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The Chitin-Binding Cladosporium fulvum Effector Protein Avr4 Is a Virulence Factor

September 2007 , Volume 20 , Number  9
Pages  1,092 - 1,101

H. Peter van Esse,1 Melvin D. Bolton,1,2 Ioannis Stergiopoulos,1 Pierre J. G. M. de Wit,1 and Bart P. H. J. Thomma1

1Laboratory of Phytopathology, Centre for Biosystems Genomics (CBSG), Wageningen University, Binnenhaven 5, 6709 PD Wageningen, The Netherlands; 2Department of Plant Pathology, North Dakota State University, Fargo 58105-1012, U.S.A.

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Accepted 19 April 2007.

The biotrophic fungal pathogen Cladosporium fulvum (syn. Passalora fulva) is the causal agent of tomato leaf mold. The Avr4 protein belongs to a set of effectors that is secreted by C. fulvum during infection and is thought to play a role in pathogen virulence. Previous studies have shown that Avr4 binds to chitin present in fungal cell walls and that, through this binding, Avr4 can protect these cell walls against hydrolysis by plant chitinases. In this study, we demonstrate that Avr4 expression in Arabidopsis results in increased virulence of several fungal pathogens with exposed chitin in their cell walls, whereas the virulence of a bacterium and an oomycete remained unaltered. Heterologous expression of Avr4 in tomato increased the virulence of Fusarium oxysporum f. sp. lycopersici. Through tomato GeneChip analyses, we demonstrate that Avr4 expression in tomato results in the induced expression of only a few genes. Finally, we demonstrate that silencing of the Avr4 gene in C. fulvum decreases its virulence on tomato. This is the first report on the intrinsic function of a fungal avirulence protein that has a counter-defensive activity required for full virulence of the pathogen.

Additional keywords: hypersensitive response, resistance gene.

© 2007 The American Phytopathological Society