Replicase-mediated resistance to plant virus disease

Engineered plant virus resistance

Virus diseases are among the key limiting factors that cause significant yield loss and continuously threaten crop production. Resistant cultivars coupled with pesticide application are commonly used to circumvent these threats. One of the limitations of the reliance on resistant cultivars is the inevitable breakdown of resistance due to the multitude of variable virus populations. Similarly, chemical applications to control virus transmitting insect vectors are costly to the farmers, cause adverse health and environmental consequences, and often result in the emergence of resistant vector strains. Thus, exploiting strategies that provide durable and broad-spectrum resistance over diverse environments are of paramount importance. The development of plant gene transfer systems has allowed for the introgression of alien genes into plant genomes for novel disease control strategies, thus providing a mechanism for broadening the genetic resources available to plant breeders. Genetic engineering offers various options for introducing transgenic virus resistance into crop plants to provide a wide range of resistance to viral pathogens. This review examines the current strategies of developing virus resistant transgenic plants.

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.

[Plant-infecting reoviruses]

The family Reoviridae separates two subfamilies and consists of 15 genera. Fourteen viruses in three genera (Phytoreovirus, Oryzavirus, and Fijivirus) infect plants. The outbreaks of the plant-infecting reoviruses cause sometime the serious yield loss of rice and maize, and are a menace to safe and efficient food production in the Southeast Asia.　The plant-infecting reoviruses are double-shelled icosahedral particles, from 50 to 80nm in diameter, and include from 10 to 12 segmented double-stranded genomic RNAs depending on the viruses. These viruses are transmitted in a persistent manner by the vector insects and replicated in both plants and in their vectors. This review provides a brief overview of the plant-infecting reoviruses and their recent research progresses including the strategy for viral controls using transgenic rice plants.

Biological and Molecular Characterization of Two Distinct Tomato Strains of Cucumber mosaic virus Based on Complete RNA-3 Genome and Subgroup Specific Diagnosis

Cucumber mosaic virus (CMV) is one of the most important viral pathogen infecting several plant species in India. Five isolates of CMV obtained from cucumber, muskmelon, tobacco and tomato from distinct geographical locations in India were analysed based on host-reactions and genome sequence. The majority of the isolates were very similar and only two isolates, Tfr-In and Tss-In showed distinct symptoms in tomato and high sequence diversity (77.8%) in coat protein (CP) gene. Tfr-In was isolated from tomato fruit showing grey patches in Aurangabad and Tss-In from tomato plant showing shoe-string symptoms in New Delhi. The RNA-3 genomes of Tfr-In (2,214 nt; JF279606), shared only 70.3% nucleotide sequence identity with Tss-In (2,178 nt; JF279605. The complete RNA-3 genome of Tss-In and Tfr-In were compared with that of 65 CMV isolates reported from various plants of the world, which formed four distinct subclades-IA, -IB, -IC and -II. The Tfr-In isolate clustered with the CMV subgroup-IB and Tss-In with the subgroup-II. The comparison of the RNA-3 sequence of both the isolates revealed maximum heterogeneity in the intergenic region (IR). Reverse transcriptase polymerase chain reaction (RT-PCR) based detection of CMV subgroup-I and -II was developed designing primers from flanking IR region. The specificity of the RT-PCR detection was confirmed using Tfr-In and Tss-In representing subgroup-I and -II and validated with field samples of tomato, cucurbits and chilli. This is the first report of complete RNA-3 of subgroup-IB CMV causing grey patches in tomato fruit and subgroup-II CMV causing shoe-string symptoms in tomato in India. The present and previous studies together showed that tomato in India was affected by multiple strains of CMV.

Targeting specific genes for RNA interference is crucial to the development of strong resistance to rice stripe virus

Rice stripe virus (RSV) has a serious negative effect on rice production in temperate regions of East Asia. Focusing on the putative importance of the selection of target sequences for RNA interference (RNAi), we analysed the effects of potential target sequences in each of the coding genes in the RSV genome, using transgenic rice plants that expressed a set of inverted-repeat (IR) constructs. The reactions of inoculated transgenic T(1) plants to RSV were divided subjectively into three classes, namely highly resistant, moderately resistant and lacking enhanced resistance to RSV, even though plants that harboured any constructs accumulated transgene-specific siRNAs prior to inoculation with RSV. Transgenic plants that harboured IR constructs specific for the gene for pC3, which encodes nucleocapsid protein, and for pC4, which encodes a viral movement protein, were immune to infection by RSV and were more resistant to infection than the natural resistant cultivars that have been used to control the disease in the field. By contrast, the IR construct specific for the gene for pC2, which encodes a glycoprotein of unknown function, and for p4, which encodes a major non-structural protein of unknown function, did not result in resistance. Our results indicate that not all RNAi constructs against viral RNAs are equally effective in preventing RSV infection and that it is important to identify the viral 'Achilles heel' for RNAi attack in the engineering of plants.

