, seed ABA levels are unchanged by altered SCL30a expression, indicating that the gene promotes seed germination under stress by reducing sensitivity to the phytohormone. Our results reveal a new player in ABA-mediated control of early development and stress response.

Role of Epigenetics in Modulating Phenotypic Plasticity against Abiotic Stresses in Plants

Plants being sessile are always exposed to various environmental stresses, and to overcome these stresses, modifications at the epigenetic level can prove vital for their long-term survival. Epigenomics refers to the large-scale study of epigenetic marks on the genome, which include covalent modifications of histone tails (acetylation, methylation, phosphorylation, ubiquitination, and the small RNA machinery). Studies based on epigenetics have evolved over the years especially in understanding the mechanisms at transcriptional and posttranscriptional levels in plants against various environmental stimuli. Epigenomic changes in plants through induced methylation of specific genes that lead to changes in their expression can help to overcome various stress conditions. Recent studies suggested that epigenomics has a significant potential for crop improvement in plants. By the induction and modulation of various cellular processes like DNA methylation, histone modification, and biogenesis of noncoding RNAs, the plant genome can be activated which can help in achieving a quicker response against various plant stresses. Epigenetic modifications in plants allow them to adjust under varied environmental stresses by modulating their phenotypic plasticity and at the same time ensure the quality and yield of crops. The plasticity of the epigenome helps to adapt the plants during pre- and postdevelopmental processes. The variation in DNA methylation in different organisms exhibits variable phenotypic responses. The epigenetic changes also occur sequentially in the genome. Various studies indicated that environmentally stimulated epimutations produce variable responses especially in differentially methylated regions (DMR) that play a major role in the management of stress conditions in plants. Besides, it has been observed that environmental stresses cause specific changes in the epigenome that are closely associated with phenotypic modifications. However, the relationship between epigenetic modifications and phenotypic plasticity is still debatable. In this review, we will be discussing the role of various factors that allow epigenetic changes to modulate phenotypic plasticity against various abiotic stress in plants.

Genome-wide investigation of histone acetyltransferase gene family and its responses to biotic and abiotic stress in foxtail millet (Setaria italica [L.] P. Beauv)

BACKGROUND

Modification of histone acetylation is a ubiquitous and reversible process in eukaryotes and prokaryotes and plays crucial roles in the regulation of gene expression during plant development and stress responses. Histone acetylation is co-regulated by histone acetyltransferase (HAT) and histone deacetylase (HDAC). HAT plays an essential regulatory role in various growth and development processes by modifying the chromatin structure through interactions with other histone modifications and transcription factors in eukaryotic cells, affecting the transcription of genes. Comprehensive analyses of HAT genes have been performed in Arabidopsis thaliana and Oryza sativa. However, little information is available on the HAT genes in foxtail millet (Setaria italica [L.] P. Beauv).

RESULTS

In this study, 24 HAT genes (SiHATs) were identified and divided into four groups with conserved gene structures via motif composition analysis. Phylogenetic analysis of the genes was performed to predict functional similarities between Arabidopsis thaliana, Oryza sativa, and foxtail millet; 19 and 2 orthologous gene pairs were individually identified. Moreover, all identified HAT gene pairs likely underwent purified selection based on their non-synonymous/synonymous nucleotide substitutions. Using published transcriptome data, we found that SiHAT genes were preferentially expressed in some tissues and organs. Stress responses were also examined, and data showed that SiHAT gene transcription was influenced by drought, salt, low nitrogen, and low phosphorus stress, and that the expression of four SiHATs was altered as a result of infection by Sclerospora graminicola.

CONCLUSIONS

Results indicated that histone acetylation may play an important role in plant growth and development and stress adaptations. These findings suggest that SiHATs play specific roles in the response to abiotic stress and viral infection. This study lays a foundation for further analysis of the biological functions of SiHATs in foxtail millet.

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.

Progressive Genomic Approaches to Explore Drought- and Salt-Induced Oxidative Stress Responses in Plants under Changing Climate

Drought and salinity are the major environmental abiotic stresses that negatively impact crop development and yield. To improve yields under abiotic stress conditions, drought- and salinity-tolerant crops are key to support world crop production and mitigate the demand of the growing world population. Nevertheless, plant responses to abiotic stresses are highly complex and controlled by networks of genetic and ecological factors that are the main targets of crop breeding programs. Several genomics strategies are employed to improve crop productivity under abiotic stress conditions, but traditional techniques are not sufficient to prevent stress-related losses in productivity. Within the last decade, modern genomics studies have advanced our capabilities of improving crop genetics, especially those traits relevant to abiotic stress management. This review provided updated and comprehensive knowledge concerning all possible combinations of advanced genomics tools and the gene regulatory network of reactive oxygen species homeostasis for the appropriate planning of future breeding programs, which will assist sustainable crop production under salinity and drought conditions.

