Chromatin sensing: integration of environmental signals to reprogram plant development through chromatin regulators

Chromatin regulation in eukaryotes plays pivotal roles in controlling the developmental regulatory gene network. This review explores the intricate interplay between chromatin regulators and environmental signals, elucidating their roles in shaping plant development. As sessile organisms, plants have evolved sophisticated mechanisms to perceive and respond to environmental cues, orchestrating developmental programs that ensure adaptability and survival. A central aspect of this dynamic response lies in the modulation of versatile gene regulatory networks, mediated in part by various chromatin regulators. Here, we summarized current understanding of the molecular mechanisms through which chromatin regulators integrate environmental signals, influencing key aspects of plant development.

The Arabidopsis Deubiquitylase OTU5 Suppresses Flowering by Histone Modification-Mediated Activation of the Major Flowering Repressors FLC, MAF4, and MAF5

Distinct phylogeny and substrate specificities suggest that 12 Arabidopsis Ovarian Tumor domain-containing (OTU) deubiquitinases participate in conserved or plant-specific functions. The otu5-1 null mutant displayed a pleiotropic phenotype, including early flowering, mimicking that of mutants harboring defects in subunits (e.g., ARP6) of the SWR1 complex (SWR1c) involved in histone H2A.Z deposition. Transcriptome and RT-qPCR analyses suggest that downregulated FLC and MAF4-5 are responsible for the early flowering of otu5-1. qChIP analyses revealed a reduction and increase in activating and repressive histone marks, respectively, on FLC and MAF4-5 in otu5-1. Subcellular fractionation, GFP-fusion expression, and MNase treatment of chromatin showed that OTU5 is nucleus-enriched and chromatin-associated. Moreover, OTU5 was found to be associated with FLC and MAF4-5. The OTU5-associated protein complex(es) appears to be distinct from SWR1c, as the molecular weights of OTU5 complex(es) were unaltered in arp6-1 plants. Furthermore, the otu5-1 arp6-1 double mutant exhibited synergistic phenotypes, and H2A.Z levels on FLC/MAF4-5 were reduced in arp6-1 but not otu5-1. Our results support the proposition that Arabidopsis OTU5, acting independently of SWR1c, suppresses flowering by activating FLC and MAF4-5 through histone modification. Double-mutant analyses also indicate that OTU5 acts independently of the HUB1-mediated pathway, but it is partially required for FLC-mediated flowering suppression in autonomous pathway mutants and FRIGIDA-Col.

Plant deubiquitinases: from structure and activity to biological functions

This article attempts to provide comprehensive review of plant deubiquitinases, paying special attention to recent advances in their biochemical activities and biological functions. Proteins in eukaryotes are subjected to post-translational modifications, in which ubiquitination is regarded as a reversible process. Cellular deubiquitinases (DUBs) are a key component of the ubiquitin (Ub)-proteasome system responsible for cellular protein homeostasis. DUBs recycle Ub by hydrolyzing poly-Ub chains on target proteins, and maintain a balance of the cellular Ub pool. In addition, some DUBs prefer to cleave poly-Ub chains not linked through the conventional K48 residue, which often alter the substrate activity instead of its stability. In plants, all seven known DUB subfamilies have been identified, namely Ub-binding protease/Ub-specific protease (UBP/USP), Ub C-terminal hydrolase (UCH), Machado-Joseph domain-containing protease (MJD), ovarian-tumor domain-containing protease (OTU), zinc finger with UFM1-specific peptidase domain protease (ZUFSP), motif interacting with Ub-containing novel DUB family (MINDY), and JAB1/MPN/MOV34 protease (JAMM). This review focuses on recent advances in the structure, activity, and biological functions of plant DUBs, particularly in the model plant Arabidopsis.

