Turnip mosaic virus RNA replication complex vesicles are mobile, align with microfilaments, and are each derived from a single viral genome

Nicotiana benthamiana plants were agroinoculated with an infectious cDNA clone of Turnip mosaic virus (TuMV) that was engineered to express a fluorescent protein (green fluorescent protein [GFP] or mCherry) fused to the viral 6K2 protein known to induce vesicle formation. Cytoplasmic fluorescent discrete protein structures were observed in infected cells, corresponding to the vesicles containing the viral RNA replication complex. The vesicles were motile and aligned with microfilaments. Intracellular movement of the vesicles was inhibited when cells were infiltrated with latrunculin B, an inhibitor of microfilament polymerization. It was also observed that viral accumulation in the presence of this drug was reduced. These data indicate that microfilaments are used for vesicle movement and are necessary for virus production. Biogenesis of the vesicles was further investigated by infecting cells with two recombinant TuMV strains: one expressed 6K2GFP and the other expressed 6K2mCherry. Green- and red-only vesicles were observed within the same cell, suggesting that each vesicle originated from a single viral genome. There were also vesicles that exhibited sectors of green, red, or yellow fluorescence, an indication that fusion among individual vesicles is possible. Protoplasts derived from TuMV-infected N. benthamiana leaves were isolated. Using immunofluorescence staining and confocal microscopy, viral RNA synthesis sites were visualized as punctate structures distributed throughout the cytoplasm. The viral proteins VPg-Pro, RNA-dependent RNA polymerase, and cytoplasmic inclusion protein (helicase) and host translation factors were found to be associated with these structures. A single-genome origin and presence of protein synthetic machinery components suggest that translation of viral RNA is taking place within the vesicle.

Maintenance of an old world betasatellite by a new world helper begomovirus and possible rapid adaptation of the betasatellite

Begomoviruses (family Geminiviridae) cause major losses to crops throughout the tropical regions of the world. Begomoviruses originating from the New World (NW) and the Old World (OW) are genetically distinct. Whereas the majority of OW begomoviruses have monopartite genomes and whereas most of these associate with a class of symptom-modulating satellites (known as betasatellites), the genomes of NW begomoviruses are exclusively bipartite and do not associate with satellites. Here, we show for the first time that a betasatellite (cotton leaf curl Multan betasatellite [CLCuMuB]) associated with a serious disease of cotton across southern Asia is capable of interacting with a NW begomovirus. In the presence of CLCuMuB, the symptoms of the NW cabbage leaf curl virus (CbLCuV) are enhanced in Nicotiana benthamiana. However, CbLCuV was unable to interact with a second betasatellite, chili leaf curl betasatellite. Although CbLCuV can transreplicate CLCuMuB, satellite accumulation levels in plants were low. However, progeny CLCuMuB isolated after just one round of infection with CbLCuV contained numerous mutations. Reinoculation of one such progeny CLCuMuB with CbLCuV to N. benthamiana yielded infections with significantly higher satellite DNA levels. This suggests that betasatellites can rapidly adapt for efficient transreplication by a new helper begomovirus, including begomoviruses originating from the NW. Although the precise mechanism of transreplication of betasatellites by begomoviruses remains unknown, an analysis of betasatellite mutants suggests that the sequence(s) required for maintenance of CLCuMuB by one of its cognate begomoviruses (cotton leaf curl Rajasthan virus) differs from the sequences required for maintenance by CbLCuV. The significance of these findings and, particularly, the threat that betasatellites pose to agriculture in the NW, are discussed.

Functional analysis of bipartite begomovirus coat protein promoter sequences

We demonstrate that the AL2 gene of Cabbage leaf curl virus (CaLCuV) activates the CP promoter in mesophyll and acts to derepress the promoter in vascular tissue, similar to that observed for Tomato golden mosaic virus (TGMV). Binding studies indicate that sequences mediating repression and activation of the TGMV and CaLCuV CP promoter specifically bind different nuclear factors common to Nicotiana benthamiana, spinach and tomato. However, chromatin immunoprecipitation demonstrates that TGMV AL2 can interact with both sequences independently. Binding of nuclear protein(s) from different crop species to viral sequences conserved in both bipartite and monopartite begomoviruses, including TGMV, CaLCuV, Pepper golden mosaic virus and Tomato yellow leaf curl virus suggests that bipartite begomoviruses bind common host factors to regulate the CP promoter. This is consistent with a model in which AL2 interacts with different components of the cellular transcription machinery that bind viral sequences important for repression and activation of begomovirus CP promoters.

