|Lee, Wah Keat|
Submitted to: CDFA Pierce's Disease Control Program Research Symposium
Publication Type: Abstract Only
Publication Acceptance Date: 10/15/2011
Publication Date: 11/4/2011
Citation: Backus, E.A., Lee, W. 2011. Glassy-winged sharpshooter feeding does not cause air embolisms in xylem of well-watered plants. CDFA Pierce's Disease Control Program Research Symposium. p. 19. Interpretive Summary:
Technical Abstract: Plant xylem vessels are under negative hydrostatic pressure (tension) as evapotranspiration of water from the leaf surface pulls the column of water in xylem upwards. When xylem fluid flux is under extreme tension, any puncture or breakage of the xylem vessel wall can cause formation of air embolisms that instantaneously empty the length of the xylem vessel (cavitation), disrupting xylem flow. Xylem fluid-ingesting hemipteran insects such as the glassy-winged sharpshooter (GWSS), Homalodisca vitripennis (Germar), penetrate their stylets into xylem cells and imbibe many times their body weight in xylem fluid each day. It has not been known whether GWSS stylet penetration causes cavitation, however, induction of cavitation during vector feeding has been suggested as one mechanism of xylem blockage in Pierce’s disease symptoms. To date, no method has been successful in visualizing xylem during real-time stylet penetration, to determine whether or not air embolism occurs. The present study used videography of live, feeding GWSS under X-ray phase contrast microscopy at the Argonne National Laboratory, to determine whether air was present in stylets or xylem of well-watered cowpea plants before, during, or after xylem penetration. Air is an excellent contrast agent for X-ray microscopy, and is readily visible in both plant cells and insect stylets. Insects were monitored via electrical penetration graph, to identify when their stylets had penetrated xylem in cowpea stems. After feeding was terminated, stems were cut; subsequent cavitation and entry of air into xylem was visible in stems under X-ray. X-ray videographs before, during, and after stylet penetration to and inside xylem showed no air present in stylets, and only in cut, not intact, xylem cells. It is hypothesized that salivary sheaths secreted by GWSS during stylet penetration prevent cavitation.