Affiliation:
1. Department
of Plant and Microbial Biology
2. Department of Environmental
Science, Policy and Management, University of
California, Berkeley, California 94720
Abstract
ABSTRACT
Xylella
fastidiosa
causes Pierce's disease of grapevine as well as
several other major agricultural diseases but is a benign endophyte in
most host plants.
X. fastidiosa
colonizes the xylem vessels of
host plants and is transmitted by xylem sap-feeding insect vectors. To
understand better the pattern of host colonization and its relationship
to disease, we engineered
X. fastidiosa
to express a green
fluorescent protein (Gfp) constitutively and performed confocal
laser-scanning microscopic analysis of colonization in a susceptible
host,
Vitis vinifera.
In symptomatic leaves, the fraction of
vessels colonized by
X. fastidiosa
was fivefold higher than in
nearby asymptomatic leaves. The fraction of vessels completely blocked
by
X. fastidiosa
colonies increased 40-fold in symptomatic
leaves and was the feature of colonization most dramatically linked to
symptoms. Therefore, the extent of vessel blockage by bacterial
colonization is highly likely to be a crucial variable in symptom
expression. Intriguingly, a high proportion (>80%) of
colonized vessels were not blocked in infected leaves and instead had
small colonies or solitary cells, suggesting that vessel blockage is
not a colonization strategy employed by the pathogen but, rather, a
by-product of endophytic colonization. We present evidence for
X.
fastidiosa
movement through bordered pits to neighboring vessels
and propose that vessel-to-vessel movement is a key colonization
strategy whose failure results in vessel plugging and
disease.
Publisher
American Society for Microbiology
Subject
Ecology,Applied Microbiology and Biotechnology,Food Science,Biotechnology