Ftf1 is a multi-copy gene that encodes a transcription factor specifically involved in virulence in Fusarium oxysporum

We have described a new transcription factor (ftf1), with potential to be a virulence factor in Fusarium oxysporum, as it is found only in highly virulent strains and is drastically upregulated during early stages of infection of the host plant. We have also identified a highly homologous gene to ftf1, named ftf2, which can be found in pathogenic and non pathogenic fungi. Experimental results obtained in F. oxysporum f.sp. phaseoli and analyses of the genome sequences of other Fusarium sp., show that ftf1 is a multi-copy gene only present in F. oxysporum. The four copies in F. oxysporum f.sp. phaseoli are located in a small chromosome, closely linked to copies of transposon marsu, while the nine copies found in the genome of the lycopersicy strain are scattered over different chromosomes of the optic map, but also linked to different kinds of transposons, including marsu and Fot types. A gene silencing strategy was followed to demonstrate that ftf1 is required for virulence. Agrobacterium tumefaciens-mediated transfer of a hairpinexpression vector carrying fragments of the ftf1 gene allowed efficient silencing of ftf1 expression in highly virulent strains of F. oxysporum f.sp. lycopersici and f.sp. phaseoli. Several independent silenced transformants were tested for pathogenicity and virulence in inoculation assays using tomato and common bean as host plants. Inoculated plants showed clear symptoms of vascular disease but, in tomato, growth and size were similar to the noninoculated controls, and, in common bean, the disease progression was similar to that induced by weakly virulent strains. These results demonstrate that silencing of ftf1 do no abolish pathogenicity but reduces virulence against the host plant. The high homology between the genes ftf1 and ftf2 but their different role in the infection of the host plant, poses questions on gene evolution that may be of interest in unraveling how pathogenic strains have evolved from non pathogenic ones and on the origin of the formae speciales in F. oxysporum.