Translational control in plant antiviral immunity

Turnip mosaic virus NIb weakens the function of eukaryotic translation initiation factor 6 facilitating viral infection in Nicotiana benthamiana

Viruses rely completely on host translational machinery to produce the proteins encoded by their genes. Controlling translation initiation is important for gaining translational advantage in conflicts between the host and virus. The eukaryotic translation initiation factor 4E (eIF4E) has been reported to be hijacked by potyviruses for virus multiplication. The role of translation regulation in defence and anti-defence between plants and viruses is not well understood. We report that the transcript level of eIF6 was markedly increased in turnip mosaic virus (TuMV)-infected Nicotiana benthamiana. TuMV infection was impaired by overexpression of N. benthamiana eIF6 (NbeIF6) either transiently expressed in leaves or stably expressed in transgenic plants. Polysome profile assays showed that overexpression of NbeIF6 caused the accumulation of 40S and 60S ribosomal subunits, the reduction of polysomes, and also compromised TuMV UTR-mediated translation, indicating a defence role for upregulated NbeIF6 during TuMV infection. However, the polysome profile in TuMV-infected leaves was not identical to that in leaves overexpressing NbeIF6. Further analysis showed that TuMV NIb protein, the RNA-dependent RNA polymerase, interacted with NbeIF6 and interfered with its effect on the ribosomal subunits, suggesting that NIb might have a counterdefence role. The results propose a possible regulatory mechanism at the translation level during plant-virus interaction.

Plant virology: an RNA treasure trove

Key principles pertaining to RNA biology not infrequently have their origins in plant virology. Examples have arisen from studies on viral RNA-intrinsic properties and the infection process from gene expression, replication, movement, and defense evasion to biotechnological applications. Since RNA is at the core of the central dogma in molecular biology, how plant virology assisted in the reinforcement or adaptations of this concept, while at other instances shook up elements of the doctrine, is discussed. Moreover, despite the negative effects of viral diseases in agriculture worldwide, plant viruses can be considered a scientific treasure trove. Today they remain tools of discovery for biotechnology, studying evolution, cell biology, and host-microbe interactions.

The Hypersensitive Response to Plant Viruses

Plant proteins with domains rich in leucine repeats play important roles in detecting pathogens and triggering defense reactions, both at the cellular surface for pattern-triggered immunity and in the cell to ensure effector-triggered immunity. As intracellular parasites, viruses are mostly detected intracellularly by proteins with a nucleotide binding site and leucine-rich repeats but receptor-like kinases with leucine-rich repeats, known to localize at the cell surface, have also been involved in response to viruses. In the present review we report on the progress that has been achieved in the last decade on the role of these leucine-rich proteins in antiviral immunity, with a special focus on our current understanding of the hypersensitive response.

Argonaute 1 and 5 proteins play crucial roles in the defence against cucumber green mottle mosaic virus in watermelon

RNA silencing, a core part of plants' antiviral defence, requires the ARGONAUTE, DICER-like, and RNA-dependent RNA polymerase proteins. However, how these proteins contribute to watermelon's RNA interference (RNAi) pathway response to cucumber green mottle mosaic virus (CGMMV) has not been characterized. Here, we identify seven ClAGO, four ClDCL, and 11 ClRDR genes in watermelon and analyse their expression profiles when infected with CGMMV. ClAGO1 and ClAGO5 expression levels were highly induced by CGMMV infection. The results of ClAGO1 and ClAGO5 overexpression and silencing experiments suggest that these genes play central roles in watermelon's antiviral defence. Furthermore, co-immunoprecipitation and bimolecular fluorescence complementation experiments showed that ClAGO1 interacts with ClAGO5 in vivo, suggesting that ClAGO1 and ClAGO5 co-regulate watermelon defence against CGMMV infection. We also identified the ethylene response factor (ERF) binding site in the promoters of the ClAGO1 and ClAGO5 genes, and ethylene (ETH) treatment significantly increased ClAGO5 expression. Two ERF genes (Cla97C08G147180 and Cla97C06G122830) closely related to ClAGO5 expression were identified using co-expression analysis. Subcellular localization revealed that two ERFs and ClAGO5 predominantly localize at the nucleus, suggesting that enhancement of resistance to CGMMV by ETH is probably achieved through ClAGO5 but not ClAGO1. Our findings reveal aspects of the mechanisms underlying RNA silencing in watermelon against CGMMV.

