Specificity of resistance to pea seed-borne mosaic potyvirus in transgenic peas expressing the viral replicase (Nlb) gene

Multiple integrations of a sense transgene, including a tandem inverted repeat confer stable RNA-silencing mediated virus resistance under different abiotic and biotic conditions

In a previous study, tobacco plants, transformed with a sense construct of the 57K domain of the replicase gene of tobacco rattle virus (TRV), provided resistance against genetically distant isolates of the virus. In this work, 57K-specific siRNAs were detected with RT-qPCR solely in the resistant line verifying the RNA-silencing base of the resistance. The integration sites of the transgene into the plant genome were identified with inverse-PCR. Moreover, the resistance against TRV was practically unaffected by low temperature conditions and the presence of heterologous viruses. The mechanism of the resistance was further examined by a gene expression analysis that showed increased transcript levels of genes with a key-role in the RNA silencing pathway and the basal antiviral defence. This work provides a comprehensive characterization of the robust virus resistance obtained by a sense transgene and underlines the usefulness of transgenic plants obtained by such a strategy.

Utilization of engineered resistance to viruses in crops of the developing world, with emphasis on sub-Saharan Africa

Viral diseases in crop plants constitute a major obstacle to food security in the developing world. Subsistence crops, including cassava, sweetpotato, potato, banana, papaya, common bean, rice and maize are often infected with RNA and/or DNA viruses that cannot be controlled with pesticides. Hence, healthy planting materials and virus-resistant cultivars are essential for high yields of good quality. However, resistance genes are not available for all viral diseases of crop plants. Therefore, virus resistance engineered in plants using modern biotechnology methods is an important addition to the crop production toolbox.

Transgenic resistance

Transgenic resistance to plant viruses is an important technology for control of plant virus infection, which has been demonstrated for many model systems, as well as for the most important plant viruses, in terms of the costs of crop losses to disease, and also for many other plant viruses infecting various fruits and vegetables. Different approaches have been used over the last 28 years to confer resistance, to ascertain whether particular genes or RNAs are more efficient at generating resistance, and to take advantage of advances in the biology of RNA interference to generate more efficient and environmentally safer, novel "resistance genes." The approaches used have been based on expression of various viral proteins (mostly capsid protein but also replicase proteins, movement proteins, and to a much lesser extent, other viral proteins), RNAs [sense RNAs (translatable or not), antisense RNAs, satellite RNAs, defective-interfering RNAs, hairpin RNAs, and artificial microRNAs], nonviral genes (nucleases, antiviral inhibitors, and plantibodies), and host-derived resistance genes (dominant resistance genes and recessive resistance genes), and various factors involved in host defense responses. This review examines the above range of approaches used, the viruses that were tested, and the host species that have been examined for resistance, in many cases describing differences in results that were obtained for various systems developed in the last 20 years. We hope this compilation of experiences will aid those who are seeking to use this technology to provide resistance in yet other crops, where nature has not provided such.

Virus-induced gene silencing in transgenic plants: transgene silencing and reactivation associate with two patterns of transgene body methylation

We used bisulfite sequencing to study the methylation of a viral transgene whose expression was silenced upon plum pox virus infection of the transgenic plant and its subsequent recovery as a consequence of so-called virus-induced gene silencing (VIGS). VIGS was associated with a general increase in the accumulation of small RNAs corresponding to the coding region of the viral transgene. After VIGS, the transgene promoter was not methylated and the coding region showed uneven methylation, with the 5' end being mostly unmethylated in the recovered tissue or mainly methylated at CG sites in regenerated silenced plants. The methylation increased towards the 3' end, which showed dense methylation in all three contexts (CG, CHG and CHH). This methylation pattern and the corresponding silenced status were maintained after plant regeneration from recovered silenced tissue and did not spread into the promoter region, but were not inherited in the sexual offspring. Instead, a new pattern of methylation was observed in the progeny plants consisting of disappearance of the CHH methylation, similar CHG methylation at the 3' end, and an overall increase in CG methylation in the 5' end. The latter epigenetic state was inherited over several generations and did not correlate with transgene silencing and hence virus resistance. These results suggest that the widespread CG methylation pattern found in body gene bodies located in euchromatic regions of plant genomes may reflect an older silencing event, and most likely these genes are no longer silenced.

