Cell biology of the Arabidopsis nuclear siRNA pathway for RNA-directed chromatin modification

How do plants remember drought?

Plants develop both short-term and transgenerational memory of drought stress through epigenetic regulation of transcription for a better response to subsequent exposure. Recurrent spells of droughts are more common than a single drought, with intermittent moist recovery intervals. While the detrimental effects of the first drought on plant structure and physiology are unavoidable, if survived, plants can memorize the first drought to present a more robust response to the following droughts. This includes a partial stomatal opening in the watered recovery interval, higher levels of osmoprotectants and ABA, and attenuation of photosynthesis in the subsequent exposure. Short-term drought memory is regulated by ABA and other phytohormone signaling with transcriptional memory behavior in various genes. High levels of methylated histones are deposited at the drought-tolerance genes. During the recovery interval, the RNA polymerase is stalled to be activated by a pause-breaking factor in the subsequent drought. Drought leads to DNA demethylation near drought-response genes, with genetic control of the process. Progenies of the drought-exposed plants can better adapt to drought owing to the inheritance of particular methylation patterns. However, a prolonged watered recovery interval leads to loss of drought memory, mediated by certain demethylases and chromatin accessibility factors. Small RNAs act as critical regulators of drought memory by altering transcript levels of drought-responsive target genes. Further studies in the future will throw more light on the genetic control of drought memory and the interplay of genetic and epigenetic factors in its inheritance. Plants from extreme environments can give queues to understanding robust memory responses at the ecosystem level.

Unravelling the Role of Epigenetic Modifications in Development and Reproduction of Angiosperms: A Critical Appraisal

Epigenetics are the heritable changes in gene expression patterns which occur without altering DNA sequence. These changes are reversible and do not change the sequence of the DNA but can alter the way in which the DNA sequences are read. Epigenetic modifications are induced by DNA methylation, histone modification, and RNA-mediated mechanisms which alter the gene expression, primarily at the transcriptional level. Such alterations do control genome activity through transcriptional silencing of transposable elements thereby contributing toward genome stability. Plants being sessile in nature are highly susceptible to the extremes of changing environmental conditions. This increases the likelihood of epigenetic modifications within the composite network of genes that affect the developmental changes of a plant species. Genetic and epigenetic reprogramming enhances the growth and development, imparts phenotypic plasticity, and also ensures flowering under stress conditions without changing the genotype for several generations. Epigenetic modifications hold an immense significance during the development of male and female gametophytes, fertilization, embryogenesis, fruit formation, and seed germination. In this review, we focus on the mechanism of epigenetic modifications and their dynamic role in maintaining the genomic integrity during plant development and reproduction.

Application of Genome Editing in Tomato Breeding: Mechanisms, Advances, and Prospects

Plants regularly face the changing climatic conditions that cause biotic and abiotic stress responses. The abiotic stresses are the primary constraints affecting crop yield and nutritional quality in many crop plants. The advances in genome sequencing and high-throughput approaches have enabled the researchers to use genome editing tools for the functional characterization of many genes useful for crop improvement. The present review focuses on the genome editing tools for improving many traits such as disease resistance, abiotic stress tolerance, yield, quality, and nutritional aspects of tomato. Many candidate genes conferring tolerance to abiotic stresses such as heat, cold, drought, and salinity stress have been successfully manipulated by gene modification and editing techniques such as RNA interference, insertional mutagenesis, and clustered regularly interspaced short palindromic repeat (CRISPR/Cas9). In this regard, the genome editing tools such as CRISPR/Cas9, which is a fast and efficient technology that can be exploited to explore the genetic resources for the improvement of tomato and other crop plants in terms of stress tolerance and nutritional quality. The review presents examples of gene editing responsible for conferring both biotic and abiotic stresses in tomato simultaneously. The literature on using this powerful technology to improve fruit quality, yield, and nutritional aspects in tomato is highlighted. Finally, the prospects and challenges of genome editing, public and political acceptance in tomato are discussed.

