Pre-meiotic 21-nucleotide reproductive phasiRNAs emerged in seed plants and diversified in flowering plants

Comparative RNA profiling identifies stage-specific phasiRNAs and coexpressed Argonaute genes in Bambusoideae and Pooideae species

Phased, small interfering RNAs (PhasiRNAs) play a crucial role in supporting male fertility in grasses. Earlier work in maize (Zea mays) and rice (Oryza sativa)-and subsequently many other plant species-identified premeiotic 21-nucleotide (nt) and meiotic 24-nt phasiRNAs. More recently, a group of premeiotic 24-nt phasiRNAs was discovered in the anthers of 2 Pooideae species, barley (Hordeum vulgare) and bread wheat (Triticum aestivum). Whether premeiotic 24-nt phasiRNAs and other classes of reproductive phasiRNAs are conserved across Pooideae species remains unclear. We conducted comparative RNA profiling of 3 anther stages in 6 Pooideae species and 1 Bambusoideae species. We observed complex temporal accumulation patterns of 21-nt and 24-nt phasiRNAs in Pooideae and Bambusoideae grasses. In Bambusoideae, 21-nt phasiRNAs accumulated during meiosis, whereas 24-nt phasiRNAs were present in both premeiotic and postmeiotic stages. We identified premeiotic 24-nt phasiRNAs in all 7 species examined. These phasiRNAs exhibit distinct biogenesis mechanisms and potential Argonaute effectors compared to meiotic 24-nt phasiRNAs. We show that specific Argonaute genes coexpressed with stage-specific phasiRNAs are conserved across Bambusoideae and Pooideae species. Our degradome analysis identified a set of conserved miRNA target genes across species, while 21-nt phasiRNA targets were species-specific. Cleavage of few targets was observed for 24-nt phasiRNAs. In summary, this study demonstrates that premeiotic 24-nt phasiRNAs are present across Bambusoideae and Pooideae families, and the temporal accumulation of other classes of 21-nt and 24-nt phasiRNA differs between bamboo and Pooideae species. Furthermore, targets of the 3 classes of phasiRNAs may be rapidly evolving or undetectable.

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.

Meiosis as a mechanism for epigenetic reprogramming and cellular rejuvenation

Meiosis is a hallmark of sexual reproduction because it represents the transition from one life cycle to the next and, in animals, meiosis produces gametes. Why meiosis evolved has been debated and most studies have focused on recombination of the parental alleles as the main function of meiosis. However, 40 years ago, Robin Holliday proposed that an essential function of meiosis is to oppose the consequence of successive mitoses that cause cellular aging. Cellular aging results from accumulated defective organelles and proteins and modifications of chromatin in the form of DNA methylation and histone modifications referred to collectively as epigenetic marks. Here, recent findings supporting the hypothesis that meiosis opposes cellular aging are reviewed and placed in the context of the diversity of the life cycles of eukaryotes, including animals, yeast, flowering plants and the bryophyte Marchantia.

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.

The role of microRNAs in NBS-LRR gene expression and its implications for plant immunity and crop development

Plants evolved, over millions of years, complex defense systems against pathogens. Once infected, the interaction between pathogen effector molecules and host receptors triggers plant immune responses, which include apoptosis, systemic immune response, among others. An important protein family responsible for pathogen effector recognition is the nucleotide binding site-leucine repeat rich (NBS-LRR) proteins. The NBS-LRR gene family is the largest disease resistance gene class in plants. These proteins are widely distributed in vascular plants and have a complex multigenic cluster distribution in plant genomes. To counteract the genetic load of such a large gene family on fitness cost, plants evolved a mechanism using post transcriptional gene silencing induced by small RNAs, particularly microRNAs. For the NBS-LRR gene family, the small RNAs involved in this silencing mechanism are mainly the microRNA482/2118 superfamily. This suppression mechanism is relieved upon pathogen infection, thus allowing increased NBS-LRR expression and triggering plant immunity. In this review, we will discuss the biogenesis of microRNAs and secondary RNAs involved in this silencing mechanism, biochemical and structural features of NBS-LRR proteins in response to pathogen effectors and the evolution of microRNA-based silencing mechanism with a focus on the miR482/2118 family. Furthermore, the biotechnological manipulation of microRNA expression, using both transgenic or genome editing approaches to improve cultivated plants will be discussed, with a focus on the miR482/2118 family in soybean.

