Plant immune responses against viruses: how does a virus cause disease?

REM1.3's phospho-status defines its plasma membrane nanodomain organization and activity in restricting PVX cell-to-cell movement

Plants respond to pathogens through dynamic regulation of plasma membrane-bound signaling pathways. To date, how the plant plasma membrane is involved in responses to viruses is mostly unknown. Here, we show that plant cells sense the Potato virus X (PVX) COAT PROTEIN and TRIPLE GENE BLOCK 1 proteins and subsequently trigger the activation of a membrane-bound calcium-dependent kinase. We show that the Arabidopsis thaliana CALCIUM-DEPENDENT PROTEIN KINASE 3-interacts with group 1 REMORINs in vivo, phosphorylates the intrinsically disordered N-terminal domain of the Group 1 REMORIN REM1.3, and restricts PVX cell-to-cell movement. REM1.3's phospho-status defines its plasma membrane nanodomain organization and is crucial for REM1.3-dependent restriction of PVX cell-to-cell movement by regulation of callose deposition at plasmodesmata. This study unveils plasma membrane nanodomain-associated molecular events underlying the plant immune response to viruses.

Viruses propagate in plants through membranous channels, called plasmodesmata, linking each cell to its neighboring cell. In this work, we challenge the role of the plasma membrane in the regulation of virus propagation. By studying the dynamics and the activation of a plant-specific protein called REMORIN, we found that the way this protein is organized inside the membrane is crucial to fulfill its function in the immunity of plants against viruses.

Exploring the Diversity of Mechanisms Associated With Plant Tolerance to Virus Infection

Tolerance is defined as an interaction in which viruses accumulate to some degree without causing significant loss of vigor or fitness to their hosts. Tolerance can be described as a stable equilibrium between the virus and its host, an interaction in which each partner not only accommodate trade-offs for survival but also receive some benefits (e.g., protection of the plant against super-infection by virulent viruses; virus invasion of meristem tissues allowing vertical transmission). This equilibrium, which would be associated with little selective pressure for the emergence of severe viral strains, is common in wild ecosystems and has important implications for the management of viral diseases in the field. Plant viruses are obligatory intracellular parasites that divert the host cellular machinery to complete their infection cycle. Highjacking/modification of plant factors can affect plant vigor and fitness. In addition, the toxic effects of viral proteins and the deployment of plant defense responses contribute to the induction of symptoms ranging in severity from tissue discoloration to malformation or tissue necrosis. The impact of viral infection is also influenced by the virulence of the specific virus strain (or strains for mixed infections), the host genotype and environmental conditions. Although plant resistance mechanisms that restrict virus accumulation or movement have received much attention, molecular mechanisms associated with tolerance are less well-understood. We review the experimental evidence that supports the concept that tolerance can be achieved by reaching the proper balance between plant defense responses and virus counter-defenses. We also discuss plant translation repression mechanisms, plant protein degradation or modification pathways and viral self-attenuation strategies that regulate the accumulation or activity of viral proteins to mitigate their impact on the host. Finally, we discuss current progress and future opportunities toward the application of various tolerance mechanisms in the field.

Mitogen-activated protein kinase phosphatase 1 reduces the replication efficiency of Bamboo mosaic virus in Nicotiana benthamiana

In plants, the mitogen-activated protein kinase (MAPK) cascades are the central signaling pathways of the complicated defense network triggered by the perception of pathogen-associated molecular patterns to repel pathogens. The Arabidopsis thaliana MAPK phosphatase 1 (AtMKP1) negatively regulates the activation of MAPKs. Recently, the AtMKP1 homolog of Nicotiana benthamiana (NbMKP1) was found in association with the Bamboo mosaic virus (BaMV) replication complex. This study aimed to investigate the role of NbMKP1 in BaMV multiplication in N. benthamiana. Silencing of NbMKP1 increased accumulations of the BaMV-encoded proteins and the viral genomic RNA, although the same condition reduced the infectivity of Pseudomonas syringae pv. tomato DC3000 in N. benthamiana. On the other hand, overexpression of NbMKP1 decreased the BaMV coat protein accumulation in a phosphatase activity-dependent manner in protoplasts. NbMKP1 also negatively affected the in vitro RNA polymerase activity of the BaMV replication complex. Collectively, the activity of NbMKP1 seems to reduce BaMV multiplication, inconsistent with the negatively regulatory role of MKP1 in MAPK cascades in terms of warding off fungal and bacterial invasion. In addition, silencing of NbMKP1 increased the accumulation of Foxtail mosaic virus but decreased Potato virus X. The discrepant effects exerted by NbMKP1 on different pathogens foresee the difficulty to develop plants with broad-spectrum resistance through genetically manipulating a single player in MAPK cascades.

Gas Exchange Rates Decrease and Leaf Temperature Increases in Nicotiana benthamiana Leaves Transiently Overexpressing Hemagglutinin in an Agrobacterium-Assisted Viral Vector System

In this study, gas exchange characteristics and temperature of Nicotiana benthamiana leaves transiently overexpressing hemagglutinin (HA), an influenza vaccine antigen, with an Agrobacterium tumefaciens-assisted viral vector were investigated. Inoculation of leaves with an empty viral vector not containing the HA gene decreased the net photosynthetic rate (P_n) and transpiration rate (T) from 2 to 3 days post-infiltration (DPI) in the A. tumefaciens suspension. Expression of HA with the vector decreased P_n and T to much lower levels until 4 DPI. Such significant decreases were not observed in leaves infiltrated with suspension of A. tumefaciens not carrying the viral vector or in uninfiltrated leaves. Thus, viral vector inoculation itself decreased P_n and T to a certain extent and the HA expression further decreased them. The decreases in P_n and T in empty vector-inoculated and HA expression vector-inoculated leaves were associated with decreases in stomatal conductance, suggesting that the reduction of gas exchange rates was caused at least in part by stomatal closure. More detailed gas exchange and chlorophyll fluorescence analyses revealed that in HA vector-inoculated leaves, the capacity of ribulose-1,5-bisphosphate carboxylase/oxygenase to assimilate CO₂ and the capacity of photosynthetic electron transport in planta were downregulated, which contributed also to the decrease in P_n. Leaf temperature (LT) increased in viral vector-inoculated leaves, which was associated with the decrease in T. When HA vector-inoculated leaves were grown at air temperatures (ATs) of 21, 23, and 26°C post-infiltration, HA accumulated earlier in leaves and the days required for HA content to attain its peak became shorter, as AT was higher. The highest LT was found 1-2 days earlier than the highest leaf HA content under all post-infiltration AT conditions. This phenomenon could be applicable in a non-destructive technique to detect the optimum harvesting date for individual plants to determine the day when leaf HA content reaches its maximum level, irrespective of spatiotemporal variation of AT, in a plant growth facility.

Joining the Crowd: Integrating Plant Virus Proteins into the Larger World of Pathogen Effectors

The first bacterial and viral avirulence ( avr) genes were cloned in 1984. Although virus and bacterial avr genes were physically isolated in the same year, the questions associated with their characterization after discovery were very different, and these differences had a profound influence on the narrative of host-pathogen interactions for the past 30 years. Bacterial avr proteins were subsequently shown to suppress host defenses, leading to their reclassification as effectors, whereas research on viral avr proteins centered on their role in the viral infection cycle rather than their effect on host defenses. Recent studies that focus on the multifunctional nature of plant virus proteins have shown that some virus proteins are capable of suppression of the same host defenses as bacterial effectors. This is exemplified by the P6 protein of Cauliflower mosaic virus (CaMV), a multifunctional plant virus protein that facilitates several steps in the infection, including modulation of host defenses. This review highlights the modular structure and multifunctional nature of CaMV P6 and illustrates its similarities to other, well-established pathogen effectors.

Brachypodium: A Monocot Grass Model Genus for Plant Biology

The genus Brachypodium represents a model system that is advancing our knowledge of the biology of grasses, including small grains, in the postgenomics era. The most widely used species, Brachypodium distachyon, is a C3 plant that is distributed worldwide. B. distachyon has a small genome, short life cycle, and small stature and is amenable to genetic transformation. Due to the intensive and thoughtful development of this grass as a model organism, it is well-suited for laboratory and field experimentation. The intent of this review is to introduce this model system genus and describe some key outcomes of nearly a decade of research since the first draft genome sequence of the flagship species, B. distachyon, was completed. We discuss characteristics and features of B. distachyon and its congeners that make the genus a valuable model system for studies in ecology, evolution, genetics, and genomics in the grasses, review current hot topics in Brachypodium research, and highlight the potential for future analysis using this system in the coming years.

