Ultrastructure of Rhizobium-induced infection threads in clover root hairs.

Ultrastructural studies of Rhizobium-induced infection threads in clover root hairs show that the infection thread is initiated by an invagination process. Root hair wall growth is redirected at a localized point, resulting in the formation of an open pore. There is no direct penetration through the wall, and the bacteria remain extracellular within the root hair.