Repression of flowering under a noninductive photoperiod by the HDA9-AGL19-FT module in Arabidopsis

Genomic analyses reveal the stepwise domestication and genetic mechanism of curd biogenesis in cauliflower

Cauliflower (Brassica oleracea L. var. botrytis) is a distinctive vegetable that supplies a nutrient-rich edible inflorescence meristem for the human diet. However, the genomic bases of its selective breeding have not been studied extensively. Herein, we present a high-quality reference genome assembly C-8 (V2) and a comprehensive genomic variation map consisting of 971 diverse accessions of cauliflower and its relatives. Genomic selection analysis and deep-mined divergences were used to explore a stepwise domestication process for cauliflower that initially evolved from broccoli (Curd-emergence and Curd-improvement), revealing that three MADS-box genes, CAULIFLOWER1 (CAL1), CAL2 and FRUITFULL (FUL2), could have essential roles during curd formation. Genome-wide association studies identified nine loci significantly associated with morphological and biological characters and demonstrated that a zinc-finger protein (BOB06G135460) positively regulates stem height in cauliflower. This study offers valuable genomic resources for better understanding the genetic bases of curd biogenesis and florescent development in crops.

Identification of candidate genes associated with peanut pod length by combined analysis of QTL-seq and RNA-seq

Pod length (PL) is one of the major traits determining pod size and yield of peanut. Discovering the quantitative trait loci (QTL) and identifying candidate genes associated with PL are essential for breeding high-yield peanut. In this study, quantitative trait loci sequencing (QTL-seq) was performed using the F2 population constructed by a short-pod variety Tifrunner (Tif) and a long-pod line Lps, and a 0.77 Mb genomic region on chromosome 07 was identified as the candidate region for PL. Then, the candidate region was narrowed to a 265.93 kb region by traditional QTL approach. RNA-seq analysis showed that there were four differentially expressed genes (DEGs) in the candidate region, among which Arahy.PF2L6F (AhCDC48) and Arahy.P4LK2T (AhTAA1) were speculated to be PL-related candidate genes. These results were informative for the elucidation of the underlying regulatory mechanism in peanut pod length and would facilitate further identification of valuable target genes.

Exploring the molecular regulation of vernalization-induced flowering synchrony in Arabidopsis

Many plant populations exhibit synchronous flowering, which can be advantageous in plant reproduction. However, molecular mechanisms underlying flowering synchrony remain poorly understood. We studied the role of known vernalization-response and flower-promoting pathways in facilitating synchronized flowering in Arabidopsis thaliana. Using the vernalization-responsive Col-FRI genotype, we experimentally varied germination dates and daylength among individuals to test flowering synchrony in field and controlled environments. We assessed the activity of flowering regulation pathways by measuring gene expression across leaves produced at different time points during development and through a mutant analysis. We observed flowering synchrony across germination cohorts in both environments and discovered a previously unknown process where flower-promoting and repressing signals are differentially regulated between leaves that developed under different environmental conditions. We hypothesized this mechanism may underlie synchronization. However, our experiments demonstrated that signals originating from sources other than leaves must also play a pivotal role in synchronizing flowering time, especially in germination cohorts with prolonged growth before vernalization. Our results suggest flowering synchrony is promoted by a plant-wide integration of flowering signals across leaves and among organs. To summarize our findings, we propose a new conceptual model of vernalization-induced flowering synchrony and provide suggestions for future research in this field.

HISTONE DEACETYLASE19 Controls Ovule Number Determination and Transmitting Tract Differentiation

The gynoecium is critical for the reproduction of flowering plants as it contains the ovules and the tissues that foster pollen germination, growth, and guidance. These tissues, known as the reproductive tract (ReT), comprise the stigma, style, and transmitting tract (TT). The ReT and ovules originate from the carpel margin meristem (CMM) within the pistil. SHOOT MERISTEMLESS (STM) is a key transcription factor for meristem formation and maintenance. In all above-ground meristems, including the CMM, local STM downregulation is required for organ formation. However, how this downregulation is achieved in the CMM is unknown. Here, we have studied the role of HISTONE DEACETYLASE 19 (HDA19) in Arabidopsis (Arabidopsis thaliana) during ovule and ReT differentiation based on the observation that the hda19-3 mutant displays a reduced ovule number and fails to differentiate the TT properly. Fluorescence-activated cell sorting coupled with RNA-sequencing revealed that in the CMM of hda19-3 mutants, genes promoting organ development are downregulated while meristematic markers, including STM, are upregulated. HDA19 was essential to downregulate STM in the CMM, thereby allowing ovule formation and TT differentiation. STM is ectopically expressed in hda19-3 at intermediate stages of pistil development, and its downregulation by RNA interference alleviated the hda19-3 phenotype. Chromatin immunoprecipitation assays indicated that STM is a direct target of HDA19 during pistil development and that the transcription factor SEEDSTICK is also required to regulate STM via histone acetylation. Thus, we identified factors required for the downregulation of STM in the CMM, which is necessary for organogenesis and tissue differentiation.

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.

Natural variation in the promoter of FLOWERING LOCUS T-LIKE 2 in pumpkin (Cucurbita moschata Duch.) is associated with flowering time under short-day conditions

Late flowering is a serious bottleneck in pumpkin (Cucurbita moschata Duch.) agriculture production. Although key genes governing flowering time have been reported in many species, the regulatory network of flowering in pumpkin remains largely obscure, thereby impeding the resolution of industry-wide challenges associated with delayed fruit ripening in pumpkin cultivation. Here, we report an early flowering pumpkin germplasm accession (LXX-4). Using LXX-4 and a late flowering germplasm accession (HYM-9), we constructed an F2 segregation population. A significant difference in FLOWERING LOCUS T-LIKE 2 (FTL2) expression level was identified to be the causal factor of the flowering time trait discrepancy in LXX-4 and HYM-9. Moreover, we have shown that a 21 bp InDel in the FTL2 promoter was the key reason for the waxing and waning of its transcript level. The 21 bp deletion excluded a repressor-AGL19 and recruited activators-BBX7, WRKY40 and SVP to the FTL2 promoter in LXX-4. Together, our data add a useful element to our knowledge which could be used to simplify breeding efforts for early-maturing pumpkin.

Epigenetic regulation of photoperiodic flowering in plants

In response to changeable season, plants precisely control the initiation of flowering in appropriate time of the year to ensure reproductive success. Day length (photoperiod) acts as the most important external cue to determine flowering time. Epigenetics regulates many major developmental stages in plant life, and emerging molecular genetics and genomics researches reveal their essential roles in floral transition. Here, we summarize the recent advances in epigenetic regulation of photoperiod-mediated flowering in Arabidopsis and rice, and discuss the potential of epigenetic regulation in crops improvement, and give the brief prospect for future study trends.

