MOLECULAR-GENETIC CHARACTERIZATION OF THE ASC-LOCUS OF TOMATO CONFERRING RESISTANCE TO THE FUNGAL PATHOGEN ALTERNARIA-ALTERNATA F SP LYCOPERSICI

The Alternaria stem canker disease of tomato is caused by the fungal p athogen Alternaria alternata f. sp. lycopersici and its host-selective AAL-toxins. Resistance to the pathogen and insensitivity to the toxin s are conferred by the Ase locus on chromosome 3L. Sensitivity to AAL- toxins is a relative character; the toxins inhibit development of all tested tomato tissues but susceptible cultivars are much more sensitiv e than resistant cultivars. In addition to tomato, some other plant an d animal species are sensitive to the toxins as well. The likely mode of action of AAL-toxins is interference with sphingolipid biosynthesis by specific inhibition of ceramide synthase activity. To molecularly isolate Asc, transposon tagging and positional cloning strategies are applied. As a first step, transposon insertions and restriction fragme nt length polymorphism (RFLP) markers are identified in proximity of t he Asc locus. Subsequently, the transposons are used to inactivate Asc by insertion mutagenesis, and the RFLP markers are used to identify y east artificial chromosomes (YACs) with tomato DNA inserts. Once an As c-insertion mutant and/or a YAC encompassing Asc has been obtained, ph ysical isolation and characterisation of Asc will be conceivable. Eluc idation of the molecular role of Ase will illuminate the specificity o f host recognition by Alternaria alternata f. sp. lycopersici.