Secondary siRNAs in Plants: Biosynthesis, Various Functions, and Applications in Virology

Asymmetric bulges within hairpin RNA transgenes influence small RNA size, secondary siRNA production and viral defence

Small RNAs (sRNAs) are essential for normal plant development and range in size classes of 21-24 nucleotides. The 22nt small interfering RNAs (siRNAs) and miRNAs are processed by Dicer-like 2 (DCL2) and DCL1 respectively and can initiate secondary siRNA production from the target transcript. 22nt siRNAs are under-represented due to competition between DCL2 and DCL4, while only a small number of 22nt miRNAs exist. Here we produce abundant 22nt siRNAs and other siRNA size classes using long hairpin RNA (hpRNA) transgenes. By introducing asymmetric bulges into the antisense strand of hpRNA, we shifted the dominant siRNA size class from 21nt of the traditional hpRNA to 22, 23 and 24nt of the asymmetric hpRNAs. The asymmetric hpRNAs effectively silenced a β-glucuronidase (GUS) reporter transgene and the endogenous ethylene insensitive-2 (EIN2) and chalcone synthase (CHS) genes. Furthermore, plants containing the asymmetric hpRNA transgenes showed increased amounts of 21nt siRNAs downstream of the hpRNA target site compared to plants with the traditional hpRNA transgenes. This indicates that these asymmetric hpRNAs are more effective at inducing secondary siRNA production to amplify silencing signals. The 22nt asymmetric hpRNA constructs enhanced virus resistance in plants compared to the traditional hpRNA constructs.

miRNA, phytometabolites and disease: Connecting the dots

miRNAs are tiny noncoding ribonucleotides that function as critical regulators of gene-expression in eukaryotes. A single miRNA may be involved in the regulation of several target mRNAs forming complex cellular networks to regulate diverse aspects of development in an organism. The deregulation of miRNAs has been associated with several human diseases. Therefore, miRNA-based therapeutics is gaining interest in the pharmaceutical industry as the next-generation drugs for the cure of many diseases. Medicinal plants have also been used for the treatment of several human diseases and their curative potential is attributed to their reserve in bioactive metabolites. A role for miRNAs as regulators of the phytometabolic pathways in plants has emerged in the recent past. Experimental studies have also indicated the potential of plant encoded secondary phytometabolites to act as cross-regulators of mammalian miRNAs and transcripts to regulate human diseases (like cancer). The evidence for this cross-kingdom gene regulation through miRNA has gathered considerable enthusiasm in the scientific field, even though there are on-going debates regarding the reproducibility and the effectiveness of these findings. In this review, we provide information to connect the medicinal and gene regulatory properties of secondary phytometabolites, their regulation by miRNAs in plants and their effects on human miRNAs for regulating downstream metabolic or pathological processes. While further extensive research initiatives and good clinical evidence are required to prove or disapprove these findings, understanding of these regulations will have important implications in the potential use of synthetic or artificial miRNAs as effective alternatives for providing health benefits.

Target-specific requirements for RNA interference can arise through restricted RNA amplification despite the lack of specialized pathways

Since double-stranded RNA (dsRNA) is effective for silencing a wide variety of genes, all genes are typically considered equivalent targets for such RNA interference (RNAi). Yet, loss of some regulators of RNAi in the nematode C. elegans can selectively impair the silencing of some genes. Here we show that such selective requirements can be explained by an intersecting network of regulators acting on genes with differences in their RNA metabolism. In this network, the Maelstrom domain-containing protein RDE-10, the intrinsically disordered protein MUT-16, and the Argonaute protein NRDE-3 work together so that any two are required for silencing one somatic gene, but each is singly required for silencing another somatic gene, where only the requirement for NRDE-3 can be overcome by enhanced dsRNA processing. Quantitative models and their exploratory simulations led us to find that (1) changing cis-regulatory elements of the target gene can reduce the dependence on NRDE-3, (2) animals can recover from silencing in non-dividing cells and (3) cleavage and tailing of mRNAs with UG dinucleotides, which makes them templates for amplifying small RNAs, is enriched within 'pUG zones' matching the dsRNA. Similar crosstalk between pathways and restricted amplification could result in apparently selective silencing by endogenous RNAs.