Effectiveness and stability of heterologous proteins expressed in plants by Turnip mosaic virus vector at five different insertion sites

The N-terminal (NT) regions of particular protein-coding sequences are generally used for in-frame insertion of heterologous open reading frames (ORFs) in potyviral vectors for protein expression in plants. An infectious cDNA clone of Turnip mosaic virus (TuMV) isolate YC5 was engineered at the generally used NT regions of HC-Pro and CP, and other possibly permissive sites to investigate their effectiveness to express the GFP (jellyfish green fluorescent protein) and Der p 5 (allergen from the dust mite, Dermatophagoides pteronyssinus) ORFs. The results demonstrated the permissiveness of the NT regions of P3, CIP and NIb to carry the ORFs and express the translates as part of the viral polyprotein, the processing of which released free-form proteins in the host cell milieu. However, these sites varied in their permissiveness to retain the ORFs intact and hence affect the heterologous protein expression. Moreover, strong influence of the inserted ORF and host plants in determining the permissiveness of a viral genomic context to stably carry the alien ORFs and hence to support their prolonged expression was also noticed. In general, the engineered sites were relatively more permissive to the GFP ORF than to the Der p 5 ORF. Among the hosts, the local lesion host, Chenopodium quinoa Willd. showed the highest extent of support to TuMV to stably carry the heterologous ORFs at the engineered sites and the protein expression therefrom. Among the systemic hosts, Nicotiana benthamiana Domin proved more supportive to TuMV to carry and express the heterologous ORFs than the Brassica hosts, whereas the protein expression levels were significantly higher and more stable in the plants of Brassica campestris L. var. chinensis and B. campestris L. var. ching-geeng than those in the plants of B. juncea L. and B. campestris L. var. pekinensis.

Electron-lucent inclusion bodies are structures specialized for aphid transmission of cauliflower mosaic virus

Cauliflower mosaic virus (CaMV) is transmitted by aphids. For acquisition by the vector, a transmissible complex must form, composed of the virus particle, the viral coat-associated protein P3 and the helper protein P2. However, the components of the transmissible complex are largely separated in infected plant cells: most P3 virions are confined in electron-dense inclusion bodies, whereas P2 is sequestered in electron-lucent inclusion bodies (elIBs). This spatial separation controls virus acquisition by favouring the binding of virus-free P2 to the vector first, rendering the vector competent for later uptake of P3 virions. Consequently, sequential acquisition of virus from different cells or tissues is possible, with important implications for the biology of CaMV transmission. CaMV strains Campbell and CM1841 contain a single amino acid mutation (G94R) in the helper protein P2, rendering them non-transmissible from plant to plant. However, the mutant P2-94 protein supports aphid transmission when expressed heterologously and supplied to P3-CaMV complexes in vitro. The non-transmissibility of P2-94 was re-examined in vivo and it is shown here that the non-transmissibility of this P2 mutant is not due to low accumulation levels in infected plants, as suggested previously, but more specifically to the failure to form elIBs within infected plant cells. This demonstrates that elIBs are complex viral structures specialized for aphid transmission and suggests that viral inclusion bodies other than viral factories, most often considered as 'garbage cans', can in fact exhibit specific functions.

