Genomes of the symbiotic nitrogen-fixing bacteria of legumes.

Over the last several decades, there have been a large number of studies done on the genetics, biochemistry, physiology, ecology, and agronomics of the bacteria forming nitrogen-fixing symbioses with legumes. These bacteria, collectively referred to as the rhizobia, are taxonomically and physiologically diverse members of the a and b subclasses of the Proteobacteria, and mostly comprise members of the genera Rhizobium, Bradyrhizobium,Mesorhizobium,Sinorhizobium, and Azorhizobium (Fig. 1). Most studies have focused on mutational and biochemical analyses to define bacterial genes involved in root-nodule formation, symbiotic specificity, nitrogen fixation, and plant-microbe signal exchange. More recently, however, several genomic approaches have been used to define and understand the involvement of whole bacterial genomes in the symbiotic process. Genomic analyses of the model symbiotic bacterial species Sinorhizobium meliloti, Rhizobium leguminosarum, and Bradyrhizobium japonicum have revealed a few surprises concerning genome evolution and structure, how plant and microbes communicate, and physiological diversity among the microsymbionts of legumes. In this review we discuss what is currently known about the genomes of several rhizobia and how genome-enabled studies have provided insights into the symbiotic interaction of the rhizobia and their respective host legumes.