Plants' Epigenetic Mechanisms and Abiotic Stress

Plants are sessile organisms that need to adapt to constantly changing environmental conditions. Unpredictable climate change places plants under a variety of abiotic stresses. Studying the regulation of stress-responsive genes can help to understand plants’ ability to adapt to fluctuating environmental conditions. Changes in epigenetic marks such as histone modifications and DNA methylation are known to regulate gene expression by their dynamic variation in response to stimuli. This can then affect their phenotypic plasticity, which helps with the adaptation of plants to adverse conditions. Epigenetic marks may also provide a mechanistic basis for stress memory, which enables plants to respond more effectively and efficiently to recurring stress and prepare offspring for potential future stresses. Studying epigenetic changes in addition to genetic factors is important to better understand the molecular mechanisms underlying plant stress responses. This review summarizes the epigenetic mechanisms behind plant responses to some main abiotic stresses.

Conservation and diversity of the eukaryotic SAGA coactivator complex across kingdoms

The SAGA complex is an evolutionarily conserved transcriptional coactivator that regulates gene expression through its histone acetyltransferase and deubiquitylase activities, recognition of specific histone modifications, and interactions with transcription factors. Multiple lines of evidence indicate the existence of distinct variants of SAGA among organisms as well as within a species, permitting diverse functions to dynamically regulate cellular pathways. Our co-expression analysis of genes encoding human SAGA components showed enrichment in reproductive organs, brain tissues and the skeletal muscle, which corresponds to their established roles in developmental programs, emerging roles in neurodegenerative diseases, and understudied functions in specific cell types. SAGA subunits modulate growth, development and response to various stresses from yeast to plants and metazoans. In metazoans, SAGA further participates in the regulation of differentiation and maturation of both innate and adaptive immune cells, and is associated with initiation and progression of diseases including a broad range of cancers. The evolutionary conservation of SAGA highlights its indispensable role in eukaryotic life, thus deciphering the mechanisms of action of SAGA is key to understanding fundamental biological processes throughout evolution. To illuminate the diversity and conservation of this essential complex, here we discuss variations in composition, essentiality and co-expression of component genes, and its prominent functions across Fungi, Plantae and Animalia kingdoms.

Salt stress and salt shock differently affect DNA methylation in salt-responsive genes in sugar beet and its wild, halophytic ancestor

Here we determined the impact of salt shock and salt stress on the level of DNA methylation in selected CpG islands localized in promoters or first exons of sixteen salt-responsive genes in beets. Two subspecies differing in salt tolerance were subjected for analysis, a moderately salt-tolerant sugar beet Beta vulgaris ssp. vulgaris cv. Huzar and a halophytic beet, Beta vulgaris ssp. maritima. The CpG island methylation status was determined. All target sequences were hyper- or hypomethylated under salt shock and/or salt stress in one or both beet subspecies. It was revealed that the genomic regions analyzed were highly methylated in both, the salt treated plants and untreated controls. Methylation of the target sequences changed in a salt-dependent manner, being affected by either one or both treatments. Under both shock and stress, the hypomethylation was a predominant response in sugar beet. In Beta vulgaris ssp. maritima, the hypermethylation occurred with higher frequency than hypomethylation, especially under salt stress and in the promoter-located CpG sites. Conversely, the hypomethylation of the promoter-located CpG sites predominated in sugar beet plants subjected to salt stress. This findings suggest that DNA methylation may be involved in salt-tolerance and transcriptomic response to salinity in beets.

Methylation-directed acetylation of histone H3 regulates developmental sensitivity to histone deacetylase inhibition

Hydroxamate-based lysine deacetylase inhibitors (KDACis) are approved for clinical use against certain cancers. However, intrinsic and acquired resistance presents a major problem. Treatment of cells with hydroxamates such as trichostatin A (TSA) leads to rapid preferential acetylation of histone H3 already trimethylated on lysine 4 (H3K4me3), although the importance of this H3K4me3-directed acetylation in the biological consequences of KDACi treatment is not known. We address this utilizing Dictyostelium discoideum strains lacking H3K4me3 due to disruption of the gene encoding the Set1 methyltransferase or mutations in endogenous H3 genes. Loss of H3K4me3 confers resistance to TSA-induced developmental inhibition and delays accumulation of H3K9Ac and H3K14Ac. H3K4me3-directed H3Ac is mediated by Sgf29, a subunit of the SAGA acetyltransferase complex that interacts with H3K4me3 via a tandem tudor domain (TTD). We identify an Sgf29 orthologue in Dictyostelium with a TTD that specifically recognizes the H3K4me3 modification. Disruption of the gene encoding Sgf29 delays accumulation of H3K9Ac and abrogates H3K4me3-directed H3Ac. Either loss or overexpression of Sgf29 confers developmental resistance to TSA. Our results demonstrate that rapid acetylation of H3K4me3 histones regulates developmental sensitivity to TSA. Levels of H3K4me3 or Sgf29 will provide useful biomarkers for sensitivity to this class of chemotherapeutic drug.

