Unipolar Polysaccharide-mediated Attachment of the N₂O-reducing bacterium Bradyrhizobium ottawaense SG09 to Plant Roots

Agricultural soils are an important source of nitrous oxide (N₂O), which has greenhouse and ozone-depleting effects. Bradyrhizobium ottawaense SG09 is a nitrogen-fixing rhizobium with high N₂O-reducing activity. Rhizobia form symbiotic nodules in leguminous plants. The initial physical attachment of bacteria to plant roots is a critical step in the establishment of symbiotic interactions. In the present study, we performed a microscopic anal-ysis using DsRed-expressing B. ottawaense SG09. We revealed that B. ottawaense SG09 attached to both the root surface and root hairs via single cellular poles. This polar attachment was observed not only to the symbiotic host soybean, but also to non-leguminous plants, such as Arabidopsis, rice, corn, and wheat. We identified and analyzed the unipolar polysaccharide (upp) gene cluster, which is proposed to be involved in the polar attachment of rhizobia, in the genome of B. ottawaense SG09. We established an Arabidopsis-based interaction assay and demonstrated that uppC and uppE play a critical role in attachment to both the root surface and root hairs.