The Role of Histone Modifications in Heat Signal Transduction in Plants

Global warming and the more frequent occurrence of extremly high temperatures seriously affect crop yields. Heat stress (HS) has become a major environmental factor threatening food security worldwide. Understanding how plants sense and respond to HS is of clear interest to plant scientists and crop breeders. However, it is not trivial to elucidate the underlying signaling cascade, as specific cellular responses (ranging from detrimental to systemic effects) must be disentangled. Plants respond and adapt to high temperatures in many ways. In this review, recent progress in understanding heat signal transduction and the role of histone modifications in regulating the expression of genes involved in HS responses are discussed. The outstanding issues that are crucial for understanding the interactions between plants and HS are also discussed. The study of heat signal transduction mechanisms in plants is essential to facilitate the cultivation of heat-resistant crop varieties.

Phosphorylation mediated regulation of RNA splicing in plants

For the past two decades, the study of alternative splicing (AS) and its involvement in plant development and stress response has grown in popularity. Only recently however, has the focus shifted to the study of how AS regulation (or lack-thereof) affects downstream mRNA and protein landscapes and how these AS regulatory events impact plant development and stress tolerance. In humans, protein phosphorylation represents one of the predominant mechanisms by which AS is regulated and thus the protein kinases governing these phosphorylation events are of interest for further study. Large-scale phosphoproteomic studies in plants have consistently found that RNA splicing-related proteins are extensively phosphorylated, however, the signaling pathways involved in AS regulation have not been resolved. In this mini-review, we summarize our current knowledge of the three major splicing-related protein kinase families in plants that are suggested to mediate AS phospho-regulation and draw comparisons to their metazoan orthologs. We also summarize and contextualize the phosphorylation events identified as occurring on splicing-related protein families to illustrate the high degree to which splicing-related proteins are modified, placing a new focus on elucidating the impacts of AS at the protein and PTM-level.

TM3 and STM3 Promote Flowering Together with FUL2 and MBP20, but Act Antagonistically in Inflorescence Branching in Tomato

The moment at which a plant transitions to reproductive development is paramount to its life cycle and is strictly controlled by many genes. The transcription factor SUPPRESSOR OF OVEREXPRESSION OF CONSTANS 1 (SOC1) plays a central role in this process in Arabidopsis. However, the role of SOC1 in tomato (Solanum lycopersicum) has been sparsely studied. Here, we investigated the function of four tomato SOC1 homologs in the floral transition and inflorescence development. We thoroughly characterized the SOC1-like clade throughout the Solanaceae and selected four tomato homologs that are dynamically expressed upon the floral transition. We show that of these homologs, TOMATO MADS 3 (TM3) and SISTER OF TM3 (STM3) promote the primary and sympodial transition to flowering, while MADS-BOX PROTEIN 23 (MBP23) and MBP18 hardly contribute to flowering initiation in the indeterminate cultivar Moneyberg. Protein-protein interaction assays and whole-transcriptome analysis during reproductive meristem development revealed that TM3 and STM3 interact and share many targets with FRUITFULL (FUL) homologs, including cytokinin regulators. Furthermore, we observed that mutating TM3/STM3 affects inflorescence development, but counteracts the inflorescence-branching phenotype of ful2 mbp20. Collectively, this indicates that TM3/STM3 promote the floral transition together with FUL2/MBP20, while these transcription factors have opposite functions in inflorescence development.

Negative regulation of floral transition in Arabidopsis by HOS15-PWR-HDA9 complex

Arabidopsis HOS15/PWR/HDA9 repressor complex, which is similar to the TBL1/NcoR1/HDAC complex in animals, plays a well-known role in epigenetic regulation. PWR and HDA9 have been reported to interact with each other and modulate the flowering time by repressing AGL19 expression, whereas HOS15 and HDA9, together with the photoperiodic evening complex, regulate flowering time through repression of GI transcription. However, the role of the HOS15/PWR/HDA9 core repressor complex as a functional unit in the regulation of flowering time is yet to be explored. In this study, we reported that the loss-of-function hos15-2/pwr/hda9 triple mutant accumulates higher transcript levels of AGL19 and exhibits an early flowering phenotype similar to those of hos15, pwr, and hda9 single mutants. Interestingly, the accumulation of HOS15 in the nucleus was drastically reduced in pwr and hda9 mutants. As a result, HOS15 could not perform its role in histone deacetylation or interaction with H3 in the nucleus. Furthermore, HOS15 is also associated with the same region of the AGL19 promoter known for PWR-HDA9 binding. The acetylation level of the AGL19 promoter was increased in the hos15-2 mutant, similar to the pwr and hda9 mutants. Therefore, our findings reveal that the HOS15/PWR/HDA9 repressor complex deacetylates the promoter region of AGL19, thereby negatively regulating AGL19 transcription, which leads to early flowering in Arabidopsis.

Arabidopsis HISTONE DEACETYLASE 9 Stimulates Hypocotyl Cell Elongation by Repressing GIGANTEA Expression Under Short Day Photoperiod

Developmental plasticity contributes to plant adaptation and fitness in a given condition. Hypocotyl elongation is under the tight control of complex genetic networks encompassing light, circadian, and photoperiod signaling. In this study, we demonstrate that HISTONE DEACETYLASE 9 (HDA9) mediates day length-dependent hypocotyl cell elongation. HDA9 binds to the GIGANTEA (GI) locus involved in photoperiodic hypocotyl elongation. The short day (SD)-accumulated HDA9 protein promotes histone H3 deacetylation at the GI locus during the dark period, promoting hypocotyl elongation. Consistently, HDA9-deficient mutants display reduced hypocotyl length, along with an increase in GI gene expression, only under SD conditions. Taken together, our study reveals the genetic basis of day length-dependent cell elongation in plants.

Regulatory network for FOREVER YOUNG FLOWER-like genes in regulating Arabidopsis flower senescence and abscission

FOREVER YOUNG FLOWER (FYF) has been reported to play an important role in regulating flower senescence/abscission. Here, we functionally analyzed five Arabidopsis FYF-like genes, two in the FYF subgroup (FYL1/AGL71 and FYL2/AGL72) and three in the SOC1 subgroup (SOC1/AGL20, AGL19, and AGL14/XAL2), and showed their involvement in the regulation of flower senescence and/or abscission. We demonstrated that in FYF subgroup, FYF has both functions in suppressing flower senescence and abscission, FYL1 only suppresses flower abscission and FYL2 has been converted as an activator to promote flower senescence. In SOC1 subgroup, AGL19/AGL14/SOC1 have only one function in suppressing flower senescence. We also found that FYF-like proteins can form heterotetrameric complexes with different combinations of A/E functional proteins (such as AGL6 and SEP1) and AGL15/18-like proteins to perform their functions. These findings greatly expand the current knowledge behind the multifunctional evolution of FYF-like genes and uncover their regulatory network in plants.

