STABILITY OF TRANSFORMED ANTAGONISTIC FUSARIUM-OXYSPORUM STRAINS IN-VITRO AND IN SOIL MICROCOSMS

The stability of a genetically modified strain of Fusarium oxysporum u sed as antagonist against phytopathogenic formae speciales of F. oxysp orum was evaluated both in vitro and in microcosm assays. The Escheric hia coli hygromycin B phosphotransferase gene (hph), conferring hygrom ycin B resistance, was introduced by genetic transformation into a rec ipient strain marked by benomyl resistance and a dark red pigmentation . Hybridization with the complete plasmid suggested that the integrati on had generally occurred in a multiple-tandem array at multiple sites . Among nine independent transformants tested, only three of them were mitotically stable after four rounds of vegetative growth with no sel ective pressure, while six showed various changes in the integration p attern. One transformant had lost the ability to grow in the presence of hygromycin B. In soil microcosms all the transformants maintained t he hygromycin B resistant phenotype, but six of them showed rearrangem ent of transforming DNA. Only one strain (coded T26.40) underwent no o bvious rearrangement both after in vitro growth and after recovery fro m the soil microcosm. The nine transformants were used in three biolog ical control experiments against Fusarium wilt of carnation in compari son to two untransformed reference strains and to the recipient mutant . A high degree of variability in the biocontrol activity was observed throughout the experiments and only transformant T26.40 consistently controlled the incidence of disease. The results are discussed in rela tion to risk assessment of the release of transgenic antagonistic fung i.