Mutations in the P3 protein of Soybean mosaic virus G2 isolates determine virulence on Rsv4-genotype soybean

GmCYB5-4 inhibit SMV proliferation by targeting P3 protein

Soybean mosaic virus (SMV) is a potyvirus found worldwide in soybean (Glycine max). GmCYB5-4 is a strong candidate interactor of P3. In this study, we comprehensively analyzed the GmCYB5 family in soybeans, including its distribution on chromosomes, promoter analysis, conserved motifs, phylogenetic analysis, and expression patterns. We cloned the full-length GmCYB5-4 and examined its interaction with P3 in yeast, which was later confirmed using bimolecular fluorescence complementation (BiFc). We silenced GmCYB5-4 using a bean pottle mosaic viris (BPMV) based system to generate SilCYB5-4 tissues, which surprisingly knocked down four isoforms of GmCYB5s for functional characterization. SilCYB5-4 plants were challenged with the SC3 strain to determine its involvement in SMV infection. Silencing GmCYB5-4 increased SMV accumulation, indicating that GmCYB5-4 inhibited SMV proliferation. However, further experiments are needed to elucidate the mechanism underlying the involvement of GmCYB5-4 in SMV infection.

Signaling and Resistosome Formation in Plant Innate Immunity to Viruses: Is There a Common Mechanism of Antiviral Resistance Conserved across Kingdoms?

Virus-specific proteins, including coat proteins, movement proteins, replication proteins, and suppressors of RNA interference are capable of triggering the hypersensitive response (HR), which is a type of cell death in plants. The main cell death signaling pathway involves direct interaction of HR-inducing proteins with nucleotide-binding leucine-rich repeats (NLR) proteins encoded by plant resistance genes. Singleton NLR proteins act as both sensor and helper. In other cases, NLR proteins form an activation network leading to their oligomerization and formation of membrane-associated resistosomes, similar to metazoan inflammasomes and apoptosomes. In resistosomes, coiled-coil domains of NLR proteins form Ca2+ channels, while toll-like/interleukin-1 receptor-type (TIR) domains form oligomers that display NAD+ glycohydrolase (NADase) activity. This review is intended to highlight the current knowledge on plant innate antiviral defense signaling pathways in an attempt to define common features of antiviral resistance across the kingdoms of life.

RSC3 K of soybean cv. Kefeng No.1 confers resistance to soybean mosaic virus by interacting with the viral protein P3

Soybean mosaic virus (SMV) is one of the most devastating viral pathogens of soybean (Glycine max (L.) Merr). In total, 22 Chinese SMV strains (SC1-SC22) have been classified based on the responses of 10 soybean cultivars to these pathogens. However, although several SMV-resistance loci in soybean have been identified, no gene conferring SMV resistance in the resistant soybean cultivar (cv.) Kefeng No.1 has been cloned and verified. Here, using F₂ -derived F₃ (F_2:3 ) and recombinant inbred line (RIL) populations from a cross between Kefeng No.1 and susceptible soybean cv. Nannong 1138-2, we localized the gene in Kefeng No.1 that mediated resistance to SMV-SC3 strain to a 90-kb interval on chromosome 2. To study the functions of candidate genes in this interval, we performed Bean pod mottle virus (BPMV)-induced gene silencing (VIGS). We identified a recombinant gene (which we named R_SC3 K) harboring an internal deletion of a genomic DNA fragment partially flanking the LOC100526921 and LOC100812666 reference genes as the SMV-SC3 resistance gene. By shuffling genes between infectious SMV DNA clones based on the avirulent isolate SC3 and virulent isolate 1129, we determined that the viral protein P3 is the avirulence determinant mediating SMV-SC3 resistance on Kefeng No.1. P3 interacts with RNase proteins encoded by R_SC3 K, LOC100526921, and LOC100812666. The recombinant R_SC3 K conveys much higher anti-SMV activity than LOC100526921 and LOC100812666, although those two genes also encode proteins that inhibit SMV accumulation, as revealed by gene silencing in a susceptible cultivar and by overexpression in Nicotiana benthamiana. These findings demonstrate that R_SC3 K mediates the resistance of Kefeng No.1 to SMV-SC3 and that SMV resistance of soybean is determined by the antiviral activity of RNase proteins.

The V2 Protein from the Geminivirus Tomato Yellow Leaf Curl Virus Largely Associates to the Endoplasmic Reticulum and Promotes the Accumulation of the Viral C4 Protein in a Silencing Suppression-Independent Manner

Viruses are strict intracellular parasites that rely on the proteins encoded in their genomes for the effective manipulation of the infected cell that ultimately enables a successful infection. Viral proteins have to be produced during the cell invasion and takeover in sufficient amounts and in a timely manner. Silencing suppressor proteins evolved by plant viruses can boost the production of viral proteins; although, additional mechanisms for the regulation of viral protein production likely exist. The strongest silencing suppressor encoded by the geminivirus tomato yellow leaf curl virus (TYLCV) is V2: V2 suppresses both post-transcriptional and transcriptional gene silencing (PTGS and TGS), activities that are associated with its localization in punctate cytoplasmic structures and in the nucleus, respectively. However, V2 has been previously described to largely localize in the endoplasmic reticulum (ER), although the biological relevance of this distribution remains mysterious. Here, we confirm the association of V2 to the ER in Nicotiana benthamiana and assess the silencing suppression activity-independent impact of V2 on protein accumulation. Our results indicate that V2 has no obvious influence on the localization of ER-synthesized receptor-like kinases (RLKs) or ER quality control (ERQC)/ER-associated degradation (ERAD), but dramatically enhances the accumulation of the viral C4 protein, which is co-translationally myristoylated, possibly in proximity to the ER. By using the previously described V2_C84S/86S mutant, in which the silencing suppression activity is abolished, we uncouple RNA silencing from the observed effect. Therefore, this work uncovers a novel function of V2, independent of its capacity to suppress silencing, in the promotion of the accumulation of another crucial viral protein.

