The wayward Hawaiian boy returns home

This chapter represents a travelog of my life and career and the philosophical points I acquired along the way. I was born on a sugar plantation on the island of Hawaii and early on had a stuttering problem. I attended the Kamehameha Schools and received my BS and MS degrees from the University of Hawaii and my Ph.D. from the University of California at Davis. I link my life and career to various principles and events, some of which are: the importance of positioning oneself; going for the big enchilada; music, the international language; the red zone of biotechnology; the human side of biotechnology; the transgenic papaya story; and my leadership time at USDA in Hawaii. The guiding light throughout my career were the words from Drs. Eduardo Trujillo and Robert Shepherd, respectively, "Dennis, don't just be a test tube scientist, do something to help people" and "Now tell me, what have you really accomplished?"

Screening promoters for Anthurium transformation using transient expression

KEY MESSAGE : There are multiple publications on Anthurium transformation, yet a commercial product has not been achieved. This may be due to use of non-optimum promoters here we address this problem. Different promoters and tissue types were evaluated for transient β-glucuronidase (GUS) expression in Anthurium andraeanum Hort. 'Marian Seefurth' following microprojectile bombardment. Plasmids containing the Ubiquitin 2, Actin 1, Cytochrome C1 from rice, Ubiquitin 1 from maize and 35S promoter from Cauliflower Mosaic Virus fused to a GUS reporter gene were bombarded into in vitro grown anthurium lamina, somatic embryos and roots. The number of GUS foci and the intensity of GUS expression were evaluated for each construct. Ubiquitin promoters from rice and maize resulted in the highest number of expressing cells in all tissues examined. Due to the slow growth of anthurium plants, development of transgenic anthurium plants takes years. This research has rapidly identified multiple promoters that express in various anthurium tissues facilitating the development of transformation vectors for the expression of desirable traits in anthurium plants.

First Report of Pepper mottle virus Infecting Tomato in Hawaii

In August 2011, tomato (Solanum lycopersicum L.) fruit from a University of Hawaii field trial displayed mottling symptoms similar to that caused by Tomato spotted wilt virus (TSWV) or other tospoviruses. The foliage from affected plants, however, appeared symptomless. Fruit and leaf tissue from affected plants were negative for TSWV analyzed by double antibody sandwich (DAS)-ELISA and/or TSWV ImmunoStrips (Agdia, Elkhart, IN) when performed following the manufacturer's instructions. Total RNA from a symptomatic and an asymptomatic plant was isolated using an RNeasy Plant Mini Kit (Qiagen, Valencia, CA) and reverse transcribed using Invitrogen SuperScript III reverse transcriptase (Life Technologies, Grand Island, NY) and primer 900 (5'- CACTCCCTATTATCCAGG(T)₁₆-3') following the enzyme manufacturer's instructions. The cDNA was then used as template in a universal potyvirus PCR assay using primers 900 and Sprimer, which amplify sequences encoding the partial inclusion body protein (NIb), coat protein, and 3' untranslated region of potyviruses (1). A ~1,700-bp product was amplified from the cDNA of the symptomatic plant but not the asymptomatic plant. This product was cloned using pGEM-T Easy (Promega, Madison, WI) and three clones were sequenced at the University of Hawaii's Advanced Studies in Genomics, Proteomics, and Bioinformatics laboratory. The 1,747-bp consensus sequence of the three clones was deposited in GenBank (Accession No. JQ429788) and, following primer sequence trimming, found to be 97% identical to positions 7,934 through 9,640 of Pepper mottle virus (PepMoV; family Potyviridae, genus Potyvirus) accessions from Korea (isolate '217' from tomato; EU586126) and California (isolate 'C' from pepper; M96425). To determine the incidence of PepMoV in the field trial, all 292 plants representing 14 tomato cultivars were assayed for the virus 17 weeks after planting using a PepMoV-specific DAS-ELISA (Agdia) following the manufacturer's directions. Plants were considered positive if their mean absorbance at 405 nm was greater than the mean absorbance + 3 standard deviations + 10% of the negative control samples. The virus incidence ranged from 4.8 to 47.6% for the different varieties, with an overall incidence of 19.9%. Although plant growth was not noticeably impaired by PepMoV infection, the majority of fruit from infected plants was unsaleable, making PepMoV a considerable threat to tomato production in Hawaii. PepMoV has been reported to naturally infect tomato in Guatemala (3) and South Korea (2). To our knowledge, this is the first report of this virus in Hawaii and the first report of this virus naturally infecting tomato in the United States. References: (1) J. Chen et al. Arch. Virol. 146:757, 2001. (2) M.-K. Kim et al. Plant Pathol. J. 24:152, 2008. (3) J. Th. J. Verhoeven et al. Plant Dis. 86:186, 2002.

Allergenicity assessment of the papaya ringspot virus coat protein expressed in transgenic rainbow papaya

The virus-resistant, transgenic commercial papaya Rainbow and SunUp (Carica papaya L.) have been consumed locally in Hawaii and elsewhere in the mainland United States and Canada since their release to planters in Hawaii in 1998. These papaya are derived from transgenic papaya line 55-1 and carry the coat protein (CP) gene of papaya ringspot virus (PRSV). The PRSV CP was evaluated for potential allergenicity, an important component in assessing the safety of food derived from transgenic plants. The transgene PRSV CP sequence of Rainbow papaya did not exhibit greater than 35% amino acid sequence homology to known allergens, nor did it have a stretch of eight amino acids found in known allergens which are known common bioinformatic methods used for assessing similarity to allergen proteins. PRSV CP was also tested for stability in simulated gastric fluid and simulated intestinal fluid and under various heat treatments. The results showed that PRSV CP was degraded under conditions for which allergenic proteins relative to nonallergens are purported to be stable. The potential human intake of transgene-derived PRSV CP was assessed by measuring CP levels in Rainbow and SunUp along with estimating the fruit consumption rates and was compared to potential intake estimates of PRSV CP from naturally infected nontransgenic papaya. Following accepted allergenicity assessment criteria, our results show that the transgene-derived PRSV CP does not pose a risk of food allergy.

