A genomic search for the gene conferring resistance to fusarium wilt in tomato

Fusarium wilt is an economically important disease of tomatoes, caused by the soil-born fungus Fusarium oxysporum f. sp. lycopersici. There are three host-specific races of this pathogen. The dominant tomato gene I-2 confers resistance to race 2. The I-2 fusarium resistance gene was mapped genetically to chromosome 11 of tomato, between the RFLP markers TG105 and TG36, 0.4 centiMorgan (cM) from TG105. A mean value of 43 kb for each cM was assigned in the vicinity of I-2. We have generated new RFLP markers in the region by chromosome walking from TG105 towards I-2 on lambda clones, and by subcloning a 350 kb long YAC clone (YAC 8) that contains TG105. These RFLP markers were mapped physically on YAC 8 by PFGE. The location of I-2 relative to these markers was genetically estimated using a recombinant inbred (RI) segregating population. The order of the markers according to the RI population is inconsistent with their order on the physical map. A cDNA clone, D14, that was isolated by YAC 8, turned out to be 53% similar to xanthine dehydrogenase from mammals and flies. Antibodies raised against a part of the protein encoded by D14 recognize cross reacting material of MW 80 kD, that is highly enriched in nodules of legumes, and seems to be induced by various environmental and pathogenic stress conditions.