Plant miRNAome and antiviral resistance: a retrospective view and prospective challenges

MicroRNAs (miRNAs) are small regulatory RNAs that play a defining role in post-transcriptional gene silencing of eukaryotes by either mRNA cleavage or translational inhibition. Plant miRNAs have been implicated in innumerable growth and developmental processes that extend beyond their ability to respond to biotic and abiotic stresses. Active in an organism's immune defence response, host miRNAs display a propensity to target viral genomes. During viral invasion, these virus-targeting miRNAs can be identified by their altered expression. All the while, pathogenic viruses, as a result of their long-term interaction with plants, have been evolving viral suppressors of RNA silencing (VSRs), as well as viral-encoded miRNAs as a counter-defence strategy. However, the gene silencing attribute of miRNAs has been ingeniously manipulated to down-regulate the expression of any gene of interest, including VSRs, in artificial miRNA (amiRNA)-based transgenics. Since we currently have a better understanding of the intricacies of miRNA-mediated gene regulation in plant-virus interactions, the majority of miRNAs manipulated to confer antiviral resistance to date are in plants. This review will share the insights gained from the studies of plant-virus combat and from the endeavour to manipulate miRNAs, including prospective challenges in the context of the evolutionary dynamics of the viral genome. Next generation sequencing technologies and bioinformatics analysis will further delineate the molecular details of host-virus interactions. The need for appropriate environmental risk assessment principles specific to amiRNA-based virus resistance is also discussed.

Transgenic strategies to confer resistance against viruses in rice plants

Rice (Oryza sativa L.) is cultivated in more than 100 countries and supports nearly half of the world's population. Developing efficient methods to control rice viruses is thus an urgent necessity because viruses cause serious losses in rice yield. Most rice viruses are transmitted by insect vectors, notably planthoppers and leafhoppers. Viruliferous insect vectors can disperse their viruses over relatively long distances, and eradication of the viruses is very difficult once they become widespread. Exploitation of natural genetic sources of resistance is one of the most effective approaches to protect crops from virus infection; however, only a few naturally occurring rice genes confer resistance against rice viruses. Many investigators are using genetic engineering of rice plants as a potential strategy to control viral diseases. Using viral genes to confer pathogen-derived resistance against crops is a well-established procedure, and the expression of various viral gene products has proved to be effective in preventing or reducing infection by various plant viruses since the 1990s. RNA interference (RNAi), also known as RNA silencing, is one of the most efficient methods to confer resistance against plant viruses on their respective crops. In this article, we review the recent progress, mainly conducted by our research group, in transgenic strategies to confer resistance against tenuiviruses and reoviruses in rice plants. Our findings also illustrate that not all RNAi constructs against viral RNAs are equally effective in preventing virus infection and that it is important to identify the viral "Achilles' heel" gene to target for RNAi attack when engineering plants.

Sharka: the past, the present and the future

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

Delay in virus accumulation and low virus transmission from transgenic rice plants expressing Rice tungro spherical virus RNA

Rice tungro, a devastating viral disease of rice in South and Southeast Asia, is caused by the joint infection of a DNA virus, Rice tungro bacilliform virus (RTBV) and an RNA virus Rice tungro spherical virus (RTSV). RTBV and RTSV are transmitted exclusively by the insect vector Green leafhopper (GLH). RTSV is necessary for the transmission of RTBV. To obtain transgenic resistance against RTSV, indica rice plants were transformed using DNA constructs designed to express an untranslatable sense or anti-sense RTSV RNA. Progeny of primary transformants showing low copies of the integrated transgenes and accumulating the corresponding transcripts at low levels were challenged with viruliferous GLH. Three out of four transgenic plant lines expressing untranslatable RTSV RNA in the sense orientation and two out of the four lines expressing an RTSV gene in the anti-sense orientation showed delayed buildup of RTSV RNA over time. Transmission of RTBV from the above lines was reduced significantly.

The cytosolic protein response as a subcomponent of the wider heat shock response in Arabidopsis

In common with a range of environmental and biological stresses, heat shock results in the accumulation of misfolded proteins and a collection of downstream consequences for cellular homeostasis and growth. Within this complex array of responses, the sensing of and responses to misfolded proteins in specific subcellular compartments involves specific chaperones, transcriptional regulators, and expression profiles. Using biological (ectopic protein expression and virus infection) and chemical triggers for misfolded protein accumulation, we have profiled the transcriptional features of the response to misfolded protein accumulation in the cytosol (i.e., the cytoplasmic protein response [CPR]) and identified the effects as a subcomponent of the wider effects induced by heat shock. The CPR in Arabidopsis thaliana is associated with the heat shock promoter element and the involvement of specific heat shock factors (HSFs), notably HSFA2, which appears to be regulated by alternative splicing and non-sense-mediated decay. Characterization of Arabidopsis HSFA2 knockout and overexpression lines showed that HSFA2 is one of the regulatory components of the CPR.

