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, almond leaf scorch, phony peach disease and many other diseases. For many plant species there is no resistant germplasm. Xylem vessels may be filled with exopolysaccharides produced by the bacterium, and pectins, gums and tyloses produced by the plant. There has been considerable interest in the production of exopolysaccharides as a component of disease symptomatology, whereas the formation of gums and tyloses has sometimes been considered an adaptive host plant response to infection (Fry and Milholland 1990). The resultant plant/bacterium interaction between bacterium and plants results in xylem dysfunction, water stress and leaf necrosis, which are characteristic of PD. Cell multiplication, formation of aggregates and biofilm may be early components of PD. The stimuli for cell multiplication and the formation of aggregates and biofilm may involve specific plant/bacterium interactions and may involve the plant nutrient status of xylem fluid. With the recent development of simple chemically-defined media for Xf, it is possible to study the effects of nutrition in vitro. We have found that certain chemically-defined media (3G10R and CHARD2) developed in our laboratory promote the development of aggregates and biofilm. The chemistry of xylem fluid typically consists of 95-98% water; amino acids, organic acids, sugars and inorganic ions are the major components of total osmolality. Chemical profiles developed from xylem chemistry of Vitis spp. are also being used to test whether xylem chemistry, and specifically xylem nutrient status is related to PD-resistance/tolerance. The chemistry of xylem fluid may be a function of temperature, fertilization and diurnal/temporal alterations (Andersen and Brodbeck 1989 a b, 1991, Andersen et al., 1995). The manipulation of xylem chemistry, whether it is based on the primary compounds or proteins in xylem fluid, is one possible method to affect PD-resistance. An alternative would be the development of transgenic plants with genes encoding for the production of lytic peptides. Transgenic V. vinifera cv. Thompson Seedless grapevines have been developed (Scorza et al. 1996) and the technology for the production of these plants has been patented (Scorza and Gray 2001). A hypothetical model was proposed to explain how Xf adheres to xylem vessels (Leite et al., 2002). In this model, a presumed negatively charged surface of Xf could be attributed to the presence of sulfur in outer membrane proteins (OMPs). Interaction between bacteria and the formation of aggregates can be facilitated by the formation of disulfide bonds between OMPs. Adhesion may occur between the bacterial cell surface and the negatively charged entities of the xylem vessel wall. Adhesion, in this hypothetical model, may possibly result from the interaction of this negatively charged xylem walls to negatively charged bacterial cell surface via calcium and magnesium bridges (Leite et al., 2002). The presence of Xf OMPs with sulfur-containing amino acids (cysteine and methionine) residues with domains localized in the outer membrane region, are being investigated.
• Publication Date: Aug 2003
• Journal: 2003 Pierce's Disease Research Symposium