Digs out and characterization of the resistance gene accountable to soybean mosaic virus in soybean (Glycine max (L.) Merrill)

A putative candidate gene conferring resistance to SMV strain SC1 was identified on chromosome 2, and the linked marker was validated in soybean cultivars Soybean mosaic, caused by the soybean mosaic virus, is the most common disease in soybean and a significant impediment to soybean production in the Huanghuai and Yangtze River regions of China. Kefeng No.1, a soybean cultivar, showed high resistance to soybean mosaic virus strain (SC1) collected from Huanghuai and Yangtze River regions. Genetic analysis based on the Mendelian genic population derived from the cross Kefeng No.1 × Nannong 1138-2 revealed that Kefeng No.1 possesses a single dominant gene. Furthermore, genetic fine-mapping using an F₂ population containing 281 individuals delimited resistant gene to a genomic region of 186 kb flanked by SSR markers BS020610 and BS020620 on chromosome 2. Within this region, there were 14 genes based on the Williams 82 reference genome. According to sequence analysis, six of the 14 genes have amino acid differences, and one of these genes is the Rsv4 allele designated as Rsc1-DR. The functional analysis of candidate genes using the bean pod mottle virus (BPMV)-induced gene silencing (VIGS) system revealed that Rsc1-DR was accountable for Kefeng No.1's resistance to SMV-SC1. Based on the genome sequence of Rsc1-DR, an Insertion/Deletion (InDel) molecular marker, JT0212, was developed and genotyped using 100 soybean cultivars, and the coincidence rate was 89%. The study enriched our understanding of the SMV resistance mechanism. The marker developed in this study could be directly used by the soybean breeders to select the genotypes with favorable alleles for making crosses, and also it will facilitate marker-assisted selection of SMV resistance in soybean breeding.

Resistance genes on the verge of plant-virus interaction

Viruses are acellular pathogens that cause severe infections in plants, resulting in worldwide crop losses every year. The lack of chemical agents to control viral diseases exacerbates the situation. Thus, to devise proper management strategies, it is important that the defense mechanisms of plants against viruses are understood. Resistance (R) genes regulate plant defense against invading pathogens by eliciting a hypersensitive response (HR). Compatible interaction between plant R gene and viral avirulence (Avr) protein activates the necrotic cell death response at the site of infection, resulting in the cessation of disease. Here, we review different aspects of R gene-mediated dominant resistance against plant viruses in dicotyledonous plants and possible ways for developing crops with better disease resistance.

Development of Comprehensive Serological Techniques for Sensitive, Quantitative and Rapid Detection of Soybean mosaic virus

Soybean is an important grain and oil crop worldwide; however, the yield and seed quality of which are seriously affected by Soybean mosaic virus (SMV). As efficient detection technology is crucial for the field management of SMV, novel immunological detection methods were developed in the present study. According to the phylogenetic analysis, the CP coding sequence of SMV-SC7 was selected for the prokaryotic expression of the recombinant SMV-CP. Purified SMV-CP was used for the development of polyclonal antibodies (PAb) against the SMV-CP (PAb-SMV-CP) and monoclonal antibodies (MAb) against SMV-CP (MAb-SMV-CP). Subsequently, the PAb-SMV-CP was used for the development of a novel DAS- quantitative ELISA (DAS-qELISA) kit, of which the sensitivity was greater than 1:4000, and this could be used for the quantitative detection of SMV in China. Meanwhile, the MAb-SMV-CP was labeled with colloidal gold, and then was used for the development of the SMV-specific gold immunochromatography strip (SMV-GICS). The SMV-GICS gives accurate detection results through observed control lines and test lines in 5 to 10 min, sharing the same sensitivity as RT-PCR, and can be used for rapid, accurate and high-throughput field SMV detection. The DAS-qELISA kit and the SMV-GICA strip developed in this study are SMV-specific, sensitive, cheap and easy to use. These products will be conducive to the timely, efficient SMV epidemiology and detection in major soybean-producing regions in China and abroad.

Decades of Genetic Research on Soybean mosaic virus Resistance in Soybean

This review summarizes the history and current state of the known genetic basis for soybean resistance to Soybean mosaic virus (SMV), and examines how the integration of molecular markers has been utilized in breeding for crop improvement. SVM causes yield loss and seed quality reduction in soybean based on the SMV strain and the host genotype. Understanding the molecular underpinnings of SMV-soybean interactions and the genes conferring resistance to SMV has been a focus of intense research interest for decades. Soybean reactions are classified into three main responses: resistant, necrotic, or susceptible. Significant progress has been achieved that has greatly increased the understanding of soybean germplasm diversity, differential reactions to SMV strains, genotype-strain interactions, genes/alleles conferring specific reactions, and interactions among resistance genes and alleles. Many studies that aimed to uncover the physical position of resistance genes have been published in recent decades, collectively proposing different candidate genes. The studies on SMV resistance loci revealed that the resistance genes are mainly distributed on three chromosomes. Resistance has been pyramided in various combinations for durable resistance to SMV strains. The causative genes are still elusive despite early successes in identifying resistance alleles in soybean; however, a gene at the Rsv4 locus has been well validated.

Overexpression of purple acid phosphatase GmPAP2.1 confers resistance to Soybean mosaic virus in a susceptible soybean cultivar

A purple acid phosphatase, GmPAP2.1, from the soybean (Glycine max) cultivar L29 may function as a resistance factor acting against specific strains of Soybean mosaic virus (SMV). In this study, we found that overexpression of GmPAP2.1 from L29 conferred SMV resistance to a susceptible cultivar, Lee 74. We determined that GmPAP2.1 interacted with the SMV protein P1 in the chloroplasts, resulting in the up-regulation of the ICS1 gene, which in turn promoted the pathogen-induced salicylic acid (SA) pathway. SA accumulation was elevated in response to the co-expression of GmPAP2.1 and SMV, while transient knockdown of endogenous SA-related genes resulted in systemic infection by SMV strain G5H, suggesting that GmPAP2.1-derived resistance depended on the SA-pathway for the activation of a defense response. Our findings thus suggest that GmPAP2.1 purple acid phosphatase of soybean cultivar L29 functions as an SA-pathway-dependent resistance factor acting against SMV.

