THE GUS GENE FUSION SYSTEM (ESCHERICHIA-COLI BETA-D-GLUCURONIDASE GENE), A USEFUL TOOL IN STUDIES OF ROOT COLONIZATION BY FUSARIUM-OXYSPORUM

The plant-pathogenic fungus Fusarium oxysporum was successfully transf ormed with the beta-D-glucuronidase gene from Escherichia coli (gusA) (GUS system) in combination with the gene for nitrate reductase (niaD) as the selectable marker. The frequency of cotransformation, as deter mined by GUS expression on plates containing medium supplemented with 5-bromo-4-chloro-3-indolyl glucuronide (GUS+), was very high (up to 75 %). Southern hybridization analyses of GUS+ transformants revealed tha t single or multiple copies of the gusA gene were integrated into the genomes. High levels of GUS activity are expressed in some transforman ts, but activity in F. oxysporum does not appear to be correlated with the copy number of the gusA gene. Since the highest activity was foun d in a transformant with a single copy, it can be assumed that sequenc e elements of F. oxysporum integrated upstream of the gene can act as a promoter or enhancer. Expression of the gusA gene was also detected during growth of the fungus in plants, indicating that the GUS system can be used as a sensitive and easy reporter gene assay in F. oxysporu m.