Graduate Thesis Or Dissertation
 

Contracting nucleolar vacuoles in somatic tobacco cells growing in microcultures

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  • Contracting nucleolar vacuoles are a general and consistent feature of somatic tobacco cells growing in microcultures. Nucleolar vacuoles were studied in cells from callus tissues of two single-cell clones, H-196 and H-239, of hybrid tobacco (Nicotiana tabacum X N. glutinosa). Vacuoles periodically contracted either completely or incompletely, apparently releasing a fluid material into the nucleus, with a corresponding decrease in diameter of the nucleolus. The lack of a stainable material within the nucleolar vacuoles after recommended killing and fixing procedures, collapsed vacuoles in killed and fixed cells, and the fact that no solid material could be seen to move from the vacuole into the nucleus indicated the soluble, fluid nature of the vacuolar contents. In actively growing cells, nucleolar vacuoles were observed to occur in about 70 percent of the cells examined, whereas nucleolar vacuoles were present in only about 33 percent of the senescent or weak cells indicating a relationship between status of the cell and occurrence of nucleolar vacuoles. Contracting nucleolar vacuoles were also observed in callus cells of hollyhock (Althaea rosea), tomato (Lycopersicon esculentum), horse bean (Vicia faba), and in single-cell clone G-252 of Nicotiana glutinosa. Nucleolar vacuoles were also observed within living cells of incense cedar (Libocedrus decurrens), carrot (Daucus carota var. sativa), potato (Solanum tuberosum), wheat (Triticum aestivum), annual bristleweed (Haplopappus gracilis), corn (Zea mays), lettuce (Lactuca sativa), and Douglas fir (Pseudotsuga menziesii). Many reports of the occurrence of nucleolar vacuoles within nucleoli of other plants and animals are reported in the literature. Several other points were established in this study concerning the nucleoli of living tobacco cells. In many instances, there was a decrease in nucleolar volume as the tobacco cells aged, became senescent and approached death. Occasionally protuberances of solid nucleolar material formed at the periphery of the nucleolus which were released into the nucleus. The number of nucleoli within the nuclei of the living tobacco cells studied varied from 1 to 30, but usually there were from 1 to 6. In recently divided tobacco cells, nucleoli were seen to fuse following telophase to form either one large nucleolus or a few smaller nucleoli. In some instances, the number of nucleoli increased as the cells became senescent. Apparent fragmentation of nucleoli prior to death was observed. Evidence from the literature points to the fact that the nucleolus is particularly active in RNA (ribonucleic acid) metabolism and specifically in the metabolism of soluble, transfer RNA. It is therefore suggested that the contracting nucleolar vacuoles within living tobacco cells may be a mechanism for the controlled release of a soluble metabolic intermediate, perhaps transfer RNA.
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Déclaration de droits
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