- Author: Kathy Keatley Garvey
It did: their research revealed how TSWV (family Tospoviridae, order Bunyavirales) packages its RNA genome, a crucial step in virus infection.
Their newly published research, “The Genome of a Bunyavirus Cannot be Defined at the Level of the Viral Particle But Only at the Scale of the Viral Population,” appears in the current edition of the Proceedings of the National Academy of Sciences (PNAS).
The 18-member research team included scientists primarily from the French National Research Institute for Agriculture, Food and Environment (INRAE) at the Campus International de Baillarguet, Montpellier; Department of Entomology, University of Wisconsin; and the Department of Entomology and Nematology, University of California, Davis.
“Our work showed the genome of TSWV can only be defined at the population level, pointing at emerging properties when viral particles infect plants in groups,” said a key author Stéphane Blanc, research director of INRAE's Biology and Genetics of Plant-Pathogen Interactions. “As most virions contain an incomplete genome, TSWV is a multi-component viral system, where co-infection and complementation are key in the life cycle. These findings open a myriad of possibly distinct properties depending on the genetic composition of the group of virions co-infecting a cell.”
“The most challenging part of this work was to create a protocol reliably quantifying the two polarities of each segment,” said lead author Michel Yvon of INRAE. “The next important advance will be to demonstrate that co-infection of cells by a group of particles is key to the spread of infection.”
Ullman, an international authority on orthotospoviruses and one of the four main authors, took a sabbatical to work on the project. “My interest was in understanding how TSWV packaged its RNA genome,” she said. “While this sounds like a simple goal, it is quite complex because TSWV has negative sense and ambisense viral strands and many research tools common to studying other viruses, such as infectious clones were not available.”
“It was a delight to work with the fantastic team of scientists that Stéphane assembled, all very talented with skills in virology, cryoelectron microscopy and nanopore PCR,” Ullman commented. “I cannot imagine a more talented and diverse group of people to conduct this difficult work. I learned a great deal about virus purification from Michel Yvon, whose leadership, skills in virology, and patient teaching really moved our project forward."
German, professor emeritus and former chair of both the Departments of Plant Pathology and Entomology at the University of Wisconsin, died Aug. 27, 2023 at age 82.
“I am indebted to my husband, Jean-Marc Leininger who frequently drove me to the laboratory in Avignon where I was able to rear thousands of virus-infected plants and to store TSWV isolate,” Ullman added. “Jean-Marc not only transported me and my virus specimens, but also learned to mechanically inoculate plants and helped with every inoculation and virus harvest.”
UC Davis postdoctoral scholar Sulley Ben-Mahmoud of the Ullman lab was among the co-authors.
Funding was provided by grants from Montpellier University of Excellence (MUSE); Floriculture and Nursery Research Initiative, U.S. Department of Agriculture, Agricultural Research Service (USDA-ARS); and the Fulbright Scholar Program. The authors also acknowledged support from
- Santé des Plantes et Environnementor Plant Health and Environment (SPE)
- Centre National de la Recherche Scientifique (CNRS)
- Institute of Research for Development (IRD)
- Institut national de la santé et de la recherche médicale (INSERM)
- Centre for International Cooperation in Agronomic Research for Development (CIRAD)
- Plant Health Institute of Montpelier (PHIM)
Ullman noted the importance of the research in her Fulbright application: “Sustainable management of insect-transmitted pathogens is a key concern for food production in France and the United States. Both countries grow many of the same crops and growers face similar challenges from insect-transmitted plant viruses. Current management strategies rely heavily on pesticides that may cause significant health and environmental concerns, including damage to bees and other pollinators, as shown with neonicotinoid pesticides. Clearly, better knowledge about these insect-transmitted viral systems…has potential to reduce pesticide use by providing novel and innovative technologies to manage orthotospoviruses and thrips in France and the United States.”
Ullman, a former chair of the Department of Entomology and a former associate dean of the UC Davis College of Agricultural and Environmental Sciences, anticipates strong research relationships between UC Davis and Montpellier that will lead to grant applications for international research and scholarly exchange opportunities for scientists, students, and post-doctoral scholars.
