Volume 2 Number 1 2008
CONTENTS AND ABSTRACTS
Corina P. D. Brussaard, Joaquin Martinez Martinez (The Netherlands) Algal Bloom Viruses (pp 1-13)
Invited Review: Although viral infections of vascular plants have been studied intensively for a long time already, it is only relatively recent that the ecological importance of algal viruses was recognized. The scientific field of phycovirology is exciting and rapidly changing as new viruses are isolated on a regular basis. The biology of algal viruses is diverse; including DNA and RNA viruses, double-stranded as well as single-stranded, ranging in genome size from a few to over 500 kb. The various sorts of algae play significant roles in aquatic ecology, with the so-called algal bloom forming species being able to occur in high abundance. Many of the algal viruses in culture infect bloom-forming algal hosts, probably as a result of the extra effort put into studying algal blooms for socio-political and economical reasons, and the high abundance of host cells. In the following review, a synopsis of the available information on viruses infecting bloom-forming algal species will be provided. It will describe the discovery, characteristics and molecular biology of the algal bloom viruses. The ecological impact of these algal bloom viruses on the hostfs population dynamics, ecosystem functioning and biogeochemical cycling will be outlined. Finally, challenges for the future of algal virology will be discussed.
Claire Peltier, Kamal Hleibieh (France), Heike Thiel (Germany), Elodie Klein (France), Claude Bragard (Belgium), David Gilmer (France) Molecular Biology of the Beet necrotic yellow vein virus (pp 14-24)
Invited Review: Beet necrotic yellow vein virus is the etiological agent of one of the most devastating sugar beet viral diseases: rhizomania. This review describes the molecular biology of the rhizomania disease, the functions of the BNYVV encoded proteins, the consequences of their expression as well as the biology of the BNYVV vector, Polymyxa betae. Root proliferation is an important part of the well-known characteristics of the viral infection that leads to yield and sugar losses. The extensive use of sugar beet cultivars displaying partial resistance or tolerance against virus infection allows containment of sugar yields. However, such extensive uses also permit viruliferous vector to be amplified and therefore the appearance of resistance breaking isolates. We review as well the defence strategies that may be used against rhizomania.
Seema Mandal, Bikash Mandal, Qazi Mohd. Rizwanul Haq, Anupam Varma (India) Properties, Diagnosis and Management of Cucumber green mottle mosaic virus (pp 25-34)
Invited Review: Cucumber green mottle mosaic virus (CGMMV), a species under the genus Tobamovirus, is an important viral pathogen affecting cucurbit crops in Asia and Europe. The virion is rod shaped particle of 300 ~ 15 nm containing positive sense ssRNA of 6.4 kb as its genome. No insect vector is known to transmit the virus in a specific manner. The virus is highly stable, contagious and sap transmissible. Several isolates have been described from different countries in Eurasian regions infecting important cucurbits such as bottle gourd (Lagenaria siceraria), cucumber (Cucumis sativus), gherkin (Cucumis anguria), muskmelon (Cucumis melo) and watermelon (Citrullus vulgaris). Symptomatology of the virus is characterized by systemic green mottle mosaic on foliage. Some of the isolates induce serious fruit symptoms such as pulp deterioration (eblood fleshf disease) in watermelon. The complete genome of CGMMV is composed of 6424 nucleotides containing four open reading frames encoding replicase proteins, movement protein and coat protein. The CGMMV isolates form a distinct evolutionary clade separated from the other cucurbit-infecting tobamovirus species. Several sensitive methods are available for reliable diagnosis of CGMMV of which ELISA and RT-PCR are commonly used. Immunofilter paper assay and bioelectric recognition assay have been shown for rapid test of CGMMV. Control of CGMMV is difficult as commercial cultivars are susceptible. Limited source of resistance is known in muskmelon and wild Cucumis spp. The transgenic approach holds a greater potential in developing resistant varieties against CGMMV in any cucurbit.
Marcelo Agenor Pavan, Renate Krause-Sakate, Norberto da Silva, Francisco Murilo Zerbini (Brazil), Olivier Le Gall (France) Virus Diseases of Lettuce in Brazil (pp 35-41)
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Invited Review: Several viruses have been reported to infect lettuce. The most important is Lettuce mosaic virus (LMV), a potyvirus found worldwide transmitted by seeds and aphids, in a non-persistent manner. LMV causes quite variable symptoms, including mosaic, dwarfing, failure to form proper heads, and sometimes necrotic reactions. Cultivars carrying the mo11 and mo12 genes have resistance to the common strains, although most (mo breaking seed transmitted) strains can overcome this resistance. At least three species of tospovirus, including Tomato spotted wilt virus (TSWV), Tomato chlorotic spot virus (TCSV) and Groundnut ringspot virus (GRSV) causes significant losses, especially during summer, in which high populations of thrips vectors can be found in the field. Tospoviruses causes systemic necrosis and plant death. During the cooler season (May to September), two viruses have been found associated with big-vein disease, Mirafiori lettuce big-vein virus (MLBVV) which belongs to the genus Ophiovirus and Lettuce big-vein associated virus (LBVaV; genus Varicosavirus). LBVaV and MLBVV are both transmitted by the soil-borne fungus Olpidium brassicae. Lettuce mottle virus (LeMoV, genus Sequivirus); Cucumber mosaic virus (CMV, genus Cucumovirus), and Bidens mosaic virus (BiMV, genus Potyvirus) are also found to cause mosaic symptoms on lettuce, although their incidence in lettuce fields is low throughout the year. Epidemiological aspects, variability of viruses, methods of control, genetic variability for lettuce resistance and breeding programs will be discussed.
