[Scientific Papers] http://www.scipapers.com    2007-11-16  

Plants, thrips, Tospoviruses ¨C the enigmatic triad

Mound L

CSIRO Entomology, Canberra, Australia.

Correspondence: Laurence.Mound@csiro.au

This talk will examine the inter-relationships between the three elements involved in this crop protection triangle, emphasizing the dependence of tospoviruses on thrips despite less than 0.2% of the 5500 known thrips species being recorded as vectors. However, the members of each of the three elements are remarkably variable within and between populations, thus making investigations unusually complex. The inter-relationships between the three types of biologists involved in such studies will therefore be considered, and the effectiveness of their collaborative research efforts. Finally consideration will be given to how we can investigate collaboratively the origins and trajectory of this horticultural equivalent of the AIDS epidemic.

A bunyaviral-type virus with unusual genome structure is associated with the ringspot disease of European mountain ash (Sorbus aucuparia L.)

Muehlbach HP, Mielke N, Benthack W, Schlatermund N

University of Hamburg, Biocentre Klein Flottbek and Botanical Garden, Ohnhorststrasse 18, D-22609 Hamburg, Germany.

Correspondence: muehlbach@botanik.uni-hamburg.de

European mountain ash trees (Sorbus aucuparia L.) suffer from a widespread disease, which is characterized by ringspot and mottling symptoms on leaves and by a gradual decay in general. We could isolate double stranded RNA (dsRNA) from symptomatic tissue, which is indicative of viral infection. Cloning and sequencing of putative viral RNAs allowed the characterization of a new virus associated with the mountain ash disease (Benthack et al. 2005. Archives of Virology 150: 37¨C52.). Fractions of dsRNA were extracted by column chromatography. A pattern of four dsRNA bands of approximately 7 kb, 2.3 kb, 1.5 kb, and 1.3 kb, respectively, was found in leaf samples of symptomatic mountain ash trees from various sites in Germany. No dsRNA was detected in asymptomatic trees. By random primed reverse transcription, DOP-PCR (degenerate oligonucleotide primed PCR), cloning and sequencing, dsRNA-specific cDNA fragments were obtained. Using 5¡ä-RACE analyses (rapid amplification of cDNA ends), modified by biotin labelling and magnetic separation, longer cDNAs could be enriched. With our cloning strategy a cDNA of 7.0 kb in length was obtained first. The corresponding RNA harbours one ORF with homology to the RNA dependent RNA polymerase (RdRP) of members of the family Bunyaviridae. It shows all conserved sequence motifs of the bunyaviral RdRP and also the typical terminus sequences at its 5¡ä- and 3¡ä-end. Primers derived from the terminus sequences allowed the subsequent identification of three further RNAs of 2.3, 1.5 and 1.3 kb. The corresponding ORFs encode a putative glycoprotein precursor, a putative nucleocapsid protein, and a protein of unknown function. In situ hybridization studies using digoxigenin labelled riboprobes for the viral RNA 1 and RNA 3 showed a scattered pattern of virus accumulation in the mesophyll of mountain ash leaves. The dsRNA pattern, the sequence information and the present image of the viral genome organisation strongly indicate that a new plant RNA virus with some similarity to Bunyaviridae is associated with the mountain ash ringspot disease. (Benthack W et al. 2005)

Environmental factors affecting Tomato spotted wilt virus (TSWV) symptom expression

Mullis S, Csinos A, Gitaitis R

Department of Plant Pathology, University of Georgia, P.O.Box 748, Tifton GA 31793

Correspondence: swmullis@uga.edu

Many environmental factors play a vital role in the expression of the Tomato spotted wilt virus (TSWV) disease. Temperature, rainfall, soil types, farmscape and thrips population density are factors that have been shown to have an impact on the expression of TSWV symptoms. While many plants can remain non-symptomatic, disease expression is the devastating aspect of TSWV infections. The degree and severity of the symptom expression is a complex set of events and factors that include the species of plant, plant age, plant condition, viral strain, and environmental conditions. Over a three year period, the timing of TSWV infections and symptom expression in tobacco plants were correlated with the overlying environmental factors affecting expression of the disease. Test plots were observed for a myriad of symptoms that ranged from single local lesions to plant desiccation due to TSWV. DAS-ELISA was used to determine the presence TSWV infection, and any symptoms that were present were documented for severity, type, and location. Rainfall, temperature, soil types, farmscape layout, and thrips density were evaluated for relation to disease expression. Although some of these factors are directly related to the ability of thrips to transmit TSWV to susceptible plants, this study concentrated on how these conditions affected TSWV disease expression.

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Complete sequence of the glycoprotein genes of four tospovirus species belonging to the American cluster

Nagata T¹, Lovato FA³, Carvalho KR¹, Sodr RA², Inoue-Nagata AK⁴, Ávila AC⁴, Resende RO³

¹Ci¨ºncias Genômicas e Biotecnologia, Universidade Cat¨®lica de Bras¨ªlia, DF, Brazil. ²Ci¨ºncias Biol¨®gicas, Universidade Cat¨®lica de Bras¨ªlia, DF, Brazil.

