Regulatory mechanism of heat-active retrotransposons by the SET domain protein SUVH2.

New transposon insertions are deleterious to genome stability. The RNA-directed DNA methylation (RdDM) pathway evolved to regulate transposon activity via DNA methylation. However, current studies have not yet clearly described the transposition regulation. ONSEN is a heat-activated retrotransposon that is activated at 37°C. The plant-specific SUPPRESSOR OF VARIEGATION 3-9 HOMOLOG (SUVH) family proteins function downstream of the RdDM pathway. The SUVH protein families are linked to TE silencing by two pathways, one through DNA methylation and the other through chromatin remodeling. In this study, we analyzed the regulation of ONSEN activity by SUVH2. We observed that ONSEN transcripts were increased; however, there was no transpositional activity in Arabidopsis suvh2 mutant. The suvh2 mutant produced siRNAs from the ONSEN locus under heat stress, suggesting that siRNAs are involved in suppressing transposition. These results provide new insights into the regulatory mechanisms of retrotransposons that involve siRNA in the RdDM pathway.

Environmental stress and transposons in plants.

Transposons were once thought to be junk repetitive DNA in the genome. However, their importance gradually became apparent as it became clear that they regulate gene expression, which is essential for organisms to survive, and that they are important factors in the driving force of evolution. Since there are multiple transposons in the genomes of all organisms, transposons have likely been activated and increased in copy number throughout their long history. This review focuses on environmental stress as a factor in transposon activation, paying particular attention to transposons in plants that are activated by environmental stresses. It is now known that plants respond to environmental stress in various ways, and correspondingly, many transposons respond to stress. The relationship between environmental stress and transposons is reviewed, including the mechanisms of their activation and the effects of transposon activation on host plants.

Epigenetic regulation of ecotype-specific expression of the heat-activated transposon ONSEN.

Transposable elements are present in a wide variety of organisms; however, our understanding of the diversity of mechanisms involved in their activation is incomplete. In this study, we analyzed the transcriptional activation of the ONSEN retrotransposon, which is activated by high-temperature stress in Arabidopsis thaliana. We found that its transcription is significantly higher in the Japanese ecotype Kyoto. Considering that transposons are epigenetically regulated, DNA methylation levels were analyzed, revealing that CHH methylation was reduced in Kyoto compared to the standard ecotype, Col-0. A mutation was also detected in the Kyoto CMT2 gene, encoding a CHH methyltransferase, suggesting that it may be responsible for increased expression of ONSEN. CHH methylation is controlled by histone modifications through a self-reinforcing loop between DNA methyltransferase and histone methyltransferase. Analysis of these modifications revealed that the level of H3K9me2, a repressive histone marker for gene expression, was lower in Kyoto than in Col-0. The level of another repressive histone marker, H3K27me1, was decreased in Kyoto; however, it was not impacted in a Col-0 cmt2 mutant. Therefore, in addition to the CMT2 mutation, other factors may reduce repressive histone modifications in Kyoto.

The novel activity of Argonautes in intron splicing: A transcriptome-wide survey in plants.

The importance of the evolutionarily conserved Argonaute (AGO) proteins has been well recognized for their involvement in the RNA interference pathways. Recent discoveries in animals demonstrated that AGOs also participate in alternative splicing (AS). Motivated by the question whether the AGO proteins are also functional in RNA splicing in plants, we searched for the introns excised through an AGO-dependent manner in Arabidopsis (Arabidopsis thaliana). RNA sequencing (RNA-seq) data analysis uncovered hundreds of the introns up- or down-regulated in the ago1 and ago4 mutants, respectively. For different genes, AGOs might play either a positive or a negative role in intron excision, which was further validated by reverse transcription-polymerase chain reaction (RT-PCR). Some introns were specifically regulated by one of the AGO proteins, while some were regulated by both AGOs. Besides, a large portion of the AGO-dependent introns were organ-specifically regulated. RNA immunoprecipitation combined with high-throughput sequencing (RIP-seq) revealed that both AGOs preferentially bound to the intronic regions, supporting their high intron binding affinities. Immunoprecipitation followed by mass spectrometry (IP-MS) was performed to identify the proteins potentially interacting with the two AGOs. Six novel interactors (two interacting with AGO1 and four with both AGOs) involved in mRNA binding were uncovered, which might facilitate AGO-intron recognition. Analysis of the RNA-seq data from the rice (Oryza sativa) ago18 mutants revealed that hundreds of the introns were expressed in an AGO18-dependent manner. In summary, our results point to the novel role of the plant AGOs in intron splicing, paving a way for further studies on the mechanisms underlying AGO-mediated RNA splicing.

