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Submitted to: American Phytopathological (APS) Vectors of Plant Pathogens
Publication Type: Book / Chapter Publication Acceptance Date: 5/14/2012 Publication Date: 6/1/2016 Citation: Backus, E.A. 2016. Sharpshooter feeding behavior in relation to transmission of Xylella fastidiosa: a model for foregut-borne transmission mechanisms. In: Brown, J.K., editor. Vector-Mediated Transmission of Plant Pathogens. St. Paul, MN: American Phytopathological Society Press. p. 175-194. Interpretive Summary: Xylella fastidiosa is a bacterial plant pathogen that causes scorch diseases in important crops, such as Pierce’s disease of grape. The bacterium lives only inside plant xylem cells and xylem-feeding insect vectors such as the glassy-winged sharpshooter (Homalodisca vitripennis). Bacteria colonize the foregut, forming a dense layer. Exactly how the insect injects bacteria from the foregut into xylem cells is not known, but is thought to involve mechanical flow of mixed plant fluid and saliva from the foregut. Nonetheless, exact feeding behaviors that control injection of bacteria are unknown. The most rigorous method to identify sharpshooter feeding behaviors is electrical penetration graph (EPG) monitoring. The purpose of this chapter is to review progress made towards identifying those feeding behaviors leading to X. fastidiosa transmission. Specific objectives are to: 1) summarize work of the last ten years to identify and define the EPG waveforms representing feeding behaviors of sharpshooters, 2) describe a model for xylem cell acceptance behaviors during feeding, and 3) present the Salivation-Egestion Hypothesis for X. fastidiosa inoculation, in which salivation combined with egestion [outward fluid flow] carries bacteria into the xylem. Understanding the inoculation mechanism will aid development of grape varieties resistant to inoculation of X. fastidiosa by sharpshooter vectors. Technical Abstract: Xylella fastidiosa is a xylem-limited bacterium that causes several scorch diseases in important crops, such as Pierce’s disease of grape. Bacteria form a dense biofilm on the foregut cuticle of the glassy-winged sharpshooter, H. vitripennis, and other xylem-feeding vectors. Bacteria are inoculated directly from sites in the foregut into a host plant during sharpshooter feeding (i.e. probing of the mouthparts, stylets, into the plant). However, despite nearly 70 years of research, no one has yet definitively associated specific sharpshooter stylet probing behaviors with such inoculation. The most rigorous method to identify sharpshooter feeding behaviors is electrical penetration graph (EPG) monitoring. The purpose of this chapter is to review progress made towards identifying those feeding behaviors leading to X. fastidiosa transmission. Specific objectives are to: 1) summarize work of the last ten years to identify and define EPG waveforms representing feeding behaviors of sharpshooters, 2) describe a model for xylem cell acceptance behaviors during feeding, and 3) present the Salivation-Egestion Hypothesis for X. fastidiosa inoculation, in which salivation combined with egestion [outward fluid flow] carries bacteria into the xylem. Understanding the inoculation mechanism will aid development of grape varieties resistant to inoculation of X. fastidiosa by sharpshooter vectors. |