Significance
In their significance statement, the authors wrote: “Bunyaviruses infect animals, plants, fungi, and protists. Despite their importance, fundamental aspects of their biology as basic as the definition of their genome remain elusive. The viral genome consists of several negative or ambisense RNA segments, and virions often miss segments and/or package complementary strands. We formally quantify this heterogeneity on the species Tomato spotted wilt orthotospovirus. Within individual virus particles, the number, the identity, and the polarity of the segments are widely variable. In contrast, we show that a stable genetic composition is an emerging property of the viral population, each of the RNA segments/polarities accumulating reproducibly at a specific frequency. This resembles the genome formula of multipartite viruses, suggesting that bunyaviruses may also function as multicomponent viral systems.”
Their abstract: “Bunyaviruses are enveloped negative or ambisense single-stranded RNA viruses with a genome divided into several segments. The canonical view depicts each viral particle packaging one copy of each genomic segment in one polarity named the viral strand. Several opposing observations revealed nonequal ratios of the segments, uneven number of segments per virion, and even packaging of viral complementary strands. Unfortunately, these observations result from studies often addressing other questions, on distinct viral species, and not using accurate quantitative methods. Hence, what RNA segments and strands are packaged as the genome of any bunyavirus remains largely ambiguous. We addressed this issue by first investigating the virion size distribution and RNA content in populations of the tomato spotted wilt virus (TSWV) using microscopy and tomography. These revealed heterogeneity in viral particle volume and amount of RNA content, with a surprising lack of correlation between the two. Then, the ratios of all genomic segments and strands were established using RNA sequencing and qRT-PCR. Within virions, both plus and minus strands (but no mRNA) are packaged for each of the three L, M, and S segments, in reproducible nonequimolar proportions determined by those in total cell extracts. These results show that virions differ in their genomic content but together build up a highly reproducible genetic composition of the viral population. This resembles the genome formula described for multipartite viruses, with which some species of the order Bunyavirales may share some aspects of the way of life, particularly emerging properties at a supravirion scale.”
- Author: Kathy Keatley Garvey
They're pests of fruits, vegetable and horticultural crops, including tomatoes, grapes, strawberries and soybeans. They're barely visible to the naked eye, but oh, how disastrous they can be. Some thrips transmit plant diseases, such as the tomato spotted wilt virus.
Enter Professors George Kennedy and Diane Ullman.
Kennedy, the William Neal Reynolds Distinguished Professor of Agriculture at North Carolina State University. Raleigh, N.C., will speak on "Modeling the Epidemiology of Tomato Spotted Wilt: Understanding the Role of Thrips Population Dynamics and Virus Inoculum Sources" from 12:10 to 1 p.m., Wednesday, April 1 in 122 Briggs Hall, as part of the UC Davis Department of Entomology and Nematology's winter seminar series.
Colleague/collaborator DianeUllman will introduce him.
"Effective management of this virus in commercial crop production systems requires an understanding of the factors that determine the timing and magnitude of virus spread. This seminar will discuss the ways in which seasonal weather events influence the dispersal dynamics of vector thrips populations, the abundance of virus inoculum sources in the landscape, and ultimately the timing and magnitude of TSWV spread into susceptible crops. It will further provide an illustration of how efforts to model these relationships improved understanding of the epidemiology of TSWV and led to the development of a TSWV risk prediction tool that is now being used in pest management decision making."
Kennedy, past president of the Entomological Society of America (ESA), holds a bachelor's degree in entomology from Oregon State University and a doctorate in entomology from Cornell University. See his biosketch on the ESA website.
Ullman and Kennedy are among those organizing two consecutive mid-May conferences at the Asilomar Conference Center, Pacific Grove. International scientists, Extension specialists, and agricultural industry professionals will target thrips and insect-vectored pathogens.
