POTENTIAL APPLICATION OF GENETIC TRANSFORMANTS OF FUSARIUM-OXYSPORUM-F SP RAPHANI FOR ASSESSING FUNGAL AUTECOLOGY

To pursue the behavior of a soil-borne plant pathogen, Fusarium oxysporum f. Sp. raphani, in soils, a genetically-transformed form of the pathogen was used as a model fungus. Strain SSI of the pathogen was transformed to resistance to hygromycin B (Hyg) by introducing the vector plasmid, pDH25, into the fungal chromosome. Transformant PEG-4 among the transformants was chosen as a model inoculum, because it possessed the same abilities of vegetative growth and conidiation as the wild-type strain, as well as considerable resistance to Hyg and mitotic stability of the marker phenotype. Population dynamics of the transformant was periodically monitored after inoculation into a soil. By using Komada's selective medium containing Hyg, PEG-4 could be specifically detected even at a low density of several CFU g-1 soil. Within 3 days after inoculation, the number of transformant propagules increased rapidly within soil aggregates, and then the population remained stable in this soil fraction during 90 days of incubation. Hyg-resistant fusaria recovered from the soil were confirmed to be from propagules of transformant PEG-4 by Southern blot analysis of their genomic DNA. These results suggest that the use of genetic transformants of this Fusarium pathogen makes it possible to monitor sensitively the behavior of the fungus in soils and to assess fungal autecology.