MOLECULAR CHARACTERIZATION OF THE INTERACTION BETWEEN THE FUNGAL PATHOGEN CLADOSPORIUM-FULVUM AND TOMATO

The fungus Cladosprium fulvum is a biotrophic leaf pathogen of tomato. The fungus develops in the intercellular space without forming specia lized feeding structures and does not affect the leaf tissue. The outc ome of the C. fulvum-tomato interaction can be described by the gene-f or-gene model. Failure of infection is expressed by a hypersensitive r esponse. Two fungal proteins, ECP1 and ECP2, have been isolated and th eir corresponding genes have been cloned. In a compatible interaction including many physiological races ECP1 and ECP2 are highly produced a nd a role in pathogenicity is suggestive. The ecp1 gene shows some hom ology with tumor necrosis factor receptors (TNFRs) while the ecp2 gene shows no homology with sequences known in data bases. However, disrup tion of one of the two genes showed no reduced pathogenicity of the fu ngus. Two race-specific elicitors, AVR4 and AVR9, have been isolated a nd their corresponding genes have been cloned. The avirulence genes Av r4 and Avr9 are only present in C. fulvum avirulent on Cf-4 and Cf-9 c ultivars, respectively. The expression of these two genes is, like the expression of the ecp genes, highly induced when the fungus grows in planta. Disruption of the Avr9 gene in wild type avirulent races leads to virulence on tomato genotypes carrying the complementary resistanc e gene Cf-9. A single base-pair change in the avirulence gene Avr4 lea ds to virulence on tomato genotypes carrying the Cf-4 resistance gene. Isolation, characterization and possible function of ECP1, ECP2, AVR4 , and AVR9 will be discussed.