Identification of Pathogenicity Determinants of Erwinia carotovora subsp. carotovora by Transposon Mutagenesis. Minna Pirhonen. Department of Molecular Genetics, Uppsala Genetic Center, Swedish University of Agricultural Sciences, S-750 07 Uppsala, Sweden. Hannu Saarilahti, Maj-Britt Karlsson, and E. Tapio Palva. Department of Molecular Genetics, Uppsala Genetic Center, Swedish University of Agricultural Sciences, S-750 07 Uppsala, Sweden.. MPMI 4:276-283. Accepted 8 February 1991. Copyright 1991 The American Phytopathological Society.

Transposon mutagenesis and large-scale virulence screening of the isolated mutants on axenic tobacco seedlings were employed to identify pathogenicity determinants of the soft rot pathogen Erwinia carotovora subsp. carotovora. Screening of 6,200 prototrophic transposon mutants resulted in the isolation of several classes of strains exhibiting reduced virulence or avirulence. Characterization of these mutants showed that the majority of the strains were defective in motility and showed a reduced virulence phenotype. The mutants in the other main category were avirulent and were affected in exoenzyme production and growth on pectin. Some of these mutants appeared to produce reduced amounts of pectin lyase (Pnl‾), but most of the strains were defective in either secretion (Out‾) or production (Exp‾) of several extracellular enzymes. The Exp? mutants represent a novel class of exoenzyme-negative mutants showing a pleiotropic defect in production and secretion of pectic enzymes, cellulase, and protease. The wild-type allele for one class of Exp‾ mutants was cloned by complementation after bacteriophage T4GT7 transduction of a genomic library.