Virus-derived transgenes expressing hairpin RNA give immunity to Tobacco mosaic virus and Cucumber mosaic virus

BACKGROUND

An effective method for obtaining resistant transgenic plants is to induce RNA silencing by expressing virus-derived dsRNA in plants and this method has been successfully implemented for the generation of different plant lines resistant to many plant viruses.

RESULTS

Inverted repeats of the partial Tobacco mosaic virus (TMV) movement protein (MP) gene and the partial Cucumber mosaic virus (CMV) replication protein (Rep) gene were introduced into the plant expression vector and the recombinant plasmids were transformed into Agrobacterium tumefaciens. Agrobacterium-mediated transformation was carried out and three transgenic tobacco lines (MP16-17-3, MP16-17-29 and MP16-17-58) immune to TMV infection and three transgenic tobacco lines (Rep15-1-1, Rep15-1-7 and Rep15-1-32) immune to CMV infection were obtained. Virus inoculation assays showed that the resistance of these transgenic plants could inherit and keep stable in T₄ progeny. The low temperature (15 °C did not influence the resistance of transgenic plants. There was no significant correlation between the resistance and the copy number of the transgene. CMV infection could not break the resistance to TMV in the transgenic tobacco plants expressing TMV hairpin MP RNA.

CONCLUSIONS

We have demonstrated that transgenic tobacco plants expressed partial TMV movement gene and partial CMV replicase gene in the form of an intermolecular intron-hairpin RNA exhibited complete resistance to TMV or CMV infection.

Characterization, genetic diversity, and evolutionary link of Cucumber mosaic virus strain New Delhi from India

The genome of Cucumber mosaic virus New Delhi strain (CMV-ND) from India, obtained from tomato, was completely sequenced and compared with full genome sequences of 14 known CMV strains from subgroups I and II, for their genetic diversity. Sequence analysis suggests CMV-ND shares maximum sequence identity at the nucleotide level with a CMV strain from Taiwan. Among all 15 strains of CMV, the encoded protein 2b is least conserved, whereas the coat protein (CP) is most conserved. Sequence identity values and phylogram results indicate that CMV-ND belongs to subgroup I. Based on the recombination detection program result, it appears that CMV is prone to recombination, and different RNA components of CMV-ND have evolved differently. Recombinational analysis of all 15 CMV strains detected maximum recombination breakpoints in RNA2; CP showed the least recombination sites.

Molecular characterization of Cucumber mosaic virus infecting Gladiolus, revealing its phylogeny distinct from the Indian isolate and alike the Fny strain of CMV

The majority of Gladiolus plants growing in the botanical garden at NBRI, Lucknow, India and adjoining areas exhibited symptoms of mosaic, color breaking, stunting of spikes and reduction in flower size. The occurrence of Cucumber mosaic virus (CMV) was suspected in symptomatic Gladiolus plants. Cucumber mosaic virus, the type species of the genus Cucumovirus of the family Bromoviridae, is an important plant virus worldwide, which infects many plants and causes quantity and quality losses. For virus characterization, total RNA was isolated from leaves of infected plants and used in reverse transcriptase polymerase chain reaction with a primer set designed in the Cucumber mosaic virus coat protein region. Viral amplicons of the expected 657 bp size were obtained from infected plants. No viral amplicon was obtained from healthy control plants. Viral amplicons were cloned and sequenced (DQ295914). Molecular characterization was performed and phylogenetic relationship determined by the comparison of coat protein gene nucleotide and amino acid sequences with other Cucumber mosaic virus isolates reported from India and worldwide. The nucleotide and amino acid percentage comparison and phylogenetic tree results revealed that Cucumber mosaic virus infecting Gladiolus show resemblance with the Fny strain, which is not common in the Asian continent.

Monoclonal antibodies specific to Cucumber mosaic virus coat protein possess DNA-hydrolyzing activity

Monoclonal antibodies (mAbs) specific to Cucumber mosaic virus coat protein (CMV-CP) were designed from cDNA and deduced amino acid sequences of the light chain genes of 10 out of 14 different hybridoma cell lines. Ten of these mAbs revealed a very restricted germline family VkappaII, within which gene bd2 has identical amino acid sequences with VIPase and an i41SL 1-2 catalytic antibody light chain, both of which possess peptidase activity. Four out of the 14 mAbs illustrated another germline family VkappaIA, within which gene bb1.1 had high homology with BV04-01 light chain mAb, which hydrolyses ssDNA. Interestingly, our mAbs showed DNA-hydrolytic activity at an optimum pH of 4-5, which is a typical pattern of autoimmune diseases in which autoantibodies hydrolyze supercoiled plasmid DNA. This is the first evidence ever that CMV-CP could stimulate catalytic antibodies, which have an identical sequence homology with autoantibodies. Furthermore, the CMV-CP-specific mAbs will be important for isolating antibodies specific to the CPs of bacteria, viruses, cancer cells, etc. that could be used for medical therapy.