Arabidopsis LSH10 transcription factor and OTLD1 histone deubiquitinase interact and transcriptionally regulate the same target genes

Histone ubiquitylation/deubiquitylation plays a major role in the epigenetic regulation of gene expression. In plants, OTLD1, a member of the ovarian tumor (OTU) deubiquitinase family, deubiquitylates histone 2B and represses the expression of genes involved in growth, cell expansion, and hormone signaling. OTLD1 lacks the intrinsic ability to bind DNA. How OTLD1, as well as most other known plant histone deubiquitinases, recognizes its target genes remains unknown. Here, we show that Arabidopsis transcription factor LSH10, a member of the ALOG protein family, interacts with OTLD1 in living plant cells. Loss-of-function LSH10 mutations relieve the OTLD1-promoted transcriptional repression of the target genes, resulting in their elevated expression, whereas recovery of the LSH10 function results in down-regulated transcription of the same genes. We show that LSH10 associates with the target gene chromatin as well as with DNA sequences in the promoter regions of the target genes. Furthermore, without LSH10, the degree of H2B monoubiquitylation in the target promoter chromatin increases. Hence, our data suggest that OTLD1-LSH10 acts as a co-repressor complex potentially representing a general mechanism for the specific function of plant histone deubiquitinases at their target chromatin.

Lipid-mediated activation of plasma membrane-localized deubiquitylating enzymes modulate endosomal trafficking

The abundance of plasma membrane-resident receptors and transporters has to be tightly regulated by ubiquitin-mediated endosomal degradation for the proper coordination of environmental stimuli and intracellular signaling. Arabidopsis OVARIAN TUMOR PROTEASE (OTU) 11 and OTU12 are plasma membrane-localized deubiquitylating enzymes (DUBs) that bind to phospholipids through a polybasic motif in the OTU domain. Here we show that the DUB activity of OTU11 and OTU12 towards K63-linked ubiquitin is stimulated by binding to lipid membranes containing anionic lipids. In addition, we show that the DUB activity of OTU11 against K6- and K11-linkages is also stimulated by anionic lipids, and that OTU11 and OTU12 can modulate the endosomal degradation of a model cargo and the auxin efflux transporter PIN2-GFP in vivo. Our results suggest that the catalytic activity of OTU11 and OTU12 is tightly connected to their ability to bind membranes and that OTU11 and OTU12 are involved in the fine-tuning of plasma membrane proteins in Arabidopsis.

Investigating Interactions Between Histone Modifying Enzymes and Transcription Factors in vivo by Fluorescence Resonance Energy Transfer

Epigenetic regulation of gene expression is commonly affected by histone modifying enzymes (HMEs) that generate heterochromatic or euchromatic histone marks for transcriptional repression or activation, respectively. HMEs are recruited to their target chromatin by transcription factors (TFs). Thus, detecting and characterizing direct interactions between HMEs and TFs are critical for understanding their function and specificity better. These studies would be more biologically relevant if performed in vivo within living tissues. Here, a protocol is described for visualizing interactions in plant leaves between a plant histone deubiquitinase and a plant transcription factor using fluorescence resonance energy transfer (FRET), which allows the detection of complexes between protein molecules that are within <10 nm from each other. Two variations of the FRET technique are presented: SE-FRET (sensitized emission) and AB-FRET (acceptor bleaching), in which the energy is transferred non-radiatively from the donor to the acceptor or emitted radiatively by the donor upon photobleaching of the acceptor. Both SE-FRET and AB-FRET approaches can be adapted easily to discover other interactions between other proteins in planta.