Rewired RNAi-mediated genome surveillance in house dust mites

House dust mites are common pests with an unusual evolutionary history, being descendants of a parasitic ancestor. Transition to parasitism is frequently accompanied by genome rearrangements, possibly to accommodate the genetic change needed to access new ecology. Transposable element (TE) activity is a source of genomic instability that can trigger large-scale genomic alterations. Eukaryotes have multiple transposon control mechanisms, one of which is RNA interference (RNAi). Investigation of the dust mite genome failed to identify a major RNAi pathway: the Piwi-associated RNA (piRNA) pathway, which has been replaced by a novel small-interfering RNA (siRNA)-like pathway. Co-opting of piRNA function by dust mite siRNAs is extensive, including establishment of TE control master loci that produce siRNAs. Interestingly, other members of the Acari have piRNAs indicating loss of this mechanism in dust mites is a recent event. Flux of RNAi-mediated control of TEs highlights the unusual arc of dust mite evolution.

Investigation of small RNA populations in dust mites revealed absence of the piwi-associated RNA (piRNA) pathway. Apart from several nematode and platyhelminths lineages, piRNAs are an essential component of animal genome surveillance, actively targeting and silencing transposable elements. In dust mites, expansion of Dicer produced small-interfering RNA (siRNA) biology compensates for loss of piRNAs. The dramatic difference we find in dust mites is likely a consequence of their evolutionary history, which is marked by descent from a parasite to the current free-living form. Our study highlights a correlation between perturbation of transposon surveillance and shifts in ecology.

Identification of Pol IV and RDR2-dependent precursors of 24 nt siRNAs guiding de novo DNA methylation in Arabidopsis

In Arabidopsis thaliana, abundant 24 nucleotide small interfering RNAs (24 nt siRNA) guide the cytosine methylation and silencing of transposons and a subset of genes. 24 nt siRNA biogenesis requires nuclear RNA polymerase IV (Pol IV), RNA-dependent RNA polymerase 2 (RDR2) and DICER-like 3 (DCL3). However, siRNA precursors are mostly undefined. We identified Pol IV and RDR2-dependent RNAs (P4R2 RNAs) that accumulate in dcl3 mutants and are diced into 24 nt RNAs by DCL3 in vitro. P4R2 RNAs are mostly 26-45 nt and initiate with a purine adjacent to a pyrimidine, characteristics shared by Pol IV transcripts generated in vitro. RDR2 terminal transferase activity, also demonstrated in vitro, may account for occasional non-templated nucleotides at P4R2 RNA 3' termini. The 24 nt siRNAs primarily correspond to the 5' or 3' ends of P4R2 RNAs, suggesting a model whereby siRNAs are generated from either end of P4R2 duplexes by single dicing events.

The splicing machinery promotes RNA-directed DNA methylation and transcriptional silencing in Arabidopsis

DNA methylation in transposons and other DNA repeats is conserved in plants as well as in animals. In Arabidopsis thaliana, an RNA-directed DNA methylation (RdDM) pathway directs de novo DNA methylation. We performed a forward genetic screen for suppressors of the DNA demethylase mutant ros1 and identified a novel Zinc-finger and OCRE domain-containing Protein 1 (ZOP1) that promotes Pol IV-dependent siRNA accumulation, DNA methylation, and transcriptional silencing. Whole-genome methods disclosed the genome-wide effects of zop1 on Pol IV-dependent siRNA accumulation and DNA methylation, suggesting that ZOP1 has both RdDM-dependent and -independent roles in transcriptional silencing. We demonstrated that ZOP1 is a pre-mRNA splicing factor that associates with several typical components of the splicing machinery as well as with Pol II. Immunofluorescence assay revealed that ZOP1 overlaps with Cajal body and is partially colocalized with NRPE1 and DRM2. Moreover, we found that the other development-defective splicing mutants tested including mac3a3b, mos4, mos12 and mos14 show defects in RdDM and transcriptional silencing. We propose that the splicing machinery rather than specific splicing factors is involved in promoting RdDM and transcriptional silencing.