Translation Arrest: A Key Player in Plant Antiviral Response

Plants evolved several mechanisms to protect themselves against viruses. Besides recessive resistance, where compatible host factors required for viral proliferation are absent or incompatible, there are (at least) two types of inducible antiviral immunity: RNA silencing (RNAi) and immune responses mounted upon activation of nucleotide-binding domain leucine-rich repeat (NLR) receptors. RNAi is associated with viral symptom recovery through translational repression and transcript degradation following recognition of viral double-stranded RNA produced during infection. NLR-mediated immunity is induced upon (in)direct recognition of a viral protein by an NLR receptor, triggering either a hypersensitive response (HR) or an extreme resistance response (ER). During ER, host cell death is not apparent, and it has been proposed that this resistance is mediated by a translational arrest (TA) of viral transcripts. Recent research indicates that translational repression plays a crucial role in plant antiviral resistance. This paper reviews current knowledge on viral translational repression during viral recovery and NLR-mediated immunity. Our findings are summarized in a model detailing the pathways and processes leading to translational arrest of plant viruses. This model can serve as a framework to formulate hypotheses on how TA halts viral replication, inspiring new leads for the development of antiviral resistance in crops.

BLB8, an antiviral protein from Brevibacillus laterosporus strain B8, inhibits Tobacco mosaic virus infection by triggering immune response in tobacco

BACKGROUND

Tobacco mosaic virus (TMV) is one of destructive plant viruses, causing serious economic losses in the world. Using antiviral proteins or elicitors to inhibit viral infection or promote plant immunity is one of the efficient strategies against TMV. Our previous study identified that the fermentation broth of Brevibacillus laterosporus strain B8 showed strong antiviral activity against TMV. However, the active antiviral ingredient is still unclear.

RESULTS

Here, BLB8 (B. laterosporus strain B8 protein, BLB8), an antiviral protein from B. laterosporus strain B8 was isolated and characterized. BLB8 showed protective, inactive and curative effects against TMV, and the inhibition rate reached up to 63%, 83% and 55%, respectively. BLB8 infiltrated around the infection site of the recombinant virus TMV-GFP inhibited the systemic extend and movement of TMV. Pretreatment of the bottom leaves with BLB8 inhibited the spread and accumulation of TMV in upper systemic leaves. Furthermore, BLB8 caused hypersensitive response (HR) in a dose-dependent way, promoted H2 O2 accumulation, and induced the expression of defense-relative genes in Nicotiana benthamiana.

CONCLUSION

The antiviral protein BLB8 from B. laterosporus strain B8 effectively inhibits TMV infection in inactivation, protective and curative effects through triggering plant immunity in tobacco. Therefore, the present study provides a new antiviral agent for prevention and control of viral disease. © 2021 Society of Chemical Industry.

Geminivirus-Host Interactions: Action and Reaction in Receptor-Mediated Antiviral Immunity

In plant−virus interactions, the plant immune system and virulence strategies are under constant pressure for dominance, and the balance of these opposing selection pressures can result in disease or resistance. The naturally evolving plant antiviral immune defense consists of a multilayered perception system represented by pattern recognition receptors (PRR) and resistance (R) proteins similarly to the nonviral pathogen innate defenses. Another layer of antiviral immunity, signaling via a cell surface receptor-like kinase to inhibit host and viral mRNA translation, has been identified as a virulence target of the geminivirus nuclear shuttle protein. The Geminiviridae family comprises broad-host range viruses that cause devastating plant diseases in a large variety of relevant crops and vegetables and hence have evolved a repertoire of immune-suppressing functions. In this review, we discuss the primary layers of the receptor-mediated antiviral immune system, focusing on the mechanisms developed by geminiviruses to overcome plant immunity.

