Root and stem nodule bacteria of legumes

Rhizobia are nitrogen-fixing bacteria that form root and sometimes stem nodules on leguminous plants and belong to many genera of alpha- and beta-proteobacteria. Formation of nitrogen-fixing nodules is dependent upon the exchange of a series of molecular signals between rhizobia and their host legumes. Within the nodules, rhizobia convert atmospheric dinitrogen gas into ammonia, and this fixed nitrogen is subsequently assimilated by the host plant resulting in improved plant growth and productively, even under N-limiting environmental conditions. While this has obvious importance for agricultural productivity, it also influences the global N cycle, is ecologically beneficial, and reduces the use of our limited fossil fuel resources. Because of the practical benefits of nodulation and nitrogen fixation, the rhizobia have been extensively studied, particularly with respect to the genetic basis for their symbiotic interactions, their host specificity, and nitrogen-fixation potential. In this chapter, we review the accumulated studies about the ecology, genetics, and application of root and stem bacteria of legumes and their importance to plant growth and productivity.