Pathogenicity and genome-wide sequence analysis reveals relationships between soybean mosaic virus strains

Soybean mosaic virus (SMV) is the most prevalent viral pathogen in soybean. In China, the SMV strains SC and N are used simultaneously in SMV resistance assessments of soybean cultivars, but the pathogenic relationship between them is unclear. In this study, SMV strains N1 and N3 were found to be the most closely related to SC18. Moreover, N3 was found to be more virulent than N1. A global pathotype classification revealed the highest level of genetic diversity in China. The N3 type was the most frequent and widespread worldwide, implying that SMV possibly originated in China and spread across continents through the dissemination of infected soybean. It also suggests that the enhanced virulence of N3 facilitated its spread and adaptability in diverse geographical and ecological regions worldwide. Phylogenetic analysis revealed prominent geographical associations among SMV strains/isolates, and genomic nucleotide diversity analysis and neutrality tests demonstrated that the whole SMV genome is under negative selection, with the P1 gene being under the greatest selection pressure. The results of this study will facilitate the nationwide use of SMV-resistant soybean germplasm and could provide useful insights into the molecular variability, geographical distribution, phylogenetic relationships, and evolutionary history of SMV around the world.

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.

Soybean Resistance to Soybean Mosaic Virus

Soybean mosaic virus (SMV) occurs in all soybean-growing areas in the world and causes huge losses in soybean yields and seed quality. During early viral infection, molecular interactions between SMV effector proteins and the soybean resistance (R) protein, if present, determine the development of resistance/disease in soybean plants. Depending on the interacting strain and cultivar, R-protein in resistant soybean perceives a specific SMV effector, which triggers either the extreme silent resistance or the typical resistance manifested by hypersensitive responses and induction of salicylic acid and reactive oxygen species. In this review, we consider the major advances that have been made in understanding the soybean–SMV arms race. We also focus on dissecting mechanisms SMV employs to establish infection and how soybean perceives and then responds to SMV attack. In addition, progress on soybean R-genes studies, as well as those addressing independent resistance genes, are also addressed.

Precise Exchange of the Helper-Component Proteinase Cistron Between Soybean mosaic virus and Clover yellow vein virus: Impact on Virus Viability and Host Range Specificity

Soybean mosaic virus and Clover yellow vein virus are two definite species of the genus Potyvirus within the family Potyviridae. Soybean mosaic virus-N (SMV-N) is well adapted to cultivated soybean (Glycine max) genotypes and wild soybean (G. soja), whereas it remains undetectable in inoculated broad bean (Vicia faba). In contrast, clover yellow vein virus No. 30 (ClYVV-No. 30) is capable of systemic infection in broad bean and wild soybean; however, it infects cultivated soybean genotypes only locally. In this study, SMV-N was shown to also infect broad bean locally; hence, broad bean is a host for SMV-N. Based on these observations, it was hypothesized that lack of systemic infection by SMV-N in broad bean and by ClYVV-No. 30 in cultivated soybean is attributable to the incompatibility of multifunctional helper-component proteinase (HC-Pro) in these hosts. The logic of selecting the HC-Pro cistron as a target is based on its established function in systemic movement and being a relevant factor in host range specificity of potyviruses. To test this hypothesis, chimeras were constructed with precise exchanges of HC-Pro cistrons between SMV-N and ClYVV-No. 30. Upon inoculation, both chimeras were viable in infection, but host range specificity of the recombinant viruses did not differ from those of the parental viruses. These observations suggest that (i) HC-Pro cistrons from SMV-N and ClYVV-No. 30 are functionally compatible in infection despite 55.6 and 48.9% nucleotide and amino acid sequence identity, respectively, and (ii) HC-Pro cistrons from SMV-N and ClYVV-No. 30 are not the determinants of host specificity on cultivated soybean or broad beans, respectively.

Soybean antiviral immunity conferred by dsRNase targets the viral replication complex

Eukaryotic positive-strand RNA viruses replicate their genomes in membranous compartments formed in a host cell, which sequesters the dsRNA replication intermediate from antiviral immune surveillance. Here, we find that soybean has developed a way to overcome this sequestration. We report the positional cloning of the broad-spectrum soybean mosaic virus resistance gene Rsv4, which encodes an RNase H family protein with dsRNA-degrading activity. An active-site mutant of Rsv4 is incapable of inhibiting virus multiplication and is associated with an active viral RNA polymerase complex in infected cells. These results suggest that Rsv4 enters the viral replication compartment and degrades viral dsRNA. Inspired by this model, we design three plant-gene-derived dsRNases that can inhibit the multiplication of the respective target viruses. These findings suggest a method for developing crops resistant to any target positive-strand RNA virus by fusion of endogenous host genes.

Recessive Resistance Governed by a Major Quantitative Trait Locus Restricts Clover Yellow Vein Virus in Mechanically but Not Graft-Inoculated Cultivated Soybeans

Clover yellow vein virus (ClYVV) infects and causes disease in legume plants. However, here, we found that ClYVV isolate No. 30 (ClYVV-No.30) inefficiently multiplied or spread via cell-to-cell movement in mechanically inoculated leaves of a dozen soybean (Glycine max) cultivars and resulted in failure to spread systemically. Soybean plants also had a similar resistance phenotype against additional ClYVV isolates. In contrast, all but one of 24 tested accessions of wild soybeans (G. soja) were susceptible to ClYVV-No.30. Graft inoculation of cultivated soybean TK780 with ClYVV-No.30-infected wild soybean B01167 scion resulted in systemic infection of the cultivated soybean rootstock. This suggests that, upon mechanical inoculation, the cultivated soybean inhibits ClYVV-No.30, at infection steps prior to the systemic spread of the virus, via vascular systems. Systemic infection of all F1 plants from crossing between TK780 and B01167 and of 68 of 76 F2 plants with ClYVV-No.30 indicated recessive inheritance of the resistance. Further genetic analysis using 64 recombinant inbred lines between TK780 and B01167 detected one major quantitative trait locus, designated d-cv, for the resistance that was positioned in the linkage group D1b (chromosome 2). The mapped region on soybean genome suggests that d-cv is not an allele of the known resistance genes against soybean mosaic virus.

