The dual role of the potyvirus P3 protein of Turnip mosaic virus as a symptom and avirulence determinant in brassicas

P3N-PIPO but not P3 is the avirulence determinant in melon carrying the Wmr resistance against watermelon mosaic virus, although they contain a common genetic determinant

Viruses employ a series of diverse translational strategies to expand their coding capacity, which produces viral proteins with common domains and entangles virus-host interactions. P3N-PIPO, which is a transcriptional slippage product from the P3 cistron, is a potyviral protein dedicated to intercellular movement. Here, we show that P3N-PIPO from watermelon mosaic virus (WMV) triggers cell death when transiently expressed in Cucumis melo accession PI 414723 carrying the Wmr resistance gene. Surprisingly, expression of the P3N domain, shared by both P3N-PIPO and P3, can alone induce cell death, whereas expression of P3 fails to activate cell death in PI 414723. Confocal microscopy analysis revealed that P3N-PIPO targets plasmodesmata (PD) and P3N associates with PD, while P3 localizes in endoplasmic reticulum in melon cells. We also found that mutations in residues L35, L38, P41, and I43 of the P3N domain individually disrupt the cell death induced by P3N-PIPO, but do not affect the PD localization of P3N-PIPO. Furthermore, WMV mutants with L35A or I43A can systemically infect PI 414723 plants. These key residues guide us to discover some WMV isolates potentially breaking the Wmr resistance. Through searching the NCBI database, we discovered some WMV isolates with variations in these key sites, and one naturally occurring I43V variation enables WMV to systemically infect PI 414723 plants. Taken together, these results demonstrate that P3N-PIPO, but not P3, is the avirulence determinant recognized by Wmr, although the shared N terminal P3N domain can alone trigger cell death.IMPORTANCEThis work reveals a novel viral avirulence (Avr) gene recognized by a resistance (R) gene. This novel viral Avr gene is special because it is a transcriptional slippage product from another virus gene, which means that their encoding proteins share the common N-terminal domain but have distinct C-terminal domains. Amazingly, we found that it is the common N-terminal domain that determines the Avr-R recognition, but only one of the viral proteins can be recognized by the R protein to induce cell death. Next, we found that these two viral proteins target different subcellular compartments. In addition, we discovered some virus isolates with variations in the common N-terminal domain and one naturally occurring variation that enables the virus to overcome the resistance. These results show how viral proteins with common domains interact with a host resistance protein and provide new evidence for the arms race between plants and viruses.

Complete genome characterization of chilli veinal mottle virus associated with mosaic and mottling disease of tomato and development of LAMP assay for quick detection

Chilli veinal mottle virus (ChiVMV) is a potyvirus known to cause havoc in many solanaceous crops. Samples from tomato plants exhibiting typical mosaic and mottling symptoms in two locations from farmers' fields were collected and tested using DAC ELISA for the presence of ChiVMV and other viruses known to infect tomato. ChiVMV Gauribidanur isolate from infected tomato was mechanically inoculated to Datura metel, Nicotiana tabacum, Nicotiana benthamiana, Nicotiana glutinosa, chilli, and tomato plants which exhibited systemic mosaic and mottling symptoms 10 days post-inoculation. This results were further confirmed by RT-PCR and DAC ELISA using CP gene-specific primers and ChiVMV antisera, respectively. Transmission electron microscopy revealed the presence of long filamentous particles (800 × 11 nm) resembling viruses in the Potyviridae family. The complete genome of ChiVMV comprised 9716 nucleotides except for poly A tail, with a predicted open reading frame spanning 9270 nucleotides encoding polyproteins of 3089 amino acids. Comparative analysis revealed that ChiVMV-tomato isolates reported across the world shared maximum nucleotide identity (93-96.7%) with chilli isolates from India and Pakistan. These results were well supported by sequence demarcation analysis. Further, the Neibhor-Net network analysis of the complete genome of ChiVMV-tomato, along with other host isolates, formed a reticular network phylogenetic tree suggesting recombination events. Subsequently, RDP5 detected intra-specific recombination breakpoints at the positions 1656-5666 nucleotides with major parent ChiVMV (MN508960) Uravakonda and minor parent ChiVMV (MN508956) with a significant average p value of 1.905 × 10-22. The LAMP assay using ChiVMV-specific primers resulted in ladder-like amplified products on electrophoresed gel and a distinct red colour pattern with hydroxy naphthalene blue, indicating a positive reaction for the presence of ChiVMV in infected tomato samples. To validate LAMP-designed primers, RNA extracted from ChiVMV-infected tomato, chilli, datura, and tobacco samples were subjected to LAMP assay and it accurately detected the presence of ChiVMV in infected plant samples. Overall, this study provides holistic information of ChiVMV infecting tomato, spanning diagnosis, transmission, genetic characterization, and detection of recombination events, which collectively contribute to effective disease management, crop protection, and informed decision-making in agricultural practices.

Soybean 40S Ribosomal Protein S8 (GmRPS8) Interacts with 6K1 Protein and Contributes to Soybean Susceptibility to Soybean Mosaic Virus

Soybean mosaic virus (SMV), a member of Potyvirus, is the most destructive and widespread viral disease in soybean production. Our earlier studies identified a soybean 40S ribosomal protein S8 (GmRPS8) using the 6K1 protein of SMV as the bait to screen a soybean cDNA library. The present study aims to identify the interactions between GmRPS8 and SMV and characterize the role of GmRPS8 in SMV infection in soybean. Expression analysis showed higher SMV-induced GmRPS8 expression levels in a susceptible soybean cultivar when compared with a resistant cultivar, suggesting that GmRPS8 was involved in the response to SMV in soybean. Subcellular localization showed that GmRPS8 was localized in the nucleus. Moreover, the yeast two-hybrid (Y2H) experiments showed that GmRPS8 only interacted with 6K1 among the eleven proteins encoded by SMV. The interaction between GmRPS8 and 6K1 was further verified by a bimolecular fluorescence complementation (BiFC) assay, and the interaction was localized in the nucleus. Furthermore, knockdown of GmRPS8 by a virus-induced gene silencing (VIGS) system retarded the growth and development of soybeans and inhibited the accumulation of SMV in soybeans. Together, these results showed that GmRPS8 interacts with 6K1 and contributes to soybean susceptibility to SMV. Our findings provide new insights for understanding the role of GmRPS8 in the SMV infection cycle, which could help reveal potyviral replication mechanisms.

6K1, NIa-VPg, NIa-Pro, and CP of Wheat Streak Mosaic Virus Are Collective Determinants of Wheat Streak Mosaic Disease in Wheat

Wheat streak mosaic virus (WSMV; genus Tritimovirus, family Potyviridae) is the causal agent of the most economically important wheat streak mosaic disease of wheat (Triticum aestivum) in the Great Plains region of the United States. WSMV determinants responsible for wheat streak mosaic disease in wheat are unknown. Triticum mosaic virus (TriMV), a wheat-infecting virus, was used as an expression vector for the transient expression of each of the WSMV-encoded cistrons in wheat. WSMV-encoded 6K1, NIa-VPg, NIa-Pro, and CP cistrons in TriMV elicited symptoms specific to different stages of wheat streak mosaic disease without significantly affecting the genomic RNA accumulation. WSMV 6K1 produced early wheat streak mosaic disease-like symptoms of severe chlorotic streaks and patches. NIa-VPg and CP caused severe chlorotic streaks, followed by moderate stunting (only with NIa-VPg) of wheat, mimicking early- and mid-stage symptoms of wheat streak mosaic disease. WSMV NIa-Pro caused mild chlorotic streaks, followed by dark green leaves with severe stunting, representing the late symptoms of wheat streak mosaic disease. Collectively, these data suggest that cumulative effects of WSMV-encoded 6K1, NIa-VPg, NIa-Pro, and CP are responsible for different stages of wheat streak mosaic disease symptoms in wheat. Furthermore, deletion analysis of wheat streak mosaic disease determinants revealed that complete 6K1 and NIa-Pro, amino acids 3 to 60 and 121 to 197 of NIa-VPg, and amino acids 101 to 294 of CP are responsible for wheat streak mosaic disease-like symptoms in wheat. This study suggests that management strategies for wheat streak mosaic disease in wheat should target WSMV determinants of the disease phenotype.