Resistance of transgenic tobacco plants incorporating the putative 57-kDa polymerase read-through gene of Tobacco rattle virus against rub-inoculated and nematode-transmitted virus

Nicotiana tabacum plants were transformed with the 57-kDa read-through domain of the replicase gene of Tobacco rattle virus (TRV) isolate SYM. From a total of six lines containing the viral transgene, four displayed various levels of resistance to TRV infection. Transgenic plants from line 81G were highly resistant to foliar rub-inoculation with the homologous isolate, or with isolates TRV-PpK20 and TRV-PLB, which are almost identical to TRV-SYM in RNA1 sequence. Moreover, 81G plants were moderately resistant to the serologically and genetically distinct, highly pathogenic isolate TRV-GR. Resistance characteristics of line 81G remained stable over six generations. No unambiguous correlation was established between number of transgene insertion loci and level of resistance. Transgene-specific mRNA was clearly detected in plants from susceptible lines but only at an early developmental stage in resistant plants, indicating the operation of a RNA silencing resistance mechanism. Following challenge using viruliferous vector nematodes carrying TRV-PpK20 or by rub inoculation of roots, 81G plants did not show any symptoms and virus was not detected in leaves. However, virus was detected in roots but without apparent effects on plant growth and often at low concentration. When challenged with nematodes carrying TRV-GR, symptoms in aerial parts of 81G plants were less severe and much delayed compared to non-transgenic plants, although younger plants showed less resistance than older ones. No difference was detected in transgene transcript accumulation between leaves and roots of 81G plants. This is the first work reporting a broad level of pathogen derived resistance against two geographically and genetically distinct TRV isolates transmitted directly by their nematode vectors and provides further insight into the expression of transgenic resistance against naturally transmitted soil-borne viruses.

Heat shock 70 protein interaction with Turnip mosaic virus RNA-dependent RNA polymerase within virus-induced membrane vesicles

Tandem affinity purification was used in Arabidopsis thaliana to identify cellular interactors of Turnip mosaic virus (TuMV) RNA-dependent RNA polymerase (RdRp). The heat shock cognate 70-3 (Hsc70-3) and poly(A)-binding (PABP) host proteins were recovered and shown to interact with the RdRp in vitro. As previously shown for PABP, Hsc70-3 was redistributed to nuclear and membranous fractions in infected plants and both RdRp interactors were co-immunoprecipitated from a membrane-enriched extract using RdRp-specific antibodies. Fluorescently tagged RdRp and Hsc70-3 localized to the cytoplasm and the nucleus when expressed alone or in combination in Nicotiana benthamiana. However, they were redistributed to large perinuclear ER-derived vesicles when co-expressed with the membrane binding 6K-VPg-Pro protein of TuMV. The association of Hsc70-3 with the RdRp could possibly take place in membrane-derived replication complexes. Thus, Hsc70-3 and PABP2 are potentially integral components of the replicase complex and could have important roles to play in the regulation of potyviral RdRp functions.

Tobacco rattle virus 16-kilodalton protein encodes a suppressor of RNA silencing that allows transient viral entry in meristems

RNA silencing is a host defense mechanism that limits the accumulation and spread of viruses in infected plants. Correspondingly, plant viruses encode suppressors of silencing. In the positive-strand RNA virus Tobacco rattle virus (TRV), the suppressor of silencing is a 16-kDa (16K) protein encoded by RNA1. The suppressor action of the 16K protein is transient and weaker than that of the P19 suppressor, encoded by tomato bushy stunt virus. Mutant TRV that does not produce its suppressor, unlike other suppressor-defective viruses, is competent to accumulate and spread systemically in the infected plant. However, this mutant virus does not exhibit the transient invasion of the meristem that is characteristic of the wild-type virus. Based on this analysis, we propose that the 16K suppressor of silencing allows TRV to transiently invade the meristem. Our data are consistent with a mechanism of long-term meristem virus exclusion that is dependent on a transient invasion of the meristem early in the infection cycle. This novel mechanism of meristem exclusion may be associated with the phenomenon of recovery in virus-infected plants in which upper leaves have little or no virus and are immune to secondary infection by the same virus.

