Coated and smooth vesicles in the biogenesis of cell walls, plasma membranes, infection threads and peribacteroid membranes in root hairs and nodules of white clover.

Abstract

ABSTRACT Involvement of coated and smooth vesicles in the infection by rhizobia of root hairs and nodule cells of white clover was examined using electron microscopy of thin sections. Coated vesicles, about 100 nm in diameter including coat, were observed free in the plant cell cytoplasm and associated with root hair plasma membranes, infection thread membranes and peribac-teroid membranes. The number of coated vesicles seen associated with segments of the plasma membrane at the tip of the root hair was about 20-fold greater than the number associated with the plasma membrane towards the base of the root hair. Similarly, the number seen associated with segments of the infection thread membrane near the tip of the thread was about 20-fold greater than the number associated with the membrane towards the base of the thread. The number seen associated with segments of the peribacteroid membranes in recently infected nodule cells was comparable to the number associated with the plasma membrane towards the base of the root hair or with the infection thread membrane towards the base of the thread. Coated vesicles were observed in the vicinity of and associated with both proximal and distal faces of the Golgi bodies; they often possessed, while still attached to the Golgi cistemae, shorter coats than those observed on coated vesicles free in the cytoplasm. Coated vesicles contained electron-dense material that did not stain positively with Thiéry stain for polysaccharides, and which appeared to have been released into the region of the cell wall or peribacteroid space as a result of fusion of the vesicles with plasma membranes or peribacteroid membranes. Smooth vesicles were also observed in the vicinity of and associated with Golgi bodies, and in association with root hair plasma membranes, infection thread membranes and peribacteroid membranes. The smooth vesicles commonly contained material of varying electron density, which sometimes occurred in the form of droplets. Similar droplets seen in the region of the cell wall and in the peribacteroid space appeared to have resulted from the fusion of smooth vesicles with these membranes. It is proposed that both smooth and coated vesicles derived from Golgi bodies are involved in the biogenesis of cell walls, plasma membranes, infection threads and peribacteroid membranes in root hairs and nodules of white clover.