miR403a and SA Are Involved in NbAGO2 Mediated Antiviral Defenses Against TMV Infection in Nicotiana benthamiana

Function and regulation of plant ARGONAUTE proteins in response to environmental challenges: a review

Environmental stresses diversely affect multiple processes related to the growth, development, and yield of many crops worldwide. In response, plants have developed numerous sophisticated defense mechanisms at the cellular and subcellular levels to react and adapt to biotic and abiotic stressors. RNA silencing, which is an innate immune mechanism, mediates sequence-specific gene expression regulation in higher eukaryotes. ARGONAUTE (AGO) proteins are essential components of the RNA-induced silencing complex (RISC). They bind to small noncoding RNAs (sRNAs) and target complementary RNAs, causing translational repression or triggering endonucleolytic cleavage pathways. In this review, we aim to illustrate the recently published molecular functions, regulatory mechanisms, and biological roles of AGO family proteins in model plants and cash crops, especially in the defense against diverse biotic and abiotic stresses, which could be helpful in crop improvement and stress tolerance in various plants.

Transcriptomic and metabolic profiling of watermelon uncovers the role of salicylic acid and flavonoids in the resistance to cucumber green mottle mosaic virus

Understanding the mechanisms underlying plant resistance to virus infections is crucial for viral disease management in agriculture. However, the defense mechanism of watermelon (Citrullus lanatus) against cucumber green mottle mosaic virus (CGMMV) infection remains largely unknown. In this study, we performed transcriptomic, metabolomic, and phytohormone analyses of a CGMMV susceptible watermelon cultivar 'Zhengkang No.2' ('ZK') and a CGMMV resistant wild watermelon accession PI 220778 (PI) to identify the key regulatory genes, metabolites, and phytohormones responsible for CGMMV resistance. We then tested several phytohormones and metabolites for their roles in watermelon CGMMV resistance via foliar application, followed by CGMMV inoculation. Several phenylpropanoid metabolism-associated genes and metabolites, especially those involved in the flavonoid biosynthesis pathway, were found to be significantly enriched in the CGMMV-infected PI plants compared with the CGMMV-infected 'ZK' plants. We also identified a gene encoding UDP-glycosyltransferase (UGT) that is involved in kaempferol-3-O-sophoroside biosynthesis and controls disease resistance, as well as plant height. Additionally, salicylic acid (SA) biogenesis increased in the CGMMV-infected 'ZK' plants, resulting in the activation of a downstream signaling cascade. SA levels in the tested watermelon plants correlated with that of total flavonoids, and SA pre-treatment up-regulated the expression of flavonoid biosynthesis genes, thus increasing the total flavonoid content. Furthermore, application of exogenous SA or flavonoids extracted from watermelon leaves suppressed CGMMV infection. In summary, our study demonstrates the role of SA-induced flavonoid biosynthesis in plant development and CGMMV resistance, which could be used to breed for CGMMV resistance in watermelon.

Argonaute 5-mediated antiviral defense and viral counter-defense in Nicotiana benthamiana

The argonaute (AGO) family proteins play a crucial role in preventing viral invasions through the plant antiviral RNA silencing pathway, with distinct AGO proteins recruited for specific antiviral mechanisms. Our previous study revealed that Nicotiana benthamiana AGO5 (NbAGO5) expression was significantly upregulated in response to bamboo mosaic virus (BaMV) infection. However, the roles of NbAGO5 in antiviral mechanisms remained to be explored. In this research, we examined the antiviral functions of NbAGO5 in the infections of different viruses. It was found that the accumulation of NbAGO5 was induced not only at the RNA but also at the protein level following the infections of BaMV, potato virus X (PVX), tobacco mosaic virus (TMV), and cucumber mosaic virus (CMV) in N. benthamiana. To explore the antiviral mechanism and regulatory function of NbAGO5, we generated NbAGO5 overexpression (OE-NbAGO5) and knockout (nbago5) transgenic N. benthamiana lines. Our findings reveal that NbAGO5 provides defense against BaMV, PVX, TMV, and a mutant CMV deficient in 2b gene, but not against the wild-type CMV and turnip mosaic virus (TuMV). Through affinity purification and small RNA northern blotting, we demonstrated that NbAGO5 exerts its antiviral function by binding to viral small interfering RNAs (vsiRNAs). Moreover, we observed that CMV 2b and TuMV HC-Pro interact with NbAGO5, triggering its degradation via the 26S proteasome and autophagy pathways, thereby allowing these viruses to overcome NbAGO5-mediated defense. In addition, TuMV HC-Pro provides another line of counter-defense by interfering with vsiRNA binding by NbAGO5. Our study provides further insights into the antiviral RNA interference mechanism and the complex interplay between NbAGO5 and plant viruses.

