Recovery of mutants impaired in pathogenicity after transposition of Impala in Fusarium oxysporum f. sp melonis

The ability of transposon impala to inactivate genes involved in pathogenic ity was tested in Fusarium oxysporum f. sp. melonis. Somatic excision of an impala copy inserted in the nitrate reductase-encoding niaD gene was posit ively selected through a phenotypic assay based on the restoration of nitra te reductase activity. Independent excision events were analyzed molecularl y and shown to carry reinserted impala in more than 70% of the cases. Mappi ng of reinserted impala elements on large NotI-restriction fragments showed that impala transposes randomly. By screening 746 revertants on plants, a high proportion (3.5%) of mutants impaired in their pathogenic potential wa s recovered. According to the kinetics of wilt symptom development, the str ains that were impaired in pathogenicity were clustered in three classes: c lass 1 grouped two strains that never induced Fusarium wilt symptoms on the host plant; class 2 and class 3 grouped 15 and 9 revertants which caused s ymptoms more than 50 and 30 days after inoculation, respectively. The first results demonstrate the efficiency of transposition in generating mutants affected in pathogenicity, which are usually difficult to obtain by classic al mutagenesis, and open the possibility to clone the altered genes with im pala as a tag.