Changes to DNA methylation and homologous recombination frequency in the progeny of stressed plants

Plants undergo changes in response to biotic and abiotic stresses that help them adjust and survive. Some of these changes may even be passed on to progeny and eventually lead to adaptive evolution. Transgenerational changes in response to stress include alterations in DNA methylation and changes in homologous recombination frequency (HRF). The progeny of plants that were stressed often show elevated HRF as well as genomic hypermethylation, although specific loci that are beneficial in times of stress may be hypomethylated. One of the possible mechanisms responsible for passing the memory to the progeny involves small interfering RNAs; Dicer-like proteins, DCL2 and DCL3, are in part required for this process. However, while epigenetic modifications are often present in the untreated progeny of stressed plants, they are not usually sustained for multiple unexposed generations. Still, transgenerational inheritance of such changes has already begun to provide evidence for an important role of epigenetics in enhancing stress resistance.

Plant memory: a tentative model

All memory functions have molecular bases, namely in signal reception and transduction, and in storage and recall of information. Thus, at all levels of organisation living organisms have some kind of memory. In plants one may distinguish two types. There are linear pathways from reception of signals and propagation of effectors to a type of memory that may be described by terms such as learning, habituation or priming. There is a storage and recall memory based on a complex network of elements with a high degree of integration and feedback. The most important elements envisaged are calcium waves, epigenetic modifications of DNA and histones, and regulation of timing via a biological clock. Experiments are described that document the occurrence of the two sorts of memory and which show how they can be distinguished. A schematic model of plant memory is derived as emergent from integration of the various modules. Possessing the two forms of memory supports the fitness of plants in response to environmental stimuli and stress.

Intron retention and 3'-UTR analysis of Arabidopsis Dicer-like 2 transcripts

Arabidopsis thaliana Dicer-like protein 2 (AtDCL2) plays an essential role in the RNA interference pathway. The function of AtDCL2 and other DCLs has been much studied but little has been done to characterize the DCLs transcripts before they are translated into proteins. Here, we investigated AtDCL2 transcripts and showed that all 21 introns of AtDCL2 except intron 9, 18, 20 and 21 could be retained although spliced sequences usually predominated. Intron 10 was more frequently retained and transient expression assays in Nicotiana benthamiana leaves showed that when AG/C at the 3' splicing site of the intron was changed to AG/G, the intron was more frequently spliced out. Conversely, a high retention of intron 18 was obtained if the AG/G at the 3' splicing site was changed to AG/C. These results suggest that the sequence at the 3' splicing site affects the efficiency of intron splicing. The 3'-UTRs of AtDCL2 had lengths between 54 and 154 nts, and the different 3'-UTRs differentially affected the transcriptional levels of fused GFP expressed transiently in N. benthamiana. Further comparisons and mutation experiments suggested that a putative SBF-1 binding site and an AU-rich element in the 3'-UTR both down-regulated expression of the upstream GFP fused to the 3'-UTR. Conversely, a second poly(A) consensus signal sequence in one 3'-UTR up-regulated gene expression. Our results provide insight into the character of AtDCL2 transcripts and demonstrate the potential complexity of factors that affect the frequency and patterns of alternative splicing.

Small RNA diversity in plants and its impact in development

MicroRNAs are a class of non-coding RNAs involved in post-transcriptional control of gene expression, either via degradation or translational inhibition of target mRNAs. Both experimental and computational approaches have been used to identify miRNAs and their target genes. In plants, deep sequencing methods have recently allowed the analysis of small RNA diversity in different species and/or mutants. Most sequencing efforts have been concentrated on the identification of miRNAs and their mRNA targets have been predicted based on complementarity criteria. The recent demonstration that certain plant miRNAs could act partly via inhibition of protein translation certainly opens new fields of analysis for plant miRNA function on a broader group of targets. The roles of conserved miRNAs on target mRNA stability have been analysed in different species and defined common mechanisms in development and stress responses. In contrast, much less is known about expression patterns or functions of non-conserved miRNAs. In this review, we focus on the comparative analyses of plant small RNA diversity and the action of si/miRNAs in post-transcriptional regulation of some key genes involved in root development.

