Global Co-transcriptional Splicing in Arabidopsis and the Correlation with Splicing Regulation in Mature RNAs

A plant-specific clade of serine/arginine-rich proteins regulates RNA splicing homeostasis and thermotolerance in tomato

Global warming poses a threat for crops, therefore, the identification of thermotolerance mechanisms is a priority. In plants, the core factors that regulate transcription under heat stress (HS) are well described and include several HS transcription factors (HSFs). Despite the relevance of alternative splicing in HS response and thermotolerance, the core regulators of HS-sensitive alternative splicing have not been identified. In tomato, alternative splicing of HSFA2 is important for acclimation to HS. Here, we show that several members of the serine/arginine-rich family of splicing factors (SRSFs) suppress HSFA2 intron splicing. Individual-nucleotide resolution UV cross-linking and immunoprecipitation (iCLIP) combined with RNA-Seq revealed that RS2Z35 and RS2Z36, which make up a plant-specific clade of SR proteins, not only regulate HSFA2 but approximately 50% of RNAs that undergo HS-sensitive alternative splicing, with preferential binding to purine-rich RNA motifs. Single and double CRISPR rs2z mutant lines show a dysregulation of splicing and exhibit lower basal and acquired thermotolerance compared to wild type plants. Our results suggest that RS2Z35 and RS2Z36 have a central role in mitigation of the negative effects of HS on RNA splicing homeostasis, and their emergence might have contributed to the increased capacity of plants to acclimate to high temperatures.

Transcriptional and post-transcriptional regulation of chloroplast development by nuclear-localized XAP5 CIRCADIAN TIMEKEEPER

Chlorophyll biosynthesis and breakdown, important cellular processes for photosynthesis, occur in the chloroplast. As a semi-autonomous organelle, chloroplast development is mainly regulated by nuclear-encoded chloroplast proteins and proteins encoded by itself. However, the knowledge of chloroplast development regulated by other organelles is limited. Here, we report that the nuclear-localized XAP5 CIRCADIAN TIMEKEEPER (XCT) is essential for chloroplast development in Arabidopsis. In this study, significantly decreased chlorophyll content phenotypes of cotyledons and subsequently emerging organs from shoot apical meristem were observed in xct-2. XCT is constitutively expressed in various tissues and localized in the nuclear with speckle patterns. RNA-seq analysis identified 207 differently spliced genes and 1511 differently expressed genes, in which chloroplast development-, chlorophyll metabolism- and photosynthesis-related genes were enriched. Further biochemical assays suggested that XCT was co-purified with the well-known splicing factors and transcription machinery, suggesting dual functions of XCT in gene transcription and splicing. Interestingly, we also found that the chlorophyll contents in xct-2 significantly decreased under high temperature and high light condition, indicating XCT integrates temperature and light signals to fine-tune the chlorophyll metabolism in Arabidopsis. Therefore, our results provide new insights into chloroplast development regulation by XCT, a nuclear-localized protein, at the transcriptional and post-transcriptional level.

Concordant Gene Expression and Alternative Splicing Regulation under Abiotic Stresses in Arabidopsis

The current investigation endeavors to identify differentially expressed alternatively spliced (DAS) genes that exhibit concordant expression with splicing factors (SFs) under diverse multifactorial abiotic stress combinations in Arabidopsis seedlings. SFs serve as the post-transcriptional mechanism governing the spatiotemporal dynamics of gene expression. The different stresses encompass variations in salt concentration, heat, intensive light, and their combinations. Clusters demonstrating consistent expression profiles were surveyed to pinpoint DAS/SF gene pairs exhibiting concordant expression. Through rigorous selection criteria, which incorporate alignment with documented gene functionalities and expression patterns observed in this study, four members of the serine/arginine-rich (SR) gene family were delineated as SFs concordantly expressed with six DAS genes. These regulated SF genes encompass cactin, SR1-like, SR30, and SC35-like. The identified concordantly expressed DAS genes encode diverse proteins such as the 26.5 kDa heat shock protein, chaperone protein DnaJ, potassium channel GORK, calcium-binding EF hand family protein, DEAD-box RNA helicase, and 1-aminocyclopropane-1-carboxylate synthase 6. Among the concordantly expressed DAS/SF gene pairs, SR30/DEAD-box RNA helicase, and SC35-like/1-aminocyclopropane-1-carboxylate synthase 6 emerge as promising candidates, necessitating further examinations to ascertain whether these SFs orchestrate splicing of the respective DAS genes. This study contributes to a deeper comprehension of the varied responses of the splicing machinery to abiotic stresses. Leveraging these DAS/SF associations shows promise for elucidating avenues for augmenting breeding programs aimed at fortifying cultivated plants against heat and intensive light stresses.

Unveiling unique alternative splicing responses to low temperature in Zoysia japonica through ZjRTD1.0, a high-quality reference transcript dataset

Inadequate reference databases in RNA-seq analysis can hinder data utilization and interpretation. In this study, we have successfully constructed a high-quality reference transcript dataset, ZjRTD1.0, for Zoysia japonica, a widely-used turfgrass with exceptional tolerance to various abiotic stress, including low temperatures and salinity. This dataset comprises 113,089 transcripts from 57,143 genes. BUSCO analysis demonstrates exceptional completeness (92.4%) in ZjRTD1.0, with reduced proportions of fragmented (3.3%) and missing (4.3%) orthologs compared to prior datasets. ZjRTD1.0 enables more precise analyses, including transcript quantification and alternative splicing assessments using public datasets, which identified a substantial number of differentially expressed transcripts (DETs) and differential alternative splicing (DAS) events, leading to several novel findings on Z. japonica's responses to abiotic stresses. First, spliceosome gene expression influenced alternative splicing significantly under abiotic stress, with a greater impact observed during low-temperature stress. Then, a significant positive correlation was found between the number of differentially expressed genes (DEGs) encoding protein kinases and the frequency of DAS events, suggesting the role of protein phosphorylation in regulating alternative splicing. Additionally, our results suggest possible involvement of serine/arginine-rich (SR) proteins and heterogeneous nuclear ribonucleoproteins (hnRNPs) in generating inclusion/exclusion isoforms under low-temperature stress. Furthermore, our investigation revealed a significantly enhanced overlap between DEGs and differentially alternatively spliced genes (DASGs) in response to low-temperature stress, suggesting a unique co-regulatory mechanism governing transcription and splicing in the context of low-temperature response. In conclusion, we have proven that ZjRTD1.0 will serve as a reliable and useful resource for future transcriptomic analyses in Z. japonica.