The second conference is the Xth International Symposium on Thysanoptera and Tospoviruses, to be held May 16-20. "This meeting is the tenth in a series of international symposia that, over 30 years, have grown to be the dominant vehicle and venue for information exchange between scientists investigating problems related to thrips and tospoviruses around the world," Ullman said. "These symposia have been instrumental in extending knowledge and producing new solutions and innovations in thrips and tospovirus management worldwide, by providing a forum for sharing research findings and integrating fundamental and applied knowledge.
And about those thrips...
The tiny insects pierce hundreds of species of plants, sucking the nutrients and causing billions of dollars in damage to U.S. agricultural crops. As mentioned, they transmit plant viruses in the genus Tospovirus, such as the tomato spotted wilt virus. Read more on the UC IPM website.
“There are 23 additional approved and emerging tospovirus genotypes transmitted by at least 14 thrips species (Thysanoptera: Thripidae),” said Ullman, who has been researching thrips and tospoviruses since 1987.
That's something to think about when you're planting your tomatoes this spring...
- Author: Kathy Keatley Garvey
He will be hosted by colleague and collaborator Diane Ullman, professor of entomology at UC Davis.
"Thrips-transmitted, tomato spotted wilt virus (TSWV), which has an extremely broad host range and is transmitted exclusively by thrips, ranks among the most economically important plant viruses affecting crops worldwide," Kennedy says in his abstract.
"Effective management of this virus in commercial crop production systems requires an understanding of the factors that determine the timing and magnitude of virus spread. This seminar will discuss the ways in which seasonal weather events influence the dispersal dynamics of vector thrips populations, the abundance of virus inoculum sources in the landscape, and ultimately the timing and magnitude of TSWV spread into susceptible crops. It will further provide an illustration of how efforts to model these relationships improved understanding of the epidemiology of TSWV and led to the development of a TSWV risk prediction tool that is now being used in pest management decision making."
Kennedy, past president of the Entomological Society of America (ESA), is an ESA fellow, recipient of the ESA's Award for Excellence in Entomology and chaired the Entomological Foundation, a non-profit educational foundation whose mission is “to excite and educate young people about science through insects.”
He also served as program manager for the National Research Initiative, affiliated with the U.S. Department of Agriculture (USDA), and on numerous advisory panels for the USDA, U.S. Environmental Protection Agency and the National Research Council, addressing issues relating to pesticides, pest resistance, integrated pest management, and biotechnology.
Kennedy holds a bachelor's degree in entomology from Oregon State University and a doctorate in entomology from Cornel University. He served as assistant professor of entomology at UC Riverside from 1974-1976, before joining the faculty at North Carolina State University.
See his biosketch on the ESA website.
Kennedy writes on his website: "Research in my program focuses on understanding the ecology and life systems of arthropods affecting agricultural crops and applying that understanding to improve the effectiveness and sustainability of arthropod management in vegetable crops. We study fundamental interactions and processes that influence pest status, population dynamics and the insect/crop interactions that result in damage. We apply the resulting information in combination with new technologies to enhance IPM. Areas of emphasis include insect-plant interactions, resistance management, landscape scale population dynamics, and epidemiology and management of insect transmitted plant viruses. Current research projects focus on understanding the determinants of tospovirus transmission by thrips in relation to epidemiology and management of tomato spotted wilt virus and on the development of reduced risk arthropod management systems for fruiting vegetables. These efforts include both field and laboratory research and collaborations with faculty in Entomology, Horticulture and Plant Pathology at NCSU and colleagues at other institutions. We also work closely with extension colleagues, growers and the agribusiness community to facilitate implementation of new pest management practices."
The seminar will be recorded for later viewing on UCTV Seminars.
- Author: Lynn M. Sosnoskie
According to a previous ANR blog post, the 2013 processing tomato crop in California is experiencing worse-than-usual curly top problems, which is caused by a complex of Beet Curly Top Viruses (BCTVs) (Processing tomatoes face critical threat: curly top). Infections are generally lethal to young transplants. Older, more developmentally mature plants will become chlorotic and stunted with cupped leaves and curled petioles. Leaf veins will often turn purple. Fruit set can be significantly impacted (Heflebower et al. 2008).
