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Submitted to: CDFA Pierce's Disease Control Program Research Symposium
Publication Type: Proceedings Publication Acceptance Date: 10/14/2010 Publication Date: 12/15/2010 Citation: Backus, E.A. 2010. Support for the salivation-egestion hypothesis for Xylella fastidiosa inoculation: bacterial cells can penetrate vector saliva in xylem. CDFA Pierce's Disease Control Program Research Symposium. p.3. Interpretive Summary: Although the inoculation mechanism of Xylella fastidiosa (Xf) has been sought by scientists for nearly 70 years, it is still not known exactly how Xf is injected into a healthy plant by feeding sharpshooter vectors. This lack of knowledge hampers development of a novel means of host plant resistance to PD, i.e. selection of plant traits that reduce the likelihood of inoculation by GWSS. Similar resistance to vector inoculation has been successfully developed in several crops with insect-vectored plant pathogens. The salivation-egestion hypothesis for inoculation proposes that Xf bacterial cells are forcefully egested (expelled) out of the vector’s mouth parts in saliva secreted into xylem cells. To determine whether bacteria could move into or out of saliva, GWSS were allowed to feed for 24 hours on a grape stem, depositing saliva in xylem cells. After the insects were removed, the stem was needle-inoculated with Xf near the feeding sites, then antibody-stained for Xf and examined by confocal microscopy. Results showed that Xf entered xylem cells and penetrated into several salivary globs already present in xylem. Bacterial penetration into such hardened saliva suggests that Xf should be able to penetrate out of newly-secreted, soft saliva. This finding provides indirect support for the egestion-salivation hypothesis. Technical Abstract: Research is underway to develop varieties of grape that are resistant to Pierce’s disease (PD) caused by Xylella fastidiosa (Xf). PD has become economically important since introduction of an exotic vector, glassy-winged sharpshooter (GWSS). However, achieving resistance to vector acquisition and inoculation is hampered by lack of knowledge of the Xf inoculation mechanism, despite nearly 70 years of research on this topic. Recent work suggests that bacterial cells spread from the cibarium (the retention reservoir) into the precibarium, from which inoculation occurs. The salivation-egestion hypothesis for inoculation proposes that saliva is brought up into the precibarium, where it is mechanically swept across taste organs by the precibarial valve. This mechanical action, combined with the enzymatic action of the saliva, dislodges bacteria in the precibarium, which are then forcefully egested (expelled) out of the stylets in a bolus of saliva. The present study allowed GWSS to feed for 24 hours on a grape stem, depositing salivary sheaths. After the insects were removed, the stem was mechanically inoculated with Xf in the vicinity of the salivary sheaths, then immunoprobed for Xf and examined by confocal laser scanning microscopy. Results showed that Xf entered xylem cells and penetrated into several salivary sheaths encountered in xylem. Bacterial penetration into hardened sheaths suggests that Xf should be able to penetrate out of newly-secreted, soft saliva, thereby providing indirect support for the egestion-salivation hypothesis. |
