Chromatin dynamics and RNA metabolism are double-edged swords for the maintenance of plant genome integrity.

Maintenance of genome integrity is an essential process in all organisms. Mechanisms avoiding the formation of DNA lesions or mutations are well described in animals because of their relevance to human health and cancer. In plants, they are of growing interest because DNA damage accumulation is increasingly recognized as one of the consequences of stress. Although the cellular response to DNA damage is mostly studied in response to genotoxic treatments, the main source of DNA lesions is cellular activity itself. This can occur through the production of reactive oxygen species as well as DNA processing mechanisms such as DNA replication or transcription and chromatin dynamics. In addition, how lesions are formed and repaired is greatly influenced by chromatin features and dynamics and by DNA and RNA metabolism. Notably, actively transcribed regions or replicating DNA, because they are less condensed and are sites of DNA processing, are more exposed to DNA damage. However, at the same time, a wealth of cellular mechanisms cooperate to favour DNA repair at these genomic loci. These intricate relationships that shape the distribution of mutations along the genome have been studied extensively in animals but much less in plants. In this Review, we summarize how chromatin dynamics influence lesion formation and DNA repair in plants, providing a comprehensive view of current knowledge and highlighting open questions with regard to what is known in other organisms.

CRUMPLED LEAF supports plastid OUTER ENVELOPE PROTEIN OF 80 KDA complex formation in Arabidopsis.

Embedded ß-barrel proteins in the outer envelope membrane mediate most cellular trafficking between the cytoplasm and plastids. Although the TRANSLOCON AT THE OUTER ENVELOPE MEMBRANE OF CHLOROPLASTS 75-V (TOC75-V)/OUTER ENVELOPE PROTEIN OF 80 KDA (OEP80) complex has been implicated in the insertion and assembly of ß-barrel proteins in the outer envelope membrane of Arabidopsis (Arabidopsis thaliana) chloroplasts, relatively little is known about this process. CRUMPLED LEAF (CRL) encodes a chloroplast outer envelope membrane-localized protein, and its loss-of-function mutation results in pleiotropic defects, including altered plant morphogenesis, growth retardation, suppression of plastid division, and spontaneous light intensity-dependent localized cell death. A suppressor screen conducted on mutagenized crl mutants revealed that a missense mutation in OEP80 suppresses the pleiotropic defects of crl. Furthermore, we found that OEP80 complex formation is compromised in crl. Additionally, we demonstrated that CRL interacts with OEP80 in vivo and that a portion of CRL is present at the same molecular weight as the OEP80 complex. Our results suggest that CRL interacts with OEP80 to facilitate its complex formation. CRL is involved in plastid protein import; therefore, the pleiotropic defects in crl are likely due to the combined effects of decreased plastid protein import and altered membrane integration of ß-barrel proteins in the outer envelope membrane. This study sheds light on the mechanisms that allow ß-barrel protein integration into the plastid outer envelope membrane and the importance of this finding for plant cellular processes.

The LRR receptor-like kinase ALR1 is a plant aluminum ion sensor.

Plant survival requires an ability to adapt to differing concentrations of nutrient and toxic soil ions, yet ion sensors and associated signaling pathways are mostly unknown. Aluminum (Al) ions are highly phytotoxic, and cause severe crop yield loss and forest decline on acidic soils which represent ∼30% of land areas worldwide. Here we found an Arabidopsis mutant hypersensitive to Al. The gene encoding a leucine-rich-repeat receptor-like kinase, was named Al Resistance1 (ALR1). Al ions binding to ALR1 cytoplasmic domain recruits BAK1 co-receptor kinase and promotes ALR1-dependent phosphorylation of the NADPH oxidase RbohD, thereby enhancing reactive oxygen species (ROS) generation. ROS in turn oxidatively modify the RAE1 F-box protein to inhibit RAE1-dependent proteolysis of the central regulator STOP1, thus activating organic acid anion secretion to detoxify Al. These findings establish ALR1 as an Al ion receptor that confers resistance through an integrated Al-triggered signaling pathway, providing novel insights into ion-sensing mechanisms in living organisms, and enabling future molecular breeding of acid-soil-tolerant crops and trees, with huge potential for enhancing both global food security and forest restoration.

The plant POLYMERASE-ASSOCIATED FACTOR1 complex links transcription and H2B monoubiquitination genome wide.