CRISPR/Cas, Multiomics, and RNA Interference in Virus Disease Management

Plant viruses infect a wide range of commercially important crop plants and cause significant crop production losses worldwide. Numerous alterations in plant physiology related to the reprogramming of gene expression may result from viral infections. Although conventional integrated pest management-based strategies have been effective in reducing the impact of several viral diseases, continued emergence of new viruses and strains, expanding host ranges, and emergence of resistance-breaking strains necessitate a sustained effort toward the development and application of new approaches for virus management that would complement existing tactics. RNA interference-based techniques, and more recently, clustered regularly interspaced short palindromic repeats (CRISPR)-based genome editing technologies have paved the way for precise targeting of viral transcripts and manipulation of viral genomes and host factors. In-depth knowledge of the molecular mechanisms underlying the development of disease would further expand the applicability of these recent methods. Advances in next-generation/high-throughput sequencing have made possible more intensive studies into host-virus interactions. Utilizing the omics data and its application has the potential to expedite fast-tracking traditional plant breeding methods, as well as applying modern molecular tools for trait enhancement, including virus resistance. Here, we summarize the recent developments in the CRISPR/Cas system, transcriptomics, endogenous RNA interference, and exogenous application of dsRNA in virus disease management.

Small RNAs: Efficient and miraculous effectors that play key roles in plant-microbe interactions

Plants' response to pathogens is highly complex and involves changes at different levels, such as activation or repression of a vast array of genes. Recently, many studies have demonstrated that many RNAs, especially small RNAs (sRNAs), are involved in genetic expression and reprogramming affecting plant-pathogen interactions. The sRNAs, including short interfering RNAs and microRNAs, are noncoding RNA with 18-30 nucleotides, and are recognized as key genetic and epigenetic regulators. In this review, we summarize the new findings about defence-related sRNAs in the response to pathogens and our current understanding of their effects on plant-pathogen interactions. The main content of this review article includes the roles of sRNAs in plant-pathogen interactions, cross-kingdom sRNA trafficking between host and pathogen, and the application of RNA-based fungicides for plant disease control.

A Jasmonic Acid-Related Mechanism Affects ARGONAUTE5 Expression and Antiviral Defense Against Potato Virus X in Arabidopsis thaliana

During virus infection, Argonaute (AGO) proteins bind to Dicer-produced virus small interfering RNAs and target viral RNA based on sequence complementarity, thereby limiting virus proliferation. The Arabidopsis AGO2 protein is important for resistance to multiple viruses, including potato virus X (PVX). In addition, AGO5 is important in systemic defense against PVX. Normally AGO5 is expressed only in reproductive tissues, and its induction by virus infection is thought to be important for its participation in antiviral defense. However, it is unclear what mechanisms induce AGO5 expression in response to virus infection. Here, we show that dde2-2, a mutant compromised in jasmonic acid (JA) biosynthesis, displays constitutive upregulation of AGO5. This mutant also showed increased resistance to PVX and this resistance was dependent on a functional AGO5 gene. Furthermore, methyl jasmonate treatment ablated AGO5 expression in leaves during virus infection and resulted in increased susceptibility to virus. Our results further support a role for AGO5 in antiviral RNA silencing and a negative regulation by JA, a plant hormone associated with defense against plant-feeding arthropods, which are often the vectors of plant viruses. [Formula: see text] Copyright © 2023 The Author(s). This is an open access article distributed under the CC BY-NC-ND 4.0 International license.

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.

