Submitted to: Phytopathology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/6/2014
Publication Date: 5/20/2015
Citation: Backus, E.A., Shugart, H.J., Rogers, E.E., Morgan, J.K., Shatters, R.G. 2015. Direct evidence of egestion and salivation of Xylella fastidiosa suggests sharpshooters can be “flying syringes”. Phytopathology. 105:608-620. Interpretive Summary: One of the most economically important diseases of grape in California and other grape-growing regions in the United States is Pierce’s disease, caused by the bacterium Xylella fastidiosa. Grape production is impractical in the southeastern United States primarily due to X. fastidiosa. Development of new disease management methods is currently hindered by lack of understanding of the mechanism of transmission (acquisition and inoculation of the bacteria) by sharpshooter vectors of X. fastidiosa. Bacteria are acquired during feeding by adhering to surfaces of the vector’s foregut and multiplying; they are inoculated by being directly expelled from the foregut and mouthparts. Green fluorescent protein-expressing X. fastidiosa or fluorescent nanoparticles were acquired from artificial diets by glassy-winged sharpshooters, Homalodisca vitripennis. The path of the bacteria then was traced out of the insects as they were transferred to a series of new, clean diets (lacking bacteria). Findings from this study provide the first conclusive evidence: 1) for egestion (outward fluid flow from the insect’s foregut) as a feeding process in hemipteran insects, and 2) that egested bacteria become mixed into simultaneously expelled saliva. In addition, results suggest that sharpshooters can hold a column of fluid, potentially containing suspended bacteria, in their foregut during transit from plant to plant. Therefore, sharpshooters could be true “flying syringes.” Such capability by vectors would explain several features of bacterial spread (such as absence of a latent period) that can improve epidemiological modeling of disease progress to ultimately develop better disease management.
Technical Abstract: Despite nearly 70 years of research, the inoculation mechanism of Xylella fastidiosa by its sharpshooter vectors is still unproven. X. fastidiosa is unique among insect-transmitted plant pathogens because it does not circulate in the vector’s hemolymph, yet it is propagative because it adheres to and multiplies on the cuticular lining of the anterior foregut. Thus X. fastidiosa is termed ‘foregut-borne.’ A non-circulative mechanism for inoculation of X. fastidiosa must explain how bacterial cells exit the vector’s stylets directly into the plant from the foregut via the food canal in the stylets. It has been proposed that the mechanism for this expulsion is egestion, putative outward flow of fluid from the foregut via hypothesized bidirectional pumping of the cibarium (part of the foregut). Inoculation of X. fastidiosa has been proposed to be via a combination of egestion and salivation. The present study traced green fluorescent protein-expressing X. fastidiosa or fluorescent nanoparticles acquired from artificial diets by glassy-winged sharpshooters, Homalodisca vitripennis, as they were egested into simultaneously secreted saliva on a series of new, clean diets. Results provide the first direct, conclusive evidence for the existence of egestion by any hemipteran insect. In addition, they definitively demonstrate that egested X. fastidiosa or nanoparticles become mixed into gelling saliva to form fluorescent deposits on the parafilm surface of artificial diets and salivary sheaths extending through the parafilm. Therefore, the present results strongly support an egestion-salivation mechanism of X. fastidiosa inoculation. In addition, the findings suggest an additional model for inoculation: that a column of fluid, potentially containing suspended bacteria, may be held in the foregut during the vector’s transit from plant to plant. Thus, sharpshooters could be true “flying syringes.”