Genome sequences and structures of two biologically distinct strains of Grapevine leafroll-associated virus 2 and sequence analysis

Grapevine leafroll-associated virus 2 (GLRaV-2), a member of the genus Closterovirus within Closteroviridae, is implicated in several important diseases of grapevines including "leafroll", "graft-incompatibility", and "quick decline" worldwide. Several GLRaV-2 isolates have been detected from different grapevine genotypes. However, the genomes of these isolates were not sequenced or only partially sequenced. Consequently, the relationship of these viral isolates at the molecular level has not been determined. Here, we group the various GLRaV-2 isolates into four strains based on their coat protein gene sequences. We show that isolates "PN" (originated from Vitis vinifera cv. "Pinot noir"), "Sem" (from V. vinifera cv. "Semillon") and "94/970" (from V. vinifera cv. "Muscat of Alexandria") belong to the same strain, "93/955" (from hybrid "LN-33") and "H4" (from V. rupestris "St. George") each represents a distinct strain, while Grapevine rootstock stem lesion-associated virus.

Translation of papaya ringspot virus RNA in vitro: detection of a possible polyprotein that is processed for capsid protein, cylindrical-inclusion protein, and amorphous-inclusion protein

The genomic RNA of papaya ringspot virus (PRV), a member of the potyvirus group, was translated in a rabbit reticulocyte cell-free system as an approach to determining the translation strategy of the virus. The RNA directed synthesis of more than 20 distinct polypeptides ranging from apparent molecular weight of 26,000 (26K) to 220K. Antiserum to PRV capsid protein (CP) reacted with a subset of these polypeptides, including a 36K protein that comigrated with PRV CP during electrophoresis. Immunoprecipitation with antiserum to PRV cylindrical-inclusion protein (CIP) defined another set of polypeptides including 70K, 108K, 205K, and 220K proteins as major precipitates. The 70K protein comigrated with authentic CIP, and the 205K and 220K proteins were related to both CP and CIP. Immunoprecipitation with antiserum to PRV amorphous-inclusion protein (AIP) defined a unique set of polypeptides which contained a 112K protein as the major precipitate and 51K, 65K, and 86K proteins as minor precipitates. The 51K protein comigrated with authentic AIR A major product of 330K was observed when translation was done without the reducing agent, dithiothreitol. Immunological analyses and kinetic studies indicated that the 330K protein zone was related to the presumed CP, CIP, and AIP zones and 330K possibly is the common precursor for these viral proteins. The presence of a polyprotein of Mr corresponding to the entire coding capacity of the genomic RNA and its likely precursor relationship to the other polypeptides suggest that proteolytic processing is involved in the translation of PRV RNA.

Resistance to Grapevine leafroll associated virus-2 is conferred by post-transcriptional gene silencing in transgenic Nicotiana benthamiana

Grapevine leafroll-associated virus-2 (GLRaV-2) is an important component of the leafroll disease complex in grapevine. We have previously sequenced the GLRaV-2 genome and identified the coat protein (CP) gene. The objective of this study is to test the concept of pathogen-derived resistance against a closterovirus associated with grapevine leafroll disease. Because GLRaV-2 is capable of infecting Nicotiana benthamiana, we decided to test the concept on this herbaceous host. Thirty-seven T(0) transgenic N. benthamiana plants expressing the GLRaV-2 CP gene were regenerated following Agrobacterium-mediated transformation. Disease resistance was evaluated in greenhouse-grown T(1) and T(2) plants by mechanical inoculation with GLRaV-2. Although all the inoculated non-transgenic plants showed symptoms 2-4 weeks post inoculation, various numbers of transgenic plants (16-100%) in 14 of 20 T(1) lines tested were not infected. In these resistant plants, GLRaV-2 was not detectable by enzyme linked immunosorbent assay. Although virus resistance was confirmed in T(2) progenies, the percentage of resistant plants was generally lower (0-63%) than that of the corresponding T(1) lines (0-100%). Northern blot and nuclear run-off results showed that virus resistance in the transgenic plants was consistently associated with the low level of transgene RNA transcript suggesting a post-transcriptional gene silencing. The success of pathogen-derived resistance to GLRaV-2 in transgenic N. benthamiana plants represents the first step towards eventual control of the leafroll disease in grapevines using this strategy.

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.

The draft genome of the transgenic tropical fruit tree papaya (Carica papaya Linnaeus)

Papaya, a fruit crop cultivated in tropical and subtropical regions, is known for its nutritional benefits and medicinal applications. Here we report a 3x draft genome sequence of 'SunUp' papaya, the first commercial virus-resistant transgenic fruit tree to be sequenced. The papaya genome is three times the size of the Arabidopsis genome, but contains fewer genes, including significantly fewer disease-resistance gene analogues. Comparison of the five sequenced genomes suggests a minimal angiosperm gene set of 13,311. A lack of recent genome duplication, atypical of other angiosperm genomes sequenced so far, may account for the smaller papaya gene number in most functional groups. Nonetheless, striking amplifications in gene number within particular functional groups suggest roles in the evolution of tree-like habit, deposition and remobilization of starch reserves, attraction of seed dispersal agents, and adaptation to tropical daylengths. Transgenesis at three locations is closely associated with chloroplast insertions into the nuclear genome, and with topoisomerase I recognition sites. Papaya offers numerous advantages as a system for fruit-tree functional genomics, and this draft genome sequence provides the foundation for revealing the basis of Carica's distinguishing morpho-physiological, medicinal and nutritional properties.