HDACs Gene Family Analysis of Eight Rosaceae Genomes Reveals the Genomic Marker of Cold Stress in Prunus mume

Histone deacetylases (HDACs) play important roles in plant growth, development, and stress response. However, the pattern of how they are expressed in response to cold stress in the ornamental woody plant Prunus mume is poorly understood. Here, we identify 121 RoHDACs from eight Rosaceae plants of which 13 PmHDACs genes are from P. mume. A phylogenetic analysis suggests that the RoHDACs family is classified into three subfamilies, HDA1/RPD3, HD2, and SIR2. We identify 11 segmental duplication gene pairs of RoHDACs and find, via a sequence alignment, that the HDACs gene family, especially the plant-specific HD2 family, has experienced gene expansion and contraction at a recent genome evolution history. Each of the three HDACs subfamilies has its own conserved domains. The expression of PmHDACs in mei is found to be tissue-specific or tissue-wide. RNA-seq data and qRT-PCR experiments in cold treatments suggest that almost all PmHDACs genes-especially PmHDA1/6/14, PmHDT1, and PmSRT1/2-significantly respond to cold stress. Our analysis provides a fundamental insight into the phylogenetic relationship of the HDACs family in Rosaceae plants. Expression profiles of PmHDACs in response to cold stress could provide an important clue to improve the cold hardiness of mei.

Epigenetic regulation of thermomorphogenesis and heat stress tolerance

Many environmental conditions fluctuate and organisms need to respond effectively. This is especially true for temperature cues that can change in minutes to seasons and often follow a diurnal rhythm. Plants cannot migrate and most cannot regulate their temperature. Therefore, a broad array of responses have evolved to deal with temperature cues from freezing to heat stress. A particular response to mildly elevated temperatures is called thermomorphogenesis, a suite of morphological adaptations that includes thermonasty, formation of thin leaves and elongation growth of petioles and hypocotyl. Thermomorphogenesis allows for optimal performance in suboptimal temperature conditions by enhancing the cooling capacity. When temperatures rise further, heat stress tolerance mechanisms can be induced that enable the plant to survive the stressful temperature, which typically comprises cellular protection mechanisms and memory thereof. Induction of thermomorphogenesis, heat stress tolerance and stress memory depend on gene expression regulation, governed by diverse epigenetic processes. In this Tansley review we update on the current knowledge of epigenetic regulation of heat stress tolerance and elevated temperature signalling and response, with a focus on thermomorphogenesis regulation and heat stress memory. In particular we highlight the emerging role of H3K4 methylation marks in diverse temperature signalling pathways.

Ethylene response factor MdERF4 and histone deacetylase MdHDA19 suppress apple fruit ripening through histone deacetylation of ripening-related genes

Histone deacetylase enzymes participate in the regulation of many aspects of plant development. However, the genome-level targets of histone deacetylation during apple (Malus domestica) fruit development have not been resolved in detail, and the mechanisms of regulation of such a process are unknown. We previously showed that the complex of ethylene response factor 4 (MdERF4) and the TOPLESS co-repressor (MdTPL4; MdERF4-MdTPL4) is constitutively active during apple fruit development (Hu et al., 2020), but whether this transcriptional repression complex is coupled to chromatin modification is unknown. Here, we show that a histone deacetylase (MdHDA19) is recruited to the MdERF4-MdTPL4 complex, thereby impacting fruit ethylene biosynthesis. Transient suppression of MdHDA19 expression promoted fruit ripening and ethylene production. To identify potential downstream target genes regulated by MdHDA19, we conducted chromatin immunoprecipitation (ChIP) sequencing of H3K9 and ChIP-quantitative polymerase chain reaction assays. We found that MdHDA19 affects ethylene production by facilitating H3K9 deacetylation and forms a complex with MdERF4-MdTPL4 to directly repress MdACS3a expression by decreasing the degree of acetylation. We demonstrate that an early-maturing-specific acetylation H3K9ac peak in MdACS3a and expression of MdACS3a were specifically up-regulated in fruit of an early-maturing, but not a late-maturing, cultivar. We provide evidence that a C-to-G mutation in the ethylene-responsive element binding factor-associated amphiphilic repression motif of MdERF4 reduces the repression of MdACS3a by the MdERF4-MdTPL4-MdHDA19 complex. Taken together, our results reveal that the MdERF4-MdTPL-MdHDA19 repressor complex participates in the epigenetic regulation of apple fruit ripening.

Genetic architecture underlying light and temperature mediated flowering in Arabidopsis, rice, and temperate cereals

Timely flowering is essential for optimum crop reproduction and yield. To determine the best flowering-time genes (FTGs) relevant to local adaptation and breeding, it is essential to compare the interspecific genetic architecture of flowering in response to light and temperature, the two most important environmental cues in crop breeding. However, the conservation and variations of FTGs across species lack systematic dissection. This review summarizes current knowledge on the genetic architectures underlying light and temperature-mediated flowering initiation in Arabidopsis, rice, and temperate cereals. Extensive comparative analyses show that most FTGs are conserved, whereas functional variations in FTGs may be species specific and confer local adaptation in different species. To explore evolutionary dynamics underpinning the conservation and variations in FTGs, domestication and selection of some key FTGs are further dissected. Based on our analyses of genetic control of flowering time, a number of key issues are highlighted. Strategies for modulation of flowering behavior in crop breeding are also discussed. The resultant resources provide a wealth of reference information to uncover molecular mechanisms of flowering in plants and achieve genetic improvement in crops.

Beyond the Genetic Pathways, Flowering Regulation Complexity in Arabidopsis thaliana

Flowering is one of the most critical developmental transitions in plants’ life. The irreversible change from the vegetative to the reproductive stage is strictly controlled to ensure the progeny’s success. In Arabidopsis thaliana, seven flowering genetic pathways have been described under specific growth conditions. However, the evidence condensed here suggest that these pathways are tightly interconnected in a complex multilevel regulatory network. In this review, we pursue an integrative approach emphasizing the molecular interactions among the flowering regulatory network components. We also consider that the same regulatory network prevents or induces flowering phase change in response to internal cues modulated by environmental signals. In this sense, we describe how during the vegetative phase of development it is essential to prevent the expression of flowering promoting genes until they are required. Then, we mention flowering regulation under suboptimal growing temperatures, such as those in autumn and winter. We next expose the requirement of endogenous signals in flowering, and finally, the acceleration of this transition by long-day photoperiod and temperature rise signals allowing A. thaliana to bloom in spring and summer seasons. With this approach, we aim to provide an initial systemic view to help the reader integrate this complex developmental process.

