Symbiosis Between Rhizobia and Legumes

Abstract

The term of rhizobia, or root nodule bacteria, is the common collective name for diverse symbiotic nitrogen-fixing soil bacteria that can induce root and/or stem nodules on the legume plants. Although this common name was derived from the genus name Rhizobium (Frank 1889), bacteria distributed in diverse species within different genera, families and classes have been confirmed as microsymbionts of nodules of distinct legume species; therefore, the word “rhizobia” has lost its taxonomic meaning but refers to the symbiotic nitrogen-fixing bacteria associated with nodules of legumes. Inside the nodules, the cells of these bacteria differentiate into bacteroids, which are polymorphic cells with modified cell walls, enclosed by plant cell membrane to form a structure called the symbiosome. This structure provides the conditions that allow the function of nitrogenase, the enzyme that reduces elemental nitrogen to ammonia in the bacteroids. From this association, both the legumes and the rhizobia can benefit: the host plants supply the rhizobia with C4-dicarboxylic acids as carbon and energy source, and the rhizobia offer the plants nitrogen nutrients produced by reducing atmospheric nitrogen and incorporating it into amino acids such as alanine (Poole and Allaway 2000).