Development of tapetum and orbicules of Catharanthus roseus (Apocynaceae)

Abstract

Abstract The development of the tapetum and orbicules in Catharanthus roseus (L.) G. Don has been examined using transmission electron microscopy (TEM). Precursors of orbicules are formed as spheroidal vesicles within the tapetal cytoplasm and are associated with the endoplasmic reticulum. The pro-orbicules are extruded through the cell membrane of the tapetal cell and this coincides with the dissolution of the ta petal cell wall and the callosic wall of the microspores. The orbicules are developed by irregular deposition of sporopollenin on the pro-orbicules. A thin membranous layer is developed between the orbicules, forming the orbicular membrane. At free microspore stage globular bodies are seen in the cytoplasm of the tapetal cells. Some of these bodies are large and coated with a thin electron-dense layer, others are small bodies, electron-dense and gathered in groups. At vacuolate stage the tapetal cells extend into the anther locule, acting as a periplasmodial tapetum. At late vacuolate stage the tapetal cells retreat from the anther locule and possess an organized and apparently functional structure. The tapetal cells start to degenerate just before an thesis of the pollen grains. The diversity in the structure of orbicules and tapetum may suggest different functions at different stages of development and may have arisen by remarkable processes of adaptation.