According to the principal control point (PCP) hypothesis, experiments with excised, carbohydrate-starved stationary root meristems of Vicia faba var. minor have demonstrated that cells which previously divided asynchronously were preferentially blocked in G1 (PCP1) and G2 (PCP2) phases. When stationary phase meristems are supplied with exogenous carbohydrate (2 % sucrose), the G1- and G2-arrested cells start out DNA replication and mitotic divisions, respectively. The resumption of DNA synthesis and mitosis is not immediate and the delays of Gi - and G2-arrested cells are found different. Using this model, we examined the effects of 4 pulse incubations with okadaic acid (OA), a specific inhibitor of PP1 and PP2A, on the duration of intervals elapsing between the provision of sucrose and the first appearance of S- and M-phase cells. We have found that depending on the period during which OA had been applied, the release from G1 and G2 phase arrest-points becomes prolonged, showing different time-course modifications The obtained data provide evidence that activation of PP1 and PP2A is required to allow the cells for both PCP1→S and PCP2→M transitions in root meristems of V. faba.
Root meristem nucleoli from soybean (Glycine max. cv. Aldana) seedlings germinated for 3 days at 25°C and then for 4 days at 10°C or still at 25°C (control) were examined. Chill was observed to reduce root meristematic zone growth 15-fold. Nucleoli doubled their volume at 10°C. Autoradiographic studies showed that after 20 min 3H-uridine incubation at 10°C, incorporation of this precursor (postincubation time 0) into nucleoli of chilled seedlings was 4.7 times weaker than in the control. After 80 min postincubation in nonradioactive medium, the cytoplasm became the most intensely labelled cell area in the control material, while in chilled roots the nucleoli were still most intensely labelled and the cytoplasm was 11 times less labelled than in the control. The increase in nucleoli volume at 10°C is suggested to result from greater cold-induced inhibition of the dynamics of maturation and transport of ribosome subunits than of rRNA synthesis dynamics. Ultrastructural studies of chilled seedling nucleoli showed a significant decrease in the fibrillar component and an increase in the granular component, forming characteristic clusters. They are supposed to correspond to shortened and condensed pre-rRNA transcription complexes (compacted "Christmas trees").
Experiments with 3H uridine incorporation showed that a 144-hr treatment with cadmium, chromium or lead, each at a concentration of 10-4 M, as well as the addition of calcium (10 -3 M), affected RNA in the nucleolus, nucleoplasm and cytoplasm in cortex cells of pea root meristem. Disturbances in RNA transport (especially from the nucleolus to the cytoplasm) appearing due to toxic metals and calcium suggest that this RNA was mostly of nucleolar origin.
Hyphae growing for 1 h in the presence of 0.05% and 0.1% N,N-bis(3-aminopropylo)dodecyloamine (APDA) showed malformations in the nuclear structure, including changes in shape, budding, progressive disappearance of the nucleolus and chromatin, and damage of the nuclear envelope. The Allium test revealed that at both concentrations used (0.005%, 0.01%) the tested substance lowered the frequency of dividing cells and induced mitotic abnormalities such as c-metaphases, anaphase-telophase bridges and sticky chromosomes, lagging chromosomes, micronuclei, binucleate cells, budding nuclei and partial extrusion of the nucleolus from the nucleus. These data indicate that APDA has cytotoxic, mitodepressive and turbogenic effects, especially at the higher dose. The results showed that APDA may be safely used as a microbicidal agent to destroy microorganisms developing on experimental material.