Management of Pierce’s Disease of Grape by Interfering with Cell-Cell Communication in Xylella Fastidiosa
Xylella fastidiosa is an “endophytic” bacterium?one that colonizes the internal tissues of the host plant. A key determinant of success for an endophyte (and symptom formation in the plant) is the ability to spread within the plant from the initial site of infection because the bacteria use up local resources as their population grows. As a large group of bacteria accumulates, they start behaving differently. There is strong evidence that Xylella, like its related plant pathogenic bacteria such as Xanthomonas campestris, secretes a gum that protects it, and produces degradative enzymes such as cellulases that break down plant tissues, but only after a large number of cells have developed at a site. These virulence traits, which allow them to expand into new vessels in a plant, would be ineffective for solitary cells. Hence the pathogen must repress the production of such traits at low cell numbers when it would be futile, and express such traits only when it has reached higher cell density, basically “ganging-up” on the plant. The bacteria keep tabs on their population size by communicating via a small diffusible signal molecule. Many different types of signals are employed by different species of bacteria, however more than one species often employ the same signal. In addition, some bacteria are capable of degrading the signal of another species, or blocking it with a “look-alike” analog. Our research involves finding other bacteria that can block or confuse the signals Xylella uses to spread in the plant. The first step in this research is to identify the signal used by Xylella. We have found that Xylella employs the same signal molecule, DSF, as the related plant-pathogen Xanthomonas. We have engineered a collection of DSF “signal-sensing” strains of Xylella to be used as molecular tools that will allow us to monitor signaling in Xylella itself. These bacteria glow when they sense DSF and will indicate which genes in Xylella are being expressed in such a cell-density-dependent fashion. We are fortunate in finding that Xylella shares the same cell signaling system as Xanthomonas, since we can use a fluorescent “signal-sensing” strain of Xanthomonas to help us find other bacteria that grow with Xylella in the plant that can disrupt the signal. We have engineered a mutant strain of Xylella that should be unable to make the signal, but are still characterizing it. When finished, we will use this strain to verify the importance of signaling for survival and spread of the bacteria in the plant. We expect that the mutant will have a smaller population size and will be less efficient at spreading throughout the plant. We have collected potential DSF degrader and DSF analog producer strains of bacteria from infected grapevines and are preparing to test them with our signal-sensing strains to see if they can disrupt Xylella signaling. Such strains should be effective in reducing disease when introduced into plants as endophytes or if the genetic ability to produce these compounds or degradative enzymes is introduced into grape.
PDF: Management of Pierce’s Disease of Grape by Interfering with Cell-Cell Communication in Xylella Fastidiosa