CLAVIBACTER-MICHIGANENSIS SUBSP SEPEDONICUS ELICITS A HYPERSENSITIVE RESPONSE IN TOBACCO AND SECRETES HYPERSENSITIVE RESPONSE-INDUCING PROTEIN(S)

Strains of Clavibacter michiganensis subsp. sepedonicus, causal agent of bacterial ring rot of potato, showed marked differences in virulenc e on host plants. When infiltrated into tobacco leaves, virulent strai ns caused a rapid localized necrotic response (within 24 to 48 h) char acteristic of the hypersensitive response (HR), whereas nonpathogenic strains did not. Concentrated cell-free culture supernatants (CCS) fro m 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 metaboli sm suppressed the necrotic reaction of tobacco to CCS. No necrotic res ponse 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 concen tration of 18 mu g/ml, whereas those from a nonpathogenic strain did n ot, even at a concentration of 180 mu g/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.