Phosphorylation of the termini of Cauliflower mosaic virus precapsid protein is important for productive infection

Cauliflower mosaic virus (CaMV) coat protein precursor (pre-CP) has 489 amino acids (p57) and is processed by the viral proteinase into three major forms: p44, p39, and p37. The N- and C-terminal extensions of pre-CP are released during maturation by the virus-encoded proteinase. We showed that these extensions are phosphorylated at several sites by host casein kinase II (CKII). We have identified the phosphorylated amino acids using an in vitro phosphorylation assay and tested the effect of mutation of these sites on viral infectivity. Mutation of serines S66, S68, and S72 to alanine in the N-terminal extension abolished phosphorylation of the protein in vitro. Also, mutation of all S and T residues in the C-terminus (450 to 489) made this region insensitive to CKII. Amino acid substitutions also were introduced into a full-length infectious clone of CaMV. Mutated forms of the virus with S66, S68, and S72 substituted with A or D showed a delay in symptom development and affected the infectivity of the virus. However, a mutant with an A substitution of all the S and T residues of the C-terminal extension of CP was not infectious. These results suggest that phosphorylation of the N- and C-termini of CaMV pre-CP plays an important role in the initiation of viral infection.

Visualization of the interaction between the precursors of VPg, the viral protein linked to the genome of turnip mosaic virus, and the translation eukaryotic initiation factor iso 4E in Planta

The RNA genome of Turnip mosaic virus is covalently linked at its 5' end to a viral protein known as VPg. This protein binds to the translation eukaryotic initiation factor iso 4E [eIF(iso)4E]. This interaction has been shown to be important for virus infection, although its exact biological function(s) has not been elucidated. In this study, we investigated the subcellular site of the VPg-eIF(iso)4E interaction using bimolecular fluorescence complementation (BiFC). As a first step, eIF(iso)4E, 6K-VPg-Pro, and VPg-Pro were expressed as full-length green fluorescent protein (GFP) fusions in Nicotiana benthamiana, and their subcellular localizations were visualized by confocal microscopy. eIF(iso)4E was predominantly associated with the endoplasmic reticulum (ER), and VPg-Pro was observed in the nucleus and possibly the nucleolus, while 6K-VPg-Pro-GFP induced the formation of cytoplasmic vesicles budding from the ER. In BiFC experiments, reconstituted green fluorescence was observed throughout the nucleus, with a preferential accumulation in subnuclear structures when the GFP split fragments were fused to VPg-Pro and eIF(iso)4E. On the other hand, the interaction of 6K-VPg-Pro with eIF(iso)4E was observed in cytoplasmic vesicles embedded in the ER. These data suggest that the association of VPg with the translation factor might be needed for two different functions, depending of the VPg precursor involved in the interaction. VPg-Pro interaction with eIF(iso)4E may be involved in perturbing normal cellular functions, while 6K-VPg-Pro interaction with the translation factor may be needed for viral RNA translation and/or replication.

A PERK-like receptor kinase interacts with the geminivirus nuclear shuttle protein and potentiates viral infection

The nuclear shuttle protein (NSP) from bipartite geminiviruses facilitates the intracellular transport of viral DNA from the nucleus to the cytoplasm and acts in concert with the movement protein (MP) to promote the cell-to-cell spread of the viral DNA. A proline-rich extensin-like receptor protein kinase (PERK) was found to interact specifically with NSP of Cabbage leaf curl virus (CaLCuV) and of tomato-infecting geminiviruses through a yeast two-hybrid screening. The PERK-like protein, which we designated NsAK (for NSP-associated kinase), is structurally organized into a proline-rich N-terminal domain, followed by a transmembrane segment and a C-terminal serine/threonine kinase domain. The viral protein interacted stably with defective versions of the NsAK kinase domain, but not with the potentially active enzyme, in an in vitro binding assay. In vitro-translated NsAK enhanced the phosphorylation level of NSP, indicating that NSP functions as a substrate for NsAK. These results demonstrate that NsAK is an authentic serine/threonine kinase and suggest a functional link for NSP-NsAK complex formation. This interpretation was corroborated by in vivo infectivity assays showing that loss of NsAK function reduces the efficiency of CaLCuV infection and attenuates symptom development. Our data implicate NsAK as a positive contributor to geminivirus infection and suggest it may regulate NSP function.