Geminiviral Triggers and Suppressors of Plant Antiviral Immunity

Geminiviruses are circular single-stranded DNA plant viruses encapsidated into geminate virion particles, which infect many crops and vegetables and, hence, represent significant agricultural constraints worldwide. To maintain their broad-range host spectrum and establish productive infection, the geminiviruses must circumvent a potent plant antiviral immune system, which consists of a multilayered perception system represented by RNA interference sensors and effectors, pattern recognition receptors (PRR), and resistance (R) proteins. This recognition system leads to the activation of conserved defense responses that protect plants against different co-existing viral and nonviral pathogens in nature. Furthermore, a specific antiviral cell surface receptor signaling is activated at the onset of geminivirus infection to suppress global translation. This review highlighted these layers of virus perception and host defenses and the mechanisms developed by geminiviruses to overcome the plant antiviral immunity mechanisms.

Extreme Resistance to Viruses in Potato and Soybean

Plant pathogens, including viruses, negatively impact global crop production. Plants have evolved complex immune responses to pathogens. These responses are often controlled by nucleotide-binding leucine-rich repeat proteins (NLRs), which recognize intracellular, pathogen-derived proteins. Genetic resistance to plant viruses is often phenotypically characterized by programmed cell death at or near the infection site; a reaction termed the hypersensitive response. Although visualization of the hypersensitive response is often used as a hallmark of resistance, the molecular mechanisms leading to the hypersensitive response and associated cell death vary. Plants with extreme resistance to viruses rarely exhibit symptoms and have little to no detectable virus replication or spread beyond the infection site. Both extreme resistance and the hypersensitive response can be activated by the same NLR genes. In many cases, genes that normally provide an extreme resistance phenotype can be stimulated to cause a hypersensitive response by experimentally increasing cellular levels of pathogen-derived elicitor protein(s). The molecular mechanisms of extreme resistance and its relationship to the hypersensitive response are largely uncharacterized. Studies on potato and soybean cultivars that are resistant to strains of Potato virus Y (PVY), Potato virus X (PVX), and Soybean mosaic virus (SMV) indicate that abscisic acid (ABA)-mediated signaling and NLR nuclear translocation are important for the extreme resistance response. Recent research also indicates that some of the same proteins are involved in both extreme resistance and the hypersensitive response. Herein, we review and synthesize published studies on extreme resistance in potato and soybean, and describe studies in additional species, including model plant species, to highlight future research avenues that may bridge the gaps in our knowledge of plant antiviral defense mechanisms.

Perturbation of nuclear-cytosolic shuttling of Rx1 compromises extreme resistance and translational arrest of potato virus X transcripts

Many plant intracellular immune receptors mount a hypersensitive response (HR) upon pathogen perception. The concomitant localized cell death is proposed to trap pathogens, such as viruses, inside infected cells, thereby preventing their spread. Notably, extreme resistance (ER) conferred by the potato immune receptor Rx1 to potato virus X (PVX) does not involve the death of infected cells. It is unknown what defines ER and how it differs from HR-based resistance. Interestingly, Rx1 can trigger an HR, but only upon artificial (over)expression of PVX or its avirulence coat protein (CP). Rx1 has a nucleocytoplasmic distribution and both pools are required for HR upon transient expression of a PVX-GFP amplicon. It is unknown whether mislocalized Rx1 variants can induce ER upon natural PVX infection. Here, we generated transgenic Nicotiana benthamiana producing nuclear- or cytosol-restricted Rx1 variants. We found that these variants can still mount an HR. However, nuclear- or cytosol-restricted Rx1 variants can no longer trigger ER or restricts viral infection. Interestingly, unlike the mislocalized Rx1 variants, wild-type Rx1 was found to compromise CP protein accumulation. We show that the lack of CP accumulation does not result from its degradation but is likely to be linked with translational arrest of its mRNA. Together, our findings suggest that translational arrest of viral genes is a major component of ER and, unlike the HR, is required for resistance to PVX.

