1Department of Plant Pathology, University of California, Riverside, CA 92521 U.S.A.; 2Departamento de Bioquímica, Facultad de Química, Conj. E. UNAM. Cd. Universitaria, Coyoacán, México 04510 D.F. México; 3Laboratoire de pathologie végétale, UMR 217 INRA-INAP/G-Paris 6, 16 rue Claude Bernard, 75005 Paris, France; 4Department of Animal Health and Biomedical Sciences, University of Wisconsin, Madison, WI 53706 U.S.A.
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Accepted 4 January 2002.
We developed improved virulence assays for Erwinia chrysanthemi 3937 on African violet varieties and devised a new method for the construction of precise bacterial gene knockouts. These methods were tested by constructing mutations in genes suspected to be involved with plant interactions. The virulence of the hrpG and hrcC mutant strains (both gene products presumed to be involved in protein secretion) was greatly reduced on leaves of semitolerant African violet varieties. An hrpN mutant strain produced delayed symptoms on African violet leaves and an hrpN Δpel (Δpel = five major pectate lyase genes deleted) double mutant was nonpathogenic. The hrcC and hrpG mutants did not produce a rapid hypersensitive response (HR) in tobacco, unlike the wild-type bacterium, and the hrpN mutant gave a reduced HR. The results, therefore, establish the importance of hrp genes in the virulence of E. chrysanthemi and their ability to elicit HR on nonhosts. The data also suggest that other effector proteins secreted by the Hrp system are required for full virulence and HR elicitation.
bacterial plant pathogens
polymerase chain reaction
type III secretion pathway.
© 2002 The American Phytopathological Society