Potyviral helper-component proteinase expressed in transgenic plants enhances titers of Potato leaf roll virus but does not alleviate its phloem limitation

Interplay between a polerovirus and a closterovirus decreases aphid transmission of the polerovirus

Multi-infection of plants by viruses is very common and can change drastically infection parameters such as virus accumulation, distribution, and vector transmission. Sugar beet is an important crop that is frequently co-infected by the polerovirus beet chlorosis virus (BChV) and the closterovirus beet yellows virus (BYV), both vectored by the green peach aphid (Myzus persicae). These phloem-limited viruses are acquired while aphids ingest phloem sap from infected plants. Here we found that co-infection decreased transmission of BChV by ~50% but had no impact on BYV transmission. The drastic reduction of BChV transmission was due to neither lower accumulation of BChV in co-infected plants nor reduced phloem sap ingestion by aphids from these plants. Using the signal amplification by exchange reaction fluorescent in situ hybridization technique on plants, we observed that 40% of the infected phloem cells were co-infected and that co-infection caused redistribution of BYV in these cells. The BYV accumulation pattern changed from distinct intracellular spherical inclusions in mono-infected cells to a diffuse form in co-infected cells. There, BYV co-localized with BChV throughout the cytoplasm, indicative of virus-virus interactions. We propose that BYV-BChV interactions could restrict BChV access to the sieve tubes and reduce its accessibility for aphids and present a model of how co-infection could alter BChV intracellular movement and/or phloem loading and reduce BChV transmission.IMPORTANCEMixed viral infections in plants are understudied yet can have significant influences on disease dynamics and virus transmission. We investigated how co-infection with two unrelated viruses, BChV and BYV, affects aphid transmission of the viruses in sugar beet plants. We show that co-infection reduced BChV transmission by about 50% without affecting BYV transmission, despite similar virus accumulation rates in co-infected and mono-infected plants. Follow-up experiments examined the localization and intracellular distribution of the viruses, leading to the discovery that co-infection caused a redistribution of BYV in the phloem vessels and altered its repartition pattern within plant cells, suggesting virus-virus interactions. In conclusion, the interplay between BChV and BYV affects the transmission of BChV but not BYV, possibly through direct or indirect virus-virus interactions at the cellular level. Understanding these interactions could be crucial for managing virus propagation in crops and preventing yield losses.

Inter-virus relationships in mixed infections and virus-drought relationships in plants: a quantitative review

Inter-virus relationships in mixed infections and virus-drought relationships are important in agriculture and natural vegetation. In this quantitative review, we sampled published factorial experiments to probe for relationships against the null hypothesis of additivity. Our sample captured antagonistic, additive and synergistic inter-virus relationships in double infections. Virus-drought relationships in our sample were additive or antagonistic, reinforcing the notion that viruses have neutral or positive effects on droughted plants, or that drought enhances plant tolerance to viruses. Both inter-virus and virus-drought relationships vary with virus species, host plant to the level of cultivar or accession, timing of infection, plant age and trait and growing conditions. The trait-dependence of these relationships has implications for resource allocation in plants. Owing to lagging theories, more experimental research in these fields is bound to return phenomenological outcomes. Theoretical work can advance in two complementary directions. First, the effective theory models the behaviour of the system without specifying all the underlying causes that lead to system state change. Second, mechanistic theory based on a nuanced view of the plant phenotype that explicitly considers downward causation; the influence of the plant phenotype on inter-virus relations and vice versa; the impact of timing, intensity and duration of drought interacting with viruses to modulate the plant phenotype; both the soil (moisture) and atmospheric (vapour pressure deficit) aspects of drought. Theories should scale in time, from short term to full growing season, and in levels of organisation up to the relevant traits: crop yield in agriculture and fitness in nature.

Coinfection of Cotton Plants with Watermelon Mosaic Virus and a Novel Polerovirus in China

Cotton is the most important fiber crop worldwide. To determine the presence of viruses in cotton plants showing leaf roll and vein yellowing symptoms in Henan Province of China, a small RNA-based deep sequencing approach was performed. Analysis of the de novo-assembled contigs followed by reverse transcription PCR allowed the reconstruction of watermelon mosaic virus and an unknown virus. The genome of the unknown virus was determined to be 5870 nucleotides in length, and has a genomic organization with characteristic features of previously reported poleroviruses. Sequence analysis revealed that the virus was closely related to, but significantly different from, cotton leafroll dwarf virus, a polerovirus of the family Solemoviridae. This virus had less than 90% amino acid sequence identity in the products of both ORF0 and ORF1. According to the polerovirus species demarcation criteria set by the International Committee on Taxonomy of Viruses, this virus should be assigned to a new polerovirus species, for which we propose the name “cotton leaf roll virus”.

Viral Hacks of the Plant Vasculature: The Role of Phloem Alterations in Systemic Virus Infection

For plant viruses, the ability to load into the vascular phloem and spread systemically within a host is an essential step in establishing a successful infection. However, access to the vascular phloem is highly regulated, representing a significant obstacle to virus loading, movement, and subsequent unloading into distal uninfected tissues. Recent studies indicate that during virus infection, phloem tissues are a source of significant transcriptional and translational alterations, with the number of virus-induced differentially expressed genes being four- to sixfold greater in phloem tissues than in surrounding nonphloem tissues. In addition, viruses target phloem-specific components as a means to promote their own systemic movement and disrupt host defense processes. Combined, these studies provide evidence that the vascular phloem plays a significant role in the mediation and control of host responses during infection and as such is a site of considerable modulation by the infecting virus. This review outlines the phloem responses and directed reprograming mechanisms that viruses employ to promote their movement through the vasculature.

Estimation of the functions of viral RNA silencing suppressors by apple latent spherical virus vector

Apple latent spherical virus (ALSV) is a latent virus with wide host range of plant species. In the present study, we prepared ALSV vectors expressing RNA silencing suppressors (RSSs) from eight plant viruses: P19 of carnation Italian ring spot virus (tombusvirus), 2b of peanut stunt virus (cucumovirus), NSs of tomato spotted wilt virus (tospovirus), HC-Pro of bean yellow mosaic virus (potyvirus), γb of barley stripe mosaic virus (hordeivirus), P15 of peanut clump virus (pecluvirus), P1 of rice yellow mottle virus (sobemovirus), or P21 of beet yellows virus (closterovirus). These vectors were inoculated to Nicotiana benthamiana to investigate the effects of RSSs on the virulence and accumulation of ALSV. Among the vectors, ALSV expressing NSs (ALSV-NSs) developed severe mosaic symptoms in newly developed leaves followed by plant death. Infection of ALSV-γb induced characteristic concentric ringspot symptoms on leaves, and plants infected with ALSV-HC-Pro showed mosaic and dwarf symptoms. Infection of the other five ALSV vectors did not show symptoms. ELISA and immunoblot assay indicated that virus titer increased in leaves infected with ALSV-NSs, γb, HC-Pro, or P19. RT-qPCR indicated that the amount of ALSV in plants infected with ALSV-NSs was increased by approximately 45 times compared with that of wtALSV without expression of any RSS. When ALSV-P19, NSs, or HC-Pro was inoculated to Cucumis sativus plants, none of these ALSV vectors induced symptoms, but accumulation of ALSV in plants infected with ALSV-NSs was increased, suggesting that functions of RSSs on virulence and accumulation of ALSV depend on host species.