Exitrons: offering new roles to retained introns-the novel regulators of protein diversity and utility

Exitrons are exonic introns. This subclass of intron retention alternative splicing does not contain a Pre-Terminating stop Codon. Therefore, when retained, they are always a part of a protein. Intron retention is a frequent phenomenon predominantly found in plants, which results in either the degradation of the transcripts or can serve as a stable intermediate to be processed upon induction by specific signals or the cell status. Interestingly, exitrons have coding ability and may confer additional attributes to the proteins that retain them. Therefore, exitron-containing and exitron-spliced isoforms will be a driving force for creating protein diversity in the proteome of an organism. This review establishes a basic understanding of exitron, discussing its genesis, key features, identification methods and functions. We also try to depict its other potential roles. The present review also aims to provide a fundamental background to those who found such exitronic sequences in their gene(s) and to speculate the future course of studies.

Comparative analysis of nascent RNA sequencing methods and their applications in studies of cotranscriptional splicing dynamics

High-throughput detection of nascent RNA is critical for studies of transcription and much more challenging than that of mRNA. Recently, several massively parallel nascent RNA sequencing methods were established in eukaryotic cells. Here, we systematically compared 3 classes of methods on the same pure or crude nuclei preparations: GRO-seq for sequence nuclear run-on RNAs, pNET-seq for sequence RNA polymerase II-associated RNAs, and CB RNA-seq for sequence chromatin-bound (CB) RNAs in Arabidopsis (Arabidopsis thaliana). To improve the resolution of CB RNAs, 3'CB RNA-seq was established to sequence the 3' ends of CB RNAs. In addition, we modified pNET-seq to establish the Chromatin Native Elongation Transcript sequencing (ChrNET) method using chromatin as the starting material for RNA immunoprecipitation. Reproducibility, sensitivity and accuracy in detecting nascent transcripts, experimental procedures, and costs were analyzed, which revealed the strengths and weaknesses of each method. We found that pNET and GRO methods best detected active RNA polymerase II. CB RNA-seq is a simple and cost-effective alternative for nascent RNA studies, due to its high correlation with pNET-seq and GRO-seq. Compared with pNET, ChrNET has higher specificity for nascent RNA capture and lower sequencing cost. 3'CB is sensitive to transcription-coupled splicing. Using these methods, we identified 1,404 unknown transcripts, 4,482 unannotated splicing events, and 60 potential recursive splicing events. This comprehensive comparison of different nascent/chromatin RNA sequencing methods highlights the strengths of each method and serves as a guide for researchers aiming to select a method that best meets their study goals.

Plant immunity suppressor SKRP encodes a novel RNA-binding protein that targets exon 3' end of unspliced RNA

The regulatory roles of RNA splicing in plant immunity are emerging but still largely obscure. We reported previously that Phytophthora pathogen effector Avr3c targets a soybean protein SKRP (serine/lysine/arginine-rich protein) to impair soybean basal immunity by regulating host pre-mRNA alternative splicing, while the biochemical nature of SKRP remains unknown. Here, by using Arabidopsis as a model, we studied the mechanism of SKRP in regulating pre-mRNA splicing and plant immunity. AtSKRP confers impaired plant immunity against Phytophthora capsici and associates with spliceosome component PRP8 and splicing factor SR45, which positively and negatively regulate plant immunity, respectively. Enhanced crosslinking and immunoprecipitation followed by high-throughput sequencing (eCLIP-seq) showed AtSKRP is a novel RNA-binding protein that targets exon 3' end of unspliced RNA. Such position-specific binding of SKRP is associated with its activity in suppressing intron retention, including at positive immune regulatory genes UBP25 and RAR1. In addition, we found AtSKRP self-interact and forms oligomer, and these properties are associated with its function in plant immunity. Overall, our findings reveal that the immune repressor SKRP is a spliceosome-associated protein that targets exon 3' end to regulate pre-mRNA splicing in Arabidopsis.

Alternative Splicing Variation: Accessing and Exploiting in Crop Improvement Programs

Alternative splicing (AS) is a gene regulatory mechanism modulating gene expression in multiple ways. AS is prevalent in all eukaryotes including plants. AS generates two or more mRNAs from the precursor mRNA (pre-mRNA) to regulate transcriptome complexity and proteome diversity. Advances in next-generation sequencing, omics technology, bioinformatics tools, and computational methods provide new opportunities to quantify and visualize AS-based quantitative trait variation associated with plant growth, development, reproduction, and stress tolerance. Domestication, polyploidization, and environmental perturbation may evolve novel splicing variants associated with agronomically beneficial traits. To date, pre-mRNAs from many genes are spliced into multiple transcripts that cause phenotypic variation for complex traits, both in model plant Arabidopsis and field crops. Cataloguing and exploiting such variation may provide new paths to enhance climate resilience, resource-use efficiency, productivity, and nutritional quality of staple food crops. This review provides insights into AS variation alongside a gene expression analysis to select for novel phenotypic diversity for use in breeding programs. AS contributes to heterosis, enhances plant symbiosis (mycorrhiza and rhizobium), and provides a mechanistic link between the core clock genes and diverse environmental clues.

