A new look at thrips salivary glands and virus dissemination pathways

Read the full article on ScienceDirect.

Salivary gland morphology, tissue tropism and the progression of tospovirus infection in Frankliniella occidentalis

Text by Anna Whitfield

 

Thrips-transmitted tospoviruses are among the most important plant viruses threatening global food security. Tomato spotted wilt virus (TSWV) is the best-studied tospovirus and its principal thrips vector, the western flower thrips (WFT, Frankliniella occidentalis), is considered a “super vector” transmitting multiple tospoviruses and other plant pathogens to an extraordinarily large number of plant species. Despite WFT’s importance as pests and vectors, basic biology and pathogen-insect interactions are largely unknown. Tospoviruses replicate in their vectors and are transmitted persistently. During acquisition, the virus first infects the midgut, replicating, spreading and then quickly moving to visceral muscle cells and tubular salivary glands (TSG). Infection of the principal salivary glands (PSGs) is required for inoculation to plants. Our goal was to describe WFT anatomy with specific focus on tissues important for virus dissemination i.e. the midgut, TSGs and PSGs.

 

We revealed the link between virus dissemination and thrips development by localizing virus in these tissues in early larval to adult stages. In spite of their small size and the difficulty of preserving delicate connective structures (ligaments and TSGs) during dissection, our findings support the hypothesis that TSGs provide a direct pathway from the midgut to the PSGs. Infection of TSGs occurred in the first instar larva and preceded virus dissemination into the PSGs; however, PSG infection did occur until the very late second larval instar and were most highly infected in adults. From these results we conclude that tissue tropism plays a key role in the dissemination of virus to the PSGs. Our finding that three distinct regions occur in the PSGs opens the door to hypotheses around specialized functions these cells may play during virus infection and insect feeding (Figure 1). The development of techniques to label viral proteins and observe cytoskeletal structures revealed a surprising diversity of infection patterns in the PSG (Figure 2), including infection of a single PSG cell to widespread infection of one or both PSGs.

Previously, we found virus abundance in larval thrips is a determinant of transmission efficiency, leading to the hypothesis that virus abundance in early first instar larvae may determine the extent of salivary gland infection in adults. Our work describing the virus dissemination pathway in WFT and identifying the role of tissue tropism in vector competence lays the foundation for examining factors determining movement between tissues and specific cellular components allowing infection of the PSG.

Figure 1

Fig.Regions-CellTypes_blog

 

Principal Salivary Glands of the western flower thrips (WFT, Frankliniella occidentalis). Different cell types and regions were observed in the principal salivary glands (PSGs) of WFT. Each PSG lobe consists of three regions, a central region (CR) composed of large binucleated cells (a, b) and smaller basal (BR) and apical (AR) regions with cells that appear to have a single nucleus (a and insets in b). Red color corresponds to actin marker and blue color to a nuclei marker. Images were obtained by confocal fluorescent microscopy. ED = efferent duct, N = nucleus, TSG = Tubular Salivary Gland, Lig = ligament-like structure, * = a second filament-like structure. Scale bars represent 50 µm in a and b; and 10 µm in insets.

Figure 2

PSG-differential infection_blog

Differential infection of PSGs. Tomato spotted wilt virus (TSWV) infection (green antibody labeling and arrows) varied in WFT principal salivary glands (PSGs). Virus may be observed mainly at the lumen (L) of the PSG (a), at one single cell in the PSG lobe (b), generalized throughout the PSG cells (c), or in discrete different cell types (binucleated and uninucleated) in a PSG (d and detail in e). Red color corresponds to actin marker and blue color to a nuclei marker. ED = efferent duct, N = nucleus, TSG = Tubular Salivary Gland, Lig = ligament-like structure, * = a second filament-like structure. Scale bars represent 10 µm in a, b, c and e; and 50 µm in d.

Introducing the authors

Authors_Final

Anna E. Whitfield is a Professor of Plant Pathology at Kansas State University. Her research focuses on the biology vector-borne viruses infecting food crops and development of new antiviral strategies.

 

Mauricio Montero-Astúa is an Associate Professor at the University of Costa Rica. He studies plant viruses, their vectors and the use of molecular tools for taxonomy.

 

Diane E. Ullman is a Professor of Entomology at the University of California-Davis. She studies insect/virus/plant interactions and development of management strategies for insect-transmitted plant pathogens.

About the research

Salivary gland morphology, tissue tropism and the progression of tospovirus infection in Frankliniella occidentalis

Mauricio Montero-Astúa, Diane E. Ullman, Anna E. Whitfield

Virology, Volume 493, June 2016, Pages 39–51