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Loss of avirulence and reduced pathogenicity of a gamma-irradiated mutant of Fusarium oxysporum f. sp. lycopersici

Mes, J.J., Wit, R., Testerink, C.S., Groot, F. de., Haring, M.A., Cornelissen, B.J.C.
Phytopathology 1999 v.89 no.12 pp. 1131-1137
Fusarium oxysporum f. sp. lycopersici, virulence, pathogenicity, mutants, gamma radiation, irradiation, mutagenesis, genetic resistance, genes, genetic transformation, beta-glucuronidase, genetic markers, reporter genes, genetic polymorphism, chromosome translocation, Solanum lycopersicum var. lycopersicum, random amplified polymorphic DNA technique, amplified fragment length polymorphism
The tomato Fusarium resistance gene 1-2 confers resistance to F. oxysporum f. sp. lycopersici race 2 which expresses the corresponding avirulence gene avrI-2. To elucidate the molecular basis of this gene-for-gene interaction, we initiated a search for the avrI-2 gene. Gamma irradiation mutagenesis, using 137Cs, was performed to generate an avrI-2 mutant of F. oxysporum f. sp. lycopersici. To this end, a race 2 isolate was first transformed with a phleomycine resistance gene and a GUS marker gene in order to distinguish mutants from contaminating isolates. A total of 21,712 mutagenized colonies was tested for loss of avirulence on I-2-containing tomato seedlings. One mutant was selected that showed the expected loss of avirulence but, surprisingly, also showed reduced pathogenicity toward susceptible tomato plants. DNA analysis was subsequently used to visualize genomic changes in the mutant. Southern analysis on contour-clamped homogeneous electrophoretic field blots demonstrated a translocation of a 3.75-Mb chromosome in the mutant. Random amplified polymorphic DNA and amplified fragment length polymorphism analysis identified at least nine polymorphisms between the wild-type and mutant isolates. Most of these polymorphisms appeared as extra fragments in the mutant and contained repetitive DNA sequences.