Ultrastructure of soybean nodules formed from electron transport mutants of bradyrhizobium japonicum 61A76

The differentiation and developmental changes of cells and organelles of soybean [Glycine max cv. Williams] nodules infected with Bradyrhizobium japonicum 61A76 wild type or electron transport mutant strains were studied by transmission electron microscopy. The three 61A76 mutant strains; namely, Na1, Na2 and Na0, although totally deficient in C-and aa3-type cytochromes were symbiotically infective but formed ineffective nodules. Geedlings were maintained in Leonard jars in sterilized sand on a N-free nutrient solution. Eleven days after inoculation cells in both effective and ineffective nodules had the features of typical meristematic cells. The three mutant strains provoked different and unusual responses in the cytoplasm of plant cells. A feature induced by strain Na1 was electron-dense cytoplasm; cell vacuoles were not common. The most remarkable structural changes occurred in host mitochondria, which appeared elongated, disrupted or dumbbell-shaped, mitochondrion-amyloplast associations were recorded. The cytoplasm in cells of nodules induced by strain Na3 was heterogeneous, variously sized vesicles and vacuoles were scattered throughout the cytoplasm. Fewer abnormalities were observed in nodules induced by strain Na0. 35-45 days after inoculation, major differences between wild type and mutant strains were observed in bacteroid development. In ineffective nodules, bacterial cells were either still enclosed within intercellular spaces, enveloped in masses of polysaccharide-like material or trapped in large infection thread vesicles surrounded by massive cell walls