The Transcriptional Adaptor Protein ADA3a Modulates Flowering of Arabidopsis thaliana

Histone acetylation is directly related to gene expression. In yeast, the acetyltransferase general control nonderepressible-5 (GCN5) targets histone H3 and associates with transcriptional co-activators alteration/deficiency in activation-2 (ADA2) and alteration/deficiency in activation-3 (ADA3) in complexes like SAGA. Arabidopsis thaliana has two genes encoding proteins, designated ADA3a and ADA3b, that correspond to yeast ADA3. We investigated the role of ADA3a and ADA3b in regulating gene expression during flowering time. Specifically, we found that knock out mutants ada3a-2 and the double mutant ada3a-2 ada3b-2 lead to early flowering compared to the wild type plants under long day (LD) conditions and after moving plants from short days to LD. Consistent with ADA3a being a repressor of floral initiation, FLOWERING LOCUS T (FT) expression was increased in ada3a mutants. In contrast, other genes involved in multiple pathways leading to floral transition, including FT repressors, players in GA signaling, and members of the SPL transcriptional factors, displayed reduced expression. Chromatin immunoprecipitation analysis revealed that ADA3a affects the histone H3K14 acetylation levels in SPL3, SPL5, RGA, GAI, and SMZ loci. In conclusion, ADA3a is involved in floral induction through a GCN5-containing complex that acetylates histone H3 in the chromatin of flowering related genes.

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

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

The biochemical and genetic discovery of the SAGA complex

One of the major advances in our understanding of gene regulation in eukaryotes was the discovery of factors that regulate transcription by controlling chromatin structure. Prominent among these discoveries was the demonstration that Gcn5 is a histone acetyltransferase, establishing a direct connection between transcriptional activation and histone acetylation. This breakthrough was soon followed by the purification of a protein complex that contains Gcn5, the SAGA complex. In this article, we review the early genetic and biochemical experiments that led to the discovery of SAGA and the elucidation of its multiple activities.

Phylogenetic and expression analysis of histone acetyltransferases in Brachypodium distachyon

Histone acetylation is an important post-translational modification in eukaryotes and is regulated by two antagonistic enzymes, namely histone acetyltransferase (HAT) and histone deacetylase (HDAC). However, little has been done on the HAT superfamily in Brachypodium distachyon (B. distachyon), a new model plant of Poaceae. In this study, eight HATs were identified from B. distachyon and classified into four major families. Subcellular localization analysis showed that a majority of BdHATs were predominantly localized in the nucleus. Syntenic and phylogenetic analysis indicated there may be two common ancestral CREB-binding protein (p300/CBP, HAC) genes prior to the separation of monocots and dicots. Expression analysis revealed that the potential roles of BdHATs in B. distachyon development and responses to four abiotic stresses. Protein-protein network analysis identified some potential interactive genes with BdHATs. Thus, our results will provide solid basis for further study the function of HAT genes in B. distachyon and other monocot plants.

Redox Balance-DDR-miRNA Triangle: Relevance in Genome Stability and Stress Responses in Plants

Plants are continuously faced with complex environmental conditions which can affect the oxidative metabolism and photosynthetic efficiency, thus leading to the over-production of reactive oxygen species (ROS). Over a certain threshold, ROS can damage DNA. DNA damage, unless repaired, can affect genome stability, thus interfering with cell survival and severely reducing crop productivity. A complex network of pathways involved in DNA damage response (DDR) needs to be activated in order to maintain genome integrity. The expression of specific genes belonging to these pathways can be used as indicators of oxidative DNA damage and effective DNA repair in plants subjected to stress conditions. Managing ROS levels by modulating their production and scavenging systems shifts the role of these compounds from toxic molecules to key messengers involved in plant tolerance acquisition. Oxidative and anti-oxidative signals normally move among the different cell compartments, including the nucleus, cytosol, and organelles. Nuclei are dynamically equipped with different redox systems, such as glutathione (GSH), thiol reductases, and redox regulated transcription factors (TFs). The nuclear redox network participates in the regulation of the DNA metabolism, in terms of transcriptional events, replication, and repair mechanisms. This mainly occurs through redox-dependent regulatory mechanisms comprising redox buffering and post-translational modifications, such as the thiol-disulphide switch, glutathionylation, and S-nitrosylation. The regulatory role of microRNAs (miRNAs) is also emerging for the maintenance of genome stability and the modulation of antioxidative machinery under adverse environmental conditions. In fact, redox systems and DDR pathways can be controlled at a post-transcriptional level by miRNAs. This review reports on the interconnections between the DDR pathways and redox balancing systems. It presents a new dynamic picture by taking into account the shared regulatory mechanism mediated by miRNAs in plant defense responses to stress.

Synergistic action of GCN5 and CLAVATA1 in the regulation of gynoecium development in Arabidopsis thaliana

In Arabidopsis thaliana the CLAVATA1 (CLV1) receptor and GENERAL CONTROL NON DEREPRESSIBLE 5 (GCN5) histone acetyltransferase both regulate inflorescence meristem size and affect the expression of the meristem-promoting transcription factor WUSCHEL (WUS). Single and multiple mutants of GCN5 and CLAVATA members, were analysed for their gynoecium development, using morphological, physiological, genetic and molecular approaches. The clv1-1gcn5-1 double mutants exhibited novel phenotypes including elongated gynoecia with reduced valves and enlarged stigma and style, indicating a synergistic action of CLAVATA signaling and GCN5 action in the development of the gynoecium. Reporter line and gene expression analysis showed that clv1-1gcn5-1 plants have altered auxin and cytokinin response, distribution and ectopic overexpression of WUS. WUS expression was found in the style of wild-type gynoecia stage 10-13, suggesting a possible novel role for WUS in the development of the style. CLV1 and GCN5 are regulators of apical-basal and mediolateral polarity of the Arabidopsis gynoecium. They affect gynoecium morphogenesis through the negative regulation of auxin biosynthesis and promotion of polar auxin transport. They also promote cytokinin signaling in the carpel margin meristem and negatively regulate it at the stigma. Finally, they synergistically suppress WUS at the centre of the gynoecium.