Characterization of Virus-Inducible Orchid Argonaute 5b Promoter and Its Functional Characterization in Nicotiana benthamiana during Virus Infection

Plant ARGONAUTES (AGOs) play a significant role in the defense against viral infection. Previously, we have demonstrated that AGO5s encoded in Phalaenopsis aphrodite subsp. formosana (PaAGO5s) took an indispensable part in defense against major viruses. To understand the underlying defense mechanism, we cloned PaAGO5s promoters (pPaAGO5s) and analyzed their activity in transgenic Nicotiana benthamiana using β-glucuronidase (GUS) as a reporter gene. GUS activity analyses revealed that during Cymbidium mosaic virus (CymMV) and Odontoglossum ringspot virus (ORSV) infections, pPaAGO5b activity was significantly increased compared to pPaAGO5a and pPaAGO5c. Analysis of pPaAGO5b 5′-deletion revealed that pPaAGO5b_941 has higher activity during virus infection. Further, yeast one-hybrid analysis showed that the transcription factor NbMYB30 physically interacted with pPaAGO5b_941 to enhance its activity. Overexpression and silencing of NbMYB30 resulted in up- and downregulation of GUS expression, respectively. Exogenous application and endogenous measurement of phytohormones have shown that methyl jasmonate and salicylic acid respond to viral infections. NbMYB30 overexpression and its closest related protein, PaMYB30, in P. aphrodite subsp. formosana reduced CymMV accumulation in P. aphrodite subsp. formosana. Based on these discoveries, this study uncovers the interaction between virus-responsive promoter and the corresponding transcription factor in plants.

NbNAC42 and NbZFP3 Transcription Factors Regulate the Virus Inducible NbAGO5 Promoter in Nicotiana benthamiana

Plant argonautes (AGOs) play important roles in the defense responses against viruses. The expression of Nicotiana benthamiana AGO5 gene (NbAGO5) is highly induced by Bamboo mosaic virus (BaMV) infection; however, the underlying mechanisms remain elusive. In this study, we have analyzed the potential promoter activities of NbAGO5 and its interactions with viral proteins by using a 2,000 bp fragment, designated as PN1, upstream to the translation initiation of NbAGO5. PN1 and seven serial 5'-deletion mutants (PN2-PN8) were fused with a β-glucuronidase (GUS) reporter and introduced into the N. benthamiana genome by Agrobacterium-mediated transformation for further characterization. It was found that PN4-GUS transgenic plants were able to drive strong GUS expression in the whole plant. In the virus infection tests, the GUS activity was strongly induced in PN4-GUS transgenic plants after being challenged with potexviruses. Infiltration of the transgenic plants individually with BaMV coat protein (CP) or triple gene block protein 1 (TGBp1) revealed that only TGBp1 was crucial for inducing the NbAGO5 promoter. To identify the factors responsible for controlling the activity of the NbAGO5 promoter, we employed yeast one-hybrid screening on a transcription factor cDNA library. The result showed that NbNAC42 and NbZFP3 could directly bind the 704 bp promoter regions of NbAGO5. By using overexpressing and virus-induced gene silencing techniques, we found that NbNAC42 and NbZFP3 regulated and downregulated, respectively, the expression of the NbAGO5 gene. Upon virus infection, NbNAC42 played an important role in regulating the expression of NbAGO5. Together, these results provide new insights into the modulation of the defense mechanism of N. benthamiana against viruses. This virus inducible promoter could be an ideal candidate to drive the target gene expression that could improve the anti-virus abilities of crops in the future.

A cluster of atypical resistance genes in soybean confers broad-spectrum antiviral activity

Soybean mosaic virus (SMV) is a severe soybean (Glycine max) pathogen. Here we characterize a soybean SMV resistance cluster (SRC) that comprises five resistance (R) genes. SRC1 encodes a Toll/interleukin-1 receptor and nucleotide-binding site (TIR-NBS [TN]) protein, SRC4 and SRC6 encode TIR proteins with a short EF-hand domain, while SRC7 and SRC8 encode TNX proteins with a noncanonical basic secretory protein (BSP) domain at their C-termini. We mainly studied SRC7, which contains a noncanonical BSP domain and gave full resistance to SMV. SRC7 possessed broad-spectrum antiviral activity toward several plant viruses including SMV, plum pox virus, potato virus Y, and tobacco mosaic virus. The TIR domain alone was both necessary and sufficient for SRC7 immune signaling, while the NBS domain enhanced its activity. Nuclear oligomerization via the interactions of both TIR and NBS domains was essential for SRC7 function. SRC7 expression was transcriptionally inducible by SMV infection and salicylic acid (SA) treatment, and SA was required for SRC7 triggered virus resistance. SRC7 expression was posttranscriptionally regulated by miR1510a and miR2109, and the SRC7-miR1510a/miR2109 regulatory network appeared to contribute to SMV-soybean interactions in both resistant and susceptible soybean cultivars. In summary, we report a soybean R gene cluster centered by SRC7 that is regulated at both transcriptional and posttranscriptional levels, possesses a yet uncharacterized BSP domain, and has broad-spectrum antiviral activities. The SRC cluster is special as it harbors several functional R genes encoding atypical TIR-NBS-LRR (TNL) type R proteins, highlighting its importance in SMV-soybean interaction and plant immunity.

