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Molecular Plant Pathology

Variation in Cutinase, Esterase, and Chromosome Patterns in Nop Mutants of a Transformed Pathogenic Strain of Phytophthora capsici. G. L. Mena, Research assistant, Departamento de Ingeniería Genética de Plantas, CINVESTAV, Unidad Irapuato, Apartado Postal 629, 36500 Irapuato, Gto, México; C. I. Muñoz, P. A. Guzmán, and A. M. Bailey. graduate research assistant, associate professor, and professor, respectively, Departamento de Ingeniería Genética de Plantas, CINVESTAV, Unidad Irapuato, Apartado Postal 629, 36500 Irapuato, Gto, México. Phytopathology 84:502-508. Accepted for publication 2 February 1994. Copyright 1994 The American Phytopathological Society. DOI: 10.1094/Phyto-84-502.

We have designed a strategy to isolate nonpathogenic (Nop) mutants of the Oomycete fungus Phytophthora capsici. This strategy involves direct enrichment of Nop mutants by exposure of mutagenized zoospores to pepper (Capsicum annuum) seedlings and the isolation of germinating zoospores that show chemotaxis but are unable to penetrate the host tissue. After enrichment, 13 Nop mutants were isolated at a frequency of 2–5 × 10^–2 by N-methyl-N’-nitro-N-nitrosoguanidine mutagenesis. The nonpathogenic phenotype of the mutants was tested by inoculating pepper seedlings as well as pepper fruits with zoospores and mycelia; in all cases no disease symptoms were observed. The Nop phenotype was stable for at least 25 asexual generations. In addition to the Nop phenotype, the Nop mutants showed reduced cutinase and esterase activities, and the electrophoretic karyotype of mutant strains was changed compared to the progenitor.