Deep RNA sequencing at single base-pair resolution reveals high complexity of the rice transcriptome

PFusionDB: a comprehensive database of plant-specific fusion transcripts

Fusion transcripts (FTs) are well known cancer biomarkers, relatively understudied in plants. Here, we developed PFusionDB (www.nipgr.ac.in/PFusionDB), a novel plant-specific fusion-transcript database. It is a comprehensive repository of 80,170, 39,108, 83,330, and 11,500 unique fusions detected in 1280, 637, 697, and 181 RNA-Seq samples of Arabidopsis thaliana, Oryza sativa japonica, Oryza sativa indica, and Cicer arietinum respectively. Here, a total of 76,599 (Arabidopsis thaliana), 35,480 (Oryza sativa japonica), 72,099 (Oryza sativa indica), and 9524 (Cicer arietinum) fusion transcripts are non-recurrent i.e., only found in one sample. Identification of FTs was performed by using a total of five tools viz. EricScript-Plants, STAR-Fusion, TrinityFusion, SQUID, and MapSplice. At PFusionDB, available fundamental details of fusion events includes the information of parental genes, junction sequence, expression levels of fusion transcripts, breakpoint coordinates, strand information, tissue type, treatment information, fusion type, PFusionDB ID, and Sequence Read Archive (SRA) ID. Further, two search modules: 'Simple Search' and 'Advanced Search', along with a 'Browse' option to data download, are present for the ease of users. Three distinct modules viz. 'BLASTN', 'SW Align', and 'Mapping' are also available for efficient query sequence mapping and alignment to FTs. PFusionDB serves as a crucial resource for delving into the intricate world of fusion transcript in plants, providing researchers with a foundation for further exploration and analysis. Database URL: www.nipgr.ac.in/PFusionDB.

Supplementary Information

The online version contains supplementary material available at 10.1007/s13205-024-04132-1.

Conserved features and diversity attributes of chimeric RNAs across accessions in four plants

As a non-collinear expression form of genetic information, chimeric RNAs increase the complexity of transcriptome in diverse organisms. Although chimeric RNAs have been identified in plants, few common features have been revealed. Here, we systemically explored the landscape of chimeric RNAs across multi-accession and multi-tissue using pan-genome and transcriptome data of four plants: rice, maize, soybean, and Arabidopsis. Among the four species, conserved characteristics of breakpoints and parental genes were discovered. In each species, chimeric RNAs displayed a high level of diversity among accessions, and the clustering of accessions using chimeric events was generally concordant with clustering based on genomic variants, implying a general relationship between genetic variations and chimeric RNAs. Through mass spectrometry, we confirmed a fusion protein OsNDC1-OsGID1L2 and observed its subcellular localization, which differed from the original proteins. Phenotypic cues in transgenic rice suggest the potential functions of OsNDC1-OsGID1L2. Moreover, an intriguing chimeric event Os01g0216500-Os01g0216900, generated by a large deletion in basmati rice, also exists in another accession without the deletion, demonstrating its convergence in evolution. Our results illuminate the characteristics and hint at the evolutionary implications of plant chimeric RNAs, which serve as a supplement to genetic variations, thus expanding our understanding of genetic diversity.

Integrative analyses of long and short-read RNA sequencing reveal the spliced isoform regulatory network of seedling growth dynamics in upland cotton

The polyploid genome of cotton has significantly increased the transcript complexity. Recent advances in full-length transcript sequencing are now widely used to characterize the complete landscape of transcriptional events. Such studies in cotton can help us to explore the genetic mechanisms of the cotton seedling growth. Through long-read single-molecule RNA sequencing, this study compared the transcriptomes of three yield contrasting genotypes of upland cotton. Our analysis identified different numbers of spliced isoforms from 31,166, 28,716, and 28,713 genes in SJ48, Z98, and DT8 cotton genotypes, respectively, most of which were novel compared to previous cotton reference transcriptomes, and showed significant differences in the number of exon structures and coding sequence length due to intron retention. Quantification of isoform expression revealed significant differences in expression in the root and leaf of each genotype. An array of key isoform target genes showed protein kinase or phosphorylation functions, and their protein interaction network contained most of the circadian oscillator proteins. Spliced isoforms from the GIGANTEA (GI) protien were differentially regulated in each genotype and might be expected to regulate translational activities, including the sequence and function of target proteins. In addition, these spliced isoforms generate diurnal expression profiles in cotton leaves, which may alter the transcriptional regulatory network of seedling growth. Silencing of the novel spliced GI isoform Gh_A02G0645_N17 significantly affected biomass traits, contributed to variable growth, and increased transcription of the early flowering pathway gene ELF in cotton. Our high-throughput hybrid sequencing results will be useful to dissect functional differences among spliced isoforms in the polyploid cotton genome.

Phytopathogens Reprogram Host Alternative mRNA Splicing

Alternative splicing (AS) is an evolutionarily conserved cellular process in eukaryotes in which multiple messenger RNA (mRNA) transcripts are produced from a single gene. The concept that AS adds to transcriptome complexity and proteome diversity introduces a new perspective for understanding how phytopathogen-induced alterations in host AS cause diseases. Recently, it has been recognized that AS represents an integral component of the plant immune system during parasitic, commensalistic, and symbiotic interactions. Here, I provide an overview of recent progress detailing the reprogramming of plant AS by phytopathogens and the functional implications on disease phenotypes. Additionally, I discuss the vital function of AS of immune receptors in regulating plant immunity and how phytopathogens use effector proteins to target key components of the splicing machinery and exploit alternatively spliced variants of immune regulators to negate defense responses. Finally, the functional association between AS and nonsense-mediated mRNA decay in the context of plant-pathogen interface is recapitulated.

Global analysis of key post-transcriptional regulation in early leaf development of Limonium bicolor identifies a long non-coding RNA that promotes salt gland development and salt resistance

Limonium bicolor, known horticulturally as sea lavender, is a typical recretohalophyte with salt glands in its leaf epidermis that secrete excess Na+ out of the plant. Although many genes have been proposed to contribute to salt gland initiation and development, a detailed analysis of alternative splicing, alternative polyadenylation patterns, and long non-coding RNAs (lncRNAs) has been lacking. Here, we applied single-molecule long-read mRNA isoform sequencing (Iso-seq) to explore the complexity of the L. bicolor transcriptome in leaves during salt gland initiation (stage A) and salt gland differentiation (stage B) based on the reference genome. We identified alternative splicing events and the use of alternative poly(A) sites unique to stage A or stage B, leading to the hypothesis that they might contribute to the differentiation of salt glands. Based on the Iso-seq data and RNA in situ hybridization of candidate genes, we selected the lncRNA Btranscript_153392 for gene editing and virus-induced gene silencing to dissect its function. In the absence of this transcript, we observed fewer salt glands on the leaf epidermis, leading to diminished salt secretion and salt tolerance. Our data provide transcriptome resources for unraveling the mechanisms behind salt gland development and furthering crop transformation efforts towards enhanced survivability in saline soils.

Relevance and regulation of alternative splicing in plant secondary metabolism: current understanding and future directions

The secondary metabolism of plants is an essential life process enabling organisms to navigate various stages of plant development and cope with ever-changing environmental stresses. Secondary metabolites, abundantly found in nature, possess significant medicinal value. Among the regulatory mechanisms governing these metabolic processes, alternative splicing stands out as a widely observed post-transcriptional mechanism present in multicellular organisms. It facilitates the generation of multiple mRNA transcripts from a single gene by selecting different splicing sites. Selective splicing events in plants are widely induced by various signals, including external environmental stress and hormone signals. These events ultimately regulate the secondary metabolic processes and the accumulation of essential secondary metabolites in plants by influencing the synthesis of primary metabolites, hormone metabolism, biomass accumulation, and capillary density. Simultaneously, alternative splicing plays a crucial role in enhancing protein diversity and the abundance of the transcriptome. This paper provides a summary of the factors inducing alternative splicing events in plants and systematically describes the progress in regulating alternative splicing with respect to different secondary metabolites, including terpenoid, phenolic compounds, and nitrogen-containing compounds. Such elucidation offers critical foundational insights for understanding the role of alternative splicing in regulating plant metabolism and presents novel avenues and perspectives for bioengineering.