Premeiotic 24-nt phasiRNAs are present in the Zea genus and unique in biogenesis mechanism and molecular function

Reproductive phasiRNAs (phased, small interfering RNAs) are broadly present in angiosperms and play crucial roles in sustaining male fertility. While the premeiotic 21-nt (nucleotides) phasiRNAs and meiotic 24-nt phasiRNA pathways have been extensively studied in maize (Zea mays) and rice (Oryza sativa), a third putative category of reproductive phasiRNAs-named premeiotic 24-nt phasiRNAs-have recently been reported in barley (Hordeum vulgare) and wheat (Triticum aestivum). To determine whether premeiotic 24-nt phasiRNAs are also present in maize and related species and begin to characterize their biogenesis and function, we performed a comparative transcriptome and degradome analysis of premeiotic and meiotic anthers from five maize inbred lines and three teosinte species/subspecies. Our data indicate that a substantial subset of the 24-nt phasiRNA loci in maize and teosinte are already highly expressed at the premeiotic phase. The premeiotic 24-nt phasiRNAs are similar to meiotic 24-nt phasiRNAs in genomic origin and dependence on DCL5 (Dicer-like 5) for biogenesis, however, premeiotic 24-nt phasiRNAs are unique in that they are likely i) not triggered by microRNAs, ii) not loaded by AGO18 proteins, and iii) not capable of mediating PHAS precursor cleavage. In addition, we also observed a group of premeiotic 24-nt phasiRNAs in rice using previously published data. Together, our results indicate that the premeiotic 24-nt phasiRNAs constitute a unique class of reproductive phasiRNAs and are present more broadly in the grass family (Poaceae) than previously known.

Premeiotic 24-nt phasiRNAs are present in the Zea genus and unique in biogenesis mechanism and molecular function

Reproductive phasiRNAs are broadly present in angiosperms and play crucial roles in sustaining male fertility. While the premeiotic 21-nt phasiRNAs and meiotic 24-nt phasiRNA pathways have been extensively studied in maize (Zea mays) and rice (Oryza sativa), a third putative category of reproductive phasiRNAs-named premeiotic 24-nt phasiRNAs-have recently been reported in barley (Hordeum vulgare) and wheat (Triticum aestivum). To determine whether premeiotic 24-nt phasiRNAs are also present in maize and related species and begin to characterize their biogenesis and function, we performed a comparative transcriptome and degradome analysis of premeiotic and meiotic anthers from five maize inbred lines and three teosinte species/subspecies. Our data indicate that a substantial subset of the 24-nt phasiRNA loci in maize and teosinte are already highly expressed at premeiotic phase. The premeiotic 24-nt phasiRNAs are similar to meiotic 24-nt phasiRNAs in genomic origin and dependence on DCL5 for biogenesis, however, premeiotic 24-nt phasiRNAs are unique in that they are likely (i) not triggered by microRNAs, (ii) not loaded by AGO18 proteins, and (iii) not capable of mediating cis-cleavage. In addition, we also observed a group of premeiotic 24-nt phasiRNAs in rice using previously published data. Together, our results indicate that the premeiotic 24-nt phasiRNAs constitute a unique class of reproductive phasiRNAs and are present more broadly in the grass family (Poaceae) than previously known.

MicroRNA482/2118 is lineage-specifically involved in gibberellin signalling via the regulation of GID1 expression by targeting noncoding PHAS genes and subsequently instigated phasiRNAs