Silencing by RNAi of the gene for Pns12, a viroplasm matrix protein of Rice dwarf virus, results in strong resistance of transgenic rice plants to the virus

The non-structural protein Pns12 of Rice dwarf virus is one of the early proteins expressed in cultured insect cells, and it is one of 12 proteins that initiate the formation of the viroplasm, the putative site of viral replication. Pns4 is also a non-structural protein, visible as minitubules after nucleation of the viroplasm. We introduced Pns12- and Pns4-specific RNA interference (RNAi) constructs into rice plants. The resultant transgenic plants accumulated short interfering RNAs specific to the constructs. The progeny of rice plants with Pns12-specific RNAi constructs, after self-fertilization, were strongly resistant to viral infection. By contrast, resistance was less apparent in the case of rice plants with Pns4-specific RNAi constructs, and delayed symptoms appeared in some plants of each line. Our results suggest that interference with the expression of a protein that is critical for viral replication, such as the viroplasm matrix protein Pns12, might be a practical and effective way to control viral infection in crop plants.

A cucumber mosaic virus mutant lacking the 2b counter-defence protein gene provides protection against wild-type strains

Several plant virus mutants, in which genes encoding silencing suppressor proteins have been deleted, are known to induce systemic or localized RNA silencing against themselves and other RNA molecules containing homologous sequences. Thus, it is thought that many cases of cross-protection, in which infection with a mild or asymptomatic virus mutant protects plants against challenge infection with closely related virulent viruses, can be explained by RNA silencing. We found that a cucumber mosaic virus (CMV) mutant of the subgroup IA strain Fny (Fny-CMVDelta2b), which cannot express the 2b silencing suppressor protein, cross-protects tobacco (Nicotiana tabacum) and Nicotiana benthamiana plants against disease induction by wild-type Fny-CMV. However, protection is most effective only if inoculation with Fny-CMVDelta2b and challenge inoculation with wild-type CMV occurs on the same leaf. Unexpectedly, Fny-CMVDelta2b also protected plants against infection with TC-CMV, a subgroup II strain that is not closely related to Fny-CMV. Additionally, in situ hybridization revealed that Fny-CMVDelta2b and Fny-CMV can co-exist in the same tissues but these tissues contain zones of Fny-CMVDelta2b-infected host cells from which Fny-CMV appears to be excluded. Taken together, it appears unlikely that cross-protection by Fny-CMVDelta2b occurs by induction of systemic RNA silencing against itself and homologous RNA sequences in wild-type CMV. It is more likely that protection occurs through either induction of very highly localized RNA silencing, or by competition between strains for host cells or resources.

Characterisation of several heterogeneous species of defective RNAs derived from RNA 3 of cucumber mosaic virus

Preparations of double-stranded RNAs (dsRNAs) extracted from Nicotiana tabacum cv Xanthi plants infected with a subgroup IB isolate of Cucumber mosaic virus (CMV) were found to contain a heterogeneous population of defective RNAs (D-RNAs) derived from RNA 3. Characterised D-RNAs ranged in size from 1.5 to 1.9 kb and were derived either by a single in-frame deletion within the 3a or 3b genes or by means of double in-frame deletions within both genes. Also, northern blot hybridisation showed two other types of RNA derived from RNA 3: (a) RNA species of ca. 0.7 kb containing the 3'-terminus but lacking the 5'-terminus, which could be 3'-coterminal subgenomic of D-RNAs derived from the 3b gene and (b) RNA species of unknown origin of ca. 0.8 kb containing the 5'-terminus but lacking the 3'-terminus.

Expression of artificial microRNAs in transgenic Arabidopsis thaliana confers virus resistance

Plant microRNAs (miRNAs) regulate the abundance of target mRNAs by guiding their cleavage at the sequence complementary region. We have modified an Arabidopsis thaliana miR159 precursor to express artificial miRNAs (amiRNAs) targeting viral mRNA sequences encoding two gene silencing suppressors, P69 of turnip yellow mosaic virus (TYMV) and HC-Pro of turnip mosaic virus (TuMV). Production of these amiRNAs requires A. thaliana DICER-like protein 1. Transgenic A. thaliana plants expressing amiR-P69(159) and amiR-HC-Pro(159) are specifically resistant to TYMV and TuMV, respectively. Expression of amiR-TuCP(159) targeting TuMV coat protein sequences also confers specific TuMV resistance. However, transgenic plants that express both amiR-P69(159) and amiR-HC-Pro(159) from a dimeric pre-amiR-P69(159)/amiR-HC-Pro(159) transgene are resistant to both viruses. The virus resistance trait is displayed at the cell level and is hereditable. More important, the resistance trait is maintained at 15 degrees C, a temperature that compromises small interfering RNA-mediated gene silencing. The amiRNA-mediated approach should have broad applicability for engineering multiple virus resistance in crop plants.

