Heterotrophic tobacco cell cultures during greening

Structural changes in cells and plastids are described that occur during the greening of an initially dark‐grown cell suspension of tobacco ( Nicotiana tabacum L. cv. Xanthi). The pattern of cell growth during greening, expressed in dry weight or cell number, showed a classical sigmoid curve with a lag phase (T 0 –T 2 ), an exponential phase (T 3 –T 9 ) and a stationary phase (T 10 –T 21 ). Achlorophyllous vacuolated cells (T 0 ), obtained after 3 culture cycles in the dark, contained amyloplasts devoid of lamellae. Exposure to light brought about an enrichment in cytoplasm and an amyloplast to proplastid transformation (starch loss) accompanied by chlorophyll synthesis. By T 3 , many cells appeared meristematic and contained dividing proplastids with rudimentary single lamellae typical of those in intact meristematic leaf cells. As cell division occurred (T 3 to T 9 ), plastids replicated and their internal membrane system developed progressively into defined grana‐intergrana thylakoids. By the stationary phase of cell growth (T 14 ), the lamellar system had reached a highly structured grana‐intergrana network typical of higher plant chloroplasts. We have emphasized the analogies between the sequence of events (proplastid to chloroplast transition) during the greening of tobacco cells and that in developing intact leaves; in this respect the cell cultures provide a useful material for studies dealing with the biogenesis of structural or physiological events.