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

• Author(s): Lindow, Steven; Chatterjee, Subhadeep; Newman, Karyn; Purcell, Alexander;
• Abstract: Xylella fastidiosa (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. We have investigated DSF-mediated cell-cell signaling in Xf with the aim of developing cell-cell signaling disruption as a means of controlling Pierces disease (PD). The rpfF gene is necessary and sufficient for DSF signal synthesis and 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 wild-type 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 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 DSF-degrading strains greatly reduced the incidence and severity of disease in grape; DSF-producing strains consistently were the most effective in reducing disease. Disease was also reduced when some of these strains were simply sprayed onto grape before inoculation with Xf, indicating that they can alter behavior of the pathogen even when not co-inoculated. To verify that disease control is due to DSF interference, we have constructed mutants of these strains that disrupt the ability of these strains to produce or degrade DSF and show that the mutants are deficient in PD control. Both mutants unable to produce DSF as well as mutants deficient in degradation of DSF exhibited less ability to control PD when co-inoculated with Xf, suggesting that altering DSF abundance within the plant was a major factor contributing to disease control by these DSF-interfering strains. Given that DSF overabundance appears to mediate an attenuation of virulence in Xf we have transformed grape with the rpfF gene of Xf to enable DSF production in planta. Transgenic plants are being assayed for DSF production and susceptibility to Xf infection. The bacterial genes required for DSF degradation have been cloned and identified in antagonist Pseudomonas strain G, enabling their exploitation for disease control by over-expression in various bacterial endophytes of grape as well as by expression within plants themselves. 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. PD was reduced in plants after topical application of a DSF-producing strain of Erwinia herbicola (E. herbicola).
• Publication Date: Dec 2005
• Journal: 2005 Pierce's Disease Research Symposium