Trans-Species Mobility of RNA Interference between Plants and Associated Organisms

Trans-species RNA interference (RNAi) occurs naturally when small RNAs (sRNAs) silence genes in species different from their origin. This phenomenon has been observed between plants and various organisms including fungi, animals and other plant species. Understanding the mechanisms used in natural cases of trans-species RNAi, such as sRNA processing and movement, will enable more effective development of crop protection methods using host-induced gene silencing (HIGS). Recent progress has been made in understanding the mechanisms of cell-to-cell and long-distance movement of sRNAs within individual plants. This increased understanding of endogenous plant sRNA movement may be translatable to trans-species sRNA movement. Here, we review diverse cases of natural trans-species RNAi focusing on current theories regarding intercellular and long-distance sRNA movement. We also touch on trans-species sRNA evolution, highlighting its research potential and its role in improving the efficacy of HIGS.

The Emerging Role of Non-Coding RNAs (ncRNAs) in Plant Growth, Development, and Stress Response Signaling

Plant species utilize a variety of regulatory mechanisms to ensure sustainable productivity. Within this intricate framework, numerous non-coding RNAs (ncRNAs) play a crucial regulatory role in plant biology, surpassing the essential functions of RNA molecules as messengers, ribosomal, and transfer RNAs. ncRNAs represent an emerging class of regulators, operating directly in the form of small interfering RNAs (siRNAs), microRNAs (miRNAs), long noncoding RNAs (lncRNAs), and circular RNAs (circRNAs). These ncRNAs exert control at various levels, including transcription, post-transcription, translation, and epigenetic. Furthermore, they interact with each other, contributing to a variety of biological processes and mechanisms associated with stress resilience. This review primarily concentrates on the recent advancements in plant ncRNAs, delineating their functions in growth and development across various organs such as root, leaf, seed/endosperm, and seed nutrient development. Additionally, this review broadens its scope by examining the role of ncRNAs in response to environmental stresses such as drought, salt, flood, heat, and cold in plants. This compilation offers updated information and insights to guide the characterization of the potential functions of ncRNAs in plant growth, development, and stress resilience in future research.

Harnessing Viral Promoters for Targeted RNAi: Engineering Phloem-Specific Resistance Against Tomato Yellow Leaf Curl Thailand Virus

Phloem-specific promoter efficiently triggers graft-transmissible RNA interference (gtRNAi). We leveraged a phloem-specific promoter derived from the Rice tungro bacilliform virus, optimizing the RNAi mechanism's efficiency and specificity. Here, we detail the construction of phloem-specific promoter-based gtRNAi system and its application through grafting experiments, demonstrating its effectiveness in inducing tomato yellow leaf curl Thailand virus (TYLCHTV) resistance in non-transgenic scions. This strategy presents a practical application for protecting crops against viruses without genetically modifying the entire plant.

Dicer-like2b suppresses the wiry leaf phenotype in tomato induced by tobacco mosaic virus

