UC Davis distinguished professor Dianne Ullman, outside Sauve, France. Photo courtesy of UC Davis.

DAVIS, CA (MPG) - When UC Davis distinguished professor Diane Ullman of the Department of Entomology and Nematology headed to France on a 2018-19 Fulbright grant to conduct research on tomato spotted wilt virus (TSWV) with her colleagues, she hoped it would lead to important discoveries to help combat plant viruses that attack agricultural crops.

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.”

TSWV is transmitted by thrips, tiny insects with fringed wings. “Orthotospoviruses cause serious damage to many important crops, including tomatoes, lettuce, peppers, peanuts, and many others,” Ullman said. “The orthotospoviruses are enveloped negative or ambisense single stranded RNA viruses with a genome divided into several segments. For many years, graphical views of viral particles have shown a virion packaging one copy of each genomic segment in a polarity named the viral strand. Various observations suggested this scenario may not represent reality, but these studies were often focused on other questions or did not use accurate quantitative methods.”

“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.”

“The intellectual and research contributions of our colleague, the late Thomas German of the University of Wisconsin (second author) were too numerous to recount,” Ullman said. “His cloning of the six possible RNA segments made reliable quantification of the two polarities of each segment, key to this research, possible. In addition, his enthusiasm for science motivated the entire research team and propelled us all forward. The outcomes of our collaboration challenge dogma around how these viruses infect plants and insects, how their populations evolve, and even the terminology describing their biology.”

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.

See more at https://tinyurl.com/yeyuxy5b

See PNAS paper at https://tinyurl.com/5hj5a2tv