LINKING THE MODEL OF THE DEVELOPMENT OF PIERCES DISEASE IN GRAPEVINES TO AN UNDERSTANDING OF THE DYNAMICS OF GLASSY-WINGED SHARPSHOOTER TRANSMISSION OF XYLELLA FASTIDIOSA TO GRAPEVINES AND GRAPEVINE GENE EXPRESSION MARKERS OF PIERCES DISEASE

• Author(s): Backus, Elaine; Labavitch, John; Matthews, Mark; Shackel, Ken; Cocroft, Rex; Greve, Carl; Kirkpatrick, Bruce; Morgan, David; Perez, Alonso; Shugart, Holly;
• Abstract: For several years we have been studying the development of Pierces disease (PD) in grapevines. Our studies have been guided by a model of PD development proposed with our initial application for funding. The Model proposed several steps in disease development following introduction of the PD causal agent, the bacterium Xylella fastidiosa (Xf): Xf introduction to vessels =>vessel cavitation =>initial water deficit => Xf population increase => production of enzymes by Xf =>cell wall digestion => oligosaccharide signals => ethylene synthesis rise => a "wave" of vessel occlusion beyond the infection site => collapse of vine water transport => leaf abscission => vine death Our hypotheses have proven quite accurate, although aspects of the model are still being tested. We have shown that xylem vessel obstruction (tyloses, plant cell wall component-derived gels, and bacterial extracellular polysaccharides) and consequent reductions in stem water transport capacity are early consequences of infection with Xf, before bacterial populations are substantial and have spread far from the inoculation point. We have shown that ethylene treatment of vines also triggers vessel obstruction development and reduced water movement and that ethylene emanation from vines may increase following infection. We have also developed data for xylem vessel length distributions in grapevines and shown that Xf must pass through vessel pit membranes if the bacterial population is to develop systemically, thus suggesting that digestion of cell wall polymers in the pit membranes is likely to be important to disease spread. These findings are reported in several reports at the annual PD Symposium (Labavitch et al., 2001, 2002, 2004, 2005; Labavitch and Matthews, 2003) and, more recently, at disciplinary scientific society meetings (Perez et al., 2004; Roper et al., 2004) and in refereed reports (Stevenson et al., 2004). We describe herein the continuing studies that have made clear that the Xf genome contains genes that encode cell wall-degrading polygalacturonase (PG) and endo--1,4-glucanase (BGase) and that these two enzymes are sufficient to open the pit membrane network, suggesting that this is the mechanism used by the pathogen to permit systemic development in infected grapevines.
• Publication Date: Nov 2006
• Journal: 2006 Pierce's Disease Research Symposium