Molecular basis of pathogenicity in the interaction between the fungal pathogen Cladosporium fulvum and tomato

The fungus Cladosporium fulvum is a leaf pathogen of tomato. During colonization of the extracellular spaces of the mesophyll it produces several proteins of low molecular weight that are secreted in the leaf apoplastic compartment. Some were found to be produced by all fungal strains and were named ECPs for ExtraCellularProteins. The importance of two of these, ECP1 and ECP2, in pathogenicity was tested by gene knock out studies. Although still pathogenic, the deficient strains exhibit lower parasitic fitness. So, though they are not essential, both proteins must be important factors for the establishment of the fungus in its host. Two other proteins were found to be race-specific elicitors as they are sufficient to trigger a necrosis response on tomato genotypes carrying the corresponding resistance genes Cf-4 and Cf-9 and were named AVR4 and AVR9, respectively for AViRulence factors 4 and 9. Partial information about their involvement in pathogenicity was obtained from studies on strains that have overcome the plant resistance. AVR9 absence appears to be linked with a natural reduction of aggressiveness allowing to hypothesize that this protein plays a role in pathogenicity. The situation of AVR4 is more ambiguous, as it seems rare that strains lose the encoding gene. However, at least one strain does not produce any AVR4 homolog protein although it retains high aggressiveness.