The role of epigenetic processes in controlling flowering time in plants exposed to stress

Plants interact with their environment by modifying gene expression patterns. One mechanism for this interaction involves epigenetic modifications that affect a number of aspects of plant growth and development. Thus, the epigenome is highly dynamic in response to environmental cues and developmental changes. Flowering is controlled by a set of genes that are affected by environmental conditions through an alteration in their expression pattern. This ensures the production of flowers even when plants are growing under adverse conditions, and thereby enhances transgenerational seed production. In this review recent findings on the epigenetic changes associated with flowering in Arabidopsis thaliana grown under abiotic stress conditions such as cold, drought, and high salinity are discussed. These epigenetic modifications include DNA methylation, histone modifications, and the production of micro RNAs (miRNAs) that mediate epigenetic modifications. The roles played by the phytohormones abscisic acid (ABA) and auxin in chromatin remodelling are also discussed. It is shown that there is a crucial relationship between the epigenetic modifications associated with floral initiation and development and modifications associated with stress tolerance. This relationship is demonstrated by the common epigenetic pathways through which plants control both flowering and stress tolerance, and can be used to identify new epigenomic players.

Transgenerational adaptation of Arabidopsis to stress requires DNA methylation and the function of Dicer-like proteins

Epigenetic states and certain environmental responses in mammals and seed plants can persist in the next sexual generation. These transgenerational effects have potential adaptative significance as well as medical and agronomic ramifications. Recent evidence suggests that some abiotic and biotic stress responses of plants are transgenerational. For example, viral infection of tobacco plants and exposure of Arabidopsis thaliana plants to UVC and flagellin can induce transgenerational increases in homologous recombination frequency (HRF). Here we show that exposure of Arabidopsis plants to stresses, including salt, UVC, cold, heat and flood, resulted in a higher HRF, increased global genome methylation, and higher tolerance to stress in the untreated progeny. This transgenerational effect did not, however, persist in successive generations. Treatment of the progeny of stressed plants with 5-azacytidine was shown to decrease global genomic methylation and enhance stress tolerance. Dicer-like (DCL) 2 and DCL3 encode Dicer activities important for small RNA-dependent gene silencing. Stress-induced HRF and DNA methylation were impaired in dcl2 and dcl3 deficiency mutants, while in dcl2 mutants, only stress-induced stress tolerance was impaired. Our results are consistent with the hypothesis that stress-induced transgenerational responses in Arabidopsis depend on altered DNA methylation and smRNA silencing pathways.

Northern blotting techniques for small RNAs

In eukaryotes, RNA silencing encompasses a range of biochemical processes mediated by approximately 20-25 nt small RNAs (smRNAs). This chapter describes northern blot hybridization techniques optimized for detection of such smRNAs, whether extracted from plant or animal tissues. The basic protocol is described, and control blots illustrate the detection specificity and sensitivity of this method using DNA oligonucleotide probes. Known endogenous smRNAs are analyzed in samples prepared from several model plant species, including Arabidopsis thaliana, Nicotiana benthamiana, Oryza sativa, Zea mays, and Physcomitrella patens, as well as the animals Drosophila melanogaster and Mus musculus. Finally, the usefulness of northern blotting in dissecting smRNA biogenesis is shown for the particular case of DNA virus infection.

Active DNA demethylation mediated by DNA glycosylases

Active DNA demethylation is involved in many vital developmental and physiological processes of plants and animals. Recent genetic and biochemical studies in Arabidopsis have demonstrated that a subfamily of DNA glycosylases function to promote DNA demethylation through a base excision-repair pathway. These specialized bifunctional DNA glycosylases remove the 5-methylcytosine base and then cleave the DNA backbone at the abasic site, resulting in a gap that is then filled with an unmethylated cytosine nucleotide by as yet unknown DNA polymerase and ligase enzymes. Evidence suggests that active DNA demethylation in mammalian cells is also mediated at least in part by a base excision repair pathway where the AID/Apobec family of deaminases convert 5-methylcytosine to thymine followed by G/T mismatch repair by the DNA glycosylase MBD4 or TDG. This review also discusses other possible mechanisms of active DNA demethylation, how genome DNA methylation status might be sensed to regulate the expression of demethylase genes, and the targeting of demethylases by small RNAs.

