Co-invasion by Australian Acacia species and rhizobium mutualists

The over-representation of legumes in the world's alien foras has been partly attributed to their ability to form mutualistic interactions with nitrogen-fxing bacteria known as rhizobia. The extent to which rhizobia enhance establishment success for alien legumes is infuenced by how dependent the plants are on these symbionts and their availability in new locations. Australian Acacia species ('wattles') have become a model taxon for studying the factors that determine the associations between introduced legumes and rhizobia in new habitats, including: (i) the dependence and specialization of wattle species on rhizobia in the genus Bradyrhizobium (bradyrhizobia); (ii) the geographic distribution of Bradyrhizobium strains that successfully nodulate wattles; and (iii) the degree to which associations between alien wattles and rhizobia arise due to novel associations or the co-invasion of rhizobia from the native range of wattles in Australia. This chapter synthesizes fndings from studies investigating the distribution of wattle-compatible rhizobia in the native and non-native ranges of wattles. While the wide availability of rhizobia to wattle species implies generalism in the association, results from genetic studies have revealed that they overwhelmingly nodulate with slow-growing Bradyrhizobium strains, particularly from the endemic Australian Clade I Bradyrhizobium, implying some degree of specifcity. Consistent evidence points to co-invasion of Clade I Bradyrhizobium with wattle species. However, the pathway of co-invasion remains unclear, and apparent differences in the specifcity of wattle-rhizobium associations between native and introduced locations remain unexplained. Co-invasion of wattles and bradyrhizobia may impact native legume-rhizobium associations by local enrichment of Bradyrhizobium at the expense of native rhizobia, and modifcations of native rhizobia via horizontal gene transfer are possible. Many questions surrounding wattle-rhizobium interactions remain unanswered and are priorities for future research.