Abstract:Cells of Stevia rebaudiana callus tissue contain chloroplasts when cultured in a liquid medium in the light. Transfer of the callus into the fresh culture medium results in the gradual decrease or disappearance of photosynthetic lamellae and the eventual dedifferentiation of chloroplasts to form proplastid-like structures. This decrease or disappearance of photosynthetic lamellae is found to be correlated with the degradation of internal membranes, coupled With the diluting out of stroma constituents and thylakoid membranes due to chloroplast divisions, especially unequal constriction. The process is not completely synchronized so that some plastids still retain a few intact lamellae while other plastids contain degenerating lamellae or have lost the lamellae completely. In addition, some plastids observed at this stage contain one to several starch grains although starch is very rarely found in the inocula. A stricking feature at this stage is the apparent grouping of organells in the vicinity of the nudeus. The cytoplasm has increared in quantity and crosses the cell in bridge-like extensions partially filling in the cell center. At this stage, well defined proplastids are recognized. After 7 days of continuous culture, many Cells appear meristematic in that the cytoplasm, rich in organelles, fills most of the cell. The majority of the plastids found in the cell at this stage are proplastids. They may be ptesent as budded dividing forms or as oval spheroids. Their internal structure consists of randomly positioned single thylakoids resembling those of proplastids in intact development leaves. During the stationary phase of cell growth, the internal membranes of plastids are organized into stacked grana interconnected by stroma thylakoids, and at the same time the plastid ribosomes increase. A correlation was discussed between the dedifferentiation of the highly vacuolate cells induced by the subculturing process and a dedifferentiation of the chloroplasts with the cells.
