Signal compound specificity in agrobacterium tumefaciens
Agrobacterium tumefaciens , a soil-borne gram negative bacterium, is the causative agent of crown gall disease and one of the most promising vectors for genetic engineering in plants. It is known to respond to the presence of certain plant-derived phenolic compounds by expressing an essential set of genes for virulence (vir) (Bolton et al., 1986; Stachel et al., 1985-a). However, only one report has described the isolation and identification of virulence inducing phytochemicals producecd by a
... ls producecd by a host plant (Stachel etal., 1985-a). These compounds are acetosyringone (AS) and a-hydroxyacetosyringone (OH-AS), or 3,5-dimethoxy-4-hydroxy- and α-hydroxy-3,5-dimethoxy-4-hydroxy-acetophenone, respectively. Since these compounds have never previously been reported from plant tissues and are not likely to be of widespread occurrence, it seemed unlikely that these were the only signal compounds for this wide host range pathogen. In addition, the results of Bolton et al. (1986), who found that a mixture of lower molecular weight phenolics could also induce vir gene expression, raised the question as to exactly which chemical structures could act as vir -inducers. This thesis reports a quantitative re-examination of the results of Bolton et al. (1986), describes more fully the structure-activity specificity of vir -induction in a wide host range (WHR) strain of A. tumefaciens than did Stachel et al. (1985-a), and presents results which indicate that WHR Agrobacterium is capable of detecting phytochemicals which are ubiquitous or at least widespread amongst susceptible hosts. The relative vir - inducing activities of the lignin precursors coniferyl and sinapyl alcohols, a variety of cinnamic acid derivatives and two chalcones are presented and discussed in terms of the early events in crown gall tumorogenesis and the sophisticated use of Agrobacterium in Ti plasmid-mediated transformation of plants.