Comprehensive Analysis of Soybean Mosaic Virus P3 Protein Interactors and Hypersensitive Response-Like Lesion-Inducing Protein Function

Soybean mosaic virus (SMV) is one of the most prevalent and important pathogens of soybean, which produces 11 proteins, and the third protein, P3, was suggested to be involved in virus movement and replication, as well as host infection. During the virus infection, host proteins are essential in the virus cycle. However, there is no comprehensive report on the network of host proteins that interact with P3. Fifty-one interactors were identified by using the P3 protein as the bait against the SMV SC15 strain-challenged soybean cDNA library. These proteins were classified into five groups, including transport and protein transport-related proteins, defense and disease-related proteins, photosynthesis proteins, cellular metabolic proteins, and unknown proteins. Among these proteins, the protein defined as hypersensitive response-like lesion-inducing (HRLI) appeared multiple times and showed strong affinity with P3, which indicated its important role in SMV infection. Thus, it was chosen for further investigation. Phylogenetic classification showed that paralog proteins GmHRLI-1 and GmHRLI-2 clustered together and shared 90% homologous identity. Bimolecular fluorescence complementation (BiFC) assay was carried out to confirm the interaction, and fluorescence was detected at the cell periplasmic as well as at the nucleus. Subcellular localization showed that GmHRLI was localized to the cell periplasmic, while the co-localization of GmHRLI and P3 signals was also observed in the nucleus, suggesting that GmHRLI could interact with P3 and promoted the translation of P3 to the nucleus. Moreover, the gene expression of GmHRLI was abundant in the roots, leaves, and flowers, and could be induced by SMV infection, suggesting its involvement in SMV infection. Our results together lay the foundation to explore the mechanisms of P3 in the HR process and the HRLI protein function in SMV response.

Genetic evolutionary analysis of soybean mosaic virus populations from three geographic locations in China based on the P1 and CP genes

Soybean mosaic virus (SMV) is one of the major pathogens causing serious soybean losses. Little is known about the genetic structure and evolutionary biology of the SMV population in southwestern China. In this study, 29 SMV isolates were obtained from Sichuan Province, and the genomic regions encoding the first protein (P1) and coat protein (CP) were sequenced. Combined with SMV isolates from the southeastern and northeastern regions of China, the genetic and molecular evolution of SMV was studied. Recombination analysis revealed that intraspecific and interspecific recombination had occurred in the SMV population. A phylogenetic tree based on the P1 gene reflected the geographic origin of the non-interspecific recombinant SMV (SMV-NI), while a tree based on the CP gene did not. Though frequent gene flow of the SMV-NI populations was found between the southeastern and northeastern populations, the southwestern population was relatively independent. Genetic differentiation was significant between the SMV interspecific recombinant (SMV-RI) and the non-interspecific recombinant (SMV-NI) populations. It was interesting to note that there was an almost identical recombination breakpoint in SMV-RI and Watermelon mosaic virus (WMV). Population dynamics showed that SMV-RI might be in an expanding state, while the SMV-NI population is relatively stable.

A Two-Amino Acid Difference in the Coat Protein of Satellite panicum mosaic virus Isolates Is Responsible for Differential Synergistic Interactions with Panicum mosaic virus

Panicum mosaic virus (PMV) (genus Panicovirus, family Tombusviridae) and its molecular parasite, Satellite panicum mosaic virus (SPMV), synergistically interact in coinfected proso and pearl millet (Panicum miliaceum L.) plants resulting in a severe symptom phenotype. In this study, we examined synergistic interactions between the isolates of PMV and SPMV by using PMV-NE, PMV85, SPMV-KS, and SPMV-Type as interacting partner viruses in different combinations. Coinfection of proso millet plants by PMV-NE and SPMV-KS elicited severe mosaic, chlorosis, stunting, and eventual plant death compared with moderate mosaic, chlorotic streaks, and stunting by PMV85 and SPMV-Type. In reciprocal combinations, coinfection of proso millet by either isolate of PMV with SPMV-KS but not with SPMV-Type elicited severe disease synergism, suggesting that SPMV-KS was the main contributor for efficient synergistic interaction with PMV isolates. Coinfection of proso millet plants by either isolate of PMV and SPMV-KS or SPMV-Type caused increased accumulation of coat protein (CP) and genomic RNA copies of PMV, compared with infections by individual PMV isolates. Additionally, CP and genomic RNA copies of SPMV-KS accumulated at substantially higher levels, compared with SMPV-Type in coinfected proso millet plants with either isolate of PMV. Hybrid viruses between SPMV-KS and SPMV-Type revealed that SPMV isolates harboring a CP fragment with four differing amino acids at positions 18, 35, 59, and 98 were responsible for differential synergistic interactions with PMV in proso millet plants. Mutation of amino acid residues at these positions in different combinations in SPMV-KS, similar to those as in SPMV-Type or vice-versa, revealed that A35 and R98 in SPMV-KS CP play critical roles in enhanced synergistic interactions with PMV isolates. Taken together, these data suggest that the two distinct amino acids at positions 35 and 98 in the CP of SPMV-KS and SPMV-Type are involved in the differential synergistic interactions with the helper viruses.

