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

To determine whether the gusA gene, which encodes beta-glucuronidase (GUS) and lacZ gene, which encodes beta-galactosidase are suitable for tracing ni trogen-fixing bacteria in the infection process, Bradyrhizobium japonicum s trains labelled with each gene were constructed. Both introduced genes were expressed in rhizobia, but it was difficult to specify the sites where lac Z-labelled bacteria were present, since endogenous beta-galactosidase level s were high in soybean root tissues. On the other hand, endogenous P-glucur onidase activity has not been detected in soybean root tissues. The gusA-ma rked Bradyrhizobium, Rhizobium and Azospirillum strains were constructed fo r assessing the use of their GUS-marked bacteria to trace the presence of i ntroduced bacteria manifested by colonization on the root surface, as well as infection sites, invasion modes and nodulation competitiveness between b acteria. Bradyrhizobium japonicum inoculated to soybean colonized in the fo rm 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 (B rady)rhizobium and Azospirillum strains which we constructed.