STUDIES ON THE LOCALIZATION OF A PROTEIN, IMMUNOLOGICALLY SIMILAR TO A 62-KILODALTON SUCROSE-BINDING PROTEIN ISOLATED FROM DEVELOPING SOYBEAN COTYLEDONS, IN THE SHOOT AND ROOT OF SPINACH

Previous work, using antisera developed against a 62 kDa sucrose-binding protein isolated from developing soybean cotyledons, resulted in the identification of an immunologically similar protein on the sieve-tube plasma membranes of the small minor veins of spinach (Spinacia oleracea L. var. Bloomsdale Dark Green). These studies have been expanded to determine the extent of the immunologically similar protein throughout the shoot and root of the mature spinach plant. Indirect immunogold labelling of resin-embedded leaf and stem tissues resulted in the specific localization of colloidal gold on the plasma membrane of mature sieve tubes in the major veins and large minor veins of the lamina as well as sieve tubes of the petiole and stem. The amount of label associated with the petiole and stem sieve tubes was less than in the sieve tubes of the lamina. In all cases, the label was evenly distributed along the plasma membrane and, except for a small amount of background label on the cell walls, was not associated with any other cellular component including organelles, membranes or membrane systems. Other cell types of the vascular and ground tissues were also essentially free of label. Control sections treated with non-immune rabbit IgG were free of label. Sections of root tissue treated with the 62 kDa antisera contained immunolabel on the plasma membrane of the sieve tubes and, in addition, the plasma membrane of various parenchymatic elements including companion cells and vascular parenchyma cells. In both root and shoot tissues, the plasma membrane of sieve tubes considered to be structurally and physiologically immature did not contain label. These results are discussed in relation to the apoplastic and symplastic transport of solutes in source and sink tissues.