Protection against detrimental effects of potyvirus infection in transgenic tobacco plants expressing the papaya ringspot virus coat protein gene

Aphid Transmission of Potyvirus: The Largest Plant-Infecting RNA Virus Genus

Potyviruses are the largest group of plant infecting RNA viruses that cause significant losses in a wide range of crops across the globe. The majority of viruses in the genus Potyvirus are transmitted by aphids in a non-persistent, non-circulative manner and have been extensively studied vis-à-vis their structure, taxonomy, evolution, diagnosis, transmission, and molecular interactions with hosts. This comprehensive review exclusively discusses potyviruses and their transmission by aphid vectors, specifically in the light of several virus, aphid and plant factors, and how their interplay influences potyviral binding in aphids, aphid behavior and fitness, host plant biochemistry, virus epidemics, and transmission bottlenecks. We present the heatmap of the global distribution of potyvirus species, variation in the potyviral coat protein gene, and top aphid vectors of potyviruses. Lastly, we examine how the fundamental understanding of these multi-partite interactions through multi-omics approaches is already contributing to, and can have future implications for, devising effective and sustainable management strategies against aphid-transmitted potyviruses to global agriculture.

Synthesis and antiphytoviral activity of α-aminophosphonates containing 3, 5-diphenyl-2-isoxazoline as potential papaya ringspot virus inhibitors

α-Aminophosphonates compounds containing 3,5-diphenyl-2-isoxazoline were synthesized and evaluated for their bioactivity. Seventeen of them showed good bioactivity (protection effect > 50%) in vivo against papaya ringspot virus, while two of them (V29 and V45) exhibited excellent antiviral activity (both 77.8%). In the latter case, the antiviral activity was close to that of antiphytovirucides ningnanmycin and dufulin (both 83.3%) at 500 mg/L. The preliminary structure-activity relationships indicated that the bioactivity was strongly influenced by the substituents.

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.

Development and validation of a multiplex reverse transcription PCR assay for simultaneous detection of three papaya viruses

Papaya ringspot virus (PRSV), Papaya leaf distortion mosaic virus (PLDMV), and Papaya mosaic virus (PapMV) produce similar symptoms in papaya. Each threatens commercial production of papaya on Hainan Island, China. In this study, a multiplex reverse transcription PCR assay was developed to detect simultaneously these three viruses by screening combinations of mixed primer pairs and optimizing the multiplex RT-PCR reaction conditions. A mixture of three specific primer pairs was used to amplify three distinct fragments of 613 bp from the P3 gene of PRSV, 355 bp from the CP gene of PLDMV, and 205 bp from the CP gene of PapMV, demonstrating the assay's specificity. The sensitivity of the multiplex RT-PCR was evaluated by showing plasmids containing each of the viral target genes with 1.44 × 103, 1.79 × 103, and 1.91 × 102 copies for the three viruses could be detected successfully. The multiplex RT-PCR was applied successfully for detection of three viruses from 341 field samples collected from 18 counties of Hainan Island, China. Rates of single infections were 186/341 (54.5%), 93/341 (27.3%), and 3/341 (0.9%), for PRSV, PLDMV, and PapMV, respectively; 59/341 (17.3%) of the samples were co-infected with PRSV and PLDMV, which is the first time being reported in Hainan Island. This multiplex RT-PCR assay is a simple, rapid, sensitive, and cost-effective method for detecting multiple viruses in papaya and can be used for routine molecular diagnosis and epidemiological studies in papaya.

Reverse transcription loop-mediated isothermal amplification assay for rapid detection of Papaya ringspot virus

Papaya ringspot virus (PRSV) and Papaya leaf distortion mosaic virus (PLDMV), which causes disease symptoms similar to PRSV, threaten commercial production of both non-transgenic-papaya and PRSV-resistant transgenic papaya in China. A reverse transcription loop-mediated isothermal amplification (RT-LAMP) assay to detect PLDMV was developed previously. In this study, the development of another RT-LAMP assay to distinguish among transgenic, PRSV-infected and PLDMV-infected papaya by detection of PRSV is reported. A set of four RT-LAMP primers was designed based on the highly conserved region of the P3 gene of PRSV. The RT-LAMP method was specific and sensitive in detecting PRSV, with a detection limit of 1.15×10(-6)μg of total RNA per reaction. Indeed, the reaction was 10 times more sensitive than one-step RT-PCR. Field application of the RT-LAMP assay demonstrated that samples positive for PRSV were detected only in non-transgenic papaya, whereas samples positive for PLDMV were detected only in commercialized PRSV-resistant transgenic papaya. This suggests that PRSV remains the major limiting factor for non-transgenic-papaya production, and the emergence of PLDMV threatens the commercial transgenic cultivar in China. However, this study, combined with the earlier development of an RT-LAMP assay for PLDMV, will provide a rapid, sensitive and cost-effective diagnostic power to distinguish virus infections in papaya.