Transgenic resistance to Citrus tristeza virus in grapefruit

Grapefruit (Citrus paradisi) transgenic plants transformed with a variety of constructs derived from the Citrus tristeza virus (CTV) genome were tested for their resistance to the virus. Most transgenic lines were susceptible (27 lines), a few were partially resistant (6 lines) and only one line, transformed with the 3' end of CTV was resistant. Transgene expression levels and siRNA accumulation were determined to identify whether the resistance observed was RNA-mediated. The responses were varied. At least one resistant plant from a partially resistant line showed no steady-state transgene mRNA, siRNA accumulation and no viral RNA, implicating posttranscriptional gene silencing (PTGS) as the mechanism of resistance. The most resistant line showed no transgene mRNA accumulation and promoter methylation of cytosines in all contexts, the hallmark of RNA-directed DNA methylation and transcriptional gene silencing (TGS). The variety of responses, even among clonally propagated plants, is unexplained but is not unique to citrus. The genetics of CTV, host response or other factors may be responsible for this variability.

RNA interference-based gene silencing as an efficient tool for functional genomics in hexaploid bread wheat

Insertional mutagenesis and gene silencing are efficient tools for the determination of gene function. In contrast to gain- or loss-of-function approaches, RNA interference (RNAi)-induced gene silencing can possibly silence multigene families and homoeologous genes in polyploids. This is of great importance for functional studies in hexaploid wheat (Triticum aestivum), where most of the genes are present in at least three homoeologous copies and conventional insertional mutagenesis is not effective. We have introduced into bread wheat double-stranded RNA-expressing constructs containing fragments of genes encoding Phytoene Desaturase (PDS) or the signal transducer of ethylene, Ethylene Insensitive 2 (EIN2). Transformed plants showed phenotypic changes that were stably inherited over at least two generations. These changes were very similar to mutant phenotypes of the two genes in diploid model plants. Quantitative real-time polymerase chain reaction revealed a good correlation between decreasing mRNA levels and increasingly severe phenotypes. RNAi silencing had the same quantitative effect on all three homoeologous genes. The most severe phenotypes were observed in homozygous plants that showed the strongest mRNA reduction and, interestingly, produced around 2-fold the amount of small RNAs compared to heterozygous plants. This suggests that the effect of RNAi in hexaploid wheat is gene-dosage dependent. Wheat seedlings with low mRNA levels for EIN2 were ethylene insensitive. Thus, EIN2 is a positive regulator of the ethylene-signaling pathway in wheat, very similar to its homologs in Arabidopsis (Arabidopsis thaliana) and rice (Oryza sativa). Our data show that RNAi results in stably inherited phenotypes and therefore represents an efficient tool for functional genomic studies in polyploid wheat.

Unravelling the genetic diversity of the three main viruses involved in Sweet Potato Virus Disease (SPVD), and its practical implications

SUMMARY Sweetpotato (Ipomoea batatas) is a widely grown food crop, in which the most important diseases are caused by viruses. Genetic variability of three widely distributed sweetpotato viruses was analysed using data from 46 isolates of Sweet potato feathery mottle virus (SPFMV), 16 isolates of Sweet potato mild mottle virus (SPMMV) and 25 isolates of Sweet potato chlorotic stunt virus (SPCSV), of which 19, seven and six isolates, respectively, are newly characterized. Division of SPFMV into four genetic groups (strains) according to phylogenetic analysis of coat protein (CP) encoding sequences revealed that strain EA contained the East African isolates of SPFMV but none from elsewhere. In contrast, strain RC contained ten isolates from Australia, Africa, Asia and North America. Strain O contained six heterogeneous isolates from Africa, Asia and South America. The seven strain C isolates from Australia, Africa, Asia, and North and South America formed a group that was genetically distant from the other SPFMV strains. SPMMV isolates showed a high level of variability with no discrete strain groupings. SPCSV isolates from East Africa were phylogenetically distant to SPCSV isolates from elsewhere. Only from East Africa were adequate data available for different isolates of the three viruses to estimate the genetic variability of their local populations. The implications of the current sequence information and the need for more such information from most sweetpotato-growing regions of the world are discussed in relation to virus diagnostics and breeding for virus resistance.

Construction and properties of a gene-silencing vector based on Poplar mosaic virus (genus Carlavirus)

