Infection and Root-Nodule Development inStylosanthesSpecies byRhizobium

Root nodules of the tropical forage legume Stylosanthes occurred only at lateral root junctions and resulted from direct invasion by rhizobia through spaces between epidermal cells. No infection threads were present in either the root hairs or nodules. Invasion of the host cortical cells was through structurally altered cell walls. The bacteria reached the site of nodule initiation in the lateral root cortex by progressive collapse of the initially invaded cells which were compressed by neighbouring cells to form intercellular thread-like infection zones. The bacteria multiplied in the invaded cells of the nodule initial which divided repeatedly to form the nodule. Bacteroids formed only when the host cells ceased to divide. Some abnormal associations occurred in S. capltata and S. hamata 40264A. Division of invaded cells was restricted in S. capitata and the bacteria became enlarged and grossly deformed. In S. hamata restricted cell division was immediastely followed by the brcakdown of the host cells and, although the bacteria multiplied, no bacteroids were formed. Bacteria isolated from these nodules formed both effective and abnormal nodules when inoculated onto the same host.