Regulatory mechanism of a heat-activated retrotransposon by DDR complex in Arabidopsis thaliana.

The RNA-directed DNA methylation (RdDM) pathway plays an essential role in the transposon silencing mechanism; the DDR complex, consisting of DRD1, DMS3, and RDM1, is an essential component of the RdDM pathway. ONSEN, identified in Arabidopsis, is a retrotransposon activated by heat stress at 37°C; however, studies on the regulation of ONSEN are limited. In this study, we analyzed the regulation of ONSEN activity by the DDR complex in Arabidopsis. We elucidated that loss of any component of the DDR complex increased ONSEN transcript levels. Transgenerational transposition of ONSEN was observed in the DDR-complex mutants treated with heat stress for 48 h. Furthermore, the DDR complex components DRD1, DMS3, and RDM1 played independent roles in suppressing ONSEN transcription and transposition. Moreover, we found that the duration of heat stress affects ONSEN activity. Therefore, the results of this study provide new insights into the retrotransposon regulatory mechanisms of the DDR complex in the RdDM pathway.

Mixed-mode oscillations from a constrained extended Bonhoeffer-van der Pol oscillator with a diode.

An extended Bonhoeffer-van der Pol (BVP) oscillator is a circuit that is naturally extended to a three-variable system from a two-variable BVP oscillator. A BVP oscillator is known to exhibit a canard explosion, and the extended BVP oscillator generates mixed-mode oscillations (MMOs). In this work, we considered a case study where the nonlinear conductor in the extended BVP oscillator includes an idealized diode. The idealized case corresponds to a degenerate case where one of the parameters tends to infinity, and circuit dynamics are represented using a constrained equation, and at the expense of the model's naturalness, i.e., in a case in which the solutions of the dynamics are defined only forward in time, the Poincaré return maps are constructed as one-dimensional (1D). Using these 1D return maps, we explain various phenomena, such as simple MMOs and MMO-incrementing bifurcations. In this oscillator, there exists a small amplitude oscillation, which emerges as a consequence of supercritical Hopf bifurcation, and there exists large relaxation oscillation which appears via canard explosion by changing the bifurcation parameter. Between these small and large amplitude oscillations, the MMO bifurcations exhibit asymmetric Farey trees. Furthermore, these theoretical results were verified using laboratory measurements and experiments.

DRD1, a SWI/SNF-like chromatin remodeling protein, regulates a heat-activated transposon in Arabidopsis thaliana.

ONSEN is a heat-activated LTR retrotransposon in Arabidopsis thaliana. Screens to identify transcriptional regulatory factors of ONSEN revealed a SWI/SNF-like chromatin remodeling protein, DRD1, which cooperates with plant-specific RNA polymerase and is involved in RNA-directed DNA methylation. ONSEN transcript level was increased in the drd1 mutant relative to wild-type under heat stress, indicating that DRD1 plays a significant role in the silencing of activated ONSEN under the stress condition. The transcript level of HsfA2, which is directly involved in transcriptional activation of ONSEN, was not higher in the drd1 mutant than in the wild-type. Interestingly, no transgenerational transposition of ONSEN was observed in the drd1 mutant, even though DNA methylation levels were significantly reduced and expression levels were increased compared to the wild-type. These results suggest that other factors are involved in the regulation of ONSEN transposition in addition to the transcript level of ONSEN.

DNA methyltransferase CHROMOMETHYLASE3 prevents ONSEN transposon silencing under heat stress.

