Early events in the infection of soybean by Rhizobium japonicum. Time course and cytology of the initial infection process

The time course of early infection events in Glycine max following inoculation with Rhizobium japonicum is described. Bacteria became attached to epidermal cells and root hairs within minutes of inoculation. Marked root hair curling occurred within 12 h. Infection thread formation was visible at the light microscope level of resolution about 24 h after inoculation. Infections were observed in short, tightly curled root hairs. These root hairs had not yet emerged at the time of inoculation. Infection threads appeared to originate in pockets formed by contact of the cell wall of the curled root hair with itself. Infection threads in the hairs were multiple and (or) branched. By 48 h, the infection thread(s) had progressed to the base of the root hair but had not yet penetrated into the cortex. Increases in cortical cell cytoplasm and in mitotic division occurred in advance of the penetrating infection thread. A nodule meristem developed in the outer cortex next to the infected root hair by 4 days and was accompanied by cell division across the cortex.