Dicer-like (DCL) proteins are principal components of RNA silencing, a major defense mechanism against plant virus infections. However, their functions in suppressing virus-induced disease phenotypes remain largely unknown. Here, we identified a role for tomato (Solanum lycopersicum) DCL2b in regulating the wiry leaf phenotype during defense against tobacco mosaic virus (TMV). Knocking out SlyDCL2b promoted TMV accumulation in the leaf primordium, resulting in a wiry phenotype in distal leaves. Biochemical and bioinformatics analyses showed that 22-nt virus-derived small interfering RNAs (vsiRNAs) accumulated less abundantly in slydcl2b mutants than in wild-type plants, suggesting that SlyDCL2b-dependent 22-nt vsiRNAs are required to exclude virus from leaf primordia. Moreover, the wiry leaf phenotype was accompanied by upregulation of Auxin Response Factors (ARFs), resulting from a reduction in trans-acting siRNAs targeting ARFs (tasiARFs) in TMV-infected slydcl2b mutants. Loss of tasiARF production in the slydcl2b mutant was in turn caused by inhibition of miRNA390b function. Importantly, silencing SlyARF3 and SlyARF4 largely restored the wiry phenotype in TMV-infected slydcl2b mutants. Our work exemplifies the complex relationship between RNA viruses and the endogenous RNA silencing machinery, whereby SlyDCL2b protects the normal development of newly emerging organs by excluding virus from these regions and thus maintaining developmental silencing.

A Non-Canonical Pathway Induced by Externally Applied Virus-Specific dsRNA in Potato Plants

The external application of double-stranded RNA (dsRNA) has recently been developed as a non-transgenic approach for crop protection against pests and pathogens. This novel and emerging approach has come to prominence due to its safety and environmental benefits. It is generally assumed that the mechanism of dsRNA-mediated antivirus RNA silencing is similar to that of natural RNA interference (RNAi)-based defence against RNA-containing viruses. There is, however, no direct evidence to support this idea. Here, we provide data on the high-throughput sequencing (HTS) analysis of small non-coding RNAs (sRNA) as hallmarks of RNAi induced by infection with the RNA-containing potato virus Y (PVY) and also by exogenous application of dsRNA which corresponds to a fragment of the PVY genome. Intriguingly, in contrast to PVY-induced production of discrete 21 and 22 nt sRNA species, the externally administered PVY dsRNA fragment led to generation of a non-canonical pool of sRNAs, which were present as ladders of ~18-30 nt in length; suggestive of an unexpected sRNA biogenesis pathway. Interestingly, these non-canonical sRNAs are unable to move systemically and also do not induce transitive amplification. These findings may have significant implications for further developments in dsRNA-mediated crop protection.

AGO2a but not AGO2b mediates antiviral defense against infection of wild-type cucumber mosaic virus in tomato

Evolutionarily conserved antiviral RNA interference (RNAi) mediates a primary antiviral innate immunity preventing infection of broad-spectrum viruses in plants. However, the detailed mechanism in plants is still largely unknown, especially in important agricultural crops, including tomato. Varieties of pathogenic viruses evolve to possess viral suppressors of RNA silencing (VSRs) to suppress antiviral RNAi in the host. Due to the prevalence of VSRs, it is still unknown whether antiviral RNAi truly functions to prevent invasion by natural wild-type viruses in plants and animals. In this research, for the first time we applied CRISPR-Cas9 to generate ago2a, ago2b, or ago2ab mutants for two differentiated Solanum lycopersicum AGO2s, key effectors in antiviral RNAi. We found that AGO2a but not AGO2b was significantly induced to inhibit the propagation of not only VSR-deficient Cucumber mosaic virus (CMV) but also wild-type CMV-Fny in tomato; however, neither AGO2a nor AGO2b regulated disease induction after infection with either virus. Our findings firstly reveal a prominent role of AGO2a in antiviral RNAi innate immunity in tomato and demonstrate that antiviral RNAi evolves to defend against infection of natural wild-type CMV-Fny in tomato. However, AGO2a-mediated antiviral RNAi does not play major roles in promoting tolerance of tomato plants to CMV infection for maintaining health.

