Ultrastructure of soybean nodules. I: release of rhizobia from the infection thread.

Root nodules on soybeans (var. Clark 63) were examined by transmission electron microscopy 10-12 days after seed inoculation and planting. The cell infection process appeared identical in both effective nodules, induced by Rhizobium japonicum strain 138 (USDA) and in ineffective nodules, induced by strain 8-0 (Iowa). Electron micrographs are presented which suggest that rhizobia are freed from the infection thread by disintegration of the thread wall and compartmentalization of the distintegrated wall material in membrane-bound vesicles derived from the membrane surrounding the thread. As the thread wall is removed in this manner, the bacteria are released into the host cytoplasm by a process which encloses each in an envelope also dervide from the thread membrane. Any thread wall material remaining around a bacterium after it has dissociated from the thread is removed from the envelope space by vesiculation of the membrane envelope. thus, it appears that endocytosis of both the bacteria and the material composing the infection thread wall occurs during release of rhizobia into the host cell.