Plant Genome IV Conference Plant Genome IV Conference
Fusarium oxysporum is a soil-borne pathogen causing diseases in many agricultural and floricultural crops. F. oxysporum f. sp. lycopersici, the casual agent of fusarium wilt of tomato (Lycopersicon esculentum), is subdivided into three races (race 1,2 and 3). We have cloned the dominant I2 gene from tomato conferring resistance against F. oxysporum f. sp. lycopersici race 2, using a positional cloning strategy. Flanking markers were used to screen a 2.2 genome equivalent tomato YAC library. Several YAC clones were identified forming a YAC contig of 2.5 Mb in size. AFLP markers were derived from the YAC containing the I2 locus (size 750 kb) to saturate the region with new markers. Based on the analysis of recombinants of genetic crosses with the additional AFLP markers, the size of the I2 locus could be narrowed down to about 130 kb. A cosmid library of the 750 kb YAC in a binary vector was constructed. Individual cosmid clones corresponding to the 130 kb I2 locus were transferred to a susceptible tomato line for complementation analysis. Three overlapping cosmids, sharing a DNA segment of 8 kb in size, were found to give a resistant phenotype in tomato. Fusarium tests were performed on R1 seedlings and confirmation of the resistance was accomplished via crosses to a susceptible plant. The nucleotide sequence of the 8 kb DNA segment was determined and one large open reading frame could be deduced showing the typical characteristics of recently cloned plant disease resistance genes.