To Turnip Mosaic Virus Research Articles

EIF2Bβ confers resistance to Turnip mosaic virus by recruiting ALKBH9B to modify viral RNA methylation.

Eukaryotic translation initiation factors (eIFs) are the primary targets for overcoming RNA virus resistance in plants. In a previous study, we mapped a BjeIF2Bβ from Brassica juncea representing a new class of plant virus resistance genes associated with resistance to Turnip mosaic virus (TuMV). However, the mechanism underlying eIF2Bβ-mediated virus resistance remains unclear. In this study, we discovered that the natural variation of BjeIF2Bβ in the allopolyploid B. juncea was inherited from one of its ancestors, B. rapa. By editing of eIF2Bβ, we were able to confer resistance to TuMV in B. juncea and in its sister species of B. napus. Additionally, we identified an N6-methyladenosine (m6A) demethylation factor, BjALKBH9B, for interaction with BjeIF2Bβ, where BjALKBH9B co-localized with both BjeIF2Bβ and TuMV. Furthermore, BjeIF2Bβ recruits BjALKBH9B to modify the m6A status of TuMV viral coat protein RNA, which lacks the ALKB homologue in its genomic RNA, thereby affecting viral infection. Our findings have applications for improving virus resistance in the Brassicaceae family through natural variation or genome editing of the eIF2Bβ. Moreover, we uncovered a non-canonical translational control of viral mRNA in the host plant.

Glutathione Contribution in Interactions between Turnip mosaic virus and Arabidopsis thaliana Mutants Lacking Respiratory Burst Oxidase Homologs D and F.

Respiratory burst oxidase homologs (Rbohs) play crucial and diverse roles in plant tissue-mediated production of reactive oxygen species during the development, growth, and response of plants to abiotic and biotic stress. Many studies have demonstrated the contribution of RbohD and RbohF in stress signaling in pathogen response differentially modulating the immune response, but the potential role of the Rbohs-mediated response in plant-virus interactions remains unknown. The present study analyzed, for the first time, the metabolism of glutathione in rbohD-, rbohF-, and rbohD/F-transposon-knockout mutants in response to Turnip mosaic virus (TuMV) infection. rbohD-TuMV and Col-0-TuMV interactions were characterized by susceptible reaction to TuMV, associated with significant activity of GPXLs (glutathione peroxidase-like enzymes) and induction of lipid peroxidation in comparison to mock-inoculated plants, with reduced total cellular and apoplastic glutathione content observed at 7-14 dpi and dynamic induction of apoplast GSSG (oxidized glutathione) at 1-14 dpi. Systemic virus infection resulted in the induction of AtGSTU1 and AtGSTU24, which was highly correlated with significant downregulation of GSTs (glutathione transferases) and cellular and apoplastic GGT (γ-glutamyl transferase) with GR (glutathione reductase) activities. On the contrary, resistant rbohF-TuMV reactions, and especially enhanced rbohD/F-TuMV reactions, were characterized by a highly dynamic increase in total cellular and apoplastic glutathione content, with induction of relative expression of AtGGT1, AtGSTU13, and AtGSTU19 genes. Moreover, virus limitation was highly correlated with the upregulation of GSTs, as well as cellular and apoplastic GGT with GR activities. These findings clearly indicate that glutathione can act as a key signaling factor in not only susceptible rbohD reaction but also the resistance reaction presented by rbohF and rbohD/F mutants during TuMV interaction. Furthermore, by actively reducing the pool of glutathione in the apoplast, GGT and GR enzymes acted as a cell first line in the Arabidopsis-TuMV pathosystem response, protecting the cell from oxidative stress in resistant interactions. These dynamically changed signal transductions involved symplast and apoplast in mediated response to TuMV.