Plant signal transduction and defense against viral pathogens

Viral infection of plants is a complex process whereby the virus parasitizes the host and utilizes its cellular machinery to multiply and spread. In turn, plants have evolved signaling mechanisms that ultimately limit the ingress and spread of viral pathogens, resulting in resistance. By dissecting the interaction between host and virus, knowledge of signaling pathways that are deployed for resistance against these pathogens has been gained. Advances in this area have shown that resistance signaling against viruses does not follow a prototypic pathway but rather different host factors may play a role in resistance to different viral pathogens. Some components of viral resistance signaling pathways also appear to be conserved with those functioning in signaling pathways operational against other nonviral pathogens, however, these pathways may or may not overlap. This review aims to document the advances that have improved our understanding of plant resistance to viruses.

Coat protein sequence shows that Cucumber mosaic virus isolate from geraniums (Pelargonium spp.) belongs to subgroup II

A viral disease was identified on geraniums (Pelargonium spp.) grown in a greenhouse at the Institute of Himalayan Bioresource Technology (IHBT), Palampur, exhibiting mild mottling and stunting. The causal virus (Cucumber mosaic virus, CMV) was identified and characterized on the basis of host range, aphid transmission, enzyme linked immunosorbent assay (ELISA), DNA-RNA hybridization and reverse transcription polymerase chain reaction (RT-PCR). A complete coat protein (CP) gene was amplified using degenerate primers and sequenced. The CP gene showed nucleotide and amino acid homology up to 97%-98% and 96%-99%, respectively with the sequences of CMV subgroup II. The CP gene also showed homologies of 75%-97% in nucleotide and 77%-96% in amino acid with the CMV Indian isolates infecting various crops. On the basis of sequence homology, it was concluded that CMV-infecting geraniums in India belong to subgroup II.

Transgenic cucumbers harboring the 54-kDa putative gene of Cucumber fruit mottle mosaic tobamovirus are highly resistant to viral infection and protect non-transgenic scions from soil infection

Cucumber fruit mottle mosaic tobamovirus (CFMMV) causes severe mosaic symptoms and yellow mottling on leaves and fruits and, occasionally, severe wilting of cucumber (Cucumis sativus L.) plants. No genetic source of resistance against this virus has been identified in cucumber. The gene coding for the putative 54-kDa replicase gene of CFMMV was cloned into an Agrobacterium tumefaciens binary vector, and transformation was performed on cotyledon explants of a parthenocarpic cucumber cultivar. R1 seedlings were screened for resistance to CFMMV by symptom expression, back inoculation on an alternative host and ELISA. From a total of 14 replicase-containing R1 lines, eight resistant lines were identified. Line 144--homozygous for the putative 54-kDa replicase gene--was immune to CFMMV infection by mechanical and graft inoculation, and to root infection following planting in CFMMV-infested soil. A substantial delay of symptom appearance was observed following infection by three additional cucurbit-infecting tobamoviruses. When used as a rootstock, line I44 protected susceptible cucumber scions from soil infection by CFMMV. This paper is the first report on protection of a susceptible cultivar against a soil-borne viral pathogen, by grafting onto a transgenic rootstock.

Recent advances in rice biotechnology--towards genetically superior transgenic rice

Rice biotechnology has made rapid advances since the first transgenic rice plants were produced 15 years ago. Over the past decade, this progress has resulted in the development of high frequency, routine and reproducible genetic transformation protocols for rice. This technology has been applied to produce rice plants that withstand several abiotic stresses, as well as to gain tolerance against various pests and diseases. In addition, quality improving and increased nutritional value traits have also been introduced into rice. Most of these gains were not possible through conventional breeding technologies. Transgenic rice system has been used to understand the process of transformation itself, the integration pattern of transgene as well as to modulate gene expression. Field trials of transgenic rice, especially insect-resistant rice, have recently been performed and several other studies that are prerequisite for safe release of transgenic crops have been initiated. New molecular improvisations such as inducible expression of transgene and selectable marker-free technology will help in producing superior transgenic product. It is also a step towards alleviating public concerns relating to issues of transgenic technology and to gain regulatory approval. Knowledge gained from rice can also be applied to improve other cereals. The completion of the rice genome sequencing together with a rich collection of full-length cDNA resources has opened up a plethora of opportunities, paving the way to integrate data from the large-scale projects to solve specific biological problems.

