Noncoding transcription by alternative RNA polymerases dynamically regulates an auxin-driven chromatin loop

Stress-responsive regulation of long non-coding RNA polyadenylation in Oryza sativa

Recently, long non-coding RNAs (lncRNAs) have been demonstrated to be involved in many biological processes of plants; however, a systematic study on transcriptional and, in particular, post-transcriptional regulation of stress-responsive lncRNAs in Oryza sativa (rice) is lacking. We sequenced three types of RNA libraries (poly(A)+, poly(A)- and nuclear RNAs) under four abiotic stresses (cold, heat, drought and salt). Based on an integrative bioinformatics approach and ~200 high-throughput data sets, ~170 of which have been published, we revealed over 7000 lncRNAs, nearly half of which were identified for the first time. Notably, we found that the majority of the ~500 poly(A) lncRNAs that were differentially expressed under stress were significantly downregulated, but approximately 25% were found to have upregulated non-poly(A) forms. Moreover, hundreds of lncRNAs with downregulated polyadenylation (DPA) tend to be highly conserved, show significant nuclear retention and are co-expressed with protein-coding genes that function under stress. Remarkably, these DPA lncRNAs are significantly enriched in quantitative trait loci (QTLs) for stress tolerance or development, suggesting their potential important roles in rice growth under various stresses. In particular, we observed substantially accumulated DPA lncRNAs in plants exposed to drought and salt, which is consistent with the severe reduction of RNA 3'-end processing factors under these conditions. Taken together, the results of this study reveal that polyadenylation and subcellular localization of many rice lncRNAs are likely to be regulated at the post-transcriptional level. Our findings strongly suggest that many upregulated/downregulated lncRNAs previously identified by traditional RNA-seq analyses need to be carefully reviewed to assess the influence of post-transcriptional modification.

A new class of genic nuclear RNA species in Arabidopsis

Targeting of ArabidopsisPHABULOSA (PHB) mRNA by miR166 has been implicated in gene body methylation at the PHB locus. We report that the PHB locus produces an array of stable nuclear RNA species that are neither polyadenylated nor capped. Their biogenesis requires neither RNA polymerases IV/V nor miR166-guided cleavage. The PHB RNAs are insensitive to mutation of nuclear RNA decay pathways and are conserved in several Brassicaceae species, suggesting functional relevance. Similar RNA species are also produced by another body-methylated locus encoding the miR414 target eIF2. Our data reveal the existence of a new class of genic nuclear RNA species.

Visualizing nuclear-localized RNA using transient expression system in plants

By modifying the existing cytosolic RNA visualization tool pioneered by Schönberger, Hammes, and Dresselhaus (2012), we developed a method to visualize nuclear-localized RNA. Our method uses (i) an RNA component that consists of an RNA of interest that is fused to a bacteriophage-derived MS2 sequence; and (ii) GFP fused to MS2 coat protein (MSCP), which binds specifically to MS2 as is also the case in the method for cytosolic RNA visualization. The nuclear localization sequence (NLS) at the C-terminal of MSCP-GFP tethers the probe to the nucleus. To reduce background signals in the nucleus, we replaced the NLS with a nuclear export sequence (NES) that anchors the MSCP-GFP probe in the cytosol. Our nuclear RNA visualization method differs from previous methods in two aspects: (i) We used an NES to reduce nuclear background signal so that the MSCP-GFP probe localizes in the cytosol by default; (ii) We added mCherry as a visual marker in the RNA component to increase its efficient usage in a transient system.

RNA Immunoprecipitation Protocol to Identify Protein-RNA Interactions in Arabidopsis thaliana

The role of RNA-binding proteins in the regulation of epigenetic processes has received increasing attention in the past decades. In particular noncoding RNAs have been shown to play a role in chromatin loop formation, recruitment of chromatin modifiers and RNA-dependent DNA methylation. In plants, the identification of specific RNA-protein interactions is now rising, facilitated by the development of specific approaches for plant tissues. Here, we present a simple one-day RNA immunoprecipitation (RIP) protocol adapted for Arabidopsis, suited for the identification of RNAs that are associated with a protein-of-interest in planta.

Technical Review: A Hitchhiker's Guide to Chromosome Conformation Capture

The introduction of chromosome conformation capture (3C) technologies boosted the field of 3D-genome research and significantly enhanced the available toolset to study chromosomal architecture. 3C technologies not only offer increased resolution compared to the previously dominant cytological approaches but also allow the simultaneous study of genome-wide 3D chromatin contacts, thereby enabling a candidate-free perspective on 3D-genome architecture. Since its introduction in 2002, 3C technologies evolved rapidly and now constitute a collection of tools, each with their strengths and pitfalls with respect to specific research questions. This chapter aims at guiding 3C novices through the labyrinth of potential applications of the various family members, hopefully providing a valuable basis for choosing the appropriate strategy for different research questions.

Unraveling the Complex Epigenetic Mechanisms that Regulate Gene Activity

Our understanding of the epigenetic mechanisms that regulate gene expression has been largely increased in recent years by the development and refinement of different techniques. This has revealed that gene transcription is highly influenced by epigenetic mechanisms, i.e., those that do not involve changes in the genome sequence, but rather in nuclear architecture, chromosome conformation and histone and DNA modifications. Our understanding of how these different levels of epigenetic regulation interact with each other and with classical transcription-factor based gene regulation to influence gene transcription has just started to emerge. This review discusses the latest advances in unraveling the complex interactions between different types of epigenetic regulation and transcription factor activity, with special attention to the approaches that can be used to study these interactions.

Genome-Wide Identification and Characterization of Long Non-Coding RNA in Wheat Roots in Response to Ca2+ Channel Blocker

It remains unclear whether plant lncRNAs are responsive to Ca²⁺-channel blocking. When using the Ca²⁺-channel blocker, LaCl₃, to treat germinated wheat seeds for 24 h, we found that both root length and mitosis were inhibited in the LaCl₃-treated groups. The effect of the Ca²⁺-channel blocker was verified in three ways: a [Ca²⁺]_cyt decrease detected using Fluo-3/AM staining, a decrease in the Ca content measured using inductively coupled plasma mass spectrometry, and an inhibition of Ca²⁺ influx detected using Non-invasive Micro-test Technology. Genome-wide high throughput RNA-seq and bioinformatical methods were used to identify lncRNAs, and found 177 differentially expressed lncRNAs that might be in responsive to Ca²⁺-channel blocking. Among these, 108 were up-regulated and 69 were down-regulated. The validity of identified lncRNAs data from RNA-seq was verified using qPCR. GO and KEGG analysis indicated that a number of lncRNAs might be involved in diverse biological processes upon Ca²⁺-channel blocking. Further GO analysis showed that 23 lncRNAs might play roles as transcription factor (TF); Moreover, eight lncRNAs might participate in cell cycle regulation, and their relative expressions were detected using qPCR. This study also provides diverse data on wheat lncRNAs that can deepen our understanding of the function and regulatory mechanism of Ca²⁺-channel blocking in plants.