Soybean Mosaic Virus 6K1 Interactors Screening and GmPR4 and GmBI1 Function Characterization

Host proteins are essential during virus infection, and viral factors must target numerous host factors to complete their infectious cycle. The mature 6K1 protein of potyviruses is required for viral replication in plants. However, the interaction between 6K1 and host factors is poorly understood. The present study aims to identify the host interacting proteins of 6K1. Here, the 6K1 of Soybean mosaic virus (SMV) was used as the bait to screen a soybean cDNA library to gain insights about the interaction between 6K1 and host proteins. One hundred and twenty-seven 6K1 interactors were preliminarily identified, and they were classified into six groups, including defense-related, transport-related, metabolism-related, DNA binding, unknown, and membrane-related proteins. Then, thirty-nine proteins were cloned and merged into a prey vector to verify the interaction with 6K1, and thirty-three of these proteins were confirmed to interact with 6K1 by yeast two-hybrid (Y2H) assay. Of the thirty-three proteins, soybean pathogenesis-related protein 4 (GmPR4) and Bax inhibitor 1 (GmBI1) were chosen for further study. Their interactions with 6K1 were also confirmed by bimolecular fluorescence complementation (BiFC) assay. Subcellular localization showed that GmPR4 was localized to the cytoplasm and endoplasmic reticulum (ER), and GmBI1 was located in the ER. Moreover, both GmPR4 and GmBI1 were induced by SMV infection, ethylene and ER stress. The transient overexpression of GmPR4 and GmBI1 reduced SMV accumulation in tobacco, suggesting their involvement in the resistance to SMV. These results would contribute to exploring the mode of action of 6K1 in viral replication and improve our knowledge of the role of PR4 and BI1 in SMV response.

Identification of viral genes involved in pepper mottle virus replication and symptom development in Nicotiana benthamiana

Pepper mottle virus (PepMoV) infects primarily Capsicum species, including pepper and bell pepper which are important vegetable and spice crops in Korea. We have previously collected 13 PepMoV isolates from nine regions comprising five provinces, causing different symptoms on inoculated indicator host plants in Korea. To further identify the responsible symptom determinant(s) and explore viral protein functions of PepMoV, two out of 13 isolates, including 134 and 205136, were used in this study. Isolate 134 causes necrosis and yellowing, while 205136 causes severe mottle and yellowing symptoms on Nicotiana benthamiana. All chimeric and site-directed mutants contain the PepMoV 134 genome as a backbone with specific regions switched for those from counterparts of PepMoV 205136. Effects of all mutants compared with 134 after inoculation onto N. benthamiana by agroinfiltration. Results from our study provide direct evidence that the helper component-proteinase (HC-Pro) and the nuclear inclusion protein b (NIb)-coat protein (CP) regions are involved in virus accumulation and symptom determinants. In addition, we mapped to amino acid residues tyrosine, glycine, and leucine at position 360, 385, and 527, respectively, in the HC-Pro region participate in faster viral accumulation or movement in the plant. The residue valine at position 2773 of NIb plays an essential role in isolate 134 symptom development. As part of this study, we seek to gain insight into viral factors involved in the PepMoV infection cycle and a better understanding of plant-virus interactions. These findings complement the insufficiency of the gene function study of the PepMoV virus and provide a novel perspective for the protein function study of the Potyvirus.

Genome-Wide Identification and Expression Analysis of eIF Family Genes from Brassica rapa in Response to TuMV Resistance

Brassica rapa is one of the most important leafy vegetables worldwide, and has a long history of cultivation. However, it has not been possible to completely control the damage of turnip mosaic virus (TuMV), a serious virus in B. rapa, to production. In this study, the genome-wide identification and expression detection of eIF family genes from B. rapa in response to TuMV resistance were analyzed, including the identification of eIF family genes, chromosomal distribution, three-dimensional (3D) structure and sequence logo analyses, and the expression characterization as well as differential metabolite analysis of eIF family genes in resistant/susceptible lines, which may further prove the whole-genome tripling (WGT) event in B. rapa evolution and provide evidence for the functional redundancy and functional loss of multicopy eIF genes in evolution. A qRT-PCR analysis revealed that the relative expressions of eIF genes in a susceptible line (80461) were higher than those in a resistant line (80124), which may prove that, when TuMV infects host plants, the eIF genes can combine with the virus mRNA 5′ end cap structure and promote the initiation of virus mRNA translation in the susceptible B. rapa line. In addition, the metabolite substances were detected, the differences in metabolites between disease-resistant and disease-susceptible plants were mainly manifested by altered compounds such as flavonoids, jasmonic acid, salicylic acid, ketones, esters, etc., which inferred that the different metabolite regulations of eIF family genes and reveal the resistance mechanisms of eIF genes against TuMV in brassica crops. This study may lay a new theoretical foundation for revealing eIF family gene resistance to TuMV in B. rapa, as well as advancing our understanding of virus–host interactions.

Mapping and identification of a new potential dominant resistance gene to turnip mosaic virus in Brassica rapa

By constructing an F₂ population, a new potential dominant resistance gene to TuMV in Brassica rapa was mapped and identified. Brassica rapa is the most widely grown vegetable crop in China, and turnip mosaic virus (TuMV) is a great threat to its production. Hence, it is a very important work to excavate more and novel resistance genes in B. rapa. In this study, the resistant line B80124 and the susceptible line B80450 were used to construct the F₂ populations, and through genetic analysis, the resistance to TuMV was found to be controlled by a dominant gene. Bulked segregant analysis sequence (BSA-seq) was used for the primary mapping, and an intersection (22.25-25.03 Mb) was obtained. After fine mapping using single nucleotide polymorphisms (SNP) markers, the candidate region was narrowed to 330 kb between the SNP markers A06S11 and A06S14, including eight genes relating to disease resistance. Using the transcriptome analysis and sequence identification, BraA06g035130.3C was screened as the final candidate gene, and it contained two deletion mutations, leading to frameshift in the susceptible line B80450. In addition, the phylogenetic analysis, hydrophilia and hydrophobicity analysis, subcellular location prediction analysis, amino acid bias analysis, and 3D modeling structures of BraA06g035130.3C were conducted to predict its functions. This study was conducive to the identification of a new TuMV resistance gene in B. rapa, which is of important scientific significance and application value for the improvement of TuMV resistance traits and molecular design breeding for Brassica crops.

Pepper Mottle Virus and Its Host Interactions: Current State of Knowledge

Pepper mottle virus (PepMoV) is a destructive pathogen that infects various solanaceous plants, including pepper, bell pepper, potato, and tomato. In this review, we summarize what is known about the molecular characteristics of PepMoV and its interactions with host plants. Comparisons of symptom variations caused by PepMoV isolates in plant hosts indicates a possible relationship between symptom development and genetic variation. Researchers have investigated the PepMoV–plant pathosystem to identify effective and durable genes that confer resistance to the pathogen. As a result, several recessive pvr or dominant Pvr resistance genes that confer resistance to PepMoV in pepper have been characterized. On the other hand, the molecular mechanisms underlying the interaction between these resistance genes and PepMoV-encoded genes remain largely unknown. Our understanding of the molecular interactions between PepMoV and host plants should be increased by reverse genetic approaches and comprehensive transcriptomic analyses of both the virus and the host genes.

Research Advances in Potyviruses: From the Laboratory Bench to the Field

Potyviruses (viruses in the genus Potyvirus, family Potyviridae) constitute the largest group of known plant-infecting RNA viruses and include many agriculturally important viruses that cause devastating epidemics and significant yield losses in many crops worldwide. Several potyviruses are recognized as the most economically important viral pathogens. Therefore, potyviruses are more studied than other groups of plant viruses. In the past decade, a large amount of knowledge has been generated to better understand potyviruses and their infection process. In this review, we list the top 10 economically important potyviruses and present a brief profile of each. We highlight recent exciting findings on the novel genome expression strategy and the biological functions of potyviral proteins and discuss recent advances in molecular plant-potyvirus interactions, particularly regarding the coevolutionary arms race. Finally, we summarize current disease control strategies, with a focus on biotechnology-based genetic resistance, and point out future research directions.

