VIRULENCE ANALYSIS OF THE PIERCES DISEASE AGENT XYLELLA FASTIDIOSA

• Author(s): Bruening, George; Civerolo, Ed; Gilchrist, David; Kirkpatrick, Bruce;
• Abstract: The bacterium, Xylella fastidiosa (Xf), is accepted as the causative agent of Pierces disease of grape. In our survey of plants for their reaction to Xf cell suspensions pressure infiltrated into leaves, we found that Chenopodium quinoa (Cq) developed a chlorosis in 24-48hr that conformed to the area infiltrated with suspensions of 106 to 108 Xf cells/mL. Comparisons of infiltrated opposite leaf halves for the intensity of the developed chlorosis provided a useful semi-quantitative assessment of the relative potency of Xf-derived preparations. The chlorosis-inducing activity was associated with Xf cells, not washings of cells, and heating Xf cells at 100C for 6 min slightly enhanced the activity. We observed that the chlorosis-inducing activity survived treatment with sodium dodecyl sulfate (SDS). Although the indicated stabilities do not suggest a protein as the active agent, the chlorosis-inducing activity was sensitive to each of three proteases and was lost after treatment with chloroform or acetic acid. These results suggested that Xf possesses a protein elicitor that is recognized in the intercellular spaces of Cq plants even when the protein is in a denatured state. Presumably metabolic events of Cq, subsequent to recognition, result in chlorosis. Chenopodium ambrosioides (Ca) is known to be a natural host of Xf and a source of Xf inoculum that can be transmitted to grape under experimental conditions (Freitag 1951). We were able to infect Ca with Xf after inoculation by petiole injection. However, infiltrated leaves of Ca failed to develop chlorosis or other reaction. In contrast, Cq, which developed chlorosis after infiltration, did not become detectably infected after inoculation with Xf. In several systems, a pathogen protein that acts as an elicitor in one species may act as a virulence factor in a closely or distantly related line or species (De Wit, Joosten et al. 1994; van't Slot and Knogge 2002). Therefore, the Xf elictor of Cq chlorosis may be a virulence factor in other, susceptible plant species, e.g., Ca and Vitis vinifera.
• Publication Date: Dec 2002
• Journal: 2002 Pierce's Disease Research Symposium