Recent Trends in Identification and Molecular Characterization of Rhizobia Species

Abstract

Nutrient enrichment of soils by nitrogen-fixing symbiotic bacteria present in legumes has been known for centuries. The symbiosis between the root nodule bacteria of the genus Rhizobium and legumes results in the fixation of atmospheric nitrogen in root nodules. This symbiotic relationship is of special significance to legume husbandry as seed inoculation with effective strains of Rhizobium can meet the nitrogen requirements of the legume to achieve increased yields. Rhizobia are Gram-negative soil bacteria which are able specifically to induce nitrogen-fixing nodules on the roots of leguminous plants. A myriad of advances have been made in the understanding of both plant and bacterial genomes, the biochemistry of the symbiosis, plant and bacterial signaling, and the measurement of nitrogen fixation. However, knowledge of the ecology of the bacterial symbiont has lagged behind, mainly due to a lack of practical techniques that could be used to monitor and assess the performance of these bacteria in the field. The development of practical techniques, while providing knowledge of the success or failure of specific strains in a range of environments, has not allowed insight into the nature of the pre-existing rhizobial populations in these sites, nor the interaction between marked strains and the background population. The advent of molecular techniques has revolutionized the study of Rhizobium. In addition, molecular techniques have increased the basic knowledge of how individual strains and populations of root nodule bacteria respond to changes in the environment and how genetic diversity evolves in field sites over time. This chapter focuses on recently developed molecular techniques and other advanced technologies of genomics, proteomics, and transcriptomics that hold promise for continuing to develop our understanding of Rhizobium ecology and how these can be used to address a range of applied problems to yield new insights into rhizobial life in soil and as legume symbionts.