Hortense A. Diallo (Ivory Coast), Edwin J. Anderson, Rose C. Gergerich (USA) Early Events in Cowpea Stunt Disease: Symptomatology, Virus Concentration and Localization (pp 42-51)
Original Research Paper: Cowpea (Vigna unguiculata (L.) subsp. unguiculata) cv. eCoronetf seedlings inoculated with crude extracts from plants infected with Cucumber mosaic cucumovirus (CMV), Blackeye cowpea mosaic potyvirus (BlCMV), and a mixture of both extracts were observed for symptom development over a period of time. By eight days after inoculation, plants doubly infected with CMV and BlCMV developed more severe foliar symptom in addition to stem and petiole necrosis, characteristic of cowpea stunt disease. A time-course enzyme-linked immunosorbent assay (ELISA) conducted showed that at 15 days post-inoculation, the concentration of the non-Potyvirus increased in the mixed infection. Surprisingly, during early stages of infection (between six and eight days after inoculation), although plants mixedly-infected with CMV and BlCMV displayed more severe symptoms than either single infection, no enhancement of CMV accumulation occurred in the plant parts tested (i.e., primary leaves, first trifoliolate and stems). Therefore, increased symptom severity in the early stage of cowpea stunt disease was not correlated with an increase in CMV accumulation. Light microscopical studies indicated that the necrosis in the mixed infection was internal (pith and medullary ray) as well as external (epidermis and cortex). Electron microscopy studies showed that in the stems of mixedly infected plants, CMV and BlCMV particles were present in both epidermal and cortical cells. Most cortical cells were collapsed and densely stained. Cells adjacent to the collapsed areas showed increased vacuolation with large numbers of BlCMV particles, an increase in the number of hypertrophied mitochondria and abnormal chloroplasts. The increase in symptom severity observed during early infection was not due to the viruses infecting different tissues in single versus mixed infection.
Hortense A. Diallo (Ivory Coast), Wendy Monger (UK), Nazaire K. Kouassi, Thierry D. Yoro (Ivory Coast), Phil Jones (UK) Occurrence of Papaya ringspot virus Infecting Papaya in Ivory Coast (pp 52-57)
Original Research Paper: Papaya orchards located in different production areas in Ivory Coast as well as papaya plants growing in the district of Abidjan were surveyed in early February 2006. A wide range of leaf symptoms including mosaic, yellowing, chlorotic-line patterns, downward and upward curving of margins, and shoe-stringing was observed in the field. Ringspots were also observed on some stems and fruits. Sixty leaf samples of which fifty seven symptomatic samples were collected in the field for virus identification. In order to test for the presence of Papaya ringspot virus (PRSV) and Papaya mosaic virus (PapMV), the double antibody sandwich-enzyme linked immuno-sorbent assay (DAS-ELISA) was conducted using leaf extracts made from dried samples. None of the samples was positive for PaMV. Nine samples out of the 60 tested, coming from three locations, were positive for PRSV. Transmission electron microscope observation of leaf-dip preparation of the ELISA-positive samples revealed flexuous particles characteristic of potyviruses. To confirm the identity of PRSV, a 676 bp RNA fragment representing part of the coat protein gene with the 3f-untranslated region, was amplified by reverse transcription-polymerase chain reaction (RT-PCR) from one PRSV-positive sample (An 1) using appropriate primers. The nucleotide sequence of the PCR product (Genbank Accession Number DQ84023) confirmed the taxonomy of the virus as being PRSV (Diallo et al. 2007). The PRSV Anyama isolate (An1) of PRSV identified in this study shows 97 % identity with PRSV strain P (PAYP) (Genbank Accession Number D00595). There is a possibility of other papaya infecting viruses present in the country.
Pamela D. McLaughlin, Wayne A. McLaughlin, Douglas P. Maxwell, Marcia E. Roye (Jamaica) Preliminary Identification of Begomoviruses Infecting Crops and Weeds in Belize (pp 58-63)
Original Research Paper: Cultivated plants, including pepper, red kidney bean, squash, string bean and tomato, as well as weeds with viral symptoms were collected from five districts in Belize over a three year period with the aim of determining the diversity of the begomoviruses present. Sixty five percent (111 / 171) of the samples screened via DNA hybridization produced signals indicative of begomovirus infection. Subsequent PCR amplifications and nucleotide sequence analyses revealed the presence of four begomoviruses in Belize. Pepper golden mosaic virus and Tomato mottle virus were found associated with tomato and sweet pepper and the former was also isolated from hot pepper. Merremia mosaic leaf curl virus was found infecting hot pepper, sweet pepper and the weed species Euphorbia heterophylla. Euphorbia mosaic virus was found in hot pepper and Euphorbia. This is the first report of the identification of begomoviruses in Belize.