Protein-coding cis-natural antisense transcripts have high and broad expression in Arabidopsis

Phototropin2 3'UTR overlaps with the AT5G58150 gene encoding an inactive RLK kinase

BACKGROUND

This study examines the biological implications of an overlap between two sequences in the Arabidopsis genome, the 3'UTR of the PHOT2 gene and a putative AT5G58150 gene, encoded on the complementary strand. AT5G58150 is a probably inactive protein kinase that belongs to the transmembrane, leucine-rich repeat receptor-like kinase family. Phot2 is a membrane-bound UV/blue light photoreceptor kinase. Thus, both proteins share their cellular localization, on top of the proximity of their loci.

RESULTS

The extent of the overlap between 3'UTR regions of AT5G58150 and PHOT2 was found to be 66 bp, using RACE PCR. Both the at5g58150 T-DNA SALK_093781C (with insertion in the promoter region) and 35S::AT5G58150-GFP lines overexpress the AT5G58150 gene. A detailed analysis did not reveal any substantial impact of PHOT2 or AT5G58150 on their mutual expression levels in different light and osmotic stress conditions. AT5G58150 is a plasma membrane protein, with no apparent kinase activity, as tested on several potential substrates. It appears not to form homodimers and it does not interact with PHOT2. Lines that overexpress AT5G58150 exhibit a greater reduction in lateral root density due to salt and osmotic stress than wild-type plants, which suggests that AT5G58150 may participate in root elongation and formation of lateral roots. In line with this, mass spectrometry analysis identified proteins with ATPase activity, which are involved in proton transport and cell elongation, as putative interactors of AT5G58150. Membrane kinases, including other members of the LRR RLK family and BSK kinases (positive regulators of brassinosteroid signalling), can also act as partners for AT5G58150.

CONCLUSIONS

AT5G58150 is a membrane protein that does not exhibit measurable kinase activity, but is involved in signalling through interactions with other proteins. Based on the interactome and root architecture analysis, AT5G58150 may be involved in plant response to salt and osmotic stress and the formation of roots in Arabidopsis.

Long noncoding RNAs in Brassica rapa L. following vernalization

Brassica rapa L. is an important agricultural crop that requires a period of prolonged cold for flowering. This process is known as vernalization. Studies have shown that long noncoding RNAs (lncRNAs) play important roles in abiotic stress responses and several cold-responsive noncoding RNAs have been suggested to be involved in vernalization. We examined the transcriptome of the Chinese cabbage inbred line (B. rapa L. var. pekinensis) RJKB-T24, and identified 1,444 long intergenic noncoding RNAs (lincRNAs), 551 natural antisense transcripts (NATs), and 93 intronic noncoding RNAs (incRNAs); 549 of the 2,088 lncRNAs significantly altered their expression in response to four weeks of cold treatment. Most differentially expressed lncRNAs did not lead to a change of expression levels in mRNAs covering or near lncRNAs, suggesting that the transcriptional responses to four weeks of cold treatment in lncRNA and mRNA are independent. However, some differentially expressed mRNAs had NATs with expression altered in the same direction. These genes were categorized as having an abiotic stress response, suggesting that the paired-expression may play a role in the transcriptional response to vernalization or cold treatment. We also identified short-term cold treatment induced NATs in BrFLC and BrMAF genes, which are involved in vernalization. The lncRNAs we identified differed from those reported in Arabidopsis thaliana, suggesting the role of lncRNAs in vernalization differ between these two species.

The Involvement of Long Noncoding RNAs in Response to Plant Stress

Plant growth and productivity are greatly impacted by environmental stresses. Therefore, plants have evolved mechanisms which allow them to adapt to abiotic stresses through alterations in gene expression and metabolism. In recent years, studies have investigated the role of long noncoding RNA (lncRNA) in regulating gene expression in plants and characterized their involvement in various biological functions through their regulation of DNA methylation, DNA structural modifications, histone modifications, and RNA-RNA interactions. Genome-wide transcriptome analyses have identified various types of noncoding RNAs (ncRNAs) that respond to abiotic stress. These ncRNAs are in addition to the well-known housekeeping ncRNAs, such as rRNAs, tRNAs, snoRNAs, and snRNAs. In this review, recent research pertaining to the role of lncRNAs in the response of plants to abiotic stress is summarized and discussed.

