engleski

Ultrastructure of plastoglobules in chromoplasts of pansy petals

In many flowers, massive synthesis of carotenoids during petal development is crucial factor which determines flower pigmentation. The large quantities of carotenoids accumulate in photosynthetically inactive plastids, chromoplasts, in which these pigments are sequestered inside discrete plastid substructures [1]. The simplest carotenoid-bearing substructures in chromoplasts are spherical lipoprotein bodies, plastoglobules. It has been proposed that surface of plastoglobules is covered by thin monolayer of polar lipids and proteins, while apolar components are buried in the plastoglobule interior [2]. Here we ultrastructurally analysed plastoglobules in petals of yellow pansy (Viola x wittrockiana Gams.). Samples for ultrastructural analyses were taken from freshly collected petals at several developmental stages. Small pieces of tissue were fixed in 2% glutaraldehyde and postfixed with 1% osmium tetroxide. After dehydration in a graded series of ethanol, the tissue was embedded in Spurr's resin. Ultrathin sections were stained with uranyl acetate and lead citrate and examined using a Zeiss EM10A electron microscope. Development of typical chromoplasts was observed in epidermal cells. In adaxial epidermis, chromoplast differentiation started from morphologically simple plastids containing several membranes, while, in abaxial epidermis, chromoplasts differentiated from young chloroplasts. In young petals, plastids in both epidermises contained a few small plastoglobules (up to 70 nm in diameter). During petal development, plastoglobules increased in number and size. In early stages of chromoplast differentiation, the interior of plastoglobules was homogenously stained, but showed different levels of osmiophilicity (Figure 1A). The periphery of many plastoglobules showed darkly stained layer. During further stages, one large or several smaller electron-translucent areas often appeared in the interior of plastoglobules (Figure 1B). These areas were more or less roundish in shape. In fully developed chromoplasts of open flowers, plastoglobules reached up to 760 nm in diameter. They were, in some cases, homogeneously stained, while, in the other cases, had one or more, electron-translucent areas (Figure 1C). Some plastoglobules did not have typical roundish shape, but showed one or more concave areas at their periphery (Figure 1D). Also, some plastoglobules appeared to be partially fused (Figure 1C). Our ultrastructural studies revealed diverse characteristics of plastoglobules in chromoplasts of pansy petals, indicating that chromoplast plastoglobules could be more complex and dynamic structures then had been generally considered.

chromoplasts, plastoglobules, Viola x wittrockiana, petals

nije evidentirano