Utilization of rhizobia by the invasive legume, Sericea Lespedeza (Lespedeza cunteata)

Background/Question/Methods

In the late 1800s, sericea lespedeza (Lespedeza cuneata) was introduced in North America. It became a noxious weed in Kansas due to its ability to invade and degrade native tallgrass prairie, and is currently one of the largest threats to existing tallgrass prairie. Because sericea lespedeza is a legume, it forms mutualistic relationships with nitrogen fixing bacteria called rhizobia, but since this legume is not native to North America, it must utilize existing rhizobia available in its environment.  Previous studies in invasion ecology have suggested that mutualisms may play important roles in the invasive success of certain species. Legume species differ in their specificity of allowing symbiosis to occur with different species of rhizobia. Some legumes are highly specific, and only allow a limited number of rhizobia species to form symbioses while other legumes are highly promiscuous. The ability to form a symbiotic relationship is primarily controlled by the legume through its recognition of the nodulation gene products. The objective of our study was to evaluate the range of rhizobia species/strains that form symbiotic relationships with sericea lespedeza. For each plant, we extracted a minimum of 20 nodules, and cultured the rhizobia on YMA plates. DNA from isolated rhizobia was extracted, amplified using PCR, and sequenced.  

Results/Conclusions

Results from a search of the BLAST database indicates that the rhizobia sericea lespedeza utilizes belong to the genus Bradyrhizobium. These preliminary results have also shown that only 2 species of Bradyrhizobium infect sericea, Bradyrhizobium liaonigense and Bradyrhizobium betea strain PL7HG1.  Bradyrhizobium liaonigense is commonly associated with soybeans, but may also form symbiotic relationships with other native legumes. Little is known about the rhizobia that most non-agricultural legumes utilize. The second species, Bradyrhizobium betea strain PL7HG1, has only been isolated from tumor-like root formations on sugar beets, and has an unknown symbiotic status. These results also suggest that sericea lespedeza is relatively specific to the rhizobia species it allows to infect its roots. This specificity also suggests that future control of this invasive plant could be developed by creating a mechanism to block the infection pathway. We need to do further work to see if this relationship holds at multiple locations.