CLARIFICATION OF XYLEM FEATURES LIMITING THE ENTRY AND SYSTEMIC SPREAD OF XYLELLA FASTIDIOSA IN GRAPEVINES Principal Investigator:

Grapevine Pierce’s disease (PD) caused by the bacterium Xyllela fastidiosa (Xf) is a devastating vascular disease and is jeopardizing the grape and wine industries in the United States due to the PD susceptibility of most important commercial grape genotypes. In our recent study, we compared the PD-susceptible commercial grapevine genotypes with some PDresistant genotypes newly obtained from traditional breeding program. Comparisons included intervessel pit membrane (PM) integrity, bacterial distribution, and tylose development, which are the most important factors determining the vascular disease resistance of a host plant. Our data indicate that Xf-infected PD-resistant genotypes could well maintain the integrity of their intervessel PMs and were found to have a very localized distribution of Xf cells while infected PD-susceptible genotypes were observed to have very porous or broken intervessel PMs and a systemic distribution of Xf cells. This demonstrates the strong correlation of intervessel PM integrity and limited bacterial distribution in infected vines. It is also revealed that grape genotypes with different PD resistances could develop tyloses in response to Xf infection. However, tylose development occurred to different spatial and quantitative extents among the genotypes, with an intensive tylose development throughout a vine in PD-susceptible genotypes but only small amounts of tyloses, these close to the inoculation site in PD-resistant genotypes. This also indicates that tylose development in PD-resistant genotypes does not seriously affect the water status of infected vines and suggests that the tylose development in PD-susceptible genotypes should make the PD symptom development worse by blocking vine water transport. These results provide information for identifying the factors affecting grape resistance to PD and, most likely, have identified vine characteristics useful to efforts aimed at the development and evaluation of an efficient approach to control this terrible disease. LAYPERSON SUMMARY Efficient approaches in control of Pierce’s disease (PD) based on the understanding of the PD-resistance mechanism of grapevine are being sought as strategies for management of PD in vineyards. This work investigated grapevine genotypes with different PD resistances by focusing on three important factors that may affect the pathogen's spread in a host grapevine and water status of an infected grapevine. Our data indicate that pit membranes (PMs; barriers of the bacterial spread through a grapevine’s water conducting system) and tyloses (occluding tissues of the water-conducting tissue) may be related to the PD resistance of the host grapevine. In PD-susceptible grapevines, the causal pathogen can become systemic by breaking the PMs in the vine water conducting system and PD disease symptoms may become more extensive due to tylose blockage of the water conducting tissue in infected vines. In PD-resistant grapevines, the PMs were well maintained and restricted bacterial spread and tyloses did not significantly affect the water transport in infected vines. The information is essential for the understanding and evaluation of PD resistance of new grape germplasm obtained from traditional breeding programs. INTRODUCTION Pierce's disease (PD) of grapevines has caused tremendous economic losses to the wine and table grape industries in the United States. The causal Xylella fastidiosa (Xf) is a xylem-limited bacterium that spreads only through the vessel system of a host grapevine (Purcell and Hopkins, 1996), thus, any factors affecting the systemic expansion of the Xf population that has been introduced initially into one or very few vessels should be relevant to the resistance vs. susceptibility of the infected vine. As the only avenue for the pathogen’s spread, the vessel system of grapevine has attracted a lot of research attention (e.g., Chatelet et al., 2006; Sun et al., 2006, 2007; Thorne et al., 2006). Individual vessels in a grapevine’s secondary xylem are relatively short (average length of 3-4 cm, Thorne et al., 2006), thus systemic movement of water, minerals or bacteria requires passage through multiple adjacently interconnected vessels. Movement from one vessel to the next requires passage through pit pairs, specialized wall structures that connect a vessel to its neighbors. In grapevines, contact with neighboring vessels occurs at multiple locations along the vessel's length and scalariform (i.e., organized in a ladder-like pattern) pit pairs