A gene-silencing vector based on a full-length genomic clone of Poplar mosaic virus (PopMV) was constructed, with coat protein and movement protein genes removed, and containing instead, the coding sequence for green fluorescent protein (GFP). This paper demonstrates that the PopMV-derived gene-silencing vector was able to silence GFP expression in GFP transgenic Nicotiana benthamiana plants. The full-length genome of an Oxford isolate of PopMV (PV275) was cloned and sequenced. A full-length PopMV clone, under transcriptional control of the 35SCaMV promoter was then constructed, and the clone was able to replicate locally in Nicotiana species. Several autonomous plant RNA and DNA viruses have been converted into vectors and implemented for virus-induced gene-silencing (VIGS) of transgenes and endogenous genes [Burton, R., Gibeaut, D., Bacic, A., Findlay, K., Roberts, K., Hamilton, A., Baulcombe, D., Fincher, G., 2000. Virus-induced silencing of a plant cellulose synthase gene. Plant Cell 12, 691-706; Dalmay, T., Horsefield, R., Braunstein, T.H., Baulcombe, D.C., 2001. SDE3 encodes an RNA helicase required for post-transcriptional gene silencing in Arabidopsis. EMBO J. 20, 2069-2077; Gossele, V., Fache, I., Meulewaeter, F., Cornelissen, M., Metzlaff, M., 2002. SVISS--a novel transient gene silencing system for gene function discovery and validation in tobacco plants. Plant J. 32, 859-866; Holzberg, S., Brosio, P., Gross, C., Pogue, G.P., 2002. Barley stripe mosaic virus-induced gene silencing in a monocot plant. Plant J. 30, 315-327; Ratcliff, F., Martin-Hernandez, A., Baulcombe, D., 2000. Tobacco rattle virus as a vector for analysis of gene function by silencing. Plant J. 25, 237-245; Ruiz, M., Vionnet, O., Baulcombe, D., 1998. Initiation and maintenance of virus-induced gene silencing. Plant Cell 10, 937-946]. The use of a virus that naturally infects trees as a gene-silencing vector has not been demonstrated before. The ability to systemically silence a plant transgene following the production of a gene-silencing signal from a locally replicating viral-construct derived from a carlavirus has not to our knowledge been shown before.

Peanut stripe potyvirus resistance in peanut (Arachis hypogaea L.) plants carrying viral coat protein gene sequences

Peanut (Arachis hypogaea L.) lines exhibiting high levels of resistance to peanut stripe virus (PStV) were obtained following microprojectile bombardment of embryogenic callus derived from mature seeds. Fertile plants of the commercial cultivars Gajah and NC7 were regenerated following co-bombardment with the hygromycin resistance gene and one of two forms of the PStV coat protein (CP) gene, an untranslatable, full length sequence (CP2) or a translatable gene encoding a CP with an N-terminal truncation (CP4). High level resistance to PStV was observed for both transgenes when plants were challenged with the homologous virus isolate. The mechanism of resistance appears to be RNA-mediated, since plants carrying either the untranslatable CP2 or CP4 had no detectable protein expression, but were resistant or immune (no virus replication). Furthermore, highly resistant, but not susceptible CP2 T0 plants contained transgene-specific small RNAs. These plants now provide important germplasm for peanut breeding, particularly in countries where PStV is endemic and poses a major constraint to peanut production.

RNA Silencing of the Introduced Coat Protein Gene of Turnip mosaic virus Confers Broad-Spectrum Resistance in Transgenic Arabidopsis

ABSTRACT The coat protein (CP) gene derived from Turnip mosaic virus (TuMV) isolate JO was introduced into Arabidopsis thaliana and the resulting transgenic progenies were analyzed for resistance to TuMV. Transgenic Arabidopsis plants with no detectable transcripts of the introduced CP gene exhibited complete resistance to TuMV. There was no significant correlation between the resistance and the copy number of the transgene. Instead, small interfering RNAs (siRNAs) were detected in these resistant plants, indicating that the resistance is attributed to RNA silencing. The RNA-mediated resistance was not only inherited over successive generations but also effective against 17 worldwide TuMV isolates with different pathogenicity. Comparative analysis of the CP genes among the 17 TuMV isolates revealed that the 380-nt in the 3' region is highly conserved, suggesting the importance of the 3' conserved region for broad-spectrum resistance. These results indicate that introduction of the TuMV-CP gene into the target Brassicaceae plants followed by selecting transformants that show RNA silencing for the transgenes can be an effective and reliable strategy for developing crucifer crops with a broad spectrum of resistance to TuMV.

Pathogen-derived resistance in potato to Potato virus Y—aspects of stability and biosafety under field conditions

Plants of three different potato cultivars/lines were transformed via Agrobacterium tumefaciens with a truncated NIb gene of a necrotic strain of Potato virus Y (PVY(N)) which had been C-terminally fused to enhanced blue-fluorescing protein. Resistance of the resulting transgenic clones was evaluated under glass house conditions using an NTN-strain of PVY. Four clones with the highest levels of resistance were chosen for further experiments. Their type of resistance was either recovery or extreme resistance. These clones and their resistance types were also characterised at the molecular level. Mechanisms other than post-transcriptional gene silencing seemed to be involved in the resistance which was not dependent on sequence homology between transgene and challenging virus. Stability of resistance was tested under field conditions. The plants usually became infected with PVY. Tubers of the clone with extreme resistance did not recover from infection whereas those from clones with the recovery type did. No influence of transgenic potatoes was apparent on aphid population numbers in test plots. Recombination events could not be detected at the RNA level between transgene and challenging virus.