Varying genetic diversity of Papaya ringspot virus isolates from two time-separated outbreaks in Jamaica and Venezuela

Coat protein sequences of 22 Papaya ringspot virus isolates collected from different locations in Jamaica and Venezuela in 1999 and 2004, respectively, were determined and compared with sequences of isolates from earlier epidemics in 1990 and 1993. Jamaican isolates collected in 1999 exhibited nucleotide sequence identities between 98 and 100% but shared lower identities of 92.2% with an isolate collected in 1990. Isolates from the 2004 epidemic in Venezuela exhibited more heterogeneity, with identities between 88.7 and 98.8%. However, isolates collected in 1993 were more closely related (97.7%). The viral populations of the two countries are genetically different and appear to be changing at different rates; presumably driven by introductions, movement of plant materials, geographical isolation, and disease management practices.

Development of genetically engineered resistant papaya for papaya ringspot virus in a timely manner: a comprehensive and successful approach

Papaya orchards throughout most of the world are severely damaged by the destructive disease caused by the papaya ringspot virus (PRSV). PRSV-resistant papaya expressing the coat protein gene (CP) of PRSV have been used in Hawaii to control PRSV since 1998. This chapter presents the experimental steps involved in the development of transgenic papaya, including transgene construction, transformation, and analysis for virus resistance of the transformed papaya. We also describe the important factors that enabled deregulation, commercialization, and adoption of transgenic papaya to occur in Hawaii in a timely manner. Transfer of this technology to other countries with the similar goal and the development of transgenic papaya in other regions of the world also are described.

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

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

Comparative spatial spread overtime of Zucchini Yellow Mosaic Virus (ZYMV) and Watermelon Mosaic Virus (WMV) in fields of transgenic squash expressing the coat protein genes of ZYMV and WMV, and in fields of nontransgenic squash

The spatial and temporal patterns of aphid-vectored spread of Zucchini Yellow Mosaic Virus (ZYMV) and Watermelon Mosaic Virus (WMV) were monitored over two consecutive years in plantings of nontransgenic and transgenic squash ZW-20H (commercial cv. Freedom II) and ZW-20B, both expressing the coat protein genes of ZYMV and WMV. All test plants were surrounded by nontransgenic plants that were mechanically inoculated with ZYMV or WMV, and served as primary virus source. Across all trials, none of the transgenic plants exhibited systemic symptoms upon infection by ZYMV and WMV but a few of them developed localized chlorotic dots and/or blotches, and had low mixed infection rates [4% (6 of 139) of ZW-20H and 9% (13 of 139) of ZW-20B], as shown by ELISA. Geostatistical analysis of ELISA positive transgenic plants indicated, (i) a lack of spatial relationship on spread of ZYMV and WMV for ZW-20H with flat omnidirectional experimental semivariograms that fitted poorly theoretical models, and (ii) some extent of spatial dependence on ZYMV spread for ZW-20B with a well structured experimental semivariogram that fitted poorly theoretical models during the first but not the second growing season. In contrast, a strong spatial dependence on spread of ZYMV and WMV was found for nontransgenic plants, which developed severe systemic symptoms, had prevalent mixed infection rates (62%, 86 of 139), and well-defined omnidirectional experimental semivariograms that fitted a spherical model. Geostatistical data were sustained by virus transmission experiments with Myzus persicae in screenhouses, showing that commercial transgenic squash ZW-20H alter the dynamics of ZYMV and WMV epidemics by preventing secondary plant-to-plant spread.

Incidence, Spatial Patterns, and Associations Among Viruses in Snap Bean and Alfalfa in New York

Recent epidemics in snap bean (Phaseolus vulgaris) characterized by virus-like symptoms prompted a survey of commercial fields for Alfalfa mosaic virus (AMV), Cucumber mosaic virus (CMV), and the Bean yellow mosaic virus (BYMV)/Clover yellow vein virus (ClYVV) complex in 2002 and 2003. Snap bean fields were either remote from or adjacent to alfalfa (Medicago sativa), a putative source of these viruses. Bean fields were sampled at the bloom stage in both years. Model-adjusted mean incidences of infection by AMV, BYMV/ClYVV, and CMV were 41.96, 6.56, and 6.69%, respectively, in alfalfa, and 6.66, 6.38, and 17.20% in snap bean. In 2002, 25.9% of snap bean plants were infected by more than one virus; <1% had more than one virus in 2003. Virus incidences did not differ between snap bean adjacent to or remote from alfalfa, but incidence of infection by AMV and BYMV/ClYVV was significantly higher in snap bean planted later in the season rather than earlier. In 2002, there was a positive association between AMV and CMV in the tendency to find both viruses in the same snap bean plant. In some years, infection by aphid-transmitted viruses can become widespread in snap bean in New York.

Update on the development of virus-resistant papaya: virus-resistant transgenic papaya for people in rural communities of Thailand

Papaya (Carica papaya L.) is one of the most important and preferred crops in rural communities in Thailand. Papaya ringspot virus (PRSV) is a serious disease of papaya throughout Thailand. Efforts to control the virus by various methods either have not been successful or have not resulted in sustainable control. In 1995, collaborative research by the Department of Agriculture of Thailand and Cornell University to develop transgenic papaya resistant to PRSV was initiated. Two local Thai cultivars were transformed by microprojectile bombardment with the use of a nontranslatable coat protein gene of PRSV from Khon Kaen. Numerous kanamycin-resistantplants were regenerated and were inoculated with the PRSV Khon Kaen isolate for selection of resistant lines. Since 1997, promising RO transgenic lines have been transferred to the research station at Thapra for subsequent screenhouse tests and selection of the most PRSV-resistant lines. In selection set 1, three R3 lines initially derived from Khaknuan papaya showed excellent resistance to PRSV (97% to 100%) and had a yield of fruit 70 times higher than nontransgenic Khaknuan papaya. In selection set 2, one R3 line initially derived from Khakdam papaya showed 100% resistance. Safety assessments of these transgenic papayas have so far found no impact on the surrounding ecology. No natural crossing between transgenic and nonmodified papaya was observed beyond a distance of 10 m from the test plots. Analysis of the nutritional composition found no differences in nutrient levels in comparison with the nonmodified counterparts. Molecular characterization by Southern blotting revealed three copies of the transgene presented; however, no coat protein product was expressed. Data on additional topics, such as the effects offeeding the transgenic papaya to rats and the stability of the gene inserts, are currently being gathered.