The evolutionarily conserved POLYMERASE-ASSOCIATED FACTOR1 complex (Paf1C) participates in transcription, and research in animals and fungi suggests that it facilitates RNA POLYMERASE II (RNAPII) progression through chromatin. We examined the genomic distribution of the EARLY FLOWERING7 (ELF7) and VERNALIZATION INDEPENDENCE3 subunits of Paf1C in Arabidopsis (Arabidopsis thaliana). The occupancy of both subunits was confined to thousands of gene bodies and positively associated with RNAPII occupancy and the level of gene expression, supporting a role as a transcription elongation factor. We found that monoubiquitinated histone H2B, which marks most transcribed genes, was strongly reduced genome wide in elf7 seedlings. Genome-wide profiling of RNAPII revealed that in elf7 mutants, RNAPII occupancy was reduced throughout the gene body and at the transcription end site of Paf1C-targeted genes, suggesting a direct role for the complex in transcription elongation. Overall, our observations suggest a direct functional link between Paf1C activity, monoubiquitination of histone H2B, and the transition of RNAPII to productive elongation. However, for several genes, Paf1C may also act independently of H2Bub deposition or occupy these genes more stable than H2Bub marking, possibly reflecting the dynamic nature of Paf1C association and H2Bub turnover during transcription.

Long noncoding RNA-mediated epigenetic regulation of auxin-related genes controls shade avoidance syndrome in Arabidopsis.

The long noncoding RNA (lncRNA) AUXIN-REGULATED PROMOTER LOOP (APOLO) recognizes a subset of target loci across the Arabidopsis thaliana genome by forming RNA-DNA hybrids (R-loops) and modulating local three-dimensional chromatin conformation. Here, we show that APOLO regulates shade avoidance syndrome by dynamically modulating expression of key factors. In response to far-red (FR) light, expression of APOLO anti-correlates with that of its target BRANCHED1 (BRC1), a master regulator of shoot branching in Arabidopsis thaliana. APOLO deregulation results in BRC1 transcriptional repression and an increase in the number of branches. Accumulation of APOLO transcription fine-tunes the formation of a repressive chromatin loop encompassing the BRC1 promoter, which normally occurs only in leaves and in a late response to far-red light treatment in axillary buds. In addition, our data reveal that APOLO participates in leaf hyponasty, in agreement with its previously reported role in the control of auxin homeostasis through direct modulation of auxin synthesis gene YUCCA2, and auxin efflux genes PID and WAG2. We show that direct application of APOLO RNA to leaves results in a rapid increase in auxin signaling that is associated with changes in the plant response to far-red light. Collectively, our data support the view that lncRNAs coordinate shade avoidance syndrome in A. thaliana, and reveal their potential as exogenous bioactive molecules. Deploying exogenous RNAs that modulate plant-environment interactions may therefore become a new tool for sustainable agriculture.

LHP1-mediated epigenetic buffering of subgenome diversity and defense responses confers genome plasticity and adaptability in allopolyploid wheat.

Polyploidization is a major driver of genome diversification and environmental adaptation. However, the merger of different genomes may result in genomic conflicts, raising a major question regarding how genetic diversity is interpreted and regulated to enable environmental plasticity. By analyzing the genome-wide binding of 191 trans-factors in allopolyploid wheat, we identified like heterochromatin protein 1 (LHP1) as a master regulator of subgenome-diversified genes. Transcriptomic and epigenomic analyses of LHP1 mutants reveal its role in buffering the expression of subgenome-diversified defense genes by controlling H3K27me3 homeostasis. Stripe rust infection releases latent subgenomic variations by eliminating H3K27me3-related repression. The simultaneous inactivation of LHP1 homoeologs by CRISPR-Cas9 confers robust stripe rust resistance in wheat seedlings. The conditional repression of subgenome-diversified defenses ensures developmental plasticity to external changes, while also promoting neutral-to-non-neutral selection transitions and adaptive evolution. These findings establish an LHP1-mediated buffering system at the intersection of genotypes, environments, and phenotypes in polyploid wheat. Manipulating the epigenetic buffering capacity offers a tool to harness cryptic subgenomic variations for crop improvement.

Salinity stress-induced phosphorylation of INDETERMINATE-DOMAIN 4 (IDD4) by MPK6 regulates plant growth adaptation in Arabidopsis.

