MANAGEMENT OF PIERCES DISEASE OF GRAPE BY INTERFERING WITH CELL-CELL COMMUNICATION IN XYLELLA FASTIDIOSA

• Author(s): Lindow, Steven; Newman, Karyn; Purcell, Alexander;
• Abstract: Xylella fastidiosa (Xf) is an endophyte that is restricted to the xylem, a network of vessels for water transport, in which it forms an aggregated biofilm. It is transmitted from plant to plant by xylem sap-feeding insects, and forms a polar biofilm in these insects foreguts. In other systems, biofilms are characterized by community behavior under the control of cell densitydependent gene expression, which requires cell-cell signaling. Xf has homologs of the cell-cell signaling genes found in the important plant pathogen Xanthomonas campestris pathovar campestris (Xcc) and produces a similar alpha,beta unsaturated fatty acid signal molecule called DSF that coordinates gene expression in a community (2, 7). We have investigated DSFmediated cell-cell signaling in Xf with the aim of developing cell-cell signaling disruption as a means of controlling Pierces disease. We have determined that the rpfF gene is necessary and sufficient for DSF signal synthesis and that rpfF mutants of Xf are hypervirulent and non-transmissible. Lack of transmissibility was linked to an inability of the rpfF mutant to form a biofilm in the insect foregut; while taken up by insects, the mutant strain is not retained. Xf strains that overproduce DSF produce disease symptoms in grape, but only at the site of inoculation and the cells do not move within the plant as do wildtype strains. Thus elevating DSF levels in plants should reduce movement of Xf in the plant and also reduce the likelihood of transmission by sharpshooters. We screened several collections of bacterial strains isolated from plants and identified bacterial strains that can interfere with Xf signaling both by producing large amounts of DSF, by degrading DSF, or by in some way interfering with recognition of DSF. When co-inoculated into grape with Xf, both DSF-producing strains and DFS degrading strains greatly reduced the indicidence of disease in grape; DSF-producing strains consistently were the most effective in reducing disease. Given that DSF appears to mediate an attenuation of virulence in Xf we are in the process of transforming grape with the rpfF gene to enable DSF production in planta. Preliminary results indicate that transient expression of rpfF in Nicotiana benthamiana following infiltration with appropriate Agrobacterium tumefaciens strains resulted in high levels of DSF production, suggesting that it is likely that grape cells will produce DSF when transformed with the bacterial rpfF gene. Non-endophytic bacterial species were also established in high numbers inside grape leaves and petioles following spray application to plants with a high concentration of a silicon-based surfactant with a low surface tension, suggesting that it may be possible to produce protective compounds such as DSF in plants by a variety of bacteria.
• Publication Date: Dec 2004
• Journal: 2004 Pierce's Disease Research Symposium