Coupling of co-transcriptional splicing and 3' end Pol II pausing during termination in Arabidopsis

BACKGROUND

In Arabidopsis, RNA Polymerase II (Pol II) often pauses within a few hundred base pairs downstream of the polyadenylation site, reflecting efficient transcriptional termination, but how such pausing is regulated remains largely elusive.

RESULT

Here, we analyze Pol II dynamics at 3' ends by combining comprehensive experiments with mathematical modelling. We generate high-resolution serine 2 phosphorylated (Ser2P) Pol II positioning data specifically enriched at 3' ends and define a 3' end pause index (3'PI). The position but not the extent of the 3' end pause correlates with the termination window size. The 3'PI is not decreased but even mildly increased in the termination deficient mutant xrn3, indicating 3' end pause is a regulatory step early during the termination and before XRN3-mediated RNA decay that releases Pol II. Unexpectedly, 3'PI is closely associated with gene exon numbers and co-transcriptional splicing efficiency. Multiple exons genes often display stronger 3' end pauses and more efficient on-chromatin splicing than genes with fewer exons. Chemical inhibition of splicing strongly reduces the 3'PI and disrupts its correlation with exon numbers but does not globally impact 3' end readthrough levels. These results are further confirmed by fitting Pol II positioning data with a mathematical model, which enables the estimation of parameters that define Pol II dynamics.

CONCLUSION

Our work highlights that the number of exons via co-transcriptional splicing is a major determinant of Pol II pausing levels at the 3' end of genes in plants.

Co-condensation with photoexcited cryptochromes facilitates MAC3A to positively control hypocotyl growth in Arabidopsis

Cryptochromes (CRYs) are blue light receptors that mediate plant photoresponses through regulating gene expressions. We recently reported that Arabidopsis CRY2 could form light-elicited liquid condensates to control RNA methylation. However, whether CRY2 condensation is involved in other gene expression-regulatory processes remains unclear. Here, we show that MOS4-associated complex subunits 3A and 3B (MAC3A/3B) are CRY-interacting proteins and assembled into nuclear CRY condensates. mac3a3b double mutants exhibit hypersensitive photoinhibition of hypocotyl elongation, suggesting that MAC3A/3B positively control hypocotyl growth. We demonstrate the noncanonical activity of MAC3A as a DNA binding protein that modulates transcription. Genome-wide mapping of MAC3A-binding sites reveals that blue light enhances the association of MAC3A with its DNA targets, which requires CRYs. Further evidence indicates that MAC3A and ELONGATED HYPOCOTYL 5 (HY5) occupy overlapping genomic regions and compete for the same targets. These results argue that photocondensation of CRYs fine-tunes light-responsive hypocotyl growth by balancing the opposed effects of HY5 and MAC3A.

Genome-wide analysis of the serine carboxypeptidase-like protein family reveals Ga09G1039 is involved in fiber elongation in cotton

The Gossypium is a model genus for understanding polyploidy and the evolutionary pattern of inheritance. This study aimed to investigate the characteristics of SCPLs in different cotton species and their role in fiber development. A total of 891 genes from one typical monocot and ten dicot species were naturally divided into three classes based on phylogenetic analysis. The SCPL gene family in cotton has undergone intense purifying selection with some functional variation. Segmental duplication and whole genome duplication were shown to be the two main reasons for the increase in the number of genes during cotton evolution. The identification of Gh_SCPL genes exhibiting differential expression in particular tissues or response to environmental stimuli provides a new measure for the in-depth characterization of selected genes of importance. Ga09G1039 was involved in the developmental process of fibers and ovules, and it is significantly different from proteins from other cotton species in terms of phylogenetic, gene structure, conserved protein motifs and tertiary structure. Overexpression of Ga09G1039 significantly increased the length of stem trichomes. Ga09G1039 may be a serine carboxypeptidase protein with hydrolase activity, according to functional region, prokaryotic expression, and western blotting analysis. The results provide a comprehensive overview of the genetic basis of SCPLs in Gossypium and further our knowledge in understanding the key aspects of SCPLs in cotton with their potential role in fiber development and stress resistance.

Hyponastic Leaves 1 Interacts with RNA Pol II to Ensure Proper Transcription of MicroRNA Genes

Hyponastic Leaves 1 (HYL1) [also known as Double-stranded RNA-Binding protein 1 (DRB1)] is a double-stranded RNA-binding protein involved in microRNA (miRNA) processing in plants. It is a core component of the Microprocessor complex and enhances the efficiency and precision of miRNA processing by the Dicer-Like 1 protein. In this work, we report a novel function of the HYL1 protein in the transcription of miRNA (MIR) genes. HYL1 colocalizes with RNA polymerase II and affects its distribution along MIR genes. Moreover, proteomic experiments revealed that the HYL1 protein interacts with many transcription factors. Finally, we show that the action of HYL1 is not limited to MIR genes and impacts the expression of many other genes, a majority of which are involved in plastid organization. These discoveries indicate HYL1 as an additional player in gene regulation at the transcriptional level, independent of its role in miRNA biogenesis.

Cotranscriptional RNA processing and modification in plants

The activities of RNA polymerases shape the epigenetic landscape of genomes with profound consequences for genome integrity and gene expression. A fundamental event during the regulation of eukaryotic gene expression is the coordination between transcription and RNA processing. Most primary RNAs mature through various RNA processing and modification events to become fully functional. While pioneering results positioned RNA maturation steps after transcription ends, the coupling between the maturation of diverse RNA species and their transcription is becoming increasingly evident in plants. In this review, we discuss recent advances in our understanding of the crosstalk between RNA Polymerase II, IV, and V transcription and nascent RNA processing of both coding and noncoding RNAs.