Recovery from virus infection: plant's armory in action

MAIN CONCLUSION

This review focuses on different factors involved in promoting symptom recovery in plants post-virus infection such as epigenetics, transcriptional reprogramming, phytohormones with an emphasis on RNA silencing as well as role of abiotic factors such as temperature on symptom recovery. Plants utilize several different strategies to defend themselves in the battle against invading viruses. Most of the viral proteins interact with plant proteins and interfere with molecular dynamics in a cell which eventually results in symptom development. This initial symptom development is countered by the plant utilizing various factors including the plant's adaptive immunity to develop a virus tolerant state. Infected plants can specifically target and impede the transcription of viral genes as well as degrade the viral transcripts to restrict their proliferation by the production of small-interfering RNA (siRNA) generated from the viral nucleic acid, known as virus-derived siRNA (vsiRNA). To further escalate the degradation of viral nucleic acid, secondary siRNAs are generated. The production of virus-activated siRNA (vasiRNA) from the host genome causes differential regulation of the host transcriptome which plays a major role in establishing a virus tolerant state within the infected plant. The systemic action of vsiRNAs, vasiRNA, and secondary siRNAs with the help of defense hormones like salicylic acid can curb viral proliferation, and thus the newly emerged leaves develop fewer symptoms, maintaining a state of tolerance.

Tweaking the Small Non-Coding RNAs to Improve Desirable Traits in Plant

Plant transcriptome contains an enormous amount of non-coding RNAs (ncRNAs) that do not code for proteins but take part in regulating gene expression. Since their discovery in the early 1990s, much research has been conducted to elucidate their function in the gene regulatory network and their involvement in plants' response to biotic/abiotic stresses. Typically, 20-30 nucleotide-long small ncRNAs are a potential target for plant molecular breeders because of their agricultural importance. This review summarizes the current understanding of three major classes of small ncRNAs: short-interfering RNAs (siRNAs), microRNA (miRNA), and transacting siRNAs (tasiRNAs). Furthermore, their biogenesis, mode of action, and how they have been utilized to improve crop productivity and disease resistance are discussed here.

Retrotransposons: How the continuous evolutionary front shapes plant genomes for response to heat stress

Long terminal repeat retrotransposons (LTR retrotransposons) are the most abundant group of mobile genetic elements in eukaryotic genomes and are essential in organizing genomic architecture and phenotypic variations. The diverse families of retrotransposons are related to retroviruses. As retrotransposable elements are dispersed and ubiquitous, their "copy-out and paste-in" life cycle of replicative transposition leads to new genome insertions without the excision of the original element. The overall structure of retrotransposons and the domains responsible for the various phases of their replication is highly conserved in all eukaryotes. The two major superfamilies of LTR retrotransposons, Ty1/Copia and Ty3/Gypsy, are distinguished and dispersed across the chromosomes of higher plants. Members of these superfamilies can increase in copy number and are often activated by various biotic and abiotic stresses due to retrotransposition bursts. LTR retrotransposons are important drivers of species diversity and exhibit great variety in structure, size, and mechanisms of transposition, making them important putative actors in genome evolution. Additionally, LTR retrotransposons influence the gene expression patterns of adjacent genes by modulating potential small interfering RNA (siRNA) and RNA-directed DNA methylation (RdDM) pathways. Furthermore, comparative and evolutionary analysis of the most important crop genome sequences and advanced technologies have elucidated the epigenetics and structural and functional modifications driven by LTR retrotransposon during speciation. However, mechanistic insights into LTR retrotransposons remain obscure in plant development due to a lack of advancement in high throughput technologies. In this review, we focus on the key role of LTR retrotransposons response in plants during heat stress, the role of centromeric LTR retrotransposons, and the role of LTR retrotransposon markers in genome expression and evolution.