Epigenetic Changes in the Regulation of Nicotiana tabacum Response to Cucumber Mosaic Virus Infection and Symptom Recovery through Single-Base Resolution Methylomes

Plants have evolved multiple mechanisms to respond to viral infection. These responses have been studied in detail at the level of host immune response and antiviral RNA silencing (RNAi). However, the possibility of epigenetic reprogramming has not been thoroughly investigated. Here, we identified the role of DNA methylation during viral infection and performed reduced representation bisulfite sequencing (RRBS) on tissues of Cucumber mosaic virus (CMV)-infected Nicotiana tabacum at various developmental stages. Differential methylated regions are enriched with CHH sequence contexts, 80% of which are located on the gene body to regulate gene expression in a temporal style. The methylated genes depressed by methyltransferase inhibition largely overlapped with methylated genes in response to viral invasion. Activation in the argonaute protein and depression in methyl donor synthase revealed the important role of dynamic methylation changes in modulating viral clearance and resistance signaling. Methylation-expression relationships were found to be required for the immune response and cellular components are necessary for the proper defense response to infection and symptom recovery.

NtTPN1: a RPP8-like R gene required for Potato virus Y-induced veinal necrosis in tobacco

Potato virus Y (PVY) is one of the most damaging viruses of tobacco. In particular, aggressive necrotic strains (PVYN ) lead to considerable losses in yield. The main source of resistance against PVY is linked to the va locus. However, va-overcoming PVY isolates inducing necrotic symptoms were observed in several countries. In this context, it is important to find va-independent protection strategies. In a previous study, the phenotyping of 162 tobacco varieties revealed 10 accessions that do not carry the va allele and do not exhibit typical PVYN -induced veinal necrosis. Despite the absence of necrotic symptoms, normal viral accumulation in these plants suggests a va-independent mechanism of tolerance to PVYN -induced systemic veinal necrosis. Fine mapping of the genetic determinant(s) was performed in a segregating F2 population. The tolerance trait is inherited as a single recessive gene, and allelism tests demonstrated that eight of the 10 tolerant varieties carry the same determinant. Anchoring the linkage map to the tobacco genome physical map allowed the identification of a RPP8-like R gene, called NtTPN1 (for Nicotiana tabacum Tolerance to PVY-induced Necrosis1), with the same single-nucleotide polymorphism in the eight tolerant accessions. Functional assays using homozygous NtTPN1 EMS mutants confirmed the role of NtTPN1 in the tolerance phenotype. PVYN -induced systemic veinal necrosis in tobacco likely represents an inefficient defense response with hypersensitive response-like characteristics. The identification of NtTPN1 opens breeding options to minimize the impact of emerging and so far uncontrolled va-breaking necrotic PVY isolates.

Polysaccharide Peptide-Induced Virus Resistance Depends on Ca2+ Influx by Increasing the Salicylic Acid Content and Upregulating the Leucine-Rich Repeat Gene in Arabidopsis thaliana

Plant viral diseases cause severe economic losses in agricultural production. The development of biosource-derived antiviral agents provides an alternative strategy to efficiently control plant viral diseases. We previously reported that the exogenous application of polysaccharide peptide (PSP) exerts significant inhibitive effects on Tobacco mosaic virus infection in Nicotiana tabacum. In this study, we studied in additional detail the mechanism by which PSP can induce virus resistance in Arabidopsis thaliana. We found that PSP significantly induced Ca²⁺ influx and increased the accumulation of hydrogen peroxide and salicylic acid (SA) in the A. thaliana cells. A gene with a toll interleukin 1 receptor-nucleotide binding site-leucine-rich repeat domain (LRR) was obtained by RNA sequencing in combination with the screening of the gene-deletion mutants of A. thaliana. The LRR gene was deleted, and the inductive response of A. thaliana to PSP was significantly attenuated after mutation. After the heterologous overexpression of the LRR gene in N. benthamiana, the SA content and PR1 gene expression in N. benthamiana were significantly increased. Through analyses of the LRR gene expression and the ability of A. thaliana to resist Cucumber mosaic virus following the treatments of PSP and PSP + ethyleneglycol-bis (beta-aminoethylether)-N,N'-tetraacetic acid, it was shown that PSP enhanced the virus resistance of A. thaliana by inducing Ca²⁺ influx and subsequently improving expression of the LRR gene, which further increased the SA content.

Viruses and Phytoparasitic Nematodes of Cicer arietinum L.: Biotechnological Approaches in Interaction Studies and for Sustainable Control

Cicer arietinum L. (chickpea) is the world's fourth most widely grown pulse. Chickpea seeds are a primary source of dietary protein for humans, and chickpea cultivation contributes to biological nitrogen fixation in the soil, given its symbiotic relationship with rhizobia. Therefore, chickpea cultivation plays a pivotal role in innovative sustainable models of agro-ecosystems inserted in crop rotation in arid and semi-arid environments for soil improvement and the reduction of chemical inputs. Indeed, the arid and semi-arid tropical zones of Africa and Asia have been primary areas of cultivation and diversification. Yet, nowadays, chickpea is gaining prominence in Canada, Australia, and South America where it constitutes a main ingredient in vegetarian and vegan diets. Viruses and plant parasitic nematodes (PPNs) have been considered to be of minor and local impact in primary areas of cultivation. However, the introduction of chickpea in new environments exposes the crop to these biotic stresses, compromising its yields. The adoption of high-throughput genomic technologies, including genome and transcriptome sequencing projects by the chickpea research community, has provided major insights into genome evolution as well as genomic architecture and domestication. This review summarizes the major viruses and PPNs that affect chickpea cultivation worldwide. We also present an overview of the current state of chickpea genomics. Accordingly, we explore the opportunities that genomics, post-genomics and novel editing biotechnologies are offering in order to understand chickpea diseases and stress tolerance and to design innovative control strategies.

Stereoisomers of the Bacterial Volatile Compound 2,3-Butanediol Differently Elicit Systemic Defense Responses of Pepper against Multiple Viruses in the Field

The volatile compound 2,3-butanediol, which is produced by certain strains of root-associated bacteria, consists of three stereoisomers, namely, two enantiomers (2R,3R- and 2S,3S-butanediol) and one meso compound (2R,3S-butanediol). The ability of 2,3-butanediol to induce plant resistance against pathogenic fungi and bacteria has been investigated; however, little is known about its effects on induced resistance against viruses in plants. To investigate the effects of 2,3-butanediol on plant systemic defense against viruses, we evaluated the disease control capacity of each of its three stereoisomers in pepper. Specifically, we investigated the optimal concentration of 2,3-butanediol to use for disease control against Cucumber mosaic virus and Tobacco mosaic virus in the greenhouse and examined the effects of drench application of these compounds in the field. In the field trial, treatment with 2R,3R-butanediol and 2R,3S-butanediol significantly reduced the incidence of naturally occurring viruses compared with 2S,3S-butanediol and control treatments. In addition, 2R,3R-butanediol treatment induced the expression of plant defense marker genes in the salicylic acid, jasmonic acid, and ethylene signaling pathways to levels similar to those of the benzothiadiazole-treated positive control. This study reports the first field trial showing that specific stereoisomers of 2,3-butanediol trigger plant immunity against multiple viruses.

AV2 protein of tomato leaf curl Palampur virus promotes systemic necrosis in Nicotiana benthamiana and interacts with host Catalase2

Tomato leaf curl Palampur virus (ToLCPalV) is a whitefly-transmitted, bipartite begomovirus. Here, we demonstrated that ectopic expression of AV2 from a Potato virus X (PVX)-based vector accelerated systemic necrosis and reactive oxygen species (ROS) accumulation in Nicotiana benthamiana. Furthermore, 10 amino acids from N-terminal region of AV2 were found to be associated with the systemic necrosis symptom/phenotype. Mutational studies of ToLCPalV infectious clones lacking the AV2 revealed that AV2 is essential for the systemic movement of DNA-A, symptom severity and viral DNA accumulation. In a yeast two-hybrid assay, Catalase2 (Cat2) was found to associate with AV2 protein. Further, silencing of Cat2 resulted in appearance of necrotic lesions on N. benthamiana and these plants were highly susceptible to ToLCPalV infection in comparison to control plants. Infection ToLCPalV on Solanum lycopersicum resulted in downregulation of Cat2 transcripts, followed by accumulation of ROS and stress marker transcripts. The AV2 protein also suppressed virus-induced gene silencing (VIGS) of the Phytoene desaturase (PDS) gene. Our results show that AV2 is essential for the pathogenicity, systemic movement and suppression of gene silencing in the host. Altogether, our findings suggest that interactions between AV2 and Cat2 might play a crucial role in the establishment of ToLCPalV infection.

