Jack-of-all-trades and master of many? How does associated rhizobial diversity influence the colonization success of Australian Acacia species?

Aim To evaluate the role of rhizobial diversity, and symbiotic promiscuity, on the invasive ability of Australian acacias (Acacia species in subgenus Phyllodineae native to Australia). Location Global. Methods A bibliographic review of the rhizobial diversity associated with Australian Acacia species was performed to assess symbiotic promiscuity for invasive and non-invasive species. The rhizobial diversity associated with Acacia dealbata and A. saligna in Australia and Portugal and with A. pycnantha in Australia and South Africa was assessed by 16S rDNA and intergenic spacer sequencing of bacteria isolated from field-collected nodules. Results All studied Australian acacias are nodulated by strains in the genus Bradyrhizobium, which appears to be the dominant group of acacia symbionts in native and non-native soils. Both literature and experimental data from this study suggest that Australian bradyrhizobia might have been co-introduced with acacias to new geographical regions. The studied Acacia species can also harbour other root-nodulating alpha and betaproteobacteria genera, although these are less abundant than Bradyrhizobium. Main conclusions There is no clear difference in the diversity of rhizobial species associated with invasive and non-invasive Australian acacias. All studied invasive acacias nodulate in both native and non-native regions, harbouring predominantly Bradyrhizobium strains but showing some degree of symbiotic promiscuity. The co-introduction of compatible root-nodulating bacteria from Australia might explain the establishment of invasive populations, but novel associations with rhizobia from the invaded soils are also possible. Invasive legumes might use both strategies but species with low symbiotic promiscuity would become invasive only if compatible bacteria are co-introduced in the new regions. The progress of invasion and the impacts on the invaded ecosystems might also differ depending on the nodulation strategy.