The plant siRNA landscape

Global knowledge mapping and emerging research trends in non-coding RNAs related to animal and plant male sterility: A visual analysis of CiteSpace maps

Animal and plant male sterility is a complex and closely studied phenomenon that significantly impacts species survival and reproduction. Advances in biotechnology and molecular biology have deepened our understanding of gene expression regulation, particularly the role of noncoding RNAs (ncRNAs). This study aims to systematically review and analyze the published literature on ncRNAs in relation to both animal and plant sterility using bibliometric methods. A bibliometric analysis was conducted with CiteSpace 6.2.R6, Scimago Graphica, VOSviewer 1.6.18, and Microsoft Excel 2016 to identify research hotspots, key developments, and emerging trends. Data were retrieved from the Web of Science Core Collection on March 3, 2024, covering publications from 2005 to 2023. The analysis revealed a consistent increase in annual publications on ncRNA research in both plant and animal fields, with China and the United States leading in publication volume. Notable scholars include Professor Abu-Halima, a prominent figure in ncRNA research related to animal male sterility, and Professor Meyers, a key contributor to plant male sterility research. Journals such as PLoS ONE serve as major platforms for disseminating findings on animal male sterility, while The Plant Cell plays a similar role for plant male sterility. Analysis of cited literature and keyword trends highlighted significant themes, including gene regulation and the application of novel technologies. At present, new technologies, model organisms, and gene regulation remain major research hotspots. Meanwhile, disease diagnosis, disease treatment, and crop improvement are emerging as important directions for future research.

Cross-kingdom RNAi induced by a beneficial endophytic fungus to its host requires transitivity and amplification of silencing signals

Cross-kingdom transfer of small RNA (sRNA) molecules has been identified as a means of communication between plants and interacting microorganisms, but the mechanistic details of this sRNA-based interaction remain elusive. We have previously shown that the beneficial root-colonizing fungus Fusarium solani strain K (FsK) translocates sRNAs to its host, Nicotiana benthamiana (Nb), leading to systemic silencing of a reporter gene. Here, we investigated the mechanistic details of the endophyte-induced systemic silencing using an RNAi sensor system. We inoculated three Nb GFP expressing lines with conidia of an FsK transformant containing a transgene that targets host GFP (FsK-hpGF). The efficiency of silencing mediated by FsK-hpGF was monitored both phenotypically under ultraviolet light as well as quantitatively by RT-qPCR. sRNA sequencing was performed to evaluate the production of sRNAs targeting host GFP. Finally, bisulfite sequencing was used to assess plant GFP methylation levels. We show that the translocated fungal sRNAs induced production of secondary sRNAs, mainly of 22-24-nt in size, with the conspicuous absence of 21-nt sRNAs. Importantly, systemic silencing could not be induced in an RNA-DEPENDENT RNA POLYMERASE 6 (RDR6) CRISPR/Cas knockout background, nor in an intron-containing target gene. Overall, our data show that endophyte-induced silencing in the host requires RDR6-mediated transitivity and amplification of silencing signals. Despite being based on an artificial RNAi sensor system, our observations may reflect a more generalized and so far unexplored facet of cross-kingdom RNAi, with RDR6-based transitivity influencing the way symbionts and pathogens elicit systemic phenotypes in their host plants.

Epigenetics in the modern era of crop improvements

Epigenetic mechanisms are integral to plant growth, development, and adaptation to environmental stimuli. Over the past two decades, our comprehension of these complex regulatory processes has expanded remarkably, producing a substantial body of knowledge on both locus-specific mechanisms and genome-wide regulatory patterns. Studies initially grounded in the model plant Arabidopsis have been broadened to encompass a diverse array of crop species, revealing the multifaceted roles of epigenetics in physiological and agronomic traits. With recent technological advancements, epigenetic regulations at the single-cell level and at the large-scale population level are emerging as new focuses. This review offers an in-depth synthesis of the diverse epigenetic regulations, detailing the catalytic machinery and regulatory functions. It delves into the intricate interplay among various epigenetic elements and their collective influence on the modulation of crop traits. Furthermore, it examines recent breakthroughs in technologies for epigenetic modifications and their integration into strategies for crop improvement. The review underscores the transformative potential of epigenetic strategies in bolstering crop performance, advocating for the development of efficient tools to fully exploit the agricultural benefits of epigenetic insights.

