Molecular Genetic Analyses of Host Range in Crown Gall Tumors

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The limited host range (LHR) biovar strains of Agrobacterium tumefaciens form tumors on grapevine but on virtually no other plants. Wide host range strains (WHR) fail to induce tumors on grapevine, but often induce necrosis at the site of inoculation. We hope to understand the molecular basis of these several different interactions of LHR and WHR Agrobacterium strains with grapevines as a model system for plant microbe interactions. These studies build on the physical and functional divergence of the VirA protein in each strain and the critical role this protein plays in plant signal transduction and plant cell transformation. To distinguish transformation from tumor formation, the biological assays for plant cell transformation will be performed using a binary vector system carrying a B-glucuronidase gene behind a 35A CaMV promoter. To demonstrate the significance of the four domains of VirA with host range, we will exchange domains between WHR and LHR strains and observe the biological consequences. We will isolate and identify compounds in grapevines that induce vir genes in LHR and WHR strains and determine their sites of interaction with the VirA protein. The locus responsible for inducing the necrotic response in grapevines will be identified by mutant analysis of T-DNA.