From Player to Pawn: Viral Avirulence Factors Involved in Plant Immunity

In the plant immune system, according to the 'gene-for-gene' model, a resistance (R) gene product in the plant specifically surveils a corresponding effector protein functioning as an avirulence (Avr) gene product. This system differs from other plant-pathogen interaction systems, in which plant R genes recognize a single type of gene or gene family because almost all virus genes with distinct structures and functions can also interact with R genes as Avr determinants. Thus, research conducted on viral Avr-R systems can provide a novel understanding of Avr and R gene product interactions and identify mechanisms that enable rapid co-evolution of plants and phytopathogens. In this review, we intend to provide a brief overview of virus-encoded proteins and their roles in triggering plant resistance, and we also summarize current progress in understanding plant resistance against virus Avr genes. Moreover, we present applications of Avr gene-mediated phenotyping in R gene identification and screening of segregating populations during breeding processes.

Resistance to Turnip Mosaic Virus in the Family Brassicaceae

Resistance to diseases caused by turnip mosaic virus (TuMV) in crop species of the family Brassicaceae has been studied extensively, especially in members of the genus Brassica. The variation in response observed on resistant and susceptible plants inoculated with different isolates of TuMV is due to a combination of the variation in the plant resistome and the variation in the virus genome. Here, we review the breadth of this variation, both at the level of variation in TuMV sequences, with one eye towards the phylogeny and evolution of the virus, and another eye towards the nature of the various responses observed in susceptible vs. different types of resistance responses. The analyses of the viral genomes allowed comparisons of pathotyped viruses on particular indicator hosts to produce clusters of host types, while the inclusion of phylogeny data and geographic location allowed the formation of the host/geographic cluster groups, the derivation of both of which are presented here. Various studies on resistance determination in particular brassica crops sometimes led to further genetic studies, in many cases to include the mapping of genes, and in some cases to the actual identification of the genes. In addition to summarizing the results from such studies done in brassica crops, as well as in radish and Arabidopsis (the latter as a potential source of candidate genes for brassica and radish), we also summarize work done using nonconventional approaches to obtaining resistance to TuMV.

Association between flower stalk elongation, an Arabidopsis developmental trait, and the subcellular location and movement dynamics of the nonstructural protein P3 of Turnip mosaic virus

Virus infections affect plant developmental traits but this aspect of the interaction has not been extensively studied so far. Two strains of Turnip mosaic virus differentially affect Arabidopsis development, especially flower stalk elongation, which allowed phenotypical, cellular, and molecular characterization of the viral determinant, the P3 protein. Transiently expressed wild-type green fluorescent protein-tagged P3 proteins of both strains and selected mutants of them revealed important differences in their behaviour as endoplasmic reticulum (ER)-associated peripheral proteins flowing along the reticulum, forming punctate accumulations. Three-dimensional (3D) model structures of all expressed P3 proteins were computationally constructed through I-TASSER protein structure predictions, which were used to compute protein surfaces and map electrostatic potentials to characterize the effect of amino acid changes on features related to protein interactions and to phenotypical and subcellular results. The amino acid at position 279 was the main determinant affecting stalk development. It also determined the speed of ER-flow of the expressed proteins and their final location. A marked change in the protein surface electrostatic potential correlated with changes in subcellular location. One single amino acid in the P3 viral protein determines all the analysed differential characteristics between strains differentially affecting flower stalk development. A model proposing a role of the protein in the intracellular movement of the viral replication complex, in association with the viral 6K2 protein, is proposed. The type of association between both viral proteins could differ between the strains.

Soybean Endo-1,3-Beta-Glucanase (GmGLU) Interaction With Soybean mosaic virus-Encoded P3 Protein May Contribute to the Intercelluar Movement

Soybean mosaic virus (SMV), a member of the genus Potyvirus, is a prevalent and devastating viral pathogen in soybean-growing regions worldwide. Potyvirus-encoded P3 protein is reported to participate in virus replication, movements, and pathogenesis. This study provides evidence that the soybean (Glycine max) endo-1,3-beta-glucanase protein (designated as GmGLU) interacts with SMV-P3 by using a yeast two-hybrid system to screen a soybean cDNA library. A bimolecular fluorescence complementation assay further confirmed the interaction, which occurred on the cytomembrane in Nicotiana benthamiana cells. Subcellular localization experiment indicated that GmGLU localized in cytomembrane and could co-localized at PD with PD marker. The transient expression of GmGLU promoted the coupling of Turnip mosaic virus replication and cell-to-cell movement in N. benthamiana. Meanwhile, qRT-PCR experiment demonstrated that the expression of GmGLU which involved in callose regulation increased under SMV infection. Under SMV infection, callose deposition at PD was observed obviously by staining with aniline blue, which raise a physical barrier restricting cell-to-cell movement of SMV. When overexpression the GmGLU into the leaves under SMV infection, the callose induced by SMV was degraded. Coexpression the GmGLU and SMV in soybean leaves, callose was not found, whereas a large amount of callose deposition on soybean leaves which were only under SMV infection. The results show that GmGLU can degrade the callose induced by SMV infection and indicate that GmGLU may be an essential host factor involvement in potyvirus infection.

Correlation Analysis of Expression Profile and Quantitative iTRAQ-LC-MS/MS Proteomics Reveals Resistance Mechanism Against TuMV in Chinese Cabbage (Brassica rapa ssp. pekinensis)

The arms race between plants and viruses never ceases. Chinese cabbage, an important type of Brassica vegetable crop, is vulnerable to plant virus infection, especially to Turnip mosaic virus (TuMV). To better examine the molecular mechanisms behind the virus infection, we conducted the correlation analysis of RNA-Seq and quantitative iTRAQ-LC-MS/MS in TuMV-infected and in healthy Chinese cabbage leaves. There were 757 differentially expressed genes and 75 differentially expressed proteins that were screened in Chinese cabbage plants infected with TuMV. These genes were enriched in many pathways, and among them, the plant hormone signal transduction, plant-pathogen interaction, and protein processing in the endoplasmic reticulum pathways were suggested to be closely related pathways. The correlation analysis between RNA-Seq and quantitative iTRAQ-LC-MS/MS was then further explored. Finally, we obtained a preliminary network of several candidate genes associated with TuMV infection, and we found that they mainly belonged to calcium signaling pathways, heat shock proteins, WRKY transcription factors, and non-specific lipid transfer proteins. These results may lead to a better understanding of antiviral mechanisms and of disease-resistant breeding.

Application of Reverse Genetics in Functional Genomics of Potyvirus

Numerous potyvirus studies, including virus biology, transmission, viral protein function, as well as virus–host interaction, have greatly benefited from the utilization of reverse genetic techniques. Reverse genetics of RNA viruses refers to the manipulation of viral genomes, transfection of the modified cDNAs into cells, and the production of live infectious progenies, either wild-type or mutated. Reverse genetic technology provides an opportunity of developing potyviruses into vectors for improving agronomic traits in plants, as a reporter system for tracking virus infection in hosts or a production system for target proteins. Therefore, this review provides an overview on the breakthroughs achieved in potyvirus research through the implementation of reverse genetic systems.

