Isolation and Characterization of Secretory Vesicles in Germinated Pollen of Lilium longiflorum

Abstract

Secretory vesicles containing polysaccharide were isolated from germinated pollen of Lilium longiflorum and characterized by biochemical and ultrastructural investigation. Pollen tubes exhibit a secretory pathway in which the vesicles concentrated in the tube apex are produced by the Golgi apparatus and contributed to the cell wall at the apex upon fusion of the vesicle membrane with the plasma membrane. Secretory vesicles were isolated by a method involving the size discrimination of cytoplasmic components using Millipore filters. Cells were disrupted under conditions which minimized membrane vesiculation. Identification was made by electron-microscopic comparison of the periodic acid-silver hexamine (PASH) reactivities of in situ and isolated secretory vesicles. The secretory vesicles contained polysaccharides which were high in galacturonic acid and similar in sugar composition to those of the hot-water-soluble fraction of pollen tube cell wall. A hot-water-insoluble, non-cellulosic glucan was the major component of the cell wall. Less than 7% of the wall was cellulosic. Chitin was absent. Similarities in the ultrastructure and PASH staining of apical secretory vesicles and an amorphous component of the cell wall support a precursor-product relationship between these 2 cell components. Ultrastructural investigations revealed complexes of the endoplasmic reticulum (ER) associated with electron-translucent regions of cytoplasm, suggesting a possible function of the ER in cell wall formation. Additionally, patterns of PASH staining show that changes in polysaccharides occur in secretory vesicles after vesicles have been formed by dictyosomes. Therefore, secretory vesicles may have a role in polysaccharide synthesis as well as in membrane and product transport.