DNA Methylation and the Evolution of Developmental Complexity in Plants

All land plants so far examined use DNA methylation to silence transposons (TEs). DNA methylation therefore appears to have been co-opted in evolution from an original function in TE management to a developmental function (gene regulation) in both phenotypic plasticity and in normal development. The significance of DNA methylation to the evolution of developmental complexity in plants lies in its role in the management of developmental pathways. As such it is more important in fine tuning the presence, absence, and placement of organs rather than having a central role in the evolution of new organs. Nevertheless, its importance should not be underestimated as it contributes considerably to the range of phenotypic expression and complexity available to plants: the subject of the emerging field of epi-evodevo. Furthermore, changes in DNA methylation can function as a "soft" mutation that may be important in the early stages of major evolutionary novelty.

Active DNA demethylation: mechanism and role in plant development

Active DNA demethylation (enzymatic removal of methylated cytosine) regulates many plant developmental processes. In Arabidopsis, active DNA demethylation entails the base excision repair pathway initiated by the Repressor of silencing 1/Demeter family of bifunctional DNA glycosylases. In this review, we first present an introduction to the recent advances in our understanding about the mechanisms of active DNA demethylation. We then focus on the role of active DNA demethylation in diverse developmental processes in various plant species, including the regulation of seed development, pollen tube formation, stomatal development, fruit ripening, and nodule development. Finally, we discuss future directions of research in the area of active DNA demethylation.

Disruption of an RNA-binding hinge region abolishes LHP1-mediated epigenetic repression

In this study, Berry et al. investigated the functions of the different domains of LIKE HETEROCHROMATIN PROTEIN 1 (LHP1) in Arabidopsis. They show that LHP1 binds RNA in vitro through the intrinsically disordered hinge region and show that both the hinge region and H3K27me3 recognition facilitate LHP1 localization and H3K27me3 maintenance.

Epigenetic maintenance of gene repression is essential for development. Polycomb complexes are central to this memory, but many aspects of the underlying mechanism remain unclear. LIKE HETEROCHROMATIN PROTEIN 1 (LHP1) binds Polycomb-deposited H3K27me3 and is required for repression of many Polycomb target genes in Arabidopsis. Here we show that LHP1 binds RNA in vitro through the intrinsically disordered hinge region. By independently perturbing the RNA-binding hinge region and H3K27me3 (trimethylation of histone H3 at Lys27) recognition, we found that both facilitate LHP1 localization and H3K27me3 maintenance. Disruption of the RNA-binding hinge region also prevented formation of subnuclear foci, structures potentially important for epigenetic repression.

The antiphasic regulatory module comprising CDF5 and its antisense RNA FLORE links the circadian clock to photoperiodic flowering

Circadian rhythms of gene expression are generated by the combinatorial action of transcriptional and translational feedback loops as well as chromatin remodelling events. Recently, long noncoding RNAs (lncRNAs) that are natural antisense transcripts (NATs) to transcripts encoding central oscillator components were proposed as modulators of core clock function in mammals (Per) and fungi (frq/qrf). Although oscillating lncRNAs exist in plants, their functional characterization is at an initial stage. By screening an Arabidopsis thaliana lncRNA custom-made array we identified CDF5 LONG NONCODING RNA (FLORE), a circadian-regulated lncRNA that is a NAT of CDF5. Quantitative real-time RT-PCR confirmed the circadian regulation of FLORE, whereas GUS-staining and flowering time evaluation were used to determine its biological function. FLORE and CDF5 antiphasic expression reflects mutual inhibition in a similar way to frq/qrf. Moreover, whereas the CDF5 protein delays flowering by directly repressing FT transcription, FLORE promotes it by repressing several CDFs (CDF1, CDF3, CDF5) and increasing FT transcript levels, indicating both cis and trans function. We propose that the CDF5/FLORE NAT pair constitutes an additional circadian regulatory module with conserved (mutual inhibition) and unique (function in trans) features, able to fine-tune its own circadian oscillation, and consequently, adjust the onset of flowering to favourable environmental conditions.

Global analysis of ribosome-associated noncoding RNAs unveils new modes of translational regulation

Noncoding RNAs are an underexplored reservoir of regulatory molecules in eukaryotes. We analyzed the environmental response of roots to phosphorus (Pi) nutrition to understand how a change in availability of an essential element is managed. Pi availability influenced translational regulation mediated by small upstream ORFs on protein-coding mRNAs. Discovery, classification, and evaluation of long noncoding RNAs (lncRNAs) associated with translating ribosomes uncovered diverse new examples of translational regulation. These included Pi-regulated small peptide synthesis, ribosome-coupled phased small interfering RNA production, and the translational regulation of natural antisense RNAs and other regulatory RNAs. This study demonstrates that translational control contributes to the stability and activity of regulatory RNAs, providing an avenue for manipulation of traits.

Eukaryotic transcriptomes contain a major non–protein-coding component that includes precursors of small RNAs as well as long noncoding RNA (lncRNAs). Here, we utilized the mapping of ribosome footprints on RNAs to explore translational regulation of coding and noncoding RNAs in roots of Arabidopsis thaliana shifted from replete to deficient phosphorous (Pi) nutrition. Homodirectional changes in steady-state mRNA abundance and translation were observed for all but 265 annotated protein-coding genes. Of the translationally regulated mRNAs, 30% had one or more upstream ORF (uORF) that influenced the number of ribosomes on the principal protein-coding region. Nearly one-half of the 2,382 lncRNAs detected had ribosome footprints, including 56 with significantly altered translation under Pi-limited nutrition. The prediction of translated small ORFs (sORFs) by quantitation of translation termination and peptidic analysis identified lncRNAs that produce peptides, including several deeply evolutionarily conserved and significantly Pi-regulated lncRNAs. Furthermore, we discovered that natural antisense transcripts (NATs) frequently have actively translated sORFs, including five with low-Pi up-regulation that correlated with enhanced translation of the sense protein-coding mRNA. The data also confirmed translation of miRNA target mimics and lncRNAs that produce trans-acting or phased small-interfering RNA (tasiRNA/phasiRNAs). Mutational analyses of the positionally conserved sORF of TAS3a linked its translation with tasiRNA biogenesis. Altogether, this systematic analysis of ribosome-associated mRNAs and lncRNAs demonstrates that nutrient availability and translational regulation controls protein and small peptide-encoding mRNAs as well as a diverse cadre of regulatory RNAs.

