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 ottawaenseSG09 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 this study, we performed microscopic analysis using DsRed-expressingB. ottawaenseSG09. We revealed thatB. ottawaenseSG09 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 asArabidopsis, rice, corn, and wheat. We identified and analyzed theunipolar polysaccharide(upp) gene cluster, which is proposed to be involved in polar attachment of rhizobia, in the genome ofB. ottawaenseSG09. We established anArabidopsis-based interaction assay and demonstrated thatuppCanduppEplay a critical role in attachment to both the root surface and root hairs.