Carbohydrate binding activities of Bradyrhizobium japonicum. III. Lectin expression, bacterial binding, and nodulation efficiency.

In previous studies, evidence that the Bradyrhizobium japonicum lectin, designated BJ38, mediated the observed carbohydrate-specific binding activities of the bacteria, including the saccharide-specific adhesion to soybean root cells was presented. In the present study, it is found that both B. japonicum, as well as the purified BJ38, bind predominantly to young emergent root hairs of soybean roots and, to a much lesser extent, to the root cap, mature root hairs, epicotyl or hypocotyl regions. Thus, the region of preferential binding for both the bacteria and the isolated lectin coincide with the region of the soybean root most susceptible to B. japonicum infection. The importance of bacterial binding for the nodulation process was studied by comparing the nodulation efficiency of binding-deficient mutants N4 and N6 to the wild-type. These mutants had been shown to be defective in carbohydrate recognition, as represented by their diminished ability to bind to soybean roots. BJ38 was immunolocalized to one pole of the cell surface of wild-type B. japonicum, but no surface labeling could be detected on either mutant. Moreover, both N4 and N6 showed a substantial decrease in nodulation activity, relative to the wild-type. These results provide additional evidence that the carbohydrate binding activity of B. japonicum, most probably mediated by BJ38, may play an important role(s) in the initial phases of the infection process.