A histone H4 gene prevents drought-induced bolting in Chinese cabbage by attenuating the expression of flowering genes

Flowering is an important trait in Chinese cabbage, because premature flowering reduces yield and quality of the harvested products. Water deficit, caused by drought or other environmental conditions, induces early flowering. Drought resistance involves global reprogramming of transcription, hormone signaling, and chromatin modification. We show that a histone H4 protein, BrHIS4.A04, physically interacts with a homeodomain protein BrVIN3.1, which was selected during the domestication of late-bolting Chinese cabbage. Over-expression of BrHIS4.A04 resulted in premature flowering under normal growth conditions, but prevented further premature bolting in response to drought. We show that the expression of key abscisic acid (ABA) signaling genes, and also photoperiodic flowering genes was attenuated in BrHIS4.A04-overexpressing (BrHIS4.A04OE) plants under drought conditions. Furthermore, the relative change in H4-acetylation at these gene loci was reduced in BrHIS4.A04OE plants. We suggest that BrHIS4.A04 prevents premature bolting by attenuating the expression of photoperiodic flowering genes under drought conditions, through the ABA signaling pathway. Since BrHIS4.A04OE plants displayed no phenotype related to vegetative or reproductive development under laboratory-induced drought conditions, our findings contribute to the potential fine-tuning of flowering time in crops through genetic engineering without any growth penalty, although more data are necessary under field drought conditions.

PuHox52-mediated hierarchical multilayered gene regulatory network promotes adventitious root formation in Populus ussuriensis

Adventitious root (AR) formation is critically important in vegetative propagation through cuttings in some plants, especially woody species. However, the underlying molecular mechanisms remain elusive. Here, we report the identification of a poplar homeobox gene, PuHox52, which was induced rapidly (within 15 min) at the basal ends of stems upon cutting and played a key regulatory role in adventitious rooting. We demonstrated that overexpression of PuHox52 significantly increased the number of ARs while suppression of PuHox52 had the opposite effect. A multilayered hierarchical gene regulatory network (ML-hGRN) mediated by PuHox52 was reverse-engineered and demonstrated to govern AR formation. PuHox52 regulated AR formation through upregulation of nine hub regulators, including a jasmonate signaling pathway gene, PuMYC2, and an auxin signaling pathway gene, PuAGL12. We also identified coherent type 4 feed-forward loops within this ML-hGRN; PuHox52 repressed PuHDA9, which encodes a histone deacetylase, and led to an increase in acetylation and presumably expression of three hub regulators, PuWRKY51, PuLBD21 and PuIAA7. Our results indicate that the ML-hGRN mediated by PuHox52 governs AR formation at the basal ends of stem cuttings from poplar trees.

The diverse and unanticipated roles of histone deacetylase 9 in coordinating plant development and environmental acclimation

Plants tightly control gene transcription to adapt to environmental conditions and steer growth and development. Different types of epigenetic modifications are instrumental in these processes. In recent years, an important role for the chromatin-modifying RPD3/HDA1 class I HDAC HISTONE DEACETYLASE 9 (HDA9) emerged in the regulation of a multitude of plant traits and responses. HDACs are widely considered transcriptional repressors and are typically part of multiprotein complexes containing co-repressors, DNA, and histone-binding proteins. By catalyzing the removal of acetyl groups from lysine residues of histone protein tails, HDA9 negatively controls gene expression in many cases, in concert with interacting proteins such as POWERDRESS (PWR), HIGH EXPRESSION OF OSMOTICALLY RESPONSIVE GENES 15 (HOS15), WRKY53, ELONGATED HYPOCOTYL 5 (HY5), ABA INSENSITIVE 4 (ABI4), and EARLY FLOWERING 3 (ELF3). However, HDA9 activity has also been directly linked to transcriptional activation. In addition, following the recent breakthrough discovery of mutual negative feedback regulation between HDA9 and its interacting WRKY-domain transcription factor WRKY53, swift progress in gaining understanding of the biology of HDA9 is expected. In this review, we summarize knowledge on this intriguing versatile-and long under-rated-protein and propose novel leads to further unravel HDA9-governed molecular networks underlying plant development and environmental biology.

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

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

Characterization of floral morphoanatomy and identification of marker genes preferentially expressed during specific stages of cotton flower development

Characterization of anther and ovule developmental programs and expression analyses of stage-specific floral marker genes in Gossypium hirsutum allowed to build a comprehensive portrait of cotton flower development before fiber initiation. Gossypium hirsutum is the most important cotton species that is cultivated worldwide. Although cotton reproductive development is important for fiber production, since fiber is formed on the epidermis of mature ovules, cotton floral development remains poorly understood. Therefore, this work aims to characterize the cotton floral morphoanatomy by performing a detailed description of anther and ovule developmental programs and identifying stage-specific floral marker genes in G. hirsutum. Using light microscopy and scanning electron microscopy, we analyzed anther and ovule development during 11 stages of flower development. To better characterize the ovule development in cotton, we performed histochemical analyses to evaluate the accumulation of phenolic compounds, pectin, and sugar in ovule tissues. After identification of major hallmarks of floral development, three key stages were established in G. hirsutum floral development: in stage 1 (early-EF), sepal, petal, and stamen primordia were observed; in stage 2 (intermediate-IF), primordial ovules and anthers are present, and the differentiating archesporial cells were observed, marking the beginning of microsporogenesis; and in stage 6 (late-LF), flower buds presented initial anther tapetum degeneration and microspore were released from the tetrad, and nucellus and both inner and outer integuments are developing. We used transcriptome data of cotton EF, IF and LF stages to identify floral marker genes and evaluated their expression by real-time quantitative PCR (qPCR). Twelve marker genes were preferentially expressed in a stage-specific manner, including the putative homologs for AtLEAFY, AtAPETALA 3, AtAGAMOUS-LIKE 19 and AtMALE STERILITY 1, which are crucial for several aspects of reproductive development, such as flower organogenesis and anther and petal development. We also evaluated the expression profile of B-class MADS-box genes in G. hirsutum floral transcriptome (EF, IF, and LF). In addition, we performed a comparative analysis of developmental programs between Arabidopsis thaliana and G. hirsutum that considered major morphoanatomical and molecular processes of flower, anther, and ovule development. Our findings provide the first detailed analysis of cotton flower development.