MicroRNA482/2118 (miR482/2118) is a 22-nt miRNA superfamily, with conserved functions in disease resistance and plant development. It usually instigates the production of phased small interfering RNAs (phasiRNAs) from its targets to expand or reinforce its silencing effect. Using a new high-quality reference genome sequence and comprehensive small RNA profiling, we characterized a newly evolved regulatory pathway of miR482/2118 in litchi. In this pathway, miR482/2118 cleaved a novel noncoding trans-acting gene (LcTASL1) and triggered phasiRNAs to regulate the expression of gibberellin (GA) receptor gene GIBBERELLIN INSENSITIVE DWARF1 (GID1) in trans; another trans-acting gene LcTASL2, targeted by LcTASL1-derived phasiRNAs, produced phasiRNAs as well to target LcGID1 to reinforce the silencing effect of LcTASL1. We found this miR482/2118-TASL-GID1 pathway was likely involved in fruit development, especially the seed development in litchi. In vivo construction of the miR482a-TASL-GID1 pathway in Arabidopsis could lead to defects in flower and silique development, analogous to the phenotype of gid1 mutants. Finally, we found that a GA-responsive transcription factor, LcGAMYB33, could regulate LcMIR482/2118 as a feedback mechanism of the sRNA-silencing pathway. Our results deciphered a lineage-specifically evolved regulatory module of miR482/2118, demonstrating the high dynamics of miR482/2118 function in plants.

mRNA cleavage by 21-nucleotide phasiRNAs determines temperature-sensitive male sterility in rice

Temperature-sensitive male sterility is one of the core components for hybrid rice (Oryza sativa) breeding based on the 2-line system. We previously found that knockout of ARGONAUTE 1d (AGO1d) causes temperature-sensitive male sterility in rice by influencing phased small interfering RNA (phasiRNA) biogenesis and function. However, the specific phasiRNAs and their targets underlying the temperature-sensitive male sterility in the ago1d mutant remain unknown. Here, we demonstrate that the ago1d mutant displays normal female fertility but complete male sterility at low temperature. Through a multiomics analysis of small RNA (sRNA), degradome, and transcriptome, we found that 21-nt phasiRNAs account for the greatest proportion of the 21-nt sRNA species in rice anthers and are sensitive to low temperature and markedly downregulated in the ago1d mutant. Moreover, we found that 21-nt phasiRNAs are essential for the mRNA cleavage of a set of fertility- and cold tolerance-associated genes, such as Earlier Degraded Tapetum 1 (EDT1), Tapetum Degeneration Retardation (TDR), OsPCF5, and OsTCP21, directly or indirectly determined by AGO1d-mediated gene silencing. The loss of function of 21-nt phasiRNAs can result in upregulation of their targets and causes varying degrees of defects in male fertility and grain setting. Our results highlight the essential functions of 21-nt phasiRNAs in temperature-sensitive male sterility in rice and suggest their promising application in 2-line hybrid rice breeding in the future.

Rice transformation treatments leave specific epigenome changes beyond tissue culture

During transgenic plant production, tissue culture often carries epigenetic, and genetic changes that underlie somaclonal variations, leading to unpredictable phenotypes. Additionally, specific treatments for rice (Oryza sativa) transformation processes may individually or jointly contribute to somaclonal variations, but their specific impacts on rice epigenomes toward transcriptional variations remain unknown. Here, the impact of individual transformation treatments on genome-wide DNA methylation and the transcriptome were examined. In addition to activating stress-responsive genes, individual transformation components targeted different gene expression modules that were enriched in specific functional categories. The transformation treatments strongly impacted DNA methylation and expression; 75% were independent of tissue culture. Furthermore, our genome-wide analysis showed that the transformation treatments consistently resulted in global hypo-CHH methylation enriched at promoters highly associated with downregulation, particularly when the promoters were colocalized with miniature inverted-repeat transposable elements. Our results clearly highlight the specificity of impacts triggered by individual transformation treatments during rice transformation with the potential association between DNA methylation and gene expression. These changes in gene expression and DNA methylation resulting from rice transformation treatments explain a significant portion of somaclonal variations, that is, way beyond the tissue culture effect.