The INDETERMINATE DOMAIN (IDD) family belongs to a group of plant-specific transcription factors that coordinates plant growth/development and immunity. However, the function and mode of action of IDDs during abiotic stress, such as salt, are poorly understood. We used idd4 transgenic lines and screened them under salt stress to find the involvement of IDD4 in salinity stress tolerance The genetic disruption of IDD4 increases salt-tolerance, characterized by sustained plant growth, improved Na+/K+ ratio, and decreased stomatal density/aperture. Yet, IDD4 overexpressing plants were hypersensitive to salt-stress with an increase in stomatal density and pore size. Transcriptomic and ChIP-seq analyses revealed that IDD4 directly controls an important set of genes involved in abiotic stress/salinity responses. Interestingly, using anti-IDD4-pS73 antibody we discovered that IDD4 is specifically phosphorylated at serine-73 by MPK6 in vivo under salinity stress. Analysis of plants expressing the phospho-dead and phospho-mimicking IDD4 versions proved that phosphorylation of IDD4 plays a crucial role in plant transcriptional reprogramming of salt-stress genes. Altogether, we show that salt stress adaption involves MPK6 phosphorylation of IDD4 thereby regulating IDD4 DNA-binding and expression of target genes.

The canonical E2Fs together with RETINOBLASTOMA-RELATED are required to establish quiescence during plant development.

Maintaining stable and transient quiescence in differentiated and stem cells, respectively, requires repression of the cell cycle. The plant RETINOBLASTOMA-RELATED (RBR) has been implicated in stem cell maintenance, presumably by forming repressor complexes with E2F transcription factors. Surprisingly we find that mutations in all three canonical E2Fs do not hinder the cell cycle, but similarly to RBR silencing, result in hyperplasia. Contrary to the growth arrest that occurs when exit from proliferation to differentiation is inhibited upon RBR silencing, the e2fabc mutant develops enlarged organs with supernumerary stem and differentiated cells as quiescence is compromised. While E2F, RBR and the M-phase regulatory MYB3Rs are part of the DREAM repressor complexes, and recruited to overlapping groups of targets, they regulate distinct sets of genes. Only the loss of E2Fs but not the MYB3Rs interferes with quiescence, which might be due to the ability of E2Fs to control both G1-S and some key G2-M targets. We conclude that collectively the three canonical E2Fs in complex with RBR have central roles in establishing cellular quiescence during organ development, leading to enhanced plant growth.

Ethylene produced in carpel primordia controls CmHB40 expression to inhibit stamen development.

Sex determination evolved to control the development of unisexual flowers. In agriculture, it conditions how plants are cultivated and bred. We investigated how female flowers develop in monoecious cucurbits. We discovered in melon, Cucumis melo, a mechanism in which ethylene produced in the carpel is perceived in the stamen primordia through spatially differentially expressed ethylene receptors. Subsequently, the CmEIN3/CmEIL1 ethylene signalling module, in stamen primordia, activates the expression of CmHB40, a transcription factor that downregulates genes required for stamen development and upregulates genes associated with organ senescence. Investigation of melon genetic biodiversity revealed a haplotype, originating in Africa, altered in EIN3/EIL1 binding to CmHB40 promoter and associated with bisexual flower development. In contrast to other bisexual mutants in cucurbits, CmHB40 mutations do not alter fruit shape. By disentangling fruit shape and sex-determination pathways, our work opens up new avenues in plant breeding.

RTEL1 is required for silencing and epigenome stability.

Transcriptional silencing is an essential mechanism for controlling the expression of genes, transgenes and heterochromatic repeats through specific epigenetic marks on chromatin that are maintained during DNA replication. In Arabidopsis, silenced transgenes and heterochromatic sequences are typically associated with high levels of DNA methylation, while silenced genes are enriched in H3K27me3. Reactivation of these loci is often correlated with decreased levels of these repressive epigenetic marks. Here, we report that the DNA helicase REGULATOR OF TELOMERE ELONGATION 1 (RTEL1) is required for transcriptional silencing. RTEL1 deficiency causes upregulation of many genes enriched in H3K27me3 accompanied by a moderate decrease in this mark, but no loss of DNA methylation at reactivated heterochromatic loci. Instead, heterochromatin exhibits DNA hypermethylation and increased H3K27me3 in rtel1. We further find that loss of RTEL1 suppresses the release of heterochromatin silencing caused by the absence of the MOM1 silencing factor. RTEL1 is conserved among eukaryotes and plays a key role in resolving DNA secondary structures during DNA replication. Inducing such aberrant DNA structures using DNA cross-linking agents also results in a loss of transcriptional silencing. These findings uncover unappreciated roles for RTEL1 in transcriptional silencing and in stabilizing DNA methylation and H3K27me3 patterns.

Distinctive and complementary roles of E2F transcription factors during plant replication stress responses.