LncRNA FLAIL affects alternative splicing and represses flowering in Arabidopsis

How the noncoding genome affects cellular functions is a key biological question. A particular challenge is to distinguish the effects of noncoding DNA elements from long noncoding RNAs (lncRNAs) that coincide at the same loci. Here, we identified the flowering-associated intergenic lncRNA (FLAIL) in Arabidopsis through early flowering flail mutants. Expression of FLAIL RNA from a different chromosomal location in combination with strand-specific RNA knockdown characterized FLAIL as a trans-acting RNA molecule. FLAIL directly binds to differentially expressed target genes that control flowering via RNA-DNA interactions through conserved sequence motifs. FLAIL interacts with protein and RNA components of the spliceosome to affect target mRNA expression through co-transcriptional alternative splicing (AS) and linked chromatin regulation. In the absence of FLAIL, splicing defects at the direct FLAIL target flowering gene LACCASE 8 (LAC8) correlated with reduced mRNA expression. Double mutant analyses support a model where FLAIL-mediated splicing of LAC8 promotes its mRNA expression and represses flowering. Our study suggests lncRNAs as accessory components of the spliceosome that regulate AS and gene expression to impact organismal development.

A role for heritable transcriptomic variation in maize adaptation to temperate environments

Background

Transcription bridges genetic information and phenotypes. Here, we evaluated how changes in transcriptional regulation enable maize (Zea mays), a crop originally domesticated in the tropics, to adapt to temperate environments.

Result

We generated 572 unique RNA-seq datasets from the roots of 340 maize genotypes. Genes involved in core processes such as cell division, chromosome organization and cytoskeleton organization showed lower heritability of gene expression, while genes involved in anti-oxidation activity exhibited higher expression heritability. An expression genome-wide association study (eGWAS) identified 19,602 expression quantitative trait loci (eQTLs) associated with the expression of 11,444 genes. A GWAS for alternative splicing identified 49,897 splicing QTLs (sQTLs) for 7614 genes. Genes harboring both cis-eQTLs and cis-sQTLs in linkage disequilibrium were disproportionately likely to encode transcription factors or were annotated as responding to one or more stresses. Independent component analysis of gene expression data identified loci regulating co-expression modules involved in oxidation reduction, response to water deprivation, plastid biogenesis, protein biogenesis, and plant-pathogen interaction. Several genes involved in cell proliferation, flower development, DNA replication, and gene silencing showed lower gene expression variation explained by genetic factors between temperate and tropical maize lines. A GWAS of 27 previously published phenotypes identified several candidate genes overlapping with genomic intervals showing signatures of selection during adaptation to temperate environments.

Conclusion

Our results illustrate how maize transcriptional regulatory networks enable changes in transcriptional regulation to adapt to temperate regions.

Supplementary information

The online version contains supplementary material available at 10.1186/s13059-023-02891-3.

Alternative Splicing in the Regulatory Circuit of Plant Temperature Response

As sessile organisms, plants have evolved complex mechanisms to rapidly respond to ever-changing ambient temperatures. Temperature response in plants is modulated by a multilayer regulatory network, including transcriptional and post-transcriptional regulations. Alternative splicing (AS) is an essential post-transcriptional regulatory mechanism. Extensive studies have confirmed its key role in plant temperature response, from adjustment to diurnal and seasonal temperature changes to response to extreme temperatures, which has been well documented by previous reviews. As a key node in the temperature response regulatory network, AS can be modulated by various upstream regulations, such as chromatin modification, transcription rate, RNA binding proteins, RNA structure and RNA modifications. Meanwhile, a number of downstream mechanisms are affected by AS, such as nonsense-mediated mRNA decay (NMD) pathway, translation efficiency and production of different protein variants. In this review, we focus on the links between splicing regulation and other mechanisms in plant temperature response. Recent advances regarding how AS is regulated and the following consequences in gene functional modulation in plant temperature response will be discussed. Substantial evidence suggests that a multilayer regulatory network integrating AS in plant temperature response has been unveiled.

A Bibliometric Study for Plant RNA Editing Research: Trends and Future Challenges

RNA editing is a post-transcriptional process that introduces changes in RNA sequences encoded by nuclear, mitochondrial, or plastid genomes. To understand the research progress of plant RNA editing, we comprehensively analyze the articles on plant RNA editing from 2001 to 2022 through bibliometric methods. Nucleic Acids Research, Plant Journal and Plant cell are the journals that deserve attention with their high production, total local citation scores (TLCS), and h-indexes. The USA, China, and Germany are the top three countries with highly productive publications. Ulm University, Cornell University, and Chinese Acad Sci are excellent cooperative institutions with a high level of influence in the field, and KNOOP V and TAKENAKA M are good partnership. Plant RNA editing researches concentrate on the subject categories of Biochemistry & Molecular Biology, Plant Sciences, Genetics & Heredity, etc. Plant mitochondria, genome editing and messenger-RNA may be the research hotspots in the future. The main plant RNA editing research tools are JACUSA, SPRINT, and REDO, and the main databases are REDIdb, PED, and dbRES. At present, the research streams are (1) RNA editing sites; (2) Pentapeptide repeat protein (PPR) involved in RNA editing; (3) RNA editing factors. Overall, this article summarizes the research overview of plant RNA editing until 2022 and provides theoretical implications for its possible future directions.

A simple and robust method for isolating and analyzing chromatin-bound RNAs in Arabidopsis

BACKGROUND

Chromatin-bound RNAs are the primary product of transcription that undergo on-chromatin processing such as capping, splicing, and polyadenylation. These processing steps then determine the fate of RNAs. Albeit its vital importance, a simple and robust method for isolating different fractions of chromatin-bound RNAs is missing in plants.

RESULT

Here, we describe our updated method and the associated step-by-step protocol for chromatin-bound RNAs isolation in A. thaliana. The chromatin-bound RNAs isolation is based on the 1 M UREA wash that removes the majority of non-chromatin-associated proteins from the nucleus, as previously developed in mammalian cells. On-demand, the isolated chromatin-bound RNAs can be either used directly for gene-specific analysis or subject to further rRNA removal and also the optional polyadenylated RNA removal, followed by high-throughput sequencing. Detailed protocols for these procedures are also provided. Comparison of sequencing results of chromatin-bound RNAs with and without polyadenylated RNA removal revealed that a small fraction of CB-RNAs is polyadenylated but not yet fully spliced, representing RNA-processing intermediate on-chromatin.