Tiered tests to assess the environmental risk of fitness changes in hybrids between transgenic crops and wild relatives: the example of virus resistant Brassica napus

Over the last 20 years, there has been much research aimed at improving environmental risk assessment of transgenic crops. Despite large amounts of data, decisions to allow or prohibit the release of transgenic crops remain confused and controversial. We argue that part of the reason for confusion is the lack of clear definitions of components of the environment that should be protected, and, as a consequence, there is no way to judge the relevance of data collected under the auspices of 'environmental risk assessment'. Although this criticism applies to most aspects of environmental risk assessment of transgenic crops, it is most pertinent to effects that might result from an increase in plant fitness, often referred to as increased weediness. Environmental risk assessment of weediness is regarded as complicated: an increase in the fitness of a transgenic plant compared with non-transgenic counterparts will be the result of an interaction between the altered plant phenotype and an enormous number of environmental variables. This has led to the idea that risk assessment of weediness needs to "understand" these interactions, with the implication that exhaustive data are required. Here we argue that environmental risk assessment of the weediness of transgenic plants need not be complicated. Analysis of the conditions that must be met for increased weediness to occur suggests a series of studies that starts with simple tests in the laboratory under "worst case" assumptions, and becomes increasingly complex and realistic should the simpler studies not indicate negligible risk with sufficient certainty. We illustrate how the approach might work for assessing the risks of increased weediness using the example of possible introgression of a gene for Turnip mosaic virus (TuMV) resistance from oilseed rape to certain wild Brassica species.

The VPgPro protein of Turnip mosaic virus: in vitro inhibition of translation from a ribonuclease activity

A role for viral encoded genome-linked (VPg) proteins in translation has often been suggested because of their covalent attachment to the 5′ end of the viral RNA, reminiscent of the cap structure normally present on most eukaryotic mRNAs. We tested the effect of Turnip mosaic virus (TuMV) VPgPro on translation of reporter RNAs in in vitro translation systems. The presence of VPgPro in either wheat germ extract or rabbit reticulocyte lysate systems lead to inhibition of translation. The inhibition did not appear to be mediated by the interaction of VPg with the eIF(iso)4E translation initiation factor since a VPg mutant that does not interact with eIF(iso)4E still inhibited translation. Monitoring the fate of RNAs revealed that they were degraded as a result of addition of TuMV VPgPro or of Norwalk virus (NV) VPg protein. The RNA degradation was not the result of translation being arrested and was heat labile and partially EDTA sensitive. The capacity of TuMV VPgPro and of (NV) VPg to degrade RNA suggests that these proteins have a ribonucleolytic activity which may contribute to the host RNA translation shutoff associated with many virus infections.

Simultaneous production of two foreign proteins from a polyvirus-based vector

With the aim of developing a biotechnological tool for the production of foreign proteins in plants, we first engineered an infectious turnip mosaic virus (TuMV) cDNA that contained the jellyfish green fluorescent protein (GFP) gene or the bacterial beta-glucuronidase (GUS) gene (uidA). Two insertion sites were assessed, either between P1 and HCPro cistrons or Pol and CP cistrons. In each construct, the junctions flanking the inserted gene coded for P1 and/or VPg-Pro cleavage recognition site sequences, to produce free GUS or GFP. After transfection by particle bombardment on Brassica perviridis, characteristic symptoms for TuMV infection appeared and Western blot analyses showed that GFP and GUS had been excised from the viral polyprotein. No significant differences in expression level were noticed between the two insertion sites. By RT-PCR, gfp was found to be stable over 30 days post-transfection (dpt) while uidA was gradually lost at 15 dpt. We also created two constructs containing either gene at each insertion sites on the same molecule. Attenuated systemic symptoms were observed after particle bombardment on B. perviridis and Western blot analyses showed that both foreign proteins were produced. Also, the same stability/instability as for the single-gene constructs were observed. These results indicate that it is possible to produce at least two foreign proteins simultaneously in a TuMV-based vector.

Can plants use an entomopathogenic virus as a defense against herbivores?

It is by now well established that plants use various strategies to defend themselves against herbivores. Besides conventional weapons such as spines and stinging hairs and sophisticated chemical defenses, plants can also involve the enemies of the herbivores in their defense. It has been suggested that plants could even use entomopathogens as part of their defense strategies. In this paper, we show that Brassica oleraceae plants that are attacked by Myzus persicae aphids infected with an entomopathogenic parvovirus (M. persicae densovirus) transport the virus through the phloem locally and systematically. Moreover, healthy aphids that fed on the same leaf, but separated from infected aphids were infected via the plant. Hence, this is proof of the principle that plants can be vectors of an insect virus and can possibly use this virus as a defense against herbivores.

