Minisymposium 17: Water

Abs # 32005: Xylella fastidiosa infection and ethylene exposure cause xylem disruption and impaired water movement in grapevine shoots

Presenter:	Pérez, Alonso G, agperez@ucdavis.edu
Authors	Pérez, Alonso G (A)   Greve, L. Carl  (A)   Walton, Jeffrey H (B)   Shackel, Ken A (A)   Thorne, Eleanor T (C)   Matthews, Mark M (C)   Lurie, Susan  (D)   Labavitch, John M (A)
Affiliations:	(A): University of California, Pomology Department (B): University of California, Nuclear Magnetic Resonance Facility (C): University of California, Viticulture and Enology Department (D): The Volcani Center, Bet-Dagan, Israel

Pierce’s Disease (PD) is a serious bacterial disease of grapevines in California that is caused by the xylem-limited bacterium Xylella fastidiosa (Xf). Vines affected by PD become nonproductive, declining in vigor and ultimately dying from the disease within two years. Currently, there is no known cure for the disease. Until recently, it was thought that multiplication of the bacteria within the xylem-vessels was the sole factor responsible for the blockage of the water movement in the plant. However, results from our studies have provided substantial support for the idea that vessel obstructions, and likely other aspects of the PD syndrome, result from the grapevine’s active responses to the presence of Xf, rather than to the direct action of the bacterium. The use of Magnetic Resonance Imaging (MRI) to observe the distribution of water within the xylem has allowed us to test the role of the plant hormone ethylene in promoting xylem obstruction development and consequent reduction in vine water transport. Thus far, MRI has shown a clear, progressive, and localized deterioration of water movement capacity, in both, grapevines infected with Xf and grapevines treated with ethylene in the absence of Xf. Stem cross section MRIs of control (healthy) plants show defined xylem rays, in which individual vessels can be clearly observed. Whereas MRIs of plants infected with Xf or exposed to ethylene become less sharp, making it more difficult to discriminate structure, particularly, individual vessels. In addition, MRIs of infected and ethylene-exposed plants show that major sectors of the xylem appear air-embolized (the magnetic signal is lost in cavitated vessels). Experiments in which cavitations are first induced and vessels are subsequently re-filled have shown the suitability of MRI for revealing cavitations. The loss of xylem function, assessed by MRI, has been also correlated with a decrease in stem hydraulic conductivity and the presence of tyloses (balloon-like outgrowths from living parenchyma cells that expand into and fill adjacent water conduits). Carbohydrate-rich gels that can plug the vessels could also be the cause of physical xylem obstructions. We think that ethylene may be involved in a series of cellular events that allow the plant to sense the presence of Xf and stimulate a plant defense response that includes the production of tyloses and gels, perhaps in an effort to slow systemic movement of the bacteria.