Interplay between coding and non-coding regulation drives the Arabidopsis seed-to-seedling transition.

Translation of seed stored mRNAs is essential to trigger germination. However, when RNAPII re-engages RNA synthesis during the seed-to-seedling transition has remained in question. Combining csRNA-seq, ATAC-seq and smFISH in Arabidopsis thaliana we demonstrate that active transcription initiation is detectable during the entire germination process. Features of non-coding regulation such as dynamic changes in chromatin accessible regions, antisense transcription, as well as bidirectional non-coding promoters are widespread throughout the Arabidopsis genome. We show that sensitivity to exogenous ABSCISIC ACID (ABA) during germination depends on proximal promoter accessibility at ABA-responsive genes. Moreover, we provide genetic validation of the existence of divergent transcription in plants. Our results reveal that active enhancer elements are transcribed producing non-coding enhancer RNAs (eRNAs) as widely documented in metazoans. In sum, this study defining the extent and role of coding and non-coding transcription during key stages of germination expands our understanding of transcriptional mechanisms underlying plant developmental transitions.

Causal role of a promoter polymorphism in natural variation of the Arabidopsis floral repressor gene FLC.

Noncoding polymorphism frequently associates with phenotypic variation, but causation and mechanism are rarely established. Noncoding single-nucleotide polymorphisms (SNPs) characterize the major haplotypes of the Arabidopsis thaliana floral repressor gene FLOWERING LOCUS C (FLC). This noncoding polymorphism generates a range of FLC expression levels, determining the requirement for and the response to winter cold. The major adaptive determinant of these FLC haplotypes was shown to be the autumnal levels of FLC expression. Here, we investigate how noncoding SNPs influence FLC transcriptional output. We identify an upstream transcription start site (uTSS) cluster at FLC, whose usage is increased by an A variant at the promoter SNP-230. This variant is present in relatively few Arabidopsis accessions, with the majority containing G at this site. We demonstrate a causal role for the A variant at -230 in reduced FLC transcriptional output. The G variant upregulates FLC expression redundantly with the major transcriptional activator FRIGIDA (FRI). We demonstrate an additive interaction of SNP-230 with an intronic SNP+259, which also differentially influences uTSS usage. Combinatorial interactions between noncoding SNPs and transcriptional activators thus generate quantitative variation in FLC transcription that has facilitated the adaptation of Arabidopsis accessions to distinct climates.

Is winter coming? Impact of the changing climate on plant responses to cold temperature.

Climate change is causing alterations in annual temperature regimes worldwide. Important aspects of this include the reduction of winter chilling temperatures as well as the occurrence of unpredicted frosts, both significantly affecting plant growth and yields. Recent studies advanced the knowledge of the mechanisms underlying cold responses and tolerance in the model plant Arabidopsis thaliana. However, how these cold-responsive pathways will readjust to ongoing seasonal temperature variation caused by global warming remains an open question. In this review, we highlight the plant developmental programmes that depend on cold temperature. We focus on the molecular mechanisms that plants have evolved to adjust their development and stress responses upon exposure to cold. Covering both genetic and epigenetic aspects, we present the latest insights into how alternative splicing, noncoding RNAs and the formation of biomolecular condensates play key roles in the regulation of cold responses. We conclude by commenting on attractive targets to accelerate the breeding of increased cold tolerance, bringing up biotechnological tools that might assist in overcoming current limitations. Our aim is to guide the reflection on the current agricultural challenges imposed by a changing climate and to provide useful information for improving plant resilience to unpredictable cold regimes.

Mechanisms of epigenetic regulation of transcription by lncRNAs in plants.

Long noncoding RNAs (lncRNAs) are an ubiquitous feature of eukaryotic genomes and in recent decades have been shown to be highly abundant and varied. Many prominent examples have been described as having essential roles in regulating the expression of genes in different developmental and environmental contexts. As a result, much work has been done on elucidating the mechanisms by which they modulate the expression of protein coding genes. In this review, we focus on those which have been characterized in plants. We specifically examine common epigenetic mechanisms that regulate gene expression at the level of transcription. In this regard, we focus on the lncRNAs in plants that have primarily been associated with controlling the chromatin environment of genes at the level of modifications, RNA POLYMERASE II (RNAPII) processivity and efficiency of transcription, and mediating the formation of transcriptionally activating and repressive chromatin loops. We discuss open questions in plant lncRNA epigenetic regulation and opportunities for future study of functionally significant lncRNAs with yet-unknown epigenetic mechanisms.