Endoplasmic Reticulum Plays a Critical Role in Integrating Signals Generated by Both Biotic and Abiotic Stress in Plants

Most studies of environmental adaptations in plants have focused on either biotic or abiotic stress factors in an attempt to understand the defense mechanisms of plants against individual stresses. However, in the natural ecosystem, plants are simultaneously exposed to multiple stresses. Stress-tolerant crops developed in translational studies based on a single stress often fail to exhibit the expected traits in the field. To adapt to abiotic stress, recent studies have identified the need for interactions of plants with various microorganisms. These findings highlight the need to understand the multifaceted interactions of plants with biotic and abiotic stress factors. The endoplasmic reticulum (ER) is an organelle that links various stress responses. To gain insight into the molecular integration of biotic and abiotic stress responses in the ER, we focused on the interactions of plants with RNA viruses. This interaction points toward the relevance of ER in viral pathogenicity as well as plant responses. In this mini review, we explore the molecular crosstalk between biotic and abiotic stress signaling through the ER by elaborating ER-mediated signaling in response to RNA viruses and abiotic stresses. Additionally, we summarize the results of a recent study on phytohormones that induce ER-mediated stress response. These studies will facilitate the development of multi-stress-tolerant transgenic crops in the future.

Dynamin-Like Proteins of Endocytosis in Plants Are Coopted by Potyviruses To Enhance Virus Infection

Endocytosis and endosomal trafficking regulate the proteins targeted to the plasma membrane and play essential roles in diverse cellular processes, including responses to pathogen attack. Here, we report the identification of Glycine max (soybean) endocytosis dynamin-like protein 5A (GmSDL5A) associated with purified soybean mosaic virus (SMV) virions from soybean using a bottom-up proteomics approach. Knockdown of GmSDL5A and its homologous gene GmSDL12A inhibits SMV infection in soybean. The role of analogous dynamin-like proteins in potyvirus infection was further confirmed and investigated using the Arabidopsis/turnip mosaic virus (TuMV) pathosystem. We demonstrate that dynamin-related proteins 2A and 2B in Arabidopsis thaliana (AtDRP2A, AtDRP2B), homologs of GmSDL5A, are recruited to the virus replication complex (VRC) of TuMV. TuMV infection is inhibited in both A. thalianadrp2a (atdrp2a) and atdrp2b knockout mutants. Overexpression of AtDRP2 promotes TuMV replication and intercellular movement. AtRDP2 interacts with TuMV VPg, CP, CI, and 6K2. Of these viral proteins, VPg, CP, and CI are essential for viral intercellular movement, and 6K2, VPg, and CI are critical components of the VRC. We reveal that VPg and CI are present in the punctate structures labeled by the endocytic tracer FM4-64, suggesting that VPg and CI can be endocytosed. Treatment of plant leaves with a dynamin-specific inhibitor disrupts the delivery of VPg and CI to endocytic structures and suppresses TuMV replication and intercellular movement. Taken together, these data suggest that dynamin-like proteins are novel host factors of potyviruses and that endocytic processes are involved in potyvirus infection.IMPORTANCE It is well known that animal viruses enter host cells via endocytosis, whereas plant viruses require physical assistance, such as human and insect activities, to penetrate the host cell to establish their infection. In this study, we report that the endocytosis pathway is also involved in virus infection in plants. We show that plant potyviruses recruit endocytosis dynamin-like proteins to support their infection. Depletion of them by knockout of the corresponding genes suppresses virus replication, whereas overexpression of them enhances virus replication and intercellular movement. We also demonstrate that the dynamin-like proteins interact with several viral proteins that are essential for virus replication and cell-to-cell movement. We further show that treatment of a dynamin-specific inhibitor disrupts endocytosis and inhibits virus replication and intercellular movement. Therefore, the dynamin-like proteins are novel host factors of potyviruses. The corresponding genes may be manipulated using advanced biotechnology to control potyviral diseases.

Soybean mosaic virus: a successful potyvirus with a wide distribution but restricted natural host range

TAXONOMY

Soybean mosaic virus (SMV) is a species within the genus Potyvirus, family Potyviridae, which includes almost one-quarter of all known plant RNA viruses affecting agriculturally important plants. The Potyvirus genus is the largest of all genera of plant RNA viruses with 160 species.

PARTICLE

The filamentous particles of SMV, typical of potyviruses, are about 7500 Å long and 120 Å in diameter with a central hole of about 15 Å in diameter. Coat protein residues are arranged in helices of about 34 Å pitch having slightly less than nine subunits per turn.

GENOME

The SMV genome consists of a single-stranded, positive-sense, polyadenylated RNA of approximately 9.6 kb with a virus-encoded protein (VPg) linked at the 5' terminus. The genomic RNA contains a single large open reading frame (ORF). The polypeptide produced from the large ORF is processed proteolytically by three viral-encoded proteinases to yield about 10 functional proteins. A small ORF, partially overlapping the P3 cistron, pipo, is encoded as a fusion protein in the N-terminus of P3 (P3N + PIPO).

BIOLOGICAL PROPERTIES

SMV's host range is restricted mostly to two plant species of a single genus: Glycine max (cultivated soybean) and G. soja (wild soybean). SMV is transmitted by aphids non-persistently and by seeds. The variability of SMV is recognized by reactions on cultivars with dominant resistance (R) genes. Recessive resistance genes are not known.

GEOGRAPHICAL DISTRIBUTION AND ECONOMIC IMPORTANCE

As a consequence of its seed transmissibility, SMV is present in all soybean-growing areas of the world. SMV infections can reduce significantly seed quantity and quality (e.g. mottled seed coats, reduced seed size and viability, and altered chemical composition).

CONTROL

The most effective means of managing losses from SMV are the planting of virus-free seeds and cultivars containing single or multiple R genes.

KEY ATTRACTIONS

The interactions of SMV with soybean genotypes containing different dominant R genes and an understanding of the functional role(s) of SMV-encoded proteins in virulence, transmission and pathogenicity have been investigated intensively. The SMV-soybean pathosystem has become an excellent model for the examination of the genetics and genomics of a uniquely complex gene-for-gene resistance model in a crop of worldwide importance.

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.

The Current Status of the Soybean-Soybean Mosaic Virus (SMV) Pathosystem

Soybean mosaic virus (SMV) is one of the most devastating pathogens that cost huge economic losses in soybean production worldwide. Due to the duplicated genome, clustered and highly homologous nature of R genes, as well as recalcitrant to transformation, soybean disease resistance studies is largely lagging compared with other diploid crops. In this review, we focus on the major advances that have been made in identifying both the virulence/avirulence factors of SMV and mapping of SMV resistant genes in soybean. In addition, we review the progress in dissecting the SMV resistant signaling pathways in soybean, with a special focus on the studies using virus-induced gene silencing. The soybean genome has been fully sequenced, and the increasingly saturated SNP markers have been identified. With these resources available together with the newly developed genome editing tools, and more efficient soybean transformation system, cloning SMV resistant genes, and ultimately generating cultivars with a broader spectrum resistance to SMV are becoming more realistic than ever.