Multiple strategies, including 6mA methylation, affecting plant alternative splicing in allopolyploid peanut

Alternative splicing (AS), an important post-transcriptional regulation mechanism in eukaryotes, can significantly increase transcript diversity and contribute to gene expression regulation and many other complicated developmental processes. While plant gene AS events are well described, few studies have investigated the comprehensive regulation machinery of plant AS. Here, we use multi-omics to analyse peanut AS events. Using long-read isoform sequencing, 146 464 full-length non-chimeric transcripts were obtained, resulting in annotation corrections for 1782 genes and the identification of 4653 new loci. Using Iso-Seq RNA sequences, 271 776 unique splice junctions were identified, 82.49% of which were supported by transcriptome data. We characterized 50 977 polyadenylation sites for 23 262 genes, 12 369 of which had alternative polyadenylation sites. AS allows differential regulation of the same gene by miRNAs at the isoform level coupled with polyadenylation. In addition, we identified many long non-coding RNAs and fusion transcripts. There is a suppressed effect of 6mA on AS and gene expression. By analysis of chromatin structures, the genes located in the boundaries of topologically associated domains, proximal chromosomal telomere regions, inter- or intra-chromosomal loops were found to have more unique splice isoforms, higher expression, lower 6mA and more transposable elements (TEs) in their gene bodies than the other genes, indicating that chromatin interaction, 6mA and TEs play important roles in AS and gene expression. These results greatly refine the peanut genome annotation and contribute to the study of gene expression and regulation in peanuts. This work also showed AS is associated with multiple strategies for gene regulation.

Physiological and Transcriptomic Responses of Bok Choy to Heat Stress

High temperatures have adverse effects on the yield and quality of vegetables. Bok choy, a popular vegetable, shows varying resistance to heat. However, the mechanism underlying the thermotolerance of bok choy remains unclear. In this study, 26 bok choy varieties were identified in screening as being heat-resistant at the seedling stage; at 43 °C, it was possible to observe obvious heat damage in different bok choy varieties. The physiological and biochemical reactions of a heat-tolerant cultivar, Jinmei (J7), and a heat-sensitive cultivar, Sanyueman (S16), were analyzed in terms of the growth index, peroxide, and photosynthetic parameters. The results show that Jinmei has lower relative conductivity, lower peroxide content, and higher total antioxidant capacity after heat stress. We performed transcriptome analysis of the two bok choy varieties under heat stress and normal temperatures. Under heat stress, some key genes involved in sulfur metabolism, glutathione metabolism, and the ribosome pathway were found to be significantly upregulated in the heat-tolerant cultivar. The key genes of each pathway were screened according to their fold-change values. In terms of sulfur metabolism, genes related to protease activity were significantly upregulated. Glutathione synthetase (GSH2) in the glutathione metabolism pathway and the L3e, L23, and S19 genes in the ribosomal pathway were significantly upregulated in heat-stressed cultivars. These results suggest that the total antioxidant capacity and heat injury repair capacity are higher in Jinmei than in the heat-sensitive variety, which might be related to the specific upregulation of genes in certain metabolic pathways after heat stress.

A comparative full-length transcriptomic resource provides insight into the perennial monocarpic mass flowering

Perennial monocarpic mass flowering represents as a key developmental innovation in flowering time diversity in several biological and economical essential families, such as the woody bamboos and the shrubby Strobilanthes. However, molecular and genetic mechanisms underlying this important biodiversity remain poorly investigated. Here, we generated a full-length transcriptome resource incorporated into the BlueOmics database (http://blueomics.iflora.cn) for two Strobilanthes species, which feature contrasting flowering time behaviors. Using about 112 and 104 Gb Iso-seq reads together with ~185 and ~75 Gb strand-specific RNA seq data, we annotated 80 971 and 79 985 non-redundant full-length transcripts for the perennial polycarpic Strobilanthes tetrasperma and the perennial monocarpic Strobilanthes biocullata, respectively. In S. tetrasperma, we identified 8794 transcripts showing spatiotemporal expression in nine tissues. In leaves and shoot apical meristems at two developmental stages, 977 and 1121 transcripts were differentially accumulated in S. tetrasperma and S. biocullata, respectively. Interestingly, among the 33 transcription factors showing differential expression in S. tetrasperma but without differential expression in S. biocullata, three were involved potentially in the photoperiod and circadian-clock pathway of flowering time regulation (FAR1 RELATED SEQUENCE 12, FRS12; NUCLEAR FACTOR Y A1, NFYA1; PSEUDO-RESPONSE REGULATOR 5, PRR5), hence provides an important clue in deciphering the flowering diversity mechanisms. Our data serve as a key resource for further dissection of molecular and genetic mechanisms underpinning key biological innovations, here, the perennial monocarpic mass flowering.

How to explore what is hidden? A review of techniques for vascular tissue expression profile analysis

The evolution of plants to efficiently transport water and assimilates over long distances is a major evolutionary success that facilitated their growth and colonization of land. Vascular tissues, namely xylem and phloem, are characterized by high specialization, cell heterogeneity, and diverse cell components. During differentiation and maturation, these tissues undergo an irreversible sequence of events, leading to complete protoplast degradation in xylem or partial degradation in phloem, enabling their undisturbed conductive function. Due to the unique nature of vascular tissue, and the poorly understood processes involved in xylem and phloem development, studying the molecular basis of tissue differentiation is challenging. In this review, we focus on methods crucial for gene expression research in conductive tissues, emphasizing the importance of initial anatomical analysis and appropriate material selection. We trace the expansion of molecular techniques in vascular gene expression studies and discuss the application of single-cell RNA sequencing, a high-throughput technique that has revolutionized transcriptomic analysis. We explore how single-cell RNA sequencing will enhance our knowledge of gene expression in conductive tissues.

Comparative analysis of alternative splicing events in skeletal muscle of different sheep

This paper aims to investigate the relationship between genes with alternative splicing (AS) events and breed-specific differences in muscle development in two breeds of sheep. RNA-seq was utilized to identify genes with AS between Small-tailed Han sheep and Dorset sheep. The gene lists of differentially spliced genes were identified, and Gene Ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway enrichment analysis were conducted on these genes. In this study, 299 genes with 356 AS indicated significant differences between two diffrerent breeds. There are differences in 31 genes with 35 AS. Cassette, alt5' and alt3' exhibited the highest levels of enrichment across various significant levels. GO and KEGG enrichment analysis demonstrated a significant correlation between Wnt, TGF-beta, Notch and MAPK signaling pathways and the development of muscle in sheep. These findings indicate that genes with AS are linked to variations in muscle development in sheep. These results offer significant scientific and practical implications for improving the quality of sheep meat.

Unraveling the Guardians of Growth: A Comprehensive Analysis of the Aux/IAA and ARF Gene Families in Populus simonii

The auxin/indole-3-acetic acid (Aux/IAA) and auxin response factor (ARF) genes are two crucial gene families in the plant auxin signaling pathway. Nonetheless, there is limited knowledge regarding the Aux/IAA and ARF gene families in Populus simonii. In this study, we first identified 33 putative PsIAAs and 35 PsARFs in the Populus simonii genome. Analysis of chromosomal location showed that the PsIAAs and PsARFs were distributed unevenly across 17 chromosomes, with the greatest abundance observed on chromosomes 2. Furthermore, based on the homology of PsIAAs and PsARFs, two phylogenetic trees were constructed, classifying 33 PsIAAs and 35 PsARFs into three subgroups each. Five pairs of PsIAA genes were identified as the outcome of tandem duplication, but no tandem repeat gene pairs were found in the PsARF family. The expression profiling of PsIAAs and PsARFs revealed that several genes exhibited upregulation in different tissues and under various stress conditions, indicating their potential key roles in plant development and stress responses. The variance in expression patterns of specific PsIAAs and PsARFs was corroborated through RT-qPCR analysis. Most importantly, we instituted that the PsIAA7 gene, functioning as a central hub, exhibits interactions with numerous Aux/IAA and ARF proteins. Furthermore, subcellular localization findings indicate that PsIAA7 functions as a protein localized within the nucleus. To conclude, the in-depth analysis provided in this study will contribute significantly to advancing our knowledge of the roles played by PsIAA and PsARF families in both the development of P. simonii tissue and its responses to stress. The insights gained will serve as a valuable asset for further inquiries into the biological functions of these gene families.

Mapping intron retention events contributing to complex traits using splice quantitative trait locus

BACKGROUND

Alternative splicing (AS) of mRNA plays an important roles in transcriptome diversity, involving regulation of plant growth and stress response. Understanding the variation of AS events underlying GWAS loci in a crop population can provide insight into the molecular mechanisms of complex agronomic traits. To date, genome-wide association studies relating AS events to agronomic traits have rarely been conducted at the population level in crops.

RESULTS

Here, a pipeline was constructed to identify candidate AS events related to complex traits. Firstly, ovule transcriptome data were used to characterize intron retention (IR), the predominant type of AS in plants, on a genome-wide scale. This was done in a natural population consisting of 279 upland cotton lines. Secondly, splice quantitative trait locus (sQTL) analysis was carried out, which yielded a total of 2295 sQTLs involving 1607 genes. Of these, 14.25% (n = 427) were cis-regulatory loci. Integration with expression quantitative trait loci (eQTL) revealed that 53 (21.4%) cis-sGenes were regulated by both cis-sQTLs and cis-eQTLs. Finally, co-localization analysis integrated with GWAS loci in this population showed 32 cis-QTLs to be co-located with genetic regulatory loci related to fiber yield and quality traits, indicating that sQTLs are likely to participate in regulating cotton fiber yield and quality. An in-depth evaluation confirmed that differences in the IR rates of sQTL-regulated candidate genes such as GhLRRK1 and GhGC1 are associated with lint percentage (LP), which has potential in breeding applications.

