An Electron Microscope Study of Root-Hair Infection by Rhizobium

Abstract

SUMMARYRoot hairs of clover seedlings infected by Rhizobium were studied in the electron microscope. The appearance of the infection sites supports the invagination hypothesis proposed by Nutman. The origin of the material enclosing the bacteria within the infection thread is discussed.

Exostructures of Rhizobium meliloti

“…The function of these exocellular structures is unknown, although they have been observed in other fast-growing Rhizobium species, such as R. trifolii and R. leguminosarum (unpublished results). Perhaps the ability of these bacteria to produce polar fibrillar material helps the rod-shaped cells to attach to inert [26] or biological surfaces [27][28][29] in a polar fashion. This positioning mechanism may be important to the initial stages of the nodulation process in the Rhizobium-legume symbiosis.…”

Root Hairs

“…Various models for the mechanism of entry of rhizobia into the root hair were proposed from the few ultrastructural studies of the infection process done between 1963 (Sahlman and Fahraeus 1963. The models differ essentially in the importance which is given to the role of hydrolysing enzymes from totally absent in the earlier models such as the invagination theory of Nutman (see Napoli and Hubbell 1975) and the implication of complete cell wall degradation in the later ones .…”

Enzymes involved in legume root hair infection by rhizobia

“…Reorientation of root hair growth to create invagination was considered by Nutman (1958) to be perhaps caused by the action of lAA, formed by rhizobia, upon the processes of intussusception occurring in the host's primary cell wall. Studies of infected Trifolium parviflorum root hairs by Sahlman and Fahraeus (1963), using electron microscopy, have presented striking photographic evidence for the invagina tion of the primary cell wall at the site of initiation of the infection thread.…”

“…Action of this enzyme, assuming it is one, on the pectin in the adjacent root hair primary cell wall could result in a decrease in plasticity of a portion of the cell wall. Continuing elongation of the root hair cell wall except at the localized area where plasticity was decreased would result in the characteristic root hair curling which has been frequently illustrated (McCoy, 1932;Thornton, 1936;Nutman, 1958;and Sahlman and Fahraeus, 1963). Setterfield and Bayley (1961) have cited evidence to support an argument that calcium-pectin interactions do not control wall plasticity.…”

“…Whether root hair curling would lead to entry of rhizobia into the root hair cell by invagination, as the excellent electron micrographs of Sahlman and Fahraeus (1963) seemed to indicate, is open to conjecture.…”