Field Resistance of Coat Protein Transgenic Papaya to Papaya ringspot virus in Jamaica

Transgenic papayas (Carica papaya) containing translatable coat protein (CP_T) or nontranslatable coat protein (CP_NT) gene constructs were evaluated over two generations for field resistance to Papaya ringspot virus in a commercial papaya growing area in Jamaica. Reactions of R₀ CP_T transgenic lines included no symptoms and mild or severe leaf and fruit symptoms. All three reactions were observed in one line and among different lines. Trees of most CP_NT lines exhibited severe symptoms of infection, and some also showed mild symptoms. R₁ offspring showed reactions previously observed with parental R₀ trees; however, reactions not previously observed or a lower incidence of the reaction were also obtained. The transgenic lines appear to possess virus disease resistance that can be manipulated in subsequent generations for the development of a product with acceptable commercial performance.

Development of virus resistant transgenic papayas expressing the coat protein gene from a Brazilian isolate of Papaya ringspot virus

Translatable and nontranslatable versions of the coat protein (cp) gene of a Papaya ringspot virus (PRSV) isolate collected in the state of Bahia, Brazil, were engineered for expression in Sunrise and Sunset Solo varieties of papaya (Carica papaya). The biolistic system was used to transform secondary somatic embryo cultures derived from immature zygotic embryos. Fifty-four transgenic lines, 26 translatable and 28 nontranslatable gene versions, were regenerated, with a transformation efficiency of 2.7%. Inoculation of cloned R0 plants with PRSV BR, PRSV HA or PRSV TH, Brazilian, Hawaiian and Thai isolates, respectively, revealed lines with mono-, double-, and triple-resistance. After molecular analysis and a preliminary agronomic evaluation, 13 R1 and R2 populations were incorporated into the papaya-breeding program at Embrapa Cassava and Tropical Fruits, in Cruz das Almas, Bahia, Brazil.

Complete genome sequences of two new variants of Grapevine rupestris stem pitting-associated virus and comparative analyses

Grapevine rupestris stem pitting-associated virus (GRSPaV), a member of the genus Foveavirus within the family Flexiviridae, is the putative causal agent of the disease Rupestris stem pitting (RSP) of grapevines. GRSPaV comprises a family of variants whose pathological characteristics have not been determined. Recently, many of the indicator "St George" plants (Vitis rupestris) used throughout the world to index RSP tested positive for GRSPaV. This finding questions the validity of past biological indexing results. In this work, a representative genomic region of GRSPaV was first sequenced from ten "St George" plants from two sources and it was demonstrated that nine of them carried a new variant, GRSPaV-SG1. The genomes of GRSPaV-SG1 and GRSPaV-BS from "Bertille Seyve 5563" plants were sequenced, revealing a genome structure identical to that of GRSPaV-1. It was demonstrated experimentally that infection of "St George" plants with GRSPaV-SG1 is asymptomatic and thus it is proposed that GRSPaV-SG1 infection should not have interfered with the outcome of past indicator indexing. This represents the first attempt to link a GRSPaV variant with pathological properties.

Movement of coat protein genes from a commercial virus-resistant transgenic squash into a wild relative

We monitored pollen-mediated transgene dissemination from commercial transgenic squash CZW-3 into its wild relative Cucurbita pepo ssp. ovifera var. texana (C. texana). Transgenic squash CZW-3 expresses the neomycin phosphotransferase II (nptII) gene and the coat protein (CP) genes of Cucumber mosaic virus (CMV), Zucchini yellow mosaic virus (ZYMV), and Watermelon mosaic virus (WMV); thereby, it is resistant to these three aphid-borne viruses. The rate of NPT II and CP transgene introgression increased with overlapping flowering patterns and a high ratio of transgenic F1 hybrids (C. texana x CZW-3) to C. texana. Transgene transfer also readily occurred from transgenic F1 hybrids into C. texana over three generations in field settings where test plants grew sympatrically and viruses were not severely limiting the growth, and fruit and seed production of C. texana. In contrast, introgression of the transgenes into C. texana was not sustained under conditions of high viral disease pressure. As expected, C. texana progeny that acquired the CP transgenes exhibited resistance to CMV, ZYMV, and WMV. This is the first report on transgene dissemination from a transgenic crop that exhibits disease resistance and hybridizes with a wild plant species without loss of fertility.

Comparative development and impact of transgenic papayas in Hawaii, Jamaica, and Venezuela

We present data concerning the creation of transgenic papayas resistant to Papaya ringspot virus (PRSV) and their adoption by three different countries: the United States (e.g., Hawaii), Jamaica, and Venezuela. Although the three sets of transgenic papayas showed effective resistance to PRSV, the adoption rate in each country has varied from full utilization in Hawaii to aggressive testing but delay in deregulating of the product in Jamaica to rejection at an early stage in Venezuela. Factors that contributed to the rapid adoption in Hawaii include a timely development of the transgenic product, PRSV causing severe damage to the papaya industry, close collaboration between researchers and the industry, and the existence of procedures for deregulating a transgenic product. In Jamaica, the technology for developing the initial field-testing of the product progressed rather rapidly, but the process of deregulation has been slowed down owing to the lack of sustained governmental efforts to complete the regulatory procedures for transgenic crops. In Venezuela, the technology to develop and greenhouse test the transgenic papaya has moved abreast with the Jamaica project, but the field testing of the transgenic papaya within the country was stopped very early on by actions by people opposed to transgenic products. The three cases are discussed in an effort to provide information on factors, other than technology, that can influence the adoption of a transgenic product.