Identification and genetic inheritance of a new source of broad-spectrum extreme resistance to turnip mosaic virus (TuMV) in Brassica rapa

Brassica rapa line K185 was identified with broad-spectrum extreme resistance to turnip mosaic virus (TuMV). The resistance was effective against TuMV isolates UK 1, CZE 1, GBR 6, POL 1 and CDN 1 (representing major pathotypes 1, 3 and 4 and major resistance-breaking isolates) following mechanical inoculation. F1 plants from a cross between K185 and the rapid-cycling B. rapa ssp. trilocularis line R-o-18 (uniformly susceptible to all the above TuMV isolates), were resistant following challenge with TuMV isolates CDN 1 and GBR 6, indicating the involvement of dominant gene(s). F2 plants derived from a single F1, CDN 1- and GBR 6-resistant plant segregated for resistance to TuMV isolate GBR 6. The segregation ratio of resistant to susceptible plants was consistent with at least two loci controlling resistance and with both loci having dominant alleles for resistance. Other sources of broad-spectrum resistance to TuMV have been shown to involve the recessive allele retr01. The broad-spectrum resistance in K185 clearly involves different alleles to retr01 and provides the opportunity to introgress an alternative form of TuMV resistance into commercial B. rapa lines and thereby reduce the selection pressure for retr01 resistance breaking.

A new potyvirus from hedge mustard (Sisymbrium officinale (L.) Scop.) sheds light on the evolutionary history of turnip mosaic virus.

A novel potyvirus was identified in symptomatic hedge mustard (Sisymbrium officinale (L.) Scop.) and wild radish (Raphanus raphanistrum L.) in France. The nearly complete genome sequence of hedge mustard mosaic virus (HMMV) was determined, demonstrating that it belongs to a sister species to turnip mosaic virus (TuMV). HMMV readily infected several other members of the family Brassicaceae, including turnip, shepherd's purse (Capsella bursa-pastoris), and arabidopsis. The identification of HMMV as a Brassicaceae-infecting virus closely related to TuMV leads us to question the current scenario of TuMV evolution and suggests a possible alternative one in which transition from a monocot-adapted ancestral lifestyle to a Brassicaceae-adapted one could have occurred earlier than previously recognized.Please check and confirm that the authors and their respective affiliations have been correctly identified and amend if necessary.all OK.

Functional Analysis of BcSNX3 in Regulating Resistance to Turnip Mosaic Virus (TuMV) by Autophagy in Pak-choi (Brassica campestris ssp. chinensis)

Sorting nexin protein is a class of highly conserved eukaryotic proteins containing the PX domain. Recent studies related to SNX in plants have focused on the regulation of abiotic stress processes, and there are few studies on the involvement of SNX in biological stress processes in plants. In this paper, a YTH assay and BiFC experiments were conducted twice to show that BcSNX3 (Brassica campestris Sorting nexin 3) interacted with CP and VPg of TuMV, and the interaction between BcATG8h (Brassica campestris autophagy-related gene 8h) and BcSNX3 was also found by YTH and BiFC. The colocalization of BcSNX3 and BcATG8b (Brassica campestris autophagy-related gene 8b) revealed BcSNX3 and autophagosome at the same place in the cell. QRT-PCR analysis showed that TuMV infection promotes the expression of BcSNX3, and the overexpression of this gene hinders the expression of autophagy-related genes and facilitates TuMV infection. VIGS was used to repress the expression level of the BcSNX3 gene in pak-choi to further study the function of BcSNX3 in the infection process of TuMV. After inoculation with TuMV, it was found that the accumulation of viral RNA in BcSNX3-gene-silenced plants was significantly less than in control plants. The accumulation of TuMV virus in the Arabidopsis snx3 knockout mutant was also less than in the wild type after TuMV inoculation. These results suggest that TuMV infection facilitates the expression of BcSNX3, and this gene may promote virus infection by inhibiting autophagy degradation of the virus and interacting with the CP and VPg of the virus. These results lay the foundation for the TuMV resistance breeding of pak-choi.

When a knockout is an Achilles' heel: Resistance to one potyvirus species triggers hypersusceptibility to another one in Arabidopsis thaliana.

