Submitted to: Phytopathology
Publication Type: Peer reviewed journal
Publication Acceptance Date: 3/18/2011
Publication Date: 7/14/2011
Citation: Backus, E.A., Morgan, D.J. 2011. Spatiotemporal colonization of Xyllela fastidiosa in its vector supports two types of egestion in the inoculation mechanism of foregut-borne plant pathogens. Phytopathology. 101:912-922. Interpretive Summary: Pierce’s disease (PD) of grapevine, caused by the bacterium Xylella fastidiosa, is an economically important disease of grape in California and other grape-growing regions in the U.S. PD is the principal reason why grape production is impractical in the southeastern U.S. Development of novel methods of disease management are currently hindered by lack of understanding of the mechanism of transmission (acquisition and inoculation of the bacteria) by sharpshooter vectors of X. fastidiosa. Results from this study provided the first direct evidence for part of the inoculation mechanism. X. fastidiosa bacteria colonize surfaces of the vector’s foregut and are expelled during feeding. Confocal laser scanning microscopy was used to identify locations in foreguts of glassy-winged sharpshooters colonized by green fluorescent protein-expressing X. fastidiosa. Distributions of acquired X. fastidiosa were examined daily over acquisition access periods of 1 to 6 days, in both contaminated (field-collected) and clean (laboratory-reared) sharpshooters. Results showed that egestion (outward fluid flow from the insect’s mouth parts) is a critical part of the X. fastidiosa inoculation mechanism. Comparison of acquisition results for field-collected vs. laboratory-reared sharpshooters suggested that there may be competition for colonization of sites in the foregut most likely to lead to inoculation. Successful inoculation may depend, in large part, on how many bacteria are present in these sites in the foregut.
Technical Abstract: The bacterial agent that causes Pierce’s disease of grapevine, Xylella fastidiosa, is the only known arthropod-transmitted prokaryotic plant pathogen that does not circulate in the vector’s hemolymph. Instead, bacteria are foregut-borne and semi-persistent, i.e. bacteria colonize cuticular surfaces of the anterior foregut, and are lost during molting, respectively. Exactly how a sharpshooter vector expels bacteria from foregut sites, while hypothesized, is still unknown. The present study used optical sectioning via confocal laser scanning microscopy to identify locations in undissected, anterior foreguts of the glassy-winged sharpshooter colonized by green fluorescent protein-expressing X. fastidiosa. Both spatial and temporal distribution of acquired X. fastidiosa was examined daily over acquisition access periods of 1 to 6 days, for both contaminated (field-collected) and clean (laboratory-reared) sharpshooters. Results provided the first direct, empirical evidence that established colonies of X. fastidiosa can disappear from vector foreguts over time. When combined with existing knowledge on behavior and functional anatomy of sharpshooter feeding, present results supported that the disappearance is caused by outward fluid flow (egestion) not inward flow (ingestion, i.e. swallowing). Thus, egestion is a critical part of the X. fastidiosa inoculation mechanism. Furthermore, a non-linear, spatiotemporal cycle of microbial acquisition into and expulsion from the foregut provided evidence for two types of egestion, i.e. rinsing and discharging. Comparison of acquisition results for field-collected vs. laboratory-reared sharpshooters suggested that there may be competitive binding for optimum acquisition sites in the foregut. Successful inoculation may depend, in large part, on bacterial load in the vector.