When Viruses Play Team Sports: Mixed Infections in Plants

The pathological importance of mixed viral infections in plants might be underestimated except for a few well-characterized synergistic combinations in certain crops. Considering that the host ranges of many viruses often overlap and that most plant species can be infected by several unrelated viruses, it is not surprising to find more than one virus simultaneously in the same plant. Furthermore, dispersal of the majority of plant viruses relies on efficient transmission mechanisms mediated by vector organisms, mainly but not exclusively insects, which can contribute to the occurrence of multiple infections in the same plant. Recent work using different experimental approaches has shown that mixed viral infections can be remarkably frequent, up to the point that they could be considered the rule more than the exception. The purpose of this review is to describe the impact of multiple infections not only on the participating viruses themselves but also on their vectors and on the common host. From this standpoint, mixed infections arise as complex events that involve several cross-interacting players, and they consequently require a more general perspective than the analysis of single-virus/single-host approaches for a full understanding of their relevance.

Assessing the Impact on Virus Transmission and Insect Vector Behavior of a Viral Mixed Infection in Melon

Mixed viral infections in plants are common, and can result in synergistic or antagonistic interactions. Except in complex diseases with severe symptoms, mixed infections frequently remain unnoticed, and their impact on insect vector transmission is largely unknown. In this study, we considered mixed infections of two unrelated viruses commonly found in melon plants, the crinivirus cucurbit yellow stunting disorder virus (CYSDV) and the potyvirus watermelon mosaic virus (WMV), and evaluated their vector transmission by whiteflies and aphids, respectively. Their dynamics of accumulation was analyzed until 60 days postinoculation (dpi) in mixed-infected plants, documenting reduced titers of WMV and much higher titers of CYSDV compared with single infections. At 24 dpi, corresponding to the peak of CYSDV accumulation, similar whitefly transmission rates were obtained when comparing either individual or mixed-infected plants as CYSDV sources, although its secondary dissemination was slightly biased toward plants previously infected with WMV, regardless of the source plant. However, at later time points, mixed-infected plants partially recovered from the initially severe symptoms, and CYSDV transmission became significantly higher. Interestingly, aphid transmission rates both at early and late time points were unaltered when WMV was acquired from mixed-infected plants despite its reduced accumulation. This lack of correlation between WMV accumulation and transmission could result from compensatory effects observed in the analysis of the aphid feeding behavior by electrical penetration graphs. Thus, our results showed that mixed-infected plants could provide advantages for both viruses, directly favoring CYSDV dissemination while maintaining WMV transmission.

Coinfección natural de virus de ARN en cultivos de papa (Solanum tuberosum subsp. Andigena) en Antioquia (Colombia)

Las enfermedades virales son uno de los principales problemas fitopatológicos de la papa. Con el fin de determinar los virus más prevalentes en cultivos de papa var. Diacol Capiro en el oriente Antioqueño (Colombia), se evaluó mediante RT-qPCR la presencia de diez virus de ARN (PVY, PVA, PVV, TaLMV, PVS, PLRV, PYVV, PVX, ToRSV y PMTV) en 36 muestras de tejido foliar. Los resultados indicaron la ocurrencia de cinco de los diez virus evaluados, con niveles de prevalencia de 88,9 %, 75 %, 75 %, 41,7 % y 25 % para PVY, PVX, PYVV, PLRV y PVS, respectivamente. Con fines comparativos, cuatro virus también se evaluaron mediante ELISA, siendo detectados PVS (80,5 %), PVY (55 %) y PLRV (5,5 %); mientras que PVX no fue encontrado con esta prueba. La comparación de estas técnicas mediante la razón de prevalencia (RP), indicó que la RT-qPCR ofrece niveles superiores de detección con valores de RP = 1,6 y RP = 7,5 para los virus PVY y PLRV; mientras que para PVS la ELISA detectó más muestras positivas que RT-qPCR (RP = 3,22), evidenciándose la necesidad de diseñar nuevos cebadores ajustados a la diversidad de este virus en Antioquia. La coinfección mixta más frecuente fue PVY-PYVV-PVX (22,2 %), mientras que los cinco virus se encontraron en el 11,1 % de las muestras. Finalmente, utilizando secuenciación Sanger de la cápside y NGS para los genomas completos, se confirmó la circulación de todos los virus detectados en los cultivos de papa del oriente Antioqueño. Estos resultados señalan la necesidad de fortalecer los programas de manejo integrado de enfermedades virales en Antioquia.

The enemy within: phloem-limited pathogens

The growing impact of phloem-limited pathogens on high-value crops has led to a renewed interest in understanding how they cause disease. Although these pathogens cause substantial crop losses, many are poorly characterized. In this review, we present examples of phloem-limited pathogens that include intracellular bacteria with and without cell walls, and viruses. Phloem-limited pathogens have small genomes and lack many genes required for core metabolic processes, which is, in part, an adaptation to the unique phloem environment. For each pathogen class, we present multiple case studies to highlight aspects of disease caused by phloem-limited pathogens. The pathogens presented include Candidatus Liberibacter asiaticus (citrus greening), Arsenophonus bacteria, Serratia marcescens (cucurbit yellow vine disease), Candidatus Phytoplasma asteris (Aster Yellows Witches' Broom), Spiroplasma kunkelii, Potato leafroll virus and Citrus tristeza virus. We focus on commonalities in the virulence strategies of these pathogens, and aim to stimulate new discussions in the hope that widely applicable disease management strategies can be found.

Root-Knot Nematode Parasitism Suppresses Host RNA Silencing

Root-knot nematodes damage crops around the world by developing complex feeding sites from normal root cells of their hosts. The ability to initiate and maintain this feeding site (composed of individual "giant cells") is essential to their parasitism process. RNA silencing pathways in plants serve a diverse set of functions, from directing growth and development to defending against invading pathogens. Influencing a host's RNA silencing pathways as a pathogenicity strategy has been well-documented for viral plant pathogens, but recently, it has become clear that silencing pathways also play an important role in other plant pathosystems. To determine if RNA silencing pathways play a role in nematode parasitism, we tested the susceptibility of plants that express a viral suppressor of RNA silencing. We observed an increase in susceptibility to nematode parasitism in plants expressing viral suppressors of RNA silencing. Results from studies utilizing a silenced reporter gene suggest that active suppression of RNA silencing pathways may be occurring during nematode parasitism. With these studies, we provide further evidence to the growing body of plant-biotic interaction research that suppression of RNA silencing is important in the successful interaction between a plant-parasitic animal and its host.

The entry of cucumber mosaic virus into cucumber xylem is facilitated by co-infection with zucchini yellow mosaic virus

We investigated the synergistic effects of co-infection by zucchini yellow mosaic virus (ZYMV) and cucumber mosaic virus (CMV) on viral distribution in the vascular tissues of cucumber. Immunohistochemical observations indicated that ZYMV was present in both the phloem and xylem tissues. ZYMV-RNA was detected in both the xylem wash and guttation fluid of ZYMV-inoculated cucumber. Steam treatment at a stem internode indicated that ZYMV enters the xylem vessels and moves through them but does not cause systemic infection in the plant. CMV distribution in singly infected cucumbers was restricted to phloem tissue. By contrast, CMV was detected in the xylem tissue of cotyledons in plants co-infected with CMV and ZYMV. Although both ZYMV-RNA and CMV-RNA were detected in the xylem wash and upper internodes of steam-treated, co-infected cucumbers grown at 24 °C, neither virus was detected in the upper leaves using an ELISA assay. Genetically modified CMV harboring the ZYMV HC-Pro gene was distributed in the xylem and phloem tissues of singly inoculated cucumber cotyledons. These results indicate that the ZYMV HC-Pro gene facilitates CMV entry into the xylem vessels of co-infected cucumbers.

