Ultrastructural study on dynamics of plastids and mitochondria during microgametogenesis in watermelon

Ultrastructural aspects of pollen ontogeny in an endangered plant species, Pancratium maritimum L. (Amaryllidaceae)

Pollen ontogeny in Pancratium maritimum L. was studied from the sporogenous cell to mature pollen grain stages using transmission electron, scanning electron, and light microscopy to determine whether the pollen development in P. maritimum follows the basic scheme in angiosperms or not. In the course of microsporogenesis and microgametogenesis, special attention was given to the considerable ultrastructural changes that are observed in the cytoplasm of microsporocytes, microspores, and mature pollen grains throughout the successive stages of pollen development. Microsporocyte differentiation concerning number and ultrastructure of organelles facilitates the transition of microsporocytes from the sporophytic phase to the gametophytic phase. However, cytoplasmic differentiation of generative and vegetative cells supports their functional distinctness and pollen maturation. Although microsporogenesis and microgametogenesis in P. maritimum generally follow the usual angiosperm pattern, abnormalities such as formation of unreduced gametes were observed. During normal microsporogenesis, meiocytes undergo meiosis and successive cytokinesis, resulting in the formation of isobilateral, decussate, and linear tetrads. Subsequent to the development of free and vacuolated microspores, the first mitotic division occurs and bicellular monosulcate pollen grains are produced. Pollen grains are shed from the anther at binucleate stage. During pollen ontogeny, three periods of vacuolization were observed: in meiocytes, in mononucleate free microspores, and in the generative cell.

Ultramicroscopic examination of mature massulae of Habenaria arinaria (Orchidaceae)

The mature massula of H. arinaria was examined by means of transmission electron microscopy, with the aim to understand the nature of cohesion between grains, the accumulation of pollen storage reserves, and the behavior of the nucleus of the vegetative cell in this composite type of pollen. The massula was a union of a large number of polygonal pollen grains that were tightly linked together. The exine within the massula were highly simplified, consisting of a single layer of nexine-2, lacking tectum, bacula, and nexine-1, while all the four layers comprised the exine on the massula surface. The two layers of nexine-2 of adjacent grains fused into a seamless whole. Undoubtedly the fusion of the nexine-2 was the mechanism by which the grains of the massula were linked together. No starch grains, lipid bodies, or storage proteins were present in the mature massula, and so the composite pollen of this species belonged to a novel type with regard to storage reserves. The vegetative nucleus was not lobed and revealed a huge amount of highly condensed chromatin, indicating a quiescent status. The condensed status of the vegetative nuclei in this composite type of pollen system is in striking contrast to the highly decondensed status reported in the free type of pollen grains.

Ultrastructural study on dynamics of lipid bodies and plastids during ripening of chili pepper fruits

Dynamics of lipid bodies and plastids in chili pepper fruits during ripening were investigated by means of transmission electron microscopy. Mesocarp of chili pepper fruits consists of collenchyma, normal parenchyma, and huge celled parenchyma. In mature green fruits, plastids contain numerous thylakoids that are well organized into grana in collenchyma, a strikingly huge amount of starch and irregularly organized thylakoids in normal parenchyma, and simple tubes rather than thylakoids in huge celled parenchyma. These morphological features suggest that plastids are chloroplasts in collenchyma, chloroamyloplasts in normal parenchyma, proplastids in huge celled parenchyma. As fruits ripen to red, plastids in all cell types convert to chromoplasts and, concomitantly, lipid bodies accumulate in both cytoplasm and chromoplasts. Cytosolic lipid bodies are lined up in a regular layer adjacent to plasma membrane. The cytosolic lipid body consists of a core surrounded by a membrane. The core is comprised of a more electron-dense central part enclosed by a slightly less electron-dense peripheral layer. Plastidial lipid bodies in collenchyma, normal parenchyma, and endodermis initiate as plastoglobuli, which in turn convert to rod-like structures. Therefore, plastidial lipid bodies are more dynamic than cytosolic lipid bodies. Both cytosolic and plastidial lipid bodies contain rich unsaturated lipids.