CONCLUSION

This study provides a clue that AS of mRNA has an impact on crop yield, along with functional sQTLs are new genetic resources for cotton precision breeding.

Integrated transcriptome and proteome analysis reveals molecular responses of soybean anther under high-temperature stress

It is well documented that high temperature (HT) severely affects the development of soybean male reproductive organs. However, the molecular mechanism of thermo-tolerance in soybean remains unclear. To explore the candidate genes and regulatory mechanism of soybean response to HT stress and flower development, here, the anthers of two previously identified HT-tolerant (JD21) and HT-sensitive (HD14) varieties were analyzed by RNA-seq. In total, 219 (172 upregulated and 47 downregulated), 660 (405 upregulated and 255 downregulated), and 4,854 (2,662 upregulated and 2,192 downregulated) differentially expressed genes (DEGs) were identified between JD21 anthers treated with HT stress vs. JD21 anthers in the natural field conditions (TJA vs. CJA), HD14 anthers treated with HT stress vs. HD14 anthers in the natural field conditions (THA vs. CHA), and JD21 vs. HD14 anthers treated with HT stress (TJA vs. THA), respectively. The results showed that there were more DEGs upregulated in JD21; this might be the reason why JD21 was more HT-resistant than the HT-sensitive variety HD14. GO annotation and KEGG enriched analysis showed that many DEGs are mainly involved in defense response, response to biological stimuli, auxin-activated signaling pathway, plant hormone signal transduction, MAPK signaling pathway-plant, starch and sucrose metabolism, etc. The conjoint analysis of RNA-seq and previous iTRAQ results found that there were 1, 24, and 54 common DEGs/DAPs showing the same expression pattern and 1, 2, and 13 common DEGs/DAPs showing the opposite pattern between TJA vs. CJA, THA vs. CHA, and TJA vs. THA at the protein and gene level, respectively, among which HSPs, transcription factor, GSTU, and other DEGs/DAPs participated in the response to HT stress and flower development. Notably, the qRT-PCR analysis and physiological index change results coincided with the sequencing results of RNA-seq and iTRAQ. In conclusion, the HT-tolerant cultivar performed better under stress than the HT-sensitive cultivar through modulation of HSP family proteins and transcription factors, and by keeping key metabolic pathways such as plant hormone signal transduction normal. This study provided important data and some key candidate genes to better study the effect and molecular basis of HT on anther in soybean at a transcription and translation level.

Cell-type-specific alternative splicing in the Arabidopsis germline

During sexual reproduction in flowering plants, the two haploid sperm cells (SCs) embedded within the cytoplasm of a growing pollen tube are carried to the embryo sac for double fertilization. Pollen development in flowering plants is a dynamic process that encompasses changes at transcriptome and epigenome levels. While the transcriptome of pollen and SCs in Arabidopsis (Arabidopsis thaliana) is well documented, previous analyses have mostly been based on gene-level expression. In-depth transcriptome analysis, particularly the extent of alternative splicing (AS) at the resolution of SC and vegetative nucleus (VN), is still lacking. Therefore, we performed RNA-seq analysis to generate a spliceome map of Arabidopsis SCs and VN isolated from mature pollen grains. Based on our de novo transcriptome assembly, we identified 58,039 transcripts, including 9,681 novel transcripts, of which 2,091 were expressed in SCs and 3,600 in VN. Four hundred and sixty-eight genes were regulated both at gene and splicing levels, with many having functions in mRNA splicing, chromatin modification, and protein localization. Moreover, a comparison with egg cell RNA-seq data uncovered sex-specific regulation of transcription and splicing factors. Our study provides insights into a gamete-specific AS landscape at unprecedented resolution.

Comparison of alternative splicing (AS) events in adipose tissue of polled dorset versus small tail han sheep

Background

During the alternative splicing (AS), the exons of primary transcripts are spliced in various arrangements, resulting in structurally and functionally distinct mRNAs and proteins. This study aimed to examine genes with AS events from Small Tail Han sheep and Dorset sheep to explore the mechanism of adipose developments.

Methods

This study identified the genes with AS events in adipose tissues of two different sheep with next-generation sequencing. In this paper, genes with significantly different AS events were performed gene ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) enrichment analyses.

Results

364 genes with 411 A S events showed significant differences in adipose tissues between the two breeds; 108 genes with 120 A S events were extremely significant differences between the two breeds. We identified several novel genes that are related with adipose growth and development. The results of KEGG and GO analysis indicated that oocyte meiosis, mitogen-activated protein kinase (Wnt), mitogen-activated protein kinase (MAPK) signaling pathway, etc. Were closely related to the adipose tissue developments.

Conclusions

This paper revealed that the genes with AS events are important for adipose tissues in sheep, exploring the mechanisms of AS events associated with adipose tissue developments in sheep of different breeds.

Transcriptomic profiling reveals candidate allelopathic genes in rice responsible for interactions with barnyardgrass

Echinochloa crus-galli (barnyardgrass) is one of the most damaging weeds in rice fields worldwide. Allelopathy has been considered a possible application for weed management. Thus understanding its molecular mechanisms is important for rice production. This study generated transcriptomes from rice under mono- and co-culture with barnyardgrass at two-time points to identify the candidate genes controlling allelopathic interactions between rice and barnyardgrass. A total of 5,684 differentially expressed genes (DEGs) were detected, amongst which 388 genes were transcription factors. These DEGs include genes associated with momilactone and phenolic acid biosynthesis, which play critical roles in allelopathy. Additionally, we found significantly more DEGs at 3 hours than at 3 days, suggesting a quick allelopathic response in rice. Up-regulated DEGs involve diverse biological processes, such as response to stimulus and pathways related to phenylpropanoid and secondary metabolites biosynthesis. Down-regulated DEGs were involved in developmental processes, indicating a balance between growth and stress response to allelopathy from barnyardgrass. Comparison of DEGs between rice and barnyardgrass shows few common genes, suggesting different mechanisms underlying allelopathic interaction in these two species. Our results offer an important basis for identifying of candidate genes responsible for rice and barnyardgrass interactions and contribute valuable resources for revealing its molecular mechanisms.

Global Landscape of Alternative Splicing in Maize Response to Low Temperature

Maize (Zea mays L.) is an important food crop planted across the world, and low-temperature stress can affect maize germination. Alternative splicing (AS) is widely present in plants under abiotic stress; however, the response of AS to low-temperature stress in maize remains unclear. In this study, a genome-wide analysis of AS during maize response to low temperatures was performed. AS events were distributed on each chromosome, approximately 2.05-2.09 AS events per gene. Seven genes only had AS in low-temperature-resistant inbred lines. A total of 278 KEGGs and 46 GOs were enriched based on overlapping AS genes, which were associated with hormone and oxidoreductase activity. The mutant was used to verify the function of AS gene ZmWRKY48, and the RGR, RSL, RRL, and RRSA of the mutant decreased by 15.16%-19.87% compared with the normal line. These results contribute to subsequent analysis of the regulatory mechanism of maize in response to low-temperature stress.

KRAB family is involved in network shifts in response to osmotic stress in camels

A feature of the camel is its tolerance to osmotic stress. However, few studies of osmotic stress in vivo or comparative analyses between different tissues of the camel have been performed. Here, we report the roles of Krüppel-associated box domain containing zinc-finger repressor proteins (KRAB-ZFPs) in transcriptional networks under osmotic stress in camels by analyzing transcriptomes of four different tissues under various osmotic conditions. We found that 273 of 278 KRAB-ZFPs were expressed in our data set, being involved in all of the 65 identified networks and exhibiting their extensive functional diversity. We also found that 110 KRAB-ZFPs were hub genes involved in more than half of the networks. We demonstrated that the osmotic stress response is involved in network shifts and that KRAB-ZFPs mediate this process. Finally, we presented the diverse mechanisms of osmotic stress responses in different tissues. These results revealed the genetic architecture of systematic physiological response in vivo to osmotic stress in camels. Our work will lead to new directions for studying the mechanism of osmotic stress response in anti-arid mammals.