Complete nucleotide sequence and genome organization of Grapevine leafroll-associated virus 3, type member of the genus Ampelovirus

This study reports on the complete genome sequence of Grapevine leafroll-associated virus 3, the type member of the genus Ampelovirus. The genome is 17 919 nt in size and contains 13 open reading frames (ORFs). Previously, the sequence of 13 154 nt of the 3'-terminal of the genome was reported. The newly sequenced portion contains a 158 nt 5' UTR, a single papain-like protease and a methyltransferase-like (MT) domain. ORF1a encodes a large polypeptide with a molecular mass of 245 kDa. With a predicted +1 frameshift, the large fusion protein generated from ORF1a/1b would produce a 306 kDa polypeptide. Phylogenetic analysis using MT domains further supports the creation of the genus Ampelovirus for mealy-bug-transmitted viruses in the family Closteroviridae.

Engineered Resistance Against Papaya ringspot virus in Venezuelan Transgenic Papayas

Local varieties of papaya grown in the Andean foothills of Mérida, Venezuela, were transformed independently with the coat protein (CP) gene from two different geographical Papaya ringspot virus (PRSV) isolates, designated VE and LA, via Agrobacterium tumefaciens. The CP genes of both PRSV isolates show 92 and 96% nucleotide and amino acid sequence similarity, respectively. Four PRSV-resistant R0 plants were intercrossed or selfed, and the progenies were tested for resistance against the homologous isolates VE and LA, and the heterologous isolates HA (Hawaii) and TH (Thailand) in greenhouse conditions. Resistance was affected by sequence similarity between the transgenes and the challenge viruses: resistance values were higher for plants challenged with the homologous isolates (92 to 100% similarity) than with the Hawaiian (94% similarity) and, lastly, Thailand isolates (88 to 89% similarity). Our results show that PRSV CP gene effectively protects local varieties of papaya against homologous and heterologous isolates of PRSV.

Comparative fitness of a wild squash species and three generations of hybrids between wild×virus-resistant transgenic squash

We compared some fitness components of the wild squash species Cucurbita pepo spp. ovifera var. texana (C. texana) and three generations of hybrids (F1, BC1, and BC2) between C. texana and commercial transgenic squash CZW-3 over three consecutive years under field conditions of low (LDP) and high disease pressure (HDP) by Cucumber mosaic virus (CMV), Zucchini yellow mosaic virus (ZYMV) and Watermelon mosaic virus (WMV). Transgenic squash CZW-3 expresses the coat protein (CP) genes of CMV, ZYMV, and WMV, and is resistant to these three aphid-borne viruses. Across all HDP trials, transgenic BC1 and BC2 hybrids expressing the three CP genes grew more vigorously, displayed resistance to CMV, ZYMV, and WMV, and produced a greater number of mature fruits and viable seeds than nontransgenic hybrid segregants and C. texana. Transgenic F1 hybrids behaved similarly to BC1 and BC2 hybrids but grew less vigorously than C. texana. In contrast, across all LDP trials, C. texana outperformed the transgenic and nontransgenic hybrid segregants. Further, only one back cross was necessary to recover individuals with most of the C. texana characteristics and yet maintain virus resistance. Our data suggest that C. texana acquiring CP transgenes upon hybridization and introgression could have a selective advantage if CMV, ZYMV, and WMV are severely limiting the growth and reproductibility of wild squash populations.

First Detection of Rupestris stem pitting associated virus Particles by Antibody to a Recombinant Coat Protein

Rupestris stem pitting associated virus (RSPaV), a member of the genus Foveavirus, is associated with the Rupestris stem pitting component of the Rugose wood (RW) disease complex of grapevines. Heretofore, particles of RSPaV have not been visualized. In this work, flexuous rod particles approximately 723 nm in length were detected in the sap of infected grapevines by immunosorbent electron microscopy (ISEM), using a polyclonal antiserum produced to a recombinant coat protein of RSPaV. Particles of RSPaV were detected in tissue culture-, greenhouse-, and field-grown grapevines infected with RSPaV, but not in healthy control plants. Detection of virus particles by ISEM corresponded with detection of RSPaV by Western blot, enzyme-linked immunosorbent assay, and reverse transcription-polymerase chain reaction. Virus particles were decorated with the antibodies specific to RSPaV but not with antibodies to Grapevine virus A or Grapevine virus B, two other viruses believed to be associated with RW. This definitive identification of RSPaV particles will help define the etiology of RW.

Antiserum to Recombinant Virus Coat Protein Detects Rupestris stem pitting associated virus in Grapevines

Rupestris stem pitting (RSP) is the most widespread virus disease of grapevines. The genome of Rupestris stem pitting associated virus (RSPaV), the putative causal agent of RSP, was recently sequenced. Until recently, the only method to diagnose RSP was biological indexing on woody indicator plants, a process that takes 2 to 3 years to complete. This study reports on the production of a polyclonal antiserum to a recombinant coat protein of RSPaV. The antiserum was used effectively to detect RSPaV from various genotypes and tissues of grapevines by Western blot and indirect enzyme-linked immunosorbent assay. Virus antigens were consistently detected in the cambium of dormant canes and in actively growing leaves of grapevines. Moreover, plants of Vitis rupestris 'St. George', the standard biological indicator for RSP, tested positive for RSPaV. The serological methods developed in this study are advantageous as compared with biological indexing because they are more rapid, less expensive, as reliable, and more suitable for assays of a large number of samples.

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.

