XYLELLA FASTIDIOSA BACTERIAL POLYSACCHARIDES WITH A POTENTIAL ROLE IN PIERCE’S DISEASE OF GRAPES Project Leader: Cooperator:

Pierce’s disease (PD) causes symptoms of leaf scorch and fruit cluster wilt on wine, table and raisin grapes, and is caused by the bacterial pathogen Xylella fastidiosa. These fastidious, gram-negative bacteria occur only in the xylem of infected plants and are transmitted by xylem-feeding insects; leafhoppers and sharpshooters. The close association of plants with bacteria, either pathogenic or symbiotic, is often mediated by cell surface polysaccharides that may become modified during the infection process (Price, 1999). Xylella has 9 genes homologous to the gum genes of Xanthomonas campestris that direct the synthesis of a highly viscous exopolysaccharide gum (EPS). The biosynthesis and secretion of EPS is often tightly regulated either by the availability of nutrients or of specific small-molecule inducers in plant exudates. In addition, Xylella undergo developmental changes from rippled to smooth cell walls during the infection process (Huang et al., 1986), implicating a possible involvement of cell surface lipopolysaccharides (LPS). LPS consists of membrane-anchored lipid A, a core oligosaccharide, and a carbohydrate O-antigen repeat that typically contains phosphorylated, acetylated or methylated sugars that can profoundly affect its physical properties. Since it forms the outermost bacterial surface, the Xylella LPS may be an important factor in mediating interactions such as recognition and adhesion between the bacteria and host plant, or the bacteria and the insect vector.