Localization of mucilaginous polysaccharides in mulberry leaves

Mulberry tree leaves were shown to have mucilaginous polysaccharides. The extracted water-soluble mucilage was separated into three fractions via a cetylpyridinum chloride complex and purified by anion-exchange chromatography. Five acidic polysaccharides were separated from these fractions, one of which was a major polysaccharide (Mp-3) that was structurally analyzed and used for antibody preparation. The Mp-3 polysaccharide contained rhamnose, galactose, glucose, galacturonic acid, and glucuronic acid in a molar ratio of 1 : 0.2 : 0.5 : 2.3 : 1.5 as constituent monosaccharides. Methylation and gas chromatography-mass spectrometry analysis indicated that the polysaccharide was a rhamnogalacturonan mainly consisting of 1,2,3-linked rhamnose residues, 1,3,4- and 1,4-linked uronic acid residues, and terminal uronic acid residues. Its molecular weight was estimated to be 5.5 x 10(5). Immunohistological observation revealed that the Mp-3 polysaccharide is specifically localized in inner epidermal cells situated in adaxial leaves, and electron microscopy showed that its subcellular location is between the plasma membrane and the cell wall. In young leaves, numerous secretory vesicles were present in a shrunken cytoplasm that was surrounded by fibers. In mature leaves, more than 20% of total epidermal cells were these inner cells in which polysaccharide deposition was significantly increased. The deposits appeared as a rounded electron-dense mass throughout the inner cells by electron microscopy.