Seed weight differences between wild and domesticated soybeans are associated with specific changes in gene expression

KEY MESSAGE

Our study systematically explored potential genes and molecular pathways as candidates for differences in seed weight resulting from soybean domestication. In addition, potential contributions of lncRNAs to seed weight were also investigated. Soybeans have a long history of domestication in China, and there are several significant phenotypic differences between cultivated and wild soybeans, for example, seeds of cultivars are generally larger and heavier than those from wild accessions. We analyzed seed transcriptomes from thirteen soybean samples, including six landraces and seven wild accessions using strand-specific RNA sequencing. Differentially expressed genes related to seed weight were identified, and some of their homologs were associated with seed development in Arabidopsis. We also identified 1251 long intergenic noncoding RNAs (lincRNAs), 243 intronic RNAs and 81 antisense lncRNAs de novo from these soybean transcriptomes. We then profiled the expression patterns of lncRNAs in cultivated and wild soybean seeds, and found that transcript levels of a number of lncRNAs were sample-specific. Moreover, gene transcript and lincRNA co-expression network analysis showed that some soybean lincRNAs might have functional roles as they were hubs of co-expression modules. In conclusion, this study systematically explored potential genes and molecular pathways as candidates for differences in seed weight resulting from soybean domestication, and will provide a useful future resource for molecular breeding of soybeans.

First-Passage Processes in the Genome

Many essential processes in biology share a common fundamental step-establishing physical contact between distant segments of DNA. How fast this step is accomplished sets the "speed limit" for the larger-scale processes it enables, whether the process is antibody production by the immune system or tissue differentiation in a developing embryo. This naturally leads us to ask, How long does it take for DNA segments that are strung out over millions of base pairs along the chromatin fiber to find each other in the crowded cell? This question, fundamental to biology, can be recognized as the physics problem of the first-passage time, or the waiting time for the first encounter. Here, we review a number of approaches to revealing the physical principles by which cells solve, with astonishing efficiency, the first-passage problem for remote genomic interactions.

Spatio-temporal analysis of coding and long noncoding transcripts during maize endosperm development

The maize endosperm consists of three major compartmentalized cell types: the starchy endosperm (SE), the basal endosperm transfer cell layer (BETL), and the aleurone cell layer (AL). Differential genetic programs are activated in each cell type to construct functionally and structurally distinct cells. To compare gene expression patterns involved in maize endosperm cell differentiation, we isolated transcripts from cryo-dissected endosperm specimens enriched with BETL, AL, or SE at 8, 12, and 16 days after pollination (DAP). We performed transcriptome profiling of coding and long noncoding transcripts in the three cell types during differentiation and identified clusters of the transcripts exhibiting spatio-temporal specificities. Our analysis uncovered that the BETL at 12 DAP undergoes the most dynamic transcriptional regulation for both coding and long noncoding transcripts. In addition, our transcriptome analysis revealed spatio-temporal regulatory networks of transcription factors, imprinted genes, and loci marked with histone H3 trimethylated at lysine 27. Our study suggests that various regulatory mechanisms contribute to the genetic networks specific to the functions and structures of the cell types of the endosperm.

The Arabidopsis SWI/SNF protein BAF60 mediates seedling growth control by modulating DNA accessibility

BACKGROUND

Plant adaptive responses to changing environments involve complex molecular interplays between intrinsic and external signals. Whilst much is known on the signaling components mediating diurnal, light, and temperature controls on plant development, their influence on chromatin-based transcriptional controls remains poorly explored.

RESULTS

In this study we show that a SWI/SNF chromatin remodeler subunit, BAF60, represses seedling growth by modulating DNA accessibility of hypocotyl cell size regulatory genes. BAF60 binds nucleosome-free regions of multiple G box-containing genes, opposing in cis the promoting effect of the photomorphogenic and thermomorphogenic regulator Phytochrome Interacting Factor 4 (PIF4) on hypocotyl elongation. Furthermore, BAF60 expression level is regulated in response to light and daily rhythms.

CONCLUSIONS

These results unveil a short path between a chromatin remodeler and a signaling component to fine-tune plant morphogenesis in response to environmental conditions.

DNA Topoisomerase 1 Prevents R-loop Accumulation to Modulate Auxin-Regulated Root Development in Rice

R-loop structures (RNA:DNA hybrids) have important functions in many biological processes, including transcriptional regulation and genome instability among diverse organisms. DNA topoisomerase 1 (TOP1), an essential manipulator of DNA topology during RNA transcription and DNA replication processes, can prevent R-loop accumulation by removing the positive and negative DNA supercoiling that is made by RNA polymerases during transcription. TOP1 is required for plant development, but little is known about its function in preventing co-transcriptional R-loop accumulation in various biological processes in plants. Here we show that knockdown of OsTOP1 strongly affects rice development, causing defects in root architecture and gravitropism, which are the consequences of misregulation of auxin signaling and transporter genes. We found that R-loops are naturally formed at rice auxin-related gene loci, and overaccumulate when OsTOP1 is knocked down or OsTOP1 protein activity is inhibited. OsTOP1 therefore sets the accurate expression levels of auxin-related genes by preventing the overaccumulation of inherent R-loops. Our data reveal R-loops as important factors in polar auxin transport and plant root development, and highlight that OsTOP1 functions as a key to link transcriptional R-loops with plant hormone signaling, provide new insights into transcriptional regulation of hormone signaling in plants.