The translation initiation factors 4E are a small family of major susceptibility factors to potyviruses. It has been suggested that knocking out these genes could provide genetic resistance in crops when natural resistance alleles, which encode functional eIF4E proteins, are not available. Here, using the well‐characterized Arabidopsis thaliana–potyvirus pathosystem, we evaluate the resistance spectrum of plants knocked out for eIF4E1, the susceptibility factor to clover yellow vein virus (ClYVV). We show that besides resistance to ClYVV, the eIF4E1 loss of function is associated with hypersusceptibility to turnip mosaic virus (TuMV), a potyvirus known to rely on the paralog host factor eIFiso4E. On TuMV infection, plants knocked out for eIF4E1 display striking developmental defects such as early senescence and primordia development stoppage. This phenotype is coupled with a strong TuMV overaccumulation throughout the plant, while remarkably the levels of the viral target eIFiso4E remain uninfluenced. Our data suggest that this hypersusceptibility cannot be explained by virus evolution leading to a gain of TuMV aggressiveness. Furthermore, we report that a functional eIF4E1 resistance allele engineered by CRISPR/Cas9 base‐editing technology successfully circumvents the increase of TuMV susceptibility conditioned by eIF4E1 disruption. These findings in Arabidopsis add to several previous findings in crops suggesting that resistance based on knocking out eIF4E factors should be avoided in plant breeding, as it could also expose the plant to the severe threat of potyviruses able to recruit alternative eIF4E copies. At the same time, it provides a simple model that can help understanding of the homeostasis among eIF4E proteins in the plant cell and what makes them available to potyviruses.

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.

Guanosine tetraphosphate modulates salicylic acid signalling and the resistance of Arabidopsis thaliana to Turnip mosaic virus

Chloroplasts can act as key players in the perception and acclimatization of plants to incoming environmental signals. A growing body of evidence indicates that chloroplasts play a critical role in plant immunity. Chloroplast function can be regulated by the nucleotides guanosine tetraphosphate and pentaphosphate [(p)ppGpp]. In plants, (p)ppGpp levels increase in response to abiotic stress and to plant hormones which are involved in abiotic and biotic stress signalling. In this study, we analysed the transcriptome of Arabidopsis plants that over-accumulate (p)ppGpp, and unexpectedly found a decrease in the levels of a broad range of transcripts for plant defence and immunity. To determine whether (p)ppGpp is involved in the modulation of plant immunity, we analysed the susceptibility of plants with different levels of (p)ppGpp to Turnip mosaic virus (TuMV) carrying a green fluorescent protein (GFP) reporter. We found that (p)ppGpp accumulation was associated with increased susceptibility to TuMV and reduced levels of the defence hormone salicylic acid (SA). In contrast, plants with lower (p)ppGpp levels showed reduced susceptibility to TuMV, and this was associated with the precocious up-regulation of defence-related genes and increased SA content. We have therefore demonstrated a new link between (p)ppGpp metabolism and plant immunity in Arabidopsis.

Engineering of CRISPR/Cas9-mediated potyvirus resistance in transgene-free Arabidopsis plants.

SummaryMembers of the eukaryotic translation initiation factor (eIF) gene family, including eIF4E and its paralogue eIF(iso)4E, have previously been identified as recessive resistance alleles against various potyviruses in a range of different hosts. However, the identification and introgression of these alleles into important crop species is often limited. In this study, we utilise CRISPR/Cas9 technology to introduce sequence‐specific deleterious point mutations at the eIF(iso)4E locus in Arabidopsis thaliana to successfully engineer complete resistance to Turnip mosaic virus (TuMV), a major pathogen in field‐grown vegetable crops. By segregating the induced mutation from the CRISPR/Cas9 transgene, we outline a framework for the production of heritable, homozygous mutations in the transgene‐free T2 generation in self‐pollinating species. Analysis of dry weights and flowering times for four independent T3 lines revealed no differences from wild‐type plants under standard growth conditions, suggesting that homozygous mutations in eIF(iso)4E do not affect plant vigour. Thus, the established CRISPR/Cas9 technology provides a new approach for the generation of Potyvirus resistance alleles in important crops without the use of persistent transgenes.