P3N-PIPO Interacts with P3 via the Shared N-Terminal Domain To Recruit Viral Replication Vesicles for Cell-to-Cell Movement

P3N-PIPO, the only dedicated movement protein (MP) of potyviruses, directs cylindrical inclusion (CI) protein from the cytoplasm to the plasmodesma (PD), where CI forms conical structures for intercellular movement. To better understand potyviral cell-to-cell movement, we further characterized P3N-PIPO using Turnip mosaic virus (TuMV) as a model virus. We found that P3N-PIPO interacts with P3 via the shared P3N domain and that TuMV mutants lacking the P3N domain of either P3N-PIPO or P3 are defective in cell-to-cell movement. Moreover, we found that the PIPO domain of P3N-PIPO is sufficient to direct CI to the PD, whereas the P3N domain is necessary for localization of P3N-PIPO to 6K2-labeled vesicles or aggregates. Finally, we discovered that the interaction between P3 and P3N-PIPO is essential for the recruitment of CI to cytoplasmic 6K2-containing structures and the association of 6K2-containing structures with PD-located CI inclusions. These data suggest that both P3N and PIPO domains are indispensable for potyviral cell-to-cell movement and that the 6K2 vesicles in proximity to PDs resulting from multipartite interactions among 6K2, P3, P3N-PIPO, and CI may also play an essential role in this process.IMPORTANCE Potyviruses include numerous economically important viruses that represent approximately 30% of known plant viruses. However, there is still limited information about the mechanism of potyviral cell-to-cell movement. Here, we show that P3N-PIPO interacts with and recruits CI to the PD via the PIPO domain and interacts with P3 via the shared P3N domain. We further report that the interaction of P3N-PIPO and P3 is associated with 6K2 vesicles and brings the 6K2 vesicles into proximity with PD-located CI structures. These results support the notion that the replication and cell-to-cell movement of potyviruses are processes coupled by anchoring viral replication complexes at the entrance of PDs, which greatly increase our knowledge of the intercellular movement of potyviruses.

An update on the arsenal: mining resistance genes for disease management of Brassica crops in the genomic era

Brassica species include many economically important crops that provide nutrition and health-promoting substances to humans worldwide. However, as with all crops, their production is constantly threatened by emerging viral, bacterial, and fungal diseases, whose incidence has increased in recent years. Traditional methods of control are often costly, present limited effectiveness, and cause environmental damage; instead, the ideal approach is to mine and utilize the resistance genes of the Brassica crop hosts themselves. Fortunately, the development of genomics, molecular genetics, and biological techniques enables us to rapidly discover and apply resistance (R) genes. Herein, the R genes identified in Brassica crops are summarized, including their mapping and cloning, possible molecular mechanisms, and application in resistance breeding. Future perspectives concerning how to accurately discover additional R gene resources and efficiently utilize these genes in the genomic era are also discussed.

Intracellular proliferation of clover yellow vein virus is unaffected by the recessive resistance gene cyv1 of Pisum sativum

The pea cyv1 gene is a yet-to-be-identified recessive resistance gene that inhibits the infection of clover yellow vein virus (ClYVV). Previous studies confirmed that the cell-to-cell movement of ClYVV is inhibited in cyv1-carrying pea plants; however, the effect of cyv1 on viral replication remains unknown. In this study, we developed a new pea protoplast transfection method to investigate ClYVV propagation at the single-cell level. Using this method, we revealed that ClYVV accumulates to similar levels in both ClYVV-susceptible and cyv1-carrying pea protoplasts. Thus, the cyv1-mediated resistance would not suppress intracellular ClYVV replication.

A Turnip Mosaic Virus Determinant of Systemic Necrosis in Nicotiana benthamiana and a Novel Resistance-Breaking Determinant in Chinese Cabbage Identified from Chimeric Infectious Clones

Infectious clones of Korean turnip mosaic virus (TuMV) isolates KIH1 and HJY1 share 88.1% genomic nucleotides and 96.4% polyprotein amino acid identity, and they induce systemic necrosis or mild mosaic, respectively, in Nicotiana benthamiana. Chimeric constructs between these isolates exchanged the 5', central, and 3' domains of KIH1 (K) and HJY1 (H), where the order of the letters indicates the origin of these domains. KIH1 and chimeras KHH and KKH induced systemic necrosis, whereas HJY1 and chimeras HHK, HKK, and HKH induced mild symptoms, indicating the determinant of necrosis to be within the 5' 3.9 kb of KIH1; amino acid identities of the included P1, Helper component protease, P3, 6K1, and cylindrical inclusion N-terminal domain were 90.06, 98.91, 93.80, 100, and 100%, respectively. Expression of P1 or P3 from a potato virus X vector yielded symptom differences only between P3 of KIH1 and HJY1, implicating a role for P3 in necrosis in N. benthamiana. Chimera KKH infected Brassica rapa var. pekinensis 'Norang', which was resistant to both KIH1 and HJY1, indicating that two separate TuMV determinants are required to overcome the resistance. Ability of diverse TuMV isolates, chimeras, and recombinants to overcome resistance in breeding lines may allow identification of novel resistance genes.

Five Newly Collected Turnip Mosaic Virus (TuMV) Isolates from Jeju Island, Korea are Closely Related to Previously Reported Korean TuMV Isolates but Show Distinctive Symptom Development

For several years, temperatures in the Korean peninsula have gradually increased due to climate change, resulting in a changing environment for growth of crops and vegetables. An associated consequence is that emerging species of insect vector have caused increased viral transmission. In Jeju Island, Korea, occurrences of viral disease have increased. Here, we report characterization of five newly collected turnip mosaic virus (TuMV) isolates named KBJ1, KBJ2, KBJ3, KBJ4 and KBJ5 from a survey on Jeju Island in 2017. Full-length cDNAs of each isolate were cloned into the pJY vector downstream of cauliflower mosaic virus 35S and bacteriophage T7 RNA polymerase promoters. Their fulllength sequences share 98.9-99.9% nucleotide sequence identity and were most closely related to previously reported Korean TuMV isolates. All isolates belonged to the BR group and infected both Chinese cabbage and radish. Four isolates induced very mild symptoms in Nicotiana benthamiana but KBJ5 induced a hypersensitive response. Symptom differences may result from three amino acid differences uniquely present in KBJ5; Gly(382)Asp, Ile(891)Val, and Lys(2522)Glu in P1, P3, and NIb, respectively.

Catch Me If You Can! RNA Silencing-Based Improvement of Antiviral Plant Immunity

Viruses are obligate parasites which cause a range of severe plant diseases that affect farm productivity around the world, resulting in immense annual losses of yield. Therefore, control of viral pathogens continues to be an agronomic and scientific challenge requiring innovative and ground-breaking strategies to meet the demands of a growing world population. Over the last decade, RNA silencing has been employed to develop plants with an improved resistance to biotic stresses based on their function to provide protection from invasion by foreign nucleic acids, such as viruses. This natural phenomenon can be exploited to control agronomically relevant plant diseases. Recent evidence argues that this biotechnological method, called host-induced gene silencing, is effective against sucking insects, nematodes, and pathogenic fungi, as well as bacteria and viruses on their plant hosts. Here, we review recent studies which reveal the enormous potential that RNA-silencing strategies hold for providing an environmentally friendly mechanism to protect crop plants from viral diseases.

Soybean Cytochrome b5 Is a Restriction Factor for Soybean Mosaic Virus

Soybean mosaic virus (SMV) is one of the most destructive viral diseases in soybeans (Glycine max). In this study, an interaction between the SMV P3 protein and cytochrome b5 was detected by yeast two-hybrid assay, and bimolecular fluorescence complementation assay showed that the interaction took place at the cell periphery. Further, the interaction was confirmed by co-immunoprecipitation analysis. Quantitative real-time polymerase chain reaction analysis revealed that GmCYB5 gene was differentially expressed in resistant and susceptible soybean plants after inoculation with SMV-SC15 strain. To test the involvement of this gene in SMV resistance, the GmCYB5 was silenced using a bean pod mottle virus (BPMV)-based vector construct. Results showed that GmCYB5-1 was 83% and 99% downregulated in susceptible (NN1138-2) and resistant (RN-9) cultivars, respectively, compared to the empty vector-treated plants. Silencing of GmCYB5 gene promotes SMV replication in soybean plants. Our results suggest that during SMV infection, the host CYB5 protein targets P3 protein to inhibit its proliferation. Taken together, these results suggest that CYB5 is an important factor in SMV infection and replication in soybeans, which could help soybean breeders develop SMV resistant soybean cultivars.

