Early infection events in the nodulation of the non-legume Parasponia andersonii by Bradyrhizobium

Abstract

The non-legume Parasponia andersonii was found to have an infection zone close behind the growing tip of the tap root which is similar to that found in legume infections by Bradyrhizobium. An examination of spot inoculated sites in the infection zone showed that bacteria had colonized the root surface and begun to erode the mucilage layer of the primary cell wall of epidermal cells within 24 h of inoculation. Four days after inoculation swelling of the root beneath the area of surface erosion was visible and was followed by the rupturing of the overlying epidermal layer and the emergence of a mass of dividing cells by 6 days. The removal of the epidermis and the emergence of exposed cells provided a site through which bacteria could enter the root and colonize the cortex. The initial infection events of root hair curling and the formation of infection threads within root hairs found in legume and other non-legume associations were not observed in the invasion of P. andersonii. P. andersonii infection requires the initiation of cell division to form a ‘wound’ for intercellular invasion followed by the formation of infection threads from colonies within the root cortex.