Comparative reactions of recombinant papaya ringspot viruses with chimeric coat protein (CP) genes and wild-type viruses on CP-transgenic papaya

Transgenic papaya cultivars SunUp and Rainbow express the coat protein (CP) gene of the mild mutant of papaya ringspot virus (PRSV) HA. Both cultivars are resistant to PRSV HA and other Hawaii isolates through homology-dependent resistance via post-transcriptional gene silencing. However, Rainbow, which is hemizygous for the CP gene, is susceptible to PRSV isolates from outside Hawaii, while the CP-homozygous SunUp is resistant to most isolates but susceptible to the YK isolate from Taiwan. To investigate the role of CP sequence similarity in overcoming the resistance of Rainbow, PRSV HA recombinants with various CP segments of the YK isolate were constructed and evaluated on Rainbow, SunUp and non-transgenic papaya. Non-transgenic papaya were severely infected by all recombinants, but Rainbow plants developed a variety of symptoms. On Rainbow, a recombinant with the entire CP gene of YK caused severe symptoms, while recombinants with only partial YK CP sequences produced a range of milder symptoms. Interestingly, a recombinant with a YK segment from the 5' region of the CP gene caused very mild, transient symptoms, whereas recombinants with YK segments from the middle and 3' parts of the CP gene caused prominent and lasting symptoms. SunUp was resistant to all but two recombinants, which contained the entire CP gene or the central and 3'-end regions of the CP gene and the 3' non-coding region of YK, and the resulting symptoms were mild. It is concluded that the position of the heterologous sequences in the recombinants influences their pathogenicity on Rainbow.

Evidence that resistance in squash mosaic comovirus coat protein-transgenic plants is affected by plant developmental stage and enhanced by combination of transgenes from different lines

Three transgenic lines of squash hemizygous for the coat protein genes of squash mosaic virus (SqMV) were shown previously to have resistant (SqMV-127), susceptible (SqMV-22) or recovery (SqMV-3) phenotypes. Post-transcriptional gene silencing (PTGS) was the underlying mechanism for resistance of SqMV-127. Here, experiments conducted to determine the mechanism of the recovery phenotype and whether enhanced resistance could be obtained by combining transgenes from susceptible and recovery plants are reported. Upper leaves of SqMV-3 plants were sampled for Northern analysis at 17, 31 and 45 days after germination (DAG) and a proportion of plants were inoculated with SqMV. SqMV-3 plants inoculated at a young stage (17 DAG) showed susceptible or recovery phenotypes. However, a number of plants inoculated at later developmental stages (31 or 45 DAG) were resistant to infection. Resistance of recovery plants was due to PTGS that was activated at a later developmental stage, independent of virus infection. Similar results were observed with plants grown under field conditions. To investigate the interactions of transgenes, progeny of crosses between SqMV-127, -3 and -22 were inoculated with SqMV. Progeny with the transgene of line 127 were resistant. However, a number of plants with transgenes from the recovery and susceptible lines or the self-pollinated recovery line were resistant even when inoculated at a young stage. Northern analysis suggested that resistance was due to PTGS. The results reveal that the timing of PTGS and consequent resistance of the transgenic plants were affected by their developmental stage and the interaction of transgene inserts.

A single chimeric transgene derived from two distinct viruses confers multi-virus resistance in transgenic plants through homology-dependent gene silencing

We showed previously that 218 and 110 bp N gene segments of tomato spotted wilt virus (TSWV) that were fused to the non-target green fluorescent protein (GFP) gene were able to confer resistance to TSWV via post-transcriptional gene silencing (PTGS). N gene segments expressed alone did not confer resistance. Apparently, the GFP DNA induced PTGS that targetted N gene segments and the incoming homologous TSWV for degradation, resulting in a resistant phenotype. These observations suggested that multiple resistance could be obtained by replacing the GFP DNA with a viral DNA that induces PTGS. The full-length coat protein (CP) gene of turnip mosaic virus (TuMV) was linked to 218 or 110 bp N gene segments and transformed into Nicotiana benthamiana. A high proportion (4 of 18) of transgenic lines with the 218 bp N gene segment linked to the TuMV CP gene were resistant to both viruses, and resistance was transferred to R(2) plants. Nuclear run-on and Northern experiments confirmed that resistance was via PTGS. In contrast, only one of 14 transgenic lines with the TuMV CP linked to a 110 bp N gene segment yielded progeny with multiple resistance. Only a few R(1) plants were resistant and resistance was not observed in R(2) plants. These results clearly show the applicability of multiple virus resistance through the fusion of viral segments to DNAs that induce PTGS.

A minimum length of N gene sequence in transgenic plants is required for RNA-mediated tospovirus resistance

We showed previously that transgenic plants with the green fluorescent protein (GFP) gene fused to segments of the nucleocapsid (N) gene of tomato spotted wilt virus (TSWV) displayed post-transcriptional gene silencing of the GFP and N gene segments and resistance to TSWV. These results suggested that a chimeric transgene composed of viral gene segments might confer multiple virus resistance in transgenic plants. To test this hypothesis and to determine the minimum length of the N gene that could trans-inactivate the challenging TSWV, transgenic plants were developed that contained GFP fused with N gene segments of 24-453 bp. Progeny from these plants were challenged with: (i) a chimeric tobacco mosaic virus containing the GFP gene, (ii) a chimeric tobacco mosaic virus with GFP plus the N gene of TSWV and (iii) TSWV. A number of transgenic plants expressing the transgene with GFP fused to N gene segments from 110 to 453 bp in size were resistant to these viruses. Resistant plants exhibited post-transcriptional gene silencing. In contrast, all transgenic lines with transgenes consisting of GFP fused to N gene segments of 24 or 59 bp were susceptible to TSWV, even though the transgene was post-transcriptionally silenced. Thus, virus resistance and post-transcriptional gene silencing were uncoupled when the N gene segment was 59 bp or less. These results provide evidence that multiple virus resistance is possible through the simple strategy of linking viral gene segments to a silencer DNA such as GFP.

