Anatomical analysis of nodule development in soybean reveals an additional autoregulatory control point

The region of the root of soybean (Glycine max [L.] Merr. cv. Bragg) susceptible to nodule initiation by Bradyrhizobium japonicum Jordan USDA110 was examined by serial section and light microscopy to study the control of nodule development. Three successive susceptible regions separated by 24-h intervals were examined. Infection foci were catalogued within defined stages of nodule development (as defined by Calvert et al., Can. J. Bot., 62 (1984) 2375-2384) and their number and location determined after short (72 h) and long (13 days) post-inoculation periods. The overall distributions of cell division stages were similar for both short and long periods, indicating that regulatory mechanisms were active throughout root development. Early cell division foci with no visible meristems (stages I-II) appeared with similar frequency in each susceptible region. In contrast, the frequency of more mature cell division events showing developing meristems (stagesIII-IV) was decreased in the third susceptible region for both periods. The constant frequency of cell division stages I-II within the feedback regulated reduction coupled with the reduction of later stages implies that nodule initiation is slowed and that the transition from cell division stage II to stage III is also regulated.