Gain of virulence by Soybean mosaic virus on Rsv4-genotype soybeans is associated with a relative fitness loss in a susceptible host

'Gene-for-gene' theory predicts that gain of virulence by an avirulent pathogen on plants expressing resistance (R) genes is associated with fitness loss in susceptible hosts. However, the validity of this prediction has been studied in only a few plant viral pathosystems. In this study, the Soybean mosaic virus (SMV)-Rsv4 pathosystem was exploited to test this prediction. In Rsv4-genotype soybeans, P3 of avirulent SMV strains provokes an as yet uncharacterized resistance mechanism that restricts the invading virus to the inoculated leaves. A single amino acid substitution in P3 functionally converts an avirulent to a virulent strain, suggesting that the genetic composition of P3 plays a crucial role in virulence on Rsv4-genotype soybeans. In this study, we examined the impact of gain of virulence mutation(s) on the fitness of virulent variants derived from three avirulent SMV strains in a soybean genotype lacking the Rsv4 gene. Our data demonstrate that gain of virulence mutation(s) by all avirulent viruses on Rsv4-genotype soybean is associated with a relative fitness loss in a susceptible host. The implications of this finding on the durable deployment of the Rsv4 gene in soybean are discussed.

Molecular Soybean-Pathogen Interactions

Soybean hosts a wide variety of pathogens that cause significant yield losses. The importance of soybean as a major oilseed crop has led to research focused on its interactions with pathogens, such as Soybean mosaic virus, Pseudomonas syringae, Phytophthora sojae, Phakopsora pachyrhizi, and Heterodera glycines. Pioneering work on soybean's interactions with these organisms, which represent the five major pathogen groups (viruses, bacteria, oomycetes, fungi, and nematodes), has contributed to our understanding of the molecular mechanisms underlying virulence and immunity. These mechanisms involve conserved and unique features that validate the need for research in both soybean and homologous model systems. In this review, we discuss identification of effectors and their functions as well as resistance gene-mediated recognition and signaling. We also point out areas in which model systems and recent advances in resources and tools have provided opportunities to gain deeper insights into soybean-pathogen interactions.

A determinant of disease symptom severity is located in RNA2 of broad bean wilt virus 2

Broad bean wilt virus 2 (BBWV2), which belongs to the genus Fabavirus, is a destructive pathogen of many economically important horticultural and ornamental crops. In this study, we constructed infectious full-length cDNA clones of two distinct isolates of BBWV2 under control of the cauliflower mosaic virus 35S promoter. BBWV2-PAP1 isolated from paprika (Capsicum annuum var. gulosum) induces severe disease symptoms in various pepper varieties, whereas BBWV2-RP1 isolated from red pepper (Capsicum annuum L.) causes mild symptoms. Agrobacterium-mediated inoculation of the infectious cDNA clones of BBWV2-PAP1 and RP1 resulted in the same symptoms as the original virus isolates. The infectious cDNA clones of BBWV2-PAP1 and RP1 were used to examine the symptoms induced by pseudorecombinants between the two isolates to localize in which of the two genomic RNAs are the symptom severity determinants in BBWV2. The pseudorecombinant of RP1-RNA1 and PAP1-RNA2 induced severe symptoms, similar to those caused by the parental isolate PAP1, whereas the pseudorecombinant of PAP1-RNA1 and RP1-RNA2 induced mild symptoms, similar to those caused by the parental isolate RP1. Our results suggest that BBWV2 RNA2 contains a symptom determinant(s) capable of enhancing symptom severity.

The evolution of soybean mosaic virus: An updated analysis by obtaining 18 new genomic sequences of Chinese strains/isolates

Soybean mosaic virus (SMV) is widely recognized as a highly damaging pathogen of soybean, and various strains/isolates have been reported to date. However, the pathogenic differences and phylogenetic relationships of these SMV strains/isolates have not been extensively studied. In the present work, by first obtaining 18 new genomic sequences of Chinese SMV strains/isolates and further compiling these with available data, we have explored the evolution of SMV from multiple aspects. First, as in other potyviruses, recombination has occurred frequently during SMV evolution, and a total of 32 independent events were detected. Second, using a maximum-likelihood method and removing recombinant fragments, a phylogeny covering 83 SMV sequences sampled from all over the world was reconstructed and the results showed four separate SMV clades, with clade I and II recovered for the first time. Third, the population structure analysis of SMV revealed significant genetic differentiations between China and two other countries (Korea and U.S.A.). Fourth, certain SMV-encoded genes, such as P1, HC-Pro and P3, exhibited higher non-synonymous substitution rate (dN) than synonymous substitution rate (dS), indicating that positive selection has influenced these genes. Finally, four Chinese SMV strains/isolates were selected for inoculation of both USA and Chinese differential soybean cultivars, and their pathogenic phenotypes were significantly different from that of the American strains. Overall, these findings have further broadened our understanding on SMV evolution, which would assist researchers to better deal with this harmful virus.

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.

Amino acid substitution in P3 of Soybean mosaic virus to convert avirulence to virulence on Rsv4-genotype soybean is influenced by the genetic composition of P3

The modification of avirulence factors of plant viruses by one or more amino acid substitutions converts avirulence to virulence on hosts containing resistance genes. Limited experimental studies have been conducted on avirulence/virulence factors of plant viruses, in particular those of potyviruses, to determine whether avirulence/virulence sites are conserved among strains. In this study, the Soybean mosaic virus (SMV)-Rsv4 pathosystem was exploited to determine whether: (i) avirulence/virulence determinants of SMV reside exclusively on P3 regardless of virus strain; and (ii) the sites residing on P3 and crucial for avirulence/virulence of isolates belonging to strain G2 are also involved in virulence of avirulent isolates belonging to strain G7. The results confirm that avirulence/virulence determinants of SMV on Rsv4-genotype soybean reside exclusively on P3. Furthermore, the data show that sites involved in the virulence of SMV on Rsv4-genotype soybean vary among strains, with the genetic composition of P3 playing a crucial role.