De novo generation of helper virus-satellite chimera RNAs results in disease attenuation and satellite sequence acquisition in a host-dependent manner

Panicum mosaic virus (PMV) is a helper RNA virus for satellite RNAs (satRNAs) and a satellite virus (SPMV). Here, we describe modifications that occur at the 3'-end of a satRNA of PMV, satS. Co-infections of PMV+satS result in attenuation of the disease symptoms induced by PMV alone in Brachypodium distachyon and proso millet. The 375 nt satS acquires ~100-200 nts from the 3'-end of PMV during infection and is associated with decreased abundance of the PMV RNA and capsid protein in millet. PMV-satS chimera RNAs were isolated from native infections of St. Augustinegrass and switchgrass. Phylogenetic analyses revealed that the chimeric RNAs clustered according to the host species from which they were isolated. Additionally, the chimera satRNAs acquired non-viral "linker" sequences in a host-specific manner. These results highlight the dynamic regulation of viral pathogenicity by satellites, and the selective host-dependent, sequence-based pressures for driving satRNA generation and genome compositions.

The resistance of sour orange to Citrus tristeza virus is mediated by both the salicylic acid and RNA silencing defence pathways

Citrus tristeza virus (CTV) induces in the field the decline and death of citrus varieties grafted on sour orange (SO) rootstock, which has forced the use of alternative decline-tolerant rootstocks in affected countries, despite the highly desirable agronomic features of the SO rootstock. Declining citrus plants display phloem necrosis below the bud union. In addition, SO is minimally susceptible to CTV compared with other citrus varieties, suggesting partial resistance of SO to CTV. Here, by silencing different citrus genes with a Citrus leaf blotch virus-based vector, we have examined the implication of the RNA silencing and salicylic acid (SA) defence pathways in the resistance of SO to CTV. Silencing of the genes RDR1, NPR1 and DCL2/DCL4, associated with these defence pathways, enhanced virus spread and accumulation in SO plants in comparison with non-silenced controls, whereas silencing of the genes NPR3/NPR4, associated with the hypersensitive response, produced a slight decrease in CTV accumulation and reduced stunting of SO grafted on CTV-infected rough lemon plants. We also found that the CTV RNA silencing suppressors p20 and p23 also suppress the SA signalling defence, with the suppressor activity being higher in the most virulent isolates.

A Violaxanthin Deepoxidase Interacts with a Viral Suppressor of RNA Silencing to Inhibit Virus Amplification

RNA silencing plays a critical role against viral infection. To counteract this antiviral silencing, viruses have evolved various RNA silencing suppressors. Meanwhile, plants have evolved counter-counter defense strategies against RNA silencing suppression (RSS). In this study, the violaxanthin deepoxidase protein of maize (Zea mays), ZmVDE, was shown to interact specifically with the helper component-proteinase (HC-Pro; a viral RNA silencing suppressor) of Sugarcane mosaic virus (SCMV) via its mature protein region by yeast two-hybrid assay, which was confirmed by coimmunoprecipitation in Nicotiana benthamiana cells. It was demonstrated that amino acids 101 to 460 in HC-Pro and the amino acid glutamine-292 in ZmVDE mature protein were essential for this interaction. The mRNA levels of ZmVDE were down-regulated 75% to 65% at an early stage of SCMV infection. Interestingly, ZmVDE, which normally localized in the chloroplasts and cytoplasm, could relocalize to HC-Pro-containing aggregate bodies in the presence of HC-Pro alone or SCMV infection. In addition, ZmVDE could attenuate the RSS activity of HC-Pro in a specific protein interaction-dependent manner. Subsequently, transient silencing of the ZmVDE gene facilitated SCMV RNA and coat protein accumulation. Taken together, our results suggest that ZmVDE interacts with SCMV HC-Pro and attenuates its RSS activity, contributing to decreased SCMV accumulation. This study demonstrates that a host factor can be involved in secondary defense responses against viral infection in monocot plants.

The C2 protein of tomato leaf curl Taiwan virus is a pathogenicity determinant that interferes with expression of host genes encoding chromomethylases

Tomato (Solanum lycopersicum) is one of the most important crops worldwide and is severely affected by geminiviruses. Tomato leaf curl Taiwan virus (ToLCTWV), belonging to the geminiviruses, was isolated in Taiwan and causes tremendous crop loss. The geminivirus-encoded C2 proteins are crucial for a successful interaction between the virus and host plants. However, the exact functions of the viral C2 protein of ToLCTWV have not been investigated. We analyzed the molecular function(s) of the C2 protein by transient or stable expression in tomato cv. Micro-Tom and Nicotiana benthamiana. Severe stunting of tomato and N. benthamiana plants infected with ToLCTWV was observed. Expression of ToLCTWV C2-green fluorescent protein (GFP) fusion protein was predominately located in the nucleus and contributed to activation of a coat protein promoter. Notably, the C2-GFP fluorescence was distributed in nuclear aggregates. Tomato and N. benthamiana plants inoculated with potato virus X (PVX)-C2 displayed chlorotic lesions and stunted growth. PVX-C2 elicited hypersensitive responses accompanied by production of reactive oxygen species in N. benthamiana plants, which suggests that the viral C2 was a potential recognition target to induce host-defense responses. In tomato and N. benthamiana, ToLCTWV C2 was found to interfere with expression of genes encoding chromomethylases. N. benthamiana plants with suppressed NbCMT3-2 expression were more susceptible to ToLCTWV infection. Transgenic N. benthamiana plants expressing the C2 protein showed decreased expression of the NbCMT3-2 gene and pNbCMT3-2::GUS (β-glucuronidase) promoter activity. C2 protein is an important pathogenicity determinant of ToLCTWV and interferes with host components involved in DNA methylation.

Transcriptomics reveals multiple resistance mechanisms against cotton leaf curl disease in a naturally immune cotton species, Gossypium arboreum

Cotton leaf curl disease (CLCuD), caused by cotton leaf curl viruses (CLCuVs), is among the most devastating diseases in cotton. While the widely cultivated cotton species Gossypium hirsutum is generally susceptible, the diploid species G. arboreum is a natural source for resistance against CLCuD. However, the influence of CLCuD on the G. arboreum transcriptome and the interaction of CLCuD with G. arboreum remains to be elucidated. Here we have used an RNA-Seq based study to analyze differential gene expression in G. arboreum under CLCuD infestation. G. arboreum plants were infested by graft inoculation using a CLCuD infected scion of G. hirsutum. CLCuD infested asymptomatic and symptomatic plants were analyzed with RNA-seq using an Illumina HiSeq. 2500. Data analysis revealed 1062 differentially expressed genes (DEGs) in G. arboreum. We selected 17 genes for qPCR to validate RNA-Seq data. We identified several genes involved in disease resistance and pathogen defense. Furthermore, a weighted gene co-expression network was constructed from the RNA-Seq dataset that indicated 50 hub genes, most of which are involved in transport processes and might have a role in the defense response of G. arboreum against CLCuD. This fundamental study will improve the understanding of virus-host interaction and identification of important genes involved in G. arboreum tolerance against CLCuD.

A mobile loop near the active site acts as a switch between the dual activities of a viral protease/deubiquitinase

The positive-strand RNA virus Turnip yellow mosaic virus (TYMV) encodes an ovarian tumor (OTU)-like protease/deubiquitinase (PRO/DUB) protein domain involved both in proteolytic processing of the viral polyprotein through its PRO activity, and in removal of ubiquitin chains from ubiquitylated substrates through its DUB activity. Here, the crystal structures of TYMV PRO/DUB mutants and molecular dynamics simulations reveal that an idiosyncratic mobile loop participates in reversibly constricting its unusual catalytic site by adopting "open", "intermediate" or "closed" conformations. The two cis-prolines of the loop form a rigid flap that in the most closed conformation zips up against the other side of the catalytic cleft. The intermediate and closed conformations also correlate with a reordering of the TYMV PRO/DUB catalytic dyad, that then assumes a classical, yet still unusually mobile, OTU DUB alignment. Further structure-based mutants designed to interfere with the loop's mobility were assessed for enzymatic activity in vitro and in vivo, and were shown to display reduced DUB activity while retaining PRO activity. This indicates that control of the switching between the dual PRO/DUB activities resides prominently within this loop next to the active site. Introduction of mutations into the viral genome revealed that the DUB activity contributes to the extent of viral RNA accumulation both in single cells and in whole plants. In addition, the conformation of the mobile flap was also found to influence symptoms severity in planta. Such mutants now provide powerful tools with which to study the specific roles of reversible ubiquitylation in viral infection.