Sources of target specificity associated with the recovery against Pea seed-borne mosaic virus infection mediated by RNA silencing in pea

SUMMARY Transgenic peas containing a copy of the Pea seed-borne mosaic virus (PSbMV) isolate DPD1 NIb sequence develop a 'recovery' phenotype when challenged with either the homologous (DPD1) or a heterologous (NY) PSbMV isolate. However, the specificity of the subsequent resistance differs with respect to the initiation by and targeting of different virus isolates. Analysis of tissue in which recovery had been induced by either of the isolates, revealed the presence of low molecular RNA molecules (siRNA) derived from degradation of the NIb transgene mRNA. When non-transgenic scions were grafted on to transgenic stocks in which recovery had been induced, all the scions became infected, indicating that the virus can be exported from recovered tissue. Experiments in which recovered scions were grafted on to non-transgenic stocks revealed that the recovery phenotype could be maintained in the apparent absence of a source of virus. However in a number of cases, side-shoots which developed on the non-transgenic stock became infected. These results indicate that recovered tissue contains extremely low levels of infectious virus with the potential, directly or indirectly, to confer the observed resistance specificity. Indirectly, the viral genome could act as a source of specific siRNA molecules, which are present in infected tissues but are below the level of detection in recovered tissues. These could act in conjunction with siRNAs derived from the transgene mRNA to maintain a level of PTGS (post-transcriptional gene silencing) activity which is effective in preventing further accumulation of homologous or related viruses. We suggest a model to explain the differential specificity.

Broad-Spectrum Resistance to Different Geographic Strains of Papaya ringspot virus in Coat Protein Gene Transgenic Papaya

ABSTRACT Papaya ringspot virus (PRSV) is a major limiting factor for cultivation of papaya (Carica papaya) in tropical and subtropical areas throughout the world. Although the coat protein (CP) gene of PRSV has been transferred into papaya by particle bombardment and transgenic lines with high resistance to Hawaii strains have been obtained, they are susceptible to PRSV isolates outside of Hawaii. This strain-specific resistance limits the application of the transgenic lines in other areas of the world. In this investigation, the CP gene of a local strain isolated from Taiwan, designated PRSV YK, was transferred into papaya via Agrobacterium-mediated transformation. A total of 45 putative transgenic lines were obtained and the presence of the transgene in papaya was confirmed by polymerase chain reaction amplification. When the plants of transgenic lines were challenged with PRSV YK by mechanical inoculation, they showed different levels of resistance ranging from delay of symptom development to complete immunity. Molecular analysis of nine selected lines that exhibited different levels of resistance revealed that the expression level of the transgene is negatively correlated with the degree of resistance, suggesting that the resistance is manifested by a RNA-mediated mechanism. The segregation analysis showed that the transgene in the immune line 18-0-9 has an inheritance of two dominant loci and the other four highly resistant lines have a single dominant locus. Seven selected lines were tested further for resistance to three PRSV heterologous strains that originated in Hawaii, Thailand, and Mexico. Six of the seven lines showed varying degrees of resistance to the heterologous strains, and one line, 19-0-1, was immune not only to the homologous YK strain but also to the three heterologous strains. Thus, these CP-transgenic papaya lines with broad-spectrum resistance have great potential for use in Taiwan and other geographic areas to control PRSV.

Silencing of a viral RNA silencing suppressor in transgenic plants

High expression levels of the helper component proteinase (HC(pro)), a known virus suppressor of RNA silencing, were attained in Nicotiana benthamiana transformed with the HC(pro) cistron of Potato virus A (PVA, genus Potyvirus). No spontaneous silencing of the HC(pro) transgene was observed, in contrast to the PVA coat protein (CP)-encoding transgene in other transgenic lines. HC(pro)-transgenic plants were initially susceptible to PVA and were systemically infected by 14 days post-inoculation (p.i.) but, 1 to 2 weeks later, the new expanding leaves at positions +6 and +7 above the inoculated leaf showed a peculiar recovery phenotype. Leaf tips (the oldest part of the leaf) were chlorotic and contained high titres of PVA, whereas the rest of the leaf was symptomless and contained greatly reduced or non-detectable levels of viral RNA, CP and transgene mRNA. The spatial recovery phenotype suggests that RNA silencing is initiated in close proximity to meristematic tissues. Leaves at position +8 and higher were symptomless and virus-free but not completely resistant to mechanical inoculation with PVA. However, they were not infected with the virus systemically transported from the lower infected leaves, suggesting a vascular tissue-based resistance mechanism. Recovery of the HC(pro)-transgenic plants from infection with different PVA isolates was dependent on the level of sequence homology with the transgene. Methylation of the HC(pro) transgene followed recovery. These data show that the transgene mRNA for a silencing suppressor can be silenced by a presumably 'strong' silencing inducer (replicating homologous virus).

