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Title: Widespread occurrence of Sinorhizobium meliloti strains with a type IV secretion system

item CAO, YUANYUAN - Anhui Agricultural University
item Miller, Susan
item Dornbusch, Melinda - Mindy
item CASTLE, SARAH - University Of Minnesota
item Lenz, Peter
item FERGUSON, JOHN - University Of Minnesota
item SADOWSKY, MICHAEL - University Of Minnesota
item NELSON, MATTHEW - University Of Minnesota
item Klatt, Christian
item Samac, Deborah - Debby

Submitted to: Symbiosis
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/1/2018
Publication Date: 6/1/2018
Citation: Cao, Y., Miller, S.S., Dornbusch, M.R., Castle, S., Lenz, P., Ferguson, J., Sadowsky, M.J., Nelson, M.S., Klatt, C.G., Samac, D.A. 2018. Widespread occurrence of Sinorhizobium meliloti strains with a type IV secretion system. Symbiosis. 75(2):81-91.

Interpretive Summary: Perennial forage legumes are an important source of nitrogen (N) in cropping systems worldwide due to symbiosis between the plant and nitrogen fixing symbiotic bacteria. Improvements in N2 fixation capacity in alfalfa and other forage legumes would be beneficial for enhancing plant production, soil N fertility, and subsequent cash crop production. However, such improved bacteria introduced into the environment must compete with the endogenous populations of rhizobia. This study investigated the source and diversity of nitrogen fixing bacteria in alfalfa root nodules in two sites that had not been cultivated with alfalfa for more than 30 years. Populations of the bacteria in soil were low (average of 700 cells/g soil) but were sufficient to form symbiotic associations with alfalfa roots and out-competed the bacteria inoculated onto seed before planting. Approximately 37% of the soil strains contained a gene cluster that was shown to be important for accelerating speed of nodulation and increasing plant biomass. The results of this study show that populations of symbiotic bacteria can persist for long periods in the absence of a host plant and retain genes important for symbiosis. The results can be used for identifying more competitive strains for enhancing nitrogen fixation to reduce costs of crop production and to increase soil health and fertility.

Technical Abstract: Improvement in symbiotic nitrogen fixation would reduce reliance on nitrogen fertilizers but is hampered by poor establishment of introduced strains due to competition by indigenous symbiotic strains. This study investigated the origins, diversity, and competitiveness of Sinorhizobium meliloti (now Ensifer meliloti) strains isolated from root nodules of alfalfa (Medicago sativa L.) grown in soils that had not been in alfalfa cultivation for over 30 years. Diversity among strains isolated from nodules of field grown plants and from commercially inoculated seeds used for establishing field plots were examined by rep-PCR DNA fingerprinting. They were also assayed for production of indole acetic acid compounds, phosphate solubilization, speed of nodulation, and presence of a Type IV Secretion System (T4SS) gene cluster, shown to be important in strain competitiveness in M. truncatula. Field strains were highly competitive for nodule occupancy compared to strains from commercially inoculated seeds. Field strains were very diverse within and between field sites and genetically different from inoculant strains. Approximately 37% of field strains were positive for the T4SS gene cluster and deletion of these genes resulted in reduced nodulation speed and plant biomass. Field populations of S. meliloti remained capable of nodulating of alfalfa in the prolonged absence of a host. Indigenous bacteria were genetically diverse, but remained competitive and retained T4SS genes important in symbiosis in the absence of host plants.