The structural basis for infection of root hairs of Trifolium repens by Rhizobium

Root hair infection of Trifolium repens L. by Rhizobium trifolii was investigated with regard to the structural basis of infection thread origin. Most infected root hairs were shown to have in common an enclosed region at-the site of thread origin formed by specialized root hair growth or contacts. Electron micrographs of diverse infection sites showed in every case a degradation of the root hair wall at the site of thread origin within the enclosure. The thread wall is a new layer formed by apposition of material by the host cytoplasm near the penetrated wall and surrounding the break as encapsulation of the invading rhizobia. It is suggested that rhizobial enzymes provide for degradative penetration of the root hair cell wall and that localized concentrated activity of hydrolytic enzymes as well as protection from cell lysis is favored by physical constraints provided by the deformed root hair enclosures.