- 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.