Arabidopsis PIRL6 Is Essential for Male and Female Gametogenesis and Is Regulated by Alternative Splicing

Plant intracellular Ras-group leucine-rich repeat (LRR) proteins (PIRLs) are related to Ras-interacting animal LRR proteins that participate in developmental cell signaling. Systematic knockout analysis has implicated some members of the Arabidopsis (Arabidopsis thaliana) PIRL family in pollen development. However, for PIRL6, no bona fide knockout alleles have been recovered, suggesting that it may have an essential function in both male and female gametophytes. To test this hypothesis, we investigated PIRL6 expression and induced knockdown by RNA interference. Knockdown triggered defects in gametogenesis, resulting in abnormal pollen and early developmental arrest in the embryo sac. Consistent with this, PIRL6 was expressed in gametophytes: functional transcripts were detected in wild-type flowers but not in sporocyteless (spl) mutant flowers, which do not produce gametophytes. A genomic PIRL6-GFP fusion construct confirmed expression in both pollen and the embryo sac. Interestingly, PIRL6 is part of a convergent overlapping gene pair, a scenario associated with an increased likelihood of alternative splicing. We detected multiple alternative PIRL6 mRNAs in vegetative organs and spl mutant flowers, tissues that lacked the functionally spliced transcript. cDNA sequencing revealed that all contained intron sequences and premature termination codons. These alternative mRNAs accumulated in the nonsense-mediated decay mutant upf3, indicating that they are normally subjected to degradation. Together, these results demonstrate that PIRL6 is required in both male and female gametogenesis and suggest that sporophytic expression is negatively regulated by unproductive alternative splicing. This posttranscriptional mechanism may function to minimize PIRL6 protein expression in sporophyte tissues while allowing the overlapping adjacent gene to remain widely transcribed.

Widespread antisense transcription of Populus genome under drought

Antisense transcription is widespread in many genomes and plays important regulatory roles in gene expression. The objective of our study was to investigate the extent and functional relevance of antisense transcription in forest trees. We employed Populus, a model tree species, to probe the antisense transcriptional response of tree genome under drought, through stranded RNA-seq analysis. We detected nearly 48% of annotated Populus gene loci with antisense transcripts and 44% of them with co-transcription from both DNA strands. Global distribution of reads pattern across annotated gene regions uncovered that antisense transcription was enriched in untranslated regions while sense reads were predominantly mapped in coding exons. We further detected 1185 drought-responsive sense and antisense gene loci and identified a strong positive correlation between the expression of antisense and sense transcripts. Additionally, we assessed the antisense expression in introns and found a strong correlation between intronic expression and exonic expression, confirming antisense transcription of introns contributes to transcriptional activity of Populus genome under drought. Finally, we functionally characterized drought-responsive sense-antisense transcript pairs through gene ontology analysis and discovered that functional groups including transcription factors and histones were concordantly regulated at both sense and antisense transcriptional level. Overall, our study demonstrated the extensive occurrence of antisense transcripts of Populus genes under drought and provided insights into genome structure, regulation pattern and functional significance of drought-responsive antisense genes in forest trees. Datasets generated in this study serve as a foundation for future genetic analysis to improve our understanding of gene regulation by antisense transcription.

Cell-specific cis-natural antisense transcripts (cis-NATs) in the sperm and the pollen vegetative cells of Arabidopsis thaliana

Background: cis-NATs (cis-natural antisense transcripts ) are transcribed from opposite strands of adjacent genes and have been shown to regulate gene expression by generating small RNAs from the overlapping region. cis-NATs are important for plant development and resistance to pathogens and stress. Several genome-wide investigations identified a number of cis-NAT pairs, but these investigations predicted cis-NATS using expression data from bulk samples that included lots of cell types. Some cis-NAT pairs identified from those investigations might not be functional, because both transcripts of cis-NAT pairs need to be co-expressed in the same cell. Pollen only contains two cell types, two sperm and one vegetative cell, which makes cell-specific investigation of cis-NATs possible.

Methods: We investigated potential protein-coding cis-NATs in pollen and sperm using pollen RNA-seq data and TAIR10 gene models using the Integrated Genome Browser.  We then used sperm microarray data and sRNAs in sperm and pollen to determine possibly functional cis-NATs in the sperm or vegetative cell, respectively.