Genetic evidence for the involvement of Dicer-like 2 and 4 as well as Argonaute 2 in the Nicotiana benthamiana response against Pelargonium line pattern virus

In plants, RNA silencing functions as a potent antiviral mechanism. Virus-derived double-stranded RNAs (dsRNAs) trigger this mechanism, being cleaved by Dicer-like (DCL) enzymes into virus small RNAs (vsRNAs). These vsRNAs guide sequence-specific RNA degradation upon their incorporation into an RNA-induced silencing complex (RISC) that contains a slicer of the Argonaute (AGO) family. Host RNA dependent-RNA polymerases, particularly RDR6, strengthen antiviral silencing by generating more dsRNA templates from RISC-cleavage products that, in turn, are converted into secondary vsRNAs by DCLs. Previous work showed that Pelargonium line pattern virus (PLPV) is a very efficient inducer and target of RNA silencing as PLPV-infected Nicotiana benthamiana plants accumulate extraordinarily high amounts of vsRNAs that, strikingly, are independent of RDR6 activity. Several scenarios may explain these observations including a major contribution of dicing versus slicing for defence against PLPV, as the dicing step would not be affected by the RNA silencing suppressor encoded by the virus, a protein that acts via vsRNA sequestration. Taking advantage of the availability of lines of N. benthamiana with DCL or AGO2 functions impaired, here we have tried to get further insights into the components of the silencing machinery that are involved in anti-PLPV-silencing. Results have shown that DCL4 and, to lesser extent, DCL2 contribute to restrict viral infection. Interestingly, AGO2 apparently makes even a higher contribution in the defence against PLPV, extending the number of viruses that are affected by this particular slicer. The data support that both dicing and slicing activities participate in the host race against PLPV.

Improvement of host-induced gene silencing efficiency via polycistronic-tRNA-amiR expression for multiple target genes and characterization of RNAi mechanism in Mythimna separata

Host-induced gene silencing (HIGS) emerged as a new strategy for pest control. However, RNAi efficiency is reported to be low in Lepidoptera, which are composed of many important crop pests. To address this, we generated transgenic plants to develop HIGS effects in a maize pest, Mythimna separata (Lepidoptera, Noctuidae), by targeting chitinase encoding genes. More importantly, we developed an artificial microRNA (amiR) based PTA (polycistronic-tRNA-amiR) system for silencing multiple target genes. Compared with hpRNA (hairpin RNA), transgenic expression of a PTA cassette including an amiR for the gut-specific dsRNA nuclease gene MsREase, resulted in improved knockdown efficiency and caused more pronounced developmental abnormalities in recipient insects. When target gene siRNAs were analysed after HIGS and direct dsRNA/siRNA feeding, common features such as sense polarity and siRNA hotspot regions were observed, however, they differed in siRNA transitivity and major 20-24nt siRNA species. Core RNAi genes were identified in M. separata, and biochemical activities of MsAGO2, MsSID1 and MsDcr2 were confirmed by EMSA (electrophoretic mobility shift assay) and dsRNA cleavage assays, respectively. Taken together, we provide compelling evidence for the existence of the RNAi mechanism in M. separata by analysis of both siRNA signatures and RNAi machinery components, and the PTA system could potentially be useful for future RNAi control of lepidopteran pests.