RNA-Seq Transcriptome Analysis Provides Candidate Genes for Resistance to Tomato Leaf Curl New Delhi Virus in Melon

Tomato leaf curl New Delhi virus (ToLCNDV) emerged in the Mediterranean Basin in 2012 as the first DNA bipartite begomovirus (Geminiviridae family), causing severe yield and economic losses in cucurbit crops. A major resistance locus was identified in the wild melon accession WM-7 (Cucumis melo kachri group), but the mechanisms involved in the resistant response remained unknown. In this work, we used RNA-sequencing to identify disease-associated genes that are differentially expressed in the course of ToLCNDV infection and could contribute to resistance. Transcriptomes of the resistant WM-7 genotype and the susceptible cultivar Piñonet Piel de Sapo (PS) (C. melo ibericus group) in ToLCNDV and mock inoculated plants were compared at four time points during infection (0, 3, 6, and 12 days post inoculation). Different gene expression patterns were observed over time in the resistant and susceptible genotypes in comparison to their respective controls. Differentially expressed genes (DEGs) in ToLCNDV-infected plants were classified using gene ontology (GO) terms, and genes of the categories transcription, DNA replication, and helicase activity were downregulated in WM-7 but upregulated in PS, suggesting that reduced activity of these functions reduces ToLCNDV replication and intercellular spread and thereby contributes to resistance. DEGs involved in the jasmonic acid signaling pathway, photosynthesis, RNA silencing, transmembrane, and sugar transporters entail adverse consequences for systemic infection in the resistant genotype, and lead to susceptibility in PS. The expression levels of selected candidate genes were validated by qRT-PCR to corroborate their differential expression upon ToLCNDV infection in resistant and susceptible melon. Furthermore, single nucleotide polymorphism (SNPs) with an effect on structural functionality of DEGs linked to the main QTLs for ToLCNDV resistance have been identified. The obtained results pinpoint cellular functions and candidate genes that are differentially expressed in a resistant and susceptible melon line in response to ToLCNDV, an information of great relevance for breeding ToLCNDV-resistant melon cultivars.

Comparative analysis identifies amino acids critical for citrus tristeza virus (T36CA) encoded proteins involved in suppression of RNA silencing and differential systemic infection in two plant species

Complementary (c)DNA clones corresponding to the full-length genome of T36CA (a Californian isolate of Citrus tristeza virus with the T36 genotype), which shares 99.1% identity with that of T36FL (a T36 isolate from Florida), were made into a vector system to express the green fluorescent protein (GFP). Agroinfiltration of two prototype T36CA-based vectors (pT36CA) to Nicotiana benthamiana plants resulted in local but not systemic GFP expression/viral infection. This contrasted with agroinfiltration of the T36FL-based vector (pT36FL), which resulted in both local and systemic GFP expression/viral infection. A prototype T36CA systemically infected RNA silencing-defective N. benthamiana lines, demonstrating that a genetic basis for its defective systemic infection was RNA silencing. We evaluated the in planta bioactivity of chimeric pT36CA-pT36FL constructs and the results suggested that nucleotide variants in several open reading frames of the prototype T36CA could be responsible for its defective systemic infection. A single amino acid substitution in each of two silencing suppressors, p20 (S107G) and p25 (G36D), of prototype T36CA facilitated its systemic infectivity in N. benthamiana (albeit with reduced titre relative to that of T36FL) but not in Citrus macrophylla plants. Enhanced virus accumulation and, remarkably, robust systemic infection of T36CA in N. benthamiana and C. macrophylla plants, respectively, required two additional amino acid substitutions engineered in p65 (N118S and S158L), a putative closterovirus movement protein. The availability of pT36CA provides a unique opportunity for comparative analysis to identify viral coding and noncoding nucleotides or sequences involved in functions that are vital for in planta infection.

