Molecular and biochemical basis of avirulence in the interaction between tomato and the fungal pathogen Cladosporium fulvum
1996
Kooman-Gersmann, M. | Honee, G. | Weide, R. | Lauge, R. | Joosten, M.H.A.J. | Vossen, P. | Wit, P.J.G.M. de (Wageningen Agricultural Univ. (Netherlands). Dept. of Phytopathology) | Vogelsang, R. | Vervoort, J.J.M.
The interaction between the biotrophic fungal pathogen Cladosporium fulvum and tomato complies with the gene-for-gene model. Resistance, expressed as a hypersensitive response (HR) followed by other defence responses, is based on recognition of products of avirulence genes from C. fulvum (race-specific elicitors) by receptors (putative products of resistance genes) in the host plant tomato. The AVR9 elicitor, a 28 amino acid (aa) peptide and the AVR4 elicitor, a 106 aa peptide, induce HR in tomato plants carrying the complementary resistance genes Cf-9 and Cf-4, respectively. The 3-D structure of the AVR9 peptide, as determined by 1H NMR, revealed that AVR9 belongs to a family of proteins with a cystine knot motif. This motif is found in channel blockers, proteinase inhibitors and growth factors. Binding studies, using the 125I-labelled AVR9 peptide, show that a high affinity binding site for AVR9 is present on plasma membranes of tomato leaves. No difference was observed between membranes of tomato leaves with or without the Cf-9 resistance gene. Furthermore, membranes of all solanaceous plants tested show binding sites with similar affinities for AVR9 compared to tomato membranes. It is assumed that for induction of HR, at least two plant proteins interact directly or indirectly with the AVR9 peptide. This then leads to activation of various other proteins involved in downstream events eventually leading to HR. We have created several mutants of the Avr9 gene, expressed them in the potato virus X (PVX) expression system and tested their biological activity on Cf-9 genotypes of tomato. For most of the peptides, a positive correlation was observed between the biological activity of the mutant AVR9 peptides and their affinity for tomato plasma membranes. This implies that the observed binding site for AVR9 is required for HR in tomato plants that contain the Cf-9 resistance gene.
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