High-quality reference transcriptome construction improves RNA-seq quantification in Oryza sativa indica

The Reference Transcriptomic Dataset (RTD) is an accurate and comprehensive collection of transcripts originating from a given organism. It holds the key to precise transcript quantification and downstream analysis of differential expressions and regulations. Currently, transcriptome annotations for most crop plants are far from complete. For example, Oryza sativa indica (O. sativa indica) is reported to have 40,759 transcripts in the Ensembl database without alternative transcript isoforms and alternative splicing (AS) events. To generate a high-quality RTD, we conducted RNA sequencing of rice leaf samples collected at various time points during Rhizoctonia solani infection. The obtained reads were analyzed by adopting the recently developed computational analysis pipeline to assemble the RTD with increased transcript and AS diversity for O. sativa indica (IndicaRTD). After stringent quality filtering, the newly constructed transcriptome annotation was comprised of 122,968 non-redundant transcripts from 53,695 genes. This study identified many novel transcripts compared to Ensembl deposited data that are important for regulating molecular and physiological processes in the plant system. Currently, the assembled IndicaRTD must allow fast quantification of transcript and gene expression with high precision.

Transcriptome analysis revealed the expression levels of genes related to abscisic acid and auxin biosynthesis in grapevine (Vitis vinifera L.) under root restriction

The root system is essential for the stable growth of plants. Roots help anchor plants in the soil and play a crucial role in water uptake, mineral nutrient absorption and endogenous phytohormone formation. Root-restriction (RR) cultivation, a powerful technique, confines plant roots to a specific soil space. In the present study, roots of one-year-old "Muscat Hamburg" grapevine under RR and control (nR) treatments harvested at 70 and 125 days after planting were used for transcriptome sequencing, and in total, 2031 (nR7 vs. nR12), 1445 (RR7 vs. RR12), 1532 (nR7 vs. RR7), and 2799 (nR12 vs. RR12) differentially expressed genes (DEGs) were identified. Gene Ontology (GO) enrichment analysis demonstrated that there were several genes involved in the response to different phytohormones, including abscisic acid (ABA), auxin (IAA), ethylene (ETH), gibberellins (GAs), and cytokinins (CTKs). Among them, multiple genes, such as PIN2 and ERF113, are involved in regulating vital plant movements by various phytohormone pathways. Moreover, following RR cultivation, DEGs were enriched in the biological processes of plant-type secondary cell wall biosynthesis, the defense response, programmed cell death involved in cell development, and the oxalate metabolic process. Furthermore, through a combined analysis of the transcriptome and previously published microRNA (miRNA) sequencing results, we found that multiple differentially expressed miRNAs (DEMs) and DEG combinations in different comparison groups exhibited opposite trends, indicating that the expression levels of miRNAs and their target genes were negatively correlated. Furthermore, RR treatment indeed significantly increased the ABA content at 125 days after planting and significantly decreased the IAA content at 70 days after planting. Under RR cultivation, most ABA biosynthesis-related genes were upregulated, while most IAA biosynthesis-related genes were downregulated. These findings lay a solid foundation for further establishing the network through which miRNAs regulate grapevine root development through target genes and for further exploring the molecular mechanism through which endogenous ABA and IAA regulate root architecture development in grapevine.

Xylem Transcriptome Analysis in Contrasting Wood Phenotypes of Eucalyptus urophylla × tereticornis Hybrids

An investigation of the effects of two important post-transcriptional regulatory mechanisms, gene transcription and alternative splicing (AS), on the wood formation of Eucalyptusurophylla × tereticornis, an economic tree species widely planted in southern China, was carried out. We performed RNA-seq on E. urophylla × tereticornis hybrids with highly contrasting wood basic density (BD), cellulose content (CC), hemicellulose content (HC), and lignin content (LC). Signals of strong differentially expressed genes (DEGs) and differentially spliced genes (DSGs) were detected in all four groups of wood properties, suggesting that gene transcription and selective splicing may have important regulatory roles in wood properties. We found that there was little overlap between DEGs and DSGs in groups of the same trait. Furthermore, the key DEGs and DSGs that were detected simultaneously in the four groups tended to be enriched in different Gene Ontology terms, Kyoto Encyclopedia of Genes and Genomes pathways, and transcription factors. These results implied that regulation of gene transcription and AS is controlled by independent regulatory systems in wood formation. Lastly, we detected transcript levels of known wood biosynthetic genes and found that 79 genes encoding mainly enzymes or proteins such as UGT, LAC, CAD, and CESA may be involved in the positive or negative regulation of wood properties. This study reveals potential molecular mechanisms that may regulate wood formation and will contribute to the genetic improvement of Eucalyptus.

The Long Read Transcriptome of Rice (Oryza sativa ssp. japonica var. Nipponbare) Reveals Novel Transcripts

BACKGROUND

High-throughput next-generation sequencing technologies offer a powerful approach to characterizing the transcriptomes of plants. Long read sequencing has been shown to support the discovery of novel isoforms of transcripts. This approach enables the generation of full-length sequences revealing splice variants that may be important in regulating gene action. Investigation of the diversity of transcripts in the rice transcriptome including splice variants was conducted using PacBio long-read sequence data to improve the annotation of the rice genome.

RESULTS

A cDNA library was prepared from RNA extracted from leaves, roots, seeds, inflorescences, and panicles of O. sativa ssp. japonica var Nipponbare and sequenced on a PacBio Sequel platform. This produced 346,190 non-redundant full-length non-chimeric reads (FLNC) resulting in 33,504 high-quality transcripts. Half of the transcripts were multi-exonic and entirely matched with the reference transcripts. However, 14,874 novel isoforms were also identified resulting predominantly from intron retention and at least one novel splice site. Intron retention was the prevalent alternative splicing event and exon skipping was the least observed. Of 73,659 splice junctions, 12,755 (17%) represented novel splice junctions with canonical and non-canonical intron boundaries. The complexity of the transcriptome was examined in detail for 19 starch synthesis-related genes, defining 276 spliced isoforms of which 94 splice variants were novel.

CONCLUSION

The data reveal the great complexity of the rice transcriptome. The novel transcripts provide new insights that may be a key input in future research to improve the annotation of the rice genome.

Stochastic Variation in DNA Methylation Modulates Nucleosome Occupancy and Alternative Splicing in Arabidopsis thaliana

Plants use complex gene regulatory mechanisms to overcome diverse environmental challenges. For instance, cold stress induces rapid and massive transcriptome changes via alternative splicing (AS) to confer cold tolerance in plants. In mammals, mounting evidence suggests chromatin structure can regulate co-transcriptional AS. Recent evidence also supports co-transcriptional regulation of AS in plants, but how dynamic changes in DNA methylation and the chromatin structure influence the AS process upon cold stress remains poorly understood. In this study, we used the DNA methylation inhibitor 5-Aza-2′-Deoxycytidine (5-aza-dC) to investigate the role of stochastic variations in DNA methylation and nucleosome occupancy in modulating cold-induced AS, in Arabidopsis thaliana (Arabidopsis). Our results demonstrate that 5-aza-dC derived stochastic hypomethylation modulates nucleosome occupancy and AS profiles of genes implicated in RNA metabolism, plant hormone signal transduction, and of cold-related genes in response to cold stress. We also demonstrate that cold-induced remodelling of DNA methylation regulates genes involved in amino acid metabolism. Collectively, we demonstrate that sudden changes in DNA methylation via drug treatment can influence nucleosome occupancy levels and modulate AS in a temperature-dependent manner to regulate plant metabolism and physiological stress adaptation.

Transcriptome analysis provides new insights into cold adaptation of corsac fox ( Vulpes Corsac )

Vulpesare widely distributed throughout the world and have undergone drastic physiological and phenotypic changes in response to their environment. However, little is known about the underlying genetic causes of these traits, especially Vulpes corsac. In this study, RNA‐Seq was used to obtain a comprehensive dataset for multiple pooled tissues of corsac fox, and selection analysis of orthologous genes was performed to identify the genes that may be influenced by the low‐temperature environment. More than 6.32 Gb clean reads were obtained and assembled into a total of 173,353 unigenes with an average length of 557 bp for corsac fox. Selective pressure analysis showed that 16 positively selected genes (PSGs) were identified in corsac fox, red fox, and arctic fox. Enrichment analysis of PSGs showed that the LRP11 gene was enriched in several pathways related to the low‐temperature response and might play a key role in response to environmental stimuli of foxes. In addition, several positively selected genes were related to DNA damage repair (ELP2 and CHAF1A), innate immunity (ARRDC4 and S100A12), and the respiratory chain (NDUFA5), and these positively selected genes might play a role in adaptation to harsh wild fox environments. The results of common orthologous gene analysis showed that gene flow or convergent evolution might be an important factor in promoting regional differentiation of foxes. Our study provides a valuable transcriptomic resource for the evolutionary history of the corsac fox and the adaptations to the extreme environments.