BCTVs are members of the Geminiviridae family of viruses (specifically, the Curtovirus genera). Viruses in this family have circular, single-stranded DNA genomes that are encapsulated in a 'twinned' (hence the name 'Geminiviridae') set of icosahedral protein shells. According to Chen et al. (2010), three distinct viruses are responsible for curly top disease in the western United States: Beet Curly Top Virus (BCTV, formerly the Cal/Logan strain), Beet Mild Curly Top Virus (BMCTV, formerly the Worland strain) and Beet Severe Curly Top Virus (BCSTV, formerly the CFH strain). BMCTV and BSCTV were the predominant pathogens associated with curly top in processing and fresh market tomatoes in California, as determined by surveys conducted between 2002 and 2008 (Chen et al. 2010).
The viruses that cause curly top have an extremely wide host range (>300 species), including crops (i.e. beets, tomatoes, peppers, cucumbers, watermelon, spinach), weeds (i.e. filaree, perennial pepperweed, Russian thistle and mustard species), and native shrubs (i.e. buck brush [Ceanthus cuneatus]) (Chen et al. 2010; Creamer et al. 1996; Davis et al. 1998). Symptomology can vary dramatically among species; crops tend to exhibit obvious signs of infection, whereas weeds and perennial shrubs are generally asymptomatic (Creamer et al. 1996; Davis et al. 1998). Differences in disease pressure (and viral concentration) are likely due to the varying degrees of virulence among the virus strains, pathogen/host interations that affect viral replication and leafhopper feeding preferences.
Similar to the virus, its vector, the beet leafhopper (Circulifer tenellus), also has a wide host range (Chen et al. 2010; Creamer et al. 1996; Davis et al. 1998; Heflebower et al. 2008). Adult leafhoppers migrate into the coastal range foothills in late fall to overwinter on perennial weeds (i.e. Russian thistle). Their eggs are oviposited on annual species (i.e. buckhorn plantain) that germinate in the following spring. The leafhopper nymphs feed on infected plants, thus acquiring the virus. Mature beet leafhoppers carry the virus to the Central Valley during their spring flights. Successive generations of leafhoppers will feed on both weeds and crops (transmitting the virus while doing so) before the remaining adults migrate back to the foothills for winter. Warm and windy conditions in the springtime increase virus problems, as this facilitates the 'dry-down' of the foothills reservoir species and brings about the start of the migratory cycle (Chen et al. 2010; Creamer et al. 1996; Davis et al. 1998; Heflebower et al. 2008). Additionally, both Chen et al. (2010) and Creamer et al. (1996) suggested that the viruses could survive, year round, in host plants located within the San Joaquin Valley, thereby precluding the need for yearly re-introductions.
Curly top can be confused with tomato spotted wilt, which is caused by the Tomato Spotted Wilt Virus (TSWV, genus Tospovirus, family Bunyviridae). Tomato Spotted Wilt Virus is extremely complex, relative to the BCTVs. The TSWV virion consists of three negative, single-standed, ribonucleoprotein-encapsulated RNAs (the Large [L], Middle [M] and Small [S] stands), which are enclosed, along with an RNA-dependent polymerase, in a host-derived membrane bilayer (Sherwood et al. 2003). TSWV can infect more than 1000 species across 85 families, including: tomato, pepper, potato, eggplant, radicchio, begonia, geranium, sowthistle, cheeseweed, purslane and prickly lettuce. Symptoms of TSWV vary with respect to host plant identity, plant age and environmental conditions, although the most common symptoms on tomatoes include: chlorosis, wilting, bronzing and the development of necrotic spots. Concentric rings may develop on infected fruit. The tospoviruses are vectored exclusively by thrips (Thysanoptera: Thripidae). Frankliniella occidentatlis, the western flower thrips, is considered to be the most important TSWV vector species because of its global distribution (Sherwood et al. 2003).