CONCLUSION

This optimized chromatin-bound RNAs purification method is simple and robust and can be used to study transcription and its-coupled RNA processing in plants.

Genome-wide characterization of nascent RNA processing in plants

Following transcription initiation, RNA polymerase II (Pol II) elongates through the genic region and terminates after the polyadenylation signal. This process is accompanied by splicing, 3' cleavage, and polyadenylation, to eventually form a mature mRNA. Recent advances in short-read and long-read high-throughput sequencing methods have shed light on the global landscape of these co-transcriptional events at nucleotide resolution. In this mini review, we summarize recent developments in genome-wide approaches that broadened our understanding of nascent RNA processing in plants.

Alternative splicing as a key player in the fine-tuning of the immunity response in Arabidopsis

Plants, like animals, are constantly exposed to abiotic and biotic stresses, which often inhibit plant growth and development, and cause tissue damage, disease, and even plant death. Efficient and timely response to stress requires appropriate co- and posttranscriptional reprogramming of gene expression. Alternative pre-mRNA splicing provides an important layer of this regulation by controlling the level of factors involved in stress response and generating additional protein isoforms with specific features. Recent high-throughput studies have revealed that several defence genes undergo alternative splicing that is often affected by pathogen infection. Despite extensive work, the exact mechanisms underlying these relationships are still unclear, but the contribution of alternative protein isoforms to the defence response and the role of regulatory factors, including components of the splicing machinery, have been established. Modulation of gene expression in response to stress includes alternative splicing, chromatin remodelling, histone modifications, and nucleosome occupancy. How these processes affect plant immunity is mostly unknown, but these facets open new regulatory possibilities. Here we provide an overview of the current state of knowledge and recent findings regarding the growing importance of alternative splicing in plant response to biotic stress.

Comprehensive profiling of alternative splicing landscape during secondary dormancy in oilseed rape (Brassica napus L.)

Alternative splicing is a general mechanism that regulates gene expression at the post-transcriptional level, which increases the transcriptomic diversity. Oilseed rape (Brassica napus L.), one of the main oil crops worldwide, is prone to secondary dormancy. However, how alternative splicing landscape of oilseed rape seed changes in response to secondary dormancy is unknown. Here, we analyzed twelve RNA-seq libraries from varieties "Huaiyou-SSD-V1" and "Huaiyou-WSD-H2" which exhibited high (> 95%) and low (< 5%) secondary dormancy potential, respectively, and demonstrated that alternative splicing changes led to a significant increase with the diversity of the transcripts in response to secondary dormancy induction via polyethylene glycol 6000 (PEG6000) treatment. Among the four basic alternative splicing types, intron retention dominates, and exon skipping shows the rarest frequency. A total of 8% of expressed genes had two or more transcripts after PEG treatment. Further analysis revealed that global isoform expression percentage variations in alternative splicing in differently expressed genes (DEGs) is more than three times as much as those in non-DEGs, suggesting alternative splicing change is associated with the transcriptional activity change in response to secondary dormancy induction. Eventually, 342 differently spliced genes (DSGs) associated with secondary dormancy were identified, five of which were validated by RT-PCR. The number of the overlapped genes between DSGs and DEGs associated with secondary dormancy was much less than that of either DSGs or DEGs, suggesting that DSGs and DEGs may independently regulates secondary dormancy. Functional annotation analysis of DSGs revealed that spliceosome components are overrepresented among the DSGs, including small nuclear ribonucleoprotein particles (snRNPs), serine/arginine-rich (SR) proteins, and other splicing factors. Thus, it is proposed that the spliceosome components could be exploited to reduce secondary dormancy potential in oilseed rape.

Supplementary Information

The online version contains supplementary material available at 10.1007/s11032-022-01314-8.

Phase separation of HRLP regulates flowering time in Arabidopsis

RNA binding proteins mediate posttranscriptional RNA metabolism and play regulatory roles in many developmental processes in eukaryotes. Despite their known effects on the floral transition from vegetative to reproductive growth in plants, the underlying mechanisms remain largely obscure. Here, we show that a hitherto unknown RNA binding protein, hnRNP R-LIKE PROTEIN (HRLP), inhibits cotranscriptional splicing of a key floral repressor gene FLOWERING LOCUS C (FLC). This, in turn, facilitates R-loop formation near FLC intron I to repress its transcription, thereby promoting the floral transition in Arabidopsis thaliana. HRLP, together with the splicing factor ARGININE/SERINE-RICH 45, forms phase-separated nuclear condensates with liquid-like properties, which is essential for HRLP function in regulating FLC splicing, R-loop formation, and RNA Polymerase II recruitment. Our findings reveal that inhibition of cotranscriptional splicing of FLC by nuclear HRLP condensates constitutes the molecular basis for down-regulation of FLC transcript levels to ensure the reproductive success of Arabidopsis.

Relevance and Regulation of Alternative Splicing in Plant Heat Stress Response: Current Understanding and Future Directions

Alternative splicing (AS) is a major mechanism for gene expression in eukaryotes, increasing proteome diversity but also regulating transcriptome abundance. High temperatures have a strong impact on the splicing profile of many genes and therefore AS is considered as an integral part of heat stress response. While many studies have established a detailed description of the diversity of the RNAome under heat stress in different plant species and stress regimes, little is known on the underlying mechanisms that control this temperature-sensitive process. AS is mainly regulated by the activity of splicing regulators. Changes in the abundance of these proteins through transcription and AS, post-translational modifications and interactions with exonic and intronic cis-elements and core elements of the spliceosomes modulate the outcome of pre-mRNA splicing. As a major part of pre-mRNAs are spliced co-transcriptionally, the chromatin environment along with the RNA polymerase II elongation play a major role in the regulation of pre-mRNA splicing under heat stress conditions. Despite its importance, our understanding on the regulation of heat stress sensitive AS in plants is scarce. In this review, we summarize the current status of knowledge on the regulation of AS in plants under heat stress conditions. We discuss possible implications of different pathways based on results from non-plant systems to provide a perspective for researchers who aim to elucidate the molecular basis of AS under high temperatures.