Single-Cell RNA Sequencing for Plant Research: Insights and Possible Benefits

In recent years, advances in single-cell RNA sequencing (scRNA-seq) technologies have continued to change our views on biological systems by increasing the spatiotemporal resolution of our analysis to single-cell resolution. Application of scRNA-seq to plants enables the comprehensive characterization of both common and rare cell types and cell states, uncovering new cell types and revealing how cell types relate to each other spatially and developmentally. This review provides an overview of scRNA-seq methodologies, highlights the application of scRNA-seq in plant science, justifies why scRNA-seq is a master player of sequencing, and explains the role of single-cell transcriptomics technologies in environmental stress adaptation, alongside the challenges and prospects of single-cell transcriptomics. Collectively, we put forward a central role of single-cell sequencing in plant research.

Physiological Analysis and Transcriptome Sequencing Reveal the Effects of Salt Stress on Banana (Musa acuminata cv. BD) Leaf

The salinization of soil is a widespread environmental problem. Banana (Musa acuminata L.) is a salt-sensitive plant whose growth, development, and production are constrained by salt stresses. However, the tolerance mechanism of this salt-sensitive banana to salt stress is still unclear. This study aimed to investigate the influence of NaCl treatment on phenotypic, physiological, and transcriptome changes in bananas. We found that the content of root activity, MDA, Pro, soluble sugar, soluble protein, and antioxidant enzymes activity in salt-stress treatment were significantly higher than the control in bananas. Transcriptome sequencing result identified an overall of 3,378 differentially expressed genes (DEGs) in banana leaves, and the Kyoto Encyclopedia of Genes and Genomes analysis indicated that these DEGs were involved in phenylpropanoid biosynthesis process, ribosome process, starch and sucrose metabolism, amino sugar process, and plant hormone signal transduction process that had simultaneously changed their expression under salt stress, which indicated these DEGs may play a role in promoting BD banana growth under salt treatments. The genes which were enriched in the phenylpropanoid biosynthesis process, starch and sucrose metabolism process, amino sugar process, and plant hormone signal transduction process were specifically regulated to respond to the salt stress treatments. Here, totally 48 differentially expressed transcription factors (TFs), including WRKY, MYB, NAC, and bHLH, were annotated in BD banana under salt stress. In the phenylpropane biosynthesis pathway, all transcripts encoding key enzymes were found to be significantly up-regulated, indicating that the genes in these pathways may play a significant function in the response of BD banana to salt stress. In conclusion, this study provides new insights into the mechanism of banana tolerance to salt stress, which provides a potential application for the genetic improvement of banana with salt tolerance.

Full-length transcriptome reconstruction reveals genetic differences in hybrids of Oryza sativa and Oryza punctata with different ploidy and genome compositions

BACKGROUND

Allopolyploid breeding is an efficient technique for improving the low seed setting rate of autotetraploids in plant breeding and one of the most promising breeding methods. However, there have been few comprehensive studies of the posttranscriptional mechanism in allopolyploids.

RESULTS

By crossing cultivated rice (Oryza sativa, genome AA) with wild rice (Oryza punctata, genome BB), we created hybrid rice lines with different ploidy and genome compositions [diploid hybrid F01 (AB), allotetraploid hybrid F02 (AABB) and F03 (AAAB)]. The genetic differences of the hybrids and the mechanism of allopolyploid breeding dominance were revealed through morphological and cytological observations and single molecule real-time sequencing techniques. The tissues and organs of allotetraploid hybrid F02 exhibited "gigantism" and the highest levels of fertility. The numbers of non-redundant transcripts, gene loci and new isoforms in the polyploid rice lines were higher and the isoform lengths greater than those of the diploid line. Moreover, alternative splicing (AS) events occurred twice as often in the polyploid rice lines than the diploid line. During these events, intron retention dominated. Furthermore, a large number of new genes and isoforms specific to the lines of different ploidy were discovered.

CONCLUSIONS

The results indicated that alternative polyadenylation (APA) and AS events contributed to the complexity and superiority of polyploids in the activity of translation regulators, nucleic acid binding transcription factor activities and the regulation of molecular function. Therefore, these APA and AS events in allopolyploid rice were found to play a role in regulation. Our study provides new germplasm for polyploid rice breeding and reveals complex regulatory mechanisms that may be related to heterosis and fertility.

Rapid Regulation of Alternative Splicing in Response to Environmental Stresses

Plants overcome the changing environmental conditions through diverse strategies and complex regulations. In addition to direct regulation of gene transcription, alternative splicing (AS) also acts as a crucial regulatory mechanism to cope with various stresses. Generating from the same pre-mRNA, AS events allow rapid adjustment of the abundance and function of key stress-response components. Mounting evidence has indicated the close link between AS and plant stress response. However, the mechanisms on how environmental stresses trigger AS are far from understood. The advancing high-throughput sequencing technologies have been providing useful information, whereas genetic approaches have also yielded remarkable phenotypic evidence for AS control of stress responses. It is important to study how stresses trigger AS events for both fundamental science and applications. We review current understanding of stress-responsive AS in plants and discuss research challenges for the near future, including regulation of splicing factors, epigenetic modifications, the shared targets of splice isoforms, and the stress-adjusting ratios between splicing variants.

Integration of metabolomics and transcriptomics data to further characterize Gliricidia sepium (Jacq.) Kunth under high salinity stress

Soil salinity is one abiotic stress that threatens agriculture in more than 100 countries. Gliricidia [Gliricidia sepium (Jacq.) Kunth] is a multipurpose tree known for its ability to adapt to a wide range of soils; however, its tolerance limits and responses to salt stress are not yet well understood. In this study, after characterizing the morphophysiological responses of young gliricidia plants to salinity stress, leaf metabolic and transcription profiles were generated and submitted to single and integrated analyses. RNA from leaf samples were subjected to RNA sequencing using an Illumina HiSeq platform and the paired-end strategy. Polar and lipidic fractions from leaf samples were extracted and analyzed on an ultra-high-performance liquid chromatography (UHPLC) coupled with electrospray ionization quadrupole time-of-flight high-resolution mass spectrometry (MS) system. Acquired data were analyzed using the OmicsBox, XCMS Online, MetaboAnalyst, and Omics Fusion platforms. The substrate salinization protocol used allowed the identification of two distinct responses to salt stress: tolerance and adaptation. Single analysis on transcriptome and metabolome data sets led to a group of 5,672 transcripts and 107 metabolites differentially expressed in gliricidia leaves under salt stress. The phenylpropanoid biosynthesis was the most affected pathway, with 15 metabolites and three genes differentially expressed. Results showed that the differentially expressed metabolites and genes from this pathway affect mainly short-term salt stress (STS). The single analysis of the transcriptome identified 12 genes coding for proteins that might play a role in gliricidia response at both STS and long-term salt stress (LTS). Further studies are needed to reveal the mechanisms behind the adaptation response.

The activation of gene expression and alternative splicing in the formation and evolution of allopolyploid Brassica napus

Allopolyploids contain two or more sets of subgenomes. To establish a compatible relationship between subgenomes, a series of gene expression changes occurred in allopolyploids. What evolutionary changes of transcripts have taken place in Brassica napus during the early establishment and subsequent evolution was a fascinating scientific question. Here, we study this issue using a set of materials (natural, resynthesized B. napus and their progenitors/parents) by long-read RNA sequencing technology. The results showed that more genes were up-regulated in resynthesized B. napus compared with its two parents, and more up-regulated expressed genes were observed in natural B. napus compared with resynthesized B. napus. The presence of up-regulation genes in organism may help it adapt to the influence of "genomic shock" and cope with natural environment. Isoforms are produced from precursor mRNAs by alternative splicing (AS) events, and more than 60% of novel isoforms were identified in all materials, which could improve the reference genome information of B. napus. We found that the isoform numbers, the number of genes potentially involved in AS and alternative polyadenylation increased in B. napus after evolution, which may involve in the adaptation of plants to natural environment. In addition, all identified isoforms were functional annotated by searching 7 databases. In general, this study could improve our overall understanding of the full-length transcriptome of B. napus, and help us recognize the significant gene expression changes and isoform abundance changes occurred in allopolyploid B. napus during evolution.

Improved Reference Genome Annotation of Brassica rapa by Pacific Biosciences RNA Sequencing

The species Brassica rapa includes several important vegetable crops. The draft reference genome of B. rapa ssp. pekinensis was completed in 2011, and it has since been updated twice. The pangenome with structural variations of 18 B. rapa accessions was published in 2021. Although extensive genomic analysis has been conducted on B. rapa, a comprehensive genome annotation including gene structure, alternative splicing (AS) events, and non-coding genes is still lacking. Therefore, we used the Pacific Biosciences (PacBio) single-molecular long-read technology to improve gene models and produced the annotated genome version 3.5. In total, we obtained 753,041 full-length non-chimeric (FLNC) reads and collapsed these into 92,810 non-redundant consensus isoforms, capturing 48% of the genes annotated in the B. rapa reference genome annotation v3.1. Based on the isoform data, we identified 830 novel protein-coding genes that were missed in previous genome annotations, defined the untranslated regions (UTRs) of 20,340 annotated genes and corrected 886 wrongly spliced genes. We also identified 28,564 AS events and 1,480 long non-coding RNAs (lncRNAs). We produced a relatively complete and high-quality reference transcriptome for B. rapa that can facilitate further functional genomic research.