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

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

OsHDA710-Mediated Histone Deacetylation Regulates Callus Formation of Rice Mature Embryo

Histone deacetylases (HDACs) play important roles in the regulation of eukaryotic gene expression. The role of HDACs in specialized transcriptional regulation and biological processes is poorly understood. In this study, we evaluated the global expression patterns of genes related to epigenetic modifications during callus initiation in rice. We found that the repression of HDAC activity by trichostatin A (TSA) or by OsHDA710 mutation (hda710) results in impaired callus formation of rice mature embryo and increased global histone H3 acetylation levels. The HDAC inhibition decreased auxin response and cell proliferation in callus formation. Meanwhile, the transcriptional repressors OsARF18 and OsARF22 were upregulated in the callus of hda710. The chromatin immunoprecipitation-quantitative PCR (ChIP-qPCR) analysis demonstrated that the callus of hda710 exhibited enhanced histone H3 acetylation levels at the chromatin regions of OsARF18 and OsARF22. Furthermore, we found that OsARF18 and OsARF22 were regulated through OsHDA710 recruitment to their target loci. In addition, overexpression of OsARF18 decreased the transcription of downstream genes PLT1 and PLT2 and inhibited callus formation of the mature embryo. These results demonstrate that OsHDA710 regulates callus formation by suppressing repressive OsARFs via histone deacetylation during callus formation of rice mature embryo. This indicates that OsHDA710-mediated histone deacetylation is an epigenetic regulation pathway for maintaining auxin response during cell dedifferentiation.

Linking signaling pathways to histone acetylation dynamics in plants

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

The delayed senescence of postharvest buds in salt ions was related to antioxidant activity, HDA9 and CCX1 in broccoli (Brassica oleracea L. var. Italic Planch.)

Epigenetic regulation and salt ions play essential roles in senescence control, but the underlying regulatory mechanism of senescence has not been thoroughly revealed in broccoli postharvest buds. Here, we found 200 mmol·L^-1 NaCl, 400 mmol·L^-1 KCl, 40 mmol·L^-1 CaCl₂ and 0.5 μmol·L^-1 Trichostatin-A (TSA, a histone deacetylase inhibitor) delayed the bud senescence. They resulted in significantly inhibiting the malondialdehyde (MDA) content, and dramatically promoting the contents of superoxide dismutase (SOD), peroxidase (POD) and Chlorophyll. Furthermore, the expression of PHEOPHYTINASE (PPH) and NONYELLOWING (NYE1), but not SUPPRESSOR OF OVEREXPRESSION OF CONSTANS 1 (SOC1), were remarkably repressed by salt ions and TSA. Interestingly, HISTONE DEACETYLASE 9 (HDA9) and CATION/Ca²⁺ EXCHANGER 1 (CCX1) were down-regulated by NaCl, CaCl₂ and TSA. Further assays demonstrated that HDA9 could not interact with CCX1 promoter. It suggested that CCX1 along with HDA9 were involved in inhibiting the senescence of broccoli buds, and regulated aging by indirect interaction.

Histone Deacetylase HDA9 and WRKY53 Transcription Factor Are Mutual Antagonists in Regulation of Plant Stress Response

Epigenetic regulation of gene expression is important for plant adaptation to environmental changes. Previous results showed that Arabidopsis RPD3-like histone deacetylase HDA9 is known to function in repressing plant response to stress in Arabidopsis. However, how HDA9 targets to specific chromatin loci and controls gene expression networks involved in plant response to stress remains largely unclear. Here, we show that HDA9 represses stress tolerance response by interacting with and regulating the DNA binding and transcriptional activity of WRKY53, which functions as a high-hierarchy positive regulator of stress response. We found that WRKY53 is post-translationally modified by lysine acetylation at multiple sites, some of which are removed by HDA9, resulting in inhibition of WRKY53 transcription activity. Conversely, WRKY53 negatively regulates HDA9 histone deacetylase activity. Collectively, our results indicate that HDA9 and WRK53 are reciprocal negative regulators of each other's activities, illustrating how the functional interplay between a chromatin regulator and a transcription factor regulates stress tolerance in plants.

HOS15 and HDA9 negatively regulate immunity through histone deacetylation of intracellular immune receptor NLR genes in Arabidopsis

Plant immune responses need to be tightly controlled for growth-defense balance. The mechanism underlying this tight control is not fully understood. Here we identify epigenetic regulation of nucleotide-binding leucine rich repeat or Nod-Like Receptor (NLR) genes as an important mechanism for immune responses. Through a sensitized genetic screen and molecular studies, we identified and characterized HOS15 and its associated protein HDA9 as negative regulators of immunity and NLR gene expression. The loss-of-function of HOS15 or HDA9 confers enhanced resistance to pathogen infection accompanied with increased expression of one-third of the 207 NLR genes in Arabidopsis thaliana. HOS15 and HDA9 are physically associated with some of these NLR genes and repress their expression likely through reducing the acetylation of H3K9 at these loci. In addition, these NLR genes are repressed by HOS15 under both pathogenic and nonpathogenic conditions but by HDA9 only under infection condition. Together, this study uncovers a previously uncharacterized histone deacetylase complex in plant immunity and highlights the importance of epigenetic regulation of NLR genes in modulating growth-defense balance.

HY5-HDA9 Module Transcriptionally Regulates Plant Autophagy in Response to Light-to-Dark Conversion and Nitrogen Starvation

Light is arguably one of the most important environmental factors that determines virtually all aspects of plant growth and development, but the molecular link between light signaling and the autophagy pathway has not been elucidated in plants. In this study, we demonstrate that autophagy is activated during light-to-dark conversion though transcriptional upregulation of autophagy-related genes (ATGs). We showed that depletion of the ELONGATED HYPOCOTYL 5 (HY5), a key component of light signaling, leads to enhanced autophagy activity and resistance to extended darkness and nitrogen starvation treatments, contributing to higher expression of ATGs. HY5 interacts with and recruits HISTONE DEACETYLASE 9 (HDA9) to ATG5 and ATG8e loci to repress their expression by deacetylation of the Lys9 and Lys27 of histone 3. Furthermore, we found that both darkness and nitrogen depletion induce the degradation of HY5 via 26S proteasome and the concomitant disassociation of HDA9 from ATG5 and ATG8e loci, leading to their depression and thereby activated autophagy. Genetic analysis further confirmed that HY5 and HDA9 act synergistically and function upstream of the autophagy pathway. Collectively, our study unveils a previously unknown transcriptional and epigenetic network that regulates autophagy in response to light-to-dark conversion and nitrogen starvation in plants.

