Functional Analysis of the Ceramide Synthase Gene ALT7, A Homolog of the Disease Resistance Gene Asc1, in the Plant Pathogen Alternaria alternata

The tomato pathotype of Alternaria alternata produces a host-specific AAL-toxin and causes Alternaria stem canker on susceptible tomato cultivars. AAL-toxin is a sphinganine-analog mycotoxin which induces apoptotic cell death in tomato cells and mammalian cells by inhibiting ceramide biosynthesis. Insensitivity to the AAL-toxin in resistant tomatoes and other plants is conferred by the Asc1 gene, a homolog of the yeast ceramide synthase gene Lag1 . The ALT7 gene, a putative acyl-CoA-dependent ceramide synthase, was found to be located in the AAL-toxin biosynthetic ( ALT ) gene cluster of the tomato pathotype of A. alternata . ALT7 and Asc1 have the TLC (TRAM/Lag1/ CLN8) domain characteristic of proteins involved in ceramide biosynthesis and are members of the LASS/Lag family. To test the hypothesis that ALT7 and Asc1 , both of which are Lag1 ceramide synthase gene homologs, might share a common biological function as toxin tolerance genes, we have cloned and characterized ALT7. ALT7 -deleted mutants were generated to investigate the effects of the deletion on vegetative growth, sporulation, toxin-sensitivity, toxin-production and pathogenicity. The deletion of ALT7 has no deleterious effect on the toxin-producing pathogen, indicating that the gene does not act as a resistance/self-tolerance factor against the toxin in the toxin biosynthetic gene cluster.