The Adaptor Protein ENY2 Is a Component of the Deubiquitination Module of the Arabidopsis SAGA Transcriptional Co-activator Complex but not of the TREX-2 Complex

The conserved nuclear protein ENY2 (Sus1 in yeast) is involved in coupling transcription and mRNA export in yeast and metazoa, as it is a component both of the transcriptional co-activator complex SAGA and of the mRNA export complex TREX-2. Arabidopsis thaliana ENY2 is widely expressed in the plant and it localizes to the nucleoplasm, but unlike its yeast/metazoan orthologs, it is not enriched in the nuclear envelope. Affinity purification of ENY2 in combination with mass spectrometry revealed that it co-purified with SAGA components, but not with the nuclear pore-associated TREX-2. In addition, further targeted proteomics analyses by reciprocal tagging established the composition of the Arabidopsis SAGA complex consisting of the four modules HATm, SPTm, TAFm and DUBm, and that several SAGA subunits occur in alternative variants. While the HATm, SPTm and TAFm robustly co-purified with each other, the deubiquitination module (DUBm) appears to associate with the other SAGA modules more weakly/dynamically. Consistent with a homology model of the Arabidopsis DUBm, the SGF11 protein interacts directly with ENY2 and UBP22. Plants depleted in the DUBm components, SGF11 or ENY2, are phenotypically only mildly affected, but they contain increased levels of ubiquitinated histone H2B, indicating that the SAGA-DUBm has histone deubiquitination activity in plants. In addition to transcription-related proteins (i.e., transcript elongation factors, Mediator), many splicing factors were found to associate with SAGA, linking the SAGA complex and ongoing transcription with mRNA processing.

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.

Comparative analysis of Histone modifications and DNA methylation at OsBZ8 locus under salinity stress in IR64 and Nonabokra rice varieties

Rice being an important cereal crop is highly sensitive to salinity stress causing growth retardation and loss in productivity. However, certain rice genotypes like Nonabokra and Pokkali show a high level of tolerance towards salinity stress compared to IR64 variety. This differential response of tolerant varieties towards salinity stress may be a cumulative effect of genetic and epigenetic factors. In this study, we have compared the salinity-induced changes in chromatin modifications at the OsBZ8 locus in salt-tolerant Nonabokra and salt-sensitive IR64 rice varieties. Expression analysis indicates that the OsBZ8 gene is highly induced in Nonabokra plants even in the absence of salt stress, whereas in IR64, the expression significantly increases only during salt stress. Sequence analysis and nucleosomal arrangement within the region -2000 to +1000 of OsBZ8 gene show no difference between the two rice varieties. However, there was a considerable difference in histone modifications and DNA methylation at the locus between these varieties. In Nonabokra, the upstream region was hyperacetylated at H3K9 and H3K27, and this acetylation did not change during salt stress. However, in IR64, histone acetylation was observed only during salt stress. Moreover, the upstream region of OsBZ8 gene has highly dynamic nucleosome arrangement in Nonabokra, compared to IR64. Furthermore, loss of DNA methylation was observed at OsBZ8 locus in Nonabokra control plants along with low H3K27me3 and high H3K4me3. Control IR64 plants show high DNA methylation and enriched H3K27me3. Collectively these results indicate a significant difference in chromatin modifications between the rice varieties that regulates differential expression of OsBZ8 gene during salt stress.

Epigenetics and RNA Processing: Connections to Drought, Salt, and ABA?

There have been great research advances in epigenetics, RNA splicing, and mRNA processing over recent years. In parallel, there have been many advances in abiotic stress and Abscisic Acid (ABA) signaling. Here we overview studies that have examined stress-induced changes in the epigenome and RNA processing as well as cases where disrupting these processes changes the plant response to abiotic stress. We also highlight some examples where specific connections of stress or ABA signaling to epigenetics or RNA processing have been found. By implication, this also points out cases where such mechanistic connections are likely to exist but are yet to be characterized. In the absence of such specific connections to stress signaling, it should be kept in mind that stress sensitivity phenotypes of some epigenetic or RNA processing mutants maybe the result of indirect, pleiotropic effects and thus may perhaps not indicate a direct function in stress acclimation.

