Estimation of biotic and abiotic factors that affect migration of rhizobia

Rhizobia are soil bacteria that form nitrogen-fixing nodules on leguminous plants. Three possible ways have been estimated for the long-distance transport of these bacteria: by introduction of legume seeds or plants that are accompanied naturally by rhizobia, by application of rhizobial inoculants, and by rivers that carry rhizobia downstream. In all these three forms, or any other possible way, rhizobia are moved passively and they have to face biotic and abiotic factors in a new environment. So, a successful migration of rhizobia mainly depends on whether they can colonize in the new environment after transportation. Since rhizobia are symbiotic bacteria associated with legumes, their colonization in new environments depends on both their host plants and environmental factors. The colonization of rhizobia is easier when the new environment is similar to the old and the host plants have restricted nodulation ability, as with the introduction of Amorpha fruticosa from USA to China. In contrast, the introduced rhizobia may be eliminated if the new environment is quite different and the hosts can nodulate with a wide range of rhizobia, as in the case of soybean. In any event, a successful migration of rhizobia can affect the local microbial community by the establishment of a new bacterial population and by the lateral gene transfer of symbiotic genes or other genetic elements. Here, some investigations relative to rhizobial migration will be reviewed.