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

Abstract

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.

Mutualistic Interactions and Biological Invasions

“…Except for a few cases (Rodríguez-Echeverría et al 2011), no barriers to establishment exist for most legumes in new areas (Birnbaum et al 2012). Most show high levels of nodulation in invaded ranges, indicating either that effective rhizobia are widely distributed between continents and/or that many legumes can nodulate with a wide range of rhizobia strains (Rodríguez-Echeverría et al 2011, Ndlovu et al 2013. For some invasive legumes, however, novel mutualisms appear to be less effective in terms of nodulation, nitrogenase activity, and plant growth than the interactions of plants and bacteria of the same origin (Rodríguez-Echeverría et al 2012).…”

Gradualism in Acacia dealbata Link invasion: Impact on soil chemistry and microbial community over a chronological sequence

“…This fact could lead to a change in the structure of microbial community, favoring species with positive feedback with A. dealbata (like species related to N 2 -fixation). In fact, associated rhizobia communities in Acacia invasions can also be exotic, probably co-introduced with the invasive legume (Rodríguez-Echeverría et al, 2007;Rodríguez-Echevarría, 2010;Ndlovu et al, 2013). On the other hand, the fungal community structure was initially altered but gradually seemed to become similar to non-invaded soils (>25 years and control were closely related).…”

Reduced availability of rhizobia limits the performance but not invasiveness of introduced Acacia

“…Compatible rhizobia also appear to be present in many locations where Acacia have been introduced, with species recorded nodulating in Europe (Rodríguez-Echeverría et al 2009), Asia (Midgley & Vivekanandan 1987;Le Roux et al 2009;Ma et al 2012), Africa (Mohamed et al 2000;Joubert 2002;Rodríguez-Echeverría 2010;Boukhatem et al 2012) and the Americas (Aronson, Ovalle & Avendano 1992), as well as outside their native range in Australia (Birnbaum et al 2012). Although it is not clear what facilitates nodulation outside the native range, the widespread occurrence of cosmopolitan rhizobia (Weir et al 2004;Birnbaum et al 2012) and the co-introduction of compatible rhizobia from the native range have both been implicated (Rodríguez-Echeverría 2010; Birnbaum et al 2012;Ndlovu et al 2013).…”