Spatial analysis for exclusive interactions between subgroups I and II of Cucumber mosaic virus in cowpea

The dynamics of virus interference in Cucumber mosaic virus (CMV) infection in cowpea were investigated by tissue-blotting and in situ hybridization. Using co-inoculation assays, we discovered that spatial competition between CMV-LE (subgroup I) and CMV-m2 (subgroup II) occurred in the inoculated leaves. Interestingly, competitive interactions between the two viruses also could be observed in the non-inoculated upper leaf tissues of the plants. Furthermore, the pattern of exclusive distribution was observed between challenge and protecting viruses in the serially inoculated leaves. Taken together, it is suggested that the dynamics of competitive interactions between the two subgroups could be characterized by exclusive infection and multiplication of the individual viruses in cowpea plants.

Resistance mechanisms to plant viruses: an overview

To obtain virus-resistant host plants, a range of operational strategies can be followed nowadays. While for decades plant breeders have been able to introduce natural resistance genes in susceptible genotypes without knowing precisely what these resistance traits were, currently a growing number of (mostly) dominant resistance genes have been cloned and analyzed. This has led not only to a better understanding of the plant's natural defence systems, but also opened the way to use these genes beyond species borders. Besides using natural resistance traits, also several novel, "engineered" forms of virus resistance have been developed over the past 15 years. The first successes were obtained embarking from the principle of pathogen-derived resistance (PDR) by transforming host plants with viral genes or sequences with the purpose to block a specific step during virus multiplication in the plant. As an unforeseen spin-off of these investments, the phenomenon of post-translational gene silencing (PTGS) was discovered, which to date is by far the most successful way to engineer resistance. It is generally believed that PTGS reflects a natural defence system of the plant, and part of the hypothesized components required for PTGS have been identified. As counteracting strategy, and confirming PTGS to be a natural phenomenon, a considerable number of viruses have acquired gene functions by which they can suppress PTGS. In addition to PDR and PTGS, further strategies for engineered virus resistance have been explored, including the use of pokeweed antiviral protein (PAP), 2',5'-oligoadenylate synthetase and "plantibodies". This paper will give a brief overview of the major strategies that have become operational during the past 10 years.

RNA-mediated virus resistance

Post-transcriptional gene silencing is an RNA degradation mechanism that can be induced by viruses. Recent evidence indicates that silencing may also be involved in virus synergism, tissue limitation of virus spread, non-host resistance, virus transmission through seeds and in more general mechanisms of defense such as that mediated by salicylic acid. The analysis of Arabidopsis mutants, and of viruses carrying silencing suppressors, has led to a greater understanding of post-transcriptional gene silencing pathways. Much still remains to be discovered, however, not least to allow the successful exploitation of gene silencing in conferring pathogen resistance to transgenic plants.

Inter- and intramolecular recombinations in the cucumber mosaic virus genome related to adaptation to alstroemeria

In four distinct alstroemeria-infecting cucumber mosaic virus (CMV) isolates, additional sequences of various lengths were present in the 3' nontranslated regions of their RNAs 2 and 3, apparently the result of intra- and intermolecular recombination events. Competition experiments revealed that these recombined RNA 2 and 3 segments increased the biological fitness of CMV in alstroemeria.

A cucumber mosaic virus (CMV) RNA 1 transgene mediates suppression of the homologous viral RNA 1 constitutively and prevents CMV entry into the phloem

Resistance to Cucumber mosaic virus (CMV) in tobacco lines transformed with CMV RNA 1 is characterized by reduced virus accumulation in the inoculated leaf, with specific suppression of accumulation of the homologous viral RNA 1, and by the absence of systemic infection. We show that the suppression of viral RNA 1 occurs in protoplasts from resistant transgenic plants and therefore is not due to a host response activated by the cell-to-cell spread of virus. In contrast, suppression of Tobacco rattle virus vectors carrying CMV RNA 1 sequences did not occur in protoplasts from resistant plants. Furthermore, steady-state levels of transgene mRNA 1 were higher in resistant than in susceptible lines. Thus, the data indicate that sequence homology is not sufficient to induce suppression. Grafting experiments using transgenic resistant or susceptible rootstocks and scions demonstrated that the resistance mechanism exhibited an additional barrier to phloem entry, preventing CMV from moving a long distance in resistant plants. On the other hand, virus from susceptible rootstocks could systemically infect grafted resistant scions via the phloem. Analysis of viral RNA accumulation in the infected scions showed that the mechanism that suppresses the accumulation of viral RNA 1 at the single-cell level was overcome. The data indicate that this transgene-mediated systemic resistance probably is not based on a posttranscriptional gene-silencing mechanism.