Chemical inhibition of the histone acetyltransferase activity in Arabidopsis thaliana

Chemical inhibition of chromatin regulators provides an effective approach to investigate the roles of chromatin modifications in plant and animals. In this work, chemical inhibition of the Arabidopsis histone acetyltransferase activity by γ-butyrolactone (MB-3), the inhibitor of the catalytic activity of mammalian GENERAL CONTROL NON-REPRESSIBLE 5 (GCN5) is evaluated. Arabidopsis seedlings were germinated in LS medium supplemented with different concentrations of MB-3, and inhibition in the root length and yellowed leaves were observed. The yellowed leaves phenotype of the plants grown in 100 μM of MB-3 was reverted when plants were additionally treated with 1 μM of TSA, a histone deacetylase inhibitor. Using an immunoblot assay with specific antibodies revealed a reduction of H3K14 acetylation levels at 3 and 24 h post-treatment. At 24 h post-treatment a reduction of H3K9 acetylation levels was observed. Targets of GCN5 related to stress were downregulated at 3 h post-treatment but no change was observed in target genes related to developmental transition. Our results indicate that MB-3 is a chemical inhibitor of the histone acetyltransferase in Arabidopsis and suggest that this inhibitor could function in other plants species.

Chromatin-Based Epigenetic Regulation of Plant Abiotic Stress Response

Plants are continuously exposed to various abiotic and biotic factors limiting their growth and reproduction. In response, they need various sophisticated ways to adapt to adverse environmental conditions without compromising their proper development, reproductive success and eventually survival. This requires an intricate network to regulate gene expression at transcriptional and post-transcriptional levels, including epigenetic switches. Changes in chromatin modifications such as DNA and histone methylation have been observed in plants upon exposure to several abiotic stresses. In the present review, we highlight the changes of DNA methylation in diverse plants in response to several abiotic stresses such as salinity, drought, cold and heat. We also discuss the progresses made in understanding how these DNA methylation changes might contribute to the abiotic stress tolerance.

Histone Acetylation and Plant Development

Reversible histone acetylation and deacetylation at the N-terminus of histone tails play a crucial role in regulation of gene activity. Hyperacetylation of histones relaxes chromatin structure and is associated with transcriptional activation, whereas hypoacetylation of histones induces chromatin compaction and gene repression. Histone acetylation and deacetylation are catalyzed by histone acetyltransferases (HATs) and histone deacetylases (HDACs), respectively. Emerging evidences revealed that plant HATs and HDACs play essential roles in regulation of gene expression in plant development and plant responses to environmental stresses. Furthermore, HATs and HDACs were shown to interact with various chromatin-remodeling factors and transcription factors involved in transcriptional regulation of multiple developmental processes.

Plant responses to abiotic stress: The chromatin context of transcriptional regulation

The ability of plants to cope with abiotic environmental stresses such as drought, salinity, heat, cold or flooding relies on flexible mechanisms for re-programming gene expression. Over recent years it has become apparent that transcriptional regulation needs to be understood within its structural context. Chromatin, the assembly of DNA with histone proteins, generates a local higher-order structure that impacts on the accessibility and effectiveness of the transcriptional machinery, as well as providing a hub for multiple protein interactions. Several studies have shown that chromatin features such as histone variants and post-translational histone modifications are altered by environmental stress, and they could therefore be primary stress targets that initiate transcriptional stress responses. Alternatively, they could act downstream of stress-induced transcription factors as an integral part of transcriptional activity. A few experimental studies have addressed this 'chicken-and-egg' problem in plants and other systems, but to date the causal relationship between dynamic chromatin changes and transcriptional responses under stress is still unclear. In this review we have collated the existing information on concurrent epigenetic and transcriptional responses of plants to abiotic stress, and we have assessed the evidence using a simple theoretical framework of causality scenarios. This article is part of a Special Issue entitled: Plant Gene Regulatory Mechanisms and Networks, edited by Dr. Erich Grotewold and Dr. Nathan Springer.

High light intensity plays a major role in emergence of population level variation in Arabidopsis thaliana along an altitudinal gradient

Environmental conditions play an important role in the emergence of genetic variations in natural populations. We identified genome-wide patterns of nucleotide variations in the coding regions of natural Arabidopsis thaliana populations. These populations originated from 700 m to 3400 m a.m.s.l. in the Western Himalaya. Using a pooled RNA-Seq approach, we identified the local and global level population-specific SNPs. The biological functions of the SNP-containing genes were primarily related to the high light intensity prevalent at high-altitude regions. The novel SNPs identified in these genes might have arisen de novo in these populations. In another approach, the F_STs of SNP-containing genes were correlated with the corresponding climatic factors. ‘Radiation in the growing season’ was the only environmental factor found to be strongly correlated with the gene-level F_STs. In both the approaches, the high light intensity was identified as the primary abiotic stress associated with the variations in these populations. The differential gene expression analysis between field and controlled condition grown plants also showed high light intensity as the primary abiotic stress, particularly for the high altitude populations. Our results provide a genome-wide perspective of nucleotide variations in populations along altitudinal gradient and their putative role in emergence of these variations.