Viruses have much smaller genomes than their hosts. Consequently, they often encode proteins which are multifunctional. For instance, some viral proteases have a dual function, being also deubiquitinases, i.e. enzymes capable of removing ubiquitin tags grafted onto proteins and that often target them for destruction. The protease and deubiquitinase activities share a single active site that is used alternately for one function or the other, but how this switch between activities may be regulated is presently unknown. To answer this question, we studied a simple plant virus that is a useful model system for these complex molecular biology phenomena, and that encodes a simplified protease/deubiquitinase. Here, thanks to a combination of structural and functional analyses, we managed to decouple the two activities, killing the deubiquitinase activity while preserving the protease one. This successful decoupling relies on our discovery that a loop inserted next to the active site is mobile, and can thus act as a switch between the two activities. This result allowed us to demonstrate the importance of the specific deubiquinase activity in viral multiplication. In addition, viral symptoms were also severely affected by mutations affecting the loop mobility. Our data provide powerful tools for further studies, that may also be relevant for more complex or medically relevant viruses.

Hexanoic Acid Treatment Prevents Systemic MNSV Movement in Cucumis melo Plants by Priming Callose Deposition Correlating SA and OPDA Accumulation

Unlike fungal and bacterial diseases, no direct method is available to control viral diseases. The use of resistance-inducing compounds can be an alternative strategy for plant viruses. Here we studied the basal response of melon to Melon necrotic spot virus (MNSV) and demonstrated the efficacy of hexanoic acid (Hx) priming, which prevents the virus from systemically spreading. We analysed callose deposition and the hormonal profile and gene expression at the whole plant level. This allowed us to determine hormonal homeostasis in the melon roots, cotyledons, hypocotyls, stems and leaves involved in basal and hexanoic acid-induced resistance (Hx-IR) to MNSV. Our data indicate important roles of salicylic acid (SA), 12-oxo-phytodienoic acid (OPDA), jasmonic-isoleucine, and ferulic acid in both responses to MNSV. The hormonal and metabolites balance, depending on the time and location associated with basal and Hx-IR, demonstrated the reprogramming of plant metabolism in MNSV-inoculated plants. The treatment with both SA and OPDA prior to virus infection significantly reduced MNSV systemic movement by inducing callose deposition. This demonstrates their relevance in Hx-IR against MNSV and a high correlation with callose deposition. Our data also provide valuable evidence to unravel priming mechanisms by natural compounds.

Hop/Sti1 - A Two-Faced Cochaperone Involved in Pattern Recognition Receptor Maturation and Viral Infection

Perception of pathogens by host pattern recognition receptors (PRRs) or R proteins is a prerequisite to promote successful immune responses. The Hsp70/Hsp90 organizing protein Hop/Sti1, a multifunctional cochaperone, has been implicated in the maturation of a receptor-like kinase (RLK) necessary for chitin sensing. However, it remains unknown whether Hop/Sti1 is generally participating in PRR genesis. Using RNA-interference (RNAi), we silenced Hop/Sti1 expression in Nicotiana tabacum to gain further insight into the role of the cochaperone in plant defense responses. As expected, transgenic plants do not respond to chitin treatment anymore. In contrast to this, trafficking and functionality of the flagellin PRR FLS2 were unaltered, suggesting a selective involvement of Hop/Sti1 during PRR maturation. Furthermore, Hop/Sti1 was identified as a cellular determinant of Potato virus Y (PVY) symptom development in tobacco, since PVY was able to accumulate to near wild-type level without provoking the usual veinal necrosis phenotype. In addition, typical antiviral host defense responses were suppressed in the transgenic plants. These data suggest that perception of PVY is dependent on Hop/Sti1-mediated receptor maturation, while viral symptoms represent a failing attempt to restrict PVY spread. In addition, Hop/Sti1 colocalized with virus-induced membrane aggregates in wild-type plants. The retention of Hop/Sti1 in potential viral replication complexes suggests a role during viral translation/replication, explaining why RNAi-lines do not exhibit increased susceptibility to PVY. This study provides evidence for a dual role of Hop/Sti1 in PRR maturation and pathogen perception as well as in promoting viral proliferation.

A viral protein promotes host SAMS1 activity and ethylene production for the benefit of virus infection

Ethylene plays critical roles in plant development and biotic stress response, but the mechanism of ethylene in host antiviral response remains unclear. Here, we report that Rice dwarf virus (RDV) triggers ethylene production by stimulating the activity of S-adenosyl-L-methionine synthetase (SAMS), a key component of the ethylene synthesis pathway, resulting in elevated susceptibility to RDV. RDV-encoded Pns11 protein specifically interacted with OsSAMS1 to enhance its enzymatic activity, leading to higher ethylene levels in both RDV-infected and Pns11-overexpressing rice. Consistent with a counter-defense role for ethylene, Pns11-overexpressing rice, as well as those overexpressing OsSAMS1, were substantially more susceptible to RDV infection, and a similar effect was observed in rice plants treated with an ethylene precursor. Conversely, OsSAMS1-knockout mutants, as well as an osein2 mutant defective in ethylene signaling, resisted RDV infection more robustly. Our findings uncover a novel mechanism which RDV manipulates ethylene biosynthesis in the host plants to achieve efficient infection.

Rice provides food for billions of people all over the world, but diseases caused by plant-infecting viruses cause serious risks to the production of rice. As a result, there is an urgent demand for developing new and impactful ways to help defend rice plants from harmful viruses. Toward this goal, it will be important to better understand how viruses actually cause diseases in plants.

Plants make chemicals known as hormones to control their own development, and hormone production is often disturbed when viruses infect rice plants. Many viruses cause infected plants to make more of a gaseous hormone called ethylene, which benefits the viruses. Yet, it is still not known how virus infection induces the production of more ethylene.

Zhao, Hong et al. have exposed rice plants to infection with a virus called rice dwarf virus. Infected plants made more ethylene than normal, which did indeed help the virus to infect. Further experiments then showed that an enzyme that makes one of the building blocks needed to produce ethylene became more active after infection with this virus. Next, Zhao, Hong et al. engineered rice plants to make more or less of this building block – which is known as S-adenosyl-L-methionine or SAM for short. Plants with too much SAM were less able to defend themselves against the virus, while plants that lacked SAM were better able to fight off viral infection.

Zhao, Hong et al. suggest that engineering rice plants to make less of the SAM-producing enzyme could make them more resistant to viruses. Further work will also be needed to find out why ethylene benefits viral infection, and to confirm whether ethylene also performs similar roles when rice is infected with other viruses.

Identification of MAPKs as signal transduction components required for the cell death response during compatible infection by the synergistic pair Potato virus X-Potato virus Y

Systemic necrosis is one of the most severe symptoms caused in compatible plant-virus interactions and shares common features with the hypersensitive response (HR). Mitogen-activated protein kinase (MAPK) cascades are associated with responses to compatible and incompatible host-virus interactions. Here, we show that virus-induced gene silencing of the Nicotiana benthamiana MAPK genes salicylic acid-induced protein kinase (SIPK) and wound-induced protein kinase (WIPK), and the MAPK kinase (MAPKK) genes MEK1 and MKK1, partially compromised the HR-like response induced by the synergistic interaction of Potato virus X with Potato virus Y (PVX-PVY). Nevertheless, ameliorated cell death induced by PVX-PVY in the MAPK(K)-silenced plants did not facilitate virus accumulation in systemically infected leaves. Dual silencing of SIPK and of the oxylipin biosynthetic gene 9-Lipoxygenase showed that the latter was epistatic to SIPK in response to PVX-PVY infection. These findings demonstrate that SIPK, WIPK, MEK1 and MKK1 function as positive regulators of PVX-PVY-induced cell death.