Methodological Guidelines for Accurate Detection of Viruses in Wild Plant Species

Ecological understanding of disease risk, emergence, and dynamics and of the efficacy of control strategies relies heavily on efficient tools for microorganism identification and characterization. Misdetection, such as the misclassification of infected hosts as healthy, can strongly bias estimates of disease prevalence and lead to inaccurate conclusions. In natural plant ecosystems, interest in assessing microbial dynamics is increasing exponentially, but guidelines for detection of microorganisms in wild plants remain limited, particularly so for plant viruses. To address this gap, we explored issues and solutions associated with virus detection by serological and molecular methods in noncrop plant species as applied to the globally important Barley yellow dwarf virus PAV (Luteoviridae), which infects wild native plants as well as crops. With enzyme-linked immunosorbent assays (ELISA), we demonstrate how virus detection in a perennial wild plant species may be much greater in stems than in leaves, although leaves are most commonly sampled, and may also vary among tillers within an individual, thereby highlighting the importance of designing effective sampling strategies. With reverse transcription-PCR (RT-PCR), we demonstrate how inhibitors in tissues of perennial wild hosts can suppress virus detection but can be overcome with methods and products that improve isolation and amplification of nucleic acids. These examples demonstrate the paramount importance of testing and validating survey designs and virus detection methods for noncrop plant communities to ensure accurate ecological surveys and reliable assumptions about virus dynamics in wild hosts.

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.

A viral transcription factor exhibits antiviral RNA silencing suppression activity independent of its nuclear localization

Viral suppressors of RNA silencing (VSRs) are critical for the success of virus infection and efficient accumulation of virus progeny. The chrysanthemum virus B p12 protein acts as a transcription factor to regulate cell size and proliferation favourable for virus infection. Here, we showed that the p12 protein suppressed RNA silencing and was able to complement a VSR-deficient unrelated virus. Moreover, p12 counter-silencing activity could be uncoupled from its function as a transcription factor in the nucleus. The altered p12 protein, which lacked a nuclear localization signal and was not imported into the nucleus, was able to suppress RNA silencing as efficiently as the native protein. The data revealed new aspects of p12 functioning and identified a novel role for this viral zinc-finger transcription factor. The results provided a general insight into one of the activities of the p12 protein, which appeared to possess more than one function.

Modification of Tobacco rattle virus RNA1 to serve as a VIGS vector reveals that the 29K movement protein is an RNA silencing suppressor of the virus

Tobacco rattle virus (TRV) has a bipartite, positive-sense single-stranded RNA genome and is widely used for virus-induced gene silencing (VIGS) in plants. RNA1 of TRV that lacks the gene for the cysteine-rich 16K silencing-suppression protein infects plants systemically in the absence of RNA2. Here, we attempted to engineer RNA1 for use as a VIGS vector by inserting heterologous gene fragments to replace 16K. The RNA1 vector systemically silenced the phytoene desaturase (PDS) gene, although less efficiently than when the original VIGS vector system was used, which consists of wild-type RNA1 and engineered RNA2 carrying the heterologous gene. Infectious RNA1 mutants with a dysfunctional 16K suppressed silencing and enhanced transgene expression in green fluorescent protein-transgenic Nicotiana benthamiana following inoculation by agroinfiltration, unlike mutants that also lacked 29K, a movement protein (MP) gene. The 30K MP gene of Tobacco mosaic virus complemented in cis the movement defect but not the silencing suppression functions of TRV 29K. Silencing suppression by 29K occurred in the context of RNA1 replication but not in an agroinfiltration assay which tested 29K alone for suppression of sense-mediated silencing. Both 29K and 16K were needed to avoid necrotic symptoms in RNA1-infected N. benthamiana. The results shed new light on virulence factors of TRV.

An RNA virus-encoded zinc-finger protein acts as a plant transcription factor and induces a regulator of cell size and proliferation in two tobacco species

Plant viruses cause a variety of diseases in susceptible hosts. The disease symptoms often include leaf malformations and other developmental abnormalities, suggesting that viruses can affect plant development. However, little is known about the mechanisms underlying virus interference with plant morphogenesis. Here, we show that a C-4 type zinc-finger (ZF) protein, p12, encoded by a carlavirus (chrysanthemum virus B) can induce cell proliferation, which results in hyperplasia and severe leaf malformation. We demonstrate that the p12 protein activates expression of a regulator of cell size and proliferation, designated upp-L (upregulated by p12), which encodes a transcription factor of the basic/helix-loop-helix family sufficient to cause hyperplasia. The induction of upp-L requires translocation of the p12 protein into the nucleus and ZF-dependent specific interaction with the conserved regulatory region in the upp-L promoter. Our results establish the role of the p12 protein in modulation of host cell morphogenesis. It is likely that other members of the conserved C-4 type ZF family of viral proteins instigate reprogramming of plant development by mimicking eukaryotic transcriptional activators.

Ectopic expression of the p23 silencing suppressor of Citrus tristeza virus differentially modifies viral accumulation and tropism in two transgenic woody hosts

Citrus tristeza virus (CTV), a phloem-restricted closterovirus infecting citrus, encodes three different silencing suppressors (p25, p20 and p23), one of which (p23) is a pathogenicity determinant that induces aberrations resembling CTV symptoms when expressed ectopically in transgenic citrus hosts. In this article, the effect of p23 ectopic expression on virus infection was examined in sweet orange (SwO), a highly susceptible host, and sour orange (SO), which severely restricts CTV cell-to-cell movement. Transgenic plants of both species ectopically expressing p23, or transformed with an empty vector, were graft inoculated with the mild CTV isolate T385 or with CTV-BC1/GFP, a clonal strain derived from the severe isolate T36 carrying the gene for the green fluorescent protein (GFP). CTV distribution in infected tissues was assessed by direct tissue blot immunoassay and fluorescence emission, and virus accumulation was estimated by quantitative real-time reverse transcriptase-polymerase chain reaction. CTV accumulation in p23-expressing and control SwO plants was similar, whereas the viral load in transgenic SO expressing p23 was 10-10(5) times higher than in the cognate control plants. Although few infection foci composed of a single cell were observed in the phloem of CTV-infected control SO, the number of foci in p23-expressing plants was higher and usually comprised two to six cells, indicating viral cell-to-cell movement. CTV was detected in mesophyll protoplasts and cells from infected SO and SwO expressing p23, but not in similar protoplasts and cells from infected control plants. Our results show that the ectopic expression of p23 enables CTV to escape from the phloem and, in addition, facilitates systemic infection of the resistant SO host. This is the first report of a viral-encoded protein that enhances virus accumulation and distribution in woody hosts.

RNA viruses and their silencing suppressors boost Abutilon mosaic virus, but not the Old World Tomato yellow leaf curl Sardinia virus

Mixed viral infections can induce different changes in symptom development, genome accumulation and tissue tropism. These issues were investigated for two phloem-limited begomoviruses, Abutilon mosaic virus (AbMV) and Tomato yellow leaf curl Sardinia virus (TYLCSV) in Nicotiana benthamiana plants doubly infected by either the potyvirus Cowpea aphid-borne mosaic virus (CABMV) or the tombusvirus Artichoke mottled crinkle virus (AMCV). Both RNA viruses induced an increase of the amount of AbMV, led to its occasional egress from the phloem and induced symptom aggravation, while the amount and tissue tropism of TYLCSV were almost unaffected. In transgenic plants expressing the silencing suppressors of CABMV (HC-Pro) or AMCV (P19), AbMV was supported to a much lesser extent than in the mixed infections, with the effect of CABMV HC-Pro being superior to that of AMCV P19. Neither of the silencing suppressors influenced TYLCSV accumulation. These results demonstrate that begomoviruses differentially respond to the invasion of other viruses and to silencing suppression.