Transgenic Brassica rapa plants over‐expressing eIF(iso)4E variants show broad‐spectrum Turnip mosaic virus (TuMV) resistance

The protein-protein interaction between VPg (viral protein genome-linked) of potyviruses and eIF4E (eukaryotic initiation factor 4E) or eIF(iso)4E of their host plants is a critical step in determining viral virulence. In this study, we evaluated the approach of engineering broad-spectrum resistance in Chinese cabbage (Brassica rapa) to Turnip mosaic virus (TuMV), which is one of the most important potyviruses, by a systematic knowledge-based approach to interrupt the interaction between TuMV VPg and B. rapa eIF(iso)4E. The seven amino acids in the cap-binding pocket of eIF(iso)4E were selected on the basis of other previous results and comparison of protein models of cap-binding pockets, and mutated. Yeast two-hybrid assay and co-immunoprecipitation analysis demonstrated that W95L, K150L and W95L/K150E amino acid mutations of B. rapa eIF(iso)4E interrupted its interaction with TuMV VPg. All eIF(iso)4E mutants were able to complement an eIF4E-knockout yeast strain, indicating that the mutated eIF(iso)4E proteins retained their function as a translational initiation factor. To determine whether these mutations could confer resistance, eIF(iso)4E W95L, W95L/K150E and eIF(iso)4E wild-type were over-expressed in a susceptible Chinese cabbage cultivar. Evaluation of the TuMV resistance of T1 and T2 transformants demonstrated that the over-expression of the eIF(iso)4E mutant forms can confer resistance to multiple TuMV strains. These data demonstrate the utility of knowledge-based approaches for the engineering of broad-spectrum resistance in Chinese cabbage.

СОЗДАНИЕ ДИГАПЛОИДНЫХ ЛИНИЙ Brassica napus L. — ДОНОРОВ УСТОЙЧИВОСТИ К ВИРУСУ МОЗАИКИ ТУРНЕПСА

Oilseed rape is a valuable crop used for biodiesel, vegetable oil and high-protein animal feed. However, there is a need for cultivars resistant to turnip mosaic virus (TuMV), which causes a great losses. Resistant plants are the only efficient and environmentally safe way to control this virus. We have studied the resistance in collection of 64 accessions of cultivated and wild brassicas ( Brassica napus, B. rapa, B. oleracea and B. juncea to six genetically different isolates of TuMV. Assessment of visual mosaic symptoms and quantitative ELISA test identified 24 most promising accessions that were resistant to all isolates of TuMV (13 accessions) and 5 of 6 isolates (11 accessions). Then, the haploid resistant plants were obtained using microspore culture. An optimal condition of high temperature treatment to induce embryogenesis were found for 12 accessions, and doubled haploid lines, the donors of resistance to TuMV, were propagated.

TRAP markers generated with resistant gene analog sequences and their application to genetic diversity analysis of radish germplasm

TRAP is a novel molecular marker technique which has been effectively used in genetic diversity analysis of germplasm and genetic mapping. However, it has not yet been applied to radish. In this study, novel TRAP markers based on expressed sequence tag (EST) and resistance gene analog (RGAs) were developed and applied to the genetic diversity analysis of radish genotypes. With data-mining method, a total of 50 RGAs including 35 unigenes and 15 singletons were identified from the public sequence databases and employed to design the fixed primers of TRAP markers. Fifty-nine RGA-derived fixed primers were combined with five arbitrary primers, and these TRAP primer combinations were tested in two genotypes (‘NAU-YH’ and ‘NAU-DY’). Furthermore, 65 TRAP primer combinations were selected for genetic diversity analysis and 385 polymorphic fragments were produced among 30 radish genotypes. Dendrograms constructed by UPGMA method showed that these genotypes could be clustered into four groups. Interestingly, these groups were in highly accordance with the results of resistance evaluation to Turnip Mosaic Virus (TuMV). A cultivar identification diagram (CID) was made manually to discriminate the 30 radish genotypes using four polymorphic TRAP primer combinations. The results indicated that TRAP is an efficient genetic marker system, which could provide an effective tool for genetic mapping and for marker-assisted selection in radish breeding programs.

