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Clavibacter michiganensis subsp. Sepedonicus Elicits a Hypersensitive Response in Tobacco and Secretes Hypersensitive Response-Inducing Protein(s)

First, third, and ninth authors: Department of Biology, Laboratory of Plant Physiology and Molecular Biology, University of Turku, Biocity 6A, Turku, Finland; second, fourth, fifth, and eighth authors: Department of Bioagricultural Sciences and Pest Management, Colorado State University, Fort Collins 80523-1177; sixth author: TerraGen Diversity Inc., Ste. 300, 2386 E. Mall, Vancouver, BC V6T 1Z3, Canada; seventh author: Centre for Animal and Plant Health, 93 Mount Edward Rd., Charlottetown, PEI C1A 5T1, Canada

Strains of Clavibacter michiganensis subsp. sepedonicus, causal agent of bacterial ring rot of potato, showed marked differences in virulence on host plants. When infiltrated into tobacco leaves, virulent strains caused a rapid localized necrotic response (within 24 to 48 h) characteristic of the hypersensitive response (HR), whereas nonpathogenic strains did not. Concentrated cell-free culture supernatants (CCS) from virulent strains caused a necrotic reaction on tobacco, whereas CCS from nonpathogenic strains did not. The necrosis-inducing activity was heat stable and protease sensitive. Inhibitors of eukaryotic metabolism suppressed the necrotic reaction of tobacco to CCS. No necrotic response was observed when host plants were infiltrated with either cells or CCS from virulent strains. HR-inducing protein(s) from a virulent strain separated from the majority of other proteins on DEAE cellulose at 250 to 300 mM NaCl. Ammonium sulfate-precipitated proteins from a virulent strain produced a necrotic reaction at a total protein concentration of 18 μg/ml, whereas those from a nonpathogenic strain did not, even at a concentration of 180 μg/ml. We conclude that virulent strains of C. michiganensis subsp. sepedonicus elicit a typical HR in tobacco and secrete proteinaceous elicitor(s) of the nonhost HR.

Additional keywords: Corynebacterium sepedonicum.