Co-introduction of exotic rhizobia to the rhizosphere of the invasive legume Acacia saligna, an intercontinental study

Invasive woody legumes have profound impacts in the nitrogen content and cycling of invaded ecosystems due to the ability to enter into symbiosis with nitrogen-fixing bacteria. In spite of the relevance of this symbiosis, the identity and origin of the symbionts involved in invasion are not well understood. We conducted a study to assess the diversity of symbiotic root-nodulating bacteria associated with the invasive Acacia saligna, in newly colonized areas in Portugal and Australia. BOX-PCR was used to discriminate the isolated bacteria and 16S rRNA and nifD genes were sequenced to identify the different isolates and their geographic origin. Bradyrhizobium and Mesorhizobium nodulated A. saligna in Australia while only Bradyrhizobium spp. were found in Portugal. The dominant strains nodulating A. saligna were related to Bradyrhizobium liaoningense and Bradyrhizobium canariense. Co-occurring Acacia longifolia and A. saligna in Australia harbor different rhizobial communities. As an example, we found Mesorhizobium sp. and Phyllobacterium trifolii in A. saligna and A. longifolia respectively, being this the first report for this association. The analysis of the phylogeographic marker nifD clustered most of the sequences obtained in this study with sequences of Australian origin, indicating that exotic bradyrhizobia might have been co-introduced with A. saligna in Portugal. This result highlights the risks of introducing exotic inoculants that might facilitate the invasion of new areas and alter native soil bacterial communities, hindering the recovery of ecosystems.

► We study the rhizobial diversity associated with Acacia saligna in Portugal and Eastern Australia.
► We find a clear dominance of Bradyrhizobium species in both ranges.
► Invasion of A. saligna in Portugal is correlated with a parallel invasion of introduced symbionts.
► The first report of Mesorhizobium nodulating A. saligna.
► The first report of Phyllobacterium trifolii nodulating Acacia longifolia.