Comparisons of the genetic structure of populations of Turnip mosaic virus in West and East Eurasia

The genetic structure of populations of Turnip mosaic virus in Eurasia was assessed by making host range and gene sequence comparisons of 142 isolates. Most isolates collected in West Eurasia infected Brassica plants whereas those from East Eurasia infected both Brassica and Raphanus plants. Analyses of recombination sites (RSs) in five regions of the genome (one third of the full sequence) showed that the protein 1 (P1 gene) had recombined more frequently than the other gene regions in both subpopulations, but that the RSs were located in different parts of the genomes of the subpopulations. Estimates of nucleotide diversity showed that the West Eurasian subpopulation was more diverse than the East Eurasian subpopulation, but the Asian-BR group of the genes from the latter subpopulation had a greater nonsynonymous/synonymous substitution ratio, especially in the P1, viral genome-linked protein (VPg) and nuclear inclusion a proteinase (NIa-Pro) genes. These subpopulations seem to have evolved independently from the ancestral European population, and their genetic structure probably reflects founder effects.

Interaction of the movement protein NSP and the Arabidopsis acetyltransferase AtNSI is necessary for Cabbage leaf curl geminivirus infection and pathogenicity

DNA viruses can modulate the activity of cellular acetyltransferases to regulate virus gene expression and to affect cell cycle progression in order to support virus replication. A role for protein acetylation in regulating the nuclear export of the bipartite geminivirus DNA genome was recently suggested by the findings that the viral movement protein NSP, which shuttles the viral genome between the nucleus and the cytoplasm, interacts with a novel Arabidopsis acetyltransferase, AtNSI, and the increased expression of AtNSI enhances susceptibility to Cabbage leaf curl virus infection. To further investigate the interaction of NSP and AtNSI and to establish the importance of this interaction in virus infections, we used a reverse yeast two-hybrid selection and deletion analysis to identify NSP mutants that were impaired in their ability to bind AtNSI. These mutants identified a 38-amino-acid region of NSP, to which no function had so far been assigned, as being necessary for NSP-AtNSI interaction. Three NSP missense mutants were analyzed in detail and were found to be comparable to wild-type NSP in their levels of accumulation, nucleocytoplasmic shuttling, DNA binding, and cooperative interaction with the viral cell-to-cell movement protein MP. Despite this, Cabbage leaf curl virus that expressed each mutated NSP was defective in its ability to infect Arabidopsis, exhibiting lower levels of infectivity than the wild-type virus, and delayed systemic spread of the virus and attenuated disease symptoms. Our data demonstrate the importance of the interaction of NSP with AtNSI for virus infection and pathogenicity.

Interaction of the movement protein NSP and the Arabidopsis acetyltransferase AtNSI is necessary for Cabbage leaf curl geminivirus infection and pathogenicity

DNA viruses can modulate the activity of cellular acetyltransferases to regulate virus gene expression and to affect cell cycle progression in order to support virus replication. A role for protein acetylation in regulating the nuclear export of the bipartite geminivirus DNA genome was recently suggested by the findings that the viral movement protein NSP, which shuttles the viral genome between the nucleus and the cytoplasm, interacts with a novel Arabidopsis acetyltransferase, AtNSI, and the increased expression of AtNSI enhances susceptibility to Cabbage leaf curl virus infection. To further investigate the interaction of NSP and AtNSI and to establish the importance of this interaction in virus infections, we used a reverse yeast two-hybrid selection and deletion analysis to identify NSP mutants that were impaired in their ability to bind AtNSI. These mutants identified a 38-amino-acid region of NSP, to which no function had so far been assigned, as being necessary for NSP-AtNSI interaction. Three NSP missense mutants were analyzed in detail and were found to be comparable to wild-type NSP in their levels of accumulation, nucleocytoplasmic shuttling, DNA binding, and cooperative interaction with the viral cell-to-cell movement protein MP. Despite this, Cabbage leaf curl virus that expressed each mutated NSP was defective in its ability to infect Arabidopsis, exhibiting lower levels of infectivity than the wild-type virus, and delayed systemic spread of the virus and attenuated disease symptoms. Our data demonstrate the importance of the interaction of NSP with AtNSI for virus infection and pathogenicity.