Hairpin inserts in viral genomes are stable when they conform to the thermodynamic properties of viral RNA substructures

Virus-induced gene silencing (VIGS) allows for the rapid targeting of gene expression and has been instrumental in characterizing plant genes. However, foreign sequences inserted into VIGS vectors are rarely maintained for unknown reasons. Citrus yellow vein-associated umbravirus-like virus (CY1) with its solved secondary structure was converted into a VIGS vector to determine why simple hairpins inserted into non-functional, single-stranded locations are not maintained. When CY1 contained foreign hairpins with thermodynamic properties (positional entropy and/or ΔG) differing from those of natural CY1 hairpins, deletions arose within a few weeks of infecting Nicotiana benthamiana. In contrast, duplication and insertion of four natural CY1 hairpins (up to 200 nt) into the same locations were retained until plant senescence. Hairpins containing similar conformations and thermodynamic properties as natural hairpins were also retained, as were hairpins that shared thermodynamic properties but were conformationally distinct. By predicting and modulating these thermodynamic properties, a hairpin was retained by CY1 for at least 30 months in citrus. These findings strongly suggest that RNA viruses have evolved to contain substructures with specific thermodynamic properties, and hairpins containing these properties are stable when inserted into non-functional regions of the genome, opening up VIGS for long-lived trees and vines.

IMPORTANCE

Plus-strand RNA plant viruses are used as tools to introduce small interfering RNAs (siRNAs) into laboratory plants to target and silence genes. However, virus-induced gene silencing (VIGS) vectors engineered to contain foreign hairpins or other sequences for siRNA generation are not stable, and the foreign sequences are rapidly lost. We found that foreign sequences are not maintained in an umbravirus-like VIGS vector (CY1) because their physical properties conflict with the innate properties of the CY1 genome's substructures (i.e., hairpins). When natural CY1 hairpins were duplicated and inserted into locations where previous inserts were rapidly lost, the hairpins were now stable as were unrelated hairpins with the same physical properties. By mimicking the physical properties of the viral genome, one insert was stable for over 30 months. These results suggest that RNA viral genomes have evolved to have specific physical properties, and these properties appear to be similar for other plus-strand RNA viruses.

Understanding the Regulation Activities of Transposons in Driving the Variation and Evolution of Polyploid Plant Genome

Transposon is the main component of the eukaryotic genome, and more and more plant genome data show that transposons are diverse in regulating genome structure, variation, function and evolution, with different transposition mechanisms in the genome. Hybridization and polyploidy play an important role in promoting plant speciation and evolution, and recent studies have shown that polyploidy is usually accompanied by the expansion of transposons, which affect the genome size and structure of polyploid plants. Transposons can insert into genes and intergenic regions, resulting in great differences in the overall genome structure of closely related plant species, and it can also capture gene segments in the genome to increase the copy number of genes. In addition, transposons influence the epigenetic modification state of the genome and regulate the expression of the gene, while plant phenotype, biological and abiotic stress response are also regulated by transposons. Overall, transposons play an important role in the plant genome, especially polyploid plant genome, adaptation and evolution.

Identification of the co-regulatory siRNAs of "miRNA→target" in Oryza sativa

The current "small interfering RNA(siRNA)→Target" mining tools can only search for targets of known siRNAs, and cannot discover co-regulatory siRNAs of unknown sequences that may exist, which means that the "microRNA(miRNA)→Target" database obtained by these mining tools is incomplete. Using the previously developed sRNATargetDigger, we re-mined the rice "miRNA→Target" database supported by the degradome and found 86.2% of the target genes were co-regulated by one or more miRNAs\siRNAs. Besides the known miRNAs, 30 miRNA isoforms (isomiRs) and 12 siRNAs were identified to be involved in co-regulation, which play important roles in rice response to external auxin regulation, rice blast resistance, adventitious root formation, cold resistance, and tillering etc. Some isomiRs even have higher expression levels than miRNAs. In addition, we also found that the regulatory relationship between 51 miRNAs and 48 target genes in the original database could not be verified due to the low expression levels of miRNA, poor complementarity between miRNA and target, or no specific cleavage signal detected by degradome in the middle of the miRNA binding site in the targets. Four miRNAs (osa-miR530-5p,osa-miR319b,osa-miR172c and osa-miR395a) only found isomiRs involved in regulation. In addition, we also found a number of miRNA→target regulatory relationships missed in the database. This study improved the rice "miRNA→target" database which will contribute to the research of rice miRNA and molecular breeding.