Barley stripe mosaic virus-induced gene silencing in a monocot plant

RNA silencing of endogenous plant genes can be achieved by virus-mediated, transient expression of homologous gene fragments. This powerful, reverse genetic approach, known as virus-induced gene silencing (VIGS), has been demonstrated only in dicot plant species, where it has become an important tool for functional genomics. Barley stripe mosaic virus (BSMV) is a tripartite, positive-sense RNA virus that infects many agriculturally important monocot species including barley, oats, wheat and maize. To demonstrate VIGS in a monocot host, we modified BSMV to express untranslatable foreign inserts downstream of the gammab gene, in either sense or antisense orientations. Phytoene desaturase (PDS) is required for synthesizing carotenoids, compounds that protect chlorophyll from photo-bleaching. A partial PDS cDNA amplified from barley was 90, 88 and 74% identical to PDS cDNAs from rice, maize and Nicotiana benthamiana, respectively. Barley infected with BSMV expressing barley, rice or maize PDS fragments became photo-bleached and accumulated phytoene (the substrate for PDS) in a manner similar to plants treated with the chemical inhibitor of PDS, norflurazon. In contrast, barley infected with wild-type BSMV, or BSMV expressing either N. benthamiana PDS or antisense green fluorescent protein (GFP), did not photo-bleach or accumulate phytoene. Thus BSMV silencing of the endogenous PDS was homology-dependent. Deletion of the coat protein enhanced the ability of BSMV to silence PDS. This is the first demonstration of VIGS in a monocot, and suggests that BSMV can be used for functional genomics and studies of RNA-silencing mechanisms in monocot plant species.

Resistance to wheat streak mosaic virus in transgenic wheat engineered with the viral coat protein gene

Wheat (Triticum aestivum) plants were stably transformed with the coat protein (CP) gene of wheat streak mosaic virus (WSMV) by the biolistic method. Eleven independently transformed plant lines were obtained and five were analyzed for gene expression and resistance to WSMV. One line showed high resistance to inoculations of two WSMV strains. This line had milder symptoms and lower virus titer than control plants after inoculation. After infection, new growth did not show symptoms. The observed resistance was similar to the 'recovery' type resistance described previously using WSMV NIb transgene and in other systems. This line looked morphologically normal but had an unusually high transgene copy number (approximately 90 copies per 2C homozygous genome). Northern hybridization analysis indicated a high level of degraded CP mRNA expression. However, no coat protein expression was detected.

Sindbis virus-induced silencing of dengue viruses in mosquitoes

Aedes aegypti were injected intrathoracically with double subgenomic Sindbis (dsSIN) viruses with inserted sequences derived from the genome of one or more of the four dengue (DEN) virus serotypes. Mosquitoes were highly resistant to challenge with homologous DEN viruses from which the effector sequences were derived, and resistance to DEN viruses was independent of the orientation of the effector RNA. dsSIN viruses designed to express RNA derived from the premembrane coding region of DEN-2 prevented the accumulation of DEN2 RNA, and C6/36 cells were highly resistant to DEN-2 virus when challenged at 2, 5 or 8 days after the initial dsSIN virus infections, even though the dsSIN-derived RNA had sharply declined at the later time points. Initiation of resistance occurred prior to or within the first 8 h after challenge with DEN-2 virus. We conclude that DEN viruses are inhibited by a mechanism similar to post-transcriptional gene silencing (PTGS) or RNA interference (RNAi) phenomena described in plants and invertebrates, respectively. The potential occurrence of PTGS or RNAi in mosquitoes and mosquito cells suggests new ways of inhibiting the replication of arthropod-borne viruses in mosquito vectors, studying vector-virus interactions, and silencing endogenous mosquito genes.

