VIEW ARTICLE

Rhizobial Root Nodules of Soybean as Revealed by Scanning and Transmission Electron Microscopy. J. C. Tu, Biological Sciences Electron Microscope Laboratory, Faculty of Science, University of Alberta, Edmonton, Alberta, Canada T6G 2E9; Phytopathology 65:447-454. Accepted for publication 6 November 1974. DOI: 10.1094/Phyto-65-447.

Gross-structural and fine-structural organization of the soybean root nodule (incited by Rhizobium japonicum) were correlated by the application of scanning (SEM) and transmission electron microscopy (TEM). The root nodule was comprised of cortical tissue and central tissue. The latter consisted of bacteroidal cells (Rhizobium-infected) and interstitial cells (Rhizobium-free). The bacteroidal cells were arbitrarily categorized into three infection stages: early, intermediate, and advanced. In the early stage, each bacteroid was enclosed in a membrane envelope; in the intermediate stage, several bacteroids were enclosed in a membrane envelope; and in the advanced stage, the bacteroids lacked a membrane envelope and the host cellular membrane system had deteriorated. All three infection stages of bacteroidal cells were present simultaneously in all root nodules observed. It is suggested that, in the advanced stage, the nitrogen-fixing ability might be lacking. Since the number of the bacteroidal cells in the advanced stage increased with the root nodule size, the efficiency of a root nodule in nitrogen fixation should not be judged on the basis of nodule size alone. Rather, efficiency should be evaluated on the basis of the total surface area of envelope membranes.