VAL1 acts as an assembly platform co-ordinating co-transcriptional repression and chromatin regulation at Arabidopsis FLC.

Polycomb (PcG) silencing is crucial for development, but how targets are specified remains incompletely understood. The cold-induced Polycomb Repressive Complex 2 (PRC2) silencing of Arabidopsis thaliana FLOWERING LOCUS C (FLC) provides an excellent system to elucidate PcG regulation. Association of the DNA binding protein VAL1 to FLC PcG nucleation regionis an important step. VAL1 co-immunoprecipitates APOPTOSIS AND SPLICING ASSOCIATED PROTEIN (ASAP) complex and PRC1. Here, we show that ASAP and PRC1 are necessary for co-transcriptional repression and chromatin regulation at FLC. ASAP mutants affect FLC transcription in warm conditions, but the rate of FLC silencing in the cold is unaffected. PRC1-mediated H2Aub accumulation increases at the FLC nucleation region during cold, but unlike the PRC2-delivered H3K27me3, does not spread across the locus. H2Aub thus involved in the transition to epigenetic silencing at FLC, facilitating H3K27me3 accumulation and long-term epigenetic memory. Overall, our work highlights the importance of VAL1 as an assembly platform co-ordinating activities necessary for epigenetic silencing at FLC.

Unique and contrasting effects of light and temperature cues on plant transcriptional programs.

Plants have adapted to tolerate and survive constantly changing environmental conditions by reprogramming gene expression in response to stress or to drive developmental transitions. Among the many signals that plants perceive, light and temperature are of particular interest due to their intensely fluctuating nature which is combined with a long-term seasonal trend. Whereas specific receptors are key in the light-sensing mechanism, the identity of plant thermosensors for high and low temperatures remains far from fully addressed. This review aims at discussing common as well as divergent characteristics of gene expression regulation in plants, controlled by light and temperature. Light and temperature signaling control the abundance of specific transcription factors, as well as the dynamics of co-transcriptional processes such as RNA polymerase elongation rate and alternative splicing patterns. Additionally, sensing both types of cues modulates gene expression by altering the chromatin landscape and through the induction of long non-coding RNAs (lncRNAs). However, while light sensing is channeled through dedicated receptors, temperature can broadly affect chemical reactions inside plant cells. Thus, direct thermal modifications of the transcriptional machinery add another level of complexity to plant transcriptional regulation. Besides the rapid transcriptome changes that follow perception of environmental signals, plant developmental transitions and acquisition of stress tolerance depend on long-term maintenance of transcriptional states (active or silenced genes). Thus, the rapid transcriptional response to the signal (Phase I) can be distinguished from the long-term memory of the acquired transcriptional state (Phase II - remembering the signal). In this review we discuss recent advances in light and temperature signal perception, integration and memory in Arabidopsis thaliana, focusing on transcriptional regulation and highlighting the contrasting and unique features of each type of cue in the process.

Noncoding SNPs influence a distinct phase of Polycomb silencing to destabilize long-term epigenetic memory at Arabidopsis FLC.

In Arabidopsis thaliana, the cold-induced epigenetic regulation of FLOWERING LOCUS C (FLC) involves distinct phases of Polycomb repressive complex 2 (PRC2) silencing. During cold, a PHD-PRC2 complex metastably and digitally nucleates H3K27me3 within FLC On return to warm, PHD-PRC2 spreads across the locus delivering H3K27me3 to maintain long-term silencing. Here, we studied natural variation in this process in Arabidopsis accessions, exploring Lov-1, which shows FLC reactivation on return to warm, a feature characteristic of FLC in perennial Brassicaceae This analysis identifies an additional phase in this Polycomb silencing mechanism downstream from H3K27me3 spreading. In this long-term silencing (perpetuated) phase, the PHD proteins are lost from the nucleation region and silencing is likely maintained by the read-write feedbacks associated with H3K27me3. A combination of noncoding SNPs in the nucleation region mediates instability in this long-term silencing phase with the result that Lov-1 FLC frequently digitally reactivates in individual cells, with a probability that diminishes with increasing cold duration. We propose that this decrease in reactivation probability is due to reduced DNA replication after flowering. Overall, this work defines an additional phase in the Polycomb mechanism instrumental in natural variation of silencing, and provides avenues to dissect broader evolutionary changes at FLC.