Effect of temperature post viral vector inoculation on the amount of hemagglutinin transiently expressed in Nicotiana benthamiana leaves

Transient gene expression in whole plants by using viral vectors is promising as a rapid, mass production system for biopharmaceutical proteins. Recent studies have indicated that plant growth conditions such as air temperature markedly influence the accumulation levels of target proteins. Here, we investigated time course of the amount of recombinant hemagglutinin (HA), a vaccine antigen of influenza virus, in leaves of Nicotiana benthamiana plants grown at 20°C or 25°C post viral vector inoculation. The HA content per unit of leaf biomass increased and decreased from 4 to 6 days post inoculation at 20°C and 25°C, respectively, irrespective of the subcellular localization of HA. The overall HA contents were higher when HA was targeted to the endoplasmic reticulum (ER) rather than the apoplast. Necrosis of leaf tissues was specifically observed in plants inoculated with the ER-targeting vector and grown at 25°C. With the ER-targeting vector, the maximum HA contents at 20°C and 25°C were recorded at 6 and 4 days post inoculation, respectively, and were comparable to each other. HA contents thereafter decreased at both temperatures; the rate of reduction appeared faster at 25°C than at 20°C. From a practical point of view, our results indicate that the strategy of targeting HA to the ER, growing plants at a lower temperature of 20°C, and harvesting leaves at around a week after vector inoculation should be implemented to obtain a high HA yield stably and efficiently.

Multiplexed Gene Editing and Protein Overexpression Using a Tobacco mosaic virus Viral Vector

Development of CRISPR/Cas9 transient gene editing screening tools in plant biology has been hindered by difficulty of delivering high quantities of biologically active single guide RNAs (sgRNAs). Furthermore, it has been largely accepted that in vivo generated sgRNAs need to be devoid of extraneous nucleotides, which has limited sgRNA expression by delivery vectors. Here, we increased cellular concentrations of sgRNA by transiently delivering sgRNAs using a Tobacco mosaic virus-derived vector (TRBO) designed with 5' and 3' sgRNA proximal nucleotide-processing capabilities. To demonstrate proof-of-principle, we used the TRBO-sgRNA delivery platform to target GFP in Nicotiana benthamiana (16c) plants, and gene editing was accompanied by loss of GFP expression. Surprisingly, indel (insertions and deletions) percentages averaged nearly 70% within 7 d postinoculation using the TRBO-sgRNA constructs, which retained 5' nucleotide overhangs. In contrast, and in accordance with current models, in vitro Cas9 cleavage assays only edited DNA when 5' sgRNA nucleotide overhangs were removed, suggesting a novel processing mechanism is occurring in planta. Since the Cas9/TRBO-sgRNA platform demonstrated sgRNA flexibility, we targeted the N. benthamiana NbAGO1 paralogs with one sgRNA and also multiplexed two sgRNAs using a single TRBO construct, resulting in indels in three genes. TRBO-mediated expression of an RNA transcript consisting of an sgRNA adjoining a GFP protein coding region produced indels and viral-based GFP overexpression. In conclusion, multiplexed delivery of sgRNAs using the TRBO system offers flexibility for gene expression and editing and uncovered novel aspects of CRISPR/Cas9 biology.

Human Norovirus and Its Surrogates Induce Plant Immune Response in Arabidopsis thaliana and Lactuca sativa

Human norovirus is the leading cause of foodborne illness worldwide with the majority of outbreaks linked to fresh produce and leafy greens. It is essential that we thoroughly understand the type of relationship and interactions that take place between plants and human norovirus to better utilize control strategies to reduce transmission of norovirus in the field onto plants harvested for human consumption. In this study the expression of gene markers for the salicylic acid (SA) and jasmonic acid (JA) plant defense pathways was measured and compared in romaine lettuce (Lactuca sativa) and Arabidopsis thaliana Col-0 plants that were inoculated with Murine Norovirus-1, Tulane Virus, human norovirus GII.4, or Hank's Balanced Salt Solution (control). Genes involving both the SA and JA pathways were expressed in both romaine lettuce and A. thaliana for all three viruses, as well as controls. Studies, including gene expression of SA- and JA-deficient A. thaliana mutant lines, suggest that the JA pathway is more likely involved in the plant immune response to human norovirus. This research provides the first pieces of information regarding how foodborne viruses interact with plants in the preharvest environment.

Prospects for engineering and improvement of cross-protective virus strains

Mild strain cross-protection is currently an important method for the production of high quality plant products; despite challenge from severe virus isolates the initial protecting strain precludes symptom development. The mechanism of cross-protection is not yet resolved as RNA silencing does not sufficiently explain the phenomenon. Six requirements have been put forward to ensure long-lasting protection. We propose two additional requirements for effective and durable mild strain cross-protection; mild strains based on knowledge of the mechanism and consideration of impacts to consumers. Future research on predicting phenotype from genotype and understanding virus-plant and virus-vector interactions will enable improvement of cross-protective strains. Shared international databases of whole ecosystem interactions across a wide range of virus patho- and symbiotic-systems will form the basis for making step-change advances towards our collective ability to engineer and improve mild strain cross-protection.

Complete nucleotide sequences and virion particle association of two satellite RNAs of panicum mosaic virus

Over six decades ago, panicum mosaic virus (PMV) was identified as the first viral pathogen of cultivated switchgrass (Panicum virgatum). Subsequently, PMV was demonstrated to support the replication of both a satellite RNA virus (SPMV) and satellite RNA (satRNA) agents during natural infections of host grasses. In this study, we report the isolation and full-length sequences of two PMV satRNAs identified in 1988 from St. Augustinegrass (Stenotaphrum secundatum) and centipedegrass (Eremochloa ophiuroides) hosts. Each of these satellites have sequence relatedness at their 5'- and 3'-ends. In addition, satC has a region of ∼100 nt complementary to the 3'-end of the PMV genome. These agents are associated with purified virions of SPMV infections. Additionally, satS and satC RNAs contain conserved in-frame open reading frames in the complementary-sense sequences that could potentially generate 6.6- and 7.9-kDa proteins, respectively. In protoplasts and plants satS is infectious, when co-inoculated with the PMV RNA alone or PMV+SPMV RNAs, and negatively affects their accumulation.

Nitric oxide associated protein 1 is associated with chloroplast perturbation and disease symptoms in Nicotiana benthamiana infected with South African cassava mosaic virus

Nitric oxide associated 1 (NOA1) in plants is a cyclic GTPase involved in protein translation in the chloroplast and has been indirectly linked to nitric oxide (NO) accumulation and response to biotic stress. The association between NOA1 and NO accumulation in Arabidopsis noa1 mutants has been linked to the inability of noa1 mutants to accumulate carbon reserves such as fumarate, leading to chloroplast dysfunction and a pale green leaf phenotype. To understand the role played by NOA1 in response to South African cassava mosaic virus infection in Nicotiana benthamiana, the expression of NbNOA1 and the accumulation of NO in leaf samples was compared between south african cassava mosaic (SACMV)-infected and mock-infected plants at 14 and 28 dpi. Real-time qPCR was used to measure SACMV viral load which increased significantly by 20% from 14 to 28 dpi as chlorosis and symptom severity progressed. At 14 and 28 dpi, NbNOA1 expression was significantly lower than mock inoculated plants (2-fold lower at 14 dpi, p-value=0.01 and 5-fold lower at 28, p-value=0.00). At 14 dpi, NO accumulation remained unchanged in infected leaf tissue compared to mock inoculated, while at 28 dpi, NO accumulation was 40% lower (p-value=0.01). At 28 dpi, the decrease in NbNOA1 expression and NO accumulation was accompanied by chloroplast dysfunction, evident from the significant reduction in chlorophylls a and b and carotenoids in SACMV-infected leaves. Furthermore, the expression of chloroplast translation factors (chloroplast RNA binding, chloroplast elongation factor G, translation elongation factor Tu, translation initiation factor 3-2, plastid-specific ribosomal protein 6 and plastid ribosome recycling factor) were found to be repressed in infected N. benthamiana. GC-MS analysis showed a decrease in fumarate and an increase in glucose in SACMV-infected N. benthamiana in comparison to mock samples suggesting a decrease in carbon stores. Collectively, these results provide evidence that in response to SACMV infection, a decrease in photopigments and carbon stores, accompanied by an increase in glucose and decrease in fumarate, leads to a decline in NbNOA1expression and NO levels. This is manifested by suppressed translation factors and disruption of chloroplast function, thereby contributing to chlorotic disease symptoms.

Live Cell Imaging Reveals the Relocation of dsRNA Binding Proteins Upon Viral Infection

Viral infection triggers a range of plant responses such as the activation of the RNA interference (RNAi) pathway. The double-stranded RNA binding (DRB) proteins DRB3 and DRB4 are part of this pathway and aid in defending against DNA and RNA viruses, respectively. Using live cell imaging, we show that DRB2, DRB3, and DRB5 relocate from their uniform cytoplasmic distribution to concentrated accumulation in nascent viral replication complexes (VRC) that develop following cell invasion by viral RNA. Inactivation of the DRB3 gene in Arabidopsis by T-DNA insertion rendered these plants less able to repress RNA viral replication. We propose a model for the early stages of virus defense in which DRB2, DRB3, and DRB5 are invasion sensors that relocate to nascent VRC, where they bind to viral RNA and inhibit virus replication.