Long-range control of gene expression via RNA-directed DNA methylation

RNA-mediated transcriptional silencing, in plants known as RNA-directed DNA methylation (RdDM), is a conserved process where small interfering RNA (siRNA) and long non-coding RNA (lncRNA) help establish repressive chromatin modifications. This process represses transposons and affects the expression of protein-coding genes. We found that in Arabidopsis thaliana AGO4 binding sites are often located distant from genes differentially expressed in ago4. Using Hi-C to compare interactions between genotypes, we show that RdDM-targeted loci have the potential to engage in chromosomal interactions, but these interactions are inhibited in wild-type conditions. In mutants defective in RdDM, the frequency of chromosomal interactions at RdDM targets is increased. This includes increased frequency of interactions between Pol V methylated sites and distal genes that are repressed by RdDM. We propose a model, where RdDM prevents the formation of chromosomal interactions between genes and their distant regulatory elements.

Emerging roles of long non-coding RNAs in plant response to biotic and abiotic stresses

Spectacular progress in high-throughput transcriptome sequencing and expression profiling using next-generation sequencing technologies have recently revolutionized molecular biology and allowed massive advances in identifying the genomic regions and molecular mechanisms underlying transcriptional regulation of growth, development, and stress response. Through recent research, non-coding RNAs, in particular long non-coding RNAs, have emerged as key regulators of transcription in eukaryotes. Long non-coding RNAs are vastly heterogeneous groups of RNAs that execute a broad range of essential roles in various biological processes at the epigenetic, transcriptional, and post-transcriptional levels. They modulate transcription through diverse mechanisms. Recently, numerous lncRNAs have been identified to be associated with defense responses to biotic and abiotic stresses. These have been suggested to perform indispensable roles in plant immunity and adaptation to environmental conditions. However, only a few lncRNAs have been functionally characterized in plants. In this paper, we summarize the present knowledge of lncRNAs, review the recent advances in understanding regulatory functions of lncRNAs, and highlight the emerging roles of lncRNAs in regulating immune responses in plants.

Nonrandom domain organization of the Arabidopsis genome at the nuclear periphery

The nuclear space is not a homogeneous biochemical environment. Many studies have demonstrated that the transcriptional activity of a gene is linked to its positioning within the nuclear space. Following the discovery of lamin-associated domains (LADs), which are transcriptionally repressed chromatin regions, the nonrandom positioning of chromatin at the nuclear periphery and its biological relevance have been studied extensively in animals. However, it remains unknown whether comparable chromatin organizations exist in plants. Here, using a strategy using restriction enzyme-mediated chromatin immunoprecipitation, we present genome-wide identification of nonrandom domain organization of chromatin at the peripheral zone of Arabidopsis thaliana nuclei. We show that in various tissues, 10%-20% of the regions on the chromosome arms are anchored at the nuclear periphery, and these regions largely overlap between different tissues. Unlike LADs in animals, the identified domains in plants are not gene-poor or A/T-rich. These domains are enriched with silenced protein-coding genes, transposable element genes, and heterochromatic marks, which collectively define a repressed environment. In addition, these domains strongly correlate with our genome-wide chromatin interaction data set (Hi-C) by largely explaining the patterns of chromatin compartments, revealed on Hi-C maps. Moreover, our results reveal a spatial compartment of different DNA methylation pathways that regulate silencing of transposable elements, where the CHH methylation of transposable elements located at the nuclear periphery and in the interior are preferentially mediated by CMT2 and DRM methyltransferases, respectively. Taken together, the results demonstrate functional partitioning of the Arabidopsis genome in the nuclear space.

Transposable elements (TEs) contribute to stress-related long intergenic noncoding RNAs in plants

Noncoding RNAs have been extensively described in plant and animal transcriptomes by using high-throughput sequencing technology. Of these noncoding RNAs, a growing number of long intergenic noncoding RNAs (lincRNAs) have been described in multicellular organisms, however the origins and functions of many lincRNAs remain to be explored. In many eukaryotic genomes, transposable elements (TEs) are widely distributed and often account for large fractions of plant and animal genomes yet the contribution of TEs to lincRNAs is largely unknown. By using strand-specific RNA-sequencing, we profiled the expression patterns of lincRNAs in Arabidopsis, rice and maize, and identified 47 611 and 398 TE-associated lincRNAs (TE-lincRNAs), respectively. TE-lincRNAs were more often derived from retrotransposons than DNA transposons and as retrotransposon copy number in both rice and maize genomes so did TE-lincRNAs. We validated the expression of these TE-lincRNAs by strand-specific RT-PCR and also demonstrated tissue-specific transcription and stress-induced TE-lincRNAs either after salt, abscisic acid (ABA) or cold treatments. For Arabidopsis TE-lincRNA11195, mutants had reduced sensitivity to ABA as demonstrated by longer roots and higher shoot biomass when compared to wild-type. Finally, by altering the chromatin state in the Arabidopsis chromatin remodelling mutant ddm1, unique lincRNAs including TE-lincRNAs were generated from the preceding untranscribed regions and interestingly inherited in a wild-type background in subsequent generations. Our findings not only demonstrate that TE-associated lincRNAs play important roles in plant abiotic stress responses but lincRNAs and TE-lincRNAs might act as an adaptive reservoir in eukaryotes.

Chromosome conformation capture-based studies reveal novel features of plant nuclear architecture

Nuclear genome organization has recently received increasing attention due to its manifold functions in basic nuclear processes, such as replication, transcription, and the maintenance of genome integrity. Using technologies based on chromosome conformation capture, such as Hi-C, we now have the possibility to study the three-dimensional organization of the genome at unprecedented resolution, shedding light onto a previously unexplored level of nuclear architecture. In plants, research in this field is still in its infancy but a number of publications provided first insights into basic principles of nuclear genome organization and the factors that influence it. Apart from general aspects, newly discovered three-dimensional conformations, such as the KNOT, raise special interest on how nuclear organization may influence the function of the genome in previously unexpected ways.

Vernalization-Triggered Intragenic Chromatin Loop Formation by Long Noncoding RNAs

Long noncoding RNAs (lncRNAs) affect gene regulation through structural and regulatory interactions with associated proteins. The Polycomb complex often binds to lncRNAs in eukaryotes, and an lncRNA, COLDAIR, associates with Polycomb to mediate silencing of the floral repressor FLOWERING LOCUS C (FLC) during the process of vernalization in Arabidopsis. Here, we identified an additional Polycomb-binding lncRNA, COLDWRAP. COLDWRAP is derived from the repressed promoter of FLC and is necessary for the establishment of the stable repressed state of FLC by vernalization. Both COLDAIR and COLDWRAP are required to form a repressive intragenic chromatin loop at the FLC locus by vernalization. Our results indicate that vernalization-mediated Polycomb silencing is coordinated by lncRNAs in a cooperative manner to form a stable repressive chromatin structure.

