|Van Berkum, Peter|
Submitted to: Australian Journal of Soil Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/30/2007
Publication Date: 9/30/2007
Citation: Zribi, K., Badri, Y., Van Berkum, P.B., Aouani, M.E. 2007. Medicago ciliaris growing in Tunisian soils is preferentially nodulated by Sinorhizobium medicae. Australian Journal of Soil Science. 45:473-477.
Interpretive Summary: Soil bacteria known as rhizobia form a symbiosis with legume crops such as soybean and alfalfa. When in symbiosis the plants produce 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. Rhizobial isolates from the annual medic Medicago ciliaris (no common name), which is closely related to alfalfa, were of two different bacterial species. However, there were differences in nitrogen fixation capabilities between these two species and there was preferential nodulation by one when both were present in the soil. Since the annual medic completely excluded the rhizobia of one species, it should be possible to obtain full inoculation success of this legume in fields previously used to produce alfalfa that was inoculated with the commonly used inoculants. These results are important to the inoculum industry and to scientists wishing to introduce this annual medic as a new crop.
Technical Abstract: Variation in growth of Medicago ciliaris was recorded across soils from five different regions in Tunisia that represented different soil types and climatic zones. In four of these soils (Mateur, Enfidha, Rhayet and Soliman) this variation appeared to be related to the nodule number on the roots of the plants. With the exception of one isolate the rhizobia isolated from these nodules had 16S rRNA PCR-RFLP fingerprint patterns that were characteristic of Sinorhizobium medicae. Plant growth in the fifth soil (Jelma) was the poorest, plants had few nodules that yielded exclusively rhizobia with 16S rRNA fingerprint patterns characteristic of S. meliloti. In subsequent plant tests, S. medicae isolates formed effective nitrogen fixation symbioses with M. ciliaris, while S. meliloti formed small white ineffective nodules. Therefore, plant growth in Jelma soil was poor because only S. meliloti are present and this species is ineffective with M. ciliaris. In a co-inoculation experiment with M. ciliaris, S. medicae was more competitive for nodulation than S. meliloti perhaps explaining why the majority of the isolates from Enfidha and Rhayet were S. medicae since S. meliloti is present in these soils. However, it is not clear how the host influences rhizobia for nodulation by S. medicae in preference to S. meliloti when present.