Identification of a broad-spectrum recessive gene in Brassica rapa and molecular analysis of the eIF4E gene family to develop molecular markers

Two Chinese cabbage (Brassica rapa L. ssp. pekinensis) lines resistant to Turnip mosaic virus (TuMV) CHN5 were identified and found to have broad-spectrum resistance against three other TuMV strains (CHN2, 3, and 4). Genetic analysis indicated that this TuMV resistance is recessive, and a candidate gene approach was used to identify the resistance gene, which we named trs (TuMV resistance discovered at Seoul National University). Based on previous research in Arabidopsis showing that mutations in eIF(iso)4E determine TuMV resistance, the eIF(iso)4E gene was selected as a candidate for the trs gene in Brassica rapa. Three copies of eIF(iso)4E, Braiso4Ea, Braiso4Eb, and Braiso4Ec, were amplified, and polymorphisms between resistant and susceptible lines were analyzed. Sequence polymorphisms were found in Braiso4Ea and Braiso4Eb; in contrast, no sequence differences were found in Braiso4Ec between resistant and susceptible lines. A CAPS marker developed to test the linkage between Braiso4Eb and TuMV resistance displayed no linkage. A SCAR marker, trsSCAR, developed using allele-specific deletions and SNPs in Braiso4Ea, did co-segregate perfectly with trs in three F2 populations. However, the presence or absence of the Braiso4Ea sequence deletion was not consistent between resistant lines and susceptible lines, indicating that Braiso4Ea is not the actual resistance gene. Results from mapping analysis indicated that the trs is located at chromosome A04, between scaffold 000104 and scaffold 040552. This location demonstrated that trs may be another recessive resistance gene tightly linked to retr02 or another allele. The molecular markers developed in this study will be useful for breeding durable resistance.

Turnip mosaic virus induces expression of the LRR II subfamily genes and regulates the salicylic acid signaling pathway in non-heading Chinese cabbage

In order to study the defense response to turnip mosaic virus (TuMV) infection in non-heading Chinese cabbage (Brassica campestris ssp. chinensis Makino), we cloned the LRR II subfamily genes which comprises six members. They were high homologous to the function-known LRR II genes of Arabidopsis. We investigated their expression through quantitative real-time PCR analysis. TuMV infection induced the expression of these genes locally and systematically, and regulated the endogenous accumulation of salicylic acid (SA). Exogenous SA spraying was able to induce resistance to the susceptibility of the TuMV-infected plants, which might function via inhibiting the viral duplication. Though TuMV-induced SA accumulation was not the determinant in regulating gene expression, it mediated the reaction oxygen species (ROS) burst as a channel of defense.

Ethylene is involved in leafy mustard systemic resistance to Turnip mosaic virus infection through the mitochondrial alternative oxidase pathway

The role of the mitochondrial alternative oxidase (AOX) pathway and the relationship between AOX and ethylene in resistance to Turnip mosaic virus (TuMV) were investigated in systemic infected mustard leaves. Expressions of AOX1a and ethylene biosynthesis-related genes were enhanced in the upper uninoculated leaves at 6 days of postinoculation (dpi). Increased accumulation of AOX1a transcript and reduced levels of TuMV-CP were also observed after pretreatment with AA (a cytochrome pathway inhibitor) or ACC (an ethylene precursor) at non-lethal concentrations. Meanwhile, application of AVG (an ethylene biosynthesis inhibitor) mostly blocked TuMV-induced AOX activity and substantially increased TuMV susceptibility in the upper leaves. SHAM (AOX pathway inhibitor) pretreatment partially reduced the ACC-induced cyanide-resistant respiration and TuMV resistance. Conversely, AA and SHAM pretreatments had little effect on generation of ethylene. These results suggested that the TuMV-induced increase of AOX was partly involved in the ethylene pathway, and ethylene unregulated AOX in upper uninoculated mustard leaves.