Noncoding RNAs and their roles in regulating the agronomic traits of crops

Molecular breeding is one of the most effective methods for improving the performance of crops. Understanding the genome features of crops, especially the physiological functions of individual genes, is of great importance to molecular breeding. Evidence has shown that genomes of both animals and plants transcribe numerous non-coding RNAs, which are involved in almost every aspect of development. In crops, an increasing number of studies have proven that non-coding RNAs are new genetic resources for regulating crop traits. In this review, we summarize the current knowledge of non-coding RNAs, which are potential crop trait regulators, and focus on the functions of long non-coding RNAs (lncRNAs) in determining crop grain yield, phased small-interfering RNAs (phasiRNAs) in regulating fertility, small interfering RNAs (siRNAs) and microRNAs (miRNAs) in facilitating plant immune response and disease resistance, and miRNAs mediating nutrient and metal stress. Finally, we also discuss the next-generation method for ncRNA application in crop domestication and breeding.

Phylogenetic analyses of seven protein families refine the evolution of small RNA pathways in green plants

Several protein families participate in the biogenesis and function of small RNAs (sRNAs) in plants. Those with primary roles include Dicer-like (DCL), RNA-dependent RNA polymerase (RDR), and Argonaute (AGO) proteins. Protein families such as double-stranded RNA-binding (DRB), SERRATE (SE), and SUPPRESSION OF SILENCING 3 (SGS3) act as partners of DCL or RDR proteins. Here, we present curated annotations and phylogenetic analyses of seven sRNA pathway protein families performed on 196 species in the Viridiplantae (aka green plants) lineage. Our results suggest that the RDR3 proteins emerged earlier than RDR1/2/6. RDR6 is found in filamentous green algae and all land plants, suggesting that the evolution of RDR6 proteins coincides with the evolution of phased small interfering RNAs (siRNAs). We traced the origin of the 24-nt reproductive phased siRNA-associated DCL5 protein back to the American sweet flag (Acorus americanus), the earliest diverged, extant monocot species. Our analyses of AGOs identified multiple duplication events of AGO genes that were lost, retained, or further duplicated in subgroups, indicating that the evolution of AGOs is complex in monocots. The results also refine the evolution of several clades of AGO proteins, such as AGO4, AGO6, AGO17, and AGO18. Analyses of nuclear localization signal sequences and catalytic triads of AGO proteins shed light on the regulatory roles of diverse AGOs. Collectively, this work generates a curated and evolutionarily coherent annotation for gene families involved in plant sRNA biogenesis/function and provides insights into the evolution of major sRNA pathways.

Small RNA-mediated DNA methylation during plant reproduction

Reproductive tissues are a rich source of small RNAs, including several classes of short interfering (si)RNAs that are restricted to this stage of development. In addition to RNA polymerase IV-dependent 24-nt siRNAs that trigger canonical RNA-directed DNA methylation, abundant reproductive-specific siRNAs are produced from companion cells adjacent to the developing germ line or zygote and may move intercellularly before inducing methylation. In some cases, these siRNAs are produced via non-canonical biosynthesis mechanisms or from sequences with little similarity to transposons. While the precise role of these siRNAs and the methylation they trigger is unclear, they have been implicated in specifying a single megaspore mother cell, silencing transposons in the male germ line, mediating parental dosage conflict to ensure proper endosperm development, hypermethylation of mature embryos, and trans-chromosomal methylation in hybrids. In this review, we summarize the current knowledge of reproductive siRNAs, including their biosynthesis, transport, and function.

CRISPR/Cas9-targeted mutagenesis of TaDCL4, TaDCL5 and TaRDR6 induces male sterility in common wheat

Phased, small interfering RNAs (phasiRNAs) are important for plant anther development, especially for male sterility. PhasiRNA biogenesis is dependent on genes like RNA polymerase 6 (RDR6), DICER-LIKE 4 (DCL4), or DCL5 to produce 21- or 24 nucleotide (nt) double-strand small RNAs. Here, we generated mutants of DCL4, DCL5 and RDR6 using CRISPR/Cas9 system and studied their effects on plant reproductive development and phasiRNA production in wheat. We found that RDR6 mutation caused sever consequence throughout plant development starting from seed germination and the dcl4 mutants grew weaker with thorough male sterility, while dcl5 plants developed normally but exhibited male sterility. Correspondingly, DCL4 and DCL5, respectively, specified 21- and 24-nt phasiRNA biogenesis, while RDR6 contributed to both. Also, the three key genes evolved differently in wheat, with TaDCL5-A/B becoming non-functioning and TaRDR6-A being lost after polyploidization. Furthermore, we found that PHAS genes (phasiRNA precursors) identified via phasiRNAs diverged rapidly among sub-genomes of polyploid wheat. Despite no similarity being found among phasiRNAs of grasses, their targets were enriched for similar biological functions. In light of the important roles of phasiRNA pathways in gametophyte development, genetic dissection of the function of key genes may help generate male sterile lines suitable for hybrid wheat breeding.