PWR/HDA9/ABI4 Complex Epigenetically Regulates ABA Dependent Drought Stress Tolerance in Arabidopsis

Drought stress adversely affects plant growth and development and significantly reduces crop productivity and yields. The phytohormone abscisic acid (ABA) rapidly accumulates in response to drought stress and mediates the expression of stress-responsive genes that help the plant to survive dehydration. The protein Powerdress (PWR), which interacts with Histone Deacetylase 9 (HDA9), has been identified as a critical component regulating plant growth and development, flowering time, floral determinacy, and leaf senescence. However, the role and function of PWR and HDA9 in abiotic stress response had remained elusive. Here we report that a complex of PWR and HDA9 interacts with ABI4 and epigenetically regulates drought signaling in plants. T-DNA insertion mutants of PWR and HDA9 are insensitive to ABA and hypersensitive to dehydration. Furthermore, the expression of ABA-responsive genes (RD29A, RD29B, and COR15A) is also downregulated in pwr and hda9 mutants. Yeast two-hybrid assays showed that PWR and HDA9 interact with ABI4. Transcript levels of genes that are normally repressed by ABI4, such as CYP707A1, AOX1a and ACS4, are increased in pwr. More importantly, during dehydration stress, PWR and HDA9 regulate the acetylation status of the CYP707A1, which encodes a major enzyme of ABA catabolism. Taken together, our results indicate that PWR, in association with HDA9 and ABI4, regulates the chromatin modification of genes responsible for regulation of both the ABA-signaling and ABA-catabolism pathways in response to ABA and drought stress.

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.

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.

StressGenePred: a twin prediction model architecture for classifying the stress types of samples and discovering stress-related genes in arabidopsis

Background

Recently, a number of studies have been conducted to investigate how plants respond to stress at the cellular molecular level by measuring gene expression profiles over time. As a result, a set of time-series gene expression data for the stress response are available in databases. With the data, an integrated analysis of multiple stresses is possible, which identifies stress-responsive genes with higher specificity because considering multiple stress can capture the effect of interference between stresses. To analyze such data, a machine learning model needs to be built.

Results

In this study, we developed StressGenePred, a neural network-based machine learning method, to integrate time-series transcriptome data of multiple stress types. StressGenePred is designed to detect single stress-specific biomarker genes by using a simple feature embedding method, a twin neural network model, and Confident Multiple Choice Learning (CMCL) loss. The twin neural network model consists of a biomarker gene discovery and a stress type prediction model that share the same logical layer to reduce training complexity. The CMCL loss is used to make the twin model select biomarker genes that respond specifically to a single stress. In experiments using Arabidopsis gene expression data for four major environmental stresses, such as heat, cold, salt, and drought, StressGenePred classified the types of stress more accurately than the limma feature embedding method and the support vector machine and random forest classification methods. In addition, StressGenePred discovered known stress-related genes with higher specificity than the Fisher method.

Conclusions

StressGenePred is a machine learning method for identifying stress-related genes and predicting stress types for an integrated analysis of multiple stress time-series transcriptome data. This method can be used to other phenotype-gene associated studies.

HISTONE DEACETYLASE 9 stimulates auxin-dependent thermomorphogenesis in Arabidopsis thaliana by mediating H2A.Z depletion

Many plant species respond to unfavorable high ambient temperatures by adjusting their vegetative body plan to facilitate cooling. This process is known as thermomorphogenesis and is induced by the phytohormone auxin. Here, we demonstrate that the chromatin-modifying enzyme HISTONE DEACETYLASE 9 (HDA9) mediates thermomorphogenesis but does not interfere with hypocotyl elongation during shade avoidance. HDA9 is stabilized in response to high temperature and mediates histone deacetylation at the YUCCA8 locus, a rate-limiting enzyme in auxin biosynthesis, at warm temperatures. We show that HDA9 permits net eviction of the H2A.Z histone variant from nucleosomes associated with YUCCA8, allowing binding and transcriptional activation by PHYTOCHROME INTERACTING FACTOR 4, followed by auxin accumulation and thermomorphogenesis.

SIN3 LIKE genes mediate long-day induction of flowering but inhibit the floral transition in short days through histone deacetylation in Arabidopsis

Day length or photoperiod changes are crucial for plants to align the timing of the floral transition with seasonal changes. Through the photoperiod pathway, day length changes induce the expression of the florigenic FLOWERING LOCUS T (FT) to promote flowering. In the facultative long days (LDs) plant Arabidopsis thaliana, LD signals induce flowering, whereas short days (SDs) inhibit flowering. Here, we show that in Arabidopsis SIN3 LIKE (SNL) family genes, encoding a scaffold protein for assembly of histone deacetylase complexes, directly repress the expression of an FT activator and three FT repressors to regulate the transition to flowering in SDs and LDs, respectively. Under inductive LDs, SNLs including SIN3 LIKE 1 (SNL1) to SNL5, function in partial redundancy to repress the expression of three AP2 family transcription factors that repress FT expression, and therefore mediate LD induction of FT expression and promote the transition to flowering. In contrast, under non-inductive SDs SNLs act to inhibit the floral transition, partly through direct repression of a MADS box transcriptional factor that promotes FT expression. Therefore, our results reveal that SNLs, through histone deacetylation, play a dual role for the control of flowering in the LD plant Arabidopsis: inhibiting flowering when the day length is shorter and promoting the floral transition when days become longer than a threshold length.

Salicylic acid-induced transcriptional reprogramming by the HAC-NPR1-TGA histone acetyltransferase complex in Arabidopsis

Plant immunity depends on massive expression of pathogenesis-related genes (PRs) whose transcription is de-repressed by pathogen-induced signals. Salicylic acid (SA) acts as a major signaling molecule in plant immunity and systemic acquired resistance triggered by bacterial or viral pathogens. SA signal results in the activation of the master immune regulator, Nonexpressor of pathogenesis-related genes 1 (NPR1), which is thought to be recruited by transcription factors such as TGAs to numerous downstream PRs. Despite its key role in SA-triggered immunity, the biochemical nature of the transcriptional coactivator function of NPR1 and the massive transcriptional reprogramming induced by it remain obscure. Here we demonstrate that the CBP/p300-family histone acetyltransferases, HACs and NPR1 are both essential to develop SA-triggered immunity and PR induction. Indeed HACs and NPR1 form a coactivator complex and are recruited to PR chromatin through TGAs upon SA signal, and finally the HAC−NPR1−TGA complex activates PR transcription by histone acetylation-mediated epigenetic reprogramming. Thus, our study reveals a molecular mechanism of NPR1-mediated transcriptional reprogramming and a key epigenetic aspect of the central immune system in plants.