Co-transcriptional splicing efficiency is a gene-specific feature that can be regulated by TGFβ

Differential splicing efficiency of specific introns is a mechanism that dramatically increases protein diversity, based on selection of alternative exons for the final mature mRNA. However, it is unclear whether splicing efficiency of introns within the same gene is coordinated and eventually regulated as a mechanism to control mature mRNA levels. Based on nascent chromatin-associated RNA-sequencing data, we now find that co-transcriptional splicing (CTS) efficiency tends to be similar between the different introns of a gene. We establish that two well-differentiated strategies for CTS efficiency exist, at the extremes of a gradient: short genes that produce high levels of pre-mRNA undergo inefficient splicing, while long genes with relatively low levels of pre-mRNA have an efficient splicing. Notably, we observe that genes with efficient CTS display a higher level of mature mRNA relative to their pre-mRNA levels. Further, we show that the TGFβ signal transduction pathway regulates the general CTS efficiency, causing changes in mature mRNA levels. Taken together, our data indicate that CTS efficiency is a gene-specific characteristic that can be regulated to control gene expression.

Co-transcriptional splicing efficiency is a gene-specific characteristic that can be regulated by TGFβ to modulate gene expression.

Arabidopsis RBV is a conserved WD40 repeat protein that promotes microRNA biogenesis and ARGONAUTE1 loading

MicroRNAs (miRNAs) play crucial roles in gene expression regulation through RNA cleavage or translation repression. Here, we report the identification of an evolutionarily conserved WD40 domain protein as a player in miRNA biogenesis in Arabidopsis thaliana. A mutation in the REDUCTION IN BLEACHED VEIN AREA (RBV) gene encoding a WD40 domain protein led to the suppression of leaf bleaching caused by an artificial miRNA; the mutation also led to a global reduction in the accumulation of endogenous miRNAs. The nuclear protein RBV promotes the transcription of MIR genes into pri-miRNAs by enhancing the occupancy of RNA polymerase II (Pol II) at MIR gene promoters. RBV also promotes the loading of miRNAs into AGO1. In addition, RNA-seq revealed a global splicing defect in the mutant. Thus, this evolutionarily conserved, nuclear WD40 domain protein acts in miRNA biogenesis and RNA splicing.

MicroRNAs regulate gene expression through RNA cleavage or translation repression. Here the authors show that RBV, an evolutionarily conserved WD40 domain protein, acts to promote MIR transcription, pri-miRNA processing and miRNA loading into AGO1.

Pervasive misannotation of microexons that are evolutionarily conserved and crucial for gene function in plants

It is challenging to identify the smallest microexons (≤15-nt) due to their small size. Consequently, these microexons are often misannotated or missed entirely during genome annotation. Here, we develop a pipeline to accurately identify 2,398 small microexons in 10 diverse plant species using 990 RNA-seq datasets, and most of them have not been annotated in the reference genomes. Analysis reveals that microexons tend to have increased detained flanking introns that require post-transcriptional splicing after polyadenylation. Examination of 45 conserved microexon clusters demonstrates that microexons and associated gene structures can be traced back to the origin of land plants. Based on these clusters, we develop an algorithm to genome-wide model coding microexons in 132 plants and find that microexons provide a strong phylogenetic signal for plant organismal relationships. Microexon modeling reveals diverse evolutionary trajectories, involving microexon gain and loss and alternative splicing. Our work provides a comprehensive view of microexons in plants.

The small size (≤15-nt) of micorexons poses difficulties for genome annotation and identification using standard RNA sequence mapping approaches. Here, the authors develop computational pipelines to discover and predict microexons in plants and reveal diverse evolutionary trajectories via genomewide microexon modeling.

Biological Parts for Engineering Abiotic Stress Tolerance in Plants

It is vital to ramp up crop production dramatically by 2050 due to the increasing global population and demand for food. However, with the climate change projections showing that droughts and heatwaves becoming common in much of the globe, there is a severe threat of a sharp decline in crop yields. Thus, developing crop varieties with inbuilt genetic tolerance to environmental stresses is urgently needed. Selective breeding based on genetic diversity is not keeping up with the growing demand for food and feed. However, the emergence of contemporary plant genetic engineering, genome-editing, and synthetic biology offer precise tools for developing crops that can sustain productivity under stress conditions. Here, we summarize the systems biology-level understanding of regulatory pathways involved in perception, signalling, and protective processes activated in response to unfavourable environmental conditions. The potential role of noncoding RNAs in the regulation of abiotic stress responses has also been highlighted. Further, examples of imparting abiotic stress tolerance by genetic engineering are discussed. Additionally, we provide perspectives on the rational design of abiotic stress tolerance through synthetic biology and list various bioparts that can be used to design synthetic gene circuits whose stress-protective functions can be switched on/off in response to environmental cues.

Alternative splicing in plant abiotic stress responses

Modifications of the cellular proteome pool upon stress allow plants to tolerate environmental changes. Alternative splicing is the most significant mechanism responsible for the production of multiple protein isoforms from a single gene. The spliceosome, a large ribonucleoprotein complex, together with several associated proteins, controls this pre-mRNA processing, adding an additional level of regulation to gene expression. Deep sequencing of transcriptomes revealed that this co- or post-transcriptional mechanism is highly induced by abiotic stress, and concerns vast numbers of stress-related genes. Confirming the importance of splicing in plant stress adaptation, key players of stress signaling have been shown to encode alternative transcripts, whereas mutants lacking splicing factors or associated components show a modified sensitivity and defective responses to abiotic stress. Here, we examine recent literature on alternative splicing and splicing alterations in response to environmental stresses, focusing on its role in stress adaptation and analyzing the future perspectives and directions for research.

