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Content archived on 2024-05-07

Improvement of symbiotic nitrogen fixation in Chinese soybean cropp ing areas


The main objectives are defined as follows :
* To carry out quantitative analyses of the Sinorhizobium fredii and Bradyrhizobium japonicum populations in soils of four different Chinese soybean cropping areas. It will be determined the density (bacteria / gram of soil) of indigenous populations of S. fredii and B. japonicum and a collection of these bacterial strains will be made.
* To determine the importance of pH as a key factor in determining the final outcome of the competition for soybean nodulation between S. fredii and B. japonicum.
* To analyze the symbiotic capacity of different Western and Asiatic soybean cultivars in order to determine the best pairs soybean cultivar-inoculant strain.
* To isolate S. fredii and B. japonicum strains that are highly effective and / or competitive as soybean inoculants.
* To investigate the symbiotic properties of Sinorhizobium meliloti strain 042B. This bacterial strain has recently been isolated in China and it is able to nodulate effectively on both alfalfa and soybean plants.
Expected Outcome

A collection of S. fredii and B. japonicum strains isolated from Chinese soybean cropping areas will be available. New S. fredii and / or B. japonicum strains showing high symbiotic capacity with soybeans will be available. The expected result will be the isolation of soybean microsymbionts that when they are used as soybean inoculants, produce an increase of the soybean yield. The effect of pH on the competition for soybean nodulation between S. fredii and B. japonicum strains will be determined. S. meliloti 042B nodulation genes that determine specific nodulation in alfalfa and / or soybean will be studied. The structure of the nod factors produced by S. meliloti strain 042B will be known.
The key activities involved :
* Soil samples from four different Chinese counties will be collected and used as inoculants (source of soybean microsymbionts) of Asiatic and Western soybean cultivars. Isolates from soybean nodules will be classified as B. japonicum or S. fredii. Cataloguing of S. fredii and B. japonicum strains will be carried out by a combination of different standard microbiological techniques.
* Plant tests with Western and Asiatic soybean cultivars will be carried out to evaluate the symbiotic ability of the bacterial collection as soybean inoculants.
* Competition experiments will be carried out by inoculation of soybean plants with a mixture of the pairs of competitors. Competition experiments at acid, neutral and alkaline pH will reveal the importance of pH for the final outcome of the competition for nodulation. A screening for bacteriocin producers will be also carried out in the S. fredii collection. Those strains able to produce bacteriocins will be tested with soybean plants for superior competition ability in mixed coinoculations with bacteriocin sensitive strains.
* A study of the nodulation genes of S. meliloti strain 042B that are involved in the production of bacterial signals will be carried out by standard molecular genetic techniques. The molecular structure of these bacterial signals will be determined.

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