Use of lacZ and gusA reporter genes to trace the infection process of nitrogen-fixing bacteria

To determine whether the gusA gene, which encodes β-glucuronidase (GUS) and lacZ gene, which encodes β-galactosidase are suitable for tracing nitrogen-fixing bacteria in the infection process, Bradyrhizobium japonicum strains labelled with each gene were constructed. Both introduced genes were expressed in rhizobia, but it was difficult to specify the sites where lacZ-labelled bacteria were present, since endogenous β-galactosidase levels were high in soybean root tissues. On the other hand, endogenous β-glucuronidase activity has not been detected in soybean root tissues. The gusA-marked Bradyrhizobium, Rhizobium and Azospirillum strains were constructed for assessing the use of their GUS-marked bacteria to trace the presence of introduced bacteria manifested by colonization on the root surface, as well as infection sites, invasion modes and nodulation competitiveness between bacteria. Bradyrhizobium japonicum inoculated to soybean colonized in the form of spots on the root surface. In the spots, curling roots with infection threads were observed. In this report, we describe only the gusA-marked (Brady)rhizobium and Azospirillum strains which we constructed.