SummaryThe resistance of tomato (Lycopersicon esculentum) to the pathogenic fungus Cladosporium fulvum complies with the gene-for-gene concept. Host resistance is based on speci®c recognition of extracellular fungal proteins, resulting in a hypersensitive response (HR). Five proteins secreted by C. fulvum were puri®ed and the encoding cDNA clone was obtained from two novel ones among them. Various tomato breeding lines and accessions of Lycopersicon pimpinellifolium were tested for their recognitional speci®city by injection of the puri®ed proteins or potato virus X-based expression of the cDNA. We found that HR-associated recognition of one or more of these proteins, in addition to recognition of the racespeci®c elicitors AVR4 and AVR9 of C. fulvum, occurs among Lycopersicon species. Studies on the inheritance of this recognition con®rmed that single dominant genes are involved. Furthermore, one of the extracellular proteins of C. fulvum is speci®cally recognized by Nicotiana paniculata, which is not a host for C. fulvum. These results indicate that plants have a highly effective surveillance system for the presence of`foreign' proteins, which, together with the high mutation rate of pathogens, can explain the complex gene-for-gene relationships frequently observed in pathosystems.

The interaction between tomato and its fungal pathogen Cladosporium fulvum complies with the gene-forgene 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 by C. 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 the Ecp2 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 against C. fulvum producing ECP2. Resistance is based on a single dominant gene, which we have designated Cf-ECP2, for resistance to C. fulvum through recognition of ECP2. Accordingly, an Ecp2-minus strain created by gene replacement is pathogenic on Cf-ECP2 plants. However, due to lack of ECP2 the mutant strain is only weakly virulent. All strains of a worldwide collection of C. fulvum strains that were tested were found to produce a HR-inducing ECP2 protein. Because the Cf-ECP2 gene operates through recognition of an important virulence factor, we expect it will confer durable resistance against C. fulvum. A similar targeted approach should allow the discovery of new valuable resistance genes in other pathosystems.

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A partial PCR clone of a PAL gene (MEPAL) was amplified from genomic DNA of cassava (Manihot escuknta). PAL enzyme activity and MEPAL mRNA levels were measured in leaves of cultiiar MCOL 22 following mechanical wounding or inoculation with 2 bacterial pathogens. MCOL 22 is resistant to Xanthomonas c~s s~v a e , which causes cassava bacterial necrosis and susceptible to X. axonopodis pv. manihotis, which causes cassava bacterial blight. PAL enzyme activity in the resistant interaction was significantly higher than in the susceptible interaction or the control. Similarly, MEPAL transcripts were detected in leaves during the resistant interaction, but not during the Susceptible reaction. RFLP analysis with MEPAL revealed a divergence between South American and African/Asian cultivars and showed promise for MEPAL as a marker for resistance to Xanthomonads.