Heterochromatic siRNAs and DDM1 independently silence aberrant 5S rDNA transcripts in Arabidopsis

5S ribosomal RNA gene repeats are arranged in heterochromatic arrays (5S rDNA) situated near the centromeres of Arabidopsis chromosomes. The chromatin remodeling factor DDM1 is known to maintain 5S rDNA methylation patterns while silencing transcription through 5S rDNA intergenic spacers (IGS). We mapped small-interfering RNAs (siRNA) to a composite 5S rDNA repeat, revealing a high density of siRNAs matching silenced IGS transcripts. IGS transcript repression requires proteins of the heterochromatic siRNA pathway, including RNA polymerase IV (Pol IV), RNA-DEPENDENT RNA POLYMERASE 2 (RDR2) and DICER-LIKE 3 (DCL3). Using molecular and cytogenetic approaches, we show that the DDM1 and siRNA-dependent silencing effects are genetically independent. DDM1 suppresses production of the siRNAs, however, thereby limiting RNA-directed DNA methylation at 5S rDNA repeats. We conclude that DDM1 and siRNA-dependent silencing are overlapping processes that both repress aberrant 5S rDNA transcription and contribute to the heterochromatic state of 5S rDNA arrays.

Interplay of RNA Pol IV and ROS1 during post-embryonic 5S rDNA chromatin remodeling

We have investigated the chromatin structure of 5S rDNA, a heterochromatic pericentromeric tandemly repeated family, at 2, 3, 4 and 5 days post-germination. Our results revealed a large-scale reorganization of 5S rDNA chromatin that occurs during the first days of development. Unexpectedly, there is a decondensation followed by a 're'condensation of 5S rDNA chromatin, to obtain almost mature nuclei 5 d post-germination. The reorganization of 5S rDNA chromatin is accompanied by a rapid and active demethylation of 5S rDNA mediated by the ROS1 (repressor of silencing 1) demethylase, whereas the plant-specific RNA polymerase IV (Pol IV) is essential to the 5S chromatin 're'condensation. In conclusion, Pol IV and ROS1 collaborate to unlock the 5S rDNA chromatin inherited from the seed, and establish adult features.

A position effect on the heritability of epigenetic silencing

In animals and yeast, position effects have been well documented. In animals, the best example of this process is Position Effect Variegation (PEV) in Drosophila melanogaster. In PEV, when genes are moved into close proximity to constitutive heterochromatin, their expression can become unstable, resulting in variegated patches of gene expression. This process is regulated by a variety of proteins implicated in both chromatin remodeling and RNAi-based silencing. A similar phenomenon is observed when transgenes are inserted into heterochromatic regions in fission yeast. In contrast, there are few examples of position effects in plants, and there are no documented examples in either plants or animals for positions that are associated with the reversal of previously established silenced states. MuDR transposons in maize can be heritably silenced by a naturally occurring rearranged version of MuDR. This element, Muk, produces a long hairpin RNA molecule that can trigger DNA methylation and heritable silencing of one or many MuDR elements. In most cases, MuDR elements remain inactive even after Muk segregates away. Thus, Muk-induced silencing involves a directed and heritable change in gene activity in the absence of changes in DNA sequence. Using classical genetic analysis, we have identified an exceptional position at which MuDR element silencing is unstable. Muk effectively silences the MuDR element at this position. However, after Muk is segregated away, element activity is restored. This restoration is accompanied by a reversal of DNA methylation. To our knowledge, this is the first documented example of a position effect that is associated with the reversal of epigenetic silencing. This observation suggests that there are cis-acting sequences that alter the propensity of an epigenetically silenced gene to remain inactive. This raises the interesting possibility that an important feature of local chromatin environments may be the capacity to erase previously established epigenetic marks.