Splicing of Cauliflower mosaic virus 35S RNA serves to downregulate a toxic gene product

Alternative splicing usually leads to an increase in the number of gene products that can be derived from a single transcript. Here, a different and novel use of alternative splicing – as a means to control the amount of a potentially toxic gene product in the plant pararetrovirus Cauliflower mosaic virus (CaMV) – is reported. About 70 % of the CaMV 35S RNA, which serves as a substrate for both reverse transcription and polycistronic mRNA, is spliced into four additional RNA species. Splicing occurs between four donor sites – one in the 5′ untranslated region and three within open reading frame (ORF) I – and one unique acceptor site at position 1508 in ORF II. A previous study revealed that the acceptor site is vital for CaMV infectivity and expression of ORFs III and IV from one of the spliced RNA species suggested that splicing may facilitate expression of downstream CaMV ORFs. However, it is shown here that deleting the splice acceptor site and replacing ORF II with a cargo ORF that lacks splice acceptor sites does not interfere with virus proliferation. Furthermore, it is demonstrated that whenever P2 cannot accumulate in infected tissues, the splice acceptor site at position 1508 is no longer vital and has little effect on virus replication. This suggests that the vital role of splicing in CaMV is regulation of P2 expression and that P2 exhibits biological properties that, whilst indispensable for virus–vector interactions, can block in planta virus infection if this regulation is abolished.

An important determinant of the ability of Turnip mosaic virus to infect Brassica spp. and/or Raphanus sativus is in its P3 protein

Turnip mosaic virus (TuMV, genus Potyvirus, family Potyviridae) infects mainly cruciferous plants. Isolates Tu-3 and Tu-2R1 of TuMV exhibit different infection phenotypes in cabbage (Brassica oleracea L.) and Japanese radish (Raphanus sativus L.). Infectious full-length cDNA clones, pTuC and pTuR1, were constructed from isolates Tu-3 and Tu-2R1, respectively. Progeny virus derived from infections with pTuC induced systemic chlorotic and ringspot symptoms in infected cabbage, but no systemic infection in radish. Virus derived from plants infected with pTuR1 induced a mild chlorotic mottle in cabbage and infected radish systemically to induce mosaic symptoms. By exchanging genome fragments between the two virus isolates, the P3-coding region was shown to be responsible for systemic infection by TuMV and the symptoms it induces in cabbage and radish. Moreover, exchanges of smaller parts of the P3 region resulted in recombinants that induced complex infection phenotypes, especially the combination of pTuC-derived N-terminal sequence and pTuR1-derived C-terminal sequence. Analysis by tissue immunoblotting of the inoculated leaves showed that the distributions of P3-chimeric viruses differed from those of the parents, and that the origin of the P3 components affected not only virus accumulation, but also long-distance movement. These results suggest that the P3 protein is an important factor in the infection cycle of TuMV and in determining the host range of this and perhaps other potyviruses.

Two histidines of the coat protein of turnip yellow mosaic virus at the capsid interior are crucial for viability

RNA-coat protein interactions in turnip yellow mosaic virus (TYMV) have been shown to involve low pK proton-donating groups. Two different types of interaction have been proposed. In the so-called type I interaction, protonated C-residues interact with acidic amino acids at low pH, thereby providing a rationale for the high C-content (38%) of the genomic RNA. The type II interaction involves charged histidines interacting with phosphates of the RNA backbone. Site-directed mutagenesis of the TYMV coat protein and subsequent in vivo analysis were performed to distinguish between these two types of RNA-protein interaction. The results reveal a prominent role for the histidines H68 and H180, since mutation to an alanine residue inhibits symptom development on secondary leaves, indicating that spreading of the virus in the plant is blocked. Viral RNA and coat protein synthesis are not altered, showing that these two histidines may play a role in the process of RNA encapsidation. Overexpression of the TYMV coat protein in Escherichia coli leads to the formation of bona fide capsids, showing that the two histidines are not critical in capsid assembly. Mutagenesis of the acidic amino acids D11, E135, and D143 to alanine apparently did not interfere with virus viability. The functional role of the histidines during the infection cycle is discussed in terms of the structure of the coat protein, both at the level of amino acid sequence conservation among the members of the Tymoviridae family and as the three-dimensional structure of the coat protein.

