HOST-RESISTANCE TO A FUNGAL TOMATO PATHOGEN LOST BY A SINGLE BASE-PAIR CHANGE IN AN AVIRULENCE GENE

HOST genotype specificity in interactions between biotrophic pathogens and plants in most cases complies with the gene-for-gene model1; succ ess or failure of infection is determined by absence or presence of co mplementary genes, avirulence and resistance genes, in the pathogen an d host plant, respectively. Resistance, expressed by the induction of a hypersensitive response in the host, is envisaged to be based on rec ognition of the pathogen, mediated through direct interaction between products of pathogen avirulence genes (the so-called race-specific eli citors) and receptors in the host plant, the putative products of resi stance genes1. The interaction between the biotrophic fungus Cladospor ium fulvum and its only host, tomato (Lycopersicon esculentum), is a w ell-established model system for studying fungus-plant gene-for-gene r elationships1. Here we report the isolation of race-specific elicitor AVR4 of C. fulvum and the cloning of its encoding avirulence gene. We present evidence that, in nature, a single base-pair change in this av irulence gene leads to virulence of races previously avirulent on toma to genotypes carrying the complementary Cf4 resistance gene.