OsLMP1, Encoding a Deubiquitinase, Regulates the Immune Response in Rice

Lesion mimic mutants have become an effective material for understanding plant-microbe interactions and the immune mechanism in plants. Although many mechanisms responsible for the lesion mimic phenotype have been clarified in plants, the mechanism by which lesion mimic is regulated by posttranslational modification remained largely elusive, especially in rice. In this study, a mutant with the lesion mimic phenotype was obtained and named lmp1-1. Physiological measurements and quantitative real-time PCR analysis showed that the defense response was activated in the mutants. Transcriptome analysis showed that the phenylalanine ammonia lyase (PAL) pathway was activated in the mutant, causing the accumulation of salicylic acid (SA). The results of mapping based cloning showed that OsLMP1 encodes a deubiquitinase. OsLMP1 can cleave ubiquitination precursors. Furthermore, OsLMP1 epigenetically modifies SA synthetic pathway genes by deubiquitinating H₂B and regulates the immune response in rice. In summary, this study deepens our understanding of the function of OsLMP1 in the plant immune response and provides further insight into the relationship between plants and pathogenic microorganisms.

Post-translational modifications regulate the activity of the growth-restricting protease DA1

Plants are a primary food source and can form the basis for renewable energy resources. The final size of their organs is by far the most important trait to consider when seeking increased plant productivity. Being multicellular organisms, plant organ size is mainly determined by the coordination between cell proliferation and cell expansion. The protease DA1 limits the duration of cell proliferation and thereby restricts final organ size. Since its initial identification as a negative regulator of organ growth, various transcriptional regulators of DA1, but also interacting proteins, have been identified. These interactors include cleavage substrates of DA1, and also proteins that modulate the activity of DA1 through post-translational modifications, such as ubiquitination, deubiquitination, and phosphorylation. In addition, many players in the DA1 pathway display conserved phenotypes in other dicot and even monocot species. In this review, we provide a timely overview of the complex, but intriguing, molecular mechanisms that fine-tune the activity of DA1 and therefore final organ size. Moreover, we lay out a roadmap to identify and characterize substrates of proteases and frame the substrate cleavage events in their biological context.

Histone Deubiquitinase OTU1 Epigenetically Regulates DA1 and DA2, Which Control Arabidopsis Seed and Organ Size

Seeds are central to plant life cycle and to human nutrition, functioning as the major supplier of human population energy intake. To understand better the roles of enzymic writers and erasers of the epigenetic marks, in particular, histone ubiquitylation and the corresponding histone modifiers, involved in control of seed development, we identified the otubain-like cysteine protease OTU1 as a histone deubiquitinase involved in transcriptional repression of the DA1 and DA2 genes known to regulate seed and organ size in Arabidopsis. Loss-of-function mutants of OTU1 accumulate H2B monoubiquitylation and such euchromatic marks as H3 trimethylation and hyperacetylation in the DA1 and DA2 chromatin. These data advance our knowledge about epigenetic regulation of the DA1 and DA2 genes by recognizing OTU1 as a member of a putative repressor complex that negatively regulates their transcription.

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Histone ubiquitylation regulates transcription of DA1/DA2 that control seed/organ size

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OTU1 deubiquitinase is involved in deubiquitylation of the DA1/DA2 chromatin

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OTU1 acts as an epigenetic transcriptional repressor of the DA1/DA2 genes

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OTU1 is nucleocytoplasmic, indicating involvement in nuclear and cytoplasmic processes

Coordinate activation of a target gene by KDM1C histone demethylase and OTLD1 histone deubiquitinase in Arabidopsis

Potential functional coordination between histone deubiquitinases and histone lysine demethylases represents one of the least studied aspects of epigenetic control of transcriptional outcomes. Here, this question was addressed using Arabidopsis histone modification erasers deubiquitinase OTLD1 and demethylase KDM1C known to interact with each other in plant cells. Characterization of gain- and loss-of-function mutants of OTLD1 and KDM1C showed that both enzymes associate with the promoter chromatin of their target gene AN3 and function as coactivators of its expression. This transcriptional outcome was underlain by demethylation of the H3K9 repression mark, presumably by the KDM1C histone demethylase activity. Association of KDM1C and OTLD1 with the target chromatin was interdependent such that OTLD1 was not detected at the AN3 in the absence of KDM1C and KDM1C displayed a different and non-functional pattern of association in the absence of OTLD1. Thus, OTLD1 and KDM1C may crosstalk with each other to assemble a functional coactivator complex at the AN3 promoter chromatin and set the KDM1C specificity for the methylated H3K9 to determine the correct transcriptional outcome.

