MOLECULAR AND BIOCHEMICAL BASIS OF THE INTERACTION BETWEEN TOMATO ANDITS FUNGAL PATHOGEN CLADOSPORIUM-FULVUM

The interaction between the biotrophic fungal pathogen Cladosporium fu lvum and tomato complies with the gene-for-gene model. Resistance, exp ressed as a hypersensitive response (HR) followed by other defence res ponses, 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 is a 28 amino acid (aa) peptide and the AVR4 elicitor a 106 aa peptide which b oth induce HR in tomato plants carrying the complementary resistance g enes Cf9 and Cf4, respectively. The 3-D structure of the AVR9 peptide, as determined by H-1 NMR, revealed that AVR9 belongs to a family of p eptides with a cystine knot motif. This motif occurs in channel blocke rs, peptidase inhibitors and growth factors. The Cf9 resistance gene e ncodes a membrane-anchored extracellular glycoprotein which contains l eucine-rich repeats (LRRs). I-125 labeled AVR9 peptide shows the same affinity for plasma membranes of Cf9(+) and Cf9(-) tomato leaves. Memb ranes of solanaceous plants tested so far all contain homologs of the Cf9 gene and show similar affinities for AVR9. It is assumed that for induction of HR, at least two plant proteins (presumably CF9 and one o f his homologs) interact directly or indirectly with the AVR9 peptide which possibly initiates modulation and dimerisation of the receptor, and 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 an d tested their biological activity on Cf9 genotypes of tomato. A posit ive correlation was observed between the biological activity of the mu tant AVR9 peptides and their affinity for tomato plasma membranes. Rec ent results on structure and biological activity of AVR4 peptides enco ded by avirulent and virulent alleles of the Avr4 gene (based on expre ssion studies in PVX) are also discussed as well as early defence resp onses induced by elicitors in tomato leaves and tomato cell suspension s.