DNA methylation plays crucial roles in transposon silencing and genome integrity. CHROMOMETHYLASE3 (CMT3) is a plant-specific DNA methyltransferase responsible for catalyzing DNA methylation at the CHG (H = A, T, C) context. Here, we identified a positive role of CMT3 in heat-induced activation of retrotransposon ONSEN. We found that the full transcription of ONSEN under heat stress requires CMT3. Interestingly, loss-of-function CMT3 mutation led to increased CHH methylation at ONSEN. The CHH methylation is mediated by CMT2, as evidenced by greatly reduced CHH methylation in cmt2 and cmt2 cmt3 mutants coupled with increased ONSEN transcription. Furthermore, we found more CMT2 binding at ONSEN chromatin in cmt3 compared to wild-type accompanied with an ectopic accumulation of H3K9me2 under heat stress, suggesting a collaborative role of H3K9me2 and CHH methylation in preventing heat-induced ONSEN activation. In summary, this study identifies a non-canonical role of CMT3 in preventing transposon silencing and provides new insights into how DNA methyltransferases regulate transcription under stress conditions.

How to Activate Heat-Responsible Retrotransposon ONSEN in Brassicaceae Species.

A number of transposable elements are activated by environmental stress. A Ty1/copia-type retrotransposon named ONSEN is activated by heat stress in Brassicaceae species. A synthetic activation of the transposon is effective for the molecular breeding without genetic modification. Here, we described the detail procedure of heat treatment to activate ONSEN in Brassicaceae species.

ONSEN shows different transposition activities in RdDM pathway mutants.

Most transposable elements (TEs) are tightly regulated by epigenetic mechanisms such as DNA methylation. RNA-directed DNA methylation (RdDM) is a major control mechanism of TE silencing in plants. We analyzed the transposition activity of a heat-responsive retrotransposon, ONSEN, in Arabidopsis thaliana. Transgenerational transposition was observed in RdDM pathway-deficient mutants upon heat stress. The transposition frequency was higher in the mutants of the upstream processes, but lower in the mutants of the downstream steps, of RdDM. The transposition frequency was not associated with the number of extrachromosomal ONSEN copies. Constitutive heterochromatin of interphase nuclei was dispersed upon heat stress. The degree of decondensation was higher in the RdDM mutants than in wild-type plants subjected to heat stress. We discuss the possible role of RdDM in the regulation of ONSEN transposition upon heat stress.

The effect of zebularine on the heat-activated retrotransposon ONSEN in Arabidopsis thaliana and Vigna angularis.

The Ty1/copia-like retrotransposon ONSEN is conserved among Brassica species, as well as in beans, including adzuki bean (Vigna angularis (Willd.) Ohwi & Ohashi), which is one of the economically important crops in Japan. ONSEN has acquired a heat-responsive element that is recognized by plant heat stress defense factors, resulting in its transcription and the production of full-length extrachromosomal DNA under conditions with elevated temperatures. DNA methylation plays an important role in regulating the activation of this transposon in plants. Therefore, chemical inhibition of DNA methyltransferases has been utilized to study the effect of DNA methylation on transposon activation. To understand the effect of DNA methylation on ONSEN activation, Arabidopsis thaliana and adzuki bean seedlings were treated with zebularine, which is known to be an effective chemical demethylation agent. The results showed that ONSEN transcription levels were upregulated in zebularine-treated plants. Extrachromosomal DNA of ONSEN also accumulated in the treated plants.

The RNA degradome: a precious resource for deciphering RNA processing and regulation codes in plants.

The plant RNA degradome was defined as an aggregate of the RNA fragments degraded from various biochemical pathways, such as RNA turnover, maturation and quality surveillance. In recent years, the degradome sequencing (degradome-seq) libraries became a rich storehouse for researchers to study on RNA processing and regulation. Here, we provided a brief overview of the uses of degradome-seq data in plant RNA biology, especially on non-coding RNA processing and small RNA-guided target cleavages. Some novel applications in RNA research area, such as in vivo mapping of the endoribonucleolytic cleavage sites, identification of conserved motifs at the 5' ends of the uncapped RNA fragments, and searching for the protein-binding regions on the transcripts, were also mentioned. More importantly, we proposed a model for the biologists to deduce the contributions of transcriptional and/or post-transcriptional regulation to gene differential expression based on degradome-seq data. Finally, we hope that the degradome-based analytical methods could be widely applied for the studies on RNA biology in eukaryotes.

