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Rhizobia are well known for their capacity to establish a symbiosis with legumes. They inhabit root nodules, where they reduce atmospheric nitrogen and make it available to the plant. Biological nitrogen fixation is an important component of sustainable agriculture, and rhizobial inoculants have been applied frequently as biofertilizers. In this review we present recently developed technologies and strategies for selecting quality inoculant strains by taking into consideration the complex interaction between the edaphic environment with the genotypes of both the legume and its microsymbiont. Enhanced competitive ability in an inoculant strain is a key requirement for successful colonization of plant roots, nodule formation, and subsequent N2-fixation. We discuss several avenues for the management and manipulation of rhizobial competition as well as genes that influence competition in the rhizosphere. The use of molecular techniques has greatly contributed to our knowledge of nodule-bacterial diversity and phylogeny. Approaches to the study of rhizobial diversity as well as mechanisms for the evolutionary diversification of nodulating bacteria are presented. Rhizobium genomes ranging from 5.5 to 9 Mb have been sequenced recently and deposited in public databases. A comparison of sequence data has led to a better understanding of genes involved in the symbiotic process as well as possible mechanisms responsible for horizontal transfer of genetic elements and symbiosis genes among rhizobia. Furthermore, rhizobia are frequent rhizosphere colonizers of a wide range of plants and may also inhabit nonleguminous plants endophytically. In these rhizospheric and endophytic habitats they may exhibit several plant growth-promoting effects, such as hormone production, phosphate solubilization, and the suppression of pathogens.
Sessitsch et al. (Mon,) studied this question.