Decoding co-/post-transcriptional complexities of plant transcriptomes and epitranscriptome using next-generation sequencing technologies

Next-generation sequencing (NGS) technologies - Illumina RNA-seq, Pacific Biosciences isoform sequencing (PacBio Iso-seq), and Oxford Nanopore direct RNA sequencing (DRS) - have revealed the complexity of plant transcriptomes and their regulation at the co-/post-transcriptional level. Global analysis of mature mRNAs, transcripts from nuclear run-on assays, and nascent chromatin-bound mRNAs using short as well as full-length and single-molecule DRS reads have uncovered potential roles of different forms of RNA polymerase II during the transcription process, and the extent of co-transcriptional pre-mRNA splicing and polyadenylation. These tools have also allowed mapping of transcriptome-wide start sites in cap-containing RNAs, poly(A) site choice, poly(A) tail length, and RNA base modifications. The emerging theme from recent studies is that reprogramming of gene expression in response to developmental cues and stresses at the co-/post-transcriptional level likely plays a crucial role in eliciting appropriate responses for optimal growth and plant survival under adverse conditions. Although the mechanisms by which developmental cues and different stresses regulate co-/post-transcriptional splicing are largely unknown, a few recent studies indicate that the external cues target spliceosomal and splicing regulatory proteins to modulate alternative splicing. In this review, we provide an overview of recent discoveries on the dynamics and complexities of plant transcriptomes, mechanistic insights into splicing regulation, and discuss critical gaps in co-/post-transcriptional research that need to be addressed using diverse genomic and biochemical approaches.

Daily temperature cycles promote alternative splicing of RNAs encoding SR45a, a splicing regulator in maize

Elevated temperatures enhance alternative RNA splicing in maize (Zea mays) with the potential to expand the repertoire of plant responses to heat stress. Alternative RNA splicing generates multiple RNA isoforms for many maize genes, and here we observed changes in the pattern of RNA isoforms with temperature changes. Increases in maximum daily temperature elevated the frequency of the major modes of alternative splices (AS), in particular retained introns and skipped exons. The genes most frequently targeted by increased AS with temperature encode factors involved in RNA processing and plant development. Genes encoding regulators of alternative RNA splicing were themselves among the principal AS targets in maize. Under controlled environmental conditions, daily changes in temperature comparable to field conditions altered the abundance of different RNA isoforms, including the RNAs encoding the splicing regulator SR45a, a member of the SR45 gene family. We established an "in protoplast" RNA splicing assay to show that during the afternoon on simulated hot summer days, SR45a RNA isoforms were produced with the potential to encode proteins efficient in splicing model substrates. With the RNA splicing assay, we also defined the exonic splicing enhancers that the splicing-efficient SR45a forms utilize to aid in the splicing of model substrates. Hence, with rising temperatures on hot summer days, SR45a RNA isoforms in maize are produced with the capability to encode proteins with greater RNA splicing potential.

Stress-Induced Changes in Alternative Splicing Landscape in Rice: Functional Significance of Splice Isoforms in Stress Tolerance

Improvements in yield and quality of rice are crucial for global food security. However, global rice production is substantially hindered by various biotic and abiotic stresses. Making further improvements in rice yield is a major challenge to the rice research community, which can be accomplished through developing abiotic stress-resilient rice varieties and engineering durable agrochemical-independent pathogen resistance in high-yielding elite rice varieties. This, in turn, needs increased understanding of the mechanisms by which stresses affect rice growth and development. Alternative splicing (AS), a post-transcriptional gene regulatory mechanism, allows rapid changes in the transcriptome and can generate novel regulatory mechanisms to confer plasticity to plant growth and development. Mounting evidence indicates that AS has a prominent role in regulating rice growth and development under stress conditions. Several regulatory and structural genes and splicing factors of rice undergo different types of stress-induced AS events, and the functional significance of some of them in stress tolerance has been defined. Both rice and its pathogens use this complex regulatory mechanism to devise strategies against each other. This review covers the current understanding and evidence for the involvement of AS in biotic and abiotic stress-responsive genes, and its relevance to rice growth and development. Furthermore, we discuss implications of AS for the virulence of different rice pathogens and highlight the areas of further research and potential future avenues to develop climate-smart and disease-resistant rice varieties.

Dual roles of the serine/arginine-rich splicing factor SR45a in promoting and interacting with nuclear cap-binding complex to modulate the salt-stress response in Arabidopsis

Alternative splicing (AS) is emerging as a critical co-transcriptional regulation for plants in response to environmental stresses. Although multiple splicing factors have been linked to the salt-sensitive signaling network, the molecular mechanism remains unclear. We discovered that a conserved serine/arginine-rich (SR)-like protein, SR45a, as a component of the spliceosome, was involved in post-transcriptional regulation of salinity tolerance in Arabidopsis thaliana. Furthermore, SR45a was required for the AS and messenger RNA (mRNA) maturation of several salt-tolerance genes. Two alternatively spliced variants of SR45a were induced by salt stress, full-length SR45a-1a and the truncated isoform SR45a-1b, respectively. Lines with overexpression of SR45a-1a and SR45a-1b exhibited hypersensitive to salt stress. Our data indicated that SR45a directly interacted with the cap-binding complex (CBC) subunit cap-binding protein 20 (CBP20) which mediated salt-stress responses. Instead of binding to other spliceosome components, SR45a-1b promoted the association of SR45a-1a with CBP20, therefore mediating salt-stress signal transduction pathways. Additionally, the mutations in SR45a and CBP20 led to different salt-stress phenotypes. Together, these results provide the evidence that SR45a-CBP20 acts as a regulatory complex to regulate the plant response to salt stress, through a regulatory mechanism to fine-tune the splicing factors, especially in stressful conditions.