Sequence Variations Among 17 New Radish Isolates of Turnip mosaic virus Showing Differential Pathogenicity and Infectivity in Nicotiana benthamiana, Brassica rapa, and Raphanus sativus

Infectious clones were generated from 17 new Korean radish isolates of Turnip mosaic virus (TuMV). Phylogenetic analysis indicated that all new isolates, and three previously characterized Korean radish isolates, belong to the basal-BR group (indicating that the pathotype can infect both Brassica and Raphanus spp.). Pairwise analysis revealed genomic nucleotide and polyprotein amino acid identities of >87.9 and >95.7%, respectively. Five clones (HJY1, HJY2, KIH2, BE, and prior isolate R007) had lower sequence identities than other isolates and produced mild symptoms in Nicotiana benthamiana. These isolates formed three distinct sequence classes (HJY1/HJY2/R007, KIH2, and BE), and several differential amino acid residues (in P1, P3, 6K2, and VPg) were present only in mild isolates HJY1, HJY2, and R007. The remaining isolates all induced systemic necrosis in N. benthamiana. Four mild isolates formed a phylogenetic subclade separate from another subclade including all of the necrosis-inducing isolates plus mild isolate KIH2. Symptom severity in radish and Chinese cabbage genotypes was not correlated with pathogenicity in N. benthamiana; indeed, Chinese cabbage cultivar Norang was not infected by any isolate, whereas Chinese cabbage cultivar Chusarang was uniformly susceptible. Four isolates were unable to infect radish cultivar Iljin, but no specific amino acid residues were correlated with avirulence. These results may lead to the identification of new resistance genes against TuMV.

Host-associated selection of a P3 mutant of zucchini yellow mosaic virus affects viral infectivity in watermelon

In this study, we found that the infectivity of zucchini yellow mosaic virus (ZYMV) in watermelon lines H1 and K6 changed from partial to complete after propagation in the susceptible watermelon line ZXG637. When using cucumber infected with strain ZYMV-CH87 as an inoculum (named ZYMV-CH87C), the mean incidences of infection in lines H1 and K6 were 6% and 11%, respectively. However, when these lines were inoculated with ZXG637 infected with ZYMV-CH87C (named ZYMV-637), 100% of the plants became infected. Sequencing of ZYMV from these different inoculums revealed two nucleotide changes in the P3 cistron in ZYMV-637, which resulted in changes in the amino acids at positions 768 and 857 of the P3 protein, compared with the original strain ZYMV-CH87. We named this variant the M₇₆₈I₈₅₇-variant. The M₇₆₈I₈₅₇-variant was detected at low levels (3.9%) in ZYMV-CH87C. When ZYMV-CH87C was passaged with ZXG637, the M₇₆₈I₈₅₇-variant was selected by the host, and the original sequence was replaced entirely after two passages. These results may be explained by host-associated selection due to an unknown host-encoded factor. Using the M₇₆₈I₈₅₇-variant as an inoculum, 100% of the H1 and K6 plants showed systemic symptoms. These results suggest that (1) changing the individual amino acids at the end of the P3 N-terminus induces resistance-breaking, and (2) the P3 N-terminus may be involved in host recognition.

The interaction of soybean reticulon homology domain protein (GmRHP) with Soybean mosaic virus encoded P3 contributes to the viral infection

Soybean mosaic virus (SMV), a member of the Potyvirus genus, is a prevalent and devastating viral pathogen in soybean-growing regions worldwide. Potyvirus replication occurs in the 6K2-induced viral replication complex at endoplasmic reticulum exit sites. Potyvirus-encoded P3 is also associated with the endoplasmic reticulum and is as an essential component of the viral replication complex, playing a key role in viral replication. This study provides evidence that the soybean (Glycine max) reticulon homology domain protein (designated as GmRHP) interacts with SMV-P3 by using a two-hybrid yeast system to screen a soybean cDNA library. A bimolecular fluorescence complementation assay further confirmed the interaction, which occurred on the cytomembrane, endoplasmic reticulum and cytoskeleton in Nicotiana benthamiana cells. The transient expression of GmRHP can promote the coupling of Turnip mosaic virus replication and cell-to-cell movement in N. benthamiana. The interaction between the membrane protein SMV-P3 and GmRHP may contribute to the potyvirus infection, and GmRHP may be an essential host factor for P3's involvement in potyvirus replication.

RNAi-mediated SMV P3 cistron silencing confers significantly enhanced resistance to multiple Potyvirus strains and isolates in transgenic soybean

KEY MESSAGE

Robust RNAi-mediated resistance to multiple Potyvirus strains and isolates, but not to Secovirus BPMV, was conferred by expressing a short SMV P3 hairpin in soybean plants. Engineering resistance to multiple Potyvirus strains is of great interest because of a wide variability of the virus strains, and mixed infections of multiple viruses or strains commonly associated with field grown soybean. In this study, RNAi-mediated silencing of the soybean mosaic virus (SMV) P3 cistron, which is reported to participate in virus movements and pathogenesis and to be the putative determinant of SMV virulence, was used to induce resistance to multiple Potyvirus strains and isolates in soybean. A 302 bp inverted repeat (IR) of the P3 cistron, isolated from the SMV strain SC3, was introduced into soybean. The transgenic lines exhibited stable and enhanced resistance to SMV SC3 under field conditions over 3 consecutive years. The transgenic lines also showed significantly enhanced resistance to four other SMV strains (SC7, SC15, SC18, and SMV-R, a novel recombinant found in China), the soybean-infecting bean common mosaic virus (BCMV) and watermelon mosaic virus (WMV). Nevertheless, no significant differences were found between transgenic plants and their non-transformed (NT) counterparts in terms of resistance to bean pod mottle virus (BPMV, Secoviridae). Consistent with the results of resistance evaluations, the expression of the respective viral CP cistrons and virus accumulation were significantly lower in seven Potyvirus strains and isolates than in the NT plants, but not in BCMV-inoculated transgenic lines. The results demonstrate the effectiveness of engineering resistance to multiple Potyvirus strains and isolates via RNAi-mediated SMV P3 cistron silencing, and thus provide an effective control strategy against Potyvirus infections in soybean and other crops.

Construction of an agroinfectious clone of bean rugose mosaic virus using Gibson Assembly

Construction of agroinfectious viral clones usually requires many steps of cloning and sub-cloning and also a binary vector, which makes the process laborious, time-consuming, and frequently susceptible to some degree of plasmid instability. Nowadays, novel methods have been applied to the assembly of infectious viral clones, and here we have applied isothermal, single-step Gibson Assembly (GA) to construct an agroinfectious clone of Bean rugose mosaic virus (BRMV) using a small binary vector. The procedure has drastically reduced the cloning steps, and BRMV could be recovered from agroinfiltrated common bean twenty days after inoculation, indicating that the infectious clone could spread in the plant tissues and efficiently generate a systemic infection. The virus was also recovered from leaves of common bean and soybean cultivars mechanically inoculated with infectious clone two weeks after inoculation, confirming the efficiency of GA cloning procedure to produce the first BRMV agroinfectious clone to bean and soybean.

Molecular Characterization of the Complete Genome of Three Basal-BR Isolates of Turnip mosaic virus Infecting Raphanus sativus in China

Turnip mosaic virus (TuMV) infects crops of plant species in the family Brassicaceae worldwide. TuMV isolates were clustered to five lineages corresponding to basal-B, basal-BR, Asian-BR, world-B and OMs. Here, we determined the complete genome sequences of three TuMV basal-BR isolates infecting radish from Shandong and Jilin Provinces in China. Their genomes were all composed of 9833 nucleotides, excluding the 3'-terminal poly(A) tail. They contained two open reading frames (ORFs), with the large one encoding a polyprotein of 3164 amino acids and the small overlapping ORF encoding a PIPO protein of 61 amino acids, which contained the typically conserved motifs found in members of the genus Potyvirus. In pairwise comparison with 30 other TuMV genome sequences, these three isolates shared their highest identities with isolates from Eurasian countries (Germany, Italy, Turkey and China). Recombination analysis showed that the three isolates in this study had no "clear" recombination. The analyses of conserved amino acids changed between groups showed that the codons in the TuMV out group (OGp) and OMs group were the same at three codon sites (852, 1006, 1548), and the other TuMV groups (basal-B, basal-BR, Asian-BR, world-B) were different. This pattern suggests that the codon in the OMs progenitor did not change but that in the other TuMV groups the progenitor sequence did change at divergence. Genetic diversity analyses indicate that the PIPO gene was under the highest selection pressure and the selection pressure on P3N-PIPO and P3 was almost the same. It suggests that most of the selection pressure on P3 was probably imposed through P3N-PIPO.