Arabidopsis SGS3 is recruited to chromatin by CHR11 to select RNA that initiate siRNA production

In plants, aberrant RNAs produced by endogenous genes or transgenes are normally degraded by the nuclear and cytosolic RNA quality control (RQC) pathways. Under certain biotic or abiotic stresses, RQC is impaired, and aberrant RNAs are converted into siRNAs that initiate post-transcriptional gene silencing (PTGS) in the cytosol. How aberrant RNAs are selected and brought to the cytoplasm is not known. Here we show that the RNA-binding protein SUPPRESSOR OF GENE SILENCING (SGS)3 shuttles between the cytosol and the nucleus where it associates with the ISWI-like CHROMATIN REMODELER (CHR)11 and with RNAs transcribed from PTGS-sensitive transgene loci binding CHR11. Knocking down CHR11 and its paralog CHR17 strongly reduces transgene PTGS, suggesting that SGS3 recruitment by CHR11/17 facilitates PTGS initiation. CHR11 is also enriched at endogenous protein-coding genes (PCGs) producing nat-siRNAs and va-siRNAs under biotic or abiotic stresses, and this production is reduced in chr11 chr17 double mutants at genome-wide level. Moreover, impairing CHR11 and CHR17 rescues the lethal phenotype caused by the massive production of siRNAs from PCGs in RQC-deficient mutants. We propose that SGS3 recruitment by CHR11/17 allows exporting RNAs to the cytosol to initiate the production of siRNAs.

Small Interfering RNAs as Critical Regulators of Plant Life Process: New Perspectives on Regulating the Transcriptomic Machinery

Small interfering RNAs (siRNAs) are a distinct class of regulatory RNAs in plants and animals. Gene silencing by small interfering RNAs is one of the fundamental mechanisms for regulating gene expression. siRNAs are critical regulators during developmental processes. siRNAs have similar structures and functions to small RNAs but are derived from double-stranded RNA and may be involved in directing DNA methylation of target sequences. siRNAs are a less well-studied class than the miRNA group, and researchers continue to identify new classes of siRNAs that appear at specific developmental stages and in particular tissues, revealing a more complex mode of siRNA action than previously thought. This review characterizes the siRNA classes and their biogenesis process and focuses on presenting their known functions in the regulation of plant development and responses to biotic and abiotic stresses. The review also highlights the exciting potential for future research in this field, proposing methods for detecting plant siRNAs and a bioinformatic pathway for identifying siRNAs and their functions.

Epigenetic gene regulation in plants and its potential applications in crop improvement

DNA methylation, also known as 5-methylcytosine, is an epigenetic modification that has crucial functions in plant growth, development and adaptation. The cellular DNA methylation level is tightly regulated by the combined action of DNA methyltransferases and demethylases. Protein complexes involved in the targeting and interpretation of DNA methylation have been identified, revealing intriguing roles of methyl-DNA binding proteins and molecular chaperones. Structural studies and in vitro reconstituted enzymatic systems have provided mechanistic insights into RNA-directed DNA methylation, the main pathway catalysing de novo methylation in plants. A better understanding of the regulatory mechanisms will enable locus-specific manipulation of the DNA methylation status. CRISPR-dCas9-based epigenome editing tools are being developed for this goal. Given that DNA methylation patterns can be stably transmitted through meiosis, and that large phenotypic variations can be contributed by epimutations, epigenome editing holds great promise in crop breeding by creating additional phenotypic variability on the same genetic material.