Genome-wide analysis reveals the crucial role of lncRNAs in regulating the expression of genes controlling pollen development

The genomic location and stage-specific expression pattern of many long non-coding RNAs reveal their critical role in regulating protein-coding genes crucial in pollen developmental progression and male germ line specification. Long non-coding RNAs (lncRNAs) are transcripts longer than 200 bp with no apparent protein-coding potential. Multiple investigations have revealed high expression of lncRNAs in plant reproductive organs in a cell and tissue-specific manner. However, their potential role as essential regulators of molecular processes involved in sexual reproduction remains largely unexplored. We have used developing field mustard (Brassica rapa) pollen as a model system for investigating the potential role of lncRNAs in reproductive development. Reference-based transcriptome assembly performed to update the existing genome annotation identified novel expressed protein-coding genes and long non-coding RNAs (lncRNAs), including 4347 long intergenic non-coding RNAs (lincRNAs, 1058 expressed) and 2,045 lncRNAs overlapping protein-coding genes on the opposite strand (lncNATs, 780 expressed). The analysis of expression profiles reveals that lncRNAs are significant and stage-specific contributors to the gene expression profile of developing pollen. Gene co-expression networks accompanied by genome location analysis identified 38 cis-acting lincRNA, 31 cis-acting lncNAT, 7 trans-acting lincRNA and 14 trans-acting lncNAT to be substantially co-expressed with target protein-coding genes involved in biological processes regulating pollen development and male lineage specification. These findings provide a foundation for future research aiming at developing strategies to employ lncRNAs as regulatory tools for gene expression control during reproductive development.

Mobile ARGONAUTE 1d binds 22-nt miRNAs to generate phasiRNAs important for low-temperature male fertility in rice

Phased small interfering RNAs (phasiRNAs) are abundantly expressed in anthers and linked to environment-related male fertility in grasses, yet how they function under different environmental conditions remains unclear. Here, we identified a rice (Oryza sativa) low temperature-induced Argonaute (AGO) protein, OsAGO1d, that is responsible for generating phasiRNAs and preserving male fertility at low temperature. Loss of OsAGO1d function causes low-temperature male sterility associated with delayed programmed cell death of tapetal cells during anther development. OsAGO1d binds miR2118 and miR2275 family members and triggers phasiRNA biogenesis; it also binds 21-nt phasiRNAs with a 5' terminal U. In total, phasiRNAs from 972 loci are OsAGO1d-dependent. OsAGO1d protein moves from anther wall cells into meiocytes, where it loads miR2275 to produce 24-nt phasiRNAs. Together, our results show that OsAGO1d acts as a mobile signal to fine-tune phasiRNA production and this function is important for male fertility at low temperature.

Coexpression network and trans-activation analyses of maize reproductive phasiRNA loci

The anther-enriched phased, small interfering RNAs (phasiRNAs) play vital roles in sustaining male fertility in grass species. Their long non-coding precursors are synthesized by RNA polymerase II and are likely regulated by transcription factors (TFs). A few putative transcriptional regulators of the 21- or 24-nucleotide phasiRNA loci (referred to as 21- or 24-PHAS loci) have been identified in maize (Zea mays), but whether any of the individual TFs or TF combinations suffice to activate any PHAS locus is unclear. Here, we identified the temporal gene coexpression networks (modules) associated with maize anther development, including two modules highly enriched for the 21- or 24-PHAS loci. Comparisons of these coexpression modules and gene sets dysregulated in several reported male sterile TF mutants provided insights into TF timing with regard to phasiRNA biogenesis, including antagonistic roles for OUTER CELL LAYER4 and MALE STERILE23. Trans-activation assays in maize protoplasts of individual TFs using bulk-protoplast RNA-sequencing showed that two of the TFs coexpressed with 21-PHAS loci could activate several 21-nucleotide phasiRNA pathway genes but not transcription of 21-PHAS loci. Screens for combinatorial activities of these TFs and, separately, the recently reported putative transcriptional regulators of 24-PHAS loci using single-cell (protoplast) RNA-sequencing, did not detect reproducible activation of either 21-PHAS or 24-PHAS loci. Collectively, our results suggest that the endogenous transcriptional machineries and/or chromatin states in the anthers are necessary to activate reproductive PHAS loci.