Translationally controlled tumour protein (TCTP) from tomato and Nicotiana benthamiana is necessary for successful infection by a potyvirus

Translationally controlled tumour protein (TCTP) is a ubiquitously distributed protein in eukaryotes, involved in the regulation of several processes, including cell cycle progression, cell growth, stress protection, apoptosis and maintenance of genomic integrity. Its expression is induced during the early stages of tomato (Solanum lycopersicum) infection by the potyvirus Pepper yellow mosaic virus (PepYMV, a close relative of Potato virus Y). Tomato TCTP is a protein of 168 amino acids, which contains all the conserved domains of the TCTP family. To study the effects of TCTP silencing in PepYMV infection, Nicotiana benthamiana plants were silenced by virus-induced gene silencing (VIGS) and transgenic tomato plants silenced for TCTP were obtained. In the early stages of infection, both tomato and N. benthamiana silenced plants accumulated less virus than control plants. Transgenic tomato plants showed a drastic reduction in symptoms and no viral accumulation at 14 days post-inoculation. Subcellular localization of TCTP was determined in healthy and systemically infected N. benthamiana leaves. TCTP was observed in both the nuclei and cytoplasm of non-infected cells, but only in the cytoplasm of infected cells. Our results indicate that TCTP is a growth regulator necessary for successful PepYMV infection and that its localization is altered by the virus, probably to favour the establishment of virus infection. A network with putative interactions that may occur between TCTP and Arabidopsis thaliana proteins was built. This network brings together experimental data of interactions that occur in other eukaryotes and helps us to discuss the possibilities of TCTP involvement in viral infection.

Comprehensive Transcriptome Analyses Reveal that Potato Spindle Tuber Viroid Triggers Genome-Wide Changes in Alternative Splicing, Inducible trans-Acting Activity of Phased Secondary Small Interfering RNAs, and Immune Responses

Many pathogens express noncoding RNAs (ncRNAs) during infection processes. In the most extreme case, pathogenic ncRNAs alone (such as viroids) can infect eukaryotic organisms, leading to diseases. While a few pathogenic ncRNAs have been implicated in regulating gene expression, the functions of most pathogenic ncRNAs in host-pathogen interactions remain unclear. Here, we employ potato spindle tuber viroid (PSTVd) infecting tomato as a system to dissect host interactions with pathogenic ncRNAs, using comprehensive transcriptome analyses. We uncover various new activities in regulating gene expression during PSTVd infection, such as genome-wide alteration in alternative splicing of host protein-coding genes, enhanced guided-cleavage activities of a host microRNA, and induction of the trans-acting function of phased secondary small interfering RNAs. Furthermore, we reveal that PSTVd infection massively activates genes involved in plant immune responses, mainly those in the calcium-dependent protein kinase and mitogen-activated protein kinase cascades, as well as prominent genes involved in hypersensitive responses, cell wall fortification, and hormone signaling. Intriguingly, our data support a notion that plant immune systems can respond to pathogenic ncRNAs, which has broad implications for providing new opportunities for understanding the complexity of immune systems in differentiating "self" and "nonself," as well as lay the foundation for resolving the long-standing question regarding the pathogenesis mechanisms of viroids and perhaps other infectious RNAs.IMPORTANCE Numerous pathogens, including viruses, express pathogenic noncoding transcripts during infection. In the most extreme case, pathogenic noncoding RNAs alone (i.e., viroids) can cause disease in plants. While some work has demonstrated that pathogenic noncoding RNAs interact with host factors for function, the biological significance of pathogenic noncoding RNAs in host-pathogen interactions remains largely unclear. Here, we apply comprehensive genome-wide analyses of plant-viroid interactions and discover several novel molecular activities underlying nuclear-replicating viroid infection processes in plants, including effects on the expression and function of host noncoding transcripts, as well as the alternative splicing of host protein-coding genes. Importantly, we show that plant immunity is activated upon infection of a nuclear-replicating viroid, which is a new concept that helps to understand viroid-based pathogenesis. Our finding has broad implications for understanding the complexity of host immune systems and the diverse functions of noncoding RNAs.

The six Tomato yellow leaf curl virus genes expressed individually in tomato induce different levels of plant stress response attenuation

Tomato yellow leaf curl virus (TYLCV) is a begomovirus infecting tomato plants worldwide. TYLCV needs a healthy host environment to ensure a successful infection cycle for long periods. Hence, TYLCV restrains its destructive effect and induces neither a hypersensitive response nor cell death in infected tomatoes. On the contrary, TYLCV counteracts cell death induced by other factors, such as inactivation of HSP90 functionality. Suppression of plant death is associated with the inhibition of the ubiquitin 26S proteasome degradation and with a deactivation of the heat shock transcription factor HSFA2 pathways (including decreased HSP17 levels). The goal of the current study was to find if the individual TYLCV genes were capable of suppressing HSP90-dependent death and HSFA2 deactivation. The expression of C2 (C3 and CP to a lesser extent) caused a decrease in the severity of death phenotypes, while the expression of V2 (C1 and C4 to a lesser extent) strengthened cell death. However, C2 or V2 markedly affected stress response under conditions of viral infection. The downregulation of HSFA2 signaling, initiated by the expression of C1 and V2, was detected in the absence of virus infection, but was enhanced in infected plants, while CP and C4 mitigated HSFA2 levels only in the infected tomatoes. The dependence of analyzed plant stress response suppression on the interaction of the expressed genes with the environment created by the whole virus infection was more pronounced than on the expression of individual TYLCV genes.

Deep sequencing leads to the identification of eukaryotic translation initiation factor 5A as a key element in Rsv1-mediated lethal systemic hypersensitive response to Soybean mosaic virus infection in soybean

Rsv1, a single dominant resistance locus in soybean, confers extreme resistance to the majority of Soybean mosaic virus (SMV) strains, but is susceptible to the G7 strain. In Rsv1-genotype soybean, G7 infection provokes a lethal systemic hypersensitive response (LSHR), a delayed host defence response. The Rsv1-mediated LSHR signalling pathway remains largely unknown. In this study, we employed a genome-wide investigation to gain an insight into the molecular interplay between SMV G7 and Rsv1-genotype soybean. Small RNA (sRNA), degradome and transcriptome sequencing analyses were used to identify differentially expressed genes (DEGs) and microRNAs (DEMs) in response to G7 infection. A number of DEGs, DEMs and microRNA targets, and the interaction network of DEMs and their target mRNAs responsive to G7 infection, were identified. Knock-down of one of the identified DEGs, the eukaryotic translation initiation factor 5A (eIF5A), diminished the LSHR and enhanced viral accumulation, suggesting the essential role of eIF5A in the G7-induced, Rsv1-mediated LSHR signalling pathway. This work provides an in-depth genome-wide analysis of high-throughput sequencing data, and identifies multiple genes and microRNA signatures that are associated with the Rsv1-mediated LSHR.

The Involvement of Heat Shock Proteins in the Establishment of Tomato Yellow Leaf Curl Virus Infection

Tomato yellow leaf curl virus (TYLCV), a begomovirus, induces protein aggregation in infected tomatoes and in its whitefly vector Bemisia tabaci. The interactions between TYLCV and HSP70 and HSP90 in plants and vectors are necessity for virus infection to proceed. In infected host cells, HSP70 and HSP90 are redistributed from a soluble to an aggregated state. These aggregates contain, together with viral DNA/proteins and virions, HSPs and components of the protein quality control system such as ubiquitin, 26S proteasome subunits, and the autophagy protein ATG8. TYLCV CP can form complexes with HSPs in tomato and whitefly. Nonetheless, HSP70 and HSP90 play different roles in the viral cell cycle in the plant host. In the infected host cell, HSP70, but not HSP90, participates in the translocation of CP from the cytoplasm into the nucleus. Viral amounts decrease when HSP70 is inhibited, but increase when HSP90 is downregulated. In the whitefly vector, HSP70 impairs the circulative transmission of TYLCV; its inhibition increases transmission. Hence, the efficiency of virus acquisition by whiteflies depends on the functionality of both plant chaperones and their cross-talk with other protein mechanisms controlling virus-induced aggregation.

