Scotch broom’s nitrogen-fixing symbiotic bacteria: Are there roommates in nodules?

Background/Question/Methods

Scotch broom, Cytisus scoparius, is an invasive species affecting regions of the world ranging from the west coast of North America to Australia and the Mediterranean. Copious seed production, a persistent seed bank, and nitrogen-fixing root nodules are a few reasons for Scotch broom’s success in a variety of habitats. Most research looking at Scotch broom’s invasion biology focuses on control techniques. In fact, little work has been done with Scotch broom’s symbiotic bacteria. With these nitrogen-fixing symbionts, Scotch broom is not limited by low nitrogen levels in soils as many other plants are. While it is known that Scotch broom does have symbiotic nitrogen-fixing bacteria, the identity of these symbiotic bacteria remains unknown. A variety of bacteria including Rhizobium, Bradyrhizobium, and Burkholderia, have been identified as symbiotic nitrogen fixers for other leguminous plants like Mimosa species in Central America. The goal of our research is to identify the nodule-inhabiting bacteria of Scotch broom in an effort to better understand the invasion biology of Scotch broom. We used 16s rDNA sequencing to identify the nodulating bacteria of Scotch broom nodules from the South Puget Sound lowland prairies by comparing our sequences to those available on BLAST. 

Results/Conclusions

We identified the symbiotic bacteria of Scotch broom in the South Puget Sound lowland prairies as predominantly from two genera: Burkholderia and Rhizobium. Moreover, two of the ten nodules in which all bacterial isolates were successfully identified simultaneously contained both Burkholderia and Rhizobium.. Therefore, it is possible that multiple bacterial species can occupy a single Scotch broom nodule. This phenomenon, called multiple occupancy, has been seen before in soybeans from Japan with Bradyrhizobium and Sinorhizobium. Neither the presence of Burkholderia nor multiple nodule occupation has been previously well-documented in a temperate zone legume plant like Scotch broom. With the identity of the bacteria known, we can now explore the possibility of co-nodulation of Scotch broom nodules and of the potential for “sneaky” bacteria, that is, bacteria that do not fix nitrogen for the host plant but simply take advantage of the provided food and shelter.