Wound induced plant phenolic compounds and virulence gene expression in Agrobacterium species
Creators: name: Spencer, Paul Anthony
Publisher: name: University of British Columbia
Year: 2011

description: Crown gall disease of plants is caused by introduction of foreign DNA into susceptible plant cells by strains of Agrobacterium tumefaciens. The expression of bacterial virulence genes is triggered by chemicals present in plant wound exudates. The exudates contain a number of phenolic compounds which act as chemical signals inducing expression of a number of genes directing the DNA transfer process. These are the virulence or vir genes, and vir::lac reporter gene fusions have been widely used to assay vir gene induction in Agrobacterium tumefaciens strains. Using such strains to monitor vir gene expression, Stachel et al. (1985) isolated from Nicotiana tabacum two active acetophenones: 3,5-dimethoxy-4-hydroxyacetophenone, ("acetosyringone" or AS), and α-hydroxy-3,5-dimethoxy-4-hydroxy-acetophenone, ("hydroxyacetosyringone" or HO-AS). However, in vitro assay results suggested that other more common compounds also exhibited activity (Spencer and Towers, 1988). This analysis of structure-activity relationships of induced vir expression in A. tumefaciens was presented in a previous thesis (Paul Spencer, M.Sc. thesis). The results revealed that a variety of commonly occurring plant phenolic compounds were capable of activating vir genes. In addition to the acetophenones, a variety of benzoic and cinnamic acid derivatives, and even a few chalcones of appropriate ring substitution were active. This thesis reports the isolation and identification of a number of these compounds in plant wound exudates. Some Agrobacterium tumefaciens strains are restricted in host range to certain grapevine cultivars. Subsequent to the development of a convenient and sensitive plate-bioassay method, a strongly active component in grapevine wound exudates was purified. A newly described vir-inducing phenolic compound was isolated from a number of Vitis cultivars using gel filtration, thin layer and high pressure liquid chromatographies. This was identified as syringic acid methyl ester (3,5-dimethoxy-4-hydroxybenzoic acid, methyl ester), using mass spectrometry. However, the presence of this compound in grapevine wound exudates does not provide a simple explanation for host range limitation of grapevine strains since it induces vir gene expression in both limited and wide host range strains of A. tumefaciens. Interestingly, neither AS nor HO-AS were present in grapevine-derived extracts. A convenient polyamide column chromatographic method was subsequently developed to permit rapid purification of plant-derived vir gene inducing mixtures, which were detected using the newly developed plate bioassay. Derivatized polyamide fractions were then analysed by combined gas chromatography-mass spectrometry (GC-MS). GC-MS proved to be an ideal means for the identification of the phenolic components in partially purified extracts. Examination of wound exudates from a range of host and non-host species revealed that the production of the acetophenones is restricted to members of the Solanaceae. Some experiments focussed on the biosynthetic precursors of the acetophenones in Nicotiana species. Wound exudates of the majority of species belonging to other plant families contained benzaldehydes and/or benzoic and cinnamic acid derivatives. The induction of virE gene expression was examined in the related Agrobacterium species, A. rhizogenes. To do this, the virE::lacZ gene fusion plasmid pSM358cd was introduced into A. rhizogenes A4 by triparental mating and the strain "A4/pSM358cd" was used to analyze vir activation. Acetophenones, chalcones, benzaldehydes, and benzoic and cinnamic acid derivatives were found to activate vir genes in A. rhizogenes.