Role of non-coding RNAs in plant immunity

Crops are exposed to attacks by various pathogens that cause substantial yield losses and severely threaten food security. To cope with pathogenic infection, crops have elaborated strategies to enhance resistance against pathogens. In addition to the role of protein-coding genes as key regulators in plant immunity, accumulating evidence has demonstrated the importance of non-coding RNAs (ncRNAs) in the plant immune response. Here, we summarize the roles and molecular mechanisms of endogenous ncRNAs, especially microRNAs (miRNAs), long ncRNAs (lncRNAs), and circular RNAs (circRNAs), in plant immunity. We discuss the coordination between miRNAs and small interfering RNAs (siRNAs), between lncRNAs and miRNAs or siRNAs, and between circRNAs and miRNAs in the regulation of plant immune responses. We also address the role of cross-kingdom mobile small RNAs in plant-pathogen interactions. These insights improve our understanding of the mechanisms by which ncRNAs regulate plant immunity and can promote the development of better approaches for breeding disease-resistant crops.

Host sunflower-induced silencing of parasitism-related genes confers resistance to invading Orobanche cumana

Orobanche cumana is a holoparasitic plant that attaches to host-plant roots and seriously reduces the yield of sunflower (Helianthus annuus L.). Effective control methods are lacking with only a few known sources of genetic resistance. In this study, a seed-soak agroinoculation (SSA) method was established, and recombinant tobacco rattle virus vectors were constructed to express RNA interference (RNAi) inducers to cause virus-induced gene silencing (VIGS) in sunflower. A host target gene HaTubulin was systemically silenced in both leaf and root tissues by the SSA-VIGS approach. Trans-species silencing of O. cumana genes were confirmed for 10 out of 11 target genes with silencing efficiency of 23.43%-92.67%. Knockdown of target OcQR1, OcCKX5, and OcWRI1 genes reduced the haustoria number, and silencing of OcEXPA6 caused further phenotypic abnormalities such as shorter tubercles and necrosis. Overexpression of OcEXPA6 caused retarded root growth in alfalfa (Medicago sativa). The results demonstrate that these genes play an important role in the processes of O. cumana parasitism. High-throughput small RNA (sRNA) sequencing and bioinformatics analyses unveiled the distinct features of target gene-derived siRNAs in O. cumana such as siRNA transitivity, strand polarity, hotspot region, and 21/22-nt siRNA predominance, the latter of which was confirmed by Northern blot experiments. The possible RNAi mechanism is also discussed by analyzing RNAi machinery genes in O. cumana. Taken together, we established an efficient host-induced gene silencing technology for both functional genetics studies and potential control of O. cumana. The ease and effectiveness of this strategy could potentially be useful for other species provided they are amenable to SSA.

MicroRNA Techniques: Valuable Tools for Agronomic Trait Analyses and Breeding in Rice

MicroRNAs (miRNAs) are a class of small noncoding RNAs that regulate gene expression at the post-transcriptional level. Extensive studies have revealed that miRNAs have critical functions in plant growth, development, and stress responses and may provide valuable genetic resources for plant breeding research. We herein reviewed the development, mechanisms, and characteristics of miRNA techniques while highlighting widely used approaches, namely, the short tandem target mimic (STTM) approach. We described STTM-based advances in plant science, especially in the model crop rice, and introduced the CRISPR-based transgene-free crop breeding. Finally, we discussed the challenges and unique opportunities related to combining STTM and CRISPR technology for crop improvement and agriculture.

Recent advances in understanding plant antiviral RNAi and viral suppressors of RNAi

Molecular plant-virus interactions provide an excellent model to understanding host antiviral immunity and viral counter-defense mechanisms. The primary antiviral defense is triggered inside the infected plant cell by virus-derived small-interfering RNAs, which guide homology-dependent RNA interference (RNAi) and/or RNA-directed DNA methylation (RdDM) to target RNA and DNA viruses. In counter-defense, plant viruses have independently evolved viral suppressors of RNAi (VSRs) to specifically antagonize antiviral RNAi. Recent studies have shown that plant antiviral responses are regulated by endogenous small silencing RNAs, RNA decay and autophagy and that some known VSRs of plant RNA and DNA viruses also target these newly recognized defense responses to promote infection. This review focuses on these recent advances that have revealed multilayered regulation of plant-virus interactions.

