Vessel embolism and tyloses in early stages of Pierce's disease

Abstract

Background and Aims: Tyloses and embolisms have been reported to impair water transport during the development of Pierce's disease (PD), caused by the xylem-dwelling pathogen Xylella fastidiosa. This work investigates the relative importance of these xylem conduit obstructions in stems of inoculated Vitis vinifera plants. Methods and Results: After 18 weeks in a greenhouse, internodes from control and needle-inoculated Chardonnay vines were assessed for embolisms using non-destructive magnetic resonance imaging (MRI). Tyloses were identified by subsequent destructive histology and light microscopy examination. Embolism of individual or small sectors of vessels was detected by MRI analysis of a control shoot, usually in the proximity of the pith; virtually no tyloses were observed in the corresponding histological sections. From a symptomatic (inoculated) shoot, MRIs revealed large areas of embolised vessels extending radially towards the epidermis in most of the internodes; several individual or groups of two to three vessels appeared to be occluded with tyloses. Conclusions: In all assessed internodes, MRI-detected embolisms were found more frequently than tyloses, suggesting that over the course of PD embolism formation precedes tylose development. Significance of the Study: Embolisms appear to be the first and predominant type of occlusion in stems during early PD progression; nevertheless, their importance for explaining water transport impairment has been overlooked in most studies.