Graduate Thesis Or Dissertation
 

Molecular factors that influence host range and virulence of A. tumefaciens

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  • Agrobacterium tumefaciens is a common soil bacterium that causes crown gall disease on many dicotyledonous plant species. It has been exploited for use in genetic engineering because it is able to promote transfer of any DNA segment to plant chromosomes. Many molecular factors in both the plant and bacterium contribute to successful infection and DNA transfer. Two bacterial factors were studied for this dissertation. The first was bacterial synthesis of cytokinins due to the induced expression of the gene tzs. Cytokinin production by some A. tumefaciens strains at the time of infection was hypothesized to enhance infection or influence host range. To test this hypothesis, heterologous strains engineered to express tzs and a native strain from which tzs was deleted were evaluated for tumor induction on a number of plant species. Major differences were seen when radish roots were infected with the tzs deletion strain. Morphological differences of the galls formed suggested a possible role for tzs in T-DNA transfer efficiency. β-glucuronidase assays with a mannopine synthase promoter-uidA fusion gene were used to study T-DNA transfer early in the infection process. No biological role for tzs was confirmed with either tumorigenesis or T-DNA transfer assays. However, optimized conditions for GUS fluorometric assays were established and the infection process was characterized. The second factor studied was a β-glucosidase gene in A. tumefaciens strain 83/73. This strain is highly infective on conifer hosts, whereas others are not. It is also able to hydrolyze the conifer phenolic compound, coniferin, to an active virulence gene inducer, coniferyl alcohol. This appears to be a major host range determining factor. As a first step in proving this role, the gene for the β-glucosidase enzyme responsible for hydrolysis of coniferin in strain B3/73 was cloned and sequenced. The deduced protein sequence has homology to other β-glucosidases, including a conserved active site.
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