Rearranged Endogenized Plant Pararetroviruses as Evidence of Heritable RNA-based Immunity

Eukaryotic genomics frequently revealed historical spontaneous endogenization events of external invading nucleic acids, such as viral elements. In plants, an extensive occurrence of endogenous plant pararetroviruses (EPRVs) is usually believed to endow hosts with an additional layer of internal suppressive weaponry. However, an actual demonstration of this activity remains speculative. We analyzed the EPRV component and accompanying silencing effectors of Solanum lycopersicum, documenting that intronic/intergenic pararetroviral integrations bearing inverted-repeats fuel the plant's RNA-based immune system with suitable transcripts capable of evoking a silencing response. A surprisingly small set of rearrangements explained a substantial fraction of pararetroviral-derived endogenous small-interfering (si)RNAs, enriched in 22-nt forms typically associated with anti-viral post-transcriptional gene silencing. We provide preliminary evidence that such genetic and immunological signals may be found in other species outside the genus Solanum. Based on molecular dating, bioinformatics, and empirical explorations, we propose that homology-dependent silencing emerging from particular immuno-competent rearranged chromosomal areas that constitute an adaptive heritable trans-acting record of past infections, with potential impact against the unlocking of plant latent EPRVs and cognate-free pararetroviruses.

Exogenous Application of dsRNA for the Control of Viruses in Cucurbits

The recurrent emergence of viral diseases in intensive horticultural crops requires alternative control strategies. The topical application of double-stranded RNA (dsRNA) molecules homologous to pathogens has been proposed as a tool for virus control in plants. These dsRNAs induce the silencing mechanism, the RNA interference (RNAi), that degrades homologous dsRNAs. Cucumber green mottle mosaic virus (CGMMV) represents a serious threat to cucurbit crops. Since genetic resistance to the virus is not yet available in commercial varieties, we aimed to control this virus by RNAi. For this purpose, we obtained constructions both for expressing dsRNA in bacteria to treat cucumber plants by topical application and for agroinoculation in experiments done in the growth chamber. Besides, greenhouse tests were performed in spring and in summer when plants were challenged with the virus, and differences in several parameters were investigated, including the severity of symptoms, dry weight, total height, virus accumulation, and virus-derived small interfering RNAs (vsiRNAs). Spraying of plants with dsRNA reduced significatively CGMMV symptoms in the plants in growth chamber tests. Agroinfiltration experiments done under identical conditions were also effective in limiting the progress of CGMMV disease. In the greenhouse assay performed in spring, symptoms were significantly reduced in dsRNA-sprayed plants, and the development of the plants improved with respect to non-treated plants. Virus titers and vsiRNAs were clearly reduced in dsRNA-treated plants. The effect of protection of the dsRNA was less evident in the greenhouse assay carried out in the summer. Besides, we investigated the mobility of long (ds)RNA derived from spraying or agroinfiltrated dsRNA and found that it could be detected in local, close distal, and far distal points from the site of application. VsiRNAs were also detected in local and distal points and the differences in accumulation were compared. In parallel, we investigated the capacity of dsRNAs derived from genes of tomato leaf curl New Delhi virus (ToLCNDV), another economically important virus in cucurbits, to limit the disease in zucchini, both by agroinfiltration or by direct spraying, but found no protective effect. In view of the results, the topical application of dsRNAs is postulated as a promising strategy for CGMMV control in the cucumber.

VIGS Goes Viral: How VIGS Transforms Our Understanding of Plant Science

Virus-induced gene silencing (VIGS) has developed into an indispensable approach to gene function analysis in a wide array of species, many of which are not amenable to stable genetic transformation. VIGS utilizes the posttranscriptional gene silencing (PTGS) machinery of plants to restrain viral infections systemically and is used to downregulate the plant's endogenous genes. Here, we review the molecular mechanisms of DNA- and RNA-virus-based VIGS, its inherent connection to PTGS, and what is known about the systemic spread of silencing. Recently, VIGS-based technologies have been expanded to enable not only gene silencing but also overexpression [virus-induced overexpression (VOX)], genome editing [virus-induced genome editing (VIGE)], and host-induced gene silencing (HIGS). These techniques expand the genetic toolbox for nonmodel organisms even more. Further, we illustrate the versatility of VIGS and the methods derived from it in elucidating molecular mechanisms, using tomato fruit ripening and programmed cell death as examples. Finally, we discuss challenges of and future perspectives on the use of VIGS to advance gene function analysis in nonmodel plants in the postgenomic era.