Temperature Sensing Is Distributed throughout the Regulatory Network that Controls FLC Epigenetic Silencing in Vernalization.

Many organisms need to respond to complex, noisy environmental signals for developmental decision making. Here, we dissect how Arabidopsis plants integrate widely fluctuating field temperatures over month-long timescales to progressively upregulate VERNALIZATION INSENSITIVE3 (VIN3) and silence FLOWERING LOCUS C (FLC), aligning flowering with spring. We develop a mathematical model for vernalization that operates on multiple timescales-long term (month), short term (day), and current (hour)-and is constrained by experimental data. Our analysis demonstrates that temperature sensing is not localized to specific nodes within the FLC network. Instead, temperature sensing is broadly distributed, with each thermosensory process responding to specific features of the plants' history of exposure to warm and cold. The model accurately predicts FLC silencing in new field data, allowing us to forecast FLC expression in changing climates. We suggest that distributed thermosensing may be a general property of thermoresponsive regulatory networks in complex natural environments.

Light and temperature cues: multitasking receptors and transcriptional integrators.

Contents Summary 1029 I. Introduction 1029 II. Convergence at the receptor 1030 III. Convergence at transcriptional hubs 1031 IV. Convergence involving clock components 1033 V. Conclusions 1033 Acknowledgements 1033 References 1033 SUMMARY: The combined information provided by light and temperature cues helps to optimise plant body architecture and physiology. Plants possess elaborate systems to sense and respond to these stimuli. Simultaneous perception of light and temperature by dual receptors such as phytochrome B and phototropin leads to immediate signalling convergence. Conversely, cue asynchronies initiate separate pathways and the information of the earliest cue is stored, awaiting the arrival of the later cue to control transcription. Storage mechanisms can involve changes in the activity of selected clock components or epigenetic modifications, depending on the time delay between cues (hours, days or several months). We propose a conceptual framework in which the mechanisms of integration relate to the timing of cue sensing.

Arabidopsis transcriptional repressor VAL1 triggers Polycomb silencing at FLC during vernalization.

The determinants that specify the genomic targets of Polycomb silencing complexes are still unclear. Polycomb silencing of Arabidopsis FLOWERING LOCUS C (FLC) accelerates flowering and involves a cold-dependent epigenetic switch. Here we identify a single point mutation at an intragenic nucleation site within FLC that prevents this epigenetic switch from taking place. The mutation blocks nucleation of plant homeodomain-Polycomb repressive complex 2 (PHD-PRC2) and indicates a role for the transcriptional repressor VAL1 in the silencing mechanism. VAL1 localizes to the nucleation region in vivo, promoting histone deacetylation and FLC transcriptional silencing, and interacts with components of the conserved apoptosis- and splicing-associated protein (ASAP) complex. Sequence-specific targeting of transcriptional repressors thus recruits the machinery for PHD-PRC2 nucleation and epigenetic silencing.

ZmMBD101 is a DNA-binding protein that maintains Mutator elements chromatin in a repressive state in maize.

In maize (Zea mays), as well as in other crops, transposable elements (TEs) constitute a great proportion of the genome. Chromatin modifications play a vital role in establishing transposon silencing and perpetuating the acquired repressive state. Nucleosomes associated with TEs are enriched for dimethylation of histone H3 at lysine 9 and 27 (H3K9me2 and H3K27me2, respectively), signals of repressive chromatin. Here, we describe a chromatin protein, ZmMBD101, involved in the regulation of Mutator (Mu) genes in maize. ZmMBD101 is localized to the nucleus and contains a methyl-CpG-binding domain (MBD) and a zinc finger CW (CW) domain. Transgenic lines with reduced levels of ZmMBD101 transcript present enhanced induction of Mu genes when plants are irradiated with UV-B. Chromatin immunoprecipitation analysis with H3K9me2 and H3K27me2 antibodies indicated that ZmMBD101 is required to maintain the levels of these histone repressive marks at Mu terminal inverted repeats (TIRs) under UV-B conditions. Although Mutator inactivity is associated with DNA methylation, cytosine methylation at Mu TIRs is not affected in ZmMBD101 deficient plants. Several plant proteins are predicted to share the simple CW-MBD domain architecture present in ZmMBD101. We hypothesize that plant CW-MBD proteins may also function to protect plant genomes from deleterious transposition.

Seasonal shift in timing of vernalization as an adaptation to extreme winter.