Long Noncoding RNAs in Plants

The eukaryotic genomes are pervasively transcribed. In addition to protein-coding RNAs, thousands of long noncoding RNAs (lncRNAs) modulate key molecular and biological processes. Most lncRNAs are found in the nucleus and associate with chromatin, but lncRNAs can function in both nuclear and cytoplasmic compartments. Emerging work has found that many lncRNAs regulate gene expression and can affect genome stability and nuclear domain organization both in plant and in the animal kingdom. Here, we describe the major plant lncRNAs and how they act, with a focus on research in Arabidopsis thaliana and our emerging understanding of lncRNA functions in serving as molecular sponges and decoys, functioning in regulation of transcription and silencing, particularly in RNA-directed DNA methylation, and in epigenetic regulation of flowering time.

The Role of Long Noncoding RNAs in Plant Stress Tolerance

Plants must adapt to multiple biotic and abiotic stresses ; thus, sensing and responding to environmental signals is imperative for their survival. Moreover, understanding these responses is imperative for efforts to improve plant yield and consistency. Regulation of transcript levels is a key aspect of the plant response to environmental signals. Long noncoding RNAs (lncRNAs) have gained widespread attention in recent years with the advance of high-throughput sequencing technologies. As important biological regulators, lncRNAs have been implicated in a wide range of developmental processes and diseases in animals. However, knowledge of the role that lncRNAs play in plant stress tolerance remains limited. Here, we review recent studies on the identification, characteristics, classification, and biological functions of lncRNAs in response to various stresses, including bacterial pathogens, excess light, drought, salinity, hypoxia, extreme temperatures, and nitrogen/phosphate deficiency. We also discuss possible directions for future research.

Tug of war: adding and removing histone lysine methylation in Arabidopsis

Histone lysine methylation plays a fundamental role in the epigenetic regulation of gene expression in multicellular eukaryotes, including plants. It shapes plant developmental and growth programs as well as responses to the environment. The methylation status of certain amino-acids, in particular of the histone 3 (H3) lysine tails, is dynamically controlled by opposite acting histone methyltransferase 'writers' and histone demethylase 'erasers'. The methylation status is interpreted by a third set of proteins, the histone modification 'readers', which specifically bind to a methylated amino-acid on the H3 tail. Histone methylation writers, readers, and erasers themselves are regulated by intrinsic or extrinsic stimuli; this forms a feedback loop that contributes to development and environmental adaptation in Arabidopsis and other plants. Recent studies have expanded our knowledge regarding the biological roles and dynamic regulation of histone methylation. In this review, we will discuss recent advances in understanding the regulation and roles of histone methylation in plants and animals.

Reprogramming of DNA methylation is critical for nodule development in Medicago truncatula

The legume-Rhizobium symbiosis leads to the formation of a new organ, the root nodule, involving coordinated and massive induction of specific genes. Several genes controlling DNA methylation are spatially regulated within the Medicago truncatula nodule, notably the demethylase gene, DEMETER (DME), which is mostly expressed in the differentiation zone. Here, we show that MtDME is essential for nodule development and regulates the expression of 1,425 genes, some of which are critical for plant and bacterial cell differentiation. Bisulphite sequencing coupled to genomic capture enabled the identification of 474 regions that are differentially methylated during nodule development, including nodule-specific cysteine-rich peptide genes. Decreasing DME expression by RNA interference led to hypermethylation and concomitant downregulation of 400 genes, most of them associated with nodule differentiation. Massive reprogramming of gene expression through DNA demethylation is a new epigenetic mechanism controlling a key stage of indeterminate nodule organogenesis during symbiotic interactions.

Chromatin architecture: A new dimension in the dynamic control of gene expression

As the most recent evidence of eukaryotic cell complexity, genome architecture has astounded the scientific community and prompted a variety of technical and cognitive challenges. Several technologies have emerged and evidenced the integration of chromatin packaging and topology, epigenetic processes, and transcription for the pertinent regulation of gene expression. In the present addendum we present and discuss some of our recent research, directed toward the holistic comprehension of the processes by which plants respond to environmental and developmental stimuli. We propose that the study of genome topology and genomic interactions is essential for the understanding of the molecular mechanisms behind a phenotype. Even though our knowledge and understanding of genome architecture and hierarchy has improved substantially in the last few years -in Arabidopsis and other eukaryotes -, there is still a long way ahead in this relatively new field of study. For this, it is necessary to take advantage of the high resolution of the emerging available techniques, and perform integrative approaches with which it will be possible to depict the role of chromatin architecture in the regulation of transcription and ultimately, physiological processes.

DNA Damage-Induced Transcription of Transposable Elements and Long Non-coding RNAs in Arabidopsis Is Rare and ATM-Dependent

Induction and mobilization of transposable elements (TEs) following DNA damage or other stresses has been reported in prokaryotes and eukaryotes. Recently it was discovered that eukaryotic TEs are frequently associated with long non-coding RNAs (lncRNAs), many of which are also upregulated by stress. Yet, it is unknown whether DNA damage-induced transcriptional activation of TEs and lncRNAs occurs sporadically or is a synchronized, genome-wide response. Here we investigated the transcriptome of Arabidopsis wild-type (WT) and ataxia telangiectasia mutated (atm) mutant plants 3 h after induction of DNA damage. In WT, expression of 5.2% of the protein-coding genes is ≥2-fold changed, whereas in atm plants, only 2.6% of these genes are regulated, and the response of genes associated with DNA repair, replication, and cell cycle is largely lost. In contrast, only less than 0.6% of TEs and lncRNAs respond to DNA damage in WT plants, and the regulation of ≥95% of them is ATM-dependent. The ATM-downstream factors BRCA1, DRM1, JMJ30, AGO2, and the ATM-independent AGO4 participate in the regulation of individual TEs and lncRNAs. Remarkably, protein-coding genes located adjacent to DNA damage-responsive TEs and lncRNAs are frequently coexpressed, which is consistent with the hypothesis that TEs and lncRNAs located close to genes commonly function as controlling elements.