MepmiRDB: a medicinal plant microRNA database.

MicroRNAs (miRNAs) have been recognized as a key regulator in plant development and metabolism. Recent reports showed that the miRNAs of medicinal plants not only act as a critical modulator in secondary metabolism but also had a great potential of performing cross-kingdom gene regulation. Although several plant miRNA repositories have been publicly available, no miRNA database specific for medicinal plants has been reported to date. Here, we report the first version of MepmiRDB (medicinal plant microRNA database), which is freely accessible at http://mepmirdb.cn/mepmirdb/index.html. This database accommodates thousands of miRNA candidates belonging to 29 medicinal plant species. The miRNA information on sequences, expression patterns and regulatory networks has been included in the functional modules of the database. Specifically, the 'Sequence' module provides the sequences of the mature miRNAs and their precursors, and the structure information of the precursors. Moreover, the processing and small RNA accumulation signals on the miRNA precursors are also included in the 'Sequence' module. The organ/growth condition-specific expression information of the mature miRNAs has been stored in the 'Expression' module. The 'Interaction' module offers the information of the degradome-validated miRNA-target pairs of eight plant species. The 'Search' module enables users to search for the miRNAs by plant species and miRNA families, or by sequences. All data in this database are available for download. Taken together, the functional modules of MepmiRDB ensure its importance and timeliness for mechanistic and functional studies on the medicinal plant miRNAs.

Deciphering the non-coding RNA-level response to arsenic stress in rice (Oryza sativa).

Arsenic (As) contamination in subsoil and groundwater is a big problem, especially in many South-East Asian countries. As a staple crop growing under flooded condition in these areas, rice (Oryza sativa L.) becomes a big threat to human health through the food chain since As is highly accumulated in grains. Thus, reducing As accumulation in rice through molecular breeding and identification of rice varieties with low As content are the pressing issues. However, the current understanding on the molecular mechanism of As stress response is still limited for rice. In this study, we performed a comprehensive search for the As-responsive small RNAs (sRNAs) of rice. Briefly, 4,762 and 18,152 sRNAs were identified to be highly activated under As stress in roots and shoots respectively, while 14,603 and 8,308 sRNAs were intensively repressed by As treatment in roots and shoots, respectively. A number of the As-responsive sRNAs found their loci on tRNAs, rRNAs or long non-coding RNAs (lncRNAs). Interestingly, these loci preferentially distributed on the 5' halves of the tRNA, rRNA or lncRNA precursors. Among the above-identified As-responsive sRNAs, 252 Argonaute 1 (AGO1)-enriched sRNAs were extracted for target identification, resulting in 200 pairs of sRNA-protein-coding target interactions. Many targets are functionally involved in the development, stress response, reproduction, or lipid metabolism. Additionally, 56 lncRNAs were discovered to be targeted by nine AGO1-enriched sRNAs, indicating the potential involvement of these lncRNAs in As signaling. Taken together, our results could expand the understanding on the non-coding RNA-mediated As stress response in rice.

PmiRDiscVali: an integrated pipeline for plant microRNA discovery and validation.

BACKGROUND: MicroRNAs (miRNAs) constitute a well-known small RNA (sRNA) species with important regulatory roles. To date, several bioinformatics tools have been developed for large-scale prediction of miRNAs based on high-throughput sequencing data. However, some of these tools become invalid without reference genomes, while some tools cannot supply user-friendly outputs. Besides, most of the current tools focus on the importance of secondary structures and sRNA expression patterns for miRNA prediction, while they do not pay attention to miRNA processing for reliability check. RESULTS: Here, we reported a pipeline PmiRDiscVali for plant miRNA discovery and partial validation. This pipeline integrated the popular tool miRDeep-P for plant miRNA prediction, making PmiRDiscVali compatible for both reference-based and de novo predictions. To check the prediction reliability, we adopted the concept that the miRNA processing intermediates could be tracked by degradome sequencing (degradome-seq) during the development of PmiRDiscVali. A case study was performed by using the public sequencing data of Dendrobium officinale, in order to show the clear and concise presentation of the prediction results. CONCLUSION: Summarily, the integrated pipeline PmiRDiscVali, featured with degradome-seq data-based validation and vivid result presentation, should be useful for large-scale identification of plant miRNA candidates.

