UNDERSTANDING XYLELLA FASTIDIOSA COLONIZATION AND COMMUNICATION IN XYLEM LUMINA

• Author(s): Turner, Harvey; Turner, James;
• Abstract: Symptoms of Pierces disease of grape caused by Xylella fastidiosa are generally recognized as being caused by restricted sap flow and resultant water stress due to plugging of xylem elements (Goodwin et al. 1988; Purcell and Hopkins 1996; Mollenhauer and Hopkins 1974). Such blockage is the result of massive bacterial aggregates and associated mucilage. It is not clear whether the extracellular polymeric mucilage is of bacterial and/or plant origin. Based on the analysis of the complete genome sequence of X. fastidiosa, gums produced by the X. fastidiosa are similar to the xanthan gums produced by Xanthomonas campestris pv campestris, although they may be less viscous (Simpson et al. 2000). In addition, tylose development in xylem vessels in response to the presence of the bacterium further restricts sap flow (Mollenhauer and Hopkins 1976. These general concepts X. fastidiosa pathogenicity are readily recognized, although it is not understood how the bacterium becomes established in the turbulent habitat of a fluid conduit i.e., xylem vessels and tracheae. Bacterial spread through xylem elements is also poorly understood, albeit enzymatic degradation of pit membranes is thought to be involved (Mollenhauer and Hopkins 1976). Colony formation is likely to be influenced by the physical constraints of the xylem element surface much like the formation of bacterial biofilms is influenced by surface characteristics (microtopography, chemistry, etc.) in other aqueous and fluid environments such as medical stints and prostheses, food handling equipment, and water supply systems (Ridgway and Olson 1981; LeChevallier et al. 1987; Caldwell and Lawrence 1988; Sternberg et al. 1999). Surface microtopography of these environments influence the temporal and spatial aspects of bacterial colonization (Bremer et al. 1992; Gorman et al. 1993; Korber et al. 1997; Arnold 1999). Surfaces become colonized as cells (in this case bacteria) attach initially via physio-chemical forces, and ultimately with extracellular polysaccharides or ligand-mediated interactions. The end result is the establishment of biofilms consisting of bacteria in a polysaccharide matrix that provide a protective habitat that is conducive for continued cell growth and colony formation. The recently completed sequencing of the X. fastidiosa genome has revealed several open reading frames with putative functions that may be associated with bacterial colonization of xylem vessels and disease (Simpson et al. 2000). For example, at least one ORF with homology to the luxR family of transcriptional regulators has been identified (GenBank accession AAF83782). Such genes encode proteins (LuxR homologs) that when bound by acyl-homoserine lactone autoinducer molecules (AI), regulate transcription of diverse types of genes (Fuqua et al. 1996). Autoinducers are synthesized by enzymes that are encoded by luxI gene homologs. The luxI luxR regulatory system was first discovered in the marine bacterium Vibrio fischeri, however now related systems have been discovered in diverse species of bacteria including plant and animal pathogens (Cha et al. 1998). Autoinducers diffuse bi-directionally across bacterial membranes and reach concentrations for efficient activation of LuxR regulators in environments of high bacterial density. Thus the ability of AI to activate the LuxR regulators is a cell density-dependent response referred to as quorum-sensing or autoinduction. The discovery of luxR homologs in X. fastidiosa strongly suggests that the bacterium produces AI and regulates genes in a density-dependent manner. This finding is intriguing because it suggests that a luxI-luxR type quorumsensing regulatory system may be functioning in X. fastidiosa biofilm communities in xylem vessels. The overall goal of the proposed research is to identify factors that affect colonization and plugging of grape xylem elements by X. fastidiosa and to use this information for development of effective control strategies for Pierces disease. Our approach is to determine physical and chemical factors that influence X. fastidiosa attachment and colony development using an in vitro system, and to establish whether genes associated with these activities are regulated by quorum-sensing. The in vitro system that we propose has several advantages. It will allow the direct observation of bacterial community development in artificial vessels microfabricated to possess topographies and chemistries similar to real in planta vessels. We will be able to determine how physical and in some cases biological parameters affect biofilm formation and plugging induced by virulent and avirulent or weakly virulent strains. Furthermore, it will be possible to differentiate between plant-induced responses and those induced specifically by the pathogen.
• Publication Date: Dec 2002
• Journal: 2002 Pierce's Disease Research Symposium