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

• Author(s): Lindow, Steven; Almeida, Alexander; Newman, Karyn;
• Abstract: Endophytic bacteria such as Xylella fastidiosa (Xf) colonize the internal tissues of the host, forming a structure very similar to a fixed biofilm inside the plant. A key determinant of success for an endophyte is the ability to move within the plant, sending out scouts to colonize new areas within the host. We expect activities required for movement to be most successful when carried out by a community of cells since individual cells may be incapable of completing the feat on their own and may be detected and easily eliminated by the host. Cells assess the size of their local population via cell-cell communication and coordinately regulate the expression of genes required for such processes. Our study aims to investigate cell-cell communication in Xf to determine its role in colonization and pathogenicity in grapevines. Xf shares sequence similarity with the plant pathogen Xanthomonas campestris pathovar campestris (Xcc). In Xcc, the expression of pathogenicity genes is controlled by the Rpf system of cell-cell communication, enabling a population of cells to launch a pathogenic attack in a coordinated manner (Barber et al. 1997). Two of the Rpf proteins, RpfB and RpfF, work to produce a diffusible signal factor (DSF; Barber et al. 1997). As the population grows, the local concentration of DSF increases. Other Rpf proteins are thought to sense the increase in DSF concentration and transduce a signal, resulting in expression of pathogenicity factors (Slater et al. 2000). The Xf genome not only contains homologs of the rpf genes most essential for cell-cell signaling in Xcc, but also exhibits striking colinearity in the arrangement of these genes on the chromosome (Dow and Daniels 2000). Thus Xf likely employs a cell-cell signaling apparatus similar to that of Xcc. Based on our knowledge of density-dependent gene regulation in other species, we predict the targets of Rpf regulation would be genes necessary for colonizing the xylem and spreading from vessel to vessel. For example, expression of extracellular polysaccharides, cellulases, proteases and pectinases might be induced by the signal. Similarly, we would expect the density-dependent genes to be expressed during the time when a population of Xf is ready to move into uncolonized areas. It is conceivable that cell-cell signal interference may be used by other organisms to inhibit density-dependent behaviors, such as pathogenicity or spreading through the habitat. Several recent studies indicate that other organisms can disrupt or manipulate the cell-cell signaling system of bacteria (Leadbetter and Greenberg 2000; Manefield et al. 1999). Examination of Xf population size in plants where Xf lives as an endophyte versus those in which Xf causes the xylem blockage symptoms of Pierces disease demonstrates a positive relationship between population size and symptom development (Fry and Milholland 1990). We hypothesize that an interaction between Xf and other organisms, such as another endophyte or the host plant itself, may modulate density-dependent behaviors in Xf by interfering with cell-cell signaling.
• Publication Date: Dec 2002
• Journal: 2002 Pierce's Disease Research Symposium