An Atypical Thioredoxin Imparts Early Resistance to Sugarcane Mosaic Virus in Maize

Sugarcane mosaic virus (SCMV) causes substantial losses of grain yield and forage biomass in susceptible maize worldwide. A major quantitative trait locus, Scmv1, has been identified to impart strong resistance to SCMV at the early infection stage. Here, we demonstrate that ZmTrxh, encoding an atypical h-type thioredoxin, is the causal gene at Scmv1, and that its transcript abundance correlated strongly with maize resistance to SCMV. ZmTrxh alleles, whether they are resistant or susceptible, share the identical coding/proximal promoter regions, but vary in the upstream regulatory regions. ZmTrxh lacks two canonical cysteines in the thioredoxin active-site motif and exists uniquely in the maize genome. Because of this, ZmTrxh is unable to reduce disulfide bridges but possesses a strong molecular chaperone-like activity. ZmTrxh is dispersed in maize cytoplasm to suppress SCMV viral RNA accumulation. Moreover, ZmTrxh-mediated maize resistance to SCMV showed no obvious correlation with the salicylic acid- and jasmonic acid-related defense signaling pathways. Taken together, our results indicate that ZmTrxh exhibits a distinct defense profile in maize resistance to SCMV, differing from previously characterized dominant or recessive potyvirus resistance genes.

Plant immunity against viruses: antiviral immune receptors in focus

BACKGROUND

Among the environmental limitations that affect plant growth, viruses cause major crop losses worldwide and represent serious threats to food security. Significant advances in the field of plant-virus interactions have led to an expansion of potential strategies for genetically engineered resistance in crops during recent years. Nevertheless, the evolution of viral virulence represents a constant challenge in agriculture that has led to a continuing interest in the molecular mechanisms of plant-virus interactions that affect disease or resistance.

SCOPE AND CONCLUSION

This review summarizes the molecular mechanisms of the antiviral immune system in plants and the latest breakthroughs reported in plant defence against viruses. Particular attention is given to the immune receptors and transduction pathways in antiviral innate immunity. Plants counteract viral infection with a sophisticated innate immune system that resembles the non-viral pathogenic system, which is broadly divided into pathogen-associated molecular pattern (PAMP)-triggered immunity and effector-triggered immunity. An additional recently uncovered virus-specific defence mechanism relies on host translation suppression mediated by a transmembrane immune receptor. In all cases, the recognition of the virus by the plant during infection is central for the activation of these innate defences, and, conversely, the detection of host plants enables the virus to activate virulence strategies. Plants also circumvent viral infection through RNA interference mechanisms by utilizing small RNAs, which are often suppressed by co-evolving virus suppressors. Additionally, plants defend themselves against viruses through hormone-mediated defences and activation of the ubiquitin-26S proteasome system (UPS), which alternatively impairs and facilitates viral infection. Therefore, plant defence and virulence strategies co-evolve and co-exist; hence, disease development is largely dependent on the extent and rate at which these opposing signals emerge in host and non-host interactions. A deeper understanding of plant antiviral immunity may facilitate innovative biotechnological, genetic and breeding approaches for crop protection and improvement.

Autophagy functions as an antiviral mechanism against geminiviruses in plants

Autophagy is an evolutionarily conserved process that recycles damaged or unwanted cellular components, and has been linked to plant immunity. However, how autophagy contributes to plant immunity is unknown. Here we reported that the plant autophagic machinery targets the virulence factor βC1 of Cotton leaf curl Multan virus (CLCuMuV) for degradation through its interaction with the key autophagy protein ATG8. A V32A mutation in βC1 abolished its interaction with NbATG8f, and virus carrying βC1V32A showed increased symptoms and viral DNA accumulation in plants. Furthermore, silencing of autophagy-related genes ATG5 and ATG7 reduced plant resistance to the DNA viruses CLCuMuV, Tomato yellow leaf curl virus, and Tomato yellow leaf curl China virus, whereas activating autophagy by silencing GAPC genes enhanced plant resistance to viral infection. Thus, autophagy represents a novel anti-pathogenic mechanism that plays an important role in antiviral immunity in plants.

Harnessing host ROS-generating machinery for the robust genome replication of a plant RNA virus

As sessile organisms, plants have to accommodate to rapid changes in their surrounding environment. Reactive oxygen species (ROS) act as signaling molecules to transduce biotic and abiotic stimuli into plant stress adaptations. It is established that a respiratory burst oxidase homolog B of Nicotiana benthamiana (NbRBOHB) produces ROS in response to microbe-associated molecular patterns to inhibit pathogen infection. Plant viruses are also known as causative agents of ROS induction in infected plants; however, the function of ROS in plant-virus interactions remains obscure. Here, we show that the replication of red clover necrotic mosaic virus (RCNMV), a plant positive-strand RNA [(+)RNA] virus, requires NbRBOHB-mediated ROS production. The RCNMV replication protein p27 plays a pivotal role in this process, redirecting the subcellular localization of NbRBOHB and a subgroup II calcium-dependent protein kinase of N. benthamiana (NbCDPKiso2) from the plasma membrane to the p27-containing intracellular aggregate structures. p27 also induces an intracellular ROS burst in an RBOH-dependent manner. NbCDPKiso2 was shown to be an activator of the p27-triggered ROS accumulations and to be required for RCNMV replication. Importantly, this RBOH-derived ROS is essential for robust viral RNA replication. The need for RBOH-derived ROS was demonstrated for the replication of another (+)RNA virus, brome mosaic virus, suggesting that this characteristic is true for plant (+)RNA viruses. Collectively, our findings revealed a hitherto unknown viral strategy whereby the host ROS-generating machinery is diverted for robust viral RNA replication.

Translational control in plant antiviral immunity

Due to the limited coding capacity of viral genomes, plant viruses depend extensively on the host cell machinery to support the viral life cycle and, thereby, interact with a large number of host proteins during infection. Within this context, as plant viruses do not harbor translation-required components, they have developed several strategies to subvert the host protein synthesis machinery to produce rapidly and efficiently the viral proteins. As a countermeasure against infection, plants have evolved defense mechanisms that impair viral infections. Among them, the host-mediated translational suppression has been characterized as an efficient mean to restrict infection. To specifically suppress translation of viral mRNAs, plants can deploy susceptible recessive resistance genes, which encode translation initiation factors from the eIF4E and eIF4G family and are required for viral mRNA translation and multiplication. Additionally, recent evidence has demonstrated that, alternatively to the cleavage of viral RNA targets, host cells can suppress viral protein translation to silence viral RNA. Finally, a novel strategy of plant antiviral defense based on suppression of host global translation, which is mediated by the transmembrane immune receptor NIK1 (nuclear shuttle protein (NSP)-Interacting Kinase1), is discussed in this review.

The coat protein of Alfalfa mosaic virus interacts and interferes with the transcriptional activity of the bHLH transcription factor ILR3 promoting salicylic acid-dependent defence signalling response

During virus infection, specific viral component-host factor interaction elicits the transcriptional reprogramming of diverse cellular pathways. Alfalfa mosaic virus (AMV) can establish a compatible interaction in tobacco and Arabidopsis hosts. We show that the coat protein (CP) of AMV interacts directly with transcription factor (TF) ILR3 of both species. ILR3 is a basic helix-loop-helix (bHLH) family member of TFs, previously proposed to participate in diverse metabolic pathways. ILR3 has been shown to regulate NEET in Arabidopsis, a critical protein in plant development, senescence, iron metabolism and reactive oxygen species (ROS) homeostasis. We show that the AMV CP-ILR3 interaction causes a fraction of this TF to relocate from the nucleus to the nucleolus. ROS, pathogenesis-related protein 1 (PR1) mRNAs, salicylic acid (SA) and jasmonic acid (JA) contents are increased in healthy Arabidopsis loss-of-function ILR3 mutant (ilr3.2) plants, which implicates ILR3 in the regulation of plant defence responses. In AMV-infected wild-type (wt) plants, NEET expression is reduced slightly, but is induced significantly in ilr3.2 mutant plants. Furthermore, the accumulation of SA and JA is induced in Arabidopsis wt-infected plants. AMV infection in ilr3.2 plants increases JA by over 10-fold, and SA is reduced significantly, indicating an antagonist crosstalk effect. The accumulation levels of viral RNAs are decreased significantly in ilr3.2 mutants, but the virus can still systemically invade the plant. The AMV CP-ILR3 interaction may down-regulate a host factor, NEET, leading to the activation of plant hormone responses to obtain a hormonal equilibrium state, where infection remains at a level that does not affect plant viability.

iTRAQ analysis of the tobacco leaf proteome reveals that RNA-directed DNA methylation (RdDM) has important roles in defense against geminivirus-betasatellite infection

Geminiviruses have caused serious losses in crop production. To investigate the mechanisms underlying host defenses against geminiviruses, an isobaric tags for relative and absolute quantification (iTRAQ)-based quantitative proteomic approach was used to explore the expression profiles of proteins in Nicotiana benthamiana (N. benthamiana) leaves in response to tomato yellow leaf curl China virus (TYLCCNV) with its betasatellite (TYLCCNB) at an early phase. In total, 4155 proteins were identified and 272 proteins were changed differentially in response to TYLCCNV/TYLCCNB infection. Bioinformatics analysis indicated that S-adenosyl-l-methionine cycle II was the most significantly up-regulated biochemical process during TYLCCNV/TYLCCNB infection. The mRNA levels of three proteins in S-adenosyl-l-methionine cycle II were further analyzed by qPCR, each was found significantly up-regulated in TYLCCNV/TYLCCNB-infected N. benthamiana. This result suggested a strong promotion of the biosynthesis of available methyl groups during geminivirus infections. We further tested the potential role of RdDM in N. benthamiana by virus-induced gene silencing (VIGS) and found that a disruption in RdDM resulted in more severe infectious symptoms and higher accumulation of viral DNA after TYLCCNV/TYLCCNB infection. Although the precise functions of these proteins still need to be determined, our proteomic results enhance the understanding of plant antiviral mechanisms.

