Host-promiscuity in symbiont associations can influence legume establishment and colonization of novel ranges

Background/Question/Methods

Mutualistic interactions play an important role in species invasions. In particular, the symbiotic relationship between legumes and nitrogen-fixing soil bacteria (i.e., rhizobia) is influential in invasion success. Leguminous plant species benefit from their association with rhizobia, which provides them with a source of nitrogen. Leguminous species, including Australian Acacias, have been introduced around the world, with many becoming highly invasive in novel ranges. The specificity with which Acacia species form associations with different rhizobial strains may play a role in determining which species are successful invaders. To better understand the mechanisms by which these species establish and colonize areas abroad, we examined the host-specificity of a suite of Acacia species paired with multiple rhizobial strains. Highly specific host plants associate with few rhizobial strains, whereas promiscuous hosts associate with a wider range of symbionts. We examined host-specificity/promiscuity of Acacia species that vary globally in invasiveness (i.e., invasive, naturalized, and non-invasive species). We compared plant growth, survival, and nodulation response of Acacia species when paired individually with 12 rhizobial strains ranging in their known symbiotic effectiveness, as well as positive and negative nitrogen controls. 

Results/Conclusions

Acacia species varying in invasiveness displayed differences in growth and nodulation responses to rhizobial strains, but no differences in survival. Species categorized as invasive were consistently promiscuous hosts, associating with five out of 12 available rhizobial strains. Naturalized species were consistently less promiscuous hosts. Non-invasive species had a more varied response, with two species showing patterns of host-promiscuity, and two species being more selective rhizobial hosts. Nodulation response followed a similar pattern, with invasive species having higher nodulation success than naturalized and non-invasive species. Our results support the idea that invasive species may generally be capable of associating with a broader range of rhizobial symbionts than naturalized and non-invasive species. These species exhibited variable degrees of host-specificity, but tended to be more constrained with regard to which symbionts they could form effective associations. Our study thus indicates that host-promiscuity plays a role in the invasiveness of Acacia species. It is therefore important to monitor promiscuous host species that have not yet become invasive abroad for potential expansion in areas where they are introduced.