Results: We identified 1471 potential protein-coding cis-NAT pairs, including 131 novel pairs that were not present in TAIR10 gene models. In pollen, 872 possibly functional pairs were identified. 72 and 56 pairs were potentially functional in sperm and vegetative cells, respectively. sRNAs were detected at 794 genes, belonging to 739 pairs.

Conclusion: These potential candidates in sperm and the vegetative cell are tools for understanding gene expression mechanisms in pollen.

Evolutionarily recent, insertional fission of mitochondrial cox2 into complementary genes in bilaterian Metazoa

Background

Mitochondrial genomes (mtDNA) of multicellular animals (Metazoa) with bilateral symmetry (Bilateria) are compact and usually carry 13 protein-coding genes for subunits of three respiratory complexes and ATP synthase. However, occasionally reported exceptions to this typical mtDNA organization prompted speculation that, as in protists and plants, some bilaterian mitogenomes may continue to lose their canonical genes, or may even acquire new genes. To shed more light on this phenomenon, a PCR-based screen was conducted to assess fast-evolving mtDNAs of apocritan Hymenoptera (Arthropoda, Insecta) for genomic rearrangements that might be associated with the modification of mitochondrial gene content.

Results

Sequencing of segmental inversions, identified in the screen, revealed that the cytochrome oxidase subunit II gene (cox2) of Campsomeris (Dielis) (Scoliidae) was split into two genes coding for COXIIA and COXIIB. The COXII-derived complementary polypeptides apparently form a heterodimer, have reduced hydrophobicity compared with the majority of mitogenome-encoded COX subunits, and one of them, COXIIB, features increased content of Cys residues. Analogous cox2 fragmentation is known only in two clades of protists (chlorophycean algae and alveolates), where it has been associated with piecewise relocation of this gene into the nucleus. In Campsomeris mtDNA, cox2a and cox2b loci are separated by a 3-kb large cluster of several antiparallel overlapping ORFs, one of which, qnu, seems to encode a nuclease that may have played a role in cox2 fission.

Conclusions

Although discontinuous mitochondrial protein genes encoding fragmented, complementary polypeptides are known in protists and some plants, split cox2 of Campsomeris is the first case of such a gene arrangement found in animals. The reported data also indicate that bilaterian animal mitogenomes may be carrying lineage-specific genes more often than previously thought, and suggest a homing endonuclease-based mechanism for insertional mitochondrial gene fission.

Electronic supplementary material

The online version of this article (doi:10.1186/s12864-017-3626-5) contains supplementary material, which is available to authorized users.

Genome-wide characterization of intergenic polyadenylation sites redefines gene spaces in Arabidopsis thaliana

Background

Messenger RNA polyadenylation is an essential step for the maturation of most eukaryotic mRNAs. Accurate determination of poly(A) sites helps define the 3’-ends of genes, which is important for genome annotation and gene function research. Genomic studies have revealed the presence of poly(A) sites in intergenic regions, which may be attributed to 3’-UTR extensions and novel transcript units. However, there is no systematically evaluation of intergenic poly(A) sites in plants.

Results

Approximately 16,000 intergenic poly(A) site clusters (IPAC) in Arabidopsis thaliana were discovered and evaluated at the whole genome level. Based on the distributions of distance from IPACs to nearby sense and antisense genes, these IPACs were classified into three categories. About 70 % of them were from previously unannotated 3’-UTR extensions to known genes, which would extend 6985 transcripts of TAIR10 genome annotation beyond their 3’-ends, with a mean extension of 134 nucleotides. 1317 IPACs were originated from novel intergenic transcripts, 37 of which were likely to be associated with protein coding transcripts. 2957 IPACs corresponded to antisense transcripts for genes on the reverse strand, which might affect 2265 protein coding genes and 39 non-protein-coding genes, including long non-coding RNA genes. The rest of IPACs could be originated from transcriptional read-through or gene mis-annotations.

Conclusions

The identified IPACs corresponding to novel transcripts, 3’-UTR extensions, and antisense transcription should be incorporated into current Arabidopsis genome annotation. Comprehensive characterization of IPACs from this study provides insights of alternative polyadenylation and antisense transcription in plants.

