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

An investigation has been made of the plastids in cells of asepticallycultured carrot explants using a range of fixatives and stainingprocedures. Attention was focused upon the plastids as theyoccur in quiescent cells of the carrot root, in explants exposedto a basal medium which does not cause rapid growth, and inexplants which were subject, in both the light and the dark,to stimuli that cause cell division. These cell-division stimuliwere supplied by the use of the liquid endosperm of the coconut,which also induces free cells to behave totipotently and togrow like zygotes. Fully differentiated and functional chloroplastsare formed when cells are exposed to the coconut-milk-supplementedmedium in the light. By contrast, the development of plastidsis arrested when the cells are grown in the dark at a stagewhich, although more complex than the state of the plastidsin the storage root, nevertheless falls far short of the chloroplastwhich develops in the light. Plastids in dark-grown cells aremembrane-bounded and, from the inner membrane, imaginationspenetrate the densely granular 8troma. Two bodies, or organelles,characterize the plastid of dark-grown cells. One consists ofa membrane-bounded, spherical structure the content of whichis very electron-dense after glutaraldehyde fixation but muchless so after treatment with osmium tetroxide. Because thisbody is progressively and demonstrably transformed in the lightinto thylakoids and grana it is named the prethylakoidal body.The other plastid inclusion to which attention is directed isa spherical entity, not visibly membrane-bounded, but whichhas lipid droplets disposed on its surface; this is termed theglobular centre. The materials for the structure of the chloroplastlamellae and grana accumulate in the dark in the prethylakoidalbody; in its metamorphosis in the light this structure formslong continuous loops of spirally twisting thylakoids which,as they close down upon each other, form discrete grana. Asthe thylakoids enlarge and develop, their association with thelipid droplets of the globular centre suggests that this bodyplays an integral part in chloroplast development. Special attentionis given to the details of thylakoidal structure, in so faras these may be revealed in fixed material, and the resultsare compared with other relevant studies. Comparisons of themembranes at the surfaces of thylakoids and at the cell surface(plasma-lemma), as these are affected by different preparativeregimes, have been made and interpreted. Thus plastids of carrotrespond to the complex of factors that stimulate the growthof the cells and, in doing so, they form structures (the prethylakoidalbody and the globular centre) in the dark which, under the influenceof the light, metamorphose into the system of thylakoids andgrana. Further questions are now posed for subsequent investigations.