Transcriptional junk: Waste or a key regulator in diverse biological processes?

Plant genomes, through their evolutionary journey, have developed a complex composition that includes not only protein-coding sequences but also a significant amount of non-coding DNA, repetitive sequences, and transposable elements, traditionally labeled as "junk DNA". RNA molecules from these regions, labeled as "transcriptional junk," include non-coding RNAs, alternatively spliced transcripts, untranslated regions (UTRs), and short open reading frames (sORFs). However, recent research shows that this genetic material plays crucial roles in gene regulation, affecting plant growth, development, hormonal balance, and responses to stresses. Additionally, some of these regulatory regions encode small proteins, such as miRNA-encoded peptides (miPEPs) and microProteins (miPs), which interact with DNA or nuclear proteins, leading to chromatin remodeling and modulation of gene expression. This review aims to consolidate our understanding of the diverse roles that these so-called "transcriptional junk" regions play in regulating various physiological processes in plants.

Enjoy the silence: Canonical and non-canonical RNA silencing activity during plant sexual reproduction

Plants produce small RNAs that accomplish a surprisingly versatile number of functions. The heterogeneity of functions of plant small RNAs is evident at the tissue-specific level. In particular, in the last years, the study of their activity in reproductive tissues has unmasked an unexpected diversity in their biogenesis and roles. Here, we review recent findings about the biogenesis pathways and roles of small RNAs during plant sexual reproduction.

Engineering VIGS Vectors by Modifying Movement Proteins of the 30K Family

Virus-induced gene silencing (VIGS) represents a particularly relevant tool in agricultural species for studying gene functionality. This study presents a novel approach for utilizing viruses belonging to the 30K family of movement proteins (MPs) as VIGS vectors. The method described here employs smaller inserts (54 bp or less) than those commonly used (100-500 bp). The developed strategy involves modifying 30K family MPs to introduce heterologous sequences of the gene of interest into their coding sequence. This approach enabled the successful induction of gene silencing in Nicotiana tabacum and Nicotiana benthamiana. Three representative viruses of the MP 30K family, alfalfa mosaic virus (AMV), cucumber mosaic virus (CMV), and tobacco mosaic virus (TMV) were employed. The capacity to induce gene silencing of small inserts (18-54 bp) was investigated, enabling to establish a correlation between insert size and silencing efficacy. This allowed the system to be calibrated to achieve partial silencing levels. The relationship between viral encapsidation and the level of gene silencing was also investigated, revealing that a high efficiency of viral encapsidation results in a reduction in the level of gene silencing achieved. Considering these findings, it can be concluded that the approach carried out with AMV, CMV, and TMV could be applied to other members of the MP 30K family. The MP 30K family comprises 20 viral genera and over 500 viral species, which can infect all agronomically significant plant species. Consequently, the strategy presented in this work could be applied to a wide range of relevant hosts.

Endogenous viral elements are targeted by RNA silencing pathways in banana

Endogenous banana streak virus (eBSV) integrants derived from three distinct species, present in Musa balbisiana (B) but not Musa acuminata (A) banana genomes are able to reconstitute functional episomal viruses causing banana streak disease in interspecific triploid AAB banana hybrids but not in the diploid (BB) parent line, which harbours identical eBSV loci. Here, we investigated the regulation of these eBSV. In-depth characterization of siRNAs, transcripts and methylation derived from eBSV using Illumina and bisulfite sequencing were carried out on eBSV-free Musa acuminata AAA plants and BB or AAB banana plants with eBSV. eBSV loci produce low-abundance transcripts covering most of the viral sequence and generate predominantly 24-nt siRNAs. siRNA accumulation is restricted to duplicated and inverted viral sequences present in eBSV. Both siRNA-accumulating and nonaccumulating sequences of eBSV in BB plants are heavily methylated in all three CG, CHG and CHH contexts. Our data suggest that eBSVs are controlled at the epigenetic level in BB diploids. This regulation not only prevents their awakening and systemic infection of the plant but is also probably involved in the inherent resistance of the BB plants to mealybug-transmitted viral infection. These findings are thus of relevance to other plant resources hosting integrated viruses.

