XYLEM CHEMISTRY MEDIATION OF RESISTANCE TO PIERCES DISEASE

• Author(s): Andersen, Peter; Ishida, Maria Lucia; Leite, Breno;
• Abstract: Xylella fastidiosa (Xf) is a Gram-negative xylem-limited bacterium that causes Pierces disease (PD), plum leaf scald, phony peach disease, almond leaf scorch, citrus variegated chlorosis, and numerous other diseases. In susceptible species xylem vessels may get plugged by Xf cells and an exopolysaccharide matrix. Vessel plugging results in xylem dysfunction, water stress, and leaf necroses, which are all characteristic of PD. Cell multiplication, formation of aggregates and biofilm are early components of PD that precede visible symptoms. The stimuli for aggregation and biofilm may involve specific plant/bacterium interactions and may involve the nutrient status of xylem fluid. Xylem fluid typically consists of 95 - 98% water; amino acids, organic acids, sugars and inorganic ions are the major components of total osmolality. Recent progress in generating a simple chemically-defined media for Xf allows studies of nutritional requirements in vitro. We have found that certain chemically-defined media (3G10R and CHARD2) developed in our laboratory promote the development of cell aggregates and biofilm. Aggregation and biofilm formation of Xf in vitro is dependent on xylem fluid chemistry. For example, xylem fluid of Vitis vinifera induced a high degree of aggregation of Xf cells, whereas V. rotundifolia did not. We have also found that calcium (CaCl)2() also promoted cell aggregation in vitro. These results support the calcium bridging) hypothetical model that was proposed to explain how Xf adheres to xylem vessels (Leite et al. 2002, 2004b), which assumes that the surface of Xf cells are negatively charged due to the presence of sulfur in the outer membrane proteins (OMP). Aggregate formation may be facilitated not only by calcium bridging but also by the formation of disulfide bonds in the OMP. The chemistry of xylem fluid may be a function of temperature, fertilization and diurnal/temporal alterations (Andersen and Brodbeck 1989ab, 1991, Andersen et al. 1995, 2004), and pH (Leite et al. 2004b). It is possible that the manipulation of xylem fluid composition, whether it is based on the primary organic compounds, ions or proteins in xylem fluid, is one possible method to affect PD-resistance. The dependence of aggregation and biofilm formation on the nutrient content of xylem fluid and growth media suggests that xylem chemistry is important in the mediation of resistance/susceptibility of PD.
• Publication Date: Dec 2004
• Journal: 2004 Pierce's Disease Research Symposium