Co-invasion of South African ecosystems by an Australian legume and its rhizobial symbionts

Aim To determine and compare the taxonomic identity and diversity of root nodule and rhizospheric microbial symbionts associated with Acacia pycnantha Benth. in its native (Australian) and invasive (South African) ranges, and to establish whether these associations are general or host specific. Location The native range of A. pycnantha in Australia and invasive ranges in South Africa and Western Australia. Methods Bacteria were isolated from root nodules collected from 18 populations of A. pycnantha. Repetitive element polymerase chain reaction (REP-PCR) fingerprinting was used to assess overall bacterial diversity and clustering. Molecular phylogenies for a subset of isolates representing major REP-PCR clades were reconstructed using maximum parsimony and Bayesian phylogenetic analyses of the nuclear 16S–23S rRNA intergenic spacer (IGS), 16S rRNA, and the symbiotic nodA genes. Results Twelve clusters were identified from the REP-PCR analysis; 11 included isolates from both the native range in Australia and invasive range in South Africa, while one cluster comprised only Australian isolates. Six rhizobial species were found in association with A. pycnantha: Bradyrhizobium japonicum, Rhizobium gallicum, R. lusitanum, R. miluonense, R. multihospitium and R. tropici. We also identified three plant-growth promoting bacteria isolated from root nodules of A. pycnantha: Burkholderia caledonica, B. graminis and B. phytofirmans. Phylogenetic analysis of the IGS gene retrieved clades containing symbionts from both Australia and South Africa while others comprised only South African taxa, suggesting the introduction of bacterial lineages from Australia to South Africa. Our phylogeographic analysis of the nodA gene confirmed that A. pycnantha was co-introduced with its symbionts to South Africa. Main conclusions Acacia pycnantha is a promiscuous legume, associated with at least six different rhizobial symbionts, and forms associations with plant-growth promoting rhizosphere bacteria from the genus Burkholderia. In the invasive range of A. pycnantha in South Africa, nodules contained some symbionts of South African origin while other symbionts appear to have been co-introduced from Australia. Acacia pycnantha is associated with a wider suite of symbionts in its invasive than native range.