Novel interactions of cardamom mosaic virus VPg with cardamom histones H3 and H4

The host genome targeting potyviral proteins is sparsely reported. Viral genome-linked protein (VPg) is a multifaceted protein known for its interactions with a suite of host proteins, guides essential viral life cycle processes such as genome replication, translation, genome packing, and antiviral defence. Besides, VPg also plays a crucial role in assisting the transport of nuclear inclusion a protease (NIa protease) into the host nucleus. Apart from that, the role of VPg in the nucleus of the cognate host is not clear. Although NIa protease has been reported for DNase activity, the molecular mechanisms underlying host genome accessibility are not yet understood completely. Here, we employed yeast two hybrid assays to test the cardamom histones H3 and H4 interaction with the VPg and NIa protease of macluravirus cardamom mosaic virus (CdMV). Although CdMV NIa protease has the putative histone-binding ER motif of MYST histone acetyltransferase, it did not interact with host histones H3 and H4. Surprisingly, CdMV VPg displayed strong interaction with histone proteins H3 and H4. Leucine prototrophy and β-galactosidase assays were performed which validated VPg interaction with histones. To the best of our knowledge, this study is the first report for the multipartnered potyvirid protein VPg interaction with host histones H3 and H4.

A Begomovirus Nuclear Shuttle Protein-Interacting Immune Hub: Hijacking Host Transport Activities and Suppressing Incompatible Functions

Begomoviruses (Geminiviridae family) represent a severe constraint to agriculture worldwide. As ssDNA viruses that replicate in the nuclei of infected cells, the nascent viral DNA has to move to the cytoplasm and then to the adjacent cell to cause disease. The begomovirus nuclear shuttle protein (NSP) assists the intracellular transport of viral DNA from the nucleus to the cytoplasm and cooperates with the movement protein (MP) for the cell-to-cell translocation of viral DNA to uninfected cells. As a facilitator of intra- and intercellular transport of viral DNA, NSP is predicted to associate with host proteins from the nuclear export machinery, the intracytoplasmic active transport system, and the cell-to-cell transport complex. Furthermore, NSP functions as a virulence factor that suppresses antiviral immunity against begomoviruses. In this review, we focus on the protein-protein network that converges on NSP with a high degree of centrality and forms an immune hub against begomoviruses. We also describe the compatible host functions hijacked by NSP to promote the nucleocytoplasmic and intracytoplasmic movement of viral DNA. Finally, we discuss the NSP virulence function as a suppressor of the recently described NSP-interacting kinase 1 (NIK1)-mediated antiviral immunity. Understanding the NSP-host protein-protein interaction (PPI) network will probably pave the way for strategies to generate more durable resistance against begomoviruses.

RNA silencing of South African cassava mosaic virus in transgenic cassava expressing AC1/AC4 hp- RNA induces tolerance

Cassava mosaic disease (CMD), caused by geminiviruses, is a major hurdle to cassava production. Due to the heterozygous nature of cassava, breeding for virus resistance is difficult, but cassava has been shown to be a good candidate for genetic engineering using RNA interference (RNAi). T This study reports on the ability of a transgene-derived RNA hairpin, homologous to an overlapping region of the SACMV replication associated protein and putative virus suppressor of silencing protein (AC1/AC4), to confer tolerance in the CMD-susceptible model cassava cultivar 60444. Three of the fourteen transgenic lines expressing SACMV AC1/AC4 hairpin-derived siRNAs showed decreased symptoms and viral loads compared to untransformed control plants. Expression of SACMV AC1/AC4 homologous siRNAs showed that this tolerance is most likely associated with post-transcriptional gene silencing of the virus. This is the first report of targeting the overlapping AC1 and AC4 genes of SACMV conferring CMD tolerance in cassava.

The receptor-like kinase NIK1 targets FLS2/BAK1 immune complex and inversely modulates antiviral and antibacterial immunity

Plants deploy various immune receptors to recognize pathogens and defend themselves. Crosstalk may happen among receptor-mediated signal transduction pathways in the same host during simultaneous infection of different pathogens. However, the related function of the receptor-like kinases (RLKs) in thwarting different pathogens remains elusive. Here, we report that NIK1, which positively regulates plant antiviral immunity, acts as an important negative regulator of antibacterial immunity. nik1 plants exhibit dwarfed morphology, enhanced disease resistance to bacteria and increased PAMP-triggered immunity (PTI) responses, which are restored by NIK1 reintroduction. Additionally, NIK1 negatively regulates the formation of the FLS2/BAK1 complex. The interaction between NIK1 and FLS2/BAK1 is enhanced upon flg22 perception, revealing a novel PTI regulatory mechanism by an RLK. Furthermore, flg22 perception induces NIK1 and RPL10A phosphorylation in vivo, activating antiviral signalling. The NIK1-mediated inverse modulation of antiviral and antibacterial immunity may allow bacteria and viruses to activate host immune responses against each other.