Yellowing disease in zucchini squash produced by mixed infections of Cucurbit yellow stunting disorder virus and Cucumber vein yellowing virus

Zucchini squash is host to Cucurbit yellow stunting disorder virus (CYSDV), a member of the genus Crinivirus, and Cucumber vein yellowing virus (CVYV), a member of the genus Ipomovirus, both transmitted by the whitefly Bemisia tabaci. Field observations suggest the appearance of new symptoms observed on leaves of zucchini squash crops when both viruses were present. When infected during controlled experiments with CYSDV only, zucchini plants showed no obvious symptoms and the virus titer decreased between 15 and 45 days postinoculation (dpi), after which it was no longer detected. CVYV caused inconspicuous symptoms restricted to vein clearing on some of the apical leaves and the virus accumulated progressively between 15 and 60 dpi. Similar accumulations of virus followed single inoculations with the potyvirus Zucchini yellow mosaic virus (ZYMV) and plants showed severe stunting, leaf deformation, and mosaic yellowing. However, in mixed infections with CYSDV and CVYV, intermediate leaves showed chlorotic mottling which evolved later to rolling, brittleness, and complete yellowing of the leaf lamina, with exception of the veins. No consistent alteration of CVYV accumulation was detected but the amounts of CYSDV increased ≈100-fold and remained detectable at 60 dpi. Such synergistic effects on the titer of the crinivirus and symptom expression were not observed when co-infected with ZYMV.

Why do viruses need phloem for systemic invasion of plants?

Plant viruses use sieve elements in phloem as the route of long-distance movement and systemic infection in plants. Plants, in turn, deploy RNA silencing, R-gene mediated defence and other mechanisms to prevent phloem transport of viruses. Cell-to-cell movement of viruses from an initially infected leaf to stem and other parts of the plant could be another possibility for systemic invasion, but it is considered to be too slow. This idea is supported by observations made on viruses that are deficient in phloem loading. The leaf abscission zone forming at the base of the petiole may constitute a barrier that prevents viral cell-to-cell movement. The abscission zone and protective layer are difficult to localize in the petiole until the leaf reaches an advanced stage of senescence. Viruses tagged with the green fluorescent protein are helpful for localization and study of the developing abscission zone.

Synergistic interaction between the Potyvirus, Turnip mosaic virus and the Crinivirus, Lettuce infectious yellows virus in plants and protoplasts

Lettuce infectious yellows virus (LIYV), the type member of the genus Crinivirus in the family Closteroviridae, is specifically transmitted by the sweet potato whitefly (Bemisia tabaci) in a semipersistent manner. LIYV infections result in a low virus titer in plants and protoplasts, impeding reverse genetic efforts to analyze LIYV gene/protein functions. We found that synergistic interactions occurred in mixed infections of LIYV and Turnip mosaic virus (TuMV) in Nicotiana benthamiana plants, and these resulted in enhanced accumulation of LIYV. Furthermore, we examined the ability of transgenic plants and protoplasts expressing only the TuMV P1/HC-Pro sequence to enhance the accumulation of LIYV. LIYV RNA and protein titers increased by as much as 8-fold in these plants and protoplasts relative to control plants. LIYV infections remained phloem-limited in P1/HC-Pro transgenic plants, suggesting that enhanced accumulation of LIYV in these plants was due primarily to increased replication efficiency, not to greater spread.

Control of nuclear and nucleolar localization of nuclear inclusion protein a of picorna-like Potato virus A in Nicotiana species

The multifunctional nuclear inclusion protein a (NIa) of potyviruses (genus Potyvirus; Potyviridae) accumulates in the nucleus of virus-infected cells for unknown reasons. In this study, two regions in the viral genome-linked protein (VPg) domain of NIa in Potato virus A (PVA) were found to constitute nuclear and nucleolar localization signals (NLS) in plant cells (Nicotiana spp). Amino acid substitutions in both NLS I (residues 4 to 9) and NLS II (residues 41 to 50) prevented nuclear localization, whereas mutations in either single NLS did not. Mutations in either NLS, however, prevented nucleolar localization and prevented or diminished virus replication in protoplasts, accumulation in infected plant tissues, and/or systemic movement in plants. One NLS mutant was partially complemented by the wild-type VPg expressed in transgenic plants. Furthermore, NLS I controlled NIa accumulation in Cajal bodies. The VPg domain interacted with fibrillarin, a nucleolar protein, and depletion of fibrillarin reduced PVA accumulation. Overexpression of VPg in leaf tissues interfered with cosuppression of gene expression (i.e., RNA silencing), whereas NLS I and NLS II mutants, which exhibited reduced nuclear and nucleolar localization, showed no such activity. These results demonstrate that some of the most essential viral functions required for completion of the infection cycle are tightly linked to regulation of the NIa nuclear and nucleolar localization.

A viral suppressor P1/HC-pro increases the GFP gene expression in agrobacterium-mediated transient assay

More than 20 post-transcriptional gene silencing (PTGS) suppressors have been found since HC-Pro, the first gene silencing suppressor, was found in 1998. The silencing suppressor strongly suggested that gene silencing functions as natural defense mechanisms against viruses. It also represented a valuable tool for the dissection of the gene silencing pathway. We have used P1/HC-Pro RNA silencing suppressor activity to increase green fluorescent protein (GFP) expression in tobacco using an Agrobacterium-mediated transient expression system. P1/HC-Pro stimulated GFP-gene expression but not dsGFP-gene expression was shown by RT-PCR, Northern and Western blot analysis. Expression of the gene silencing suppressor and the target gene provided a new strategy of heterogeneous gene expressing in plants. It may be of commercial significance to produce foreign proteins using plant bioreactors.

The C terminus of the polerovirus p5 readthrough domain limits virus infection to the phloem

Poleroviruses are restricted to vascular phloem tissues from which they are transmitted by their aphid vectors and are not transmissible mechanically. Phloem limitation has been attributed to the absence of virus proteins either facilitating movement or counteracting plant defense. The polerovirus capsid is composed of two forms of coat protein, the major P3 protein and the minor P3/P5 protein, a translational readthrough of P3. P3/P5 is required for insect transmission and acts in trans to facilitate long-distance virus movement in phloem tissue. Specific potato leafroll virus mutants lacking part or all of the P5 domain moved into and infected nonvascular mesophyll tissue when the source-sink relationship of the plant (Solanum sarrachoides) was altered by pruning, with the progeny virus now being transmissible mechanically. However, in a period of months, a phloem-specific distribution of the virus was reestablished in the absence of aphid transmission. Virus from the new phloem-limited infection showed compensatory mutations that would be expected to restore the production of full-length P3/P5 as well as the loss of mechanical transmissibility. The data support our hypothesis that phloem limitation in poleroviruses presumably does not result from a deficiency in the repertoire of virus genes but rather results from P3/P5 accumulation under selection in the infected plant, with the colateral effect of facilitating transmission by phloem-feeding aphid vectors.

Role of the zinc-finger and basic motifs of chrysanthemum virus B p12 protein in nucleic acid binding, protein localization and induction of a hypersensitive response upon expression from a viral vector

The genomes of carlaviruses encode cysteine-rich proteins (CRPs) of unknown function. The 12 kDa CRP of chrysanthemum virus B (CVB), p12, has been shown previously to induce a hypersensitive response (HR) when expressed from potato virus X (PVX). This study demonstrated that a p12-induced HR was preceded by induction of a number of genes related to pathogenesis, stress and systemic acquired resistance. p12 localized predominantly to the nucleus. Interestingly, it was found that p12 bound both RNA and DNA in vitro, but notably exhibited a preference for DNA in the presence of Zn(2+) ions. Mutational analysis of the p12 conserved sequence motifs demonstrated that the basic motif is required for p12 translocation to the nucleus, thus representing part of the protein nuclear localization signal, whereas the predicted zinc finger motif is needed for both Zn(2+)-dependent DNA binding and eliciting an HR in PVX-infected leaves. Collectively, these results link, for the first time, nuclear localization of the protein encoded by a cytoplasmically replicating virus and its DNA-binding capacity with HR induction. Furthermore, these data suggest that p12 may mediate induction of the host genes by binding to the plant genomic DNA, and emphasize that CVB p12 is functionally distinct from other known nuclear-localized proteins encoded by the plant positive-stranded RNA viruses.