Electronic supplementary material

The online version of this article (doi:10.1186/s12864-015-1691-1) contains supplementary material, which is available to authorized users.

Genome-wide view of natural antisense transcripts in Arabidopsis thaliana

Natural antisense transcripts (NATs) are endogenous transcripts that can form double-stranded RNA structures. Many protein-coding genes (PCs) and non-protein-coding genes (NPCs) tend to form cis-NATs and trans-NATs, respectively. In this work, we identified 4,080 cis-NATs and 2,491 trans-NATs genome-widely in Arabidopsis. Of these, 5,385 NAT-siRNAs were detected from the small RNA sequencing data. NAT-siRNAs are typically 21nt, and are processed by Dicer-like 1 (DCL1)/DCL2 and RDR6 and function in epigenetically activated situations, or 24nt, suggesting these are processed by DCL3 and RDR2 and function in environment stress. NAT-siRNAs are significantly derived from PC/PC pairs of trans-NATs and NPC/NPC pairs of cis-NATs. Furthermore, NAT pair genes typically have similar pattern of epigenetic status. Cis-NATs tend to be marked by euchromatic modifications, whereas trans-NATs tend to be marked by heterochromatic modifications.

NERF encodes a RING E3 ligase important for drought resistance and enhances the expression of its antisense gene NFYA5 in Arabidopsis

NFYA5 is an important drought-stress inducible transcription factor gene that is targeted by miR169 in Arabidopsis. We show here that the cis-natural antisense transcript gene of NFYA5, NFYA5 Enhancing RING FINGER (NERF), can produce siRNAs from their overlapping region (OR) and affect NFYA5 transcripts by functioning together with miR169. The NERF protein functions as an E3 ligase for ubiquitination. Overexpression of NERF or OR cDNA leads to siRNANERF accumulation, miR169 repression, and NFYA5 transcript enhancement; knock-down of NERF transcripts by an artificial miRNA enhances miR169 abundance and reduces NFYA5 transcripts. Overexpression of NFYA5 does not affect the NERF mRNA level. Deep sequencing of the small RNA library from 35S::OR plants identifies 960 sequences representing 323 unique siRNAs that originate from OR; the sequences of some siRNANERF are similar/complementary to those of miR169. Overexpression of the 195- to 280-bp OR cDNA-containing siRNAs similar/complementary to miR169 also leads to the accumulation of NFYA5 transcripts. Analysis of NERF knock-down plants and NERF overexpression lines showed that, like NFYA5, NERF is important for controlling stomatal aperture and drought resistance. This regulatory model might apply to other natural antisense transcripts with positively correlated expression patterns.

Extending the sRNAome of apple by next-generation sequencing

The global importance of apple as a fruit crop necessitates investigations into molecular aspects of the processes that influence fruit quality and yield, including plant development, fruit ripening and disease resistance. In order to study and understand biological processes it is essential to recognise the range of molecules, which influence these processes. Small non-coding RNAs are regulatory agents involved in diverse plant activities, ranging from development to stress response. The occurrence of these molecules in apple leaves was studied by means of next-generation sequencing. 85 novel microRNA (miRNA) gene loci were predicted and characterized along with known miRNA loci. Both cis- and trans-natural antisense transcript pairs were identified. Although the trans-overlapping regions were enriched in small RNA (sRNA) production, cis-overlaps did not seem to agree. More than 150 phased regions were also identified, and for a small subset of these, potential miRNAs that could initiate phasing, were revealed. Repeat-associated siRNAs, which are generated from repetitive genomic regions such as transposons, were also analysed. For this group almost all available repeat sequences, associated with the apple genome and present in Repbase, were found to produce siRNAs. Results from this study extend our current knowledge on apple sRNAs and their precursors significantly. A rich molecular resource has been created and is available to the research community to serve as a baseline for future studies.