Viral Strain-Specific Differential Alterations in Arabidopsis Developmental Patterns

Turnip mosaic virus (TuMV) infections affect many Arabidopsis developmental traits. This paper analyzes, at different levels, the development-related differential alterations induced by different strains of TuMV, represented by isolates UK 1 and JPN 1. The genomic sequence of JPN 1 TuMV isolate revealed highest divergence in the P1 and P3 viral cistrons, upon comparison with the UK 1 sequence. Infectious viral chimeras covering the whole viral genome uncovered the P3 cistron as a major viral determinant of development alterations, excluding the involvement of the PIPO open reading frame. However, constitutive transgenic expression of P3 in Arabidopsis did not induce developmental alterations nor modulate the strong effects induced by the transgenic RNA silencing suppressor HC-Pro from either strain. This highlights the importance of studying viral determinants within the context of actual viral infections. Transcriptomic and interactomic analyses at different stages of plant development revealed large differences in the number of genes affected by the different infections at medium infection times but no significant differences at very early times. Biological functions affected by UK 1 (the most severe strain) included mainly stress response and transport. Most cellular components affected cell-wall transport or metabolism. Hubs in the interactome were affected upon infection.

Genome-wide identification of turnip mosaic virus-responsive microRNAs in non-heading Chinese cabbage by high-throughput sequencing

Turnip mosaic virus (TuMV) is the most prevalent viral pathogen infecting most cruciferous plants. MicroRNAs (miRNAs) are around 22 nucleotides long non-protein-coding RNAs that play key regulatory roles in plants. Recent research findings show that miRNAs are involved in plant-virus interaction. However we know little about plant defense and viral offense system networks throughout microRNA regulation pathway. In this study, two small RNA libraries were constructed based on non-heading Chinese cabbage (Brassica campestris ssp. chinensis L. Makino, NHCC) leaves infected by TuMV and healthy leaves, and sequenced using the Illumina-Solexa high-throughput sequencing technology. A total of 86 conserved miRNAs belonging to 25 known miRNA families and 45 novel ones were identified. Among them, twelve conserved and ten new miRNAs were validated by real-time fluorescence quantitative PCR (qPCR). Differential expression analysis showed that 42 miRNAs were down-regulated and 27 miRNAs were up-regulated in response to TuMV stress. A total of 271 target genes were predicted using a bioinformatics approach, these genes are mainly involved in growth and resistance to various stresses. We further selected 13 miRNAs and their corresponding target genes to explore their expression pattern under TuMV and/or cold (4°C) stresses, and the results indicated that some of the identified miRNAs could link TuMV response with cold response of NHCC. The characterization of these miRNAs could contribute to a better understanding of plant-virus interaction throughout microRNA regulation pathway. This can lead to finding new approach to defend virus infection using miRNA in Chinese cabbage.

The amino acid 419 in HC-Pro is involved in the ability of PVY isolate N605 to induce necrotic symptoms on potato tubers

The ability to induce the potato tuber necrosis ringspot disease (PTNRD) is a property shared by PVY isolates belonging to different groups (e.g. PVY(N) and PVY(O)) and variants (e.g. PVY(NTN) and PVY(N)-W). The identification of viral molecular determinant(s) involved in the expression of PTNRD symptoms is essential for (i) an easier detection of tuber necrosis isolates and (ii) an improvement of our knowledge on the epidemiology of this potato disease. A reverse genetic approach associated with a biological typing of a collection of PVY chimeras and mutants indicated that residue E419 of the HC-Pro protein is linked to the ability of PVY to induce tuber necrosis on four PTNRD-susceptible potato cultivars. Indeed, the substitution of the N-type glutamic acid (E) in O-type aspartic acid (D) at position 419 in the HC-Pro cistron prevents the expression of tuber necrosis on infected tubers without reducing the virulence of the corresponding E/D419 mutant. This result opens opportunities for the future studies on potato/PVY interactions.

Soybean actin-depolymerizing factor 2 interacts with Soybean mosaic virus-encoded P3 protein

Soybean mosaic virus (SMV), a member of the Potyvirus genus, is one of the most prevalent and devastating viral pathogens in soybean-growing regions worldwide. It is generally accepted that symptom development of a viral plant disease results from molecular interactions between the virus and its host plant. P3 protein is the most variable polyprotein in potyviruses, which potentially plays an important role in the process of the evolution of virus type specialization. However, P3 not only plays a major role in virus replication and movement, but it is also responsible for symptom development in SMV-infected plants. This study provides evidence that actin-depolymerizing factor 2 (designated as ADF2) of soybean interacts with SMV P3 via a two-hybrid yeast system by screening a soybean cDNA library. Bimolecular fluorescence complementation assay further confirmed the interaction, which occurred in both the cytomembrane and cytoskeleton of Nicotiana benthamiana cells. The results support the hypothesis that SMV P3 might have a role in virus movement within cells.

Molecular biology of potyviruses

Potyvirus is the largest genus of plant viruses causing significant losses in a wide range of crops. Potyviruses are aphid transmitted in a nonpersistent manner and some of them are also seed transmitted. As important pathogens, potyviruses are much more studied than other plant viruses belonging to other genera and their study covers many aspects of plant virology, such as functional characterization of viral proteins, molecular interaction with hosts and vectors, structure, taxonomy, evolution, epidemiology, and diagnosis. Biotechnological applications of potyviruses are also being explored. During this last decade, substantial advances have been made in the understanding of the molecular biology of these viruses and the functions of their various proteins. After a general presentation on the family Potyviridae and the potyviral proteins, we present an update of the knowledge on potyvirus multiplication, movement, and transmission and on potyvirus/plant compatible interactions including pathogenicity and symptom determinants. We end the review providing information on biotechnological applications of potyviruses.

[Plant potyvirus evolution: the survey of the genetic structure of populations]

The Potyvirus is the largest genus of the largest family of plant RNA viruses, the Potyviridae. The potyviruses infect not only dicotyledonous but also monocotyledonous plants. The potyvirus phylogeny shows that the genus probably originated from a virus of monocotyledonous plants and that it first diverged approximately 7250 years ago in Southwest Eurasia or North Africa. Turnip mosaic virus (TuMV) belongs to the genus Potyvirus and infects a wide range of plant species, most from the family Brassicaceae. TuMV is most studied a potyvirus species for molecular evolution and the genetic structure of populations. The use of computer programs for better understanding of the evolution and the genetic structures of populations of potyviruses and TuMV are illustrated.