Histone deacetylase HDA9 negatively regulates salt and drought stress responsiveness in Arabidopsis

Histone modification is an important epigenetic regulation in higher plants adapting to environment changes including salt and drought stresses. In this report, we show that the Arabidopsis RPD3-type histone deacetylase HDA9 is involved in modulating plant responses to salt and drought stresses in Arabidopsis. Loss-of-function mutants of the gene displayed phenotypes (such as seedling root growth and seed germination) insensitive to NaCl and polyethylene glycol (PEG) treatments. HDA9 mutation led to up-regulation of many genes, among which those involved in response to water deprivation stress (GO: 0009414) were enriched. These genes were much more induced in the mutants than wild-type plants when treated with PEG and NaCl. In addition, we found that in the mutants, salt and drought stresses led to much higher levels of histone H3K9 acetylation at promoters of 14 genes randomly selected from those that respond to water-deprivation stress than in wild-type plants. Our study suggested that HDA9 might be a novel chromatin protein that negatively regulates plant sensitivity to salt and drought stresses by regulating histone acetylation levels of a large number of stress-responsive genes in Arabidopsis.

Synergistic action of histone acetyltransferase GCN5 and receptor CLAVATA1 negatively affects ethylene responses in Arabidopsis thaliana

GENERAL CONTROL NON-REPRESSIBLE 5 (GCN5) is a histone acetyltransferase (HAT) and the catalytic subunit of several multicomponent HAT complexes that acetylate lysine residues of histone H3. Mutants in AtGCN5 display pleiotropic developmental defects including aberrant meristem function. Shoot apical meristem (SAM) maintenance is regulated by CLAVATA1 (CLV1), a receptor kinase that controls the size of the shoot and floral meristems. Upon activation through CLV3 binding, CLV1 signals to the transcription factor WUSCHEL (WUS), restricting WUS expression and thus the meristem size. We hypothesized that GCN5 and CLV1 act together to affect SAM function. Using genetic and molecular approaches, we generated and characterized clv gcn5 mutants. Surprisingly, the clv1-1 gcn5-1 double mutant exhibited constitutive ethylene responses, suggesting that GCN5 and CLV signaling act synergistically to inhibit ethylene responses in Arabidopsis. This genetic and molecular interaction was mediated by ETHYLENE INSENSITIVE 3/ EIN3-LIKE1 (EIN3/EIL1) transcription factors. Our data suggest that signals from the CLV transduction pathway reach the GCN5-containing complexes in the nucleus and alter the histone acetylation status of ethylene-responsive genes, thus translating the CLV information to transcriptional activity and uncovering a link between histone acetylation and SAM maintenance in the complex mode of ethylene signaling.

The Histone Deacetylase Inhibitor Suberoylanilide Hydroxamic Acid Alleviates Salinity Stress in Cassava

Cassava (Manihot esculenta Crantz) demand has been rising because of its various applications. High salinity stress is a major environmental factor that interferes with normal plant growth and limits crop productivity. As well as genetic engineering to enhance stress tolerance, the use of small molecules is considered as an alternative methodology to modify plants with desired traits. The effectiveness of histone deacetylase (HDAC) inhibitors for increasing tolerance to salinity stress has recently been reported. Here we use the HDAC inhibitor, suberoylanilide hydroxamic acid (SAHA), to enhance tolerance to high salinity in cassava. Immunoblotting analysis reveals that SAHA treatment induces strong hyper-acetylation of histones H3 and H4 in roots, suggesting that SAHA functions as the HDAC inhibitor in cassava. Consistent with increased tolerance to salt stress under SAHA treatment, reduced Na⁺ content and increased K⁺/Na⁺ ratio were detected in SAHA-treated plants. Transcriptome analysis to discover mechanisms underlying salinity stress tolerance mediated through SAHA treatment reveals that SAHA enhances the expression of 421 genes in roots under normal condition, and 745 genes at 2 h and 268 genes at 24 h under both SAHA and NaCl treatment. The mRNA expression of genes, involved in phytohormone [abscisic acid (ABA), jasmonic acid (JA), ethylene, and gibberellin] biosynthesis pathways, is up-regulated after high salinity treatment in SAHA-pretreated roots. Among them, an allene oxide cyclase (MeAOC4) involved in a crucial step of JA biosynthesis is strongly up-regulated by SAHA treatment under salinity stress conditions, implying that JA pathway might contribute to increasing salinity tolerance by SAHA treatment. Our results suggest that epigenetic manipulation might enhance tolerance to high salinity stress in cassava.

The role of promoter cis-element, mRNA capping, and ROS in the repression and salt-inducible expression of AtSOT12 in Arabidopsis

Inducible gene expression is a gene regulatory mechanism central to plant response to environmental cues. The inducible genes are often repressed under normal growth conditions while their expression levels are significantly elevated by conditions such as abiotic stresses. Induction of gene expression requires both cis-acting DNA elements and trans-acting proteins that are modulated through signal transduction pathways. Here we report several molecular events that affect salt induced expression of the Arabidopsis AtSOT12 gene. Promoter deletion analysis revealed that DNA elements residing in the 5' UTR are required for the salt induced expression of AtSOT12. Cytosine methylation in the promoter was low and salt stress slightly increased the DNA methylation level, suggesting that DNA methylation may not contribute to AtSOT12 gene repression. Co-transcriptional processing of AtSOT12 mRNA including capping and polyadenylation site selection was also affected by salt stress. The percentage of capped mRNA increased by salt treatment, and the polyadenylation sites were significantly different before and after exposure to salt stress. The expression level of AtSOT12 under normal growth conditions was markedly higher in the oxi1 mutant defective of reactive oxygen species (ROS) signaling than in the wild type. Moreover, AtSOT12 transcript level was elevated by treatments with DPI and DMTU, two chemicals preventing ROS accumulation. These results suggest that repression of AtSOT12 expression may require physiological level of ROS and ROS signaling.

