Structural and Immunocytochemical Characterization of the Synthesis and Accumulation of Starch in Sweet Potato (Ipomoea batatas Lam.) Tuberous Root

Abstract

The structural changes in the plastid-amyloplast system in the parenchyma cells of sweet potato tuberous roots during thickening were examined by electron microscopy. In the tuberous roots, proplastids and plastids that contain starch granules propagated in young parenchyma cells adjacent to the meristem, but amyloplasts did not in parenchyma cells. It was suggested that the number of amyloplasts in a parenchyma cell is determined by the propagation of the proplastids and plastids. The form of amyloplasts and the number, size and form of starch granules in them were various. Tubular membranes containing the electron-dense substance were formed in plastids and extended from the envelope membranes of plastids to the starch granules. The electron-dense substance also existed around the starch granules. Tubular membranes are converted into membrane-bound inclusion bodies as a result of loading of the electron-dense substance into these tubes in the plastids. The inclusion bodies were also at the periphery of the amyloplasts. In this study, the functions of both tubular membrane and the inclusion were discussed. The localization of starch branching enzyme in tuberous roots was examined by immunogold electron microscopy. The label for branching enzyme was localized predominantly throughout the surface of each starch granule, suggesting that this is the branching for amylopectin synthesis, but not throughout the stroma in the plastid-amyloplast system. Small and round starch granules were often formed at parts of the periphery in the amyloplast. Dense labeling for the enzyme was detected around the granules. The increase of the number of starch granules in an amyloplast is certainly made by means of the formation of new starch granules at the periphery of the amyloplast. It is likely that the new granules are intensively formed there.