The HDA19 histone deacetylase complex is involved in the regulation of flowering time in a photoperiod-dependent manner

Chromatin modifications are known to affect flowering time in plants, but little is known about how these modifications regulate flowering time in response to environmental signals like photoperiod. In Arabidopsis thaliana, HDC1, a conserved subunit of the RPD3-like histone deacetylase (HDAC) complex, was previously reported to regulate flowering time via the same mechanism as does the HDAC HDA6. Here, we demonstrate that HDC1, SNLs and MSI1 are shared subunits of the HDA6 and HDA19 HDAC complexes. While the late-flowering phenotype of the hda6 mutant is independent of photoperiod, the hda19, hdc1 and snl2/3/4 mutants flower later than or at a similar time to the wild-type in long-day conditions but flower earlier than the wild-type in short-day conditions. Our genome-wide analyses indicate that the effect of hdc1 on histone acetylation and transcription is comparable with that of hda19 but is different from that of hda6. Especially, we demonstrate that the HDA19 complex directly regulates the expression of two flowering repressor genes related to the gibberellin signaling pathway. Thus, the study reveals a photoperiod-dependent role of the HDA19 HDAC complex in the regulation of flowering time.

The EC-HDA9 complex rhythmically regulates histone acetylation at the TOC1 promoter in Arabidopsis

Circadian clocks are conserved time-keeper mechanisms in some prokaryotes and higher eukaryotes. Chromatin modification is emerging as key regulatory mechanism for refining core clock gene expression. Rhythmic changes in histone marks are closely associated to the TIMING OF CAB EXPRESSION 1 (TOC1) Arabidopsis clock gene. However, the chromatin-related modifiers responsible for these marks remain largely unknown. Here, we uncover that the chromatin modifier HISTONE DEACETYLASE 9 (HDA9) and the Evening complex (EC) component EARLY FLOWERING 3 (ELF3) directly interact to regulate the declining phase of TOC1 after its peak expression. We found that HDA9 specifically binds to the TOC1 promoter through the interaction with ELF3. The EC-HDA9 complex promotes H3 deacetylation at the TOC1 locus, contributing to suppressing TOC1 expression during the night, the time of EC function. Therefore, we have identified the mechanism by which the circadian clock intertwines with chromatin-related components to shape the circadian waveforms of gene expression in Arabidopsis.

AGL18-1 delays flowering time through affecting expression of flowering-related genes in Brassica juncea

Brassica juncea is an important vegetable and condiment crop widely grown in Asia, and the yield and quality of its product organs are affected by flowering time. AGAMOUS-LIKE18-1 (AGL18-1) belongs to a member of MADS-domain transcription factors, which play vital roles in flowering time control, but the biological role of AGL18-1 in B. juncea (BjuAGL18-1) has not been thoroughly revealed in flowering regulatory network. In this study, BjuAGL18-1 expressed highly in inflorescence and flower, but slightly in root, stem and leaf. The sense and anti-sense transgenic lines of BjuAGL18-1 were generated and showed that BjuAGL18-1 functioned as a flowering inhibitor and depressed growth of lateral branching. During the vegetative phase, BjuAGL18-1 induced another flowering repressor AGAMOUS-LIKE15 (BjuAGL15) but inhibited the flowering signal integrator of SUPPRESSOR OF OVEREXPRESSION OF CONSTANS 1 (BjuSOC1) in Brassica juncea. Whereas, during the flower developmental phase, both SOC1 and AGAMOUS-LIKE24 (AGL24) were down-regulated by BjuAGL18-1. By contrast, AGL15 was promoted by BjuAGL18-1, while SHORT VEGETATIVE PHASE (SVP) was independent of BjuAGL18-1. Additionally, HISTONE DEACETYLASE 9 (HDA9) was highly induced by BjuAGL18-1. These results will provide valuable information for clarifying the molecular mechanism of BjuAGL18-1 in mediating flowering time.

Histone deacetylases HDA6 and HDA9 coordinately regulate valve cell elongation through affecting auxin signaling in Arabidopsis

Both Histone Deacetylases HDA6 and HDA9 belong to class I subfamily of RPD3/HDA1 HDACs. Loss-of-function mutants of HDA9 form slightly blunt siliques. However, the involvement of HDA6 in regulating silique tip growth is unclear. In this study, we show that HDA6 acts redundantly with HDA9 in regulating the elongation of valve cells in the silique tip. Although the hda6 single mutant does not exhibit a detectable silique phenotype, the silique tip of hda6 hda9 double mutant displays a more severe bulge, a morphology we termed as "nock-shaped". The valve cells of the silique tip of hda9 are longer than wild-type, and loss of HDA6 in hda9 enhances the valve cell elongation phenotype. The transcript levels of auxin-signaling-related genes are mis-regulated in hda9 and hda6 hda9 siliques, and the GFP reporter driven by the auxin response promoter DR5 is weaker in hda9 or hda6 hda9 than wild-type or hda6. Thus, our findings reveal that HDA6 and HDA9 coordinately control the elongation of silique valve cells through regulating the expression of auxin-related genes in silique tips.

Arabidopsis HEAT SHOCK TRANSCRIPTION FACTORA1b regulates multiple developmental genes under benign and stress conditions

In Arabidopsis thaliana, HEAT SHOCK TRANSCRIPTION FACTORA1b (HSFA1b) controls resistance to environmental stress and is a determinant of reproductive fitness by influencing seed yield. To understand how HSFA1b achieves this, we surveyed its genome-wide targets (ChIP-seq) and its impact on the transcriptome (RNA-seq) under non-stress (NS), heat stress (HS) in the wild type, and in HSFA1b-overexpressing plants under NS. A total of 952 differentially expressed HSFA1b-targeted genes were identified, of which at least 85 are development associated and were bound predominantly under NS. A further 1780 genes were differentially expressed but not bound by HSFA1b, of which 281 were classified as having development-associated functions. These genes are indirectly regulated through a hierarchical network of 27 transcription factors (TFs). Furthermore, we identified 480 natural antisense non-coding RNA (cisNAT) genes bound by HSFA1b, defining a further mode of indirect regulation. Finally, HSFA1b-targeted genomic features not only harboured heat shock elements, but also MADS box, LEAFY, and G-Box promoter motifs. This revealed that HSFA1b is one of eight TFs that target a common group of stress defence and developmental genes. We propose that HSFA1b transduces environmental cues to many stress tolerance and developmental genes to allow plants to adjust their growth and development continually in a varying environment.