Temperature-sensitive male sterility in rice determined by the roles of AGO1d in reproductive phasiRNA biogenesis and function

Phased secondary siRNAs (phasiRNAs) are broadly present in the reproductive tissues of flowering plants, with spatial-temporal specificity. However, the ARGONAUTE (AGO) proteins associated with phasiRNAs and their miRNA triggers remain elusive. Here, through histological and high-throughput sequencing analyses, we show that rice AGO1d, which is specifically expressed in anther wall cells before and during meiosis, associates with both miR2118 and miR2275 to mediate phasiRNA biogenesis. AGO1d preferentially binds to miR2118-triggered 21-nucleotide (nt) phasiRNAs with a 5'-terminal uridine, suggesting a dual role in phasiRNA biogenesis and function. Depletion of AGO1d causes a reduction of 21- and 24-nt phasiRNAs and temperature-sensitive male sterility. At lower temperatures, anthers of the ago1d mutant predominantly show excessive tapetal cells with little starch accumulation during pollen formation, possibly caused by the dysregulation of cell metabolism. These results uncover an essential role of AGO1d in rice anther development at lower temperatures and demonstrate coordinative roles of AGO proteins during reproductive phasiRNA biogenesis and function.

Genetic architecture underlying variation in floral meristem termination in Aquilegia

Floral organs are produced by floral meristems (FMs), which harbor stem cells in their centers. Since each flower only has a finite number of organs, the stem cell activity of an FM will always terminate at a specific time point, a process termed floral meristem termination (FMT). Variation in the timing of FMT can give rise to floral morphological diversity, but how this process is fine-tuned at a developmental and evolutionary level is poorly understood. Flowers from the genus Aquilegia share identical floral organ arrangement except for stamen whorl number (SWN), making Aquilegia a well-suited system for investigation of this process: differences in SWN between species represent differences in the timing of FMT. By crossing A. canadensis and A. brevistyla, quantitative trait locus (QTL) mapping has revealed a complex genetic architecture with seven QTL. We explored potential candidate genes under each QTL and characterized novel expression patterns of select loci of interest using in situ hybridization. To our knowledge, this is the first attempt to dissect the genetic basis of how natural variation in the timing of FMT is regulated, and our results provide insight into how floral morphological diversity can be generated at the meristematic level.

The evolutionary history of small RNAs in Solanaceae

The Solanaceae or "nightshade" family is an economically important group with remarkable diversity. To gain a better understanding of how the unique biology of the Solanaceae relates to the family's small RNA (sRNA) genomic landscape, we downloaded over 255 publicly available sRNA data sets that comprise over 2.6 billion reads of sequence data. We applied a suite of computational tools to predict and annotate two major sRNA classes: (1) microRNAs (miRNAs), typically 20- to 22-nucleotide (nt) RNAs generated from a hairpin precursor and functioning in gene silencing and (2) short interfering RNAs (siRNAs), including 24-nt heterochromatic siRNAs typically functioning to repress repetitive regions of the genome via RNA-directed DNA methylation, as well as secondary phased siRNAs and trans-acting siRNAs generated via miRNA-directed cleavage of a polymerase II-derived RNA precursor. Our analyses described thousands of sRNA loci, including poorly understood clusters of 22-nt siRNAs that accumulate during viral infection. The birth, death, expansion, and contraction of these sRNA loci are dynamic evolutionary processes that characterize the Solanaceae family. These analyses indicate that individuals within the same genus share similar sRNA landscapes, whereas comparisons between distinct genera within the Solanaceae reveal relatively few commonalities.

