The Fine Structure and Development of Plastids in Cultured Cells of Daucus carota

An investigation has been made of the plastids in cells of aseptically cultured carrot explants using a range of fixatives and staining procedures. Attention was focused upon the plastids as they occur in quiescent cells of the carrot root, in explants exposed to a basal medium which does not cause rapid growth, and in explants which were subject, in both the light and the dark, to stimuli that cause cell division. These cell-division stimuli were supplied by the use of the liquid endosperm of the coconut, which also induces free cells to behave totipotently and to grow like zygotes. Fully differentiated and functional chloroplasts are formed when cells are exposed to the coconut-milk-supplemented medium in the light. By contrast, the development of plastids is arrested when the cells are grown in the dark at a stage which, although more complex than the state of the plastids in the storage root, nevertheless falls far short of the chloroplast which develops in the light. Plastids in dark-grown cells are membrane-bounded and, from the inner membrane, imaginations penetrate the densely granular 8troma. Two bodies, or organelles, characterize the plastid of dark-grown cells. One consists of a membrane-bounded, spherical structure the content of which is very electron-dense after glutaraldehyde fixation but much less so after treatment with osmium tetroxide. Because this body is progressively and demonstrably transformed in the light into thylakoids and grana it is named the prethylakoidal body. The other plastid inclusion to which attention is directed is a spherical entity, not visibly membrane-bounded, but which has lipid droplets disposed on its surface; this is termed the globular centre. The materials for the structure of the chloroplast lamellae and grana accumulate in the dark in the prethylakoidal body; in its metamorphosis in the light this structure forms long continuous loops of spirally twisting thylakoids which, as they close down upon each other, form discrete grana. As the thylakoids enlarge and develop, their association with the lipid droplets of the globular centre suggests that this body plays an integral part in chloroplast development. Special attention is given to the details of thylakoidal structure, in so far as these may be revealed in fixed material, and the results are compared with other relevant studies. Comparisons of the membranes at the surfaces of thylakoids and at the cell surface (plasma-lemma), as these are affected by different preparative regimes, have been made and interpreted. Thus plastids of carrot respond to the complex of factors that stimulate the growth of the cells and, in doing so, they form structures (the prethylakoidal body and the globular centre) in the dark which, under the influence of the light, metamorphose into the system of thylakoids and grana. Further questions are now posed for subsequent investigations.