Spatial relationships between uninfected and infected cells in root nodules of soybean

In soybean (Glycine max (L.) Merr.) the uninfected cells of the root nodule are responsible for the final steps in ureide production from recently fixed nitrogen. Stereological methods and an original quantitative method were used to investigate the organization of these cells and their spatial relationships to infected cells in the central region of nodules of soybean inoculated with Rhizobium japonicum strain USDA 3I1B110 and grown with and without nitrogen (as nitrate) in the nutrient medium. The volume occupied by the uninfected tissue was 21% of the total volume of the central infected region for nodules of plants grown without nitrate, and 31% for nodules of plants grown with nitrate. Despite their low relative volume, the uninfected cells outnumbered the much larger infected cells in nodules of plants grown both without and with nitrate. The surface density of the interface between the ininfected and infected tissue in the infected region was similar for nodules in both cases also, the total range being from 24 to 26 mm²/mm³. In nodules of plants grown without nitrate, all sampled infected cells were found to be in contact with at least one uninfected cell. The study demonstrates that although the uninfected tissue in soybean nodules occupies a relatively small volume, it is organized so as to produce a large surface area for interaction with the infected tissue.