Molecular Biology of Resistance to Alternaria alternata f.sp. lycopersici

 

Tomato Alternaria stem canker is a necrotrophic fungal disease of Lycopersicon esculentum caused by Alternaria alternata f.sp lycopersici. Although the disease is not a major threat to tomato agriculture, being confined mostly to coastal regions of California and Greece, the symptoms of the disease are intriguing. The symptoms involve programmed cell death in an apoptotic like manner, in response to toxins (AAL-toxins) secreted by the fungus. AAL-toxins are structurally related to another group of toxins, the fumonisins. These toxins are often found to contaminate maize and consequently present a significant threat to human health as consumption of these toxins is associated with oesophageal cancer. We are interested in the mechanism behind the induction of apoptosis in susceptible tomato cultivars in response to challenge by Alternaria alternata f.sp lycopersici AAL-toxins.

Resistance to Alternaria alternata f.sp lycopersici and AAL-toxins is found at a single locus on chromosome three. Through map based cloning and complementation of susceptible root cultures the Asc gene, which confers resistance to AAL-toxins, has been isolated (Brandwagt et al. Proc.Natl.Acad.Sci. USA 97 (9): 4961-4966, 2000 [PDF]). The asc allele contains a 2 base pair deletion in exon 2, which leads to early termination of translation and presumably a non-functional gene product.

Many plant pathogen resistance genes have homology to one another and can be formed into a resistance gene superfamily. The Asc gene is unique in that it does not have any of the features associated with known resistance genes. The only homologous genes with a known function are two genes from the yeast Saccharomyces cerevisiae called LAG1 and DAGT1. These genes appear to be functionally redundant and yet the presence of two homologous genes is found in C.elegans, H.sapiens and A.thaliana. In yeast these genes are involved in the ER to Golgi trafficking of a unique set of proteins which are anchored in the membrane by a glycosylphosphatidylinositol chain which is attached to the N-terminus of the protein. The role of the Asc gene product in conferring resistance to Alternaria alternata f.sp lycopersici is currently unknown.

In deciphering the molecular biology and biochemistry of resistance to Alternaria alternata f.sp. lycopersici we hope to be able to investigate some of the mechanisms by which plants regulate apoptosis. The project is using transgenic tomato and yeast models to investigate the function of the Asc gene product. We are also looking to isolate Arabidopsis plants with decreased or knocked-out Asc homologues which should provide a more powerful model for studying apoptosis in plants.