Carbon Dots Boost dsRNA Delivery in Plants and Increase Local and Systemic siRNA Production

In this work, we obtained carbon dots from glucose or saccharose as the nucleation source and passivated them with branched polyethylenimines for developing dsRNA nanocomposites. The CDs were fully characterized using hydrodynamic analyses, transmission electron microscopy, X-ray photoelectron spectroscopy and Fourier transform infrared spectroscopy. The ζ potential determined that the CDs had positive charges, good electrophoretic mobility and conductivity, and were suitable for obtaining dsRNA nanocomposites. DsRNA naked or coated with the CDs were delivered to leaves of cucumber plants by spraying. Quantitation of the dsRNA that entered the leaves showed that when coated with the CDs, 50-fold more dsRNA was detected than when naked dsRNA. Moreover, specific siRNAs derived from the sprayed dsRNAs were 13 times more abundant when the dsRNA was coated with the CDs. Systemic dsRNAs were determined in distal leaves and showed a dramatic increase in concentration when delivered as a nanocomposite. Similarly, systemic siRNAs were significantly more abundant in distal leaves when spraying with the CD-dsRNA nanocomposite. Furthermore, FITC-labeled dsRNA was shown to accumulate in the apoplast and increase its entry into the plant when coated with CDs. These results indicate that CDs obtained by hydrothermal synthesis are suitable for dsRNA foliar delivery in RNAi plant applications.

Role of phasiRNAs in plant-pathogen interactions: molecular perspectives and bioinformatics tools

The genome of an organism is regulated in concert with the organized action of various genetic regulators at different hierarchical levels. Small non-coding RNAs are one of these regulators, among which microRNAs (miRNAs), a distinguished sRNA group with decisive functions in the development, growth and stress-responsive activities of both plants as well as animals, are keenly explored over a good number of years. Recent studies in plants revealed that apart from the silencing activity exhibited by miRNAs on their targets, miRNAs of specific size and structural features can direct the phasing pattern of their target loci to form phased secondary small interfering RNAs (phasiRNAs). These trigger-miRNAs were identified to target both coding and long non-coding RNAs that act as potent phasiRNA precursors or PHAS loci. The phasiRNAs produced thereby exhibit a role in enhancing further downstream regulation either on their own precursors or on those transcripts that are distinct from their genetic source of origin. Hence, these tiny regulators can stimulate an elaborative cascade of interacting RNA networks via cis and trans-regulatory mechanisms. Our review focuses on the comprehensive understanding of phasiRNAs and their trigger miRNAs, by giving much emphasis on their role in the regulation of plant defense responses, together with a summary of the computational tools available for the prediction of the same.

RNA silencing: From discovery and elucidation to application and perspectives

RNA silencing (or RNA interference, RNAi) is a conserved mechanism for regulating gene expression in eukaryotes. The discovery of natural trans-kingdom RNAi indicated that small RNAs act as signaling molecules and enable communication between organisms in different kingdoms. The phenomenon and potential mechanisms of trans-kingdom RNAi are among the most exciting research topics. To better understand trans-kingdom RNAi, we review the history of the discovery and elucidation of RNAi mechanisms. Based on canonical RNAi mechanisms, we summarize the major points of divergence around RNAi pathways in the main eukaryotes' kingdoms, including plants, animals, and fungi. We review the representative incidents associated with the mechanisms and applications of trans-kingdom RNAi in crop protection, and discuss the critical factors that should be considered to develop successful trans-kingdom RNAi-based crop protection strategies.