Survival of living organisms is fully dependent on their maintenance of genome integrity, being permanently threatened by replication stress in proliferating cells. Although the plant DNA damage response (DDR) regulator SOG1 has been demonstrated to cope with replication defects, accumulating evidence points to other pathways functioning independent of SOG1. Here, we report the roles of the Arabidopsis E2FA and EF2B transcription factors, two well-characterized regulators of DNA replication, in plant response to replication stress. Through a combination of reverse genetics and chromatin immunoprecipitation approaches, we show that E2FA and E2FB share many target genes with SOG1, providing evidence for their involvement in the DDR. Analysis of double- and triple-mutant combinations revealed that E2FB, rather than E2FA, plays the most prominent role in sustaining plant growth in the presence of replication defects, either operating antagonistically or synergistically with SOG1. Conversely, SOG1 aids in overcoming the replication defects of E2FA/E2FB-deficient plants. Collectively, our data reveal a complex transcriptional network controlling the replication stress response in which E2Fs and SOG1 act as key regulatory factors.

Arabidopsis Topless-related 1 mitigates physiological damage and growth penalties of induced immunity.

Transcriptional corepressors of the Topless (TPL) family regulate plant hormone and immunity signaling. The lack of a genome-wide profile of their chromatin associations limits understanding of the TPL family roles in transcriptional regulation. Chromatin immunoprecipitation with sequencing (ChIP-Seq) was performed on Arabidopsis thaliana lines expressing GFP-tagged Topless-related 1 (TPR1-GFP) with and without constitutive immunity via Enhanced Disease Susceptibility 1 (EDS1). RNA-Seq profiling of the TPR1-GFP lines and pathogen-infected tpl/tpr mutants, combined with measuring immunity, growth, and physiological parameters was employed to investigate TPL/TPR roles in immunity and defense homeostasis. TPR1 was enriched at promoter regions of c. 1400 genes and c. 10% of the detected binding required EDS1 immunity signaling. In a tpr1 tpl tpr4 (t3) mutant, resistance to bacteria was slightly compromised, and defense-related transcriptional reprogramming was weakly reduced or enhanced, respectively, at early (< 1 h) and late 24 h stages of bacterial infection. The t3 plants challenged with bacteria or pathogen-associated molecular pattern nlp24 displayed photosystem II dysfunctions. Also, t3 plants were hypersensitive to phytocytokine pep1 at the level of root growth inhibition. Transgenic expression of TPR1 rescued these t3 physiological defects. We propose that TPR1 and TPL family proteins function in Arabidopsis to reduce detrimental effects associated with activated transcriptional immunity.

HSFA1a modulates plant heat stress responses and alters the 3D chromatin organization of enhancer-promoter interactions.

The complex and dynamic three-dimensional organization of chromatin within the nucleus makes understanding the control of gene expression challenging, but also opens up possible ways to epigenetically modulate gene expression. Because plants are sessile, they evolved sophisticated ways to rapidly modulate gene expression in response to environmental stress, that are thought to be coordinated by changes in chromatin conformation to mediate specific cellular and physiological responses. However, to what extent and how stress induces dynamic changes in chromatin reorganization remains poorly understood. Here, we comprehensively investigated genome-wide chromatin changes associated with transcriptional reprogramming response to heat stress in tomato. Our data show that heat stress induces rapid changes in chromatin architecture, leading to the transient formation of promoter-enhancer contacts, likely driving the expression of heat-stress responsive genes. Furthermore, we demonstrate that chromatin spatial reorganization requires HSFA1a, a transcription factor (TF) essential for heat stress tolerance in tomato. In light of our findings, we propose that TFs play a key role in controlling dynamic transcriptional responses through 3D reconfiguration of promoter-enhancer contacts.

Predictable and stable epimutations induced during clonal plant propagation with embryonic transcription factor.

