LINKING THE MODEL OF THE DEVELOPMENT OF PIERCE’S 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 PIERCE’S DISEASE Project Leaders:

INTRODUCTION For several years we have been studying the development of Pierce’s disease (PD) in grapevines. Our studies have been guided by a model of PD development that was 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 Although some aspects of the model are still being tested (the current project), our hypotheses have proven to be quite accurate. We have shown that xylem vessel obstruction (tyloses, plant cell wall component-derived gels, and, perhaps, 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; Labavitch and Matthews, 2003) and, more recently, at disciplinary scientific society meetings (Perez et al., 2004; Roper et al., 2004) and in referred reports (Stevenson et al., 2004). This research has drawn together an assortment of UC Davis (UCD) researchers, each bringing a different disciplinary research orientation to the study. In addition, through regular discussions at UCD and with other researchers who have become colleagues as a result of meetings at the annual PD Symposia, we have begun to see how important connections can be made between our studies and those of other PD researchers. In this progress report, we discuss the successes we have had in filling the gap in the portion of the model that proposes the links: Xf population increase => production of enzymes by Xf => cell wall digestion => oligosaccharide signals => ethylene synthesis rise