Phloem unloading of potato virus X movement proteins is regulated by virus and host factors

To determine the requirements for viral proteins exiting the phloem, transgenic plants expressing green fluorescent protein (GFP) fused to the Potato virus X (PVX) triple gene block (TGB)p1 and coat protein (CP) genes were prepared. The fused genes were transgenically expressed from the companion cell (CC)-specific Commelina yellow mottle virus (CoYMV) promoter. Transgenic plants were selected for evidence of GFP fluorescence in CC and sieve elements (SE) and proteins were determined to be phloem mobile based on their ability to translocate across a graft union into nontransgenic scions. Petioles and leaves were analyzed to determine the requirements for phloem unloading of the fluorescence proteins. In petioles, fluorescence spread throughout the photosynthetic vascular cells (chlorenchyma) but did not move into the cortex, indicating a specific barrier to proteins exiting the vasculature. In leaves, fluorescence was mainly restricted to the veins. However, in virus-infected plants or leaves treated with a cocktail of proteasome inhibitors, fluorescence spread into leaf mesophyll cells. These data indicate that PVX contributes factors which enable specific unloading of cognate viral proteins and that proteolysis may play a role in limiting proteins in the phloem and surrounding chlorenchyma.

EVOLUTION OF PLANT VIRUSES: ADAPTATION TO HOSTS AND VECTORS

Viruses are obligate molecular pathogens. They depend on living host cells for their multiplication, including synthesis of the viral nucleic acids and proteins. The infection cycle of viruses in plants includes three main phases: i) replication, ii) cell to cell movement via plasmodesmata, and iii) long distance movement to different parts of the plant. During all these steps of the infection cycle viruses are challenged by the genetic variability of their hosts, which requires the virus to be adjusted to minor or major differences in virus-host interactions. These adjustments require mutations in the viral genome. Most plant viruses are also dependent on vector organisms for their spread to new host plants. The changes in virus genomes for better adaptability to the host should not compromise vector-transmissibility of progeny viruses. Host adaptation and vector adaptation can therefore be seen as the main forces influencing plant virus evolution.

Transcomplementation and synergism in plants: implications for viral transgenes?

In plants, viral synergisms occur when one virus enhances infection by a distinct or unrelated virus. Such synergisms may be unidirectional or mutualistic but, in either case, synergism implies that protein(s) from one virus can enhance infection by another. A mechanistically related phenomenon is transcomplementation, in which a viral protein, usually expressed from a transgene, enhances or supports the infection of a virus from a distinct species. To gain an insight into the characteristics and limitations of these helper functions of individual viral genes, and to assess their effects on the plant-pathogen relationship, reports of successful synergism and transcomplementation were compiled from the peer-reviewed literature and combined with data from successful viral gene exchange experiments. Results from these experiments were tabulated to highlight the phylogenetic relationship between the helper and dependent viruses and, where possible, to identify the protein responsible for the altered infection process. The analysis of more than 150 publications, each containing one or more reports of successful exchanges, transcomplementation or synergism, revealed the following: (i) diverse viral traits can be enhanced by synergism and transcomplementation; these include the expansion of host range, acquisition of mechanical transmission, enhanced specific infectivity, enhanced cell-to-cell and long-distance movement, elevated or novel vector transmission, elevated viral titre and enhanced seed transmission; (ii) transcomplementation and synergism are mediated by many viral proteins, including inhibitors of gene silencing, replicases, coat proteins and movement proteins; (iii) although more frequent between closely related viruses, transcomplementation and synergism can occur between viruses that are phylogenetically highly divergent. As indicators of the interoperability of viral genes, these results are of general interest, but they can also be applied to the risk assessment of transgenic crops expressing viral proteins. In particular, they can contribute to the identification of potential hazards, and can be used to identify data gaps and limitations in predicting the likelihood of transgene-mediated transcomplementation.

Capsid protein-mediated recruitment of host DnaJ-like proteins is required for Potato virus Y infection in tobacco plants

The capsid protein (CP) of potyviruses is required for various steps during plant infection, such as virion assembly, cell-to-cell movement, and long-distance transport. This suggests a series of compatible interactions with putative host factors which, however, are largely unknown. By using the yeast two-hybrid system the CP from Potato virus Y (PVY) was found to interact with a novel subset of DnaJ-like proteins from tobacco, designated NtCPIPs. Mutational analysis identified the CP core region, previously shown to be essential for virion formation and plasmodesmal trafficking, as the interacting domain. The ability of NtCPIP1 and NtCPIP2a to associate with PVY CP could be confirmed in vitro and was additionally verified in planta by bimolecular fluorescence complementation. The biological significance of the interaction was assayed by PVY infection of agroinfiltrated leaves and transgenic tobacco plants that expressed either full-length or J-domain-deficient variants of NtCPIPs. Transient expression of truncated dominant-interfering NtCPIP2a but not of the functional protein resulted in strongly reduced accumulation of PVY in the inoculated leaf. Consistently, stable overexpression of J-domain-deficient variants of NtCPIP1 and NtCPIP2a dramatically increased the virus resistance of various transgenic lines, indicating a critical role of functional NtCPIPs during PVY infection. The negative effect of impaired NtCPIP function on viral pathogenicity seemed to be the consequence of delayed cell-to-cell movement, as visualized by microprojectile bombardment with green fluorescent protein-tagged PVY. Therefore, we propose that NtCPIPs act as important susceptibility factors during PVY infection, possibly by recruiting heat shock protein 70 chaperones for viral assembly and/or cellular spread.

The Polerovirus silencing suppressor P0 targets ARGONAUTE proteins for degradation

Plant and animal viruses encode suppressor proteins of an adaptive immunity mechanism in which viral double-stranded RNA is processed into 21-25 nt short interfering (si)RNAs. The siRNAs guide ARGONAUTE (AGO) proteins so that they target viral RNA. Most viral suppressors bind long dsRNA or siRNAs and thereby prevent production of siRNA or binding of siRNA to AGO. The one exception is the 2b suppressor of Cucumoviruses that binds to and inhibits AGO1. Here we describe a novel suppressor mechanism in which a Polerovirus-encoded F box protein (P0) targets the PAZ motif and its adjacent upstream sequence in AGO1 and mediates its degradation. F box proteins are components of E3 ubiquitin ligase complexes that add polyubiquitin tracts on selected lysine residues and thereby mark a protein for proteasome-mediated degradation. With P0, however, the targeted degradation of AGO is insensitive to inhibition of the proteasome, indicating that the proteasome is not involved. We also show that P0 does not block a mobile signal of silencing, indicating that the signal molecule does not have AGO protein components. The ability of P0 to block silencing without affecting signal movement may contribute to the phloem restriction of viruses in the Polerovirus group.