Combinatorial control of plant gene expression

Combinatorial gene regulation provides a mechanism by which relatively small numbers of transcription factors can control the expression of a much larger number of genes with finely tuned temporal and spatial patterns. This is achieved by transcription factors assembling into complexes in a combinatorial fashion, exponentially increasing the number of genes that they can target. Such an arrangement also increases the specificity and affinity for the cis-regulatory sequences required for accurate target gene expression. Superimposed on this transcription factor combinatorial arrangement is the increasing realization that histone modification marks expand the regulatory information, which is interpreted by histone readers and writers that are part of the regulatory apparatus. Here, we review the progress in these areas from the perspective of plant combinatorial gene regulation, providing examples of different regulatory solutions and comparing them to other metazoans. This article is part of a Special Issue entitled: Plant Gene Regulatory Mechanisms and Networks, edited by Dr. Erich Grotewold and Dr. Nathan Springer.

LHP1 Regulates H3K27me3 Spreading and Shapes the Three-Dimensional Conformation of the Arabidopsis Genome

Precise expression patterns of genes in time and space are essential for proper development of multicellular organisms. Dynamic chromatin conformation and spatial organization of the genome constitute a major step in this regulation to modulate developmental outputs. Polycomb repressive complexes (PRCs) mediate stable or flexible gene repression in response to internal and environmental cues. In Arabidopsis thaliana, LHP1 co-localizes with H3K27me3 epigenetic marks throughout the genome and interacts with PRC1 and PRC2 members as well as with a long noncoding RNA. Here, we show that LHP1 is responsible for the spreading of H3K27me3 towards the 3' end of the gene body. We also identified a subset of LHP1-activated genes and demonstrated that LHP1 shapes local chromatin topology in order to control transcriptional co-regulation. Our work reveals a general role of LHP1 from local to higher conformation levels of chromatin configuration to determine its accessibility to define gene expression patterns.

Control of Chromatin Structure by Long Noncoding RNA

Long noncoding RNA (lncRNA) is a pivotal factor regulating various aspects of genome activity. Genome regulation via DNA methylation and post-translational histone modifications is a well-documented function of lncRNA in plants, fungi, and animals. Here, we summarize evidence showing that lncRNA also controls chromatin structure, including nucleosome positioning and chromosome looping. We focus on data from plant experimental systems, discussed in the context of other eukaryotes. We explain the mechanisms of lncRNA-controlled chromatin remodeling and the implications of the functional interplay between noncoding transcription and several different chromatin remodelers. We propose that the unique properties of RNA make it suitable for controlling chromatin modifications and structure.

Small interfering RNAs from bidirectional transcripts of GhMML3_A12 regulate cotton fiber development

Natural antisense transcripts (NATs) are commonly observed in eukaryotic genomes, but only a limited number of such genes have been identified as being involved in gene regulation in plants. In this research, we investigated the function of small RNA derived from a NAT in fiber cell development. Using a map-based cloning strategy for the first time in tetraploid cotton, we cloned a naked seed mutant gene (N1 ) encoding a MYBMIXTA-like transcription factor 3 (MML3)/GhMYB25-like in chromosome A12, GhMML3_A12, that is associated with fuzz fiber development. The extremely low expression of GhMML3_A12 in N1 is associated with NAT production, driven by its 3' antisense promoter, as indicated by the promoter-driven histochemical staining assay. In addition, small RNA deep sequencing analysis suggested that the bidirectional transcriptions of GhMML3_A12 form double-stranded RNAs and generate 21-22 nt small RNAs. Therefore, in a fiber-specific manner, small RNA derived from the GhMML3_A12 locus can mediate GhMML3_A12 mRNA self-cleavage and result in the production of naked seeds followed by lint fiber inhibition in N1 plants. The present research reports the first observation of gene-mediated NATs and siRNA directly controlling fiber development in cotton.

Hyperosmotic stress memory in Arabidopsis is mediated by distinct epigenetically labile sites in the genome and is restricted in the male germline by DNA glycosylase activity

Inducible epigenetic changes in eukaryotes are believed to enable rapid adaptation to environmental fluctuations. We have found distinct regions of the Arabidopsis genome that are susceptible to DNA (de)methylation in response to hyperosmotic stress. The stress-induced epigenetic changes are associated with conditionally heritable adaptive phenotypic stress responses. However, these stress responses are primarily transmitted to the next generation through the female lineage due to widespread DNA glycosylase activity in the male germline, and extensively reset in the absence of stress. Using the CNI1/ATL31 locus as an example, we demonstrate that epigenetically targeted sequences function as distantly-acting control elements of antisense long non-coding RNAs, which in turn regulate targeted gene expression in response to stress. Collectively, our findings reveal that plants use a highly dynamic maternal 'short-term stress memory' with which to respond to adverse external conditions. This transient memory relies on the DNA methylation machinery and associated transcriptional changes to extend the phenotypic plasticity accessible to the immediate offspring.

Genome-wide analysis of chromatin packing in Arabidopsis thaliana at single-gene resolution

The three-dimensional packing of the genome plays an important role in regulating gene expression. We have used Hi-C, a genome-wide chromatin conformation capture (3C) method, to analyze Arabidopsis thaliana chromosomes dissected into subkilobase segments, which is required for gene-level resolution in this species with a gene-dense genome. We found that the repressive H3K27me3 histone mark is overrepresented in the promoter regions of genes that are in conformational linkage over long distances. In line with the globally dispersed distribution of RNA polymerase II in A. thaliana nuclear space, actively transcribed genes do not show a strong tendency to associate with each other. In general, there are often contacts between 5' and 3' ends of genes, forming local chromatin loops. Such self-loop structures of genes are more likely to occur in more highly expressed genes, although they can also be found in silent genes. Silent genes with local chromatin loops are highly enriched for the histone variant H3.3 at their 5' and 3' ends but depleted of repressive marks such as heterochromatic histone modifications and DNA methylation in flanking regions. Our results suggest that, different from animals, a major theme of genome folding in A. thaliana is the formation of structural units that correspond to gene bodies.

Discriminative gene co-expression network analysis uncovers novel modules involved in the formation of phosphate deficiency-induced root hairs in Arabidopsis

Cell fate and differentiation in the Arabidopsis root epidermis are genetically defined but remain plastic to environmental signals such as limited availability of inorganic phosphate (Pi). Root hairs of Pi-deficient plants are more frequent and longer than those of plants grown under Pi-replete conditions. To dissect genes involved in Pi deficiency-induced root hair morphogenesis, we constructed a co-expression network of Pi-responsive genes against a customized database that was assembled from experiments in which differentially expressed genes that encode proteins with validated functions in root hair development were over-represented. To further filter out less relevant genes, we combined this procedure with a search for common cis-regulatory elements in the promoters of the selected genes. In addition to well-described players and processes such as auxin signalling and modifications of primary cell walls, we discovered several novel aspects in the biology of root hairs induced by Pi deficiency, including cell cycle control, putative plastid-to-nucleus signalling, pathogen defence, reprogramming of cell wall-related carbohydrate metabolism, and chromatin remodelling. This approach allows the discovery of novel of aspects of a biological process from transcriptional profiles with high sensitivity and accuracy.