The management of viral diseases can be quite difficult. With respect to curly top, control strategies could include: altering the timing of tomato plantings to avoid exposing young transplants to the early migration of leafhoppers, increasing planting densities to compensate for losses and discourage visitation by leafhoppers, and eliminating weeds and tomato volunteers following harvest. The California Department of Food and Agriculture manages a statewide program (Curly Top Virus Program) designed to control the beet leafhopper. For tomato spotted wilt, growers are advised to ensure that their transplants are both virus and thrips free. Fields should be monitored for both thrips and viral symptoms. Thrips management should occur early in crop development; if reasonable, symptomatic plants should be removed. As with curly top, weeds and volunteers should be controlled. The use of resistant varieties is encouraged (the resistances of some tomato varieties to TSWV are noted here).
References:
Chen et al. 2010. Characterization of curtoviruses associated with curly top disease of tomato in California and monitoring these viruses in beet leafhoppers. Plant Disease 94:99-108
Creamer et al. 1996. Epidemiology and incidence of Beet Curly Top Geminivirus in naturally infected weed hosts. Plant Disease 80:533-535.
Davis, et al. 1998. Curly top virus found in perennial shrubs in foothills. California Agriculture. 52:38-40.
Heflebower et al. 2008. Curly top of tomato. Utah Pests Fact Sheet PLP-007. http://extension.usu.edu/files/publications/factsheet/curly-top-tomato08.pdf
Sherwood et al. 2003. Tomato spotted wilt. The Plant Health Instructor. http://www.apsnet.org/edcenter/intropp/lessons/viruses/Pages/TomatoSpottedWilt.aspx
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- Author: Steven T. Koike
Beginning in early April, 2012, the UC Cooperative Extension diagnostic lab in Salinas began to receive lettuce samples exhibiting obvious symptoms of a virus problem. Samples continued to be submitted throughout the month. All samples tested positive for the thrips-vectored Impatiens necrotic spot virus (INSV). This virus first began to cause damage to California lettuce in 2006. Since that time, INSV has occurred to a greater or lesser degree every season. However, significant INSV outbreaks usually are found beginning in late May or early June. Seeing INSV on lettuce in April is therefore unusual and growers and pest control advisors should carefully monitor these situations.
INSV-infected plants have leaves with brown to dark brown spots and dead (necrotic) areas; this necrotic tissue can resemble burn damage caused by pesticide or fertilizer applications (see photos below). Affected leaves can be distorted and twisted. Extensive necrosis can cause much of the leaf to become brown, dry, and dead. Some leaf yellowing can also be observed. Yellowing and the brown spotting tend to be observed on the newer leaves near the center of the plant’s growing point. If plants are affected with INSV early in their development, growth may be stunted. All lettuce types are susceptible, and INSV has been confirmed on iceberg, butterhead, romaine, and leaf lettuces. INSV can infect many other crops and weed species; the virus is vectored by the Western flower thrips (Frankliniella occidentalis).
Growers, PCAs, and other field personnel should exercise caution in diagnosing INSV disease because the lettuce dieback viruses cause very similar symptoms (see table below). Lettuce dieback is caused by two pathogens: Lettuce necrotic stunt virus (LNSV) and Tomato bushy stunt virus (TBSV). Infected lettuce can be severely stunted, especially if infected early in plant development. The oldest, outer leaves can be severely yellowed. Brown, necrotic spots and lesions later develop in these outer leaves. The younger, inner leaves remain dark green in color, but can be rough and leathery. LNSV/TBSV infects only romaine, butterhead, and leaf lettuces; modern cultivars of iceberg lettuce are immune. The LNSV/TBSV virus complex is a soilborne problem and no vector (insect, nematode, fungus) is known to spread these viruses.
For help in diagnosing these and other plant problems, submit samples to the UC Cooperative Extension diagnostic lab in Salinas.
Photos: Impatiens necrotic spot virus (INSV) on lettuce.