The biogenesis, regulation and functions of transitive siRNA in plants

Small RNA (sRNA)-mediated RNA interference (RNAi) is a sequence-specific gene silencing mechanism that modulates gene expression in eukaryotes. As core molecules of RNAi, various sRNAs are encoded in the plant genome or derived from invading RNA molecules, and their biogenesis depends on distinct genetic pathways. Transitive small interfering RNAs (siRNAs), which are sRNAs produced from double-strand RNA (dsRNA) in a process that depends on RNA-dependent RNA polymerases (RDRs), can amplify and spread silencing signals to additional transcripts, thereby enabling a phenomenon termed "transitive RNAi". Members of this class of siRNAs function in various biological processes ranging from development to stress adaptation. In Arabidopsis thaliana, two RDRs participate in the generation of transitive siRNAs, acting cooperatively with various siRNA generation-related factors, such as the RNA-induced silencing complex (RISC) and aberrant RNAs. Transitive siRNAs are produced in diverse subcellular locations and structures under the control of various mechanisms, highlighting the intricacies of their biogenesis and functions. In this review, we discuss recent advances in understanding the molecular events of transitive siRNA biogenesis and its regulation, with a particular focus on factors involved in RDR recruitment. We aim to provide a comprehensive description of the generalized mechanism governing the biogenesis of transitive siRNAs. Additionally, we present an overview of the diverse biological functions of these siRNAs and raise some pressing questions in this area for further investigation.

Arabidopsis AGO1 N-terminal extension acts as an essential hub for PRMT5 interaction and post-translational modifications

Plant ARGONAUTE (AGO) proteins play pivotal roles regulating gene expression through small RNA (sRNA) -guided mechanisms. Among the 10 AGO proteins in Arabidopsis thaliana, AGO1 stands out as the main effector of post-transcriptional gene silencing. Intriguingly, a specific region of AGO1, its N-terminal extension (NTE), has garnered attention in recent studies due to its involvement in diverse regulatory functions, including subcellular localization, sRNA loading and interactions with regulatory factors. In the field of post-translational modifications (PTMs), little is known about arginine methylation in Arabidopsis AGOs. In this study, we show that NTE of AGO1 (NTEAGO1) undergoes symmetric arginine dimethylation at specific residues. Moreover, NTEAGO1 interacts with the methyltransferase PRMT5, which catalyzes its methylation. Notably, we observed that the lack of symmetric dimethylarginine has no discernible impact on AGO1's subcellular localization or miRNA loading capabilities. However, the absence of PRMT5 significantly alters the loading of a subgroup of sRNAs into AGO1 and reshapes the NTEAGO1 interactome. Importantly, our research shows that symmetric arginine dimethylation of NTEs is a common process among Arabidopsis AGOs, with AGO1, AGO2, AGO3 and AGO5 undergoing this PTM. Overall, this work deepens our understanding of PTMs in the intricate landscape of RNA-associated gene regulation.

Redox regulation of epigenetic and epitranscriptomic gene regulatory pathways in plants

Developmental and environmental constraints influence genome expression through complex networks of regulatory mechanisms. Epigenetic modifications and remodelling of chromatin are some of the major actors regulating the dynamic of gene expression. Unravelling the factors relaying environmental signals that induce gene expression reprogramming under stress conditions is an important and fundamental question. Indeed, many enzymes involved in epigenetic and chromatin modifications are regulated by redox pathways, through post-translational modifications of proteins or by modifications of the flux of metabolic intermediates. Such modifications are potential hubs to relay developmental and environmental changes for gene expression reprogramming. In this review, we provide an update on the interaction between major redox mediators, such as reactive oxygen and nitrogen species and antioxidants, and epigenetic changes in plants. We detail how redox status alters post-translational modifications of proteins, intracellular epigenetic and epitranscriptional modifications, and how redox regulation interplays with DNA methylation, histone acetylation and methylation, miRNA biogenesis, and chromatin structure and remodelling to reprogram genome expression under environmental constraints.