Put your 3D glasses on: plant chromatin is on show

The three-dimensional organization of the eukaryotic nucleus and its chromosomal conformation have emerged as important features in the complex network of mechanisms behind gene activity and genome connectivity dynamics, which can be evidenced in the regionalized chromosomal spatial distribution and the clustering of diverse genomic regions with similar expression patterns. The development of chromatin conformation capture (3C) techniques has permitted the elucidation of commonalities between the eukaryotic phyla, as well as important differences among them. The growing number of studies in the field performed in plants has shed light on the structural and regulatory features of these organisms. For instance, it has been proposed that plant chromatin can be arranged into different conformations such as Rabl, Rosette-like, and Bouquet, and that both short- and long-range chromatin interactions occur in Arabidopsis. In this review, we compile the current knowledge about chromosome architecture characteristics in plants, as well as the molecular events and elements (including long non-coding RNAs, histone and DNA modifications, chromatin remodeling complexes, and transcription factors) shaping the genome three-dimensional conformation. Furthermore, we discuss the developmental outputs of genome topology-mediated gene expression regulation. It is becoming increasingly clear that new tools and techniques with higher resolution need to be developed and implemented in Arabidopsis and other model plants in order to better understand chromosome architecture dynamics, from an integrative perspective with other fields of plant biology such as development, stress biology, and finally agriculture.

Parental epigenetic asymmetry of PRC2-mediated histone modifications in the Arabidopsis endosperm

Parental genomes in the endosperm are marked by differential DNA methylation and are therefore epigenetically distinct. This epigenetic asymmetry is established in the gametes and maintained after fertilization by unknown mechanisms. In this manuscript, we have addressed the key question whether parentally inherited differential DNA methylation affects de novo targeting of chromatin modifiers in the early endosperm. Our data reveal that polycomb-mediated H3 lysine 27 trimethylation (H3K27me3) is preferentially localized to regions that are targeted by the DNA glycosylase DEMETER (DME), mechanistically linking DNA hypomethylation to imprinted gene expression. Our data furthermore suggest an absence of de novo DNA methylation in the early endosperm, providing an explanation how DME-mediated hypomethylation of the maternal genome is maintained after fertilization. Lastly, we show that paternal-specific H3K27me3-marked regions are located at pericentromeric regions, suggesting that H3K27me3 and DNA methylation are not necessarily exclusive marks at pericentromeric regions in the endosperm.

Divergent lncRNAs Regulate Gene Expression and Lineage Differentiation in Pluripotent Cells

Divergent lncRNAs that are transcribed in the opposite direction to nearby protein-coding genes comprise a significant proportion (∼20%) of total lncRNAs in mammalian genomes. Through genome-wide analysis, we found that the distribution of this lncRNA class strongly correlates with essential developmental regulatory genes. In pluripotent cells, divergent lncRNAs regulate the transcription of nearby genes. As an example, the divergent lncRNA Evx1as promotes transcription of its neighbor gene, EVX1, and regulates mesendodermal differentiation. At a single-cell level, early broad expression of Evx1as is followed by a rapid, high-level transcription of EVX1, supporting the idea that Evx1as plays an upstream role to facilitate EVX1 transcription. Mechanistically, Evx1as RNA binds to regulatory sites on chromatin, promotes an active chromatin state, and interacts with Mediator. Based on our analyses, we propose that the biological function of thousands of uncharacterized lncRNAs of this class may be inferred from the role of their neighboring adjacent genes.

The three-dimensional genome: principles and roles of long-distance interactions

The linear sequence of eukaryotic genomes is arranged in a specific manner within the three-dimensional nuclear space. Interactions between distant sites partition the genome into domains of highly associating chromatin. Interaction domains are found in many organisms, but their properties and the principles governing their establishment vary between different species. Topologically associating domains (TADs) extending over large genomic regions are found in mammals and Drosophila melanogaster, whereas other types of contact domains exist in lower eukaryotes. Here we review recent studies that explore the mechanisms by which long distance chromatin interactions determine the 3D organization of the genome and the relationship between this organization and the establishment of specific patterns of gene expression.

The age of multiplexity: recruitment and interactions of Polycomb complexes in plants

Polycomb group (PcG) proteins form distinct complexes that modify chromatin by histone H3 methylation and H2A mono-ubiquitination leading to chromatin compaction and epigenetic repression of target genes. A network of PcG protein complexes, associated partners and antagonistically acting chromatin modifiers is essential to regulate developmental transitions and cell fate in all multicellular eukaryotes. In this review, we discuss insights on the subfunctionalization of PcG complexes and their modes of recruitment to target sites based on data from the model organism Arabidopsis thaliana.

Epigenetic Modifications and Plant Hormone Action

The action of phytohormones in plants requires the spatiotemporal regulation of their accumulation and responses at various levels. Recent studies reveal an emerging relationship between the function of phytohormones and epigenetic modifications. In particular, evidence suggests that auxin biosynthesis, transport, and signal transduction is modulated by microRNAs and epigenetic factors such as histone modification, chromatin remodeling, and DNA methylation. Furthermore, some phytohormones have been shown to affect epigenetic modifications. These findings are shedding light on the mode of action of phytohormones and are opening up a new avenue of research on phytohormones as well as on the mechanisms regulating epigenetic modifications.

Global identification and analysis of long non-coding RNAs in diploid strawberry Fragaria vesca during flower and fruit development

Background

Long non-coding RNAs (lncRNAs) are a new class of regulatory molecules with roles in diverse biological processes. While much effort has been invested in the analysis of lncRNAs from established plant models Arabidopsis, maize, and rice, almost nothing is known about lncRNAs from fruit crops, including those in the Rosaceae family.