Effect of virus infection on the secondary metabolite production and phytohormone biosynthesis in plants

Plants have evolved according to their environmental conditions and continuously interact with different biological entities. These interactions induce many positive and negative effects on plant metabolism. Many viruses also associate with various plant species and alter their metabolism. Further, virus-plant interaction also alters the expression of many plant hormones. To overcome the biotic stress imposed by the virus's infestation, plants produce different kinds of secondary metabolites that play a significant role in plant defense against the viral infection. In this review, we briefly highlight the mechanism of virus infection, their influence on the plant secondary metabolites and phytohormone biosynthesis in response to the virus-plant interactions.

What does it take to be antiviral? An Argonaute-centered perspective on plant antiviral defense

RNA silencing is a major mechanism of constitutive antiviral defense in plants, mediated by a number of proteins, including the Dicer-like (DCL) and Argonaute (AGO) endoribonucleases. Both DCL and AGO protein families comprise multiple members. In particular, the AGO protein family has expanded considerably in different plant lineages, with different family members having specialized functions. Although the general mode of action of AGO proteins is well established, the properties that make different AGO proteins more or less efficient at targeting viruses are less well understood. In this report, we review methodologies used to study AGO antiviral activity and current knowledge about which AGO family members are involved in antiviral defense. In addition, we discuss what is known about the different properties of AGO proteins thought to be associated with this function.

Synthesis of Full-Length cDNA Infectious Clones of Soybean Mosaic Virus and Functional Identification of a Key Amino Acid in the Silencing Suppressor Hc-Pro

Soybean mosaic virus (SMV), which belongs to the Potyviridae, causes significant reductions in soybean yield and seed quality. In this study, both tag-free and reporter gene green fluorescent protein (GFP)-containing infectious clones for the SMV N1 strain were constructed by Gibson assembly and with the yeast homologous recombination system, respectively. Both infectious clones are suitable for agroinfiltration on the model host N. benthamiana and show strong infectivity for the natural host soybean and several other legume species. Both infectious clones were seed transmitted and caused typical virus symptoms on seeds and progeny plants. We used the SMV-GFP infectious clone to further investigate the role of key amino acids in the silencing suppressor helper component-proteinase (Hc-Pro). Among twelve amino acid substitution mutants, the co-expression of mutant 2-with an Asparagine→Leucine substitution at position 182 of the FRNK (Phe-Arg-Asn-Lys) motif-attenuated viral symptoms and alleviated the host growth retardation caused by SMV. Moreover, the Hc-Prom2 mutant showed stronger oligomerization than wild-type Hc-Pro. Taken together, the SMV infectious clones will be useful for studies of host-SMV interactions and functional gene characterization in soybeans and related legume species, especially in terms of seed transmission properties. Furthermore, the SMV-GFP infectious clone will also facilitate functional studies of both virus and host genes in an N. benthamiana transient expression system.

Ultrahigh-activity immune inducer from Endophytic Fungi induces tobacco resistance to virus by SA pathway and RNA silencing

BACKGROUND

Plant viruses cause severe economic losses in agricultural production. An ultrahigh activity plant immune inducer (i.e., ZhiNengCong, ZNC) was extracted from endophytic fungi, and it could promote plant growth and enhance resistance to bacteria. However, the antiviral function has not been studied. Our study aims to evaluate the antiviral molecular mechanisms of ZNC in tobacco.

RESULTS

Here, we used Potato X virus (PVX), wild-type tobacco and NahG transgenic tobacco as materials to study the resistance of ZNC to virus. ZNC exhibited a high activity in enhancing resistance to viruses and showed optimal use concentration at 100-150 ng/mL. ZNC also induced reactive oxygen species accumulation, increased salicylic acid (SA) content by upregulating the expression of phenylalanine ammonia lyase (PAL) gene and activated SA signaling pathway. We generated transcriptome profiles from ZNC-treated seedlings using RNA sequencing. The first GO term in biological process was positive regulation of post-transcriptional gene silencing, and the subsequent results showed that ZNC promoted RNA silencing. ZNC-sprayed wild-type leaves showed decreased infection areas, whereas ZNC failed to induce a protective effect against PVX in NahG leaves.

CONCLUSION

All results indicate that ZNC is an ultrahigh-activity immune inducer, and it could enhance tobacco resistance to PVX at low concentration by positively regulating the RNA silencing via SA pathway. The antiviral mechanism of ZNC was first revealed in this study, and this study provides a new antiviral bioagent.