Plant genetic engineering for crop improvement

Plant genetic engineering has long since left its experimental stage: transgenic plants with resistance to viruses, bacteria, fungi, various pests and abiotic stresses have already been released in their hundreds. Transgenic plants can produce better fruits and food of higher quality than wild-types, and can be used as bioreactors for the synthesis of pharmaceutically important compounds. This review portrays some of the achievements in this field of plant molecular biology.

Papaya ringspot virus-P: characteristics, pathogenicity, sequence variability and control

TAXONOMY

Papaya ringspot virus (PRSV) is an aphid-transmitted plant virus belonging to the genus Potyvirus, family Potyviridae, with a positive sense RNA genome. PRSV isolates belong to either one of two major strains, P or W. The P strains infect both papaya and cucurbits whereas the W strains infect only cucurbits.

GEOGRAPHICAL DISTRIBUTION

PRSV-P is found in all major papaya-growing areas.

PHYSICAL PROPERTIES

Virions are filamentous, non-enveloped and flexuous measuring 760-800 x 12 nm. Virus particles contain 94.5% protein and 5.5% nucleic acid. The protein component consists of the virus coat protein (CP), which has a molecular weight of about 36 kDa as estimated by Western blot analysis. Density of the sedimenting component in purified PRSV preparations is 1.32 g/cm(3) in CsCl.

GENOME

The PRSV genome consists of a unipartite linear single-stranded positive sense RNA of 10 326 nucleotides with a 5' terminus, genome-linked protein, VPg.

TRANSMISSION

The virus is naturally transmitted via aphids in a non-persistent manner. Both the CP and helper component (HC-Pro) are required for vector transmission. This virus can also be transmitted mechanically, and is typically not seed-transmitted.

HOSTS

PRSV has a limited number of hosts belonging to the families Caricaceae, Chenopodiaceae and Cucurbitaceae. Propagation hosts are: Carica papaya, Cucurbita pepo and Cucumis metuliferus cv. accession 2459. Local lesion assay hosts are: Chenopodium quinoa and Chenopodium amaranticolor.

CONTROL

Two transgenic papaya varieties, Rainbow and SunUp, with engineered resistance to PRSV have been commercially grown in Hawaii since 1998. Besides transgenic resistance, tolerant varieties, cross-protection and other cultural practices such as isolation and rogueing of infected plants are used to manage the disease.

VIRUS CODE

00.057.0.01.045.

VIRUS ACCESSION NUMBER

57010045.

USEFUL LINK

http://www.ncbi.nlm.nih.gov/ICTVdb/ICTVdB/57010045.htm.

Role of the 25-26 nt siRNA in the resistance of transgenic Prunus domestica graft inoculated with plum pox virus

The reaction of a genetically engineered plum clone (C5) resistant to plum pox virus (PPV) by graft inoculation with the virus was evaluated. The resistance in this clone has been demonstrated to be mediated through post-transcriptional gene silencing (PTGS). A single C5 plant out of 30 plants inoculated with PPV M strain by double chip-budding showed mild diffuse mosaic 'Sharka' symptom at the bottom section of the scion. The upper leaves of this PPV-infected C5 plant remained symptomless and the virus was not detected in them by either DAS-ELISA or RT-PCR. An RNA silencing associated small interfering RNA duplex, siRNA (21-26 nt), was detected in non-inoculated C5 plants and in the portions of inoculated C5 plant in which PPV could not be detected. In the PPV-infected portion of the C5 plant and in C6 PPV susceptible plants only the approximately 21-22 nt siRNAs was detected. Cytosine-methylation was confirmed in C5 plants both uninfected and showing PPV symptoms. The 25-26 nt siRNA normally present in C5 was absent in PPV-infected C5 tissues confirming the critical role of this siRNA in the resistance of clone C5 to PPV infection. We also show that this PPV infection was limited and transient. It was only detected in one plant at one of four post-dormancy sampling dates and did not appear to affect the overall PPV resistance of the C5 clone.