Tracking microRNA Processing Signals by Degradome Sequencing Data Analysis.

Degradome sequencing (degradome-seq) was widely used for cleavage site mapping on the microRNA (miRNA) targets. Here, the application value of degradome-seq data in tracking the miRNA processing intermediates was reported. By adopting the parameter "signal/noise" ratio, prominent degradome signals on the miRNA precursors were extracted. For the 15 species analyzed, the processing of many miRNA precursors were supported by the degradome-seq data. We found that the supporting ratio of the "high-confidence" miRNAs annotated in miRBase was much higher than that of the "low-confidence." For a specific species, the percentage of the miRNAs with degradome-supported processing signals was elevated by the increment of degradome sampling diversity. More interestingly, the tissue- or cell line-specific processing patterns of the miRNA precursors partially contributed to the accumulation patterns of the mature miRNAs. In this study, we also provided examples to show the value of the degradome-seq data in miRNA annotation. Based on the distribution of the processing signals, a renewed model was proposed that the stems of the miRNA precursors were diced through a "single-stranded cropping" mode, and "loop-to-base" processing was much more prevalent than previously thought. Together, our results revealed the remarkable capacity of degradome-seq in tracking miRNA processing signals.

Characterization of a heat-activated retrotransposon in Vigna angularis.

In plants, several transposable elements are conserved across species. We found a homolog of ONSEN, which is a heat-activated retrotransposon originally isolated from Arabidopsis thaliana, in Vigna. The ONSEN-like elements (VaONS) were detected in all the analyzed Japanese accessions of Vigna angularis (adzuki bean) by Southern blot analysis. However, VaONS sequences were observed to be polymorphic in the different accessions. Interestingly, extrachromosomal DNA (ecDNA) was detected in some accessions of adzuki bean, indicating the conserved heat-activation of VaONS. Furthermore, we successfully induced retrotransposition of VaONS in adzuki plant regenerated through callus. Findings of our study should provide a new tool for molecular breeding of adzuki bean.

Effects of burstiness on the air transportation system.

The effects of burstiness in complex networks have received considerable attention. In particular, the effects on temporal distance and delays in the air transportation system are significant owing to their huge impact on our society. Therefore, in this paper, the temporal distance of empirical U.S. flight schedule data is compared with that of regularized data without burstiness to analyze the effects of burstiness. The temporal distance is calculated by a graph analysis method considering flight delays, missed connections, flight cancellations, and congestion. In addition, we propose two temporal distance indexes based on passengers' behavior to quantify the effects. As a result, we find that burstiness reduces both the scheduled and the actual temporal distances for business travelers, while delays caused by missed connections and congestion are increased. We also find that the decrease of the scheduled temporal distance by burstiness is offset by an increase of the delays for leisure passengers. Moreover, we discover that the positive effect of burstiness is lost when flight schedules are overcrowded.

Characterization of a heat-activated retrotransposon in natural accessions of Arabidopsis thaliana.

Natural accessions are used for studying intraspecies genetic variation in the model plant Arabidopsis thaliana in order to address fundamental questions of evolution. Transposable elements are responsible for a wide range of mutations and play significant roles in shaping a genome over evolutionary time. In the present study, we aimed to characterize ONSEN, a heat-activated long terminal repeat (LTR) retrotransposon, in natural A. thaliana accessions. Southern blot analysis demonstrated that ONSEN was present in all the studied accessions, but the copy number was diverse. Olympia-1 contained a single ONSEN copy, located in the centromere of Chromosome 3. A premature stop codon in Olympia-1 ONSEN presumably abolishes integrase activity, which in turn presumably renders the retrotransposon non-functional. Hybridization of Col-0 with Olympia-1 showed that several ONSEN copies in Col-0 were activated by heat stress and maintained their transpositional activity in the progeny.