Differential nucleosome occupancy modulates alternative splicing in Arabidopsis thaliana

Alternative splicing (AS) is a major gene regulatory mechanism in plants. Recent evidence supports co-transcriptional splicing in plants, hence the chromatin state can impact AS. However, how dynamic changes in the chromatin state such as nucleosome occupancy influence the cold-induced AS remains poorly understood. Here, we generated transcriptome (RNA-Seq) and nucleosome positioning (MNase-Seq) data for Arabidopsis thaliana to understand how nucleosome positioning modulates cold-induced AS. Our results show that characteristic nucleosome occupancy levels are strongly associated with the type and abundance of various AS events under normal and cold temperature conditions in Arabidopsis. Intriguingly, exitrons, alternatively spliced internal regions of protein-coding exons, exhibit distinctive nucleosome positioning pattern compared to other alternatively spliced regions. Likewise, nucleosome patterns differ between exitrons and retained introns, pointing to their distinct regulation. Collectively, our data show that characteristic changes in nucleosome positioning modulate AS in plants in response to cold.

Cotranscriptional and Posttranscriptional Features of the Transcriptome in Soybean Shoot Apex and Leaf

Transcription is the first step of central dogma, in which the genetic information stored in DNA is copied into RNA. In addition to mature RNA sequencing (RNA-seq), high-throughput nascent RNA assays have been established and applied to provide detailed transcriptional information. Here, we present the profiling of nascent RNA from trifoliate leaves and shoot apices of soybean. In combination with nascent RNA (chromatin-bound RNA, CB RNA) and RNA-seq, we found that introns were largely spliced cotranscriptionally. Although alternative splicing (AS) was mainly determined at nascent RNA biogenesis, differential AS between the leaf and shoot apex at the mature RNA level did not correlate well with cotranscriptional differential AS. Overall, RNA abundance was moderately correlated between nascent RNA and mature RNA within each tissue, but the fold changes between the leaf and shoot apex were highly correlated. Thousands of novel transcripts (mainly non-coding RNA) were detected by CB RNA-seq, including the overlap of natural antisense RNA with two important genes controlling soybean reproductive development, FT2a and Dt1. Taken together, we demonstrated the adoption of CB RNA-seq in soybean, which may shed light on gene expression regulation of important agronomic traits in leguminous crops.

Processing of coding and non-coding RNAs in plant development and environmental responses

Precursor RNAs undergo extensive processing to become mature RNAs. RNA transcripts are subjected to 5′ capping, 3′-end processing, splicing, and modification; they also form dynamic secondary structures during co-transcriptional and post-transcriptional processing. Like coding RNAs, non-coding RNAs (ncRNAs) undergo extensive processing. For example, secondary small interfering RNA (siRNA) transcripts undergo RNA processing, followed by further cleavage to become mature siRNAs. Transcriptome studies have revealed roles for co-transcriptional and post-transcriptional RNA processing in the regulation of gene expression and the coordination of plant development and plant–environment interactions. In this review, we present the latest progress on RNA processing in gene expression and discuss phased siRNAs (phasiRNAs), a kind of germ cell-specific secondary small RNA (sRNA), focusing on their functions in plant development and environmental responses.

Post-transcriptional splicing of nascent RNA contributes to widespread intron retention in plants

In eukaryotes, genes are transcribed by RNA polymerase-II (Pol-II) and introns are removed by the spliceosome largely cotranscriptionally^1-3; analysis using long-read sequencing revealed that splicing occurs immediately after Pol-II passes introns in yeast^4,5. Here, we developed a Nanopore-based method to profile chromatin-bound RNA that enables the simultaneous detection of splicing status, Pol-II position and polyadenylation at the genome-wide scale in Arabidopsis. We found that more than half of the introns remain unspliced after Pol-II transcribes 1 kb past the 3' splice site, which is much slower than the rate of splicing reported in yeast^4,5. Many of the full-length chromatin-bound RNA molecules are polyadenylated, yet still contain unspliced introns at specific positions. These introns are nearly absent in the cytoplasm and are resistant to nonsense-mediated decay, suggesting that they are post-transcriptionally spliced before the transcripts are released into the cytoplasm; we therefore termed these introns post-transcriptionally spliced introns (pts introns). Analysis of around 6,500 public RNA-sequencing libraries found that the splicing of pts introns requires the function of splicing-related proteins such as PRMT5 and SKIP, and is also influenced by various environmental signals. The majority of the intron retention events in Arabidopsis are at pts introns, suggesting that chromatin-tethered post-transcriptional splicing is a major contributor to the widespread intron retention that is observed in plants, and could be a mechanism to produce fully spliced functional mRNAs for rapid response.

Global transcriptome analysis reveals circadian control of splicing events in Arabidopsis thaliana

The circadian clock of Arabidopsis thaliana controls many physiological and molecular processes, allowing plants to anticipate daily changes in their environment. However, developing a detailed understanding of how oscillations in mRNA levels are connected to oscillations in co/post-transcriptional processes, such as splicing, has remained a challenge. Here we applied a combined approach using deep transcriptome sequencing and bioinformatics tools to identify novel circadian-regulated genes and splicing events. Using a stringent approach, we identified 300 intron retention, eight exon skipping, 79 alternative 3' splice site usage, 48 alternative 5' splice site usage, and 350 multiple (more than one event type) annotated events under circadian regulation. We also found seven and 721 novel alternative exonic and intronic events. Depletion of the circadian-regulated splicing factor AtSPF30 homologue resulted in the disruption of a subset of clock-controlled splicing events. Altogether, our global circadian RNA-seq coupled with an in silico, event-centred, splicing analysis tool offers a new approach for studying the interplay between the circadian clock and the splicing machinery at a global scale. The identification of many circadian-regulated splicing events broadens our current understanding of the level of control that the circadian clock has over this co/post-transcriptional regulatory layer.