PATTERNS OF XYLELLA FASTIDIOSA INFECTION IN PLANTS AND EFFECTS ON ACQUISITION BY INSECT VECTORS

• Author(s): Lindow, Steven; Purcell, Alexander; Wistrom, Christina;
• Abstract: This project investigates the effects of host plant tolerance on insect vector acquisition and spread of Xylella fastidiosa (Xf) from plants with a range of susceptibility to Pierces disease (PD). By characterizing vascular anatomy, bacterial populations, and sharpshooter acquisition of Xf from those plants, we may assess their ability to provide inoculum for PD spread. Previously reported data from paint infusion experiments did not identify significant anatomical measures as an explanation for tolerance or resistance differences among grape varieties. There were significant differences in vascular anatomy between alternate host species, and vessels of all alternate hosts were much shorter than in grapevines. In morning glory, 71% of vessels were less than 3cm, and the other species had between 40% and 63% of vessels <3cm. The longest vessel measured in any alternate host was 32cm (sunflower), and on average, sunflower and quinoa had longer vessels than mugwort and morning glory. Mugwort had roughly twice as many vessels at the stem base than morning glory, quinoa or sunflower stems of comparable diameter and age. Sunflower, mugwort and quinoa had vascular tissues in evenly distributed bundles with wide regions of parenchyma between them. Annual morning glory had a smaller number of large vessels distributed evenly along the cambium, compared to large numbers of small vessels in mugwort. As grapevines, most susceptible to Xf infection, had the longest vessels, and sunflower, the second most susceptible, had the next-longest vessels. The relationship between vascular anatomy and Xf host status appears complicated, and additional comparisons of overall vascular area and vessel distribution are required to generalize further. Plants were inoculated with an Xf strain that continually expressed green fluorescent protein (Gfp-Xf), or, as a control, the wild-type parent strain. Six weeks after inoculation, Xf-free sharpshooters were placed on the inoculation site for a 4-day acquisition access period, and tested for bacterial acquisition. Next, the inoculation site was examined with confocal microscopy and bacterial presence determined by culture. Six percent of grapes and 17% of alternate hosts had Gfp-Xf infections, compared to 50% of grapevines and 16% of alternate hosts inoculated with wild-type Xf. There was no difference in infection rate of wild-type Xf between grape cultivars. No colonized vessels were seen in plants infected with Gfp-Xf. Wild-type Xf populations were at least seven times lower in inoculated grape stems (10(5)to 10(6)CFU/g) compared to populations in distal petioles (10(6)to 10(8) CFU/g). Data analysis from those experiments is ongoing to determine the acquisition rate of Xf from inoculated stems.
• Publication Date: Dec 2005
• Journal: 2005 Pierce's Disease Research Symposium