Coordinated Action of RTBV and RTSV Proteins Suppress Host RNA Silencing Machinery

RNA silencing is as an adaptive immune response in plants that limits the accumulation or spread of invading viruses. Successful virus infection entails countering the RNA silencing machinery for efficient replication and systemic spread in the host. The viruses encode proteins with the ability to suppress or block the host silencing mechanism, resulting in severe pathogenic symptoms and diseases. Tungro is a viral disease caused by a complex of two viruses and it provides an excellent system to understand the host and virus interactions during infection. It is known that Rice tungro bacilliform virus (RTBV) is the major determinant of the disease while Rice tungro spherical virus (RTSV) accentuates the symptoms. This study brings to focus the important role of RTBV ORF-IV in disease manifestation, by acting as both the victim and silencer of the RNA silencing pathway. The ORF-IV is a weak suppressor of the S-PTGS or stable silencing, but its suppression activity is augmented in the presence of specific RTSV proteins. Among these, RTBV ORF-IV and RTSV CP3 proteins interact with each other. This interaction may lead to the suppression of localized silencing as well as the spread of silencing in the host plants. The findings present a probable mechanistic glimpse of the requirement of the two viruses in enhancing tungro disease.

Small RNAs Participate in Plant-Virus Interaction and Their Application in Plant Viral Defense

Small RNAs are significant regulators of gene expression, which play multiple roles in plant development, growth, reproductive and stress response. It is generally believed that the regulation of plants’ endogenous genes by small RNAs has evolved from a cellular defense mechanism for RNA viruses and transposons. Most small RNAs have well-established roles in the defense response, such as viral response. During viral infection, plant endogenous small RNAs can direct virus resistance by regulating the gene expression in the host defense pathway, while the small RNAs derived from viruses are the core of the conserved and effective RNAi resistance mechanism. As a counter strategy, viruses evolve suppressors of the RNAi pathway to disrupt host plant silencing against viruses. Currently, several studies have been published elucidating the mechanisms by which small RNAs regulate viral defense in different crops. This paper reviews the distinct pathways of small RNAs biogenesis and the molecular mechanisms of small RNAs mediating antiviral immunity in plants, as well as summarizes the coping strategies used by viruses to override this immune response. Finally, we discuss the current development state of the new applications in virus defense based on small RNA silencing.

Direct Foliar Application of dsRNA Derived From the Full-Length Gene of NSs of Groundnut Bud Necrosis Virus Limits Virus Accumulation and Symptom Expression

Groundnut bud necrosis virus (GBNV) is the most significant member of the genus Orthotospovirus occurring in the Indian subcontinent. There is hardly any effective measure to prevent GBNV in crop plants. In order to develop GBNV infection prevention procedure, we examined the effect of the direct foliar application of double-stranded RNA (dsRNA) derived from the full-length NSs gene (1,320 nucleotides) of GBNV. The bacterially expressed dsRNA to the non-structural (dsNSs) gene of GBNV was purified and delivered to plants as an aqueous suspension containing 0.01% Celite for evaluating its efficacy in preventing GBNV infection in systemic host, Nicotiana benthamiana as well as in local lesion and systemic host, cowpea cv. Pusa Komal (Vigna unguiculata). The dsNSs application and challenge-inoculation were conducted in three different combinations, where plants were challenge-inoculated with GBNV a day after, immediately, and a day before the application of dsNSs. N. benthamiana plants, which were not treated with dsRNA showed severe systemic wilting and death by 9-16 days post-inoculation (dpi). The non-treated cowpea plants exhibited many chlorotic and necrotic lesions on the cotyledonary leaves followed by systemic necrosis and death of the plants by 14-16 dpi. The dsNSs treated plants in all the combinations showed significant reduction of disease severity index in both N. benthamiana and cowpea. The treatment combination where the GBNV inoculation was conducted immediately after the dsNSs treatment was found to be the most effective treatment in preventing symptom expression. The viral RNA analysis by real time PCR also showed 20 and 12.5 fold reduction of GBNV in cowpea and N. benthamiana, respectively. Our results suggest that the foliar application of dsRNA derived from the full-length NSs gene of GBNV through Celite is successful in delivering long dsRNA leading to effective prevention of GBNV infection.