Effect of temperature and sanitizers on the survival of feline calicivirus, Escherichia coli, and F-specific coliphage MS2 on leafy salad vegetables

We conducted a series of experiments to compare the survival of Escherichia coli, feline calicivirus, and F-specific coliphage MS2 on lettuce and cabbage with and without disinfection. Inoculated produce was held at 4, 25, or 37 degrees C for 21 days or was treated with different concentrations of sodium bicarbonate, chlorine bleach, peroxyacetic acid, or hydrogen peroxide. Survival was measured by the decimal reduction value (time to 90% reduction in titer) and the change in log titers of the test organisms. A stronger correlation of survival measures was observed between feline calicivirus and MS2 than between E. coli and either of the viral agents at 25 and 37 degrees C. The maximum time to detection limit for MS2 at all temperatures was 9 days, whereas feline calicivirus was detected for a maximum of 14 days at 4 degrees C. In contrast, E. coli was detectable for 21 days at 4 and 25 degrees C and for 14 days at 37 degrees C. Significant increases in E. coli titer occurred within the first 5 days, but virus titers decreased steadily throughout the experiments. E. coli was also highly susceptible to all disinfectants except 1% sodium bicarbonate and 50 ppm chlorine bleach, whereas the viruses were resistant to all four disinfectants.

Interaction of VPg-Pro of turnip mosaic virus with the translation initiation factor 4E and the poly(A)-binding protein in planta

The viral protein linked to the genome (VPg) of Turnip mosaic virus (TuMV) interacts in vitro with the translation eukaryotic initiation factor (eIF) 4E. In the present study, we investigated the consequence of TuMV infection on eIF4E expression. Two isomers are present in plants, namely eIF4E and eIF(iso)4E. Expression of the latter was detected in both TuMV-infected and mock-inoculated Brassica perviridis plants, but expression of eIF4E was found only in infected plants. Membranes from TuMV-infected or mock-inoculated tissues were separated by sucrose gradient centrifugation and fractions were collected. Immunoblot analyses showed that 6K(2)-VPg-Pro/VPg-Pro polyproteins were associated with endoplasmic reticulum membranes and were the viral forms likely to interact with eIF(iso)4E and eIF4E. In planta interaction between 6K(2)-VPg-Pro/VPg-Pro and eIF(iso)4E/eIF4E was confirmed by co-purification by metal chelation chromatography. The poly(A)-binding protein (PABP) was also found to co-purify with VPg-Pro. Direct interaction between VPg-Pro and PABP was shown by an ELISA-based binding assay. These experiments suggest that a multi-protein complex may form around VPg-Pro of TuMV.

Activity of aphids associated with lettuce and broccoli in Spain and their efficiency as vectors of Lettuce mosaic virus

This research sought to identify the aphid virus vector species associated with lettuce and broccoli crops in Spain, and to determine their population dynamics and ability to transmit Lettuce mosaic virus (LMV). Green tile traps and Moericke yellow water-pan traps were used to monitor aphid flights during the spring and autumn growing seasons of 2001. Aphid species feeding on lettuce were counted weekly. The transmission efficiencies of LMV were determined for the aphid species caught most frequently. The Moericke traps generally caught more aphid species than the tile trap, but the latter was the most suitable to estimate flight activity of species involved in virus spread. Spring aphid catches indicated that the main aphid species landing on lettuce in the regions of Madrid and Murcia was Hyperomyzus lactucae, but Brachycaudus helichrysi was also abundant in both regions. In broccoli in the Navarra region, the most abundant species in spring were Aphis fabae, B. helichrysi and H. lactucae. In autumn-sown crops, the main species landing on lettuce in the Madrid region were Hyadaphis coriandri and Aphis spiraecola. In Murcia, A. spiraecola and Myzus persicae were the most abundant, while in Navarra, Therioaphis trifolii, and various Aphis spp. were the most numerous landing on broccoli. The main aphid species colonising lettuce was Nasonovia ribisnigri, but other less abundant colonising species were Aulacorthum solani and Macrosiphum euphorbiae. The most efficient vectors of LMV were M. persicae, Aphis gossypii and M. euphorbiae, while A. fabae and H. lactucae transmitted with low efficiency, and Rhopalosiphum padi and N. ribisnigri did not transmit. Occurrence of LMV epidemics in central Spain in relation to aphid flights and the role of weeds as virus reservoirs is discussed.