VPg of Potato virus A alone does not suppress RNA silencing but affects virulence of a heterologous virus

The viral genome-linked protein (VPg) is a well-known virulence factor in potyviruses (genus Potyvirus), including Potato virus A (PVA). Its ability to suppress onset and signalling of transgene-mediated RNA silencing and accumulation of small interfering RNA (siRNA) was studied using cross-protection and Agrobacterium infiltration assays and green fluorescent protein (GFP) and PVA VPg protein-expressing transgenic Nicotiana benthamiana plants. N. benthamiana plants were also transformed with a transgene comprising the cylindrical inclusion protein (CI), nuclear inclusion protein a (NIa) and coat protein (CP) encoding regions of PVA. This transgene mRNA was expressed in the T1 progeny of the transgenic lines but all were susceptible to PVA. This result contrasted the plants transformed with the PVA P1, VPg (N-proximal part of NIa) or CP encoding regions that expressed various forms of resistance. There was little evidence for direct involvement of VPg in suppression of silencing, while other mechanisms by which VPg might interfere with transgenic resistance could not be excluded. Expression of the wild-type PVA VPg from the genome of Potato virus X (PVX, genus Potexvirus) increased symptom severity in N. benthamiana, whereas a single point mutation introduced to the VPg enhanced accumulation of the PVX chimera. These data demonstrated previously unknown virulence functions controlled by the VPg of a potyvirus.

Cucurbit aphid-borne yellows virus Is Prevalent in Field-Grown Cucurbit Crops of Southeastern Spain

Despite the importance of field-grown cucurbits in Spain, only limited information is available about the impact of disease on their production. During the 2003 and 2004 growing seasons, systematic surveys were carried out in open field melon (Cucumis melo) and squash (Cucurbita pepo) crops of Murcia Province (Spain). The fields were chosen with no previous information regarding their sanitation status, and samples were taken from plants showing viruslike symptoms. Samples were analyzed using molecular hybridization to detect Beet pseudo-yellows virus (BPYV), Cucurbit aphid-borne yellows virus (CABYV), Cucumber mosaic virus (CMV), Cucumber vein yellowing virus (CVYV), Cucurbit yellow stunting disorder virus (CYSDV), Melon necrotic spot virus (MNSV), Papaya ringspot virus (PRSV), Watermelon mosaic virus (WMV), and Zucchini yellow mosaic virus (ZYMV). We collected 924 samples from 48 field plots. Out of these, almost 90% were infected by at least one of the viruses considered, usually CABYV, which was present in 83 and 66% of the melon and squash samples, respectively. In the case of melon, CYSDV, BPYV, and WMV followed CABYV in relative importance, with frequencies of around 20 to 30%, while in squash, CVYV and BPYY showed frequencies between 28 and 21%. The number of multiple infections was very high, 66 and 56% of the infected samples of melon and squash, respectively, being afflicted. CABYV was present in all multiple infections. The high incidence of CABYV in single and multiple infections suggests that this virus may well become an important threat for cucurbit crops in the region. Restriction fragment length polymorphism (RFLP) analysis revealed that CABYV isolates can be grouped into two genetic types, both of which seemed to be present during the 2003 epidemic episode, but only one of the types was found in 2004.

Soybean mosaic virus Helper Component-Protease Alters Leaf Morphology and Reduces Seed Production in Transgenic Soybean Plants

ABSTRACT Transgenic soybean (Glycine max) plants expressing Soybean mosaic virus (SMV) helper component-protease (HC-Pro) showed altered vegetative and reproductive phenotypes and responses to SMV infection. When inoculated with SMV, transgenic plants expressing the lowest level of HC-Pro mRNA and those transformed with the vector alone initially showed mild SMV symptoms. Plants that accumulated the highest level of SMV HC-Pro mRNA showed very severe SMV symptoms initially, but after 2 weeks symptoms disappeared, and SMV titers were greatly reduced. Analysis of SMV RNA abundance over time with region-specific probes showed that the HC-Pro region of the SMV genome was degraded before the coat protein region. Transgenic soybean plants that expressed SMV HC-Pro showed dose-dependent alterations in unifoliate leaf morphologies and seed production where plants expressing the highest levels of HC-Pro had the most deformed leaves and the lowest seed production. Accumulation of microRNAs (miRNAs) and mRNAs putatively targeted by miRNAs was analyzed in leaves and flowers of healthy, HC-Pro-transgenic, and SMV-infected plants. Neither expression of SMV HC-Pro nor SMV infection produced greater than twofold changes in accumulation of six miRNAs. In contrast, SMV infection was associated with twofold or greater increases in the accumulation of four of seven miRNA-targeted mRNAs tested.

Yield Loss in Garlic Caused by Leek yellow stripe virus Argentinean Isolate

Garlic plants (Allium sativum) are naturally infected by a complex of viruses in the genera Potyvirus, Carlavirus, and Allexivirus. The yield of virus-free garlic plants (noninoculated control) was compared with that of plants infected with an Argentinean isolate of Leek yellow stripe virus (LYSV; L treatment) and garlic plants infected with the virus complex (VC). Evaluations were conducted in the field and in anti-aphid cages during two crop cycles after planting three sizes of cloves (categories). The percent plant emergence in the noninoculated control and in the L treatments (between 80 and 100%) did not differ statistically, but the percent emergence for these two treatments was double that for the VC treatment (25 to 62%). Plant height and leaf number in the L treatment were lower than in the noninoculated control during the first evaluation (year 1), but they did not differ during the second evaluation (year 2). However, both treatments produced taller plants with more leaves than those of VC in both years. The L treatment decreased bulb weight up to 28% and perimeter up to 9% when compared with those in the noninoculated control maintained in the anti-aphid cages until the end of the experiment. However, differences between these treatments were higher in the field experiments where plants were exposed to infection by other viruses (up to 36% in bulb weight and 13% in perimeter). Bulbs of the VC-infected plant treatment were reduced up to 74% in weight and 37% in perimeter. In field evaluations, a high percentage of plants were infected with Onion yellow dwarf virus (58 to 100%), whereas fewer were infected with LYSV (15 to 68%). Garlic virus A infection was high in plants previously infected with LYSV (96 and 97%), but lower in the noninoculated control (12 and 68%). These results show the high impact of the virus complex on garlic yield and the effect of LYSV as a component of the garlic virus complex.

Synergism of a DNA and an RNA virus: enhanced tissue infiltration of the begomovirus Abutilon mosaic virus (AbMV) mediated by Cucumber mosaic virus (CMV)

Replication of the begomovirus Abutilon mosaic virus (AbMV) is restricted to phloem nuclei, generating moderate levels of virus DNA. Co-infection with Cucumber mosaic virus (CMV) evidently increased AbMV titers in Nicotiana benthamiana, tobacco, and tomato, resulting in synergistic symptom enhancement. In situ hybridization revealed that in double-infected leaves an increased number of nuclei contained elevated amounts of AbMV. Additionally, the begomoviral phloem-limitation was broken. Whereas CMV 3a movement protein-expressing tobacco plants did not exert any similar influence, the presence of CMV 2b silencing suppressor protein lead to enhanced AbMV titers and numbers of infected vascular cells. The findings prove that AbMV can replicate in nonvascular cells and represent the first report on a true synergism of an RNA/ssDNA virus combination in plants, in which CMV 2b protein plays a role. They indicate considerable consequences of mixed infections between begomo- and cucumoviruses on virus epidemiology and agriculture.

Safety of virus-resistant transgenic plants two decades after their introduction: lessons from realistic field risk assessment studies

Potential safety issues have been raised with the development and release of virus-resistant transgenic plants. This review focuses on safety assessment with a special emphasis on crops that have been commercialized or extensively tested in the field such as squash, papaya, plum, grape, and sugar beet. We discuss topics commonly perceived to be of concern to the environment and to human health--heteroencapsidation, recombination, synergism, gene flow, impact on nontarget organisms, and food safety in terms of allergenicity. The wealth of field observations and experimental data is critically evaluated to draw inferences on the most relevant issues. We also express inside views on the safety and benefits of virus-resistant transgenic plants, and recommend realistic risk assessment approaches to assist their timely deregulation and release.