³Dept. Biol. Cel. Universidade Cat¨®lica de Bras¨ªlia, DF, Brazil..

⁴Empresa Brasileira de Pesquisa Agropecu¨¢ria -Hortaliças. Bras¨ªlia, DF, Brazil.

Correspondence: rresende@unb.br

Several tospovirus species are often found in Brazil causing severe economic losses in horticultural and ornamental crops. Their N genes are fully sequenced, however, some important genes as the glycoprotein precursor (G) are still to be characterized. Due to its essential roles in the virus-vector interaction, the G gene of Tomato chlorotic spot virus (TCSV), Groundnut ringspot virus (GRSV) Zucchini lethal chlorosis virus (ZLCV) and Chrysanthemum stem necrosis virus (CSNV) were cloned and sequenced. Approximately 85% of the genomic segment of the M-RNA (4 Kbps) of all the four species was amplified by RT-PCR using specific and degenerated primers and cloned in the plasmid vector, pCR4 (Invitrogen). TCSV and GRSV shared 92% identity, while with Tomato spotted wilt virus (TSWV) identity were 81 and 79%, respectively. The G sequence of ZLCV had the highest identity of 76% with CSNV and 74% with TSWV, while CSNV was closely related to TSWV sharing 78% identity. Phylogenic analysis of the glycoprotein genes corroborated the close relationship among the tospoviruses belonging to the American cluster. The parameters on tospovirus taxonomic classification are discussed. Financial Support: CNPq (CBAB), Embrapa, UnB, UCB, FINATEC

Epidemiology of Tospoviruses in South and Southeast Asia: Current status and future prospects

Naidu RA¹, Adkins S², Ravi KS³, Chiemsombat P⁴, Jain RK⁵, Savithri HS⁶, Gajanandana O⁷, Muniyappa V⁸, Riley DJ⁹

¹Washington State University-IAREC, Prosser, WA, 99350, USA.

²USDA-ARS-USHRL, Fort Pierce, FL 34945, USA

³Mahyco Research Center, Jalna 431203, India

⁴Kasetsart University, Kamphaeng Saen Campus, Nakorn Pathom 73140, Thailand; ⁵Indian Agricultural Research Institute, New Delhi 110012, India

⁶Indian Institute of Science, Bangalore 560 024, India

⁷National Center for Genetic Engineering and Biotechnology, Pathumthani 12120, Thailand

⁸University of Agricultural Sciences, Hebbal Campus, Bangalore 560 024, India

⁹The University of Georgia, Coastal Plain Experiment Station, Tifton, GA 31793, USA.

Correspondence: naidu@wsu.edu

At least seven of the fourteen established tospoviruses occur in South and Southeast Asia (S&SEA). Peanut bud necrosis virus (PBNV), Peanut yellow spot virus (PYSV) and Watermelon bud necrosis virus (WBNV) were reported from India. Among them, PBNV and WBNV belong to serogroup IV and PYSV belong to serogroup V. The occurrence of Iris yellow spot virus (IYSV) has recently been reported in onion for the first time from India. Studies on the epidemiology of PBNV have indicated that the virus has a broad geographic range and infects several economically important crops in the family Leguminoceae and Solanaceae. In addition, other crops like cotton and sesame, and several weed species were found to be naturally infected with PBNV. Serology and comparative analysis of nucleocapsid gene sequence of PBNV isolates from different host plants revealed that they are closely related. The host range of WBNV appears to be restricted to members of the family Cucurbitaceae. The host range of PYSV is not yet determined. PBNV and WBNV were reported to be vectored by Thrips palmi. Scritothrips dorsalis was shown to be the principal vector of PYSV. The presence of Thrips tabaci in India, the main vector of IYSV infecting onion in other countries, is yet to be known.

PBNV, Watermelon silver mottle virus (WSMV), Melon yellow spot virus and isolates related to Capsicum chlorosis virus (CaCV), all belonging to serogroup IV, have been documented in Thailand. Recent studies, based on serology and nucleotide sequence analyses of the nucleocapsid gene, have shown that WSMV and MYSV and tospovirus isolates related to CaCV infect several economically important crops in the family Solanaceae and Leguminoceae. Besides PBNV, tospovirus isolates related to CaCV were found in peanuts. Although T. palmi was observed predominantly in cucurbits and S. dorsalis was found to be dominant in peanuts, their ability to transmit these tospoviruses has not been studied. Recently, Ceratothrips claratris has been reported as a competent vector of a tospovirus closely related to CaCV. Frankliniella occidentalis (Western Flower Thrips), the principal vector of many tospoviruses in different countries around the world, has not yet been recorded in S&SEA. The occurrence of IYSV in India and isolates of CaCV in Thailand may suggest the expansion of tospoviruses to new geographic regions, perhaps due to changes in agricultural practices and shifting cropping patterns and international trade and commerce. Consequently, improved knowledge of tospovirus diversity and tospovirus-vector thrips species interactions is a necessary precursor for the deployment of comprehensive integrated disease management strategies to mitigate losses caused by different tospoviruses in S&SEA.