The requirement for vernalization, a need for prolonged cold to trigger flowering, aligns reproductive development with favorable spring conditions. In Arabidopsis thaliana vernalization depends on the cold-induced epigenetic silencing of the floral repressor locus FLC. Extensive natural variation in vernalization response is associated with A. thaliana accessions collected from different geographical regions. Here, we analyse natural variation for vernalization temperature requirement in accessions, including those from the northern limit of the A. thaliana range. Vernalization required temperatures above 0°C and was still relatively effective at 14°C in all the accessions. The different accessions had characteristic vernalization temperature profiles. One Northern Swedish accession showed maximum vernalization at 8°C, both at the level of flowering time and FLC chromatin silencing. Historical temperature records predicted all accessions would vernalize in autumn in N. Sweden, a prediction we validated in field transplantation experiments. The vernalization response of the different accessions was monitored over three intervals in the field and found to match that when the average field temperature was given as a constant condition. The vernalization temperature range of 0-14°C meant all accessions fully vernalized before snowfall in N. Sweden. These findings have important implications for understanding the molecular basis of adaptation and for predicting the consequences of climate change on flowering time.

Vernalizing cold is registered digitally at FLC.

A fundamental property of many organisms is an ability to sense, evaluate, and respond to environmental signals. In some situations, generation of an appropriate response requires long-term information storage. A classic example is vernalization, where plants quantitatively sense long-term cold and epigenetically store this cold-exposure information to regulate flowering time. In Arabidopsis thaliana, stable epigenetic memory of cold is digital: following long-term cold exposure, cells respond autonomously in an all-or-nothing fashion, with the fraction of cells that stably silence the floral repressor flowering locus C (FLC) increasing with the cold exposure duration. However, during cold exposure itself it is unknown whether vernalizing cold is registered at FLC in individual cells in an all-or-nothing (digital) manner or is continuously varying (analog). Using mathematical modeling, we found that analog registration of cold temperature is problematic due to impaired analog-to-digital conversion into stable memory. This disadvantage is particularly acute when responding to short cold periods, but is absent when cold temperatures are registered digitally at FLC. We tested this prediction experimentally, exposing plants to short periods of cold interrupted with even shorter warm breaks. For FLC expression, we found that the system responds similarly to both interrupted and uninterrupted cold, arguing for a digital mechanism integrating long-term temperature exposure.

Antisense COOLAIR mediates the coordinated switching of chromatin states at FLC during vernalization.

Long noncoding RNAs (lncRNAs) have been proposed to play important roles in gene regulation. However, their importance in epigenetic silencing and how specificity is determined remain controversial. We have investigated the cold-induced epigenetic switching mechanism involved in the silencing of Arabidopsis thaliana Flowering Locus C (FLC), which occurs during vernalization. Antisense transcripts, collectively named COOLAIR, are induced by prolonged cold before the major accumulation of histone 3 lysine 27 trimethylation (H3K27me3), characteristic of Polycomb silencing. We have found that COOLAIR is physically associated with the FLC locus and accelerates transcriptional shutdown of FLC during cold exposure. Removal of COOLAIR disrupted the synchronized replacement of H3K36 methylation with H3K27me3 at the intragenic FLC nucleation site during the cold. Consistently, genetic analysis showed COOLAIR and Polycomb complexes work independently in the cold-dependent silencing of FLC. Our data reveal a role for lncRNA in the coordinated switching of chromatin states that occurs during epigenetic regulation.

DDM1 and ROS1 have a role in UV-B induced- and oxidative DNA damage in A. thaliana.

Absorption of UV-B by DNA induces the formation of covalent bonds between adjacent pyrimidines. In maize and arabidopsis, plants deficient in chromatin remodeling show increased DNA damage compared to WT plants after a UV-B treatment. However, the role of enzymes that participate in DNA methylation in DNA repair after UV-B damage was not previously investigated. In this work, we analyzed how chromatin remodeling activities that have an effect on DNA methylation affects the repair of UV-B damaged DNA using plants deficient in the expression of DDM1 and ROS1. First, we analyzed their regulation by UV-B radiation in arabidopsis plants. Then, we demonstrated that ddm1 mutants accumulated more DNA damage after UV-B exposure compared to Col0 plants. Surprisingly, ros1 mutants show less CPDs and 6-4PPs than WT plants after the treatment under light conditions, while the repair under dark conditions is impaired. Transcripts for two photolyases are highly induced by UV-B in ros1 mutants, suggesting that the lower accumulation of photoproducts by UV-B is due to increased photorepair in these mutants. Finally, we demonstrate that oxidative DNA damage does not occur after UV-B exposure in arabidopsis plants; however, ros1 plants accumulate high levels of oxoproducts, while ddm1 mutants have less oxoproducts than Col0 plants, suggesting that both ROS1 and DDM1 have a role in the repair of oxidative DNA damage. Together, our data provide evidence that both DDM1 and ROS1, directly or indirectly, participate in UV-B induced- and oxidative DNA damage repair.

