FIG. 6.

Comparative AS dose-response determinations for wild-type VirA and VirAⁱ¹⁶⁹ and the effect of AS sensitivity on tumor initiation. A136 (wild-type chvE) strains carrying wild-type virA and virAⁱ¹⁶⁹ on pRG109 (pGN60 and pGN66; see the supplemental material for a description) were grown at 25°C in induction medium with 10 mM l-arabinose and glycerol at either pH 5.5 (a) or pH 7.0 (b). AS concentrations from 0 to 300 μM were used. β-Galactosidase activity was determined after 20 h, and the results are expressed in Miller units. The top right corners of panels a and b depict ED₅₀ values (the concentrations of the phenolic inducers that elicit 50% of the maximal response; calculated as described in Materials and Methods) in μM AS. Samples were assayed in triplicate and plotted as means with standard errors. Three independent experiments were performed and yielded similar results. (c) Tumorigenesis assay. A348-3 strains carrying plasmids expressing wild-type VirA, VirAⁱ¹⁶⁹, or VirAⁱ²⁴⁸ were evaluated for the ability to form tumors by cocultivation with tobacco leaf explants at different external AS concentrations on hormone-free MS medium. This graph represents the average number of tumors per leaf explant for each strain at cell concentrations of 0.5 OD₆₀₀. The error bars on the graph represent standard errors (n = 14). Two independent virulence assays were performed before conclusions were drawn.