Technical Advance. Tobacco rattle virus as a vector for analysis of gene function by silencing

Virus vectors carrying host-derived sequence inserts induce silencing of the corresponding genes in infected plants. This virus-induced gene silencing (VIGS) is a manifestation of an RNA-mediated defence mechanism that is related to post-transcriptional gene silencing (PTGS) in transgenic plants. Here we describe an infectious cDNA clone of tobacco rattle virus (TRV) that has been modified to facilitate insertion of non-viral sequence and subsequent infection to plants. We show that this vector mediates VIGS of endogenous genes in the absence of virus-induced symptoms. Unlike other RNA virus vectors that have been used previously for VIGS, the TRV construct is able to target host RNAs in the growing points of plants. These features indicate that the TRV vector will have wide application for gene discovery in plants.

Temporal and spatial activity of a promoter from a pea enzyme inhibitor gene and its exploitation for seed quality improvement

The promoter from one of the two seed-expressed genes encoding trypsin/chymotrypsin inhibitors (TI) has been isolated and characterised in transgenic pea lines, following its re-introduction by Agrobacterium-mediated transformation, as a TI promoter-beta-glucuronidase (GUS) gene fusion. The promoter from this gene (TI1) directed expression of GUS enzyme at late stages of embryogenesis, comparable to those determined for activity of the homologous native TI genes. GUS expression was detected in roots of plants subjected to drought stress conditions, indicating that the TI1 gene, normally seed-specific in its expression, can be induced under these conditions. A second gene construct utilised the TI1 gene promoter to direct expression of an antisense TI gene. Seed TI activities in some lines transformed with this construct were reduced significantly. A limitation of the pea transformation methodology for antisense manipulations, in particular, is the observed frequency of non-transmission of transgenes from primary transformants (up to 80%).

Phylogenetic analysis of two potyvirus pathogens of commercial cowpea lines: implications for obtaining pathogen-derived resistance

As a prelude to developing engineered resistance to two important potyvirus pathogens of cowpea, a phylogenetic analysis of strains of Cowpea aphid-borne mosaic virus (CAbMV) and Bean common mosaic virus--blackeye cowpea strain (BCMV-B1C) was undertaken. Nucleotide sequences for the coat protein genes and 3'-untranslated regions of four CAbMV and one BCMV-B1C strains were determined and included in an analysis with published sequences. While all the newly sequenced viruses showed strong homology with the existing respective sequences in the database, the CAbMV group showed a divergence into two subgroups. These groups differed from each other by more than some CAbMV strains differed from the South African Passiflora virus (CAbMV-SAP), which has distinct biological characteristics. The implications of the sequence analyses are discussed with respect to a strategy for the generation of engineered resistance to both groups of viruses.

Transgenic resistance to PVY(O) associated with post-transcriptional silencing of P1 transgene is overcome by PVY(N) strains that carry highly homologous P1 sequences and recover transgene expression at infection

Resistance to Potato virus Y (PVY) has been obtained in our previous studies through expression of the PVY P1 gene in sense or antisense orientation in potato cv. Pito. In the present study, the mechanism and strain specificity of the resistance were analyzed. Several features including low steady-state P1 mRNA expression in the resistant P1 plants indicated that resistance was based on post-transcriptional gene silencing (PTGS). Resistance was specific to PVY(O) isolates, the PVY strain group from which the P1 transgene was derived. However, according to group analyses, there was no distinguishing characteristic between the PVY(O) and PVY(N) strains P1 gene sequences. Therefore, the ability of the PVY(N) strains to overcome resistance could not be explained solely based on their P1 gene sequences. Infection with PVY(N) of the PVY(O)-resistant transgenic lines led to a recovery of expression of the P1 transgene. These data suggested that factors other than sequence homology are required in determination of the resistance specificity.

Gene silencing: RNA makes RNA makes no protein

A mutation that disrupts post-transcriptional gene silencing in Neurospora crassa has been found to affect the homologue of a plant-encoded RNA-dependent RNA polymerase. This enzyme may produce a specificity determinant of gene silencing and mediate an epigenetic conversion at the RNA level.

De novo methylation and co-suppression induced by a cytoplasmically replicating plant RNA virus

The relationship between co-suppression and DNA methylation was explored in transgenic plants showing inducible co-suppression following infection with a cytoplasmically replicating RNA virus. Induction resulted in a loss of transgene mRNA and resistance to further infection, factors typical of post-transcriptional gene silencing. In infected plants, de novo methylation of the transgene appeared to precede the onset of resistance and only occurred in plants where the outcome was co-suppression. The methylation was limited to sequences homologous to the viral RNA and occurred at both symmetric and non-symmetric sites on the DNA. Although methylation is predicted to occur in mitotic cells, the virus was found not to access the meristem. A diffusible sequence-specific signal may account for the epigenetic changes in those tissues.