Spt-Ada-Gcn5-Acetyltransferase (SAGA) Complex in Plants: Genome Wide Identification, Evolutionary Conservation and Functional Determination

The recruitment of RNA polymerase II on a promoter is assisted by the assembly of basal transcriptional machinery in eukaryotes. The Spt-Ada-Gcn5-Acetyltransferase (SAGA) complex plays an important role in transcription regulation in eukaryotes. However, even in the advent of genome sequencing of various plants, SAGA complex has been poorly defined for their components and roles in plant development and physiological functions. Computational analysis of Arabidopsis thaliana and Oryza sativa genomes for SAGA complex resulted in the identification of 17 to 18 potential candidates for SAGA subunits. We have further classified the SAGA complex based on the conserved domains. Phylogenetic analysis revealed that the SAGA complex proteins are evolutionary conserved between plants, yeast and mammals. Functional annotation showed that they participate not only in chromatin remodeling and gene regulation, but also in different biological processes, which could be indirect and possibly mediated via the regulation of gene expression. The in silico expression analysis of the SAGA components in Arabidopsis and O. sativa clearly indicates that its components have a distinct expression profile at different developmental stages. The co-expression analysis of the SAGA components suggests that many of these subunits co-express at different developmental stages, during hormonal interaction and in response to stress conditions. Quantitative real-time PCR analysis of SAGA component genes further confirmed their expression in different plant tissues and stresses. The expression of representative salt, heat and light inducible genes were affected in mutant lines of SAGA subunits in Arabidopsis. Altogether, the present study reveals expedient evidences of involvement of the SAGA complex in plant gene regulation and stress responses.

Stress-induced chromatin changes in plants: of memories, metabolites and crop improvement

Exposure of plants to adverse environmental conditions leads to extensive transcriptional changes. Genome-wide approaches and gene function studies have revealed the importance of chromatin-level control in the regulation of stress-responsive gene expression. Advances in understanding chromatin modifications implicated in plant stress response and identifying proteins involved in chromatin-mediated transcriptional responses to stress are briefly presented in this review. We then highlight how chromatin-mediated gene expression changes can be coupled to the metabolic status of the cell, since many of the chromatin-modifying proteins involved in transcriptional regulation depend on cofactors and metabolites that are shared with enzymes in basic metabolism. Lastly, we discuss the stability and heritability of stress-induced chromatin changes and the potential of chromatin-based strategies for increasing stress tolerance of crops.

Chromatin changes in response to drought, salinity, heat, and cold stresses in plants

Chromatin regulation is essential to regulate genes and genome activities. In plants, the alteration of histone modification and DNA methylation are coordinated with changes in the expression of stress-responsive genes to adapt to environmental changes. Several chromatin regulators have been shown to be involved in the regulation of stress-responsive gene networks under abiotic stress conditions. Specific histone modification sites and the histone modifiers that regulate key stress-responsive genes have been identified by genetic and biochemical approaches, revealing the importance of chromatin regulation in plant stress responses. Recent studies have also suggested that histone modification plays an important role in plant stress memory. In this review, we summarize recent progress on the regulation and alteration of histone modification (acetylation, methylation, phosphorylation, and SUMOylation) in response to the abiotic stresses, drought, high-salinity, heat, and cold in plants.

Composition of the SAGA complex in plants and its role in controlling gene expression in response to abiotic stresses

Protein complexes involved in epigenetic regulation of transcription have evolved as molecular strategies to face environmental stress in plants. SAGA (Spt-Ada-Gcn5 Acetyltransferase) is a transcriptional co-activator complex that regulates numerous cellular processes through the coordination of multiple post-translational histone modifications, including acetylation, deubiquitination, and chromatin recognition. The diverse functions of the SAGA complex involve distinct modules that are highly conserved between yeast, flies, and mammals. In this review, the composition of the SAGA complex in plants is described and its role in gene expression regulation under stress conditions summarized. Some of these proteins are likely involved in the regulation of the inducible expression of genes under light, cold, drought, salt, and iron stress, although the functions of several of its components remain unknown.

Involvement of histone modifications in plant abiotic stress responses

As sessile organisms, plants encounter various environmental stimuli including abiotic stresses during their lifecycle. To survive under adverse conditions, plants have evolved intricate mechanisms to perceive external signals and respond accordingly. Responses to various stresses largely depend on the plant capacity to modulate the transcriptome rapidly and specifically. A number of studies have shown that the molecular mechanisms driving the responses of plants to environmental stresses often depend on nucleosome histone post-translational modifications including histone acetylation, methylation, ubiquitination, and phosphorylation. The combined effects of these modifications play an essential role in the regulation of stress responsive gene expression. In this review, we highlight our current understanding of the epigenetic mechanisms of histone modifications and their roles in plant abiotic stress response.