Extreme resistance to Potato leafroll virus in potato cv. Russet Burbank mediated by the viral replicase gene

High levels of field resistance to Potato leafroll virus (PLRV; Genus: Polerovirus; Family: Luteoviridae) were achieved by expression of the unmodified, full-length PLRV replicase gene in potato plants cv. Russet Burbank. A high degree of resistance was also achieved, but less frequently, by expression of a truncated construct of the replicase gene. In limited testing, neither miss-frame nor antisense constructs of the replicase gene conferred resistance. The degree of resistance expressed among different transformant lines ranged from near immunity to full susceptibility. Resistance to the Colorado potato beetle (Leptinotarsa decemlineata Say) was combined with resistance to PLRV by expression of the cry3A insect control protein gene from Bacillus thuringiensis var. tenebrionis in combination with the unmodified, full-length, viral replicase gene. Resistance was expressed as a reduced incidence of infection detectable by foliage symptoms or serological tests. Reduced incidence of infection was not associated with a decrease in virus antigen concentration in the few plants of resistant lines that became infected. Virus was not detected in the foliage of symptomless plants but was detected in progeny plants produced from the tubers of inoculated but symptomless test plants of some resistant lines. The resistance was effective under natural exposure and against plant-to-plant spread of PLRV by the aphid vector, Myzus persicae Sulzer. Three of the resistant lines selected in these studies were released and are now in commercial production.

The ecology of Cucumber mosaic virus and sustainable agriculture

An account is given of the ecology of Cucumber mosaic virus (CMV) as a pertinent example of how a virus can affect the sustainability of an important crop. It is now generally accepted that the technologies used in modern agriculture should ensure that production systems are operated in such a way that the quality of the produce is maintained year after year without causing degradation of the environment. Recent experiences in countries of the Mediterranean basin demonstrate that the benefits expected from the introduction of new and highly productive plant varieties may be quickly eroded by the concomitant introduction of new virus strains which can greatly change the structure of the resident virus population. Quarantine inspection of plant propagules and genetic engineering are suggested as powerful tools to help achieve sustainability.

Advances in Understanding Plant Viruses and Virus Diseases

▪ Abstract  Plant viruses have had an impact on the science of virology and on plant pathology ever since the virus concept was discovered with Tobacco mosaic virus at the end of the nineteenth century. In this review, we highlight those discoveries. We have divided plant virus research into a "Classical Discovery Period" from 1883-1951 in which the findings were very descriptive; an "Early Molecular Era" from 1952 to about 1983, in which information was developed that described further properties of the viruses, aided by the development of a number of salient techniques; and the "Recent Period" from 1983 to the present, when techniques have been developed to modify plant virus genomes, to detect nonstructural gene products, to determine the functions of viral gene products, and to transform plants to elicit novel forms of resistance to viral diseases. In this period, plant virology has played a significant role in formulating an understanding of the mechanisms of gene silencing and recombination, plasmodesmatal function, systemic acquired resistance, and in developing methods for pathogen detection. We also attempt to predict the direction plant virology will take in the future.

Transgene translatability increases effectiveness of replicase-mediated resistance to cucumber mosaic virus

Transgenic tobacco plants expressing an altered form of the 2a replicase gene from the Fny strain of Cucumber mosaic virus (CMV) exhibit suppressed virus replication and restricted virus movement when inoculated mechanically or by aphid vectors. Additional transformants have been generated which contain replicase gene constructs designed to determine the role(s) of transgene mRNA and/or protein in resistance. Resistance to systemic disease caused by CMV, as well as delayed infection, was observed in several lines of transgenic plants which were capable of expressing either full-length or truncated replicase proteins. In contrast, among plants which contained nontranslatable transgene constructs, only one of 61 lines examined exhibited delays or resistance. Once infected, plants never recovered, regardless of transgene translatability. Transgenic plants exhibiting a range of resistance levels were examined for transgene copy number, mRNA and protein levels. Although ribonuclease protection assays demonstrated that transgene mRNA levels were very low, resistant lines had consistently more steady-state transgene mRNA than susceptible lines. Furthermore, chlorotic or necrotic local lesions developed on the inoculated leaves of transgenic lines containing translatable transgenes, but not on inoculated leaves of lines containing nontranslatable transgenes. These results demonstrate that translatability of the transgene and possibly expression of the transgene protein itself facilitates replicase-mediated resistance to CMV in tobacco.

