Influence of Vitis xylem fluid and xylem fluid plus cecropin on growth of Xylella fastidiosa
Colony growth of Xylella fastidiosa (UCLA ID and STL PD strains) was quantified after incubation for 48 h in xylem fluid of Vitis rotundifolia Michx. cv. Noble and Vitis vinifera L. cv. Chardonnay. Xylem fluid was collected from grapevines in the field (dormant and growing season) and from container-grown plants in a screen house (growing season). Colony forming units(.)ml(-1) (cfu(.)ml(-1)) were counted 15 d after plating on periwinkle wilt (PW+) medium. Colony growth was promoted or inhibited compared to PW+ medium, and was dependent on X.fastidiosa strain, plant species and source of xylem fluid. The efficacy of cecropin A and B was tested against this bacterium. Colony growth of X.fastidiosa was greatly inhibited after a I-h-exposure to cecropin A or B. The minimum inhibitory concentration (MIC) of cecropin A or B for 100 % inhibition of X.fastidiosa was < 1 muM. The activity of cecropin B in xylem fluid of V rotundifolia cv. Noble was progressively reduced over time from 0.2 to 24 h. When 2 and 10 muM concentrations of cecropin A and cecropin B were mixed with xylem fluid for 24 h, a substantial amount of bacterial growth occurred after subsequent plating; shorter time intervals did not degrade the cecropins and did not prevent colony growth. Cecropin B (1 muM) added to xylem fluid of V rotundifolia cv. Noble and V vinifera cv. Chardormay for 24,48,72 and 96 h did not prevent subsequent colony growth. Colony number tended to be higher for V rotundifolia cv. Noble than V vinifera cv. Chardonnay. Tricine-sodium dodecyl sulphate polyacrylamide gel electrophoresis (Tricine-SDS-PAGE) of cecropin B in xylem fluid showed that cecropin B degraded completely (V vinifera cv. Chardormay) or almost completely (V rotundifolia cv. Noble) after 96 h.