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The Putative Secreted Serine Protease Chp-7 Is Required for Full Virulence and Induction of a Nonhost Hypersensitive Response by Clavibacter michiganensis subsp. sepedonicus

¹Department of Applied Biology, University of Helsinki, PO Box 27, 00014 Helsinki, Finland; ²Department of Plant Pathology, University of Minnesota, St. Paul, MN, U.S.A.; ³Department of Bioagricultural Sciences and Pest Management, Colorado State University, Ft. Collins, CO, U.S.A.; ⁴Department of Biosciences, University of Helsinki, 00014 Helsinki, Finland

Molecular biological studies on Clavibacter michiganensis subsp. sepedonicus, the causal agent of bacterial ring rot of potato, have gained greater feasibility due to the recent availability of whole genomic sequences and genetic tools for related taxa. Here, we describe the first report of construction and characterization of a transposon (Tn) mutant library of C. michiganensis subsp. sepedonicus sp. strain R10. Since virulence of R10 in potato has been shown previously to be associated with elicitation of a nonhost hypersensitive response (HR), the mutant library was screened initially for loss of HR in tobacco. The screen identified two HR-negative mutants containing Tn insertions within the same gene, CMS2989 (chp-7), although at distinct locations. chp-7 is one of 11 pat-1 homologs in C. michiganensis subsp. sepedonicus. HR-negative mutants of R10 multiplied to the same extent as wild type in planta but were less virulent in potato. Complementation with chp-7 restored virulence as well as the HR phenotype. Together, these findings demonstrate a role for chp-7 in C. michiganensis subsp. sepedonicus--plant interactions.