Evidence supporting the Egestion-Salivation Hypothesis for inoculation of Xylella fastidiosa by sharpshooter vectors

Despite more than 70 years of study, the mechanism of inoculation of semipersistent, foregut-borne plant pathogens by their vectors is still unknown. The best model system for these studies is inoculation of the Pierce’s disease bacterium, Xylella fastidiosa (Xf), by vectors such as the glassy-winged sharpshooter GWSS). Research on the egestion-salivation hypothesis for Xf inoculation is presented. Two important steps in this hypothesis are: 1) uptake of saliva containing the cell wall-degrading enzyme beta-1,4 glucanase into the vector’s precibarium, causing the Xf colonies attached therein to loosen from the cuticle, followed by 2) injection of saliva containing loosened bacteria into the xylem prior to ingestion. To directly test the likelihood that Xf could survive in and migrate out of saliva following inoculation, immunohistology was used to study interactions between Xf and GWSS saliva in grapevine. Adult GWSS were confined in small cages on grapevine stems for 24 hours and allowed to probe, leaving salivary sheaths in the plant. Xf was then needle-inoculated into the same stem area; 1 hour later, the tissue was excised and prepared for immunohistology using a commercial Xf probe. Xf bacteria observed in xylem cells penetrated the semi-viscous saliva deposited during GWSS probing prior to Xf inoculation. Therefore, Xf bacteria have the ability to infiltrate gelled saliva containing salivary glucanase. Implications for the mechanism of inoculation are discussed.