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.

Type 2C protein phosphatase is a key regulator of antiviral extreme resistance limiting virus spread

Effector-triggered immunity (ETI) is an active immune response triggered by interactions between host resistance proteins and their cognate effectors. Although ETI is often associated with the hypersensitive response (HR), various R genes mediate an HR-independent process known as extreme resistance (ER). In the soybean-Soybean mosaic virus (SMV) pathosystem, the strain-specific CI protein of SMV functions as an effector of Rsv3-mediated ER. In this study, we used the soybean (Rsv3)-SMV (CI) pathosystem to gain insight into the molecular signaling pathway involved in ER. We used genome-wide transcriptome analysis to identify a subset of the type 2C protein phophatase (PP2C) genes that are specifically up-regulated in Rsv3-mediated ER. Gain-of-function analysis of the most significantly expressed soybean PP2C gene, GmPP2C3a, showed that ABA-induced GmPP2C3a functions as a key regulator of Rsv3-mediated ER. Our results further suggest that the primary mechanism of ER against viruses is the inhibition of viral cell-to-cell movement by callose deposition in an ABA signaling-dependent manner.

Control of virus diseases in soybeans

Soybean, one of the world's most important sources of animal feed and vegetable oil, can be infected by numerous viruses. However, only a small number of the viruses that can potentially infect soybean are considered as major economic problems to soybean production. Therefore, we consider management options available to control diseases caused by eight viruses that cause, or have the potential to cause, significant economic loss to producers. We summarize management tactics in use and suggest direction for the future. Clearly, the most important tactic is disease resistance. Several resistance genes are available for three of the eight viruses discussed. Other options include use of virus-free seed and avoidance of alternative virus hosts when planting. Attempts at arthropod vector control have generally not provided consistent disease management. In the future, disease management will be considerably enhanced by knowledge of the interaction between soybean and viral proteins. Identification of genes required for soybean defense may represent key regulatory hubs that will enhance or broaden the spectrum of basal resistance to viruses. It may be possible to create new recessive or dominant negative alleles of host proteins that do not support viral functions but perform normal cellular function. The future approach to virus control based on gene editing or exploiting allelic diversity points to necessary research into soybean-virus interactions. This will help to generate the knowledge needed for rational design of durable resistance that will maximize global production.

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.

Genomic segments RNA1 and RNA2 of Prunus necrotic ringspot virus codetermine viral pathogenicity to adapt to alternating natural Prunus hosts

Prunus necrotic ringspot virus (PNRSV) affects Prunus fruit production worldwide. To date, numerous PNRSV isolates with diverse pathological properties have been documented. To study the pathogenicity of PNRSV, which directly or indirectly determines the economic losses of infected fruit trees, we have recently sequenced the complete genome of peach isolate Pch12 and cherry isolate Chr3, belonging to the pathogenically aggressive PV32 group and mild PV96 group, respectively. Here, we constructed the Chr3- and Pch12-derived full-length cDNA clones that were infectious in the experimental host cucumber and their respective natural Prunus hosts. Pch12-derived clones induced much more severe symptoms than Chr3 in cucumber, and the pathogenicity discrepancy between Chr3 and Pch12 was associated with virus accumulation. By reassortment of genomic segments, swapping of partial genomic segments, and site-directed mutagenesis, we identified the 3' terminal nucleotide sequence (1C region) in RNA1 and amino acid K at residue 279 in RNA2-encoded P2 as the severe virulence determinants in Pch12. Gain-of-function experiments demonstrated that both the 1C region and K279 of Pch12 were required for severe virulence and high levels of viral accumulation. Our results suggest that PNRSV RNA1 and RNA2 codetermine viral pathogenicity to adapt to alternating natural Prunus hosts, likely through mediating viral accumulation.

SCE1, the SUMO-conjugating enzyme in plants that interacts with NIb, the RNA-dependent RNA polymerase of Turnip mosaic virus, is required for viral infection

SUMOylation, which is catalyzed by small ubiquitin-like modifier (SUMO) enzymes, is a transient, reversible posttranslational protein modification that regulates diverse cellular processes. Potyviruses, the largest group of known plant viruses, comprise many agriculturally important viruses, such as Turnip mosaic virus (TuMV). The potyviral genome encodes 11 mature proteins. To investigate if SUMOylation plays a role in potyvirus infection, a yeast two-hybrid screen was performed to examine possible interactions of each of the 11 viral proteins of TuMV with AtSCE1, the only SUMO-conjugating enzyme in Arabidopsis thaliana homologous to the key SUMO-conjugating enzyme E2 in mammalian cells or Ubc9 in yeast. A positive reaction was found between AtSCE1 and NIb, the potyviral RNA-dependent RNA polymerase. Further bimolecular fluorescence complementation (BiFC) and fluorescence resonance energy transfer (FRET) assays revealed that the NIb and AtSCE1 interaction occurred in both the cytoplasm and nuclei of epidermal cells of Nicotiana benthamiana. The interaction motif was mapped to a region encompassing NIb amino acids 171 to 300 which contains a potential negatively charged amino acid-dependent SUMOylation motif (NDSM). An Escherichia coli SUMOylation assay showed that NIb can be SUMOylated and that the lysine residue (K172) in the motif is a potent SUMOylation site. A TuMV infectious clone with an arginine (R) substitution mutation at K172 compromised TuMV infectivity in plants. In comparison with wild-type Arabidopsis plants, sce1 knockdown plants exhibited increased resistance to TuMV as well as a nonrelated RNA virus. To the best of our knowledge, this is the first report showing that the host SUMO modification system plays an essential role in infection by plant RNA viruses.