Clonal propagation is frequently used in commercial plant breeding and biotechnology programs because it minimizes genetic variation, yet it is not uncommon to observe clonal plants with stable phenotypic changes, a phenomenon known as somaclonal variation. Several studies have linked epigenetic modifications induced during regeneration with this newly acquired phenotypic variation. However, the factors that determine the extent of somaclonal variation and the molecular changes underpinning this process remain poorly understood. To address this gap in our knowledge, we compared clonally propagated Arabidopsis thaliana plants derived from somatic embryogenesis using two different embryonic transcription factors- RWP-RK DOMAIN-CONTAINING 4 (RKD4) or LEAFY COTYLEDON2 (LEC2) and from two epigenetically distinct founder tissues. We found that both the epi(genetic) status of the explant and the regeneration protocol employed play critical roles in shaping the molecular and phenotypic landscape of clonal plants. Phenotypic variation in regenerated plants can be largely explained by the inheritance of tissue-specific DNA methylation imprints, which are associated with specific transcriptional and metabolic changes in sexual progeny of clonal plants. For instance, regenerants were particularly affected by the inheritance of root-specific epigenetic imprints, which were associated with an increased accumulation of salicylic acid in leaves and accelerated plant senescence. Collectively, our data reveal specific pathways underpinning the phenotypic and molecular variation that arise and accumulate in clonal plant populations.

<b>The control of carpel determinacy pathway leads to sex determination</b> <b>in cucurbits</b>.

Male and female unisexual flowers evolved from hermaphroditic ancestors, and control of flower sex is useful for plant breeding. We isolated a female-to-male sex transition mutant in melon and identified the causal gene as the carpel identity gene <i>CRABS CLAW (CRC)</i>. We show that the master regulator of sex determination in cucurbits, the transcription factor <i>WIP1</i> whose expression orchestrates male flower development, recruits the corepressor TOPLESS to the <i>CRC</i> promoter to suppress its expression through histone deacetylation. Impairing TOPLESS-WIP1 physical interaction leads to <i>CRC</i> expression, carpel determination, and consequently the expression of the stamina inhibitor, the aminocyclopropane-1-carboxylic acid synthase 7 (<i>CmACS7</i>), leading to female flower development. Our findings suggest that sex genes evolved to interfere with flower meristematic function, leading to unisexual flower development.

Folding Features and Dynamics of 3D Genome Architecture in Plant Fungal Pathogens.

The folding and dynamics of three-dimensional (3D) genome organization are fundamental for eukaryotes executing genome functions but have been largely unexplored in nonmodel fungi. Using high-throughput sequencing coupled with chromosome conformation capture (Hi-C) data, we generated two chromosome-level assemblies for Puccinia striiformis f. sp. tritici, a fungus causing stripe rust disease on wheat, for studying 3D genome architectures of plant pathogenic fungi. The chromatin organization of the fungus followed a combination of the fractal globule model and the equilibrium globule model. Surprisingly, chromosome compartmentalization was not detected. Dynamics of 3D genome organization during two developmental stages of P. striiformis f. sp. tritici indicated that regulation of gene activities might be independent of the changes of genome organization. In addition, chromatin conformation conservation was found to be independent of genome sequence synteny conservation among different fungi. These results highlighted the distinct folding principles of fungal 3D genomes. Our findings should be an important step toward a holistic understanding of the principles and functions of genome architecture across different eukaryotic kingdoms. IMPORTANCE Previously, our understanding of 3D genome architecture has mainly come from model mammals, insects, and plants. However, the organization and regulatory functions of 3D genomes in fungi are largely unknown. In this study, we comprehensively investigated P. striiformis f. sp. tritici, a plant fungal pathogen, and revealed distinct features of the 3D genome, comparing it with the universal folding feature of 3D genomes in higher eukaryotic organisms. We further suggested that there might be distinct regulatory mechanisms of gene expression that are independent of chromatin organization changes during the developmental stages of this rust fungus. Moreover, we showed that the evolutionary pattern of 3D genomes in this fungus is also different from the cases in mammalian genomes. In addition, the genome assembly pipeline and the generated two chromosome-level genomes will be valuable resources. These results highlighted the unexplored distinct features of 3D genome organization in fungi. Therefore, our study provided complementary knowledge to holistically understand the organization and functions of 3D genomes across different eukaryotes.

The Arabidopsis APOLO and human UPAT sequence-unrelated long noncoding RNAs can modulate DNA and histone methylation machineries in plants.