Genome-Wide Expression and Physiological Profiling of Pearl Millet Genotype Reveal the Biological Pathways and Various Gene Clusters Underlying Salt Resistance

Pearl millet (Pennisetum glaucum L.) is a vital staple food and an important cereal crop used as food, feed, and forage. It can withstand heat and drought due to the presence of some unique genes; however, the mechanism of salt stress has been missing in pearl millet until now. Therefore, we conducted a comparative transcriptome profiling to reveal the differentially expressed transcripts (DETs) associated with salt stress in pearl millet at different time points, such as 1, 3, and 7 h, of salt treatment. The physiological results suggested that salt stress significantly increased proline, malondialdehyde (MDA) content, and hydrogen peroxide (H₂O₂) in pearl millet at 1, 3, and 7 h of salt treatment. In addition, pearl millet plants regulated the activities of superoxide dismutase, catalase, and peroxidase to lessen the impact of salinity. The transcriptomic results depicted that salt stress upregulated and downregulated the expression of various transcripts involved in different metabolic functions. At 1 and 7 h of salt treatment, most of the transcripts were highly upregulated as compared to the 3 h treatment. Moreover, among commonly enriched Kyoto Encyclopedia of Genes and Genomes (KEGG) pathways, the mitogen-activated protein kinase (MAPK) signaling pathway and peroxisome pathway were significantly enriched. The DETs related to hormone signaling (auxins, ethylene, gibberellin, and abscisic acid), kinases, protein modifications, and degradation were also identified, depicting the possible role of hormones and kinases to enhance plant tolerance against salt stress. Furthermore, the transcription factors, such as ethylene-responsive element binding factors (ERF), basic helix-loop-helix (bHLH), HMG box-containing protein (HBP), MADS, myeloblastosis (MYB), and WRKY, were predicted to significantly regulate different transcripts involved in salt stress responses at three different time points. Overall, this study will provide new insights to better understand the salt stress regulation mechanisms in pearl millet to improve its resistance against salinity and to identify new transcripts that control these mechanisms in other cereals.

Genome-wide differences in gene expression and alternative splicing in developing embryo and endosperm, and between F1 hybrids and their parental pure lines in sorghum

Developing embryo and endosperm of sorghum show substantial and multifaceted differences in gene expression and alternative splicing, which are potentially relevant to heterosis. Differential regulation of gene expression and alternative splicing (AS) are major molecular mechanisms dictating plant growth and development, as well as underpinning heterosis in F1 hybrids. Here, using deep RNA-sequencing we analyzed differences in genome-wide gene expression and AS between developing embryo and endosperm, and between F1 hybrids and their pure-line parents in sorghum. We uncover dramatic differences in both gene expression and AS between embryo and endosperm with respect to gene features and functions, which are consistent with the fundamentally different biological roles of the two tissues. Accordingly, F1 hybrids showed substantial and multifaceted differences in gene expression and AS compared with their pure-line parents, again with clear tissue specificities including extents of difference, genes involved and functional enrichments. Our results provide useful transcriptome resources as well as novel insights for further elucidation of seed yield heterosis in sorghum and related crops.

Full-length transcriptome analysis and identification of transcript structures in Eimeria necatrix from different developmental stages by single-molecule real-time sequencing

Background

Eimeria necatrix is one of the most pathogenic parasites, causing high mortality in chickens. Although its genome sequence has been published, the sequences and complete structures of its mRNA transcripts remain unclear, limiting exploration of novel biomarkers, drug targets and genetic functions in E. necatrix.

Methods

Second-generation merozoites (MZ-2) of E. necatrix were collected using Percoll density gradients, and high-quality RNA was extracted from them. Single-molecule real-time (SMRT) sequencing and Illumina sequencing were combined to generate the transcripts of MZ-2. Combined with the SMRT sequencing data of sporozoites (SZ) collected in our previous study, the transcriptome and transcript structures of E. necatrix were studied.

Results

SMRT sequencing yielded 21,923 consensus isoforms in MZ-2. A total of 17,151 novel isoforms of known genes and 3918 isoforms of novel genes were successfully identified. We also identified 2752 (SZ) and 3255 (MZ-2) alternative splicing (AS) events, 1705 (SZ) and 1874 (MZ-2) genes with alternative polyadenylation (APA) sites, 4019 (SZ) and 2588 (MZ-2) fusion transcripts, 159 (SZ) and 84 (MZ-2) putative transcription factors (TFs) and 3581 (SZ) and 2039 (MZ-2) long non-coding RNAs (lncRNAs). To validate fusion transcripts, reverse transcription-PCR was performed on 16 candidates, with an accuracy reaching up to 87.5%. Sanger sequencing of the PCR products further confirmed the authenticity of chimeric transcripts. Comparative analysis of transcript structures revealed a total of 3710 consensus isoforms, 815 AS events, 1139 genes with APA sites, 20 putative TFs and 352 lncRNAs in both SZ and MZ-2.

Conclusions

We obtained many long-read isoforms in E. necatrix SZ and MZ-2, from which a series of lncRNAs, AS events, APA events and fusion transcripts were identified. Information on TFs will improve understanding of transcriptional regulation, and fusion event data will greatly improve draft versions of gene models in E. necatrix. This information offers insights into the mechanisms governing the development of E. necatrix and will aid in the development of novel strategies for coccidiosis control.

Graphical Abstract

Supplementary Information

The online version contains supplementary material available at 10.1186/s13071-021-05015-7.

A dynamic intron retention program regulates the expression of several hundred genes during pollen meiosis

Intron retention is a stage-specific mechanism of functional attenuation of a subset of co-regulated, functionally related genes during early stages of pollen development. To improve our understanding of the gene regulatory mechanisms that drive developmental processes, we performed a genome-wide study of alternative splicing and isoform switching during five key stages of pollen development in field mustard, Brassica rapa. Surprisingly, for several hundred genes (12.3% of the genes analysed), isoform switching results in stage-specific expression of intron-retaining transcripts at the meiotic stage of pollen development. In such cases, we report temporally regulated switching between expression of a canonical, translatable isoform and an intron-retaining transcript that is predicted to produce a truncated and presumably inactive protein. The results suggest a new pervasive mechanism underlying modulation of protein levels in a plant developmental program. The effect is not based on gene expression induction but on the type of transcript produced. We conclude that intron retention is a stage-specific mechanism of functional attenuation of a subset of co-regulated, functionally related genes during meiosis, especially genes related to ribosome biogenesis, mRNA transport and nuclear envelope architecture. We also propose that stage-specific expression of a non-functional isoform of Brassica rapa BrSDG8, a non-redundant member of histone methyltransferase gene family, linked to alternative splicing regulation, may contribute to the intron retention observed.

Fruit Colour and Novel Mechanisms of Genetic Regulation of Pigment Production in Tomato Fruits

Fruit colour represents a genetic trait with ecological and nutritional value. Plants mainly use colour to attract animals and favour seed dispersion. Thus, in many species, fruit colour coevolved with frugivories and their preferences. Environmental factors, however, represented other adaptive forces and further diversification was driven by domestication. All these factors cooperated in the evolution of tomato fruit, one of the most important in human nutrition. Tomato phylogenetic history showed two main steps in colour evolution: the change from green-chlorophyll to red-carotenoid pericarp, and the loss of the anthocyanic pigmentation. These events likely occurred with the onset of domestication. Then spontaneous mutations repeatedly occurred in carotenoid and phenylpropanoid pathways, leading to colour variants which often were propagated. Introgression breeding further enriched the panel of pigmentation patterns. In recent decades, the genetic determinants underneath tomato colours were identified. Novel evidence indicates that key regulatory and biosynthetic genes undergo mechanisms of gene expression regulation that are much more complex than what was imagined before: post-transcriptional mechanisms, with RNA splicing among the most common, indeed play crucial roles to fine-tune the expression of this trait in fruits and offer new substrate for the rise of genetic variables, thus providing further evolutionary flexibility to the character.