The Biosynthesis Process of Small RNA and Its Pivotal Roles in Plant Development

In the realm of plant biology, small RNAs (sRNAs) are imperative in the orchestration of gene expression, playing pivotal roles across a spectrum of developmental sequences and responses to environmental stressors. The biosynthetic cascade of sRNAs is characterized by an elaborate network of enzymatic pathways that meticulously process double-stranded RNA (dsRNA) precursors into sRNA molecules, typically 20 to 30 nucleotides in length. These sRNAs, chiefly microRNAs (miRNAs) and small interfering RNAs (siRNAs), are integral in guiding the RNA-induced silencing complex (RISC) to selectively target messenger RNAs (mRNAs) for post-transcriptional modulation. This regulation is achieved either through the targeted cleavage or the suppression of translational efficiency of the mRNAs. In plant development, sRNAs are integral to the modulation of key pathways that govern growth patterns, organ differentiation, and developmental timing. The biogenesis of sRNA itself is a fine-tuned process, beginning with transcription and proceeding through a series of processing steps involving Dicer-like enzymes and RNA-binding proteins. Recent advances in the field have illuminated the complex processes underlying the generation and function of small RNAs (sRNAs), including the identification of new sRNA categories and the clarification of their involvement in the intercommunication among diverse regulatory pathways. This review endeavors to evaluate the contemporary comprehension of sRNA biosynthesis and to underscore the pivotal role these molecules play in directing the intricate performance of plant developmental processes.

Targeted suppression of siRNA biogenesis in Arabidopsis pollen promotes triploid seed viability

In plants, small-interfering RNAs (siRNAs) mediate epigenetic silencing via the RNA-directed DNA methylation (RdDM) pathway, which is particularly prominent during reproduction and seed development. However, there is limited understanding of the origins and dynamics of reproductive siRNAs acting in different cellular and developmental contexts. Here, we used the RNaseIII-like protein RTL1 to suppress siRNA biogenesis in Arabidopsis pollen, and found distinct siRNA subsets produced during pollen development. We demonstrate that RTL1 expression in the late microspore and vegetative cell strongly impairs epigenetic silencing, and resembles RdDM mutants in their ability to bypass interploidy hybridization barriers in the seed. However, germline-specific RTL1 expression did not impact transgenerational inheritance of triploid seed lethality. These results reveal the existence of multiple siRNA subsets accumulated in mature pollen, and suggest that mobile siRNAs involved in the triploid block are produced in germline precursor cells after meiosis, or in the vegetative cell during pollen mitosis.

An AGO10:miR165/6 module regulates meristem activity and xylem development in the Arabidopsis root

The RNA-silencing effector ARGONAUTE10 influences cell fate in plant shoot and floral meristems. ARGONAUTE10 also accumulates in the root apical meristem (RAM), yet its function(s) therein remain elusive. Here, we show that ARGONAUTE10 is expressed in the root cell initials where it controls overall RAM activity and length. ARGONAUTE10 is also expressed in the stele, where post-transcriptional regulation confines it to the root tip's pro-vascular region. There, variations in ARGONAUTE10 levels modulate metaxylem-vs-protoxylem specification. Both ARGONAUTE10 functions entail its selective, high-affinity binding to mobile miR165/166 transcribed in the neighboring endodermis. ARGONAUTE10-bound miR165/166 is degraded, likely via SMALL-RNA-DEGRADING-NUCLEASES1/2, thus reducing miR165/166 ability to silence, via ARGONAUTE1, the transcripts of cell fate-influencing transcription factors. These include PHABULOSA (PHB), which controls meristem activity in the initials and xylem differentiation in the pro-vasculature. During early germination, PHB transcription increases while dynamic, spatially-restricted transcriptional and post-transcriptional mechanisms reduce and confine ARGONAUTE10 accumulation to the provascular cells surrounding the newly-forming xylem axis. Adequate miR165/166 concentrations are thereby channeled along the ARGONAUTE10-deficient yet ARGONAUTE1-proficient axis. Consequently, inversely-correlated miR165/166 and PHB gradients form preferentially along the axis despite ubiquitous PHB transcription and widespread miR165/166 delivery inside the whole vascular cylinder.