Synergistic Interaction Between Tomato chlorosis virus and Tomato spotted wilt virus Results in Breakdown of Resistance in Tomato

ABSTRACT Multiple viral infections frequently are found in single plants of cultivated and wild hosts in nature, with unpredictable pathological consequences. Synergistic reactions were observed in mixed infections in tomato plants doubly infected with the positive-sense and phloem-limited single-stranded RNA (ssRNA) crinivirus Tomato chlorosis virus (ToCV) and the negative-sense ssRNA tospovirus Tomato spotted wilt virus (TSWV). Synergism in a tomato cultivar susceptible to both viruses resulted in a rapid death of plants. A pronounced enhancement of ToCV accumulation mediated by TSWV co-infection was observed with no evident egress of ToCV from phloem tissues. No consistent alteration of TSWV accumulation was detected. More remarkable was the synergism observed in tomato cultivars which carry the Sw-5 resistance gene, which are resistant to TSWV. Pre-infection with ToCV resulted in susceptibility to TSWV, whereas co-inoculations did not. This suggested that a threshold level or a time lapse is needed for ToCV to interfere or downregulate the defense response in the TSWV-resistant plants.

Assessment of the integral membrane protein topology in living cells

The bimolecular fluorescence complementation (BiFC) phenomenon has been successfully applied for in vivo protein-protein interaction studies and protein tagging analysis. Here we report a novel BiFC-based technique for investigation of integral membrane protein topology in living plant cells. This technique relies on the formation of a fluorescent complex between a non-fluorescent fragment of the yellow fluorescent protein (YFP) targeted into a specific cellular compartment and a counterpart fragment attached to the integral membrane protein N- or C-terminus or inserted into the internal loop(s). We employed this technique for topological studies of beet yellows virus-encoded p6 membrane-embedded movement protein, a protein with known topology, and the potato mop-top virus-encoded integral membrane TGBp2 protein with predicted topology. The results confirm that p6 is a type III integral transmembrane protein. Using a novel method, the central hydrophilic region of TGBp2 was localized into the ER lumen, whereas the N- and C-termini localized to the cytosol. We conclude that the BiFC-based reporter system for membrane protein topology analysis is a relatively fast and efficient method that can be used for high-throughput analysis of proteins integrated into the endoplasmic reticulum in living plant cells.

P1- and VPg-transgenic plants show similar resistance to Potato virus A and may compromise long distance movement of the virus in plant sections expressing RNA silencing-based resistance

Nicotiana benthamiana was transformed with P1 or VPg cistron of Potato virus A (PVA, genus Potyvirus). For both transgenes, T1 progeny displayed (i) resistance to PVA infection, (ii) susceptibility, or (iii) systemic infection followed by recovery of new leaves from PVA infection (RC), regardless of the transgene. In RC plants, fully recovered leaves contained no detectable PVA RNA, were highly resistant to challenge inoculation with PVA, and had barely detectable steady-state levels of transgene mRNA; transgene-homologous siRNA was not detected, in contrast to leaves undergoing recovery. Tops in RC plants and PVA-susceptible transgenic plants were replaced with scions from wild-type plants; only scions on the latter became PVA-infected. These findings suggest that vascular movement of PVA from lower, infected parts of RC plants was compromised in the recovered section expressing RNA silencing-based resistance, which adds a novel dimension to the current models for potyvirus movement.

Nucleolar localization of potato leafroll virus capsid proteins

Potato leafroll virus (PLRV) encodes two capsid proteins, major protein (CP) and minor protein (P5), an extended version of the CP produced by occasional translational 'readthrough' of the CP gene. Immunogold electron microscopy showed that PLRV CP is located in the cytoplasm and also localized in the nucleus, preferentially targeting the nucleolus. The nucleolar localization of PLRV CP was also confirmed when it was expressed as a fusion with green fluorescent protein (GFP) via an Agrobacterium vector. Mutational analysis identified a particular sequence within PLRV CP involved in nucleolar targeting [the nucleolar localization signal (NoLS)]. Minor protein P5 also contains the same NoLS, and was targeted to the nucleolus when it was expressed as a fusion with GFP from Agrobacterium. However, P5-GFP lost its nucleolar localization in the presence of replicating PLRV.

A novel insertion site inside the potyvirus P1 cistron allows expression of heterologous proteins and suggests some P1 functions

The P1 cistron encodes the first and most variable part of the polyprotein of potyviruses. A site tolerant to a pentapeptide insertion at the N-terminus of Potato virus A P1 (Genome Res. 12, 584-594) was used to express heterologous proteins (insertions up to 783 nucleotides) with or without flanking new proteolytic sites. Aequorea victoria green fluorescent protein (GFP) accumulated to high levels when proteolytically released from P1 and showed strong fluorescence in leaves systemically infected with vector virus. Deletions in GFP and adjacent viral sequences emerged 2-4 weeks after infection, revealing putative recombination hot spots. The inserts in P1 diminished infectivity host-specifically, reduced virus accumulation in protoplasts and systemically infected leaves, alleviated symptoms and reduced accumulation of mRNA and HCpro in cis in a virus-free system. This heterologous protein expression site is the first within a protein-encoding cistron and the third in the genome of potyviruses.

Viral class 1 RNase III involved in suppression of RNA silencing

Double-stranded RNA (dsRNA)-specific endonucleases belonging to RNase III classes 3 and 2 process dsRNA precursors to small interfering RNA (siRNA) or microRNA, respectively, thereby initiating and amplifying RNA silencing-based antiviral defense and gene regulation in eukaryotic cells. However, we now provide evidence that a class 1 RNase III is involved in suppression of RNA silencing. The single-stranded RNA genome of sweet potato chlorotic stunt virus (SPCSV) encodes an RNase III (RNase3) homologous to putative class 1 RNase IIIs of unknown function in rice and Arabidopsis. We show that RNase3 has dsRNA-specific endonuclease activity that enhances the RNA-silencing suppression activity of another protein (p22) encoded by SPCSV. RNase3 and p22 coexpression reduced siRNA accumulation more efficiently than p22 alone in Nicotiana benthamiana leaves expressing a strong silencing inducer (i.e., dsRNA). RNase3 did not cause intracellular silencing suppression or reduce accumulation of siRNA in the absence of p22 or enhance silencing suppression activity of a protein encoded by a heterologous virus. No other known RNA virus encodes an RNase III or uses two independent proteins cooperatively for RNA silencing suppression.

Co-infection of Beet mosaic virus with Beet Yellowing Viruses Leads to Increased Symptom Expression on Sugar Beet

Three distinct aphid-transmitted viruses associated with a yellowing disease on sugar beet were examined in single and mixed infections for the effects of virus interactions on plant weight, rate of symptom development, and virus concentration. Sugar beet lines exhibiting different degrees of susceptibility to the virus yellows complex were inoculated with either one, two, or all three viruses. Severe stunting, as measured by fresh plant biomass, was observed with mixed infections with Beet yellows virus (BYV) and Beet mosaic virus (BtMV), compared to single infections of these viruses. In addition, the overall rate of appearance of Beet western yellows virus (BWYV) symptoms increased during co-infection with BtMV. Synergistic effects on stunting severity, as measured by plant biomass, were more pronounced in susceptible beet lines, but similar patterns also were observed in lines exhibiting tolerance to virus yellows. Relative concentrations of viruses were compared among single and mixed infections using dot-blot hybridization with virus specific probes, and quantified by phosphorimage analysis. Titers of all three viruses increased as a result of co-infection compared with single infections.