Virus recovery is induced in Brome mosaic virus p2 transgenic plants showing synchronous complementation and RNA-2-specific silencing

Nicotiana benthamiana plants expressing Brome mosaic virus (BMV) p2 protein complemented replication of RNAs1 + 3 but, surprisingly, supported little or no replication of RNA-2. Despite this, the p2 transgenic plants were able to support systemic migration of RNAs-1 and -3. Kinetic analyses showed identical degradation rates for RNAs-2 and -3, greatly detracting from the concept of an induction of an RNA-2-specific degradation system. Deletion analysis identified a 200-nucleotide sequence that may contribute to silencing in a context-specific manner. When R1 progeny of a severely silencing p2 transgenic line were tested for virus resistance, three different classes of reactions were observed. In class 1 and class 3 plants, the virus moved systemically and showed various extents of RNA-2 silencing. However, in class 2 plants, there was a stochastic onset of post-transcriptional silencing in the systemic leaves that was reminiscent of virus recovery. Plants showing recovery tended to have a greater number of transgene loci than did those exhibiting component-specific silencing. The induction of silencing did not appear to be dependent solely on the combined steady state levels of the transgene and viral RNA. Some plants transformed with a p2 frameshift construct showed a complete silencing phenotype, but none showed RNA-2-specific silencing. While the relationship between the two types of silencing remains unclear, we speculate that our observations reflect early events in the induction of virus recovery.

RT-PCR detection and identification of three species of cucumoviruses with a genus-specific single pair of primers

Reverse transcription and polymerase chain reaction (RT-PCR) was used for detection and identification of three cucumoviruses (cucumber mosaic virus, CMV; peanut stunt virus, PSV; tomato aspermy virus, TAV) in various plants sources with a single pair of primers, designed as CPTALL-3 and CPTALL-5. The pair of cucumovirus genus-specific primers that flank the coat protein gene were designed and used to amplify a DNA fragment of approximately ranging from 938 to 966 bp. The RT-PCR with the set of primers specifically amplified the target size of DNA fragment in all the tested cucumoviruses (CMV S-IA, S-IB and S-II, PSV and TAV). No DNA product of any length was produced when brome mosaic virus or tobacco mosaic virus RNA was used as templates. The cucumoviruses examined were differentiated by PCR-restriction fragment length polymorphism with different enzymes. This indicates that the designed primers are only specific for the cucumoviruses and useful for reliable information of identification of members of the Cucumovirus genus.

Near immunity to rice tungro spherical virus achieved in rice by a replicase-mediated resistance strategy

ABSTRACT Rice tungro disease is caused by rice tungro bacilliform virus (RTBV), which is responsible for the symptoms, and rice tungro spherical virus (RTSV), which assists transmission of both viruses by leafhoppers. Transgenic japonica rice plants (Oryza sativa) were produced containing the RTSV replicase (Rep) gene in the sense or antisense orientation. Over 70% of the plants contained one to five copies of the Rep gene, with integration occurring at a single locus in most cases. Plants producing antisense sequences exhibited significant but moderate resistance to RTSV (60%); accumulation of antisense RNA was substantial, indicating that the protection was not of the homology-dependent type. Plants expressing the full-length Rep gene, as well as a truncated Rep gene, in the (+)-sense orientation were 100% resistant to RTSV even when challenged with a high level of inoculum. Accumulation of viral RNA was low, leading us to conclude that RTSV Rep-mediated resistance is not protein-mediated but is of the cosuppression type. Resistance was effective against geographically distinct RTSV isolates. In addition, RTSV-resistant transgenic rice plants were unable to assist transmission of RTBV. Such transgenic plants could be used in an epidemiological approach to combat the spread of the tungro disease.

Elucidation of the genome organization of tobacco mosaic virus

Proteins unique to tobacco mosaic virus (TMV)-infected plants were detected in the 1970s by electrophoretic analyses of extracts of virus-infected tissues, comparing their proteins to those generated in extracts of uninfected tissues. The genome organization of TMV was deduced principally from studies involving in vitro translation of proteins from the genomic and subgenomic messenger RNAs. The ultimate analysis of the TMV genome came in 1982 when P. Goelet and colleagues sequenced the entire genome. Studies leading to the elucidation of the TMV genome organization are described below.