Arabidopsis non-coding RNA regulation in abiotic stress responses

Plant growth and productivity are largely affected by environmental stresses. Therefore, plants have evolved unique adaptation mechanisms to abiotic stresses through fine-tuned adjustment of gene expression and metabolism. Recent advanced technologies, such as genome-wide transcriptome analysis, have revealed that a vast amount of non-coding RNAs (ncRNAs) apart from the well-known housekeeping ncRNAs such as rRNAs, tRNAs, small nuclear RNAs (snRNAs) and small nucleolar RNAs (snoRNAs) are expressed under abiotic stress conditions. These various types of ncRNAs are involved in chromatin regulation, modulation of RNA stability and translational repression during abiotic stress response. In this review, we summarize recent progress that has been made on ncRNA research in plant abiotic stress response.

Integrated detection of natural antisense transcripts using strand-specific RNA sequencing data

Pairs of RNA molecules transcribed from partially or entirely complementary loci are called cis-natural antisense transcripts (cis-NATs), and they play key roles in the regulation of gene expression in many organisms. A promising experimental tool for profiling sense and antisense transcription is strand-specific RNA sequencing (ssRNA-seq). To identify cis-NATs using ssRNA-seq, we developed a new computational method based on a model comparison framework that incorporates the inherent variable efficiency of generating perfectly strand-specific libraries. Applying the method to new ssRNA-seq data from whole-root and cell-type–specific Arabidopsis libraries confirmed most of the known cis-NAT pairs and identified 918 additional cis-NAT pairs. Newly identified cis-NAT pairs are supported by polyadenylation data, alternative splicing patterns, and RT-PCR validation. We found 209 cis-NAT pairs that have opposite expression levels in neighboring cell types, implying cell-type–specific roles for cis-NATs. By integrating a genome-wide epigenetic profile of Arabidopsis, we identified a unique chromatin signature of cis-NATs, suggesting a connection between cis-NAT transcription and chromatin modification in plants. An analysis of small-RNA sequencing data showed that ∼4% of cis-NAT pairs produce putative cis-NAT–induced siRNAs. Taken together, our data and analyses illustrate the potential for multifaceted regulatory roles of plant cis-NATs.

Natural antisense transcripts in plants: a review and identification in soybean infected with Phakopsora pachyrhizi SuperSAGE library

Natural antisense ranscripts (NAT) are RNA molecules complementary to other endogenous RNAs. They are capable of regulating the expression of target genes at different levels (transcription, mRNA stability, translation, etc.). Such a property makes them ideal for interventions in organisms' metabolism. The present study reviewed plant NAT aspects, including features, availability and genesis, conservation and distribution, coding capacity, NAT pair expression, and functions. Besides, an in silico identification of NATs pairs was presented, using deepSuperSAGE libraries of soybean infected or not with Phakopsora pachyrhizi. Results showed that around 1/3 of the 77,903 predicted trans-NATs (by PlantsNATsDB database) detected had unitags mapped in both sequences of each pair. The same 1/3 of the 436 foreseen cis-NATs showed unitags anchored in both sequences of the related pairs. For those unitags mapped in NAT pairs, a modulation expression was assigned as upregulated, downregulated, or constitutive, based on the statistical analysis (P < 0.05). As a result, the infected treatment promoted the expression of 2,313 trans-NATs pairs comprising unitags exclusively from that library (1,326 pairs had unitags only found in the mock library). To understand the regulation of these NAT pairs could be a key aspect in the ASR plant response.

Natural antisense transcripts of Trifolium repens dehydrins

The recently described complex nature of some dehydrin-coding sequences in Trifolium repens could explain the considerable variability among transcripts originating from a single gene.1 For some of the sequences the existence of natural antisense transcripts (NAT s), which could form sense-antisense (SAS) pairs, was predicted. The present study demonstrates that cis-natural antisense transcripts of 2 dehydrin types (YnKn and YnSKn) accumulate in white clover plants subjected to treatments with polyethylene glycol (PEG), abscisic acid (ABA), and high salt concentration. The isolated YnKn cis-NAT s mapped to sequence site enriched in alternative start codons. Some of the sense-antisense pairs exhibited inverse expression with differing profiles which depended on the applied stress. A natural antisense transcript coding for an ABC F family protein (a trans-NAT ) which shares short sequence homology with YnSKn dehydrin was identified in plants subjected to salt stress. Forthcoming experiments will evaluate the impact of NAT s on transcript abundances, elucidating the role of transcriptional and post-transcriptional interferences in the regulation of dehydrin levels under various abiotic stresses.