Results

Here, we present a genome-scale identification and characterization of lncRNAs from a diploid strawberry, Fragaria vesca, based on rich RNA-seq datasets from 35 different flower and fruit tissues. 5,884 Fve-lncRNAs derived from 3,862 loci were identified. These lncRNAs were carefully cataloged based on expression level and whether or not they contain repetitive sequences or generate small RNAs. About one fourth of them are termed high-confidence lncRNAs (hc-lncRNAs) because they are expressed at a level of FPKM higher than 2 and produce neither small RNAs nor contain repetitive sequence. To identify regulatory interactions between lncRNAs and their potential protein-coding (PC) gene targets, pairs of lncRNAs and PC genes with positively or negatively correlated expression trends were identified based on their expression; these pairs may be candidates of cis- or trans-acting lncRNAs and their targets. Finally, blast searches within plant species indicate that lncRNAs are not well conserved.

Conclusions

Our study identifies a large number of tissue-specifically expressed lncRNAs in F. vesca, thereby highlighting their potential contributions to strawberry flower and fruit development and paving the way for future functional studies.

Electronic supplementary material

The online version of this article (doi:10.1186/s12864-015-2014-2) contains supplementary material, which is available to authorized users.

A SWI/SNF Chromatin Remodelling Protein Controls Cytokinin Production through the Regulation of Chromatin Architecture

Chromatin architecture determines transcriptional accessibility to DNA and consequently gene expression levels in response to developmental and environmental stimuli. Recently, chromatin remodelers such as SWI/SNF complexes have been recognized as key regulators of chromatin architecture. To gain insight into the function of these complexes during root development, we have analyzed Arabidopsis knock-down lines for one sub-unit of SWI/SNF complexes: BAF60. Here, we show that BAF60 is a positive regulator of root development and cell cycle progression in the root meristem via its ability to down-regulate cytokinin production. By opposing both the deposition of active histone marks and the formation of a chromatin regulatory loop, BAF60 negatively regulates two crucial target genes for cytokinin biosynthesis (IPT3 and IPT7) and one cell cycle inhibitor (KRP7). Our results demonstrate that SWI/SNF complexes containing BAF60 are key factors governing the equilibrium between formation and dissociation of a chromatin loop controlling phytohormone production and cell cycle progression.

Analysis of non-coding transcriptome in rice and maize uncovers roles of conserved lncRNAs associated with agriculture traits

Long non-coding RNAs (lncRNAs) have recently been found to widely exist in eukaryotes and play important roles in key biological processes. To extend our knowledge of lncRNAs in crop plants we performed both non-directional and strand-specific RNA-sequencing experiments to profile non-coding transcriptomes of various rice and maize organs at different developmental stages. Analysis of more than 3 billion reads identified 22 334 long intergenic non-coding RNAs (lincRNAs) and 6673 pairs of sense and natural antisense transcript (NAT). Many lincRNA genes were associated with epigenetic marks. Expression of rice lincRNA genes was significantly correlated with that of nearby protein-coding genes. A set of NAT genes also showed expression correlation with their sense genes. More than 200 rice lincRNA genes had homologous non-coding sequences in the maize genome. Much more lincRNA and NAT genes were derived from conserved genomic regions between the two cereals presenting positional conservation. Protein-coding genes flanking or having a sense-antisense relationship to these conserved lncRNA genes were mainly involved in development and stress responses, suggesting that the associated lncRNAs might have similar functions. Integrating previous genome-wide association studies (GWAS), we found that hundreds of lincRNAs contain trait-associated SNPs (single nucleotide polymorphisms [SNPs]) suggesting their putative contributions to developmental and agriculture traits.

Long non-coding RNAs and their functions in plants

Eukaryotic genomes encode thousands of long noncoding RNAs (lncRNAs), which play important roles in essential biological processes. Although lncRNAs function in the nuclear and cytoplasmic compartments, most of them occur in the nucleus, often in association with chromatin. Indeed, many lncRNAs have emerged as key regulators of gene expression and genome stability. Emerging evidence also suggests that lncRNAs may contribute to the organization of nuclear domains. This review briefly summarizes the major types of eukaryotic lncRNAs and provides examples of their mechanisms of action, with focus on plant lncRNAs, mainly in Arabidopsis thaliana, and describes current advances in our understanding of the mechanisms of lncRNA action and the roles of lncRNAs in RNA-dependent DNA methylation and in the regulation of flowering time.

Advances in understanding cis regulation of the plant gene with an emphasis on comparative genomics

The plant gene model remains largely an extrapolation from animals, with the cis functional unit, the gene, cast as a dynamic looping structure. Molecular genetics with model plants continues to make advances; highlighted here are quantitative-occupancy results from the Arabidopsis thaliana (Arabidopsis) Phytochrome-Interacting bHLH transcription Factors (PIF) quartet. Compared to this complex snapshot, results from chromatin occupancy and other Encyclopedia of DNA Elements (ENCODE)-like approaches increase our transcription factor-motif cognate library, but regulation cannot by itself be inferred from binding. Complementary published Arabidopsis conserved noncoding sequence lists are compared, evaluated, merged, and released. Comparative genomic approaches have identified a cis modifier of a gene's expression-hypothetically, a transposon-based 'rheostat'-that works in all cells, times and places.

Viroids, infectious long non-coding RNAs with autonomous replication

Transcriptome deep-sequencing studies performed during the last years confirmed that the vast majority of the RNAs transcribed in higher organisms correspond to several types of non-coding RNAs including long non-coding RNAs (lncRNAs). The study of lncRNAs and the identification of their functions, is still an emerging field in plants but the characterization of some of them indicate that they play an important role in crucial regulatory processes like flowering regulation, and responses to abiotic stress and plant hormones. A second group of lncRNAs present in plants is formed by viroids, exogenous infectious subviral plant pathogens well known since many years. Viroids are composed of circular RNA genomes without protein-coding capacity and subvert enzymatic activities of their hosts to complete its own biological cycle. Different aspects of viroid biology and viroid-host interactions have been elucidated in the last years and some of them are the main topic of this review together with the analysis of the state-of-the-art about the growing field of endogenous lncRNAs in plants.

Chromatin in 3D: progress and prospects for plants

Methods that use high-throughput sequencing have begun to reveal features of the three-dimensional structure of genomes at a resolution that goes far beyond that of traditional microscopy. Integration of these methods with other molecular tools has advanced our knowledge of both global and local chromatin packing in plants, and has revealed how patterns of chromatin packing correlate with the genomic and epigenomic landscapes. This update reports recent progress made in this area in plants, and suggests new research directions.