Characterization of cucumber mosaic virus. VI. Generation of deletions in defective RNA 3s during passage in transgenic tobacco expressing the 3a gene

Defective mutants of cucumber mosaic virus (CMV) RNA 3, containing deletions in the 3a gene were passaged in transgenic tobacco plants expressing the CMV 3a gene. After six passages, the various progeny RNA 3 were characterized. In all but one case, the size of the deletion increased. For the NheI-fs RNA 3 mutant of the Fny-strain of CMV (with an 8 nucleotide deletion), the deletion increased in the progeny viral RNA 3 to 570 nucleotides. For a similar frameshift mutant in RNA 3 of the M strain of CMV, either single RNA 3 species (with deletions of 579 or 627 nucleotides) or mixtures of RNA 3 deletion variants were observed in different plants. The DeltaE-H mutant (with a deletion of 202 nucleotides) of Fny-CMV RNA 3 underwent further deletion resulting in the loss of the entire 3a gene and flanking sequences. The DeltaKpnI mutant (deletion of 501 nucleotides) of Fny-CMV RNA 3 underwent a further deletion of 30 nucleotides. Except for the deletion progeny of the DeltaE-H RNA 3 mutant, the other defective RNA 3s all contained inframe deletions. It is proposed that the various deletions were created by different types of recombination and that packaging may be an important factor in the selection of particular defective RNA 3 variants.

Transgenic resistance to cucumber mosaic virus in tomato: blocking of long-distance movement of the virus in lines harboring a defective viral replicase gene

ABSTRACT Tomato breeding lines were transformed with a defective replicase gene from RNA 2 of cucumber mosaic virus (CMV). A total of 63 transformants from five tomato genotypes were evaluated for resistance to CMV strains. The responses of R1 transgenic offspring fit into three categories: fully susceptible lines (44%), fully resistant lines (8%), and an intermediate-type mixture of susceptible and resistant seedlings in variable proportions (48%). Further characterization of the response of two highly resistant lines was performed by mechanical inoculation, aphid transmission, or grafting experiments. No virus was detected in noninoculated leaves from these lines, although a low level of virus accumulated initially in the inoculated leaf. The homozygous R2 plants and further generations that were evaluated (up to R5) showed resistance to the Fny-CMV strain, two Israeli isolates tentatively classified as subgroup IA, and K-CMV (a representative of subgroup IB). These lines were partially resistant to LS-CMV (a representative of subgroup II) when a high-virus-titer inoculum was used. Expression of the viral transgene was verified in these lines; however, the expected translation product was not detectable. In grafting experiments, we demonstrated that CMV virions were blocked in their ability to move from infected rootstocks of nontransformed tomato or tobacco into the transgenic scions. Interestingly, virions could not move through a transgenic intersection into the upper scion. These results provide an additional indication that replicase-mediated resistance affects long-distance movement.

Extreme Reduction of Disease in Oats Transformed with the 5' Half of the Barley Yellow Dwarf Virus-PAV Genome

ABSTRACT Barley yellow dwarf viruses (BYDVs) are the most serious and widespread viruses of oats, barley, and wheat worldwide. Natural resistance is inadequate. Toward overcoming this limitation, we engineered virus-derived transgenic resistance in oat. Oat plants were transformed with the 5' half of the BYDV strain PAV genome, which includes the RNA-dependent RNA polymerase gene. In experiments on T2- and T3-generation plants descended from the same transformation event, all BYDV-inoculated plants containing the transgene showed disease symptoms initially, but recovered, flowered, and produced seed. In contrast, all but one of the BYDV-PAV-inoculated nontransgenic segregants died before reaching 25 cm in height. Although all of the recovered transgenic plants looked similar, the amount of virus and viral RNA ranged from substantial to undetectable levels. Thus, the transgene may act either by restricting virus accumulation or by a novel transgenic tolerance phenomenon. This work demonstrates a strategy for genetically stable transgenic resistance to BYDVs that should apply to all hosts of the virus.

Cucumber mosaic virus is restricted from entering minor veins in transgenic tobacco exhibiting replicase-mediated resistance

Transgenic tobacco plants expressing an altered form of the 2a replicase gene from cucumber mosaic virus (CMV) strain Fny exhibited a suppression of viral replication and restricted viral movement when inoculated mechanically or by insect vectors. Resistant plants could be infected, however, through a graft-union with an infected nontransformed plant. The infectious entity moved quickly through intergrafts of resistant tissue, indicating that it could move without replicating in the vascular system. Viral replication continued to be suppressed in systemically infected transgenic portions of grafted plants, as demonstrated by the synthesis of lower levels of viral RNA than in systemically infected nontransformed portions of the same grafted plants. Cell-to-cell spread within this tissue also occurred much more slowly than in nontransformed tobacco. Young inoculated levels of transgenic-resistant plants exhibited limited cell-to-cell virus movement, revealed as chlorotic lesions, but no long-distance virus movement occurred. The results of in situ hybridization studies on these lesions indicated that CMV RNA does not traffic from bundle-sheath cells to vascular parenchyma or companion cells in chlorotic lesions on the inoculated leaves of transgenic-resistant tobacco plants. The inhibition of long-distance movement was a consequence of restricted entry of the infectious entity into the vascular system.