HDA9 interacts with the promoters of SOC1 and AGL24 involved in flowering time control in Brassica juncea

HDA9 (a RPD3-like histone deacetylase) belongs to the histone deacetylase family which is involved in flowering time control through repression of AGL19 and FT, but it is still elusive that whether and how HDA9 directly interacts with flowering signal integrators of SOC1 and AGL24 in Brassica juncea. In this study, BjuHDA9 (a homologous HDA9) was cloned from B. juncea and ubiquitously expressed in root, stem, cauline leaf, flower bud and opening flower. BjuHDA9 was highly induced by short-day photoperiod. Yeast two-hybrid and pull-down assays demonstrated that BjuHDA9 could not interact with BjuSOC1 and BjuAGL24 proteins. Whereas, BjuHDA9 directly interacted with promoters of BjuSOC1 and BjuAGL24 via yeast one-hybrid and Dual-Glo^® Luciferase assays. It suggested that the histone deacetylase BjuHDA9 was probably involved in flowering time control by binding to promoter regions of BjuSOC1 and BjuAGL24. This study will provide valuable information for elucidating the molecular mechanism of BjuHDA9 in regulating flowering time.

OLIGOCELLULA1/HIGH EXPRESSION OF OSMOTICALLY RESPONSIVE GENES15 Promotes Cell Proliferation With HISTONE DEACETYLASE9 and POWERDRESS During Leaf Development in Arabidopsis thaliana

Organ size regulation is dependent on the precise spatial and temporal regulation of cell proliferation and cell expansion. A number of transcription factors have been identified that play a key role in the determination of aerial lateral organ size, but their functional relationship to various chromatin modifiers has not been well understood. To understand how leaf size is regulated, we previously isolated the oligocellula1 (oli1) mutant of Arabidopsis thaliana that develops smaller first leaves than the wild type (WT) mainly due to a reduction in the cell number. In this study, we further characterized oli1 leaf phenotypes and identified the OLI1 gene as well as interaction partners of OLI1. Detailed characterizations of leaf development suggested that the cell proliferation rate in oli1 leaf primordia is lower than that in the WT. In addition, oli1 was associated with a slight delay of the progression from the juvenile to adult phases of leaf traits. A classical map-based approach demonstrated that OLI1 is identical to HIGH EXPRESSION OF OSMOTICALLY RESPONSIVE GENES15 (HOS15). HOS15/OLI1 encodes a homolog of human transducin β-like protein1 (TBL1). TBL1 forms a transcriptional repression complex with the histone deacetylase (HDAC) HDAC3 and either nuclear receptor co-repressor (N-CoR) or silencing mediator for retinoic acid and thyroid receptor (SMRT). We found that mutations in HISTONE DEACETYLASE9 (HDA9) and a switching-defective protein 3, adaptor 2, N-CoR, and transcription factor IIIB-domain protein gene, POWERDRESS (PWR), showed a small-leaf phenotype similar to oli1. In addition, hda9 and pwr did not further enhance the oli1 small-leaf phenotype, suggesting that these three genes act in the same pathway. Yeast two-hybrid assays suggested physical interactions, wherein PWR probably bridges HOS15/OLI1 and HDA9. Earlier studies suggested the roles of HOS15, HDA9, and PWR in transcriptional repression. Consistently, transcriptome analyses showed several genes commonly upregulated in the three mutants. From these findings, we propose a possibility that HOS15/OLI1, PWR, and HDA9 form an evolutionary conserved transcription repression complex that plays a positive role in the regulation of final leaf size.

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.

Plant hormone signaling in flowering: An epigenetic point of view

Reproduction is one of the most important phases in an organism's lifecycle. In the case of angiosperm plants, flowering provides the major developmental transition from the vegetative to the reproductive stage, and requires genetic and epigenetic reprogramming to ensure the success of seed production. Flowering is regulated by a complex network of genes that integrate multiple environmental cues and endogenous signals so that flowering occurs at the right time; hormone regulation, signaling and homeostasis are very important in this process. Working alone or in combination, hormones are able to promote flowering by epigenetic regulation. Some plant hormones, such as gibberellins, jasmonic acid, abscisic acid and auxins, have important effects on chromatin compaction mediated by DNA methylation and histone posttranslational modifications, which hints at the role that epigenetic regulation may play in flowering through hormone action. miRNAs have been viewed as acting independently from DNA methylation and histone modification, ignoring their potential to interact with hormone signaling - including the signaling of auxins, gibberellins, ethylene, jasmonic acid, salicylic acid and others - to regulate flowering. Therefore, in this review we examine new findings about interactions between epigenetic mechanisms and key players in hormone signaling to coordinate flowering.

Remarkable Evolutionary Conservation of Antiobesity ADIPOSE/WDTC1 Homologs in Animals and Plants

ASG2 (Altered Seed Germination 2) is a prenylated protein in Arabidopsis thaliana that participates to abscisic acid signaling and is proposed to act as a substrate adaptor for the DDB1 (DNA damage-binding protein 1)-CUL4 (Cullin 4) E3 ubiquitin ligase complex. ASG2 harbors WD40 and TetratricoPeptide Repeat (TPR) domains, and resembles the well-conserved animal gene called ADP (antiobesity factor ADIPOSE) in fly and WDTC1 (WD40 and TPR 1) in humans. Loss of function of WDTC1 results in an increase in adipocytes, fat accumulation, and obesity. Antiadipogenic functions of WDTC1 involve regulation of fat-related gene transcription, notably through its binding to histone deacetylases (HDACs). Our sequence and phylogenetic analysis reveals that ASG2 belongs to the ADP/WDTC1 cluster. ASG2 and WDTC1 share a highly conserved organization that encompasses structural and functional motifs: seven WD40 domains and WD40 hotspot-related residues, three TPR protein-protein interaction domains, DDB1-binding elements [H-box and DWD (DDB1-binding WD40 protein)-box], and a prenylatable C-terminus. Furthermore, ASG2 involvement in fat metabolism was confirmed by reverse genetic approaches using asg2 knockout Arabidopsis plants. Under limited irradiance, asg2 mutants produce "obese" seeds characterized by increased weight, oil body density, and higher fatty acid contents. In addition, considering some ASG2- and WDTC1-peculiar properties, we show that the WDTC1 C-terminus is prenylated in vitro and HDAC-binding capability is conserved in ASG2, suggesting that the regulation mechanism and targets of ADP/WDTC1-like proteins may be conserved features. Our findings reveal the remarkable evolutionary conservation of the structure and the physiological role of ADIPOSE homologs in animals and plants.