MicroRNA482/2118, a miRNA superfamily essential for both disease resistance and plant development

MicroRNAs (miRNAs) are a class of 21-24 nucleotides (nt) noncoding small RNAs ubiquitously distributed across the plant kingdom. miR482/2118, one of the conserved miRNA superfamilies originating from gymnosperms, has divergent main functions in core-angiosperms. It mainly regulates NUCLEOTIDE BINDING SITE-LEUCINE-RICH REPEAT (NBS-LRR) genes in eudicots, functioning as an essential component in plant disease resistance; in contrast, it predominantly targets numerous long noncoding RNAs (lncRNAs) in monocot grasses, which are vital for plant reproduction. Usually, miR482/2118 is 22-nt in length, which can trigger the production of phased small interfering RNAs (phasiRNAs) after directed cleavage. PhasiRNAs instigated from target genes of miR482/2118 enhance their roles in corresponding biological processes by cis-regulation on cognate genes and expands their function to other pathways via trans activity on different genes. This review summarizes the origin, biogenesis, conservation, and evolutionary characteristics of the miR482/2118 superfamily and delineates its diverse functions in disease resistance, plant development, stress responses, etc.

Big Role of Small RNAs in Female Gametophyte Development

In living organisms, sexual reproduction relies on the successful development of the gametes. Flowering plants produce gametes in the specialized organs of the flower, the gametophytes. The female gametophyte (FG), a multicellular structure containing female gametes (egg cell and central cell), is often referred to as an embryo sac. Intriguingly, several protein complexes, molecular and genetic mechanisms participate and tightly regulate the female gametophyte development. Recent evidence indicates that small RNA (sRNA) mediated pathways play vital roles in female gametophyte development and specification. Here, we present an insight into our understanding and the recent updates on the molecular mechanism of different players of small RNA-directed regulatory pathways during ovule formation and growth.

Heat-responsive microRNAs and phased small interfering RNAs in reproductive development of flax

Plants will face increased heat stress due to rising global temperatures. Heat stress affects plant reproductive development and decreases productivity; however, the underlying molecular mechanisms of these processes are poorly characterized. Plant small RNAs (sRNAs) have important regulatory roles in plant reproductive development following abiotic stress responses. We generated sRNA transcriptomes of reproductive bud stages at three different time points to identify sRNA-mediated pathways responsive to heat stress in flax (Linum usitatissimum). With added sRNA transcriptomes of vegetative tissues, we comprehensively annotated miRNA and phasiRNA-encoding genes (PHAS) in flax. We identified 173 miRNA genes, of which 42 are newly annotated. Our analysis revealed that 141 miRNA genes were differentially accumulated between tissue types, while 18 miRNA genes were differentially accumulated in reproductive tissues following heat stress, including members of miR482/2118 and miR2275 families, known triggers of reproductive phasiRNAs. Furthermore, we identified 68 21-PHAS flax loci from protein-coding and noncoding regions, four 24-PHAS loci triggered by miR2275, and 658 24-PHAS-like loci with unknown triggers, derived mostly from noncoding regions. The reproductive phasiRNAs are mostly downregulated in response to heat stress. Overall, we found that several previously unreported miRNAs and phasiRNAs are responsive to heat stress in flax reproductive tissues.

Spatiotemporal regulation and roles of reproductive phasiRNAs in plants

Since co-suppression was discovered as a pioneer silencing phenomenon of RNA interference (RNAi) in petunia in 1990, many types of small RNAs have been identified in the RNAi pathway among various eukaryotes. In plants, a large number of 21- or 24-nucleotide (nt) phased small interfering RNAs (phasiRNAs) are produced via processing of long RNA precursors by Dicer-like proteins. However, the roles of phasiRNAs remain largely unknown. The development of imaging technology and RNA profiling has clarified the spatiotemporal regulation of phasiRNAs, and subsequently the different functions of 21-nt trans-acting phasiRNAs and 24-nt cis-regulatory phasiRNAs during male organ development. This review focuses on the biogenesis, diversification, spatiotemporal expression pattern and function of phasiRNAs in plants.