Global Survey of Alternative Splicing in Rice by Direct RNA Sequencing During Reproductive Development: Landscape and Genetic Regulation

Alternative splicing is a widespread phenomenon, which generates multiple isoforms of the gene product. Reproductive development is the key process for crop production. Although numerous forms of alternative splicing have been identified in model plants, large-scale study of alternative splicing dynamics during reproductive development in rice has not been conducted. Here, we investigated alternative splicing of reproductive development of young panicles (YP), unfertilized florets (UF) and fertilized florets (F) in rice using direct RNA sequencing, small RNA sequencing, and degradome sequencing. We identified a total of 35,317 alternative splicing (AS) events, among which 67.2% splicing events were identified as novel alternative splicing events. Intron retention (IR) was the most abundant alternative splicing subtype. Splicing factors that differentially expressed and alternatively spliced could result in global alternative splicing. Global analysis of miRNAs-targets prediction revealed that alternative spliced transcripts affected miRNAs' targets during development. Degradome sequencing detected only 6.8% of the differentially alternative splicing transcripts, suggesting a productive transcripts generation during development. In addition, alternative splicing isoforms of Co-like, a transcription factor, interacted with Casein kinase 1-like protein HD1 (CKI) examined in luciferase assay, which could modulate normal male-floral organs development and flowering time. These results reveal that alternative splicing is intensely associated with developmental stages, and a high complexity of gene regulation.

Reading between the Lines: RNA-seq Data Mining Reveals the Alternative Message of the Rice Leaf Transcriptome in Response to Heat Stress

Rice (Oryza sativa L.) is a major food crop but heat stress affects its yield and grain quality. To identify mechanistic solutions to improve rice yield under rising temperatures, molecular responses of thermotolerance must be understood. Transcriptional and post-transcriptional controls are involved in a wide range of plant environmental responses. Alternative splicing (AS), in particular, is a widespread mechanism impacting the stress defence in plants but it has been completely overlooked in rice genome-wide heat stress studies. In this context, we carried out a robust data mining of publicly available RNA-seq datasets to investigate the extension of heat-induced AS in rice leaves. For this, datasets of interest were subjected to filtering and quality control, followed by accurate transcript-specific quantifications. Powerful differential gene expression (DE) and differential AS (DAS) identified 17,143 and 2162 heat response genes, respectively, many of which are novel. Detailed analysis of DAS genes coding for key regulators of gene expression suggests that AS helps shape transcriptome and proteome diversity in response to heat. The knowledge resulting from this study confirmed a widespread transcriptional and post-transcriptional response to heat stress in plants, and it provided novel candidates for rapidly advancing rice breeding in response to climate change.

Transcriptome and Small RNA Combined Sequencing Analysis of Cold Tolerance in Non-heading Chinese Cabbage

Background

Non-heading Chinese cabbage (Brassica rapa ssp. chinensis) is an important leaf vegetable grown worldwide. However, there has currently been not enough transcriptome and small RNA combined sequencing analysis of cold tolerance, which hinders further functional genomics research.

Results

In this study, 63.43 Gb of clean data was obtained from the transcriptome analysis. The clean data of each sample reached 6.99 Gb, and the basic percentage of Q30 was 93.68% and above. The clean reads of each sample were sequence aligned with the designated reference genome (Brassica rapa, IVFCAASv1), and the efficiency of the alignment varied from 81.54 to 87.24%. According to the comparison results, 1,860 new genes were discovered in Pak-choi, of which 1,613 were functionally annotated. Among them, 13 common differentially expressed genes were detected in all materials, including seven upregulated and six downregulated. At the same time, we used quantitative real-time PCR to confirm the changes of these gene expression levels. In addition, we sequenced miRNA of the same material. Our findings revealed a total of 34,182,333 small RNA reads, 88,604,604 kinds of small RNAs, among which the most common size was 24 nt. In all materials, the number of common differential miRNAs is eight. According to the corresponding relationship between miRNA and its target genes, we carried out Gene Ontology and Kyoto Encyclopedia of Genes and Genomes enrichment analysis on the set of target genes on each group of differentially expressed miRNAs. Through the analysis, it is found that the distributions of candidate target genes in different materials are different. We not only used transcriptome sequencing and small RNA sequencing but also used experiments to prove the expression levels of differentially expressed genes that were obtained by sequencing. Sequencing combined with experiments proved the mechanism of some differential gene expression levels after low-temperature treatment.

Conclusion

In all, this study provides a resource for genetic and genomic research under abiotic stress in Pak-choi.

Comparative transcriptome analysis reveals changes in gene expression in sea cucumber (Holothuria leucospilota) in response to acute temperature stress

Ambient temperature is an important abiotic factor that influences growth performance and physiological functions in sea cucumbers. To understand the molecular responses of the sea cucumber Holothuria leucospilota to acute temperature stress, we performed a de novo transcriptome analysis of body wall tissue from H. leucospilota exposed to 2 hoursh of acute heat (35 ± 1 °C) and cold stress (15 ± 1 °C). A total of 99,015 unigenes were obtained after assembly of the sequenced reads. Compared with a control group maintained at 25.0 ± 1 °C, 1169 differentially expressed unigenes (DEGs) were identified after heat stress, 781 were up-regulated and 388 were down-regulated. After cold stress, 1464 DEGs were identified; 900 were up-regulated and 564 were down-regulated. The annotation of DEGs revealed that heat shock proteins play important roles in protecting H. leucospilota from high temperature stress. Furthermore, KEGG pathway enrichment analysis showed that the categories: "Ribosome" and "Protein processing in endoplasmic reticulum" were strongly affected by heat stress. These two pathways are associated with biosynthesis and processing of proteins, and refolding of misfolded proteins. The lipid metabolism pathways "Sphingolipid metabolism" and "Ether lipid metabolism", were affected by cold stress. The RNA-Seq results for eight selected DEGs were verified the expression by quantitative real-time PCR analysis. Our results will improve the understanding of the molecular response mechanisms of H. leucospilota to ambient temperature stress.

Investigation Into Different Wood Formation Mechanisms Between Angiosperm and Gymnosperm Tree Species at the Transcriptional and Post-transcriptional Level

Enormous distinctions of the stem structure and cell types between gymnosperms and angiosperms tree species are expected to cause quite different wood physical and mechanical attributes, however, the molecular mechanisms underlying the differing wood morphology are still unclear. In this study, we compared the transcriptomes obtained by RNA-Seq between Populus alba × P. glandulosa clone 84K, and Larix kaempferi (Lamb.) Carr trees. Available genome resource served as reference for P. alba × P. glandulosa and the Iso-Seq results of a three-tissues mixture (xylem, phloem, and leaf) were used as the reference for L. kaempferi to compare the xylem-specifically expressed genes and their alternative splicing model. Through screening, we obtained 13,907 xylem-specifically expressed genes (5,954 up-regulated, 7,953 down-regulated) in the xylem of P. alba × P. glandulosa, and 2,596 xylem-specifically expressed genes (1,648 up-regulated, 948 down-regulated) in the xylem of L. kaempferi. From the GO and KEGG analyses, some genes associated with two wood formation-related pathways, namely those for phenylpropanoid biosynthesis, and starch and sucrose metabolism, were successfully screened. Then the distributions and gene expression models between P. alba × P. glandulosa and L. kaempferi in those pathways were compared, which suggested differential wood formation processes between the angiosperm and gymnosperm trees. Furthermore, a Weight Gene Co-expression Network Analysis (WGCNA) for total xylem-specifically expressed genes in two species was conducted, from which wood formation-related modules were selected to build a co-expression network for the two tree species. The genes within this co-expression network showed different co-expression relationships between the angiosperm and gymnosperm woody species. Comparing the alternative splicing events for wood formation-related genes suggests a different post-transcriptional regulation process exists between the angiosperm and gymnosperm trees. Our research thus provides the foundation for the in-depth investigation of different wood formation mechanisms of angiosperm and gymnosperm species.

Identification of MicroRNAs in Taxillus chinensis (DC.) Danser Seeds under Cold Stress

Taxillus chinensis (DC.) Danser, a parasitic plant that belongs to the Loranthaceae family, has a long history of being used in the Chinese medicine. We observed that the loranthus seeds were sensitive to temperature and could lose viability below 0°C quickly. Thus, we performed small RNA sequencing to study the microRNA (miRNA) regulation in the loranthus seeds under cold stress. In total, we identified 600 miRNAs, for the first time, in the loranthus seeds under cold stress. Then, we detected 224, 229, and 223 miRNAs (TPM > 1) in A0 (control), A1 (cold treatment for 12 h at 0°C), and A2 (cold treatment for 36 h at 0°C), respectively. We next identified 103 differentially expressed miRNAs (DEmiRs) in the loranthus seeds in response to cold. Notably, miR408 was upregulated during the cold treatment, which can regulate genes encoding phytocyanin family proteins and phytophenol oxidases. Some DEmiRs were specific to A1 and may function in early response to cold, such as gma-miR393b-3p, miR946, ath-miR779.2-3p, miR398, and miR9662. It is interesting that ICE3, IAA13, and multiple transcription factors (e.g., WRKY and CRF4 and TCP4) regulated by the DEmiRs have been reported to respond cold in other plants. We further identified 4, 3, and 4 DEmiRs involved in the pathways “responding to cold,” “responding to abiotic stimulus,” and “seed development/germination,” respectively. qRT-PCR was used to confirm the expression changes of DEmiRs and their targets in the loranthus seeds during the cold treatment. This is the first time to study cold-responsive miRNAs in loranthus, and our findings provide a valuable resource for future studies.