Using MuDR/Mu transposons in directed tagging strategies.

An introduction to MuDR/Mu transposons as mutagens is provided along with protocols for using these elements to tag maize genes. Selection for retention of Mutator activity is described as well as details for establishing and screening tagging populations efficiently.

Evolution and expression of tandem duplicated maize flavonol synthase genes.

Flavonoids are specialized compounds widely distributed and with diverse functions throughout the plant kingdom and with several benefits for human health. In particular, flavonols, synthesized by flavonol synthase (FLS), protect plants against UV-B radiation and are essential for male fertility in maize and other plants. We have recently characterized a UV-B inducible ZmFLS1, corresponding to the first to be described in monocot plants. Interestingly, the new assembly of the B73 maize genome revealed the presence of a second putative FLS gene (ZmFLS2), with very high identity with ZmFLS1. ZmFLSs expression was analyzed in different maize tissues, and by combining electrophoretic mobility shift assays and transient expression experiments, we show that both genes are direct targets of anthocyanin (C1/PL1 + R/B) and 3-deoxy flavonoid (P1) transcriptional regulators. ZmFLS expression analyses show higher levels of both transcripts in high altitude landraces than inbred lines, and both genes are regulated by UV-B radiation in all lines analyzed. Moreover, the high sequence conservation of the ZmFLS promoters between maize lines suggests that the differences observed in ZmFLS expression are due to allelic variations in the transcription factors that regulate their activities. Finally, we generated pFLS1::FLS1-RFP transgenic plants and analyzed ZmFLS1 expression in different maize tissues; we found that this enzyme is localized in the ER and the perinuclear region.

Mutator transposon activation after UV-B involves chromatin remodeling.

Spontaneous silencing of MuDR/Mu transposons occurs in approximately 10-100% of the progeny of an active plant, and once silenced reactivation is very rare. To date, only radiation treatments have reactivated silenced Mu; for example UV-B radiation reactivated Mutator activities. Here we have investigated possible mechanisms by which UV-B could reactivate Mu transposons by monitoring transcript abundance, epigenetic DNA marks, and chromatin factors associated with these elements. We demonstrate that both mudrA and B transcripts are expressed at higher levels after an 8 h-UV-B treatment, in both active Mutator and silencing plants, and that different chromatin remodeling events occur in the promoter regions of MuDR than in non-autonomous Mu1 elements. Increased transcript abundance is accompanied by an increase in histone H3 acetylation and by decreased DNA and H3K9me2 methylation. No changes in siRNA levels were detected. In contrast, the decrease in H3K9me2 present at Mu elements after UV-B is significant in silencing plants, suggesting that early changes in H3 methylation in K9, chromatin remodeling, and transcription factor binding contribute directly to transposon reactivation by UV-B in maize.

Biofilm formation, epiphytic fitness, and canker development in Xanthomonas axonopodis pv. citri.

The phytopathogenic bacterium Xanthomonas axonopodis pv. citri is responsible for the canker disease affecting citrus plants throughout the world. Here, we have evaluated the role of bacterial attachment and biofilm formation in leaf colonization during canker development on lemon leaves. Crystal violet staining and confocal laser scanning microscopy analysis of X. axonopodis pv. citri strains expressing the green fluorescent protein were used to evaluate attachment and biofilm formation on abiotic and biotic (leaf) surfaces. Wild-type X. axonopodis pv. citri attached to and formed a complex, structured biofilm on glass in minimal medium containing glucose. Similar attachment and structured biofilm formation also were seen on lemon leaves. An X. axonopodis pv. citri gumB mutant strain, defective in production of the extracellular polysaccharide xanthan, did not form a structured biofilm on either abiotic or biotic surfaces. In addition, the X. axonopodis pv. citri gumB showed reduced growth and survival on leaf surfaces and reduced disease symptoms. These findings suggest an important role for formation of biofilms in the epiphytic survival of X. axonopodis pv. citri prior to development of canker disease.