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Cladosporium fulvum Avr4 Protects Fungal Cell Walls Against Hydrolysis by Plant Chitinases Accumulating During Infection

December 2006 , Volume 19 , Number  12
Pages  1,420 - 1,430

Harrold A. van den Burg , 1 , 2 Stuart J. Harrison , 3 Matthieu H. A. J. Joosten , 2 Jacques Vervoort , 1 and Pierre J. G. M. de Wit 2

1Laboratory of Biochemistry, Wageningen University, Dreijenlaan 3, NL-6703 HA Wageningen, The Netherlands; 2Laboratory of Phytopathology, Wageningen University, Binnenhaven 5, NL-6709 PD Wageningen, The Netherlands; 3Syngenta-Mogen, Einsteinweg 97, 2333 CB Leiden, Netherlands, Syngenta Mogen, Einsteinweg 97, 2333 CB Leiden, The Netherlands

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Accepted 12 July 2006.

Resistance against the leaf mold fungus Cladosporium fulvum is mediated by the tomato Cf proteins which belong to the class of receptor-like proteins and indirectly recognize extracellular avirulence proteins (Avrs) of the fungus. Apart from triggering disease resistance, Avrs are believed to play a role in pathogenicity or virulence of C. fulvum. Here, we report on the avirulence protein Avr4, which is a chitin-binding lectin containing an invertebrate chitin-binding domain (CBM14). This domain is found in many eukaryotes, but has not yet been described in fungal or plant genomes. We found that interaction of Avr4 with chitin is specific, because it does not interact with other cell wall polysaccharides. Avr4 binds to chitin oligomers with a minimal length of three N-acetyl glucosamine residues. In vitro, Avr4 protects chitin against hydrolysis by plant chitinases. Avr4 also binds to chitin in cell walls of the fungi Trichoderma viride and Fusarium solani f. sp. phaseoli and protects these fungi against normally deleterious concentrations of plant chitinases. In situ fluorescence studies showed that Avr4 also binds to cell walls of C. fulvum during infection of tomato, where it most likely protects the fungus against tomato chitinases, suggesting that Avr4 is a counter-defensive virulence factor.

Additional keywords: defense , hypersensitive response , resistance genes .

© 2006 The American Phytopathological Society