BIOLOGICAL SIGNIFICANCE

One of the major limitations to crop growth in the worldwide is the prevalence of geminiviruses. They are able to infect food and cash crops and cause serious crop failures and economic losses worldwide, especially in Africa and Asia. Tomato yellow leaf curl China virus (TYLCCNV), which causes severe viral diseases in China, is a monopartite geminivirus associated with the betasatellite (TYLCCNB). However, the mechanisms underlying the TYLCCNV/TYLCCNB defense in plants are still not fully understood at the molecular level. In this study, the combined proteomic, bioinformatic and VIGS analyses revealed that TYLCCNV/TYLCCNB invasion caused complex proteomic alterations in the leaves of N. benthamiana involving the processes of stress and defense, energy production, photosynthesis, protein homeostasis, metabolism, cell structure, signal transduction, transcription, transportation, and cell growth/division. Promotion of available methyl groups via the S-adenosyl-l-methionine cycle II pathway in N. benthamiana appeared crucial for antiviral responses. These findings enhance our understanding in the proteomic aspects of host antiviral defenses against geminiviruses, and also demonstrate that the combination of proteomics with bioinformatics and VIGS analysis is an effective approach to investigate systemic plant responses to geminiviruses and to shed light on plant-virus interactions.

Antiviral Defenses in Plants through Genome Editing

Plant-virus interactions based-studies have contributed to increase our understanding on plant resistance mechanisms, providing new tools for crop improvement. In the last two decades, RNA interference, a post-transcriptional gene silencing approach, has been used to induce antiviral defenses in plants with the help of genetic engineering technologies. More recently, the new genome editing systems (GES) are revolutionizing the scope of tools available to confer virus resistance in plants. The most explored GES are zinc finger nucleases, transcription activator-like effector nucleases, and clustered regularly interspaced short palindromic repeats/Cas9 endonuclease. GES are engineered to target and introduce mutations, which can be deleterious, via double-strand breaks at specific DNA sequences by the error-prone non-homologous recombination end-joining pathway. Although GES have been engineered to target DNA, recent discoveries of GES targeting ssRNA molecules, including virus genomes, pave the way for further studies programming plant defense against RNA viruses. Most of plant virus species have an RNA genome and at least 784 species have positive ssRNA. Here, we provide a summary of the latest progress in plant antiviral defenses mediated by GES. In addition, we also discuss briefly the GES perspectives in light of the rebooted debate on genetic modified organisms (GMOs) and the current regulatory frame for agricultural products involving the use of such engineering technologies.

Immune Receptors and Co-receptors in Antiviral Innate Immunity in Plants

Plants respond to pathogens using an innate immune system that is broadly divided into PTI (pathogen-associated molecular pattern- or PAMP-triggered immunity) and ETI (effector-triggered immunity). PTI is activated upon perception of PAMPs, conserved motifs derived from pathogens, by surface membrane-anchored pattern recognition receptors (PRRs). To overcome this first line of defense, pathogens release into plant cells effectors that inhibit PTI and activate effector-triggered susceptibility (ETS). Counteracting this virulence strategy, plant cells synthesize intracellular resistance (R) proteins, which specifically recognize pathogen effectors or avirulence (Avr) factors and activate ETI. These coevolving pathogen virulence strategies and plant resistance mechanisms illustrate evolutionary arms race between pathogen and host, which is integrated into the zigzag model of plant innate immunity. Although antiviral immune concepts have been initially excluded from the zigzag model, recent studies have provided several lines of evidence substantiating the notion that plants deploy the innate immune system to fight viruses in a manner similar to that used for non-viral pathogens. First, most R proteins against viruses so far characterized share structural similarity with antibacterial and antifungal R gene products and elicit typical ETI-based immune responses. Second, virus-derived PAMPs may activate PTI-like responses through immune co-receptors of plant PTI. Finally, and even more compelling, a viral Avr factor that triggers ETI in resistant genotypes has recently been shown to act as a suppressor of PTI, integrating plant viruses into the co-evolutionary model of host-pathogen interactions, the zigzag model. In this review, we summarize these important progresses, focusing on the potential significance of antiviral immune receptors and co-receptors in plant antiviral innate immunity. In light of the innate immune system, we also discuss a newly uncovered layer of antiviral defense that is specific to plant DNA viruses and relies on transmembrane receptor-mediated translational suppression for defense.

Subcellular Localization and Detection of Tobacco mosaic virus ORF6 Protein by Immunoelectron Microscopy

Members of the genus Tobamovirus represent one of the best-characterized groups of plant positive, single stranded RNA viruses. Previous studies have shown that genomes of some tobamoviruses contain not only genes coding for coat protein, movement protein, and the cistron coding for different domains of RNA-polymerase, but also a gene, named ORF6, coding for a poorly conserved small protein. The amino acid sequences of ORF6 proteins encoded by different tobamoviruses are highly divergent. The potential role of ORF6 proteins in replication of tobamoviruses still needs to be elucidated. In this study, using biochemical and immunological methods, we have shown that ORF6 peptide is accumulated after infection in case of two isolates of Tobacco mosaic virus strain U1 (TMV-U1 common and TMV-U1 isolate A15). Unlike virus particles accumulating in the cytoplasm, the product of the ORF6 gene is found mainly in nuclei, which correlates with previously published data about transient expression of ORF6 isolated from TMV-U1. Moreover, we present new data showing the presence of ORF6 genes in genomes of several tobamoviruses. For example, in the genomes of other members of the tobamovirus subgroup 1, including Rehmannia mosaic virus, Paprika mild mottle virus, Tobacco mild green mosaic virus, Tomato mosaic virus, Tomato mottle mosaic virus, and Nigerian tobacco latent virus, sequence comparisons revealed the existence of a similar open reading frame like ORF6 of TMV.

Ageratum enation virus Infection Induces Programmed Cell Death and Alters Metabolite Biosynthesis in Papaver somniferum

A previously unknown disease which causes severe vein thickening and inward leaf curl was observed in a number of opium poppy (Papaver somniferum L.) plants. The sequence analysis of full-length viral genome and associated betasatellite reveals the occurrence of Ageratum enation virus (AEV) and Ageratum leaf curl betasatellite (ALCB), respectively. Co-infiltration of cloned agroinfectious DNAs of AEV and ALCB induces the leaf curl and vein thickening symptoms as were observed naturally. Infectivity assay confirmed this complex as the cause of disease and also satisfied the Koch's postulates. Comprehensive microscopic analysis of infiltrated plants reveals severe structural anomalies in leaf and stem tissues represented by unorganized cell architecture and vascular bundles. Moreover, the characteristic blebs and membranous vesicles formed due to the virus-induced disintegration of the plasma membrane and intracellular organelles were also present. An accelerated nuclear DNA fragmentation was observed by Comet assay and confirmed by TUNEL and Hoechst dye staining assays suggesting virus-induced programmed cell death. Virus-infection altered the biosynthesis of several important metabolites. The biosynthesis potential of morphine, thebaine, codeine, and papaverine alkaloids reduced significantly in infected plants except for noscapine whose biosynthesis was comparatively enhanced. The expression analysis of corresponding alkaloid pathway genes by real time-PCR corroborated well with the results of HPLC analysis for alkaloid perturbations. The changes in the metabolite and alkaloid contents affect the commercial value of the poppy plants.