The HC-Pro and P3 cistrons of an avirulent Soybean mosaic virus are recognized by different resistance genes at the complex Rsv1 locus

The complex Rsv1 locus in soybean plant introduction (PI) 'PI96983' confers extreme resistance (ER) against Soybean mosaic virus (SMV) strain N but not SMV-G7 and SMV-G7d. Both the SMV helper-component proteinase (HC-Pro) and P3 cistrons can serve as avirulence factors recognized by Rsv1. To understand the genetics underlying recognition of the two cistrons, we have utilized two soybean lines (L800 and L943) derived from crosses between PI96983 (Rsv1) and Lee68 (rsv1) with distinct recombination events within the Rsv1 locus. L800 contains a single PI96983-derived member (3gG2) of an Rsv1-associated subfamily of nucleotide-binding leucine-rich repeat (NB-LRR) genes. In contrast, although L943 lacks 3gG2, it contains a suite of five other NB-LRR genes belonging to the same family. L800 confers ER against SMV-N whereas L943 allows limited replication at the inoculation site. SMV-N-derived chimeras containing HC-Pro from SMV-G7 or SMV-G7d gained virulence on L943 but not on L800 whereas those with P3 replacement gained virulence on L800 but not on L943. In reciprocal experiments, SMV-G7- and SMV-G7d-derived chimeras with HC-Pro replacement from SMV-N lost virulence on L943 but retained virulence on L800 whereas those with P3 replacement lost virulence on L800 while remaining virulent on L943. These data demonstrate that distinct resistance genes at the Rsv1 locus, likely belonging to the NB-LRR class, mediate recognition of HC-Pro and P3.

Evaluation of North American isolates of Soybean mosaic virus for gain of virulence on Rsv-genotype soybeans with special emphasis on resistance-breaking determinants on Rsv4

Resistance to Soybean mosaic virus (SMV) in soybean is conferred by three dominant genes: Rsv1, Rsv3 and Rsv4. Over the years, scientists in the USA have utilized a set of standard pathotypes, SMV-G1 to SMV-G7, to study interaction with Rsv-genotype soybeans. However, these pathotypes were isolated from a collection of imported soybean germplasm over 30 years ago. In this study, 35 SMV field isolates collected in recent years from 11 states were evaluated for gain of virulence on soybean genotypes containing individual Rsv genes. All isolates were avirulent on L78-379 (Rsv1), whereas 19 were virulent on L29 (Rsv3). On PI88788 (Rsv4), 14 of 15 isolates tested were virulent; however, only one was capable of systemically infecting all of the inoculated V94-5152 (Rsv4). Nevertheless, virulent variants from 11 other field isolates were rapidly selected on initial inoculation onto V94-5152 (Rsv4). The P3 cistrons of the original isolates and their variants on Rsv4-genotype soybeans were sequenced. Analysis showed that virulence on PI88788 (Rsv4) was not associated, in general, with selection of any new amino acid, whereas Q1033K and G1054R substitutions were consistently selected on V94-5152 (Rsv4). The role of Q1033K and G1054R substitutions, individually or in combination, in virulence on V94-5152 (Rsv4) was confirmed on reconstruction in the P3 cistron of avirulent SMV-N, followed by biolistic inoculation. Collectively, our data demonstrate that SMV has evolved virulence towards Rsv3 and Rsv4, but not Rsv1, in the USA. Furthermore, they confirm that SMV virulence determinants on V94-5152 (Rsv4) reside on P3.

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.

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.

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.

The helper component proteinase cistron of Potato virus Y induces hypersensitivity and resistance in Potato genotypes carrying dominant resistance genes on chromosome IV

The Nc(tbr) and Ny(tbr) genes in Solanum tuberosum determine hypersensitive reactions, characterized by necrotic reactions and restriction of the virus systemic movement, toward isolates belonging to clade C and clade O of Potato virus Y (PVY), respectively. We describe a new resistance from S. sparsipilum which possesses the same phenotype and specificity as Nc(tbr) and is controlled by a dominant gene designated Nc(spl). Nc(spl) maps on potato chromosome IV close or allelic to Ny(tbr). The helper component proteinase (HC-Pro) cistron of PVY was shown to control necrotic reactions and resistance elicitation in plants carrying Nc(spl), Nc(tbr), and Ny(tbr). However, inductions of necrosis and of resistance to the systemic virus movement in plants carrying Nc(spl) reside in different regions of the HC-Pro cistron. Also, genomic determinants outside the HC-Pro cistron are involved in the systemic movement of PVY after induction of necroses on inoculated leaves of plants carrying Ny(tbr). These results suggest that the Ny(tbr) resistance may have been involved in the recent emergence of PVY isolates with a recombination breakpoint near the junction of HC-Pro and P3 cistrons in potato crops. Therefore, this emergence could constitute one of the rare examples of resistance breakdown by a virus which was caused by recombination instead of by successive accumulation of nucleotide substitutions.

Identification of a novel Soybean mosaic virus isolate in China that contains a unique 5' terminus sharing high sequence homology with Bean common mosaic virus

Two soybean viral isolates 4469-4 and Sc6 from plants in China were characterized by serological assays, pathogenicity tests, full-genome sequencing and sequence analyses. Both isolates are determined to be Soybean mosaic virus (SMV) isolates but 4469-4 is different from other known SMVs by inducing symptoms on common bean. 4469-4 has an RNA genome of 9994 nucleic acids (nt) encoding 3202 amino acids (aa), which is approximately 400 nt longer than that of Sc6 and other SMV strains. Comparison with SMV and SMV-related potyviruses suggests that 4469-4 shares high nt and aa sequence identify (>92%) with other SMV strains. However, significant diversity between 4469-4 and other SMV strains was observed in the 5' genomic region. In contrast, this region is highly similar to the corresponding region of Bean common mosaic virus (BCMV). Recombination analyses conclude that there is a recombination site near the nt 900 of 4469-4. Taken together these data suggest that 4469-4 may result from recombination between SMV and BCMV or a BCMV-like virus in the N-terminus of the genome.