Differential roles of AC2 and AC4 of cassava geminiviruses in mediating synergism and suppression of posttranscriptional gene silencing

Posttranscriptional gene silencing (PTGS) in plants is a natural defense mechanism against virus infection. In mixed infections, virus synergism is proposed to result from suppression of the host defense mechanism by the viruses. Synergistic severe mosaic disease caused by simultaneous infection with isolates of the Cameroon strain of African cassava mosaic virus (ACMV-[CM]) and East African cassava mosaic Cameroon virus (EACMCV) in cassava and tobacco is characterized by a dramatic increase in symptom severity and a severalfold increase in viral-DNA accumulation by both viruses compared to that in singly infected plants. Here, we report that synergism between ACMV-[CM] and EACMCV is a two-way process, as the presence of the DNA-A component of ACMV-[CM] or EACMCV in trans enhanced the accumulation of viral DNA of EACMCV and ACMV-[CM], respectively, in tobacco BY-2 protoplasts. Furthermore, transient expression of ACMV-[CM] AC4 driven by the Cauliflower mosaic virus 35S promoter (p35S-AC4) enhanced EACMCV DNA accumulation by approximately 8-fold in protoplasts, while p35S-AC2 of EACMCV enhanced ACMV-[CM] DNA accumulation, also by approximately 8-fold. An Agrobacterium-based leaf infiltration assay determined that ACMV-[CM] AC4 and EACMCV AC2, the putative synergistic genes, were able to suppress PTGS induced by green fluorescent protein (GFP) and eliminated the short interfering RNAs associated with PTGS, with a correlated increase in GFP mRNA accumulation. In addition, we have identified AC4 of Sri Lankan cassava mosaic virus and AC2 of Indian cassava mosaic virus as suppressors of PTGS, indicating that geminiviruses evolved differently in regard to interaction with the host. The specific and different roles played by these AC2 and AC4 proteins of cassava geminiviruses in regulating anti-PTGS activity and their relation to synergism are discussed.

Escape of a plant virus from amplicon-mediated RNA silencing is associated with biotic or abiotic stress

Strong RNA silencing was induced in plants transformed with an amplicon consisting of full-length cDNA of potato leafroll virus (PLRV) expressing green fluorescent protein (GFP), as shown by low levels of PLRV-GFP accumulation, lack of symptoms and accumulation of amplicon-specific short interfering RNAs (siRNAs). Inoculation of these plants with various viruses known to encode silencing suppressor proteins induced a striking synergistic effect leading to the enhanced accumulation of PLRV-GFP, suggesting that it had escaped from silencing. However, PLRV-GFP escape also occurred following inoculation with viruses that do not encode known silencing suppressors and treatment of silenced plants with biotic or abiotic stress agents. We propose that viruses can evade host RNA-silencing defences by a previously unrecognized mechanism that may be associated with a host response to some types of abiotic stress such as heat shock.

Expression of functionally active helper component protein of Tobacco etch potyvirus in the yeast Pichia pastoris

Tobacco etch potyvirus (TEV) is transmitted by aphids in a non-persistent manner with the assistance of a virus-encoded protein known as helper component (HC-Pro). To produce a biologically active form of recombinant TEV HC-Pro protein, heterologous expression in the methylotrophic yeast Pichia pastoris was used. A cDNA encoding the TEV HC-Pro region, fused to a Saccharomyces cerevisiae alpha-mating factor secretory peptide coding region, was inserted into the P. pastoris genome using a modified version of the pPIC9 vector. The expressed TEV HC-Pro protein was obtained directly from culture medium of recombinant yeast colonies; it was able to interact with TEV particles in a protein overlay binding assay, and also to assist aphid transmission of purified TEV particles to plants using the aphid Myzus persicae as vector. Our results indicate that P. pastoris provides a rapid and low-cost heterologous expression system that can be used to obtain biologically active potyvirus HC-Pro protein for in vitro transmission assays.

Two potato proteins, including a novel RING finger protein (HIP1), interact with the potyviral multifunctional protein HCpro

Potyviral helper-component proteinase (HCpro) is a multifunctional protein exerting its cellular functions in interaction with putative host proteins. In this study, cellular protein partners of the HCpro encoded by Potato virus A (PVA) (genus Potyvirus) were screened in a potato leaf cDNA library using a yeast two-hybrid system. Two cellular proteins were obtained that interact specifically with PVA HCpro in yeast and in the two in vitro binding assays used. Both proteins are encoded by single-copy genes in the potato genome. Analysis of the deduced amino acid sequences revealed that one (HIP1) of the two HCpro interactors is a novel RING finger protein. The sequence of the other protein (HIP2) showed no resemblance to the protein sequences available from databanks and has known biological functions.

Potato leafroll virus: a classic pathogen shows some new tricks

SUMMARY Taxonomy: PLRV is the type species of the genus Polerovirus, in the family Luteoviridae. Isolates are known from most continents, presumably all spread in potato material derived from the Andean region of South America. Physical properties: PLRV particles are isometric and c. 25 nm in diameter. They contain one major (c. 23 kDa) and one minor (c. 80 kDa) protein. The genome is a single 5.8 kb positive sense RNA that has neither a 5'-cap nor 3' poly(A) but carries a VPg.

HOST RANGE

PLRV has a limited host range; about 20 largely solanaceous species have been infected experimentally. PLRV is a common pathogen of potato, and closely related isolates are occasionally found in tomato, but no other crops are affected.

SYMPTOMS

Infection, especially from infected seed potato stocks, causes leafrolling and stunting, the extent depending on the potato cultivar. Biological properties: The biology of PLRV is that of a classic luteovirus. Its isometric particles are persistently transmitted by aphids in a non-propagative manner, it multiplies largely in phloem tissue and disease symptoms reflect this localization. A decade or so of molecular study has revealed the many features of PLRV that are characteristic of its family. Key attractions: In recent years some interesting features of PLRV have emerged that are the focus of further investigation. These are, its phloem confinement, its movement in infected plants, its ability to suppress gene silencing and new ideas about the structure of its particles. This review describes the background to PLRV and points towards these new developments.

Localization of a potyvirus and the viral genome-linked protein in wild potato leaves at an early stage of systemic infection

The upper noninoculated 'sink' leaves of the wild potato species, Solanum commersonii, were studied for distribution of Potato virus A (PVA) at an early stage of systemic infection. Viral RNA was detected by in situ hybridization, and five viral proteins were localized using immunohistochemical staining in leaf sections. Initial systemic infection foci were found at the vicinity of major and minor veins. In these infection foci, the viral coat protein, cylindrical inclusion protein, and helper component-proteinase colocalized with viral RNA in parenchyma and mesophyll cells, but none of these were detected in companion cells (CC). In contrast, VPg, which is the N-proximal half of the NIa protein (separated from the C-terminal proteinase domain, NIapro, by an autocatalytic cleavage) and acts as a viral genome-linked protein, was detected in CC in the infection foci, but only at an early stage of virus unloading. Outside the infection foci, conspicuous signals for VPg were readily and exclusively detected in CC of many veins in all vein classes in the absence of signals for NIapro, other viral proteins, and viral RNA. Taken together, our data indicate that both major and minor veins may unload PVA in the sink leaves of potato. The data suggest that VPg is translocated from inoculated source leaves to the sink leaves, where it accumulates in CC at an early stage of systemic infection. These findings suggest that VPg may be a 'phloem protein' that specifically acts in CC in the sink leaves to facilitate virus unloading.