The role of individual pectate lyases of Erwinia chrysanthemi strain 3937 in pathogenicity on saintpaulia plants

Abstract Mutants in each of the pel genes ( pelA , pelb , pelC , pelD and pelE ) which encode the five pectate lyase isozymes (PLa, PLb, PLc, PLd or PLe) of Erwinia chrysanthemi strain 3937, a pathogen of Saintpaulia ionantha , were obtained by insertion directed mutagenesis. The five mutant strains were tested for pathogenicity by inoculating into mini-saintpaulia plants. The acidic PLa and the basic PLd and PLe isozymes are the most important for pathogenicity. The mutants which were unable to produce the neutral, PLb or PLc, isozymes behaved like the wild type strain and macerated the whole miniplant. In contrast the PelA − and PelD − mutant strains were less virulent than the wild type strain, producing fewer systemically infected plants and much more limited maceration. The PelE − mutant strain was non-invasive, with maceration and bacteria being confined to the one leaf or the part of the leaf blade which was inoculated, suggesting that plant defence reactions had taken place.