The Arabidopsis ortholog of the YEATS domain containing protein YAF9a regulates flowering by controlling H4 acetylation levels at the FLC locus

Histone acetylation and complexes associated with this process are directly involved in chromatin regulation and gene expression. Among these, NuA4 complex is directly involved in acetylation of histone H4, H2A and H2A.Z. In yeast, the NuA4 complex contains the catalytic subunit, the histone acetyltransferase ESA1, and several associated components including YAF9. In this report we explored the biological role of YAF9a in Arabidopsis thaliana. Homozygous yaf9a-1 and yaf9a-3 mutants show early flowering phenotypes. Moreover, yaf9a-1 mutants displayed reduced expression of the flowering repressor FLC, whereas the expression of the flowering activators FT and SOC1 was induced in comparison to wild-type plants. Using chromatin immunoprecipitation assays with H4 tetra-acetylated antibodies we observed a positive correlation with gene expression profile of FLC and FT in yaf9a-1 mutants under long days. We therefore conclude that YAF9a in Arabidopsis is a negative regulator of flowering by controlling the H4 acetylation levels in the FLC and FT chromatin.

Sgf29p facilitates the recruitment of TATA box binding protein but does not alter SAGA's global structural integrity in vivo

Although Sgf29p has been biochemically implicated as a component of SAGA (Spt-Ada-Gcn5 acetyltransferase), its precise mechanism of action in transcription is not clearly understood in vivo. Here, using a formaldehyde-based in vivo cross-linking and chromatin immunoprecipitation (ChIP) assay in conjunction with transcriptional and mutational analyses, we show that Sgf29p along with other SAGA components is recruited to the upstream activating sequence (UAS) of a SAGA-regulated gene, GAL1, in an activation domain-dependent manner. However, Sgf29p does not alter the recruitment of Spt20p that maintains the overall structural and functional integrity of SAGA. The recruitment of other SAGA components such as TAF10p, TAF12p, and Ubp8p to the GAL1 UAS is also not altered in the absence of Sgf29p. Interestingly, we find that the recruitment of TBP (TATA box binding protein that nucleates the assembly of general transcription factors to form the preinitiation complex for transcriptional initiation) to the core promoter of GAL1 is weakened in Δsgf29. Likewise, Sgf29p also enhances the recruitment of TBP to other SAGA-regulated promoters. Such weakening of recruitment of TBP to these promoters subsequently decreases the level of transcription. Taken together, these results support the idea that SAGA-associated Sgf29p facilitates the recruitment of TBP (and hence transcription) without altering the global structural integrity of SAGA in vivo.

The role of transcriptional coactivator ADA2b in Arabidopsis abiotic stress responses

Plant growth and crop production can be greatly affected by common environmental stresses such as drought, high salinity and low temperatures. Gene expression is affected by several abiotic stresses. Stress-inducible genes are regulated by transcription factors and epigenetic mechanisms such as histone modifications. In this Mini-Review, we have explored the role of transcriptional adaptor ADA2b in Arabidopsis responses to abiotic stress. ADA2b is required for the expression of genes involved in abiotic stress either by controlling H3 and H4 acetylation in the case of salt stress or affecting nucleosome occupancy in low temperatures response.

The role of epigenetic processes in controlling flowering time in plants exposed to stress

Plants interact with their environment by modifying gene expression patterns. One mechanism for this interaction involves epigenetic modifications that affect a number of aspects of plant growth and development. Thus, the epigenome is highly dynamic in response to environmental cues and developmental changes. Flowering is controlled by a set of genes that are affected by environmental conditions through an alteration in their expression pattern. This ensures the production of flowers even when plants are growing under adverse conditions, and thereby enhances transgenerational seed production. In this review recent findings on the epigenetic changes associated with flowering in Arabidopsis thaliana grown under abiotic stress conditions such as cold, drought, and high salinity are discussed. These epigenetic modifications include DNA methylation, histone modifications, and the production of micro RNAs (miRNAs) that mediate epigenetic modifications. The roles played by the phytohormones abscisic acid (ABA) and auxin in chromatin remodelling are also discussed. It is shown that there is a crucial relationship between the epigenetic modifications associated with floral initiation and development and modifications associated with stress tolerance. This relationship is demonstrated by the common epigenetic pathways through which plants control both flowering and stress tolerance, and can be used to identify new epigenomic players.

Chromatin modifications and remodeling in plant abiotic stress responses

Sensing environmental changes and initiating a gene expression response are important for plants as sessile autotrophs. The ability of epigenetic status to alter rapidly and reversibly could be a key component to the flexibility of plant responses to the environment. The involvement of epigenetic mechanisms in the response to environmental cues and to different types of abiotic stresses has been documented. Different environmental stresses lead to altered methylation status of DNA as well as modifications of nucleosomal histones. Understanding how epigenetic mechanisms are involved in plant response to environmental stress is highly desirable, not just for a better understanding of molecular mechanisms of plant stress response but also for possible application in the genetic manipulation of plants. In this review, we highlight our current understanding of the epigenetic mechanisms of chromatin modifications and remodeling, with emphasis on the roles of specific modification enzymes and remodeling factors in plant abiotic stress responses. This article is part of a Special Issue entitled: Plant gene regulation in response to abiotic stress.