IsoSplitter: identification and characterization of alternative splicing sites without a reference genome

Long-read transcriptome sequencing is designed to sequence full-length RNA molecules and advantageous for identifying alternative splice isoforms; however, in the absence of a reference genome, it is difficult to accurately locate splice sites, because of the diversity of patterns of alternative splicing (AS). Based on long-read transcriptome data we developed a versatile tool, IsoSplitter, to reverse-trace and validate AS gene "split-sites" with the following features: (1) IsoSplitter initially invokes a modified SIM4 program to find transcript split-sites; (2) each split-site is then quantified, to reveal transcript diversity, and putative isoforms are grouped into gene clusters; (3) an optional step for aligning short-reads is provided, to validate split-sites by identifying unique junction reads, and revealing and quantifying tissue-specific alternative splice isoforms. We tested IsoSplitter AS prediction using datasets from multiple model and non-model plant species, and showed that IsoSplitter pipeline is efficient to handle different transcriptomes with high accuracy. Furthermore, we evaluated the IsoSplitter pipeline compared with that of the splice junction identification tools, Program to Assemble Spliced Alignments (PASA-software needs a reference genome for AS identification) and AStrap, using data from the model plant Arabidopsis thaliana. We found that, IsoSplitter determined more than twice as many AS events than AStrap analysis; and 94.13% of the IsoSplitter predicted AS events were also identified by the PASA analysis. Starting from a simple sequence file, IsoSplitter is an assembly-free tool for identification and characterization of AS. IsoSplitter is developed and implemented in Python 3.5 using the Linux platform and is freely available at https://github.com/Hengfu-Yin/IsoSplitter.

Characteristics of RNA alternative splicing and its potential roles in ginsenoside biosynthesis in a single plant of ginseng, Panax ginseng C.A. Meyer

RNA alternative splicing (AS) is prevalent in higher organisms and plays a paramount role in biology; therefore, it is crucial to have comprehensive knowledge on AS to understand biology. However, knowledge is limited about how AS activates in a single plant and functions in a biological process. Ginseng is one of the most widely used medicinal herbs that is abundant in a number of medicinal bioactive components, especially ginsenosides. In this study, we sequenced the transcripts of 14 organs from a 4-year-old ginseng plant and quantified their ginsenoside contents. We identified AS genes by analyzing their transcripts with the ginseng genome and verified their AS events by PCR. The plant had a total of 13,863 AS genes subjected to 30,801 AS events with five mechanisms: skipped exon, retained intron, alternative 5'splice site, alternative 3' splice site, and mutually exclusive exon. The genes that were more conserved, had more exons, and/or expressed across organs were more likely to be subjected to AS. AS genes were enriched in over 500 GO terms in the plant even though the number of AS gene-enriched GO terms varied across organs. At least 24 AS genes were found to be involved in ginsenoside biosynthesis. These AS genes were significantly up-enriched and more likely to form a co-expression network, thus suggesting the functions of AS and correlations of the AS genes in the process. This study provides comprehensive insights into the molecular characteristics and biological functions of AS in a single plant; thus, helping better understand biology.

Nanopore long-read RNAseq reveals regulatory mechanisms of thermally variable reef environments promoting heat tolerance of scleractinian coral Pocillopora damicornis

Some scleractinian corals exhibit high thermal adaptability to climate changes, although the mechanism of their adaptation is unclear. This study investigated the adaptability of scleractinian coral Pocillopora damicornis to thermally variable reef environments by applying a nanopore-based RNA sequencing method to characterize different transcription responses that promote heat tolerance of P. damicornis. We identified 1414 novel genes and optimized 6256 mis-annotated loci. Based on full-length transcriptome data, we identified complex alternative polyadenylation and alternative splicing events, which can improve our understanding of the genome annotation and gene structures of P. damicornis. Furthermore, we constructed differentially expressed lncRNA-mRNA co-expression networks, which may play a crucial role in the P. damicornis thermal adaptive response. KEGG function enrichment analysis revealed that P. damicornis from the high-temperature pool had a lower metabolic rate than that from the low-temperature pool. We hypothesize that metabolic readjustment, in the form of a lower metabolic rate, positively correlated with increased heat tolerance in P. damicornis in thermally variable reef environments. Our study provides novel insights into lncRNAs that promote thermally tolerance of scleractinian corals in the thermally variable reef environment, suggesting potential mechanisms for their adaptation to global warming in the future.

Transcriptional landscape of rice roots at the single-cell resolution

There are two main types of root systems in flowering plants, namely taproot systems of dicots and fibrous root systems found in monocots. Despite this fundamental split, our current knowledge of cellular and molecular mechanism driving root development is mainly based on studies of the dicot model Arabidopsis. However, the world major crops are monocots and little is known about the transcriptional programs underlying cell-type specification in this clade. Here, we report the transcriptomes of more than 20 000 single cells derived from root tips of two agronomically important rice cultivars. Using combined computational and experimental analyses we were able to robustly identify most of the major cell types and define novel cell-type-specific marker genes for both cultivars. Importantly, we found divergent cell types associated with specific regulatory programs, including phytohormone biosynthesis, signaling, and response, which were well conserved between the two rice cultivars. In addition, we detected substantial differences between the cell-type transcript profiles of Arabidopsis and rice. These species-specific features emphasize the importance of analyzing tissues across diverse model species, including rice. Taken together, our study provides insight into the transcriptomic landscape of major cell types of rice root tip at single-cell resolution and opens new avenues to study cell-type specification, function, and evolution in plants.

Characterization of CHARK, an unusual cytokinin receptor of rice

The signal transduction of the plant hormone cytokinin is mediated by a His-to-Asp phosphorelay. The canonical cytokinin receptor consists of an extra cytoplasmic hormone binding domain named cyclase/histidine kinase associated sensory extracellular (CHASE) and cytoplasmic histidine kinase and receiver domains. In addition to classical cytokinin receptors, a different type receptor—named CHASE domain receptor serine/threonine kinase (CHARK)—is also present in rice. It contains the same ligand binding domain as other cytokinin receptors but has a predicted Ser/Thr—instead of a His-kinase domain. Bioinformatic analysis indicates that CHARK is a retrogene and a product of trans-splicing. Here, we analyzed whether CHARK can function as a bona fide cytokinin receptor. A biochemical assay demonstrated its ability to bind cytokinin. Transient expression of CHARK in protoplasts increased their response to cytokinin. Expression of CHARK in an Arabidopsis receptor double mutant complemented its growth defects and restored the ability to activate cytokinin response genes, clearly demonstrating that CHARK functions as a cytokinin receptor. We propose that the CHARK gene presents an evolutionary novelty in the cytokinin signaling system.

PSDX: A Comprehensive Multi-Omics Association Database of Populus trichocarpa With a Focus on the Secondary Growth in Response to Stresses

Populus trichocarpa (P. trichocarpa) is a model tree for the investigation of wood formation. In recent years, researchers have generated a large number of high-throughput sequencing data in P. trichocarpa. However, no comprehensive database that provides multi-omics associations for the investigation of secondary growth in response to diverse stresses has been reported. Therefore, we developed a public repository that presents comprehensive measurements of gene expression and post-transcriptional regulation by integrating 144 RNA-Seq, 33 ChIP-seq, and six single-molecule real-time (SMRT) isoform sequencing (Iso-seq) libraries prepared from tissues subjected to different stresses. All the samples from different studies were analyzed to obtain gene expression, co-expression network, and differentially expressed genes (DEG) using unified parameters, which allowed comparison of results from different studies and treatments. In addition to gene expression, we also identified and deposited pre-processed data about alternative splicing (AS), alternative polyadenylation (APA) and alternative transcription initiation (ATI). The post-transcriptional regulation, differential expression, and co-expression network datasets were integrated into a new P. trichocarpa Stem Differentiating Xylem (PSDX) database (http://forestry.fafu.edu.cn/db/SDX), which further highlights gene families of RNA-binding proteins and stress-related genes. The PSDX also provides tools for data query, visualization, a genome browser, and the BLAST option for sequence-based query. Much of the data is also available for bulk download. The availability of PSDX contributes to the research related to the secondary growth in response to stresses in P. trichocarpa, which will provide new insights that can be useful for the improvement of stress tolerance in woody plants.