Fitness penalty in susceptible host is associated with virulence of Soybean mosaic virus on Rsv1-genotype soybean: a consequence of perturbation of HC-Pro and not P3

The multigenic Rsv1 locus in the soybean plant introduction (PI) 'PI96983' confers extreme resistance against the majority of Soybean mosaic virus (SMV) strains, including SMV-N, but not SMV-G7 and SMV-G7d. In contrast, in susceptible soybean cultivars lacking a functional Rsv1 locus, such as 'Williams82' (rsv1), SMV-N induces severe disease symptoms and accumulates to a high level, whereas both SMV-G7 and SMV-G7d induce mild symptoms and accumulate to a significantly lower level. Gain of virulence by SMV-N on Rsv1-genotype soybean requires concurrent mutations in both the helper-component proteinase (HC-Pro) and P3 cistrons. This is because of the presence of at least two resistance (R) genes, probably belonging to the nucleotide-binding leucine-rich repeat (NB-LRR) class, within the Rsv1 locus, independently mediating the recognition of HC-Pro or P3. In this study, we show that the majority of experimentally evolved mutational pathways that disrupt the avirulence functions of SMV-N on Rsv1-genotype soybean also result in mild symptoms and reduced accumulation, relative to parental SMV-N, in Williams82 (rsv1). Furthermore, the evaluation of SMV-N-derived HC-Pro and P3 chimeras, containing homologous sequences from virulent SMV-G7 or SMV-G7d strains, as well as SMV-N-derived variants containing HC-Pro or P3 point mutation(s) associated with gain of virulence, reveals a direct correlation between the perturbation of HC-Pro and a fitness penalty in Williams82 (rsv1). Collectively, these data demonstrate that gain of virulence by SMV on Rsv1-genotype soybean results in fitness loss in a previously susceptible soybean genotype, this being a consequence of mutations in HC-Pro, but not in P3.

Intra-specific variability and biological relevance of P3N-PIPO protein length in potyviruses

BACKGROUND

Pipo was recently described as a new ORF encoded within the genome of the Potyviridae family members (PNAS 105:5897-5902, 2008). It is embedded within the P3 cistron and is translated in the +2 reading frame relative to the potyviral long ORF as the P3N-PIPO fusion protein. In this work, we first collected pipo nucleotide sequences available for different isolates of 48 Potyvirus species. Second, to determine the biological implications of variation in pipo length, we measured infectivity, viral accumulation, cell-to-cell and systemic movements for two Turnip mosaic virus (TuMV) variants with pipo alleles of different length in three different susceptible host species, and tested for differences between the two variants.

RESULTS

In addition to inter-specific variation, there was high variation in the length of the PIPO protein among isolates within species (ranging from 1 to 89 amino acids). Furthermore, selection analyses on the P3 cistron did not account for the existence of stop codons in the pipo ORF, but showed that positive selection was significant in the overlapping region for Potato virus Y (PVY) and TuMV. In some cases, variability in length was associated with host species, geographic provenance and/or other strain features. We found significant empirical differences among the phenotypes associated with TuMV pipo alleles, though the magnitude and sign of the effects were host-dependent.

CONCLUSIONS

The combination of computational molecular evolution analyses and experiments stemming from these analyses provide clues about the selective pressures acting upon the different-length pipo alleles and show that variation in length may be maintained by host-driven selection.

Analysis of Soybean mosaic virus genetic diversity in Iran allows the characterization of a new mutation resulting in overcoming Rsv4-resistance

The genetic variation and population structure of Soybean mosaic virus (SMV) in Iran was analysed through the characterization of a set of isolates collected in the soybean-growing provinces of Iran. The partial nucleotide sequence of these isolates showed a single, undifferentiated population with low genetic diversity, highly differentiated from other SMV world populations. These traits are compatible with a population bottleneck associated with the recent introduction of SMV in Iran. Phylogenetic analyses suggest that SMV was introduced into Iran from East Asia, with at least three introduction events. The limited genetic diversification of SMV in Iran may be explained by strong negative selection in most viral genes eliminating the majority of mutations, together with recombination purging deleterious mutations. The pathogenicity of Iranian SMV isolates was typified on a set of soybean differential lines either susceptible or carrying different resistance genes or alleles to SMV. Two pathotypes were distinguished according to the ability to overcome Rsv4 resistance in line V94-5152. Amino acid sequence comparisons of virulent and avirulent isolates on V94-5152 (Rsv4), plus site-directed mutagenesis in a biologically active cDNA clone, identified mutation S1053N in the P3 protein as the determinant for virulence on V94-5152. Codon 1053 was shown to be under positive selection, and S1053N-determined Rsv4-virulence occurred in isolates with different genealogies. The V94-5152 (Rsv4)-virulence determinant in Iranian isolates maps into a different amino acid position in the P3 protein than those previously reported, indicating different evolutionary pathways towards resistance breaking that might be conditioned by sequence context.

Diverse amino acid changes at specific positions in the N-terminal region of the coat protein allow Plum pox virus to adapt to new hosts

Plum pox virus (PPV)-D and PPV-R are two isolates from strain D of PPV that differ in host specificity. Previous analyses of chimeras originating from PPV-R and PPV-D suggested that the N terminus of the coat protein (CP) includes host-specific pathogenicity determinants. Here, these determinants were mapped precisely by analyzing the infectivity in herbaceous and woody species of chimeras containing a fragment of the 3' region of PPV-D (including the region coding for the CP) in a PPV-R backbone. These chimeras were not infectious in Prunus persica, but systemically infected Nicotiana clevelandii and N. benthamiana when specific amino acids were modified or deleted in a short 30-amino-acid region of the N terminus of the CP. Most of these mutations did not reduce PPV fitness in Prunus spp. although others impaired systemic infection in this host. We propose a model in which the N terminus of the CP, highly relevant for virus systemic movement, is targeted by a host defense mechanism in Nicotiana spp. Mutations in this short region allow PPV to overcome the defense response in this host but can compromise the efficiency of PPV systemic movement in other hosts such as Prunus spp.

Quantitative and qualitative involvement of P3N-PIPO in overcoming recessive resistance against Clover yellow vein virus in pea carrying the cyv1 gene

In pea carrying cyv1, a recessive gene for resistance to Clover yellow vein virus (ClYVV), ClYVV isolate Cl-no30 was restricted to the initially infected cells, whereas isolate 90-1 Br2 overcame this resistance. We mapped the region responsible for breaking of cyv1-mediated resistance by examining infection of cyv1 pea with chimeric viruses constructed from parts of Cl-no30 and 90-1 Br2. The breaking of resistance was attributed to the P3 cistron, which is known to produce two proteins: P3, from the main open reading frame (ORF), and P3N-PIPO, which has the N-terminal part of P3 fused to amino acids encoded by a small open reading frame (ORF) called PIPO in the +2 reading frame. We introduced point mutations that were synonymous with respect to the P3 protein but nonsynonymous with respect to the P3N-PIPO protein, and vice versa, into the chimeric viruses. Infection of plants with these mutant viruses revealed that both P3 and P3N-PIPO were involved in overcoming cyv1-mediated resistance. Moreover, P3N-PIPO quantitatively affected the virulence of Cl-no30 in cyv1 pea. Additional expression in trans of the P3N-PIPO derived from Cl-no30, using White clover mosaic virus as a vector, enabled Cl-no30 to move to systemic leaves in cyv1 pea. Susceptible pea plants infected with chimeric ClYVV possessing the P3 cistron of 90-1 Br2, and which were therefore virulent toward cyv1 pea, accumulated more P3N-PIPO than did those infected with Cl-no30, suggesting that the higher level of P3N-PIPO in infected cells contributed to the breaking of resistance by 90-1 Br2. This is the first report showing that P3N-PIPO is a virulence determinant in plants resistant to a potyvirus.

Turnip mosaic potyvirus probably first spread to Eurasian brassica crops from wild orchids about 1000 years ago

Turnip mosaic potyvirus (TuMV) is probably the most widespread and damaging virus that infects cultivated brassicas worldwide. Previous work has indicated that the virus originated in western Eurasia, with all of its closest relatives being viruses of monocotyledonous plants. Here we report that we have identified a sister lineage of TuMV-like potyviruses (TuMV-OM) from European orchids. The isolates of TuMV-OM form a monophyletic sister lineage to the brassica-infecting TuMVs (TuMV-BIs), and are nested within a clade of monocotyledon-infecting viruses. Extensive host-range tests showed that all of the TuMV-OMs are biologically similar to, but distinct from, TuMV-BIs and do not readily infect brassicas. We conclude that it is more likely that TuMV evolved from a TuMV-OM-like ancestor than the reverse. We did Bayesian coalescent analyses using a combination of novel and published sequence data from four TuMV genes [helper component-proteinase protein (HC-Pro), protein 3(P3), nuclear inclusion b protein (NIb), and coat protein (CP)]. Three genes (HC-Pro, P3, and NIb), but not the CP gene, gave results indicating that the TuMV-BI viruses diverged from TuMV-OMs around 1000 years ago. Only 150 years later, the four lineages of the present global population of TuMV-BIs diverged from one another. These dates are congruent with historical records of the spread of agriculture in Western Europe. From about 1200 years ago, there was a warming of the climate, and agriculture and the human population of the region greatly increased. Farming replaced woodlands, fostering viruses and aphid vectors that could invade the crops, which included several brassica cultivars and weeds. Later, starting 500 years ago, inter-continental maritime trade probably spread the TuMV-BIs to the remainder of the world.