Non-Coding RNAs in Response to Drought Stress

Drought stress causes changes in the morphological, physiological, biochemical and molecular characteristics of plants. The response to drought in different plants may vary from avoidance, tolerance and escape to recovery from stress. This response is genetically programmed and regulated in a very complex yet synchronized manner. The crucial genetic regulations mediated by non-coding RNAs (ncRNAs) have emerged as game-changers in modulating the plant responses to drought and other abiotic stresses. The ncRNAs interact with their targets to form potentially subtle regulatory networks that control multiple genes to determine the overall response of plants. Many long and small drought-responsive ncRNAs have been identified and characterized in different plant varieties. The miRNA-based research is better documented, while lncRNA and transposon-derived RNAs are relatively new, and their cellular role is beginning to be understood. In this review, we have compiled the information on the categorization of non-coding RNAs based on their biogenesis and function. We also discuss the available literature on the role of long and small non-coding RNAs in mitigating drought stress in plants.

Interaction between βC1 of satellite and coat protein of Chili leaf curl virus plays a crucial role in suppression of host RNA silencing

The monopartite Chili leaf curl virus (ChiLCV) and its β-satellite (ChiLCB) have been found to co-exist in infected plants. The ability of βC1 protein to suppress RNA silencing was investigated using an in-house developed in-planta reversal of silencing assay, using Nicotiana tabacum lines harboring green fluorescent protein (GFP) silenced by short hairpin GFP (ShGFP). Transient expression of recombinant βC1 complemented and increased the suppressor activity of ChiLCV coat protein (CP), and this was confirmed by molecular analysis. In silico analysis followed by a yeast two-hybrid screen-identified ChiLCV-CP as the interacting partner of the ChiLCB-βC1 protein. Subcellular localization through confocal analysis revealed that when βC1 and ChiLCV-CP were co-present, the fluorescence was localized in the cytoplasm indicating that nuclear localization of both proteins was obstructed. The cytoplasmic compartmentalization of the two viral suppressors of RNA silencing may be responsible for the enhanced suppression of the host gene silencing. This study presents evidence on the interaction of ChiLCV-CP and βC1 proteins and indicates that ChiLCB may support the ChiLCV in overcoming host gene silencing to cause Chili leaf curl disease. KEY POINTS: • CP of ChiLCV and βC1 of ChiLCB contain RNA silencing suppression activity • The RNA silencing suppression activity of ChiLCB-βC1 complements that of ChiLCV-CP • There is a direct interaction between ChiLCB-βC1 and ChiLCV-CP.

Influence of virus-host interactions on plant response to abiotic stress

Environmental factors play a significant role in controlling growth, development and defense responses of plants. Changes in the abiotic environment not only significantly alter the physiological and molecular pathways in plants, but also result in attracting the insect pests that carry a payload of viruses. Invasion of plants by viruses triggers the RNA silencing based defense mechanism in plants. In counter defense the viruses have gained the ability to suppress the host RNA silencing activities. A new paradigm has emerged, with the recognition that plant viruses also have the intrinsic capacity to modulate host plant response to environmental cues, in an attempt to favour their own survival. Thus, plant-virus interactions provide an excellent system to understand the signals in crosstalk between biotic (virus) and abiotic stresses. In this review, we have summarized the basal plant defense responses to pathogen invasion while emphasizing on the role of RNA silencing as a front line of defense response to virus infection. The emerging knowledge indicates overlap between RNA silencing with the innate immune responses during antiviral defense. The suppressors of RNA silencing serve as Avr proteins, which can be recognized by the host R proteins. The defense signals also function in concert with the phytohormones to influence plant responses to abiotic stresses. The current evidence on the role of virus induced host tolerance to abiotic stresses is also discussed.