BACKGROUND: RNA-DNA hybrid (R-loop)-associated long noncoding RNAs (lncRNAs), including the Arabidopsis lncRNA AUXIN-REGULATED PROMOTER LOOP (APOLO), are emerging as important regulators of three-dimensional chromatin conformation and gene transcriptional activity. RESULTS: Here, we show that in addition to the PRC1-component LIKE HETEROCHROMATIN PROTEIN 1 (LHP1), APOLO interacts with the methylcytosine-binding protein VARIANT IN METHYLATION 1 (VIM1), a conserved homolog of the mammalian DNA methylation regulator UBIQUITIN-LIKE CONTAINING PHD AND RING FINGER DOMAINS 1 (UHRF1). The APOLO-VIM1-LHP1 complex directly regulates the transcription of the auxin biosynthesis gene YUCCA2 by dynamically determining DNA methylation and H3K27me3 deposition over its promoter during the plant thermomorphogenic response. Strikingly, we demonstrate that the lncRNA UHRF1 Protein Associated Transcript (UPAT), a direct interactor of UHRF1 in humans, can be recognized by VIM1 and LHP1 in plant cells, despite the lack of sequence homology between UPAT and APOLO. In addition, we show that increased levels of APOLO or UPAT hamper VIM1 and LHP1 binding to YUCCA2 promoter and globally alter the Arabidopsis transcriptome in a similar manner. CONCLUSIONS: Collectively, our results uncover a new mechanism in which a plant lncRNA coordinates Polycomb action and DNA methylation through the interaction with VIM1, and indicates that evolutionary unrelated lncRNAs with potentially conserved structures may exert similar functions by interacting with homolog partners.

Utility of Triti-Map for bulk-segregated mapping of causal genes and regulatory elements in Triticeae.

Triticeae species, including wheat, barley, and rye, are critical for global food security. Mapping agronomically important genes is crucial for elucidating molecular mechanisms and improving crops. However, Triticeae includes many wild relatives with desirable agronomic traits, and frequent introgressions occurred during Triticeae evolution and domestication. Thus, Triticeae genomes are generally large and complex, making the localization of genes or functional elements that control agronomic traits challenging. Here, we developed Triti-Map, which contains a suite of user-friendly computational packages specifically designed and optimized to overcome the obstacles of gene mapping in Triticeae, as well as a web interface integrating multi-omics data from Triticeae for the efficient mining of genes or functional elements that control particular traits. The Triti-Map pipeline accepts both DNA and RNA bulk-segregated sequencing data as well as traditional QTL data as inputs for locating genes and elucidating their functions. We illustrate the usage of Triti-Map with a combination of bulk-segregated ChIP-seq data to detect a wheat disease-resistance gene with its promoter sequence that is absent from the reference genome and clarify its evolutionary process. We hope that Triti-Map will facilitate gene isolation and accelerate Triticeae breeding.

A hierarchical transcriptional network activates specific CDK inhibitors that regulate G2 to control cell size and number in Arabidopsis.

How cell size and number are determined during organ development remains a fundamental question in cell biology. Here, we identified a GRAS family transcription factor, called SCARECROW-LIKE28 (SCL28), with a critical role in determining cell size in Arabidopsis. SCL28 is part of a transcriptional regulatory network downstream of the central MYB3Rs that regulate G2 to M phase cell cycle transition. We show that SCL28 forms a dimer with the AP2-type transcription factor, AtSMOS1, which defines the specificity for promoter binding and directly activates transcription of a specific set of SIAMESE-RELATED (SMR) family genes, encoding plant-specific inhibitors of cyclin-dependent kinases and thus inhibiting cell cycle progression at G2 and promoting the onset of endoreplication. Through this dose-dependent regulation of SMR transcription, SCL28 quantitatively sets the balance between cell size and number without dramatically changing final organ size. We propose that this hierarchical transcriptional network constitutes a cell cycle regulatory mechanism that allows to adjust cell size and number to attain robust organ growth.

The lncRNA MARS modulates the epigenetic reprogramming of the marneral cluster in response to ABA.

Clustered organization of biosynthetic non-homologous genes is emerging as a characteristic feature of plant genomes. The co-regulation of clustered genes seems to largely depend on epigenetic reprogramming and three-dimensional chromatin conformation. In this study, we identified the long non-coding RNA (lncRNA) MARneral Silencing (MARS), localized inside the Arabidopsis marneral cluster, which controls the local epigenetic activation of its surrounding region in response to abscisic acid (ABA). MARS modulates the POLYCOMB REPRESSIVE COMPLEX 1 (PRC1) component LIKE HETEROCHROMATIN PROTEIN 1 (LHP1) binding throughout the cluster in a dose-dependent manner, determining H3K27me3 deposition and chromatin condensation. In response to ABA, MARS decoys LHP1 away from the cluster and promotes the formation of a chromatin loop bringing together the MARNERAL SYNTHASE 1 (MRN1) locus and a distal ABA-responsive enhancer. The enrichment of co-regulated lncRNAs in clustered metabolic genes in Arabidopsis suggests that the acquisition of novel non-coding transcriptional units may constitute an additional regulatory layer driving the evolution of biosynthetic pathways.