Light Quantity and Photosystem Function Mediate Host Susceptibility to Turnip mosaic virus Via a Salicylic Acid–Independent Mechanism

Evidence going as far back as the early part of the 20th century suggests that both light and chloroplast function may play key roles in host susceptibility to viruses. Despite the long history of such work, confirmation of these phenomena and a determination of the underlying mechanisms remain elusive. Here, we revisited these questions using modern imaging technologies to study the susceptibility of Nicotiana benthamiana to Turnip mosaic virus (TuMV). We found that both light deficiency and photosystem impairment increased the susceptibility of N. benthamiana to TuMV infection. Time-lapse photography studies indicated that, under these conditions, rub-inoculated plants exhibited greater numbers of infection foci and more rapid foci development. The rate of systemic movement was also accelerated though cell-to-cell movement appeared unchanged. Inhibition of salicylic acid (SA)-mediated defense responses is not likely responsible for changes in susceptibility because SA and pathogen response-1 gene induction were not affected by light deficiency or chloroplast impairment and treatment of plants with SA had no measureable impact on TuMV infection. Taken together, these data suggest that both light and optimal chloroplast function influence virus infection either by limiting the cellular resources needed by TuMV to establish replication complexes or the host's ability to activate SA-independent defenses.

Cloning and Characterization of the BcTuR3 Gene Related to Resistance to Turnip Mosaic Virus (TuMV) from Non-heading Chinese Cabbage

A cDNA clone for the turnip mosaic virus (TuMV)-induced gene in non-heading Chinese cabbage (Brassica campestris ssp. chinensis Makino cv. Duanbaigeng) was isolated and characterized. The full-length cDNA clone, designated BcTuR3, was isolated during resistance response to TuMV. Sequence analysis of the cDNA clone confirmed that the translation product of the gene is homologous to other plant resistance proteins. Genomic DNA Southern blot analysis indicated that the gene represented a small multi-gene family. Northern hybridizations confirmed its elevated expression in the resistant “Duanbaigeng” and the susceptible “Aijiaohuang.” Upon inoculation with TuMV, the BcTuR3 transcript was rapidly accumulated in the infected leaves and stems. Tissue-specific expression of the gene showed that higher levels of the BcTuR3 transcript were observed in the TuMV-infected leaves, and later in the stems and roots of the resistant and susceptible varieties. These data showed that the BcTuR3 gene may be involved in plant resistance against TuMV pathogen infection.

First Report of Turnip mosaic virus in Rhubarb in Alaska.