Plants deploy numerous receptor-like kinases (RLKs) to respond to pathogens. Here the authors show that NIK1, an RLK that positively regulates antiviral immunity, negatively regulates the response to bacteria by modulating FLS2/BAK1 complex formation, suggesting crosstalk between bacterial and viral immunity.

Host factors against plant viruses

Plant virus genome replication and movement is dependent on host resources and factors. However, plants respond to virus infection through several mechanisms, such as autophagy, ubiquitination, mRNA decay and gene silencing, that target viral components. Viral factors work in synchrony with pro-viral host factors during the infection cycle and are targeted by antiviral responses. Accordingly, establishment of virus infection is genetically determined by the availability of the pro-viral factors necessary for genome replication and movement, and by the balance between plant defence and viral suppression of defence responses. Sequential requirement of pro-viral factors and the antagonistic activity of antiviral factors suggest a two-step model to explain plant-virus interactions. At each step of the infection process, host factors with antiviral activity have been identified. Here we review our current understanding of host factors with antiviral activity against plant viruses.

Nicotiana benthamiana Argonaute10 plays a pro-viral role in Bamboo mosaic virus infection

RNA silencing is a major defense mechanism against invading viruses in plants. Argonaute proteins (AGOs) are the key players in RNA silencing. The number of AGO family members involved varies depending on the plant species and they play distinct or sometimes redundant roles in antiviral defense. By using a virus-induced gene silencing technique, it was found that Nicotiana benthamiana AGO1 restricted Bamboo mosaic virus (BaMV) accumulation, but NbAGO10, the closest paralog of NbAGO1, positively regulated BaMV accumulation. Immunoprecipitation assay revealed BaMV virus-derived small interfering RNAs (vsiRNAs) in NbAGO10 complexes. Transient overexpression of NbAGO10 increased BaMV RNA accumulation, but with co-expression of NbAGO1, BaMV RNA accumulation was reduced, which suggests that NbAGO10 may have competed with NbAGO1 for sequestering BaMV vsiRNA and prevented the formation of RNA-induced silencing complexes. In addition, overexpression of NbAGO10 decreased BaMV vsiRNA accumulation. A host enzyme, small RNA degrading nuclease 1 (SDN1), also was found to interact with NbAGO10 on in vivo pull-down assay. Silencing of SDN1 elevated BaMV vsiRNA level and decreased BaMV RNA accumulation in N. benthamiana, indicating that NbAGO10 might recruit SDN1 for BaMV vsiRNA degradation. The results herein suggested that NbAGO10 plays a pro-viral role by BaMV vsiRNA sequestration and degradation.

Virus perception at the cell surface: revisiting the roles of receptor-like kinases as viral pattern recognition receptors

Activation of antiviral innate immune responses depends on the recognition of viral components or viral effectors by host receptors. This virus recognition system can activate two layers of host defence, pathogen-associated molecular pattern (PAMP)-triggered immunity (PTI) and effector-triggered immunity (ETI). While ETI has long been recognized as an efficient plant defence against viruses, the concept of antiviral PTI has only recently been integrated into virus-host interaction models, such as the RNA silencing-based defences that are triggered by viral dsRNA PAMPs produced during infection. Emerging evidence in the literature has included the classical PTI in the antiviral innate immune arsenal of plant cells. Therefore, our understanding of PAMPs has expanded to include not only classical PAMPS, such as bacterial flagellin or fungal chitin, but also virus-derived nucleic acids that may also activate PAMP recognition receptors like the well-documented phenomenon observed for mammalian viruses. In this review, we discuss the notion that plant viruses can activate classical PTI, leading to both unique antiviral responses and conserved antipathogen responses. We also present evidence that virus-derived nucleic acid PAMPs may elicit the NUCLEAR SHUTTLE PROTEIN-INTERACTING KINASE 1 (NIK1)-mediated antiviral signalling pathway that transduces an antiviral signal to suppress global host translation.

