THE BIOTROPHIC FUNGUS CLADOSPORIUM-FULVUM CIRCUMVENTS CF-4-MEDIATED RESISTANCE BY PRODUCING UNSTABLE AVR4 ELICITORS

The avirulence gene Avr4 conditions avirulence of the biotrophic fungu s Cladosporium fulvum on tomato genotypes carrying resistance gene Cf- 4 (MM-Cf4), Strains of the fungus that circumvent Cf-4-specific resist ance show various single point mutations in the coding region of the A vr4 gene. Similar to expression of the Avr4 gene, expression of the va rious virulent avr4 alleles is specifically induced during pathogenesi s. Polyclonal antibodies raised against the AVR4 elicitor, however, di d not detect AVR4 isoforms in MM-Cf4 plants infected by the different virulent strains, indicating that these isoforms are unstable, To anal yze whether the AVR4 isoforms still possess specific elicitor activity , the avr4 alleles were expressed in MM-Cf4 plants by using the potato virus X (PVX)-based expression system. Inoculation with PVX::Avr4 res ulted in the development of spreading lesions, eventually leading to p lant death, whereas the various PVX::avr4 derivatives induced symptoms ranging from severe necrosis to no lesions at all. We conclude that i nstability of the AVR4 isoforms that are produced by virulent strains is a crucial factor in circumvention of Cf-4-mediated resistance.