| In evolutionarily divergent plant species, resistance to a wide range of pathogens (viruses, bacteria, fungi, and nematodes) is mediated by structurally related Resistance (R) gene products. R gene specified resistance, known as gene-forgene resistance, occurs if and only if both plant and pathogen are of the appropriate genotype. It is now assumed that this specificity is predicated on an interaction—either direct or indirect—between a plant molecule (R gene product) and a pathogen-derived molecule (Avirulence [ Avr] gene product).; In tobacco and other solanaceous species, the N gene confers gene-for-gene resistance to tobacco mosaic virus (TMV), a pathogen that infects more than 200 plant species including most members of the Solanaceae. In common with other R gene-mediated pathways, the resistance response induced by TMV infection involves a hypersensitive response (HR) and the induction of pathogenesis-related ( PR) genes. Research in this lab and others has established that the C-terminal, 50 kDa portion of the 126 kDa viral replicase can elicit an HR—even when presented to the plant in a non-viral context. However the molecular mechanisms of recognition remain unknown. In an effort to delineate the initial events that mediate host-virus interaction, a search for plant components that interact with this elicitor was conducted. This was complimented by a deletion analysis of the elicitor protein in which a more minimal region of the replicase was identified as necessary and sufficient for elicitation of N-mediated HR.; Using N transgenic tomato we have begun to dissect the N-mediated signal transduction pathway genetically and have identified a tomato mutant that is defective in N-mediated resistance. This locus was designated Son1 for s&barbelow;uppressor o&barbelow;f N&barbelow;. Genetic characterization of the mutant son1-1 allele revealed that it is a single recessive allele, unlinked to N. Plants bearing the son1-1 mutation exhibit an enhanced susceptibility to TMV relative to non-N containing (susceptible) wild type plants. Interestingly, experiments with additional pathogens demonstrated that the son1-1 mutation impairs not only N, but multiple R gene pathways. son1-1 is the first mutation found to affect such a wide range of R genes, suggesting that Son1 is at the junction of multiple resistance signaling cascades. Using bulk segregant analysis, we have localized Son1 to tomato chromosome 6. Fine mapping in this region identified two RFLP markers flanking the son1-1 mutation and a 15kb deletion in this interval. |