Peptide-mediated broad-spectrum plant resistance to tospoviruses

Plant viruses have a significant impact on agronomic losses worldwide. A new strategy for engineering virus-resistant plants by transgenic expression of a dominant interfering peptide is presented here. This peptide of 29 aa strongly interacts with the nucleocapsid proteins (N) of different tospoviruses. Transgenic Nicotiana benthamiana lines expressing the peptide fused to a carrier protein were challenged with five different tospoviruses that have a nucleocapsid protein interacting with the peptide. In the transgenic plants, strong resistance to tomato spotted wilt virus, tomato chlorotic spot virus, groundnut ring spot virus, and chrysanthemum stem necrosis virus was observed. This therefore demonstrates the feasibility of using peptide "aptamers" as an in vivo tool to control viral infection in higher plants.

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.

Virus Coat Protein Transgenic Papaya Provides Practical Control of Papaya ringspot virus in Hawaii

Since 1992, Papaya ringspot virus (PRSV) destroyed nearly all of the papaya hectarage in the Puna district of Hawaii, where 95% of Hawaii's papayas are grown. Two field trials to evaluate transgenic resistance (TR) were established in Puna in October 1995. One trial included the following: SunUp, a newly named homozygous transformant of Sunset; Rainbow, a hybrid of SunUp, the nontransgenic Kapoho cultivar widely grown in Puna, and 63-1, another segregating transgenic line of Sunset. The second trial was a 0.4-ha block of Rainbow, simulating a near-commercial planting. Both trials were installed within a matrix of Sunrise, a PRSV-susceptible sibling line of Sunset. The matrix served to contain and trace pollen flow from TR plants, and as a secondary inoculum source. Virus infection was first observed 3.5 months after planting. At a year, 100% of the non-TR control and 91% of the matrix plants were infected, while PRSV infection was not observed on any of the TR plants. Fruit production data of SunUp and Rainbow show that yields were at least three times higher than the industry average, while maintaining percent soluble solids above the minimum of 11% required for commercial fruit. These data suggest that transgenic SunUp and Rainbow, homozygous and hemizygous for the coat protein transgene, respectively, offer a good solution to the PRSV problem in Hawaii.

[Detection of recombinant DNA from genetically modified papaya]

A method using polymerase chain reaction (PCR) was developed to detect the genetically modified (GM) papaya (55-1 line), of which the mandatory safety assessment has not been finished in Japan because of insufficient data. The papaya intrinsic papain gene was used as an internal control. The results of PCR amplification of the papain gene segment indicated that a commercial silica membrane type kit (QIAGEN DNeasy plant mini) was useful for extraction of DNA from papaya fruit, but not for extraction from canned papaya fruit. On the other hand, a commercial ion-exchange type kit (QIAGEN Genomic-tip) provided enough purified DNA for PCR from canned papaya fruit. Compared with the parental line and other commercial non-GM papayas, the DNA from GM papaya fruit provided specific amplification bands in PCR with five primer pairs (Nos. 2-6) including beta-glucuronidase and neomycin phosphotransferase II gene-specific ones. On the other hand, the primer pairs recognizing these genes showed false-positive results when we used DNAs from canned papaya. Therefore, we recommend that the primer pairs (Nos. 5 and 6) recognizing the sequences derived from two different species of organism should be used in order to detect specifically the GM papaya in canned fruits.

Post-transcriptional gene silencing in plum pox virus resistant transgenic European plum containing the plum pox potyvirus coat protein gene

Transgenic plums containing the plum pox potyvirus coat protein (PPV-CP) gene were inoculated with PPV. Infection was monitored by evaluating symptoms, ELISA, and IC-RT-PCR. Transgenic clone C5 was highly resistant to PPV during four years of testing and displayed characteristics typical of post-transcriptional gene silencing (PTGS), including a high level of transgene transcription in the nucleus, low levels of transgene mRNA in the cytoplasm, a complex multicopy transgene insertion with aberrant copies, and methylation of the silenced PPV-CP transgene. The PPV-CP transgene was also methylated in seedlings of C5 and these seedlings were resistant to PPV. Our results show, for the first time, that PTGS functions as a mechanism for virus resistance in a woody perennial species.

