|Van Berkum, Peter|
Submitted to: Canadian Journal of Microbiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/8/2006
Publication Date: 1/30/2007
Citation: Van Berkum, P.B., Badri, Y., Zribi, K., Badri, M., Aouani, M.E., Huguet, T. 2007. Comparison of rhizobia that nodulate medicago laciniata and m. truncatula present in a single tunisian arid soil. Canadian Journal of Microbiology. 53:277-283.
Interpretive Summary: Soil bacteria known as rhizobia form a symbiosis with legume crops such as soybean and alfalfa. When in symbiosis, the plants produce small spherical growths on their roots called nodules where the rhizobia are located. These rhizobia in symbiosis extract nitrogen gas from the atmosphere and convert it into a form that the plants use for growth. This process, referred to as symbiotic nitrogen fixation, results in efficient crop production since growers need not apply fertilizer, which saves money and prevents pollution of the environment. Management of the symbiosis is at sowing by inoculation of the seeds with rhizobia using precise formulations of the bacteria that are compatible with specific legume species. To maximize nitrogen fixation, the proper rhizobia must be paired with its compatible legume species. Rhizobial isolates from two different annual medic species, which are legume crops closely related to alfalfa, appeared to be the same bacterial species. However, little or no nitrogen fixation occurred when the rhizobial isolate from one annual medic, Medicago truncatula, was used to inoculate the other medic species, Medicago laciniata. This result indicated that if Medicago laciniata were sown and inoculated with it homologous rhizobia it would nodulate and efficiently fix nitrogen even in the presence of alfalfa rhizobia. These results are important to the inoculum industry and to scientists wishing to introduce Medicago laciniata into production on fields previously cropped with alfalfa.
Technical Abstract: The rhizobia present in a single arid-region Tunisian soil that nodulate Medicago laciniata and M. truncatula were compared. All isolates, 40 from each host, probably were more closely related to S. meliloti than to S. medicae based on 16S rRNA PCR-RFLP. There was no apparent relationship between Medicago host species of isolation and the nodulating rhizobial genome as determined by repetitive extragenic palandromic (REP)-PCR. The isolates of M. laciniata were distinguished from those of M. truncatula present in the same soil by variation in PCR-RFLP of nifDK, indicating that this dissimilarity is of genomic and not of geographic origin. While forming effective symbioses with their own respective isolates, both M. laciniata and M. truncatula formed ineffective true nodules, nodule-like structures or no nodules at all in cross-inoculation tests. Up to 22% of the nodules formed by M. truncatula were occupied by an isolate of M. laciniata in a competition for nodulation analysis. However, the isolate of M. truncatula did not nodulate M. laciniata indicating that field inoculation of M. lacinata with its specific rhizobia in production areas previously dedicated to alfalfa would lead to high nodule occupancy.