Reversible Histone H2B Monoubiquitination Fine-Tunes Abscisic Acid Signaling and Drought Response in Rice

Histone H2B monoubiquitination (H2Bub1) plays important roles in several physiological and developmental processes, but its roles in the regulation of plant stress responses remain elusive. Here, we report that H2Bub1 is crucially involved in abscisic acid (ABA) signaling and drought response in rice. We found that rice HISTONE MONOUBIQUITINATION2 (OsHUB2), an E3 ligase for H2Bub1, interacted with OsbZIP46, a key transcription factor regulating ABA signaling and drought response in rice. Genetic analyses suggest that OsHUB2, upregulated by drought and ABA, positively modulates ABA sensitivity and drought resistance. The H2Bub1 levels were increased in the target genes of OsbZIP46 under the drought stress and ABA treatments, which were positively correlated with their increased expression levels. Interestingly, MODD, a reported suppressor of ABA signaling and drought resistance by mediating OsbZIP46 deactivation and degradation, could reduce the H2Bub1 levels in the target genes of OsbZIP46 by recruiting a putative deubiquitinase OsOTLD1. Suppression of OsOTLD1 in vivo resulted in increased H2Bub1 levels and expression of OsbZIP46 target genes. Collectively, these findings established an elaborate mechanism of histone monoubiquitination in the fine-turning of ABA signaling and drought response by balancing H2Bub1 deposition and removal.

Activation of gene expression by histone deubiquitinase OTLD1

One of the main mechanisms of epigenetic control is post translational modification of histones, and one of the relatively less characterized, yet functionally important histone modifications is monoubiquitylation, which is reversed by histone deubiquitinases. In Arabidopsis, only two of such enzymes are known to date. One of them, OTLD1, deubiquitylates histone 2B and functions as a transcriptional repressor. But, could the same deubiquitinase act both as a repressor and an activator? Here, we addressed this question. Using gain-of-function and loss-of-function Arabidopsis alleles, we showed that OTLD1 can promote expression of a target gene. This transcriptional activation activity of OTLD1 involves occupation of the target chromatin by this enzyme, deubiquitination of monoubiquitylated H2B within the occupied regions, and formation of the euchromatic histone acetylation and methylation marks. Thus, OTLD1 can play a dual role in transcriptional repression and activation of its target genes. In these reactions, H2B ubiquitylation acts as both a repressive and an active mark whereas OTLD1 association with and deubiquitylation of the target chromatin may represent the key juncture between two opposing effects of this enzyme on gene expression.

Plant Deubiquitinases and Their Role in the Control of Gene Expression Through Modification of Histones

Selective degradation of proteins in the cell occurs through ubiquitination, which consists of post-translational deposition of ubiquitin on proteins to target them for degradation by proteases. However, ubiquitination does not only impact on protein stability, but promotes changes in their functions. Whereas the deposition of ubiquitin has been amply studied and discussed, the antagonistic activity, deubiquitination, is just emerging and the full model and players involved in this mechanism are far from being completely understood. Nevertheless, it is the dynamic balance between ubiquitination and deubiquitination that is essential for the development and homeostasis of organisms. In this review, we present a detailed analysis of the members of the deubiquitinase (DUB) superfamily in plants and its division in different clades. We describe current knowledge in the molecular and functional characterisation of DUB proteins, focusing primarily on Arabidopsis thaliana. In addition, the striking function of the duality between ubiquitination and deubiquitination in the control of gene expression through the modification of chromatin is discussed and, using the available information of the activities of the DUB superfamily in yeast and animals as scaffold, we propose possible scenarios for the role of these proteins in plants.