A WW Domain-Containing Protein Forms Immune Nuclear Bodies against Begomoviruses

The bipartite begomoviruses (Geminiviridae family), which are DNA viruses that replicate in the nucleus of infected cells, encode the nuclear shuttle protein (NSP) to facilitate the translocation of viral DNA from the nucleus to the cytoplasm via nuclear pores. This intracellular trafficking of NSP-DNA complexes is accessorized by the NSP-interacting guanosine triphosphatase (NIG) at the cytosolic side. Here, we report the nuclear redistribution of NIG by AtWWP1, a WW domain-containing protein that forms immune nuclear bodies (NBs) against begomoviruses. We demonstrated that AtWWP1 relocates NIG from the cytoplasm to the nucleus where it is confined to AtWWP1-NBs, suggesting that the NIG-AtWWP1 interaction may interfere with the NIG pro-viral function associated with its cytosolic localization. Consistent with this assumption, loss of AtWWP1 function cuased plants more susceptible to begomovirus infection, whereas overexpression of AtWWP1 enhanced plant resistance to begomovirus. Furthermore, we found that a mutant version of AtWWP1 defective for NB formation was no longer capable of interacting with and relocating NIG to the nucleus and lost its immune function against begomovirus. The antiviral function of AtWWP1-NBs, however, could be antagonized by viral infection that induced either the disruption or a decrease in the number of AtWWP1-NBs. Collectively, these results led us to propose that AtWWP1 organizes nuclear structures into nuclear foci, which provide intrinsic immunity against begomovirus infection.

Profiling of small RNAs derived from cucumber mosaic virus in infected Nicotiana benthamiana plants by deep sequencing

In plants, RNA silencing is a conserved mechanism underlying antiviral immunity. To investigate antiviral responses in Nicotiana benthamiana, we analyzed the profiles of the virus-derived small RNAs (vsRNAs) in wild-type N. benthamiana and NbRDR6 mutant plants infected with the cucumber mosaic virus (CMV) 2b-deficient mutant. We observed that NbRDR6 regulates RNA silencing by producing vsRNAs that trigger an effective antiviral response, while NbRDR1 may nonredundantly and synergistically function with NbRDR6 to mediate immune responses. The vsRNAs in N. benthamiana and NbRDR6 mutant plants mainly comprised 21 or 22 nucleotides, and mostly consisted of a 5'-terminal adenine. Additionally, NbAGO2 expression was significantly up-regulated in N. benthamiana and NbRDR6 mutant plants, suggesting that NbAGO2 is closely associated with the antiviral activities of vsRNAs. The distribution of vsRNAs in the CMV genome was biased toward RNA sense strands in both N. benthamiana and NbRDR6 mutant plants. These findings indicate the specific and conserved antiviral immunity in Nicotiana benthamiana.

Signaling Mechanisms Underlying Resistance Responses: What Have We Learned, and How Is It Being Applied?

Plants have evolved highly specific mechanisms to resist pathogens including preformed barriers and the induction of elaborate signaling pathways. Induced signaling requires recognition of the pathogen either via conserved pathogen-derived factors or specific pathogen-encoded proteins called effectors. Recognition of these factors by host encoded receptor proteins can result in the elicitation of different tiers of resistance at the site of pathogen infection. In addition, plants induce a type of systemic immunity which is effective at the whole plant level and protects against a broad spectrum of pathogens. Advances in our understanding of pathogen-recognition mechanisms, identification of the underlying molecular components, and their significant conservation across diverse plant species has enabled the development of novel strategies to combat plant diseases. This review discusses key advances in plant defense signaling that have been adapted or have the potential to be adapted for plant protection against microbial diseases.