Mapping and candidate-gene screening of the novel Turnip mosaic virus resistance gene retr02 in Chinese cabbage (Brassica rapa L.)

The extreme resistance to Turnip mosaic virus observed in the Chinese cabbage (Brassica rapa) line, BP8407, is monogenic and recessive. Bulked segregant analysis was carried out to identify simple sequence repeat and Indel markers linked to this recessive resistance gene, termed recessive Turnip mosaic virus resistance 02 (retr02). Mapping of PCR-specific Indel markers on 239 individuals of a BP8407 × Ji Zao Chun F(2) population, located this resistance gene to a 0.9-cM interval between two Indel markers (BrID10694 and BrID101309) and in scaffold000060 or scaffold000104 on chromosome A04 of the B. rapa genome. Eleven eukaryotic initiation factor 4E (eIF4E) and 14 eukaryotic initiation factor 4G (eIF4G) genes are predicted in the B. rapa genome. A candidate gene, Bra035393 on scaffold000104, was predicted within the mapped resistance locus. The gene encodes the eIF(iso)4E protein. Bra035393 was sequenced in BP8407 and Ji Zao Chun. A polymorphism (A/G) was found in exon 3 between BP8407 and Ji Zao Chun. This gene was analysed in four resistant and three susceptible lines. A correlation was observed between the amino acid substitution (Gly/Asp) in the eIF(iso)4E protein and resistance/susceptibility. eIF(iso)4E has been shown previously to interact with the TuMV genome-linked protein, VPg.

Sharka: the past, the present and the future

Members the Potyviridae family belong to a group of plant viruses that are causing devastating plant diseases with a significant impact on agronomy and economics. Plum pox virus (PPV), as a causative agent of sharka disease, is widely discussed. The understanding of the molecular biology of potyviruses including PPV and the function of individual proteins as products of genome expression are quite necessary for the proposal the new antiviral strategies. This review brings to view the members of Potyviridae family with respect to plum pox virus. The genome of potyviruses is discussed with respect to protein products of its expression and their function. Plum pox virus distribution, genome organization, transmission and biochemical changes in infected plants are introduced. In addition, techniques used in PPV detection are accentuated and discussed, especially with respect to new modern techniques of nucleic acids isolation, based on the nanotechnological approach. Finally, perspectives on the future of possibilities for nanotechnology application in PPV determination/identification are outlined.

RNA silencing and plant viral diseases

RNA silencing plays a critical role in plant resistance against viruses, with multiple silencing factors participating in antiviral defense. Both RNA and DNA viruses are targeted by the small RNA-directed RNA degradation pathway, with DNA viruses being also targeted by RNA-directed DNA methylation. To evade RNA silencing, plant viruses have evolved a variety of counter-defense mechanisms such as expressing RNA-silencing suppressors or adopting silencing-resistant RNA structures. This constant defense-counter defense arms race is likely to have played a major role in defining viral host specificity and in shaping viral and possibly host genomes. Recent studies have provided evidence that RNA silencing also plays a direct role in viral disease induction in plants, with viral RNA-silencing suppressors and viral siRNAs as potentially the dominant players in viral pathogenicity. However, questions remain as to whether RNA silencing is the principal mediator of viral pathogenicity or if other RNA-silencing-independent mechanisms also account for viral disease induction. RNA silencing has been exploited as a powerful tool for engineering virus resistance in plants as well as in animals. Further understanding of the role of RNA silencing in plant-virus interactions and viral symptom induction is likely to result in novel anti-viral strategies in both plants and animals.

Simultaneous mutations in multi-viral proteins are required for soybean mosaic virus to gain virulence on soybean genotypes carrying different R genes

BACKGROUND

Genetic resistance is the most effective and sustainable approach to the control of plant pathogens that are a major constraint to agriculture worldwide. In soybean, three dominant R genes, i.e., Rsv1, Rsv3 and Rsv4, have been identified and deployed against Soybean mosaic virus (SMV) with strain-specificities. Molecular identification of virulent determinants of SMV on these resistance genes will provide essential information for the proper utilization of these resistance genes to protect soybean against SMV, and advance knowledge of virus-host interactions in general.

METHODOLOGY/PRINCIPAL FINDINGS

To study the gain and loss of SMV virulence on all the three resistance loci, SMV strains G7 and two G2 isolates L and LRB were used as parental viruses. SMV chimeras and mutants were created by partial genome swapping and point mutagenesis and then assessed for virulence on soybean cultivars PI96983 (Rsv1), L-29 (Rsv3), V94-5152 (Rsv4) and Williams 82 (rsv). It was found that P3 played an essential role in virulence determination on all three resistance loci and CI was required for virulence on Rsv1- and Rsv3-genotype soybeans. In addition, essential mutations in HC-Pro were also required for the gain of virulence on Rsv1-genotype soybean. To our best knowledge, this is the first report that CI and P3 are involved in virulence on Rsv1- and Rsv3-mediated resistance, respectively.

CONCLUSIONS/SIGNIFICANCE

Multiple viral proteins, i.e., HC-Pro, P3 and CI, are involved in virulence on the three resistance loci and simultaneous mutations at essential positions of different viral proteins are required for an avirulent SMV strain to gain virulence on all three resistance loci. The likelihood of such mutations occurring naturally and concurrently on multiple viral proteins is low. Thus, incorporation of all three resistance genes in a soybean cultivar through gene pyramiding may provide durable resistance to SMV.

Amino acid changes in P3, and not the overlapping pipo-encoded protein, determine virulence of soybean mosaic virus on functionally immune Rsv1-genotype soybean

A small open reading frame, termed 'pipo', is embedded in the P3 cistron of potyviruses. Currently, knowledge on pipo and its role(s) in the life cycle of potyviruses is limited. The P3 and helper-component proteinase (HC-Pro) cistrons of Soybean mosaic virus (SMV) harbour determinants affecting virulence on functionally immune Rsv1-genotype soybeans. Interestingly, a key virulence determinant of SMV on Rsv1-genotype soybeans (i.e. soybeans containing the Rsv1 resistance gene) that resides at polyprotein codon 947 overlaps both P3 and a pipo-encoded codon. This raises the question of whether PIPO or P3 is the virulence factor. To answer this question, the corresponding pipo of an avirulent and two virulent strains of SMV were studied by comparative genomics, followed by syntheses and analyses of site-directed mutants. Our data demonstrate that the virulence of SMV on Rsv1-genotype soybeans is affected by P3 and not the overlapping pipo-encoded protein.

How do plant viruses induce disease? Interactions and interference with host components

Plant viruses are biotrophic pathogens that need living tissue for their multiplication and thus, in the infection-defence equilibrium, they do not normally cause plant death. In some instances virus infection may have no apparent pathological effect or may even provide a selective advantage to the host, but in many cases it causes the symptomatic phenotypes of disease. These pathological phenotypes are the result of interference and/or competition for a substantial amount of host resources, which can disrupt host physiology to cause disease. This interference/competition affects a number of genes, which seems to be greater the more severe the symptoms that they cause. Induced or repressed genes belong to a broad range of cellular processes, such as hormonal regulation, cell cycle control and endogenous transport of macromolecules, among others. In addition, recent evidence indicates the existence of interplay between plant development and antiviral defence processes, and that interference among the common points of their signalling pathways can trigger pathological manifestations. This review provides an update on the latest advances in understanding how viruses affect substantial cellular processes, and how plant antiviral defences contribute to pathological phenotypes.