Studies on the Bacterial Component of Soybean Root Nodules: Cytology and Organization in the Host Tissue

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Abstract
Light- and electron-microscope studies of bacteroids in soybean nodule sections and suspensions of isolated bacteroids have shown well-defined nuclear elements, containing tangled filaments and dense granules. The cytoplasm was poorly stained as seen in the light microscope but had considerable dye-reducing activity localized at the poles, giving the appearance of granules. However, electron micrographs revealed no electron dense granules in the cytoplasm near the poles. Cell walls were not found on bacteroids in suspensions isolated from nodules, but were present on bacteria grown in vitro, and on bacteroids in the host tissue. Electron micrographs of thin sections of mature nitrogen-fixing nodules revealed a system of double membranes enclosing groups of four or six bacteroids. These membranes appeared to originate from the host cells and their development was followed in nodules aged from 1–5 weeks; they could be centrifugally separated from the bacteroid fraction of crushed nodules. The origin of the membrane system and its possible role in nodule functions is discussed.

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