Rupestris stem pitting associated virus-1 consists of a family of sequence variants

Rupestris stem pitting (RSP) seems to be one of the most widespread virus diseases of grapevines. A virus, designated as rupestris stem pitting associated virus-1 (RSPaV-1), is consistently associated with, and likely to be the causative agent of RSP. Sequence analyses of cDNA clones derived from several RSP-affected grapevines suggested that a family of sequence variants of RSPaV-1 was associated with RSP. The genome structure of the sequence variants is identical to that of RSPaV-1 in that they had five open reading frames (ORF) and sequence identities ranging from 75 to 93% in nucleotide sequence and from 80 to 99% in amino acid sequence. ORF5 (coat protein) and the carboxyl-terminal portion of ORF1 (replicase) appeared to be the most conserved regions. The coat proteins of the sequence variants exhibited highly similar antigenic indices, suggesting serological relatedness among them. The cDNA clones obtained through reverse transcription-polymerase chain reaction from RSP-infected grapevines were heterogeneous in nt sequence with identities of 77-99% relative to RSPaV-1. Furthermore, a number of sequence variants were identified in several grapevines infected with RSP. Baselines for defining RSPaV-1 and possible mechanisms accounting for infection of grapevines with multiple sequence variants of RSPaV-1 are proposed. Findings from this study should have practical applications toward understanding the etiology of RSP and developing reliable assays to rapidly detect the disease.

Turnip mosaic potyvirus resistance in Nicotiana benthamiana derived by post-transcriptional gene silencing

The coat protein (CP) gene of turnip mosaic potyvirus isolate ESC8 (TuMV-ESC8) was cloned and sequenced. Comparisons of the 867-nucleotide (nt) CP region with those of 11 TuMV isolates showed 86.7-89.3% nucleotide identity and 92.4-95.5% amino acid identity. The CP gene was cloned into a plant expression vector and transformed into Nicotiana benthamiana plants via Agrobacterium tumefaciens-mediated leaf disk transformation. Progeny from R0 lines was screened for resistance to TuMV-ESC8. Five of 29 tested lines showed TuMV protection in more than 50% of their progeny. Interestingly, some of the resistant plants transformed with the CP gene of TuMV displayed mild mosaicism in the new growing leaves at the later stages of evaluation; but these mosaic symptoms disappeared when the leaves were fully expanded. Collective data from steady-state RNA analysis and nuclear run-on assay of a line showed that the resistance was RNA-mediated through the post-transcriptional gene silencing mechanism.

Comparative Virus Resistance and Fruit Yield of Transgenic Squash with Single and Multiple Coat Protein Genes

Five transgenic squash lines expressing coat protein (CP) genes from cucumber mosaic cucumovirus (CMV), zucchini yellow mosaic potyvirus (ZYMV), and watermelon mosaic virus 2 potyvirus (WMV 2) were analyzed in the field for their reaction to mixed infections by these three viruses and for fruit production. Test plants were exposed to natural inoculations via aphids in trials simulating the introduction of viruses by secondary spread from mechanically infected susceptible border row plants. Plants of transgenic line CZW-3 expressing the CP genes from CMV, ZYMV, and WMV 2 displayed the highest level of resistance with no systemic infection, although 64% exhibited localized chlorotic dots which were mainly confined to older leaves. CZW-3 plants had a 50-fold increase in marketable yield compared to controls and the highest predicted cash returns. Plants of transgenic line ZW-20 expressing the CP genes from ZYMV and WMV 2 displayed high levels of resistance to these two potyviruses, but 22% became infected by CMV. However, ZW-20 plants provided a 40-fold increase in marketable yield relative to controls and good estimated cash returns. Three transgenic lines expressing single CP genes from either ZYMV (line Z-33), WMV 2 (line W-164) or CMV (line C-14) developed systemic symptoms similar to those of controls but showed a delay of 2 to 4 weeks before the onset of disease. Plants of transgenic line Z-33 were highly resistant to ZYMV but not to WMV 2 and CMV. Interestingly, Z-33 plants had a 20-fold increase in marketable yield compared to controls and some predicted cash returns if market sale prices were high. This study indicates that virus-resistant transgenic lines are economically viable even if they are affected by viruses other than those to which they are resistant.

Leafroll Virus Is Common in Cultivated American Grapevines in Western New York

American grapevines (Vitis labrusca L. 'Niagara'; Vitis × labruscana L. H. Bailey 'Concord' and 'Catawba'; V. labrusca × V. riparia Michx. 'Elvira') from 24 vineyards in the New York portion of the Lake Erie production region (>13,000 ha cultivated) were tested to explore a possible relationship between virus infection and an unexplained fruit set malady in the district. One-year-old cane segments were collected 4 to 6 weeks before budbreak from 65 individual vines, which previously had been identified as malady positive or negative. Preparations from bark scrapings were tested for the presence of double-stranded (ds) RNA and for fan leaf degeneration virus, tobacco streak virus, and grapevine leafroll associated closterovirus-3 (GLRaV-3) by enzyme-linked immunosorbent assay (ELISA). Mechanical transmission of other potential viruses to Chenopodium quinoa was attempted with sap extracted from young shoots forced from intact segments of sampled canes. GLRaV-3 was detected in 17 (26%) of the sampled vines from eight (33%) of the vineyards, but there was no apparent relationship between infected vines and the fruit set malady. Vines of all four cultivars were infected. dsRNA was detected in all 17 samples positive for GLRaV-3 plus four additional samples. No other viruses were detected. Near harvest, nine vines (from two vineyards) previously testing positive for GLRaV-3 were examined and retested; all nine tested positive again, although none showed any overt symptoms of viral infection. This is believed to be the first report of GLRaV-3 from American grape vineyards in New York. The source of these infections is unknown: all vines were self rooted, the individual vineyards had been planted independently at different times, and V. vinifera and its hybrids are rare in the district. Wild grapevines (primarily V. riparia) are abundant in the region, although it has been reported that leafroll disease does not occur naturally in wild North American grapes (1). Nevertheless, our results indicate that cultivated American grapevines can be common reservoirs of GLRaV-3, and furthermore suggest the need to reassess the possibility that wild grapes also may serve as reservoirs of the virus. Trials are currently underway to determine possible effects of GLRaV-3 on cv. Concord, the most widely planted variety in the region. Reference: (1) A. C. Goheen. 1988. Leafroll. Page 52 in: Compendium of Grape Diseases. R. C. Pearson and A. C. Goheen, eds. American Phytopathological Society, St. Paul, MN.

