Isolation and characterization of an amyloplast envelope-enriched fraction from immature maize endosperm

Gardner, H. W., Miernyk, J. A., Christianson, D. D. and Khoo, U. 1987. Isolation and characterization of an amyloplast envelope-enriched fraction from immature maize endosperm. A 10000–100000 g pellet obtained by centrifugation of homogenates from immature (25 days after pollination) de-embryonated maize (Zea mays L., cv. W64A-normal and a typical hybrid) kernels was further fractionated by sedimentation on discontinuous sucrose density gradients. Particles with the highest carotenoid content (0.68% by weight carotenoids based upon total lipid) sedimented at densities of 1.083-1.106 g ml-1, coincident with the plastid envelope marker enzyme, galacto-syltransferase (EC 2.4.1.46). Lipids extracted from the carotenoid-rich fraction were mainly digalactosyldiacylglycerols, monogalactosyldiacylglycerols, phosphatidylcholines, phosphatidylinositols and phosphatidylglycerols, in order of molar abundance. With increasing particle density (>1.106 g ml1) the phospholipid and neutral lipid content increased, and the proportion of carotenoids and galactolipids decreased. Electron micrographs of the carotenoid-rich fraction revealed vesicles ranging in size from < 0.1 to 0.5 um, as well as smaller granular membranes. The carotenoid-rich membrane fraction was progressively more difficult to isolate as the endosperm matured, and freezing the immature endosperm prevented subsequent isolation. The lipid and enzyme composition and ultrastructural characteristics of the isolated fraction suggest that it is composed of amyloplast envelope vesicles.