engleski

Structure and formation of fibrils in chromoplasts of cucumber corollas

In chromoplasts of the fibrillar type, carotenoids are deposited into rodlike structures which are, in most species, 15-60 nm in diameter and can reach 10 μ m in length. It is generally considered that the carotenoid pigments are placed in the interior of the fibrils and are surrounded by a coat composed of polar lipids (glycolipids and phospholipids) and proteins. In the present work, the ultrastructure and formation of chromoplast fibrils are characterized in the petals of the cucumber. Ultrastructural analyses were performed on corollas of 15-mm-long flower buds of the cucumber (Cucumis sativus L. var. Cornishon). Small pieces of petal tissue were fixed for 30 min with 2% glutaraldehyde in 0.05 M cacodylate buffer (pH 7.2), at +1 °C. The material was washed in the same buffer and postfixed for 1 h in 1% osmium tetroxide, at +1 °C. After dehydration, the tissue was embedded in Spurr’ s resin. Ultrathin sections were stained with uranyl acetate and lead citrate. Negative staining of isolated chromoplast structures was performed using 2% phosphotungstic acid. Specimens were examined with a Zeiss EM 10A electron microscope. The chromoplasts in cucumber petals contain straight or slightly bent fibrils, which, when cross-sectioned, are polygonal in shape. By using standard staining procedures, the fibrillar coat does not appear as a uniformly contrasted layer, but is formed of darkly stained roundish subunits which are interposed by more lightly stained areas. The darkly stained sites probably correspond to the lipid constituents, since it is generally considered that osmium primarily binds to polar lipids. The central areas of the fibrils are electron-translucent and are presumably composed of carotenoid pigments which are dissolved and removed during sample preparation. Some fibrillar structures appear to be composed of incompletely separated single fibrils. Electron microscopic observations indicate that, in cucumber petals, the plastoglobules have an important role in the formation of the fibrils. This process starts with the appearance of a poorly contrasted area inside the plastoglobule. In most cases, this area is roundish in shape or forms a straight band, indicating the differentiation of a rodlike structure. Sometimes, inside the plastoglobules, a few roundish electron-translucent areas can be observed: these areas are often incompletely separated and morphologically resemble the central part of unseparated fibrils. The plastoglobules are often elongated in the direction of the long axis of the band assuming a half-moon or spindly shape, and a lightly stained band becomes the central part of a fibril which extends from the plastoglobule. These findings implicate that, inside the plastoglobules, some kind of ordered structure is organised which is presumably composed of the hydrophobic fibrillar constituents. It appears plausible that the plastoglobules create a medium favourable for the accumulation of hydrophobic fibrillar constituents, and thus for the initial steps of their morphogenesis.

kromoplastni fibrili; krastavac; ultrastruktura

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