Ultrastructure of Rhizobium-Induced Infection Threads in Clover Root Hairs1

The first stage in the establishment of the Rhizobium-legume symbiosis is the infection of the root of the host legume by the appropriate Rhizobium species. A recent and complete review of the infection process has been given by Dart (1). In the clover symbiosis, infective strains of Rhizobium trifolii enter the host through root hairs. A characteristic deformation is curling at the root hair tip to produce a "shepherd's crook" (4). The bacteria enter the root hair and are enclosed in a tubular structure, the infection thread, which is the first visible, microscopic sign of a successful infection (10). The majority of infected root hairs have the shepherd's crook at the infection thread origin; however, exceptions exist (4, 16) and will be reported in this paper. Three theories have been proposed regarding the entry of the bacteria into the root hair. Nutman (15) has advanced the hypothesis of root hair cell wall invagination. An invagination is the redirection of plant cell wall growth at a localized point, resulting in the wall growing back into the root hair to form the tubular infection thread. There is no penetration through the wall at the point of entry, and the bacteria remain extracellular; i.e., there is no contact with the host cytoplasm. Ljunggren and Fahraeus (12) have proposed a "polygalacturonase" hypothesis, in which the rhizobial exopolysaccharide increases plant pectic enzyme activity and a single bacterial cell softens and subsequently penetrates the plant cell wall without pronounced structural disruption. The infection thread is presumably initiated once the bacterium penetrates to the plant plasmalemma. Dart and Mercer (2) have proposed the entry of small coccoid forms of