In July 2003, noticeable red lesions were observed on rhubarb leaves (Rheum rhababarum cv. Kerwin) from a plant at the Arctic Plant Germplasm Research and Introduction Project in Palmer, AK. Extracts of leaf tissue tested positive for a potyvirus using indirect enzyme-linked immunosorbent assay (ELISA) and western blots with a monoclonal antibody specific to the potyvirus group (Agdia, Inc., Elkhart, IN). During the following growing season (June 2004), obvious chlorotic ringspots developed into red lesions on the same plant and an adjacent plant of the same cultivar. Partially purified particles that were isolated from the infected rhubarb plants were mechanically inoculated to an experimental host range (number of infected plants per total number of plants), resulting in lesions on leaves of Rheum palmatum (1 of 2) and Chenopodium amaranticolor (3 of 5) but none on C. quinoa (0 of 4). The leaves with local lesions from C. amaranticolor were ground in phosphate buffer (1 g of tissue per 10 ml of buffer), and the extract rubbed onto a set of plants resulting in lesions on R. hybridum (raponticum) (1 of 2), C. amaranticolor (1 of 4), and C. quinoa (1 of 4). The original diseased rhubarb plants and experimental symptomatic plants were confirmed to have a potyvirus using ELISA. Subsequent compound direct ELISA and western blot assays revealed that the virus reacted strongly to monoclonal or polyclonal antibodies to Turnip mosaic virus (TuMV) (Agdia, Inc.). Total RNA was extracted from leaves of the naturally infected rhubarb plants with an RNeasy Plant Mini Kit (Qiagen Sciences, Germantown, Maryland), and used in reverse-transcription-polymerase chain reaction (RT-PCR) with specific primers for TuMV (1) predicted to amplify a 1,134-bp 3'-terminal cDNA fragment encompassing the 3'-end of the nuclear inclusion protein gene (NIb), the coat protein gene, and the 3'-nontranslated region. A PCR product of approximately the expected size was obtained and then sequenced. Sequences (1,077 nt) that corresponded to the TuMV coat protein gene and 3'-terminal noncoding region were submitted to Genbank (Accession No. AY744930). Blast searches against NCBI (National Center for Biotechnology Information) contained high identities to many TuMV isolates with up to 96% (1,043 of 1,077) nucleotide identity (i.e., GenBank Accession No. AF169561). Similar high identities of up to 97% at the amino acid level occurred within the coat protein coding region (i.e., GenBank Accession No. BAC02892.1). Infected rhubarb plants were removed from the site and none of the remaining 109 plants tested positive for TuMV using ELISA. On the basis of the mechanical transmission to plant hosts, the definitive TuMV serology, and the consensus of sequenced regions with TuMV, we concluded that the causal agent of the diseased rhubarb plants was TuMV. Although TuMV has a wide plant host range occurring worldwide (2), to our knowledge, this is the first report of TuMV in rhubarb in Alaska and the first time that TuMV has been detected in Alaska.

Molecular Characterization of Carla‐ and Potyviruses from Narcissus in China

AbstractConserved carlavirus and potyvirus primers were used in reverse transcriptase‐polymerase chain reaction (RT‐PCR) to amplify virus fragments from Chinese narcissus (Narcissus tazetta var. chinensis) in China and the fragments were subsequently sequenced and compared in phylogenetic analyses. Samples from Fujiang province and Shanghai contained either one or two potyviruses and a carlavirus. One potyvirus (PY1) showed a distant relationship to Iris severe mosaic virus, Onion yellow dwarf virus and Shallot yellow stripe virus, while the other (only in the Fujiang sample, PY2) was most closely related to Turnip mosaic virus (TuMV). Similar experiments using glasshouse grown Narcissus originating from the UK contained Ornithogalum mosaic virus and another potyvirus (PY3) in the TuMV/PY2 cluster. Comparisons with previously determined sequence fragments indicated that PY2 was probably Narcissus yellow stripe virus and PY3 Narcissus late season yellow virus. Carlavirus fragments from both Chinese sites seemed to be of the same virus, which was most closely related in phylogenetic analyses to Potato virus M, Aconitum latent virus and Hop latent virus. It is most probably a new member of the genus Carlavirus and has been tentatively named Narcissus common latent virus.

Resistance to Turnip mosaic virus in Brassica rapa and B. napus and the analysis of genetic inheritance in selected lines

Forty‐two Brassica rapa and Brassica napus lines were tested for resistance to Turnip mosaic virus (TuMV) isolates representing the three major pathotypes in Europe. Of these lines, 11 were susceptible to all pathotypes; nine were resistant to one pathotype; eight were resistant to two pathotypes; and 14 were resistant to all three pathotypes. Of the lines tested, 23 were either able to, or had the potential to, discriminate between two different pathotype‐3 isolates. Genetic models for inheritance of resistance were proposed for four B. rapa lines: Jong Bai No. 2 had dominant resistance to pathotype 1 conferred by a single allele; PI418957C and Jin G 55 had recessive resistance to pathotype 4 where a single allele was required; PI418957C also had recessive resistance to pathotype 3 where a model with one of two epistatic, unlinked loci was proposed. Jong Bai No. 1 also had recessive resistance to pathotype 3, apparently conferred by alleles at three loci, where any two of the three loci were epistatic and required for resistance.