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Journal of Bacteriology, June 2001, p. 3597-3605, Vol. 183, No. 12
Department of Plant Pathology, University of
Wisconsin
Received 15 January 2001/Accepted 20 March 2001
Ralstonia solanacearum, a widely distributed and
economically important plant pathogen, invades the roots of diverse
plant hosts from the soil and aggressively colonizes the xylem vessels, causing a lethal wilting known as bacterial wilt disease. By examining bacteria from the xylem vessels of infected plants, we found that R. solanacearum is essentially nonmotile in planta,
although it can be highly motile in culture. To determine the role of
pathogen motility in this disease, we cloned, characterized, and
mutated two genes in the R. solanacearum flagellar
biosynthetic pathway. The genes for flagellin, the subunit of the
flagellar filament (fliC), and for the flagellar motor
switch protein (fliM) were isolated based on their
resemblance to these proteins in other bacteria. As is typical for
flagellins, the predicted FliC protein had well-conserved N- and
C-terminal regions, separated by a divergent central domain. The
predicted R. solanacearum FliM closely resembled motor
switch proteins from other proteobacteria. Chromosomal mutants lacking
fliC or fliM were created by replacing the
genes with marked interrupted constructs. Since fliM is
embedded in the fliLMNOPQR operon, the aphA
cassette was used to make a nonpolar fliM mutation. Both
mutants were completely nonmotile on soft agar plates, in minimal
broth, and in tomato plants. The fliC mutant lacked
flagella altogether; moreover, sheared-cell protein preparations from
the fliC mutant lacked a 30-kDa band corresponding to
flagellin. The fliM mutant was usually aflagellate, but
about 10% of cells had abnormal truncated flagella. In a biologically
representative soil-soak inoculation virulence assay, both nonmotile
mutants were significantly reduced in the ability to cause disease on tomato plants. However, the fliC mutant had wild-type
virulence when it was inoculated directly onto cut tomato petioles, an
inoculation method that did not require bacteria to enter the intact
host from the soil. These results suggest that swimming motility makes its most important contribution to bacterial wilt virulence in the
early stages of host plant invasion and colonization.
0021-9193/01/$04.00+0 DOI: 10.1128/JB.183.12.3597-3605.2001
Copyright © 2001, American Society for Microbiology. All rights reserved.
Ralstonia solanacearum Needs Motility
for Invasive Virulence on Tomato
Madison, Madison, Wisconsin 53706
*
Corresponding author. Mailing address: Department of
Plant Pathology, University of Wisconsin
Madison, 1630 Linden Dr.,
Madison, WI 53706. Phone: (608) 262-9578. Fax: (608) 263-2626. E-mail: cza{at}plantpath.wisc.edu.
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