Mitotic Cycle Regulation in the Meristem of Cultured Roots: The Principal Control Point Hypothesis

Cell cycle regulation during growth-dormancy cycles in pea axillary buds

Accumulation patterns of mRNAs corresponding to histones H2A and H4, ribosomal protein genes rpL27 and rpL34, MAP kinase, cdc2 kinase and cyclin B were analyzed during growth-dormancy cycles in pea (Pisum sativum cv. Alaska) axillary buds. The level of each of these mRNAs was low in dormant buds on intact plants, increased when buds were stimulated to grow by decapitating the terminal bud, decreased when buds ceased growing and became dormant, and then increased when buds began to grow again. Flow cytometry was used to determine nuclear DNA content during these developmental transitions. Dormant buds contain G1 and G2 nuclei (about 3:1 ratio), but only low levels of S phase nuclei. It is hypothesized that cells in dormant buds are arrested at three points in the cell cycle, in mid-G1, at the G1/S boundary and near the S/G2 boundary. Based on the accumulation of histone H2A and H4 mRNAs, which are markers for S phase, cells arrested at the G1/S boundary enter S within one hour of decapitation. The presence of a cell population arrested in mid-G1 is indicated by a second peak of histone mRNA accumulation 6 h after the first peak. Based on the accumulation of cyclin B mRNA, a marker for late G2 and mitosis, cells arrested at G1/S begin to divide between 12 and 18 h after decapitation. A small increase in the level of cyclin B mRNA at 6 h after decapitation may represent mitosis of the cells that has been arrested near the S/G2 boundary. Accumulation of MAP kinase, cdc2 kinase, rpL27 and rpL34 mRNAs are correlated with cell proliferation but not with a particular phase of the cell cycle.

Leaf growth of hybrid poplar following exposure to elevated CO2

Leaf extension was stimulated following exposure of three interamerican hybrid poplar clones (Populus trichocarpa P. deltoides); 'Unal', 'Boelare', and 'Beaupre' and a euramerican clone 'Primo' (Populus nigra×P. deltoides) to elevated CO₂ , in controlled environment chambers. For all three interamerican clones the evidence suggests that this was the result of increased leaf cell expansion associated with enhanced cell wall extensibility (WEx), measured as tensiomerric increases in cell wall plasticity. For the interameriean clone 'Boelare', there was also a significant increase in cell wall elasticity following exposure to elevated CO₂ (P⩽ 0.001). The effect of elevated CO₂ in stimulating cell wall extensibility was confirmed in a detailed spatial analysis of extensibility made across the lamina of expanding leaves of the clone 'Boelare'. For two of the interamerican hybrids, 'Unal' and 'Beaupre', both leaf cell water potential Ψ and turgor pressure (P) were lower in elevated than in ambient CO₂ . By contrast, no significant effects on the cell wall properties or leaf water relations for the euramerican hybrid 'Primo' were observed following exposure to elevated CO₂ . suggesting that the mechanism for increased leaf extension in elevated CO₂ , differed, depending on clone. The cumulative total length of leaves of 'Boelare' grown in elevated CO₂ , was significantly increased (P≤ 0.05) and since leaf number was not significantly increased in any inter-american clone it is hypothesized that final leaf size was stimulated in elevated CO₂ for these clones. By contrast, there was no significant effect of CO₂ on cumulative total leaf length for the euramerican clone 'Primo', but leaf number was significantly increased by elevated CO₂ . The measurements suggest that total tree leaf area was stimulated for a range of poplar hybrids exposed to elevated CO² . Given the short rotation of a coppiced crop, it is likely that increased leaf areas will result in enhanced stemwood production when hybrid poplars are grown in the CO, concentrations predicted for the next century.

Cell radiosensitivity variation in synchronously-dividing root meristems of Pisum sativum L. and Zea mays L. during the mitotic cycle

The cell divisions in pea and maize seedling root meristems were synchronized by treatment with 0.03 per cent hydroxyurea for 24 hours. At different times after block cessation (removing seedlings from hydroxyurea), i.e. according to certain phases and subphases of the mitotic cycle, the seedlings were treated with 60Co gamma-radiation at doses from 3 to 32 Gy. Evaluation of seedling radioresistance by the survival of the main root meristem on the tenth day after irradiation led to the following sequence of cell stages in order of increasing resistance. For the pea root meristem: G2, M, late G1, early S, late S, middle S and early G1. For the maize root meristem: late G2, M, G1, early S, early G2, late S and middle S.

Metabolic events in synchronised cell cultures of Acer pseudoplatanus L

Cell division was synchronised in 4-litre batch cultures of Acer pseudoplatanus L. by starvation and regrowth. Up to five consecutive cell cycles were observed in each culture. Mitosis and cytokinesis were synchronised within 0.2 cell cycles. Accumulation of extractable DNA was discontinuous and separate from cytokinesis. Correction for the degree of synchrony in the population gave: G1=13-37 h, S=15 h, G2=14-19 h and M=0.9-1.3 h. Thymidine kinase activity and [(14)C]thymidine incorporation were highest during S-phase. A peak of activity of aspartate transcarbamoylase occurred during G2. Peaks in succinate dehydrogenase activity and respiration rate were observed at the initiation of DNA synthesis and just prior to mitosis. The activity of glucose-6-phosphate dehydrogenase doubled in one step during the cell cycle. Total RNA and protein accumulated continuously through the cell cycle; the final rate being twice that observed initially.