Development of the bacteroid in the root nodule of barrel medic (Medicago tribuloides Desr.) and subterraneum clover (Trifolium subterraneum L.)

Summary The development of the bacteriod is traced from thin sections of slices of nodules fixed in KMnO₄ and OsO₄. While in the infection thread the Rhizobium cell has the ultrastructure characteristic of gram-negative bacteria, with two unit membranes bounding a granular cytoplasm containing dense bodies, a nucleoid area and inclusion granules. A 10–12 fold increase in size, a loss of inclusion granules and the formation of a membrane envelope around each Rhizobium cell follows the dispersal of the rhizobia through the host cytoplasm. As the bacteriods develop there is a loss of fibrillar material from the nucleoid region and changes occur in the distribution of ribosome-like particles in both host and bacterial cells. When fully differentiated and presumably fixing nitrogen the bacteroids from the red zone of subterraneum clover nodules but not barrel medic have a well developed intra-cytoplasmic membrane system.