engleski

Redifferentiation of chloroplasts in the spathe of Zantedeschia elliottiana

As a critical event in plant life, pollination is usually followed by perianth abscission, withering or wilting. In some plant species, however, pollination is followed by regreening of the perianth, thus causing a significant delay in flower senescence (van Doorn 1997). This is, for instance, exemplified by Zantedeschia elliottiana, whose inflorescence is surrounded by a large bract, called the spathe, which bears the function of the perianth. During its development, the spathe of Zantedeschia undergoes remarkable changes in colour. When the bud emerges, it is tightly folded, with its outer face green and its inner face pale, almost white. During maturation, the spathe gradually unfolds and turns yellow. At anthesis, both the inner and the outer face of the spathe are brightly yellow. After pollination, the spathe begins to regreen, first in its outer face, and proceeds until both faces are equally green. The macroscopic changes in the colour of the spathe are due to a reversible transformation of chloroplasts into chromoplasts in the inner and outer subepidermal cell layers of the spathe. In this work, ultrastructural changes in the course of regreening of the spathe were investigated. The ultrastructure of subepidermal plastids was investigated by conventional methods of electron microscopy. Small pieces of spathe 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 and examined with a Zeiss EM 10A and FEI Morgagni electron microscopes. Both the inner and the outer subepidermal cell layers of the young, green spathe contained typical lens-shaped chloroplasts which, during spathe maturation, turned into roundish or irregularly shaped chromoplasts. The latter had several, usually highly swollen, thylakoids which frequently formed vacuole-like structures. The conspicuous feature of chromoplasts were osmiophilic plastoglobule-like inclusions. Some of them were roundish, but the majority showed a tendency to form polygonal structures, or even various unusual shapes, such as elongated or ear-like. In the early stages of regreening, an increase in the number of thylakoids was observed in plastids of both faces of the spathe. Simultaneously, the osmiophilic inclusions became spherical, while their size and number decreased. In the later stages of regreening, the plastids in the outer subepidermal cell layers appeared as typical lens-shaped chloroplasts with a well-developed thylakoid system and small plastoglobules. At the same time, the plastids in the inner subepidermal cell layers did not considerably change. In the fully regreened spathe, the chloroplasts of the outer face contained high grana stacks oriented in different directions, while plastoglobules were scarce, small and spherical. In this developmental stage, chloroplasts also appeared in the inner face of the spathe, although they were never as abundant in thylakoids as those in the outer face. Their plastoglobules, the number of which remained rather high, were spherical and differed in size, from very small to very large. Our ultrastructural studies showed that the chromoplasts in the subepidermal cell layers of the yellow spathe are able to transform to fully functional chloroplasts. It may be postulated that the biological role of the regreening of the perianth and its delayed senescence could be a contribution to the nutrition of the developing fruit.

chloroplasts; Zantedeschia elliottiana; spathe

nije evidentirano