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United States Department of Agriculture

Agricultural Research Service


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Title: Support for the salivation-egestion hypothesis for Xylella fastidiosa inoculation: salivary glucanase is injected into xylem during vector feeding.)

item Backus, Elaine
item Andrews, Kim
item Labavitch, John
item Greve, Carl

Submitted to: Meeting Abstract
Publication Type: Abstract only
Publication Acceptance Date: 3/23/2011
Publication Date: 7/11/2011
Citation: Backus, E.A., Andrews, K., Labavitch, J., Greve, C. 2011. Support for the salivation-egestion hypothesis for Xylella fastidiosa inoculation: salivary glucanase is injected into xylem during vector feeding. Meeting Abstract. Available:

Interpretive Summary:

Technical Abstract: The salivation-egestion hypothesis for the inoculation mechanism of Xylella fastidiosa (Xf) proposes that saliva secreted into plants is taken up into the vector’s precibarium. There, saliva loosens the Xf bacterial biofilm by enzymatically degrading ß-1, 4 glucans that form the chemical backbone of the exopolysaccharide matrix binding bacteria to the insect’s cuticle. Bacteria-laden fluid is then egested into xylem vessels. In the present study, carbohydrases in glassy-winged sharpshooter (GWSS) saliva were purified and tested for activity. The most active salivary fraction was a type of cellulase, ß-1,4 glucanase. Subsequently, over 2,000 pairs of GWSS salivary glands were dissected, extracted, and the glucanase purified, then used to produce a polyclonal antibody for immunohistochemical staining of GWSS salivary sheaths. GWSS probing of grapevine petioles was recorded by electrical penetration graph (EPG), to determine when stylets had reached xylem. Grapevine tissues were excised, fixed, embedded in paraffin, sectioned, and examined using confocal scanning laser microscopy (CSLM). Red-stained glucanase-containing saliva co-localized (and therefore was secreted simultaneously) with sheath saliva for most of the probe. However, the narrowest branch(es) at the tip(s) of the salivary sheath were composed only of glucanase-containing saliva, which also was clearly injected into the xylem cell at the sheath terminus. Glucanase-containing saliva formed a ring that lined the interior of the xylem vessel and penetrated into the cell wall. Salivary lining of the xylem cell was found in adjoining sections, pulled over 200 µm downstream from the point of entry and connected to a salivary deposit in the middle of the xylem cell. Glucanase-containing saliva also was injected into non-xylem cells, accumulating in loose, flocculent masses. Results support the egestion-salivation hypothesis by showing that bacteria carried in glucanase saliva could be injected into xylem.

Last Modified: 8/24/2016
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