Epigenetics involves the heritable alteration of gene activity without changes in DNA sequence. Although clearly a repository for heritable information, what makes epigenetic states distinct is that they are far more labile than those associated with DNA sequence. The epigenetic landscape of eukaryotic genomes is far from uniform. Vast stretches of them are effectively epigenetically silenced, while other regions are largely active. The experiments described here suggest that the propensity to maintain heritable epigenetic states can vary depending on position within the genome. Because transposable elements, or transposons, move from place to place within the genome, they make an ideal probe for differences in epigenetic states at various positions. Our model system uses a single transposon, MuDR in maize, and a variant of MuDR, Mu killer (Muk). When MuDR and Muk are combined genetically, MuDR elements become epigenetically silenced, and they generally remain so even after Muk is lost in subsequent generations. However, we have identified a particular position at which the MuDR element reactivates after Muk is lost. These data show that there are some parts of the maize genome that are either competent to erase epigenetic silencing or are incapable of maintaining it. These results suggest that erasure of heritable information may be an important component of epigenetic regulation.

Roles of RNA polymerase IV in gene silencing

Eukaryotes typically have three multi-subunit enzymes that decode the nuclear genome into RNA: DNA-dependent RNA polymerases I, II and III (Pol I, II and III). Remarkably, higher plants have five multi-subunit nuclear RNA polymerases: the ubiquitous Pol I, II and III, which are essential for viability; plus two non-essential polymerases, Pol IVa and Pol IVb, which specialize in small RNA-mediated gene silencing pathways. There are numerous examples of phenomena that require Pol IVa and/or Pol IVb, including RNA-directed DNA methylation of endogenous repetitive elements, silencing of transgenes, regulation of flowering-time genes, inducible regulation of adjacent gene pairs, and spreading of mobile silencing signals. Although biochemical details concerning Pol IV enzymatic activities are lacking, genetic evidence suggests several alternative models for how Pol IV might function.

siRNA and miRNA processing: new functions for Cajal bodies

In diverse eukaryotes, micro-RNAs (miRNAs) and small interfering RNAs (siRNAs) regulate important processes that include mRNA inactivation, viral defense, chromatin modification, and transposon silencing. Recently, nucleolus-associated Cajal bodies in plants have been implicated as sites of siRNA and miRNA biogenesis, whereas in animals siRNA and miRNA dicing occurs in the cytoplasm. The plant nucleolus also contains proteins of the nonsense-mediated mRNA decay pathway that in animals are found associated with cytoplasmic processing bodies (P-bodies). P-bodies also function in the degradation of mRNAs subjected to miRNA and siRNA targeting. Collectively, these observations suggest interesting variations in the way siRNAs and miRNAs can accomplish their similar functions in plants and animals.

RNA polymerase I: a multifunctional molecular machine

In this issue, Kuhn et al. (2007) report the complete structure of the 14-subunit yeast RNA polymerase (Pol) I enzyme at 12 A resolution using cryo-electron microscopy (cryo-EM). Their study reveals that three subunits of Pol I perform functions in transcription elongation that are outsourced to the transcription factors TFIIF and TFIIS in the analogous Pol II transcription system.

Noncoding RNAs in Long-Term Memory Formation

Current research exploring the molecular basis of memory focuses mainly on proteins despite recent genomic studies reporting the abundant transcription of non-protein-coding RNA (ncRNA). Although ncRNAs are involved in a diverse range of biological processes, they are particularly prevalent within the nervous system, where they contribute towards the complexity and function of the mammalian brain. In this review, we apply recent advances in ncRNA biology to predict a critical role for ncRNAs in the molecular mechanisms underlying memory formation and maintenance. We describe the role of ncRNAs in regulating the translation, stability, and editing of mRNA populations in response to synaptic activity during memory formation and the role of ncRNAs in the epigenetic and transcriptional programs that underlie long-term memory storage. We also consider ncRNAs acting as an additional avenue of communication between neurons by their intercellular trafficking. Taken together, the emerging evidence suggests a central role for ncRNAs in memory formation and provokes novel research directions in this field. NEUROSCIENTIST 14(5):434—445, 2008. DOI: 10.1177/1073858408319187