Loss of function of the Fusarium oxysporum SNF1 gene reduces virulence on cabbage and Arabidopsis

Fusarium oxysporum pathogenicity is believed to require the activity of cell wall-degrading enzymes. Production of these enzymes in fungi is subject to carbon catabolite repression, a process that in yeast is mostly controlled by the SNF1 (sucrose non-fermenting 1) gene. To elucidate the role of cell wall-degrading enzymes in F. oxysporum pathogenicity, we cloned and disrupted its SNF1 homologue ( FoSNF1). The fosnf1 mutants had a reduced expression of several genes encoding cell wall-degrading enzymes and grew poorly on certain carbon sources. Infection assays on Arabidopsis thaliana and Brassica oleracea revealed that progression of wilt symptoms in plants infected by fosnf1 mutants was considerably delayed, in comparison with those infected by a wild-type strain. In conclusion, mutations in FoSNF1 prevent F. oxysporum from properly derepressing the production of cell wall-degrading enzymes, compromise the utilization of certain carbon sources, and reduce its virulence on A. thaliana and B. oleracea.

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

Two isolates of the potyvirus Turnip mosaic virus (TuMV), UK 1 and CDN 1, differ both in their general symptoms on the susceptible propagation host Brassica juncea and in their ability to infect B. napus lines possessing a variety of dominant resistance genes. The isolate CDN 1 produces a more extreme mosaic in infected brassica leaves than UK 1 and is able to overcome the resistance genes TuRB01, TuRB04, and TuRB05. The resistance gene TuRB03, in the B. napus line 22S, is effective against CDN 1 but not UK 1. The nucleic acid sequences of the UK 1 and CDN 1 isolates were 90% identical. The C-terminal half of the P3 protein was identified as being responsible for the differences in symptoms in B. juncea. A single amino acid in the P3 protein was found to be the avirulence determinant for TuRB03. Previous work already has identified the P3 as an avirulence determinant for TuRB04. Our results increase the understanding of the basis of plant-virus recognition, show the importance of the potyviral P3 gene as a symptom determinant, and provide a role in planta for the poorly understood P3 protein in a normal infection cycle.

Cauliflower mosaic virus is preferentially acquired from the phloem by its aphid vectors

Cauliflower mosaic virus (CaMV) is transmitted in a non-circulative manner by aphids following the helper strategy. Helper proteins P2 and P3 act as a bridge between virions and the aphid cuticle. Electronic monitoring of aphid stylet activities (EPG technique), transmission tests and electron microscopy showed that CaMV is preferentially acquired from the phloem by its most common aphid vectors, Brevycorine brassicae and Myzus persicae. We also found that CaMV is semipersistently transmitted and that the rate of acquisition does not follow a typical bimodal curve. Instead, the virus could be acquired from non-phloem tissues at a low and fairly constant rate after one or more intracellular punctures within a few minutes, but the probability of acquisition rose significantly when aphids reached the phase of committed ingestion from the phloem. The acquisition rate of CaMV did not increase with increasing number of intracellular punctures, but the total duration of intracellular puncture was one of the variables selected by the stepwise logistic regression model used to fit the data that best explained acquisition of CaMV. Furthermore, aphids reaching the phloem faster had a higher probability of acquiring the virus. Our results support the hypothesis that multiple intracellular punctures of epidermal and mesophyll cells result in loading aphids with the CaMV-encoded aphid transmission factor (P2), and that aphids, in most cases, subsequently acquire CaMV particles during phloem sap ingestion. Consistently, immunoelectron microscopy showed that P3-virions are frequently found in the sieve element lumen, whereas P2 could not be detected.