β-glucuronidase (GUS) operon fusions as a tool for studying plant-microbe interactions

We are developing the Escherichia coli gus operon as a transgenic marker for the detection and study of bacteria of the rhizosphere and phyllosphere. This operon includes genes encoding β-glucuronidase (GUS) and a glucuronide-specific membrane transporter. We have introduced the entire gus operon cloned from a natural isolate of E. coli into Agrobacterium and Khizobium strains. The operon was expressed in both species. Alfalfa nodules induced by R. meliloti strains carrying the gus operon stained blue on incubation with the GUS substrate X-Gluc. To enable introduction of GUS into bradyrhizobia, a translational fusion of the Tn5 aph gene to gusA was constructed. Blue-stained infection threads in root hairs of siratro plants inoculated with a Bradyrhizobiwn strain carrying this fusion were clearly visible after incubation with X-Gluc. (author)