What Makes the Rhizobia-Legume Symbiosis So Special?

Abstract

“We are too prone to give all credit to him who places the last piece in the puzzle and to forget that all his predecessors had prepared the way.” —Edwin W. Fred, Ira L. Baldwin, and Elizabeth McCoy (1932) Ever since the identification by Hellriegel and Wilfarth (1888) of rhizobia as the source of fixed nitrogen in root nodules of legumes, people have wondered whether or not plants outside the Fabaceae could be manipulated to associate with rhizobia. The development of nodules, the keystone of Hellriegel and Wilfarth's findings, has since become the “Holy Grail” of the field of biological nitrogen fixation. It is well known that the rhizobia-legume interaction falls into cross inoculation groups, whereby certain rhizobial strains nodulate only certain legumes. For example, Sinorhizobium melilotieffectively nodulates species of Medicago,Melilotus, and Trigonella, whereasRhizobium leguminosarum bv viciae induces nitrogen-fixing nodules on Pisum, Vicia,Lens, and Lathyrus spp. Closely related to the pea (Pisum sativum) strain is R. leguminosarum bvtrifolii, which initiates nodules only on species of clover (Trifolium).