SECRETION OF CELL-WALL POLYSACCHARIDES IN VICIA ROOT HAIRS

Root hairs of hairy winter vetch (Vicia villosa Roth) synthesize and s ecrete abundant cell wall matrix polysaccharides, making this an excel lent system for the study of secretion during tip growth. Roots with n ewly formed hairs were preserved by cryofixation and freeze substituti on. Cryofixed root hairs showed excellent structural and antigenic pre servation. Ultrastructural analyses showed numerous vesicles near the tip and a concentration of Golgi bodies in the subapical region of the hair. The distribution of polygalacturonic acid and xyloglucan in the endomembrane system and cell wall were revealed by immunolabeling, us ing previously characterized monoclonal antibodies. De-esterified poly galacturonic acid was present on the external surface of the cell wall , but was not detected within the cell, although chemical de-esterific ation revealed abundant antigen in Golgi bodies and secretory vesicles . Methyl-esterified polygalacturonic acid epitopes were detected withi n the medial and trans cisternae of Golgi bodies, in secretory vesicle s, and throughout the wall, indicating that pectin is secreted in a ne utral form and may then be de-esterified in mure. Xyloglucan was also detected within the trans cisternae of Golgi bodies, secretory vesicle s and throughout the cell wall. Double labeling experiments demonstrat ed that both polysaccharides occur simultaneously in the same Golgi bo dies, and that secretory vesicles containing both polygalacturonic aci d and xyloglucan deliver the polysaccharides to the cell wall at the g rowing tip.