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The AVR4 Elicitor Protein of Cladosporium fulvum Binds to Fungal Components with High Affinity

December 2002 , Volume 15 , Number  12
Pages  1,219 - 1,227

Nienke Westerink , 1 Ronelle Roth , 1 Harrold A. Van den Burg , 2 Pierre J. G. M. De Wit , 1 and Matthieu H. A. J. Joosten 1

1Laboratory of Phytopathology, Wageningen University, Binnenhaven 5, 6709 PD Wageningen, The Netherlands; 2Laboratory of Biochemistry, Wageningen University, Dreijenlaan 3, 6703 HA, Wageningen, The Netherlands

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Accepted 7 August 2002.

The interaction between tomato and the fungal pathogen Cladosporium fulvum complies with the gene-for-gene system. Strains of C. fulvum that produce race-specific elicitor AVR4 induce a hypersensitive response, leading to resistance, in tomato plants that carry the Cf-4 resistance gene. The mechanism of AVR4 perception was examined by performing binding studies with 125I-AVR4 on microsomal membranes of tomato plants. We identified an AVR4 high-affinity binding site (KD = 0.05 nM) which exhibited all the characteristics expected for ligand-receptor interactions, such as saturability, reversibility, and specificity. Surprisingly, the AVR4 high-affinity binding site appeared to originate from fungi present on infected tomato plants rather than from the tomato plants themselves. Detailed analysis showed that this fungus-derived, AVR4-specific binding site is heat- and proteinase K-resistant. Affinity crosslinking demonstrated that AVR4 specifically binds to a component of approximately 75 kDa that is of fungal origin. Our data suggest that binding of AVR4 to a fungal component or components is related to the intrinsic virulence function of AVR4 for C. fulvum.

© 2002 The American Phytopathological Society