Nucleotide sequence and genome structure of grapevine rupestris stem pitting associated virus-1 reveal similarities to apple stem pitting virus

Rupestris stem pitting (RSP), a component of the rugose wood complex, is one of the most widespread graft-transmissible diseases of grapevines. Here we report on the consistent association of a high molecular mass dsRNA (ca. 8.7 kbp) with RSP. The dsRNA was reverse-transcribed and cDNAs generated were cloned into Lambda ZAP II. Sequence analysis of the cDNA clones showed that the dsRNA was of viral origin and the putative virus was designated rupestris stem pitting associated virus-1 (RSPaV-1). The genome of RSPaV-1 consists of 8726 nt excluding a poly(A) tail at the 3' terminus. It has five potential open reading frames which have the capacity to code for the replicase (ORF1), the triple gene block (ORF2-4) and the coat protein (ORF5). Comparison of the genome structure and nucleotide and amino acid sequences indicated similarities of RSPaV-1 to apple stem pitting virus, and to a lesser extent, to potato virus M carlavirus. The possibility that different strains of RSPaV-1 or other viruses are associated with RSP is discussed.

Transgenic peanut plants containing a nucleocapsid protein gene of tomato spotted wilt virus show divergent levels of gene expression

The nucleocapsid protein (N) gene of the lettuce isolate of tomato spotted wilt virus (TSWV) was inserted into peanut (Arachis hypogaea L.) via microprojectile bombardment. Constructs containing the hph gene for resistance to the antibiotic hygromycin and the TSWV N gene were used for bombardment of peanut somatic embryos. High frequencies of transformation and regeneration of plants containing the N gene were obtained. Southern blot analysis of independent transgenic lines revealed that one to several copies of the N gene were integrated into the peanut genome. Northern blot, RT-PCR and ELISA analyses indicated that a gene silencing mechanism may be operating in primary transgenic lines containing multiple copy insertions of the N transgene. One transgenic plant which contained a single copy of the transgene expressed the N protein in the primary transformant, and the progeny segregated in a 3 :1 ratio based upon ELISA determination.

Nucleotide sequence of the 3'-terminal two-thirds of the grapevine leafroll-associated virus-3 genome reveals a typical monopartite closterovirus

The RNA genome of grapevine leafroll-associated closterovirus-3 (GLRaV-3) was cloned as a cDNA generated from GLRaV-3-specific dsRNA, and a partial genome sequence of 13154 nucleotides (nt) including the 3' terminus was determined. The sequenced portion contained 13 open reading frames (ORFs) potentially encoding, in the 5'-3' direction, proteins of > 77 kDa (ORF1a; helicase, HEL), 61 kDa (ORF1b; RNA-dependent RNA polymerase, RdRp), 6 kDa (ORF2), 5 kDa (ORF3, small transmembrane protein), 59 kDa (ORF4; heat shock protein 70, HSP70), 55 kDa (ORF5), 35 kDa (ORF6; coat protein, CP), 53 kDa (ORF7; diverged coat protein, CPd), 21 kDa (ORF8), 20 kDa (ORF9), 20 kDa (ORF10), 4 kDa (ORF11), 7 kDa (ORF12), and an untranslated region of 277 nt. ORF1b is probably expressed via a +1 ribosomal frameshift mechanism, most similar to that of lettuce infectious yellows virus (LIYV). Phylogenetic analysis using various gene sequences (HEL, RdRp, HSP70 and CP) clearly demonstrated that GLRaV-3, a mealybug-transmissible closterovirus, is positioned independently from aphid-transmissible monopartite closteroviruses (beet yellows, citrus tristeza and beet yellows stunt) and whitefly-transmissible bipartite closterovirus (lettuce infectious yellows, LIYV). However, another alleged mealybug-transmissible closterovirus, little cherry virus, was shown to be more closely related to the whitefly-transmissible LIYV than to GLRaV-3.

Nucleotide sequence and genome organization of grapevine leafroll-associated virus-2 are similar to beet yellows virus, the closterovirus type member

The entire genome of grapevine leafroll-associated closterovirus-2 (GLRaV-2), except the exact 5' terminus, was cloned and sequenced. The sequence encompasses nine open reading frames (ORFs) which include, in the 5' to 3' direction, an incomplete ORF1a encoding a putative viral polyprotein and eight ORFs that encode proteins of 52 kDa (ORF1b), 6 kDa (ORF2), 65 kDa (ORF3), 63 kDa (ORF4), 25 kDa (ORF5), 22 kDa (ORF6), 19 kDa (ORF7) and 24 kDa (ORF8) respectively, and 216 nucleotides of the 3' untranslated region. An incomplete ORF1a potentially encoded a large polyprotein containing the conserved domains characteristic of a papain-like protease, methyltransferase and helicase. ORF1b potentially encoded a putative RNA-dependent RNA polymerase. The expression of ORF1b may be via a +1 ribosomal frameshift mechanism, similar to other closteroviruses. A unique gene array, which is conserved in other closteroviruses, was also identified in GLRaV-2; it includes genes encoding a 6 kDa small hydrophobic protein, 65 kDa heat shock protein 70, 63 kDa protein of function unknown, 25 kDa coat protein duplicate and 22 kDa coat protein. Identification of ORF6 (22 kDa) as the coat protein gene was further confirmed by in vivo expression in E. coli and immunoblotting. Phylogenetic analysis comparing different genes of GLRaV-2 with those of other closteroviruses demonstrated a close relationship with beet yellows virus (BYV), beet yellow stunt virus and citrus tristeza virus. GLRaV-2 is the only closterovirus, so far, that matches the genome organization of the type member of the group, BYV, and thus can be unambiguously classified as a definitive member of the genus Closterovirus.