Successful search for a resistance gene in tomato targeted against a virulence factor of a fungal pathogen

The interaction between tomato and its fungal pathogenCladosporium fulvum complies with the gene-for-gene system, in which specific recognition of fungal proteins by plant genotypes with matching resistance genes results in host resistance. Two proteins, ECP1 and ECP2, secreted byC. fulvum during infection, are required for full virulence of the fungus on tomato. We chose the most important virulence factor, ECP2, for a targeted search for hypersensitive response (HR)-based resistance among a collection of tomato genotypes. By screening with recombinant potato virus X that expresses theEcp 2 gene, we identified four lines that respond with HR toward ECP2. The capacity to recognize ECP2 and induce HR is sufficient to confer resistance in tomato againstC. fulvum producing ECP2. Resistance is based on a single dominant gene, which we have designatedCf-ECP2 , for resistance toC. fulvum through recognition of ECP2. Accordingly, anEcp 2-minus strain created by gene replacement is pathogenic onCf-ECP2 plants. However, due to lack of ECP2 the mutant strain is only weakly virulent. All strains of a worldwide collection ofC. fulvum strains that were tested were found to produce a HR-inducing ECP2 protein. Because theCf-ECP2 gene operates through recognition of an important virulence factor, we expect it will confer durable resistance againstC. fulvum . A similar targeted approach should allow the discovery of new valuable resistance genes in other pathosystems.