Heteroduplex Mobility and Sequence Analyses for Assessment of Variability of Zucchini yellow mosaic virus

ABSTRACT A heteroduplex mobility assay (HMA) was used to analyze the variability among five isolates of Zucchini yellow mosaic virus (ZYMV; TW-TC1, TW-CY2, TW-TN3, TW-TNML1, and TW-NT1) collected from cucurbit fields in different areas of Taiwan. A cDNA fragment of 760 bp covering the variable region of the N terminal half of the coat protein (CP) gene was amplified by reverse transcription-polymerase chain reaction (RT-PCR) and subsequently subjected to HMA analysis for sequence variation. When TW-NT1 combined with any of the other Taiwan isolates, the heteroduplexes obtained migrated much more slowly than did the heteroduplexes obtained in combinations among the other four Taiwan isolates, indicating that TW-TC1, TW-CY2, TW-TN3, and TW-TNML1 share a high degree of sequence homology, while the TW-NT1 isolate is more distinct. The complete nucleotide sequences of the CP genes and the 3' noncoding regions of the five isolates were determined from RT-PCR-derived cDNA clones. A phylogenetic tree derived from the actual sequences of the 760-bp fragments of the five Taiwan and another six ZYMV isolates from different geographic areas revealed four genotypes. TW-TNML1, TW-TC1, TWC-Y2, and TW-TN3 were in genotype I, while TW-NT1 and U.S. isolates were in genotype II. The Singapore and Reunion Island isolates were separated into genotypes III and IV, respectively. Comparison of the CP genes of the five Taiwan isolates indicated that they share 92.8 to 98.7% nucleotide identities and 96.4 to 99.3% amino acid identities. The amino acid positions 73, 102, 109, and 149 of the CP gene, where lysine, serine, arginine, and aspartic acid reside, respectively, were uniquely conserved for genotype I Taiwan isolates. Thus, results of HMA agreed well with those of phylogenetic analysis based on the sequence data of the five Taiwan ZYMV isolates. These five ZYMV isolates of known sequence can be used as reference strains for HMA to analyze the variability of ZYMV in Taiwan.

Control of papaya ringspot virus in papaya: a case study

The papaya crop is severely affected by papaya ringspot virus (PSRV) worldwide. This review focuses on efforts to control the destructiveness of the disease caused by PSRV in Hawaii, starting from the use of cross protection to parasite-derived resistance with transgenic papaya expressing the PSRV coat protein gene. A chronology of the research effort is given and related to the development of technologies and the pressing need to control PSRV in Hawaii. The development of commercial virus-resistant transgenic papaya provides a tangible approach to control PSRV in Hawaii. Moreover, the development of transgenic papaya by other laboratories and employment of a mechanism of effective technology transfer to different countries hold promise for control of PSRV worldwide.

Coat protein-mediated resistance in transgenic plants

This review describes the proposed mechanism(s) of classical virus cross-protection in plants, followed by those suggested for coat protein-mediated resistance (CP-mediated resistance). Although both have common features, cross-protection is thought to be a complex response caused by the replication and expression of the entire viral genome, whereas the resistance conferred by the expression of a virus coat protein gene is more limited. The term genetically engineered cross-protection is frequently used because in many cases the phenotype of resistance mimics that of cross-protection. However, CP-mediated resistance, although a narrow term, more accurately describes the resistance that results from the expression of a virus CP gene in transgenic plants.

Principles and background for the construction of transgenic plants displaying multiple virus resistance

We investigated the possibility of reconstructing the 2'-5' oligoadenylate (2-5A) pathway into the plant kingdom to achieve multiple virus resistance. Differently phosphorylated 2-5A trimers and tetramers inhibited TMV RNA translation in cell-free systems. In wheat germ extracts the most potent inhibitors were nonphosphorylated forms of 2-5A. Triphosphorylated forms of 2-5A were deposphorylated and hydrolysed in plant extracts. Since we could not detect homologous DNA to mammalian 2-5A synthetase cDNA in tobacco or potato, we cloned rat 2-5A synthetase cDNA and transformed it by the Agrobacterium-mediated mechanism into tobacco and potato. Transformed tobacco plants were resistant to PVS infection and propagation of PVX was reduced. In transgenic potatoes tolerance to PVX and, in one transgenic clone, also to PVY was observed.

Cloning of the capsid protein gene from a blotch isolate of peanut stripe virus

The 3' terminal 1,367 nucleotides (nts) of a blotch isolate of the potyvirus, peanut stripe virus (PStV), were cloned and sequenced. This region included the 861 nts (287 amino acids) of the PStV capsid protein (CP). The amino acid sequence of the predicted proteinase cleavage site was identified. This region shared limited homology with other potyvirus proteinase cleavage sites. The viral CP gene sequence was followed by 254 nucleotides of 3' nontranslated sequence and a poly-A tail. Based on computer modeling, the 3' nontranslated region could form two lengthy stem-loop structures and two pseudoknot structures. Subclones containing the coding sequences of the CP (amino acids 1-287, 17-287, 17-113 and 106-287) were constructed and expressed in Escherichia coli. These polypeptides were detected using polyclonal antibodies in Western blots.

Plants that express a potyvirus proteinase gene are resistant to virus infection

Transgenic tobacco plants that express the genome-linked protein/proteinase-coding region of the potyvirus tobacco vein mottling virus (TVMV) were produced and tested for their reaction to inoculation with TVMV and two other potyviruses. These plants did not develop disease symptoms after being inoculated with large doses of TVMV but were as susceptible to infection by the other potyviruses as were control plants. Lines of tobacco that express the coat protein- or the nonstructural cylindrical inclusion protein-coding regions were also produced. The coat protein transgenic plants were protected against all three potyviruses, and the cylindrical inclusion transgenic plants were susceptible to all three potyviruses. These results indicate that some, but not all, TVMV genes can be used to confer protection against potyviruses in plants. The results also suggest that combinations of viral genes in transgenic plants might improve protection against potyviruses.

Efficient pathogen-derived resistance induced by integrated potato virus Y coat protein gene in tobacco

The coat protein (CP) gene from potato virus Y (Hungarian isolate, PVY-H) was engineered into Agrobacterium tumefaciens binary vector for expression in different tobacco lines. Three different Nicotiana tabacum breeding lines were transformed and the integration of the CP gene was confirmed by PCR technique using genomic DNA preparations. The transcription and expression of the integrated CP gene was detected by Northern and Western blots. Pathogen-derived resistance was demonstrated by inoculation of the R1 progeny of the transformed lines with purified PVY-H. The efficiency of protection varied between different transgenic plants ranging from almost complete to no protection. Five CP expressing tobacco lines were resistant to challenge infection with PVY-H as indicated by attenuation or absence of symptom development associated with reduction or lack of detectable virus accumulation. Data from Western blots showed that there is no correlation between the level of the expressed CP and the extent of protection. This suggests that the mechanism of the observed resistance is independent of the level of CP accumulation in the transgenic tobacco plants.

Untranslatable transcripts of the tobacco etch virus coat protein gene sequence can interfere with tobacco etch virus replication in transgenic plants and protoplasts

Transgenic tobacco plants which express untranslatable sense or antisense forms of the tobacco etch virus potyvirus (TEV) coat protein (CP) gene sequence have been generated. One of seven transgenic plant lines expressing a CP gene antisense transcript showed an attenuation of symptoms when inoculated with TEV. Three of ten transgenic plant lines expressing untranslatable sense transcripts did not develop symptoms when inoculated with TEV. These lines were resistant to either aphid or mechanically transmitted TEV. In contrast to CP-mediated resistance reported for other viruses, resistance was (1) mediated by an RNA molecule; (2) TEV-specific (i.e., "broad-spectrum resistance" was not observed); (3) independent of inoculum levels; (4) not dependent on plant size and; (5) due to decreased levels of virus replication. Protoplast experiments were used to demonstrate that resistant plant lines did not support the production of virus protein and progeny virus at wild-type levels.

Expression of the gene encoding the coat protein of cucumber mosaic virus (CMV) strain WL appears to provide protection to tobacco plants against infection by several different CMV strains

The gene (cp) encoding the coat protein (CP) of cucumber mosaic virus (CMV) strain WL (CMV-WL, which belongs to CMV subgroup II) was custom polymerase chain reaction (CPCR)-engineered for expression as described by Slightom [Gene 100 (1991) 251-255]. CPCR amplification was used to add 5'- and 3'-flanking NcoI sites to the CMV-WL cp gene, and cp was cloned into the expression vector, pUC18cpexp. This CMV-WL cp expression cassette was transferred into the genome of tobacco (Nicotiana tabacum cv. Havana 423) via the Agrobacterium T-DNA transfer mechanism. R0 plants that express the CMV-WL cp gene were subcloned, propagated, and challenge-inoculated with CMV-WL. Several R0 plant lines showed excellent protection against CMV-WL infection; however, plants found to accumulate the highest CP levels did not show the highest degree of protection. Thus in our case, CP levels appear not to be a useful predictor of the degree of protection. Plants from the best protected CMV-WL cp gene-expressing R0 tobacco lines were also inoculated with CMV strains belonging to the other major CMV subgroup (subgroup I), CMV-C and CMV-Chi, and compared in a parallel experiment with a transgenic tobacco plant line that expresses the CMV-C cp gene. Plants expressing the CMV-WL cp gene appeared to show a broader spectrum of protection against infection by the various CMV strains than plants expressing the CMV-C cp gene.