The recent isolation of the tomato resistance gene Cf-9, effective against the fungal pathogen Cladosporium fulvum, by transposon tagging may provide a new strategy for broad spectrum diseaseresistance. Plants that carry the corresponding Avr9 gene, and which exhibit somatic excision of Ds from Cf-9 show a variegation for defence-related necrosis. This leads to induction of the plant defence mechanisms and a broad spectrum suppression of disease. The enhanced resistance exhibited in the plants variegating for necrosis has been termed Genetic Aquired Resistance (GAR). The specific objective of this research is to engineer resistance against all races of Phytophthora infestans (late blight fungus) into existing commercial potato cultivars. Tobacco will also be transformed as a test plant .
Previous studies in tomato show that plants with a high frequency of small necrotic sectors produce the best resistance phenotype. To apply GAR to potato plants two strategies will be adopted to manipulate the frequency of somatic restoration of Cf-9 function. In System 1, an Ac element with low levels of excision will be inserted in theCf-9 sequence. In System 2, plants carrying a Cf-9 with a Ds element insertion will be crossed with plants carrying a TPase directed by different promoters. For both systems the T-DNA containing Cf-9* will also carry the 35S: Avr9 transgene.
The variegation for necrosis observed in the GAR plants will be correlated with the effectiveness of protection